Would a lighter aircraft require the same amount of power than a heavier one to fly at the same speed? I had a question come up:

  If aircraft A is identical to aircraft B, except that aircraft B weighs 200 kg more. Which of the following statements are true?

The answer I selected was wrong:

  Aircraft A would need less power to fly at the same speed as aircraft B

I pictured that with less weight, a reduced angle of attack would result in less drag and hence less power? Would anyone be able to “lighten” me with this?

Thanks for the feedback, it may have just been a typo..
The 4 multiple choices where:
1) Aircraft A would need less power to fly at the same speed as aircraft B
2) Aircraft A would need the same power to fly at the same speed as aircraft B
3) Aircraft B would have poorer climb performance
4) Aircraft B would require a longer runway for landing

You are right. The heavier aircraft needs more power. With one exception.

The power needed to keep an aircraft in the air is the product of flight speed and drag. When flying at the same speed, the drag of both aircraft is crucial to answer the question. Since both share the same geometry, it is safe to assume that both have about the same friction and parasitic drag. But the heavier aircraft needs to fly at a higher lift coefficient in order to carry its higher mass, so it will create more induced drag.

The one exception is when both aircraft have a highly laminar airfoil with a pronounced drag bucket. If the flight speed is picked such that the heavier aircraft will still fly within this drag bucket but the lighter aircraft will fly at a lift coefficient below the lower boundary of that drag bucket, the difference in friction drag could be higher than the difference in induced drag (which is small at high speed). But that exception would need to be made clear in the question.

To fully judge the correctness of the test, please provide all answers. As @David K suggests, your error might have been not to select all correct answers.


I accidentally deviated from an ATC instruction as a student pilot. How bad was it? Here's the scenario: I was a student pilot on inbound for landing at my home airport on my final solo cross country flight I needed before doing a checkride.

At roughly 8 miles out to the north west, I established contact with the tower and was told to "enter left base, runway 7, report 3 miles out." I interpreted the "report 3 miles out" as to give the tower a call when I was 3 miles away from the field, ie, when my GPS indicated I was 3 miles from the field.

Now, it was the most windy day I had flown solo in so far (15kt gusts) so I wanted to make sure I had ample time to line up with the runway on final and get a feeling for how the winds were blowing since the ATIS reported them as being variable in direction.

That said, what I ended up doing was making a wide base, except apparently I made it a little too wide because the GPS did not indicate I was 3 miles from the field until I was on final. At this point, I called the tower, reported I was on a 3 mile final and was given clearance to land. After landing, I still had no idea I had done anything incorrectly as the controller made no indication to me.

A few hours after I landed, my CFI sent me a text message asking me how "report a 3 mile base" meant "report a 3 mile final and fly a straight in approach." I'm not sure if he was on the frequency when it happened or if the tower controller called him after the fact and told him about it because I was told to "report 3 miles out", not "report 3 mile base."

I completely understand that I made my base leg too wide and that caused me to deviate from an ATC instruction. What I'm wondering is how bad of a screw up is this? My CFI's reaction was one of legitimate anger and it makes me want to stay on the ground for a while.

Well, you screwed up, but nobody got hurt, no metal was bent, and since the tower didn't yell at you presumably you didn't obviously endanger anyone's safety (though there's always some level of risk when an aircraft isn't where the tower expects it to be).

As far as deviations go, that's not good, but it's not terrible. The tower was probably expecting you to report 3 miles out, continuing in to about a mile from the field for your downwind.

Schedule an extra half hour of ground time with your CFI before your next flight to talk about what happened. Also take the time to fill out an ASRS report explaining what happened and why (It's probably unnecessary in this case, as a student the tower will almost certainly be satisfied with you getting "a good talking-to" from your instructor, but it's a good way to learn how that system works.)

There are a couple of other things you might want to do on your next few training flights:

Spend some time getting comfortable in the pattern on windy days.
You should be comfortable in a tighter pattern than 3 miles, even on a windy day (see my next item for why). This takes practice, and fortunately for you it's spring (at least in the USA) so there are plenty of windy days in most of the country!
Practice some engine-out approaches to get a feel for your plane's glide performance.
A base leg 3 miles out in a typical GA trainer is very wide -- Remember you generally want to be able to make some spot on the runway if your engine quits on downwind, and you probably can't do that from 1000 feet AGL at 3 miles out.
Take a few laps around the pattern and have your instructor pull the power at various points so you have a feeling for how the energy management works out - this will give you a good feel for how wide your pattern should be.
Turn the GPS off and practice flying visually.
The GPS is one link link in your error chain, but I think it's a big one: If you were flying visually and relying on landmarks to judge your distance rather than the 3 mile alert from your GPS you might have realized "Hey, I'm entering downwind - I should tell the tower!"


Where does this integral for a 'aerospike nozzle' come from? Today I was reading about the aerospike nozzle and found this image:

(from section 10.3.1 of this lecture)

This image tells us how to measure the thrust of this kind of nozzle. I can understand all of it except this part:

Where does this integral come from? How do they derive it?

I expect you know how a conventional rocket nozzle works: It expands the exhaust gas, and the pressure inside helps to increase thrust, pushing the nozzle forward.

Now turn the conventional nozzle inside out. The expansion happens on its outside, but there is still a pressure acting on a backward-facing surface. The graph doesn't explain it, but the ramp area $\text{A}_{\text{Ramp}}$ is the projection of the nozzle area in flight direction. The ramp pressure $\text{p}_{\text{Ramp}}$ is the pressure acting along the nozzle surface, and it must be reduced by ambient pressure $\text{p}_{\infty}$ in order to arrive at the correct thrust. The integration is needed because $\text{p}_{\text{Ramp}}$ varies over the nozzle area.


Brace position - does it really save lives? There are some people say the brace position was invented so that passengers don't experience pain when they die in a crash. The brace is designed so that you die quicker (by breaking your neck).

One source: Here

Is that true? If not, how does it provide better protection during a crash?

The FAA has a study on it you can find here the study is worth reading completely but the important parts of the conclusion seem to be,

  Current Brace Position

  This position (head against the seat back,
  hands on top of the seat back) was evaluated for three common types of
  seat backs.

  This position was only successful for locked-out type seat
  backs. For full break-over and energy absorbing type seat backs, the
  ATD’s arms pushed the seat back away, allowing the head to accelerate
  relative to the seat back, increasing the severity of the head impact.
  There was, however, no evidence that the ATD interaction with any of
  the seatback types resulted in hyper-extension of the shoulder joint.

  The “Forward” and “Vertical” leg brace positions permitted the lower
  legs to flail forward and contact the seat in front. This contact
  produced femur bending and compression (below criteria limits), and
  tibia injury risk in some cases.

  The “Aft” leg brace position reduced
  lower leg flailing and prevented femur contact with the front of the
  seat frame. This position resulted in low femur and tibia injury
  assessment values; however, it is only achievable for occupants whose
  lower legs are long enough for their feet to firmly touch the floor.

  Alternate Brace Position

  To reduce the detrimental interaction
  between the occupant’s arms and the seatback, the current position was
  modified by placing the hands down by the lower legs instead of on the
  seat back.

  This “Alternate” position was successful in reducing head
  and neck injury risk for all of the seat back types evaluated.

  Pike Position

  This position is currently recommended when the seat
  in front is too far away to support the head, or there is no seat in
  front, as in a bulkhead row.

  It was successful in reducing head and
  neck injury risk in the row-to-row scenario, as long as the struck
  seat back has a relatively soft local compliance at the point of
  impact. When seated behind a bulkhead, the effectiveness of the pike
  position was dependent on whether the head of the occupant struck the
  bulkhead. “Head-path-reducing features,” such as Y-belts, prevented
  head contact at the typical 35-in setback.

So the answer is really, it depends on the position as well as the seats in question. You find a full explanation of what the various seat types are on page 2 of the study.

Perhaps the most interesting note is that they call for a potential change in the current brace position:

  ...and the current positions recommended in AC 121-24B may need some
  adjustment to provide an equivalent level of safety for all passenger
  seat back types.


Is this aircraft legal in Canada? is an aircraft like this legal in Alberta, Canada? It's basically a human sized quadrocopter. If so do you need any license?

It's not possible to tell for certain based just on this one picture, but I don't see any reason why it wouldn't be. Unlike the US, Canada doesn't recognize the concept of an ultralight helicopter, so you'd need a regular helicopter license to actually fly it.


What is the role of discontinuous surfaces on the inside and outside elevons of the F-117? Why are there discontinuous surfaces on the inside and outside elevons of the stealth F-117? Will that not be highly reflective to the radar?

I believe you are referring to the gaps between the elevons and the other control surfaces?  Those are called cats eye gaps and they’re used to reduce radar returns between the gaps.  Virtually all stealth aircraft use this technology.


Does lift coefficient for maximum range change for a glider experiencing headwind? When a glider or unpropelled aircraft experiences headwind, I understand that the flight velocity and sink velocity for the "new" maximum possible range both increase (see velocity polar from How does wind affect the airspeed that I should fly for maximum range in an airplane?).

Intuitively I would think this means that $C_{L}$ needs to decrease so that the now higher $\frac{1}{2}\rho v^2$ can be compensated (small glide slope assumption $L \approx G$ i.e. $L >> D$).

For the theoretical $C_{L_{R_{max}}}$ equation for a symmetric $C_{L}-C_{D}$ polar, I don't see this playing a role:

$C_{L_{R_{max}}}=\sqrt{C_{D_0}\pi\Lambda e} $

...as $C_{D_0}$ to my knowledge doesn't change with velocity.

Does lift coefficient $C_{L_{R_{max}}}$ for maximum range change for a glider experiencing headwind?

Your first statement is correct, look at the lift vs AOA chart.  You go faster by pitching down (reducing lift and drag) and speeding up above Vbg(lift recovered now more drag), which effectively changes your trim and lowers AOA.  Why?  Because best gliding "Vbg" is all about covering the most distance per unit altitude, NOT time in the air.

In the most extreme case:  headwind = gliding airspeed you can see that no forward progress is made unless you glide faster.  Forward progress per unit altitude is:

(airspeed - headwind)/altitude

Your new Vbg now factors in ground gain vs increase in parasitic drag.  The result is (obviously) you can't glide as far into a headwind, but (more importantly) you can help making your landing zone in a headwind by increasing your airspeed above published Vbg.


Do airlines have freezers? In the latest twist to the case of the dead rabbit, an anonymous "airport worker" has told a reporter that it was killed by leaving it in a  freezer locker, contradicting the accounts of United executives who have claimed the rabbit was well treated. United also hastily incinerated the body without the owner's permission, preventing any kind of autopsy from occurring. This all happened apparently at Chicago O'Hare.

My question is why an airline would have a freezer at all, and why would they be putting luggage or pets into said freezers. I mean when I make connections I kind of assume they are not putting my bags in a freezer. Do airlines have freezers?

(Source) Cool room in a cargo terminal.

  The rabbit arrived fine but there was some sort of mistake and he was locked inside a freezer overnight.

Goods that are perishable and human remains awaiting transport, wait in the freezer before being loaded onto flights.

Different rooms for cargo handling can be seen here, they include:

Cool rooms
Freezer rooms
Warm rooms (for e.g. perishable flowers)
Dangerous goods rooms

There are also specialized ULD's that come with refrigeration units.

(Source) Temperature controlled ULD.

  [Depending] on the nature of the goods to be transported, [ULD's] may have built-in refrigeration units.


Do pilots ever use the aircraft's ICAO 24-bit address when communicating with ATC? Do pilots know the unique ICAO 24-bit hexadecimal code for their aircraft? Do they ever give it directly to ATC? For example, to controllers who make the flight strips with destination, departure, type of aircraft etc. I know for a fact that some flight plans are sent out that include the unique hex code for the aircraft.

Typically pilots do not know the 24-bit aircraft address of their aircraft.

The 24 bit address is intended for data protocol level of communications and is not used in voice communication. Therefor there is no need for pilots to be aware of their ICAO 24-bit aircraft address. They use the aircraft's registration if they need to identify the aircraft they are using for the flight.

The 24-bit address is used by transponders for communication with Mode-S radars, ADS-B transmissions, by TCAS for tracking traffic and coordinating Resolution Advisories and by Data-Link communications.


What is the distance unit on a navigation display? I want to learn what is the distance unit on a navigation display.

I pointed the distance marks with red circles on the navigation display. What are these 10, 15 etc. values' unit? Are they representing mile, nautical mile or kilometer?

And can it be adjusted in a different measurement unit (mile, kilometer or nautical mile) or is it a standard unit for aviation and navigation displays?

They are nautical miles, which is the standard for essentially all discussions of distance in aviation. (Visibility being an exception; it can be reported in essentially every unit except nautical miles.) The software certainly could switch the display to another scale such as kilometers, although I’m not familiar with any systems that provide that option.


Why do Boeing and Airbus have distinctively different nose designs? Boeing aircraft generally have a pointy nose, which seems to me to imply better aerodynamic qualities, less drag etc. Airbus aircraft on the other hand have rather bulbous noses.

Is this difference to give Boeing and Airbus aircraft a consistent, distinctive visual appearance, or is there any specific aerodynamic reason for the different designs?

This image is only an example and my question is not about any specific model.

The answer to this is, in part to do with corporate culture and part aerodynamics.

The corporate culture and history part is that Boeing have always built their noses that way and senior engineers have a tendency to return to designs they have used successfully before. If you look at the nose of a 747 and the nose of a B-17 you'll see some distinctive similarities that follow through all the major Boeing aircraft. This isn't surprising as lead designer of the 747, Joe Sutter, started working for Boeing on the 707 project under the direction of the engineers that designed the early flying forts.
Airbus is a newer company with a different corporate culture. They tend to embrace new design techniques like CFD computer modelling to a greater extent and this leads to slightly more efficient but less aesthetically pleasing nose and wing designs.

The science side is related to the speed that modern airliners travel at (around 0.85mach). At these speeds aircraft begin to encounter a phenomenon called 'Wave Drag' which is the incremental build-up of compressive shockwaves along the wings and fuselage as the aircraft approaches the speed of sound. The effect of wave drag is to drastically increase the overall drag affecting the aircraft, increasing the power output required from the engines.

There are several methods of combating wave drag, the first of these is the swept wing that has been a feature of trans and supersonic aircraft since WW2. The optimum angle of wing sweep is determined by the cruising speed of the aircraft. A more recent innovation is the Transonic or 'Whitcomb' Area Rule which states that:

"Two airplanes with the same longitudinal cross-sectional area distribution have the same wave drag, independent of how the area is distributed laterally"

Wave Drag can be reduced by attempting to match the cross sectional proportions of an aircraft as closely as possible to those of a Sears-Haack body (an aerodynamically perfect shape for supersonic flight) This is why Airbus planes have blunter nose. it is also why there are strange bulges under the wing roots of the A380. they are both attempts to make the planes cross-section conform to a Sears-Haack body more closely.


Do the speed brakes on an F-16 change its pitch? In level flight at 400 knots, what happens when the speed brakes are extended?

Because the speed brakes are tail mounted, and because they deploy upwards and downwards equally, pitch should not change.  The airplane's fly-by-wire system likely dampens any slight pitch changes caused by the buffeting.

The only photos I've seen of the speed brakes open are of them fully open, not partially open.  So they may open gradually with different deflections up and down, or with different resulting air loads up and down, or both, or neither.  But in any of those four cases, the FBW should correct for pitch excursions by deflecting the other control surfaces.


How much thrust is available from the Allison T-56-A-15 engine? What is the thrust availability per engine in Lockheed C130H?
The engines are Allison T-56-A-15 (SHP=4100, SFC=0.5)
Propellers = NP2000 eight Bladed(Prop Efficiency if assumed to be 90% constant at all speeds)
SHP - Shaft Horse Power

From Raymer's "Conceptual Aircraft Design", page 327, equation 13.15:

$$ \eta_p = \frac{TV}{P}$$

Where:

$\eta_P$ is the propeller efficiency
$T$ is the Thrust [kN]
$V$ is the True Airspeed [m/s]
$P$ is the engine power [kW]

Reverting the formula you get that the Thrust per engine is given by:

$$ T  = \frac{P \eta_p}{V}$$


What type of visa is required to be allowed to earn a PPL in the US? What are the visa types (H1b, L1b, H4, L2, J1, etc) accepted by TSA to learn flying for recreational purposes?

UPDATE 1

I found some further information from National association of flight instructors, According to them; Person on work visa can follow Part 61 course and get the PPL.

UPDATE 2

I have received the permission from the TSA to flow the Category 3 flight training. It took around 1 & 1/2 weeks to process and it will cost around 130 for the application and 99 for fingerprint process.

Based on this process, I understood that L1 Or L2 Or H1 Or H4 holders are eligible with the right set of document and no criminal record.

For a foreigner to come to USA to learn to fly, one would need an F-1 or M-1 visa.

This article in Flying magazine provides some information. You can get more information on from this school1. Also, you can look at this meta question: Can I ask about details of coming to the US to get a PPL?

Update:

It appears that you are already in the US on H-1b visa (it wasn't mentioned in the question), you need to go through the security clearances mentioned in Alien Flight Student Program (official version on TSA website).

AOPA has put together the related information (website). I also found about this flight school1 which has outlined the process.

If you search on Google about Alien Flight Student Program, you will find a ton of information.

1: Disclaimer I am not advertising any flight school.


What are these five indicators at the front of the 737 passenger cabin? As a passenger I noticed set of five indicators on ceiling at the front of the 737 passenger cabin. They are: red, yellow (or white), blue, white, red.

Red ones seem to be coupled together (what could indicate their higher importance and also prevents problem with failed light bulb).
On the photo, there seem to be two white indicators but on older models I almost certainly remember that one of them was yellow.

I have seen those indicators in old and new 737 models. I understand they inform the cabin crew of something but I did not find explanation so far.

What is the exact purpose of each indicator?
Are they controlled manually or automatically?
Are they Boeing-specific?
How are similar conditions communicated to cabin crew in other similar types of aircrafts (e.g. A320) which do not seem to have these indicators?

This is the forward Master Call Light Panel installed in the ceiling of the cabin. A second one is located aft. These panels exist on all large aircraft for the cabin crew members to be alerted without staying at their seats.

The number and color of lights vary according to customer requested options for the cabin. Some lights may be duplicated to indicate which side of the cabin is affected (e.g. for the passenger call). These panels are not required by regulation, it's to facilitate the work of cabin crew members.

For the B737

The non customized panel of a B737 looks like this:

Source: Boeing 737 FAM

The lights illuminates and chimes are played:

Pink, HI-LO chime: A crew member dialed the forward station number on the cabin interphone handset.
Blue, HI chime: A passenger pressed the ATTENDANT CALL switch on the Passenger Service Unit.
Amber, HI chime: A passenger in the lavatory pressed the ATTENDANT CALL button.

The white portions are not used, there is no light here.

Source: Boeing 737 NG Flight Attendant Manual (D6-27370-6/7/8/900-FAM).

For the B787

Green, HI-LO chime: Cabin crew call.
Green flashing, HI-LO chime three times: All call.
Pink, HI-LO chime: Flight deck call.
Blue, HI chime: Passenger call.
Amber, HI chime: Lavatory call.
Amber flashing, HI-LO continuous chime: Lavatory smoke.
Amber flashing, HI continuous chime: Crew rest smoke.
Red: Sterile flight deck (pilots mustn't be disturbed except for safety matters, and attendants must expedite operational activities leaving commercial activities for later).

Source

No doubt in a few years, attendants will receive these indications directly on individual portable devices with more details.

Airbus has its own similar colors and chimes, see:

What does a blue light by the toilet sign mean?


How can I find out how much a passenger window of a Boeing 737 costs? Where do I have to look to find out how much a passenger window of a Boeing 737 costs?

This page has declared values for part shipments, including one with the 140N2139-4, which is the assembly containing the seal, mid pane, and outer pane for the 737 NG. The declared value is 24,562 INR which is currently about 375 USD. A shipment of 20 also listed has a lower unit price.

For searching purposes GKN has part numbers in their catalog. You typically have to ask for a quote from suppliers.


What is the ultraviolet signature, if any, of aircraft and jet engines? The Wikipedia article on the FIM-92 B variant of the Stinger Missile says:

  In this version, the infrared seeker head was replaced by a combined IR/UV seeker that utilized rosette scanning. This resulted in achieving significantly higher resistance to enemy countermeasures (Flares) and natural disturbances. Production ran from 1981 to 1987, a total of 600 missiles were produced.

I'm assuming "IR/UV" means "infrared/ultraviolet."

Do aircraft (or jet engines) really have a strong ultraviolet signature? If so, how is it formed? Is it only evident at high airspeeds?

(Also, what on Earth could it be talking about by "natural disturbances"?)

Your assumption is correct - it is indeed about infrared (wavelength between 750 nm and 10,000 nm) and ultraviolet (wavelength between 100 nm and 400 nm) radiation. The sensor detected two specific wavelengths, one from each range.

Every body has an electromagnetic radiation which is consistent with its temperature. This radiation has an intensity distribution which shifts to shorter wavelengths with temperature. By comparing the intensity of the IR and UV parts of this radiation, the sensor can much better distinguish between radiation sources of different temperature. Flares are normally hotter than engine exhausts, so by detecting an excess of UV radiation, the sensor can distinguish between flares and exhaust pipes.

"Wiens law" by 4C - Own work, based on JPG version. Licensed under CC BY-SA 3.0 via Commons.

Natural disturbances means reflected sunlight off a polished aircraft surface or canopy, or off lakes or windows on the ground. Even after filtering by the upper atmosphere, sunlight will contain much more UV radiation than your average exhaust pipe, and glares were a big distraction to early thermal seekers. Sunlight would correspond to the green line in the diagram above, while the exhaust would be even below the dark red line.

Note that the absolute intensity also goes up with temperature, but would not help in identifying the temperature of a target if only one wavelength is observed. A more distant, hotter target would cause the same intensity than a colder, closer one. By sensing two wavelengths, the sensor could much better tell an exhaust pipe from other heat sources.

To answer the question directly: The UV signature of the aircraft itself is practically zero except for the reflection of the UV part of sunlight, and the active UV emission of a jet engine is extremely low and concentrated around its exhaust when seen from behind. Lighting the afterburner will increase the UV radiation, but it will still be much less relative to the IR radiation that that of, say, burning magnesium.


What is this black symbol on a VFR chart? What is this black symbol near KLBE VFR chart?

An obstacle, an "Outdoor theater".https://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/aero_guide/#sectional_radioaids


Do turbofan nacelles generate lift? Since they are essentially an annular wing, how much do they contribute if at all to an aircraft's total lift? Would the effect of this lift be noticeable enough to help cancel out the drag they produce?

Most jet nacelles contribute little or no lift, but there is at least one highly notable exception.
The airflow inside the nacelle is dominated by the engine and any chance to create differential pressure between upper and lower leading sections is kept negligible in order not to disrupt the intake flow.
The outside is more of a stubby fuselage than an annular wing and low drag dominates the design. The avoidance of interference with the leading edge of the wing, part way along and just above, is also a major consideration for the upper profile. Lift is not really considered, and is not present to any significant extent.
Having said that, for a large high-bypass turbofan, when placed well forward the lifting area can become significant at high angles of attack (whether positive or negative). It can be powerful enough to move the centre of lift forwards, reducing pitch stability and at steep angles leading to a tendency for the angle to diverge further. When Boeing were developing the Max variant of the 737, its new engines suffered just this problem on takeoff and landing. Boeing's solution was to introduce a Manoeuvring Characteristics Augmentation System (MCAS) which automatically operated the elevator to reduce the angle sharply if it got too steep. Tragically, in their hurry they botched the job, two commercial flights crashed fatally and the 737 Max has been grounded indefinitely until they can get a fix certified.


Did I witness an engine test for a 757 at MAN? A while ago (Summer 2020) I was walking near MAN airport (UK) and saw a 757 taking off. There was a peculiar event during takeoff and I'd love to know what it was.
Once airborne at about 1000 feet (complete guess) its starboard engine started showing flashing rings of flames and there was a loud repeating banging sound for about 20-30 seconds. The plane continued to climb up to several thousand feet.
While watching this I opened FlightRadar and saw that the flight's destination was also MAN.
Over the next 30 minutes or so the plane did a few laps and presumably came back to land.
I'd love to know what this event might have been?

See this avherald report: https://avherald.com/h?article=4db1ccdb&opt=1

A Jet2.com Boeing 757-200, registration G-LSAN performing a test flight from Manchester,EN to Manchester,EN (UK), was in the initial climb out of Manchester's runway 05L when the right hand engine (RB211) emitted a loud bang and streaks of flame. The aircraft levelled off at 5000 feet and returned to Manchester for a safe landing on runway 05R about 25 minutes after departure.

So yes, it was a test flight, but it obviously did not go according to plan, resulting in an engine failure.


Why do both east and west isogonic lines tilt toward the west as they go north? Here is a picture of the 3 degree east isogonic line just north of Tulsa, OK.

Here is a picture of the 4 degree west isogonic line just east of Nashville, TN.

They both trend westward in relation to true north as you follow the lines northward.  Why do they not go in opposite directions if one is to the east and the other to the west?  This also implies that the 0 degree isogonic line is not parallel to true north.

I checked, and sure enough, it also goes west as you follow it to the north.

Why don't the lines tilt in different directions after crossing over from zero degrees?

Because the present position of the magnetic pole pole is such, that in central and eastern US states the isogonic lines are tilted to the West. But in western parts of the U.S., those lines are tilted to the East...


What happens to EGT after a bird strike? I just watched the movie "Sully", and I really enjoyed the effort put into making the movie as accurate as possible.  As I'm working towards my PPL, I decided to apply some of my new aviation knowledge when watching the movie.  I especially focused on the center MFD.  I understood everything I saw, except for the Exhaust Gas Temperature (EGT) readings after the bird strike.  I noticed that Engine Pressure Ratio (EPR) and N2 fell, but EGT stayed in what looked like roughly the same position.  I would have expected EGT to fall after the bird strike destroyed the engines, but it didn't.  Is this normal?  Did the flames seen coming out of the engines cover up a fall in EGT?

No two bird strikes are the same when it comes to engine parameters, we can look at this incident.

Bird strike in both engines, yet only one showed above-normal EGT.

In a jet engine, the bypass air assists in cooling, damage will disrupt this air flow.

The core might still be running, or flammables (oil, fuel, etc.) might leak and catch fire.

In an engine fire, EGT-rise is one of the confirming factors.

However, according to the NTSB report for Sully's plane:

  No in-flight or postaccident fire occurred.

So if it is just an engine failure, the engine will spool down.

Even if the damage prevented the windmilling, EGT should still drop. If the EGT sensors were damaged, an indication of EGT reading failure is to be expected.

Also the engine vibration indicator should show very high vibration after the bird strike.

(Source)


How far could the Wright brother's catapult have thrown their unpowered plane? I know that the Wright brothers started to use a catapult in September 1904 to help their Flyer II take off easily even when no headwind was available.

Assuming the propellers did not turn, or they were not even mounted on their shafts, how far away the catapult could have thrown Flyer II together with its engine and pilot? 1m, 5m, 10m, 25m, 100m or more? Even a rough evaluation, based for example on an experiment with a scale model, would be good for me.

(Note: I am not interested in a pure ballistic evaluation, considering the plane the equivalent of canon ball of mass, m, that is accelerated to the speed, v, and launched at an angle alpha.)

The catapult cannot launch an unpowered Flyer II at all.

Robert's comment reaches the right conclusion using potential to kinetic energy, though his frictionless answer up to takeoff overestimates available energy and depends on aero drag not to take flight. The corrections are:

All the potential energy from the weight is not transferred to the flyer. The weight builds kinetic energy that is dissipated as it slams into the ground. Its velocity is one third that of the flyer.
Though the tower is 20ft tall, the weight only falls 16ft(5m). The rope and weight take up space.

Setting potential energy of the weight equal to kinetic energy of weight and plane:

$$ M_w\cdot H_w\cdot g=\frac{1}{2}M_f\cdot V_f^2+\frac{1}{2}M_w\cdot V_w^2$$
$$635kg\cdot 5m\cdot 9.8\frac{m}{s^2}=\frac{1}{2}\cdot 408kg\cdot V_f^2+\frac{1}{2}\cdot 635kg\left(\frac{V_f}{3}\right)^2$$
$$130\frac{m^2}{s^2}=V_f^2$$
$$V_f=11.4\frac{m}{s}=25mph$$
Since the Flyer II needed 28-30mph to become airborne, the catapult cannot launch the unpowered flyer by itself, even excluding friction and drag.


How can I certify an ultralight design as complying with part 103? Can you help me about which procedure do I need to follow in order to have a certificate that my design is part 103 ultralight.

And as a information how long would it take to get the certificate and about how much?

What would be the first step?

Thank you.

No certificate needed and "satisfactory evidence" is only needed "upon request". From the FARs:

  103.1   Applicability.

  This part prescribes rules governing the operation of ultralight vehicles in the United States. For the purposes of this part, an ultralight vehicle is a vehicle that:

  (a) Is used or intended to be used for manned operation in the air by a single occupant; >(b) Is used or intended to be used for recreation or sport purposes only; (c) Does not have any U.S. or foreign airworthiness certificate; and (d) If unpowered, weighs less than 155 pounds;

  or (e) If powered: (1) Weighs less than 254 pounds empty weight, excluding floats and safety devices which are intended for deployment in a potentially catastrophic situation; (2) Has a fuel capacity not exceeding 5 U.S. gallons; (3) Is not capable of more than 55 knots calibrated airspeed at full power in level flight; and (4) Has a power-off stall speed which does not exceed 24 knots calibrated airspeed.

  § 103.3   Inspection requirements.

  (a) Any person operating an ultralight vehicle under this part shall, upon request, allow the Administrator, or his designee, to inspect the vehicle to determine the applicability of this part.
  (b) The pilot or operator of an ultralight vehicle must, upon request of the Administrator, furnish satisfactory evidence that the vehicle is subject only to the provisions of this part.

  103.7   Certification and registration.

  (a) Notwithstanding any other section pertaining to certification of aircraft or their parts or equipment, ultralight vehicles and their component parts and equipment are not required to meet the airworthiness certification standards specified for aircraft or to have certificates of airworthiness.


How to use a set of Pressure Transducer to mimic pitot static tube for VTOL vertical airspeed indicator? May i know if i have 3 pressure transducer, 1 facing upwards and the other downwards to measure the vertical velocity of a VTOL (Vertical Takeoff and Landing aircraft) and another 1 on the nose of the aircraft, do i still need a pressure transducer to mimic a Static port to measure the climb rate? Since i already have 2 pointing upwards and downwards. For your kind advise, thanks!

You won't be measuring any static pressures because facing the transducers into the sense direction produces dynamic positive pressure which is not subject to temperature compensation or air density corrections at altitude. A Vertical Speed Indicator works by taking static pressure and a calibrated leak inside the instrument to show your climb rate. You can't replicate this using the system you describe.
Even using your suggestion of adding another transducer for static is not enough. You need a rate of change which, as stated above, is normally provided by a calibrated leak inside the VSI.
If you want to explore further options of measuring position in 3d space that don't involve pressure transducers, I suggest looking at gyros and accelerometers.


Are type-C doors plug doors? As I understand from the answer to this questions, plug doors are larger than the hole they mus fit in so that they are plugged thanks to air pressure, leading to several advantages. Then, I found this video of a type-C door operating. This door does not seems larger then the hole it must fit in and that confuses me.

Is this a plug-door?
Is there something I miss?
Is there other types of door that are not plug-door, or that are plug-door but don't look like plug-door?

No, they are not. Most (not sure if all) Airbus door are not plug-type. And neither are cargo door on most Boeing types.


Are lhe old King KX170B Nav/Com radios monaural? If so, does this mean I need to use an adaptor to play the output on stereo headphones?  My Stereo-Mono switch does not produce a two-ear audio.  Would this be solved with a converter? Would be nice to receive other pilots and ATC in both ears.

Yes. The idea of listening to anything more than scratchy AM radio via your ADF was an alien concept in the 70s, unless you had some custom system made up for you.  It's really only taken off since ANR headsets have made it quiet enough within the earcups to make music worth listening to.

You just need to get a mono-to-stereo jack adapter to hear the mono signal in both ears.


Can I attach a camera to the outside of my aircraft in the USA? I've been watching a lot of videos shot with mini cameras (like a GoPro) from outside the aircraft. Some look like they're mounted on or under wings, others from underneath the aircraft. This looks cool, I want to do it!

I know the FAA is pretty strict on unapproved modifications to aircraft and I've heard that attaching cameras can conflict with this. What should I know before I attempt to attach a camera to the outside of my aircraft?

The NTSB actually recommended the use of exterior cameras to the FAA in 2012.

It appears to be something of a grey area, but it looks like you should contact your local FSDO for approval of an external load if you're permanently fixing a camera to a plane.

The grey area comes in that if the camera is temporarily attached, you shouldn't need special approval, but in practicality it could come loose and damage the plane.

GoPro says this about affixing their cameras to planes:

  Industrial strength suction cup. Strong enough to pull dents out of a car door or stay attached to an airplane at 200mph+, proven though not endorsed by GoPro per FAA rules.

There's a forum thread where someone asked the FSDO to sign it off as a Minor Alteration, and it was determined that a short logbook entry would suffice:

  Temporarily installed a GoPro camera mounted to a standard inspection panel on (Left/Right) lower wing in place of the standard inspection panel. There are no electrical connections associated with this installation. This is a minor alteration as it has no appreciable effect on performance, flight characteristics, or other qualities affecting airworthiness (refer to FAR 1.1 Major Alteration and FAR 21.93). A net weight change of 9 oz is incurred with the temporary installation; therefore, the weight and balance change is negligible (refer to AC 43.13 1B, 10-2 (c)).


Can a plane land safely if it loses power during approach? British Airways Flight 38 experienced a loss of power during approach to Heathrow and ended up crashing 270m short of the runway.
The aircraft was on glideslope when it lost power. The name "glideslope" suggests it should be able to glide to a safe landing. So why didn't it?

Suppose a commercial aircraft (e.g. 7X7, A3X0) lost all engines (say, due to birdstrike) shortly before landing. How likely would a safe landing on the planned runway be?

It would make sense for the approach angle to be such that a safe landing is very likely. So if not, why not?

EDIT: To clarify (due to @rbp's comment): I want to know how far is the typical approach angle from being able to glide in? And why is the approach angle not chosen so that gliding is possible?

A standard “glide-slope” is 3–3.3°. 3° is 5.2% or 1:19, 3.3° is 5.8% or ~1:17.

A modern airliner will have lift to drag ratio between 17:1 and 20:1 in clean configuration and at best glide speed, which is usually somewhere in range 220–250 knots indicated, possibly even a bit more depending on weight. So it would be just barely able to follow the glide-slope if it was configured for best glide.

But on final approach it will at least need gear down and that increases the drag considerably. It will also already be slower than the best glide speed and have some flaps deployed and retracting them and accelerating to the best glide speed will take some altitude. Therefore if all engines quit on final, the aircraft is definitely landing short of the intended point.

For small aircraft I've read advice that if the runway is long enough, you should normally plan to use the middle part of the runway exactly so that you have a margin when engine quits on short final, accompanied with account of incident where it proved useful right there and another in the next chapter. However airliners need more runway, so they can't afford leaving much reserve on the near end, plus they usually follow the ILS or at least PAPI and those lead to fixed spot 1000 ft past threshold.


How do pilots maintain level flight after launching missile? I saw, on National Geographic, two well built navy guys lifting a comparatively small missile to be attached on wings of an F-18 Super Hornet.

So when one missile on left wing is launched doesn't it drastically shift CoG of jet? Who copes with this shift

Computer & other stuff internally adjusts the zero position of flight controls to attain level flight. In this case what is the basis of detecting shift in flight control made by pilot - the old zero or the new zero?
The pilot by shifting position of flight controls, using hands & feet, in non-zeroish positions to maintain level flight despite changed CoG. If yes, isn't it detrimental to pilot's performance in a dog fight?

It's kind of a mix of the two. Flight controls on aircraft have what's known as trim. All (or nearly all) airplanes have elevator trim; many also have rudder trim and aileron trim. The trimmer for each control is used to balance constant forces needed on that control.

To take an example that's more widely applicable than launching missiles: when you increase power (to climb or to reach a faster airspeed), the airplane will have a tendency to pitch up, so you might find you need to push forward on the stick to maintain the right attitude. By moving the elevator trim forward, you eliminate the needed force, effectively changing the neutral position of the stick.

On early aircraft, the trim often worked by having springs connecting the control wires to a large lever. Working the lever pulls the spring in that direction, applying a force to the controls. On modern light aircraft, there is a trim tab on each control surface. The trim tab is like a tiny control surface on the control surface. Moving the trim tab to one side or the other provides a force on it, replacing the force the pilot would provide through the controls.

Fly-by-wire aircraft, such as military jets and large passenger airplanes, have electronic trim. Since the pilot doesn't have to put much force on the controls to work them anyway, trimming just resets the neutral position of the stick as you describe. Because it's electronic, there can be different ways to control it. Some aircraft still have small levers to set the trim; some have up/down buttons in reach of the stick (such as the Su-27); and yet others simply have a single button that trims to the current position of the controls (the A-10). As Mr Çetin points out, some very modern military jets take advantage of the possibility of computer control: the aircraft is aerodynamically unstable, but a flight control system manages keeping stable flight.

Returning to the particular case of the F-18, it doesn't re-trim itself. When heavy ordnance is launched, the aircraft will roll to one side as you suspect, and the pilot has to compensate with the stick and pedals. Once he has stablised the aircraft at the desired attitude and airspeed, he can adjust the trim using a hat switch on the stick.


Why is the velocity squared in the lift equation? Why is the velocity $v$ squared in the lift equation?
$$L=C_L\cdot\frac{1}{2}\cdot\rho\cdot v^2\cdot S$$

Newton's second law is:

$$F = ma$$

The $m$ass in the lift equation is of the fluid's, mass times velocity is the fluid momentum. So the equation can be written as:

$$F = {\Delta}(mv)/{\Delta}t$$

Rearrange that equation and it becomes:

$$F = \text{constant}
\cdot
(m/t)v$$

Mass over time is the mass flow rate:

$$m/t = {\rho}vA$$

So we end up with:

$$F = \text{constant}
\cdot
{\rho}vAv$$

Which is:

$$F = \text{constant}
\cdot
{\rho}v^2A$$

Add $A$ to the constant:

$$F = \text{constant}
\cdot
{\rho}v^2$$

Reference: NASA


How big a risk is to abort take-off before V1 with uncontained engine fire and side-wind? The Blue Panorama flight 1504 had uncontained engine fire just before V1 during take-off. The pilot continued the take-off and succesfully landed the plane with one engine. In the TEDx talk the pilot Maurizio Guzzetti explained that he thought that if he had followed the official protocol he would have aborted the take-off and the heavy sidewind would have pushed the heat from engine fire to the cabin and caused the cabin to catch fire and result in casualties. Instead he decided to take the plane to air, turn the plane around and land with the other side to the wind before stopping the plane. Note that the plane was over the maximum landing weight and no ability to dump fuel.
Do you think that this was a real threat and if so, what should a pilot do with similar fire starting with slower speed? Assuming the fire cannot be contained and heavy side-wind is there a reason no believe the cabin will not burn, too? However, if you're a lot before V1, it really wouldn't make sense to try to complete take-off with only one engine.
The TEDx talk: Collaborating with machines at high altitude and speed | Maurizio Guzzetti | TEDxModenaSalon

A well made animation about the accident:
Bursting into Flames Just Before Takeoff in Rome | Season Finale [Real Audio]

If it happens at a slower speed as you ask, then maneuvering the plane on the runway as it stops would help, even if it's only a 45-degree turn. You might wonder why not do a complete 180 if the runway width allows; that turn is slow and takes valuable time, while a fully loaded plane can theoretically be evacuated in under 90 seconds with only half the exits. It's a smaller risk considering the low-energy abort. A high-energy abort adds the risk of the brakes starting another fire.
As to Flight 1504, there's no need for opinions, as the final report did not fault the commander's decision:

The management of the failure by the flight crew was carried out professionally and in compliance with the established procedures. The same applies to the air traffic controllers, who assisted the aircraft with professionalism and providing maximum cooperation. The commander decided not to interrupt the take-off run despite the right engine fire warning having intervened a few knots before V1, thus exercising the expected discretion and taking into consideration all the available elements that favored this decision. [translated from Italian; emphasis added]

Speaking of Go shortly before V1, it is advisable for tire failures due to the compromised braking:

Source: Pilot Guide to Takeoff Safety - FAA (PDF)


Why does the FMC in B737 limit the climb speed to 230 knots on the VNAV mode until the leading edge devices are fully retracted? In the VNAN climb on B737, the FMC changes the target speed from V2+20kt at liftoff, to 230kt for acceleration at the preset accleration height, then to 250kt (below 10,000ft) as the leading edge devices are fully retracted along with the trailing edge flaps.

My question is: Why is the climb speed limited to 230kt on the VNAV mode until the leading edge devices are fully retracted, when there are only operating speed limits to trailing edge flaps as below?

For your information, I've found a discussion on this topic on another aviation website but no one seems to have a definite answer to this.

https://www.pprune.org/tech-log/570676-737ng-led-s-speed-limitations-ops.html

Does anybody have a good answer to this?

The non normal checklist QRH will answer your question.  If the LE flap transit light remains illuminated I.e the leading edge flaps and slats haven’t retracted or disagree with the flap handle position, then the QRH directs you to limit the speed to  230 kits until you have rectified the issue.  VNAV is honouring that restriction.

Hope that answers your question.


Is there any data repository of discontinued commercial routes? Is there any way of knowing - for example by referring to an existing data repository or something similar - what commercial routes were operated from a given airport in a given year?

This would be a list of all commercial routes that have been discontinued.

departedflights.com has a list of historical routes both by airlines and (a few) hubs. Though it is not complete, it has a lot of data.


What is the purpose of the box between the engines of an A-10? There is a curious box between the engines of an A-10. It's seems rather rudimentary, like an afterthought, although it was present on the prototype.

I've found a few cutaways but this part is hidden. I'm fairly sure that at least the front is hollow, so perhaps related to an APU. The question is, what is it for, and why, even in the context of the somewhat inelegant A-10, why so unrefined?

source: savasucakalari

It is the intake/exhaust duct of the cooling system.

Basically, the front side consists of the ram air duct, which feeds air into the ECU precooler, with the (heated) exhaust air vented out through the rear. The image below shows a closeup of the duct.

Image description: Kenneth Thompson, 576th Aerospace Maintenance and Regeneration Squadron aircraft mechanic, aligns a portion of the pre-cooler ram air duct onto the top of an A-10A Thunderbolt on Davis-Monthan Air Force Base, Ariz., Jan. 23, 2013. Source: warthognews.blogspot.in

Here's another image that shows a cross-section of the system. The cooling system (without the box) could be seen above the APU.

Image from pintrest.


Should full flaps be deployed on takeoff? Are full flaps ever used on takeoff?

One flying book I read strongly discouraged anything more than quarter flaps on most planes due to the amount of drag produced. I was just wondering if there are any scenarios where full flaps might be necessary.

First, you have to remember on some configurations, the lower stages of flap will mostly add drag and very little lift. Sometimes that extra notch of full flaps is only there to change the camber of the wing to add a bit of a nose down attitude to help with visibility on landing. Both are items you do not require on takeoff.

Now, when selecting flaps for take-off there are two things to consider: runway length and obstacles to clear after the runway:

Runway Length

Generally when lowering flaps your $V_{\text{mu}}$ (the speed at which you become airborne, “minimum unstick speed”) will decrease. This  means a shorter runway for normal stages of flaps. As you continue to add more flaps your $V_{\text{mu}}$ will be lower, but because of the added drag, which decreases acceleration,  it will actually take a longer distance to reach that lower $V_{\text{mu}}$. There is a sweet spot in-between, though. “Take-off Flaps” is not always designed to give you the shortest takeoff roll.

Obstacle Clearance

And that is because once airborne, the more flaps you have deployed, the higher the drag, the less excess thrust available, thus the less you can climb. Simply put, you are now flying at a low speed and barely climbing. Not a good idea if there are tall trees waiting for you at the end of the runway.

It would be illegal  to take-off on full flaps if:

POH says it is prohibited,or
For commercial flying, if the flap setting does not provide the minimum required gross obstacle clearance climb gradient (depending on the type of operation and number of engines, between 2.4%-3.0% ).


Can airliners land with auto pilot in gusty wind conditions? Why does the pilot make so many movements? I don't see that plane is moving all that much, but his yoke movements are strong and fast.
For someone who is not a pilot, it almost looks like panicked flying.

Can airliners land with auto pilot during the day with strong winds and does night make landing even worse in these gusty conditions?

To the first part of your question, the airplane is not moving BECAUSE his yoke movements are "strong and fast". Experienced pilots can feel a gust and respond with a control input to counter it before it has enough time to affect the aircraft. (You really can't appreciate this from a camera fixed to the airframe.)  As mentioned in a comment, this is called "staying ahead of the airplane" or "flying by the seat of your pants". Quick and correct reactions are required to keep the approach on track when being buffeted around.
Another thing that is worth pointing out is that the magnitude of control inputs is higher at slower approach speeds because of the reduced effectiveness of the control surfaces. You wouldn't want to move the controls that much at cruise speed. Just like when driving on a bumpy road you move the steering wheel a lot more than when cruising on the freeway.
I have never flown an airplane with auto-land capability, so I cannot answer the second part. However, given the experience I do have with a couple different autopilots, my belief is that even the best systems would be stressed trying to keep up while landing in very gusty conditions. There are just some things you still need a trained and experienced human for...


Does Clearance Delivery operate in the same way for airports in Bravo, Charlie and Delta airspace? Something tells me that Clearance Delivery might offer different instructions at airports within Bravo, Charlie, and Delta airspace, e.g. if I take off VFR from KLAX vs from a class Delta out in the middle of nowhere.

Can someone give me some guidelines on what might be similar or different? I'm guessing that Clearance Delivery might give me a heading, altitude, and a departure frequency in a class Bravo. In class Delta, I might get an altitude (stay below 2500' AGL) to remain inside of the Delta?

All will be pretty similar.  In Bravo, and Charlie and Delta if they are close to the Bravo, you will typically contact Clearance for your departure directions (heading to fly, altitude to climb to or hold at after takeoff (or cleared to climb to 1, and expect clearance to higher after so many minutes, or when reaching some intersection), and Approach frequency to expect) before Ground gives you clearance to taxi from parking towards the active runway and for your runup.  If you filed a Flight Plan, expect to have the clearance (perhaps modified) read to you, which you must read back.
After switching to Gnd for Taxi directions, be ready to read those back as well. Have the Airport Diagram out, and don't be afraid to ask for Progressive Directins.  Things can be busy at a Bravo. When your runup is done, you switch to Tower if Gnd hadn't handed you over already, let them know where you are and that you're ready for takeoff.

If the Charlie and Delta are not near the Bravo, things may be a little more relaxed. You may get your Clearance from the Tower, or Gnd, and Gnd/Tower may be the same guy on the same frequency, or he may be on 2 or more frequencies, switching between Gnd and 1 or more Tower frequencies for different air sectors.  You may stay on the Tower frequency longer before being switched to Approach, or for Delta you may just be asked to advise when you are clear of the Delta airspace.

Don't fly to any Bravos until you are comfortable on the radio.  Limit flights to Delta and Charlie to start until you can follow the lingo.


Is there a standard install direction of the abradable shroud honeycomb on a CF6-80c2 engine? Is there a standard install direction of the honeycomb cells on the Fan Forward case on the CF6-80C2 abradable shroud section?

I have seen engines that the honeycomb cells run point to point front to back. I have seen engines where the cells run point to point in the direction of the circumference of the the abradable material for the Fan blades. I have searched the manuals and I cannot find where the orientation of the honeycomb is specified.

Any repairs are understandably performed with the same orientation.  I am wondering is there a MFG standard?

As noticed by Daniel Kiracofe, this applies to the turbine, but as far as orientation is concerned the logic is the same

The abradable material in the honeycomb cells can readily withstand the high temperatures encountered in the turbine stages of the engine and protects the annular steel backing from high temperatures as well as providing a smooth gas flow surface. However, under extreme operating temperatures, the turbine blades may expand sufficiently to actually rub the surface of the abradable alloy overfilling the honeycomb material, whereupon the abradable sealant material will yield so that the turbine blades will not be damaged

However, the abradable material filling a given cell of the honeycomb can become loosened and fall out due to thermal cycling, rubbing of the abradable filler against the turbine blades, engine vibrations, or can otherwise be blown out by action of the hot gas flowing across the seal.

The orientation is the one, the manufacturer found the least probabilistically susceptible to allow the abradable material to separate; internally the cells walls are not strictly perpendicular to their outer flat surface, and this angular orientation prevents to the best the falling of the abradable material. Thus indeed, as you write, the repairs are understandably performed with the same orientation, using identical material.


What is the sense of the Aerodynamic Centre for Design Process? This topic has been discussed already several times, but I do not really get it. I think this one is a quite nice explenation: enter link description here

So I understand that the Aerodynamic Centre does not change with angle of attack. But what I do not get: how this helps in a pre-design of my aircraft?

Because: for stability calculation, I have to consider the forces acting on the whole aircraft and the corresponding lenghts (moments).

So regard a stationary horizontal flying aircraft: when I make the equilibrium of moments around the centre of the gravity, I have to regard the force F_cp,w at the centre of pressure of the wing (resulting lifting force) with distance l_cp (from centre of gravity) AND the force F_cp,t (resulting lifting force at the tail) with it's lever arm l_cp,t .

The balance becomes around the centre of gravitiy: F_cp,w * l_cp,w = F_cp,t * l_cp,t

When the angle of attacks change, F_cp,w*l_cp,w will change since the centre of pressure will move and the resulting lift will change.

So what I assume: there is a trick with which I can make the moment of equilibrium around the centre of gravity and where l_cp,w remains constant = Aerodynamic Centre

But I can not imagine how this is working with the balance of moments.

Can someone help? I am really confused.

Thank you very much

Aerodynamic Centre (AC) is particularly handy when we analyse the dynamics, that is, changes. For instance, changes due to disturbances: stability.

First thing, for analysis, we split all forces into forces acting where we want plus moments caused by the fact that these forces actually act somewhere else. And then when we write our balance equation, we need to balance both the forces and the moments.

We want forces to be at CG? Fine, let's declare that lift happens at CG. But then we acknowledge that apart from just lift, there will be an additional moment with respect to CG.

Now, due to linearity of aerodynamics at small angles of attack (AoA), this moment will change linearly with AoA. This effectively means that there will be a certain fixed point at which the moment will not change; or in other words, a point at which all the additional lift (due to pure AoA) can be thought to be happening. This point is the AC.

This is great, because:

We can express this extra AoA-related lift as $L_{w\alpha} \cdot \alpha$, where $\alpha$ is AoA and $L_{w\alpha}$ is the wing lift derivative by AoA, which is an intrinsic measurable property of a wing (or airfoil/aircraft).
The moment produced by this lift (with respect to CG) is that lift times its arm $x_{wac} - x_{cg}$ ($x$ is a relative distance (coordinate) of a point with respect to some datum, typically the front of the mean aerodynamic chord (MAC), which is another fixed property). So, our moment from lift will be $L_{w\alpha} \cdot \alpha \cdot (x_{wac} - x_{cg})$.

Remember though that for all but symmetric airfoils there will also be a static moment $M_0$ due to the camber-related lift also not acting at our CG.

In the above moment equation everything except for $\alpha$ is constant for a given wing/airplane. When we analyse longitudinal dynamics, which is all about AoA changes, we need the derivative by AoA. This leaves us with a constant, which unequivocally characterises our static stability.

Let's add the tail. It's lift will be trivial, $L_{t\alpha} \cdot \alpha_t$ plus some static lift at the chosen zero AoA. The arm of the AoA-related lift with respect to CG will also be constant: the distance between CG and AC of the tail. (Given that the tail airfoil is often symmetric, the whole lift arm is simply the distance to 1/4 tail MAC).

So, when we equate the moments for a straight-and-level flight (and ignore the downwash effect for a moment, which changes the tail AoA $\alpha_t$ from the base AoA), we get
$$(L_{w\alpha} (x_{wac} - x_{cg}) + L_{t\alpha} (x_{tac} - x_{cg})) \cdot \alpha = const \Leftrightarrow M_\alpha \cdot \alpha = const$$

The $const$ represents the static moments to be trimmed out; but what's important for the dynamics analysis is that we get a constant moment derivative $M_\alpha$: everything in the brackets is constant thanks to the 'constancy' of AC. We can break it up as $M_\alpha = L_\alpha \cdot (x_{ac} - x_{cg})$, which defines the overall AC of the wing + tail system, often called neutral point (NP).

Now if we want a statically stable aircraft, i.e. the one which negates the changes to AoA, we need, by definition, a negative $M_\alpha$ (and that's regardless of the trim $const$). From there, you can easily derive the requirements for the CG position with respect to AC. This very convenient, and this is why we need AC more than anything else. (In aircraft design, unlike pure airfoil/wing aerodynamics, CP practically never appears anywhere). Finding the AC is not always trivial though.

(Of course, in practice engineers use dimensionless coefficients rather than actual forces/moments, but I didn't want to introduce more entities than necessary here).


What is the heaviest aircraft fitted with a central control stick? Primary control surfaces for roll and pitch are controlled either by a wheel (C-172, Concorde, ...) or by a center-stick (mirage 2000, most gliders, ...). Wheel tend to be used for bigger aircraft. E.g. I don't know any airliner fitted with a center stick.

What is the biggest aircraft (highest MTOW) fitted with a center-stick? (Airbus style side-stick are excluded.)

The biggest I can find is at 585,000 lb the C-17 has a center stick but depending on what you mean by "airbus style" this may be disqualified.

Other contenders:

The SR-71 had a MTOW of 172,000 lb and had a center stick
If you consider the time it spent in the atmosphere as qualifications for "aircraft" at 240,000 lb the Space Shuttle had a center stick


What is the purpose and history of the FAA's tracking system for handbooks and manuals? Handbooks and manuals published by the Federal Aviation Administration usually have a designation attached to them. For example, the current Instrument Flying Handbook is designated FAA-H-8083-15B at the time I am writing this.

One apparent purpose of this system is to keep track of what particular version of is current.

What is this designator's proper name? What is the history of this scheme and can anyone shed light on any other purposes it may have?

I must have too much time on my hands, because I actually looked this up in the handbook on handbooks that aeroalias referenced.

  5.1.2.3 Document identifiers

  The identifier for handbooks shall be placed in the upper right corner
  of the first page. The identifier as assigned by ATO-W Enterprise CM
  shall include the letters “FAA” followed by a hyphen, then any
  combination of numbers, letters, or dashes. It shall not include any
  other symbols.

  5.1.2.7 Handbook numbering.

  FAA handbooks are assigned traceability numbers by Enterprise
  Configuration Management organization following endorsement by the
  Systems Engineering organization. This alphanumeric designator will
  utilize the following format: FAA- HDBK-(XXX), where "XXX" equals a
  three digit number. The handbook number is placed in the uppermost
  right corner of the cover page.

Section 5.1.2.5.1 Superseding revisions doesn’t appear to be followed in any of the FAA handbooks I have.


What is the difference between a wing station and wing buttock line? Can someone explain (with images) what the difference between a Wing Station and Wing Buttock Line is, in terms of aircraft structures location identification?

The text I'm reading states:

  W. STA = A plane perpendicular to the wing set at 90 degrees to
  leading edge

  WBL = A plane perpendicular to the wing and parallel to the body
  buttock line

This description has utterly confused me!

The wing stations are perpendicular to the leading edge. Wing station zero is typically where the leading edge meets the fuselage. Wing ribs are typically parallel to the wing station. On swept wing aircraft, this won't match the flow direction, but it makes design and construction easier.

The wing buttlines are parallel to the airplane buttlines, which run down the length of the aircraft. Wing buttline zero is typically where the wing meets the fuselage.

This means that, unlike with other sections like the fuselage, the wing stations and buttlines are not perpendicular if the wing has sweep.

Rough illustration of the wing vs. fuselage BL and STA. Not to scale.


Why open up the window shades before takeoff and landing? On commercial airlines, I've noticed that the flight attendants always ask everyone to open up their window shades before landing or takeoff.

I've always wondered why this is. Is it something to do with the mechanics of the plane? Is it some kind of passenger-comfort thing?

I can't imagine what difference this act makes, but the attendants seemed really insistent on it. :)

I'm not sure about other countries, but in the USA there is no FAA requirement mandating that the window shades be open for takeoff and landing.

Individual airlines vary in their procedures, with some requiring them to be open while others do not.  This is set by company policy, presumably set by management because they decided that there was/was not a safety benefit in doing so.

As a pilot, I would prefer the window shades to be open all of the time so that even passengers have the chance to point out something that they think might be a problem.  For example, recently a passenger on an airliner noticed a fuel leak and told a flight attendant about it before the airplane took off.

One important thing to keep in mind is that in the event of an emergency evacuation, you should always look out the window before opening an emergency exit to make sure that there isn't a fire or other hazard just outside the door.  This is why a lot of emergency exits don't even have window shades on their windows.

This answer on Skeptics.SE also points out that emergency crews responding to a crash can see through the open windows to better respond and help the people on the airplane.  It also mentions that the passengers eyes will also be more acclimated to the lighting outside the airplane if the window shades are open.  This is also why some airlines have policies requiring the cabin lighting to match the outside light (i.e. the lights will be on bright during the day and dim/out at night).

This answer on Travel.SE also goes into more detail from a person who works in the airline safety industry.


Are old illegal air transfer methods still feasible today? A retired soldier told me he used to know people who smuggled jewelries, hard drugs, guns and even people from middle eastern countries to Europe, specifically, Germany! Via planes! In fact, he said some goods were too valuable for them to "risk it" by land transports.

He also added that the aircraft were operated by experienced World War II pilots who specialized in flying at high speed in very low altitudes to make deliveries for infantry units on distress, making them unnoticed on radars and unstoppable for air defense systems.

Are similar methods still possible to this date? if not, what has changed?

similar: How do people smuggle stuff with private airplanes?

NOTE: I have researched myself and so far couldn't find evidence that any sort of actual "improvements" were done in these 70+ years in this matter, it seems the security tightens only when it's demanded and new technologies are not consecutively being used to monitor the skies.

no duplicate of Can someone actually "fly under the radar"? (read comments)

Yes, it is possible to fly "under the radar".

The type of radio waves that are used by radar and similar equipment to track aircraft generally travel in straight lines. This means that, because of the curvature of the Earth, radars have blind spots at low altitudes when you are sufficiently far from the radar. See the top illustration (not to scale):

While radar coverage today is no doubt much better than it was during World War II, much airborne surveillance today (SSR, ADS, etc.) relies on cooperative targets - which means that tracking an aircraft is a two way communication; the radar sends out a request, and the aircraft replies. The pilot could easily turn off airborne surveillance equipment, thus becoming invisible to secondary radars, ADS and so on, but would still be visible on primary radars, which work by sending out a radio pulse which is reflected off the surface of an aircraft. My point is, primary radar coverage is limited, with more and more primary radars being phased out and replaced by secondary radars, so there are definitely plenty of areas where, if you fly low enough, you can avoid radar detection.


what reference of airfoil is used to define lift and drag? On aerofoil section, the force of lift acts perpendicular to, and the force drags acts parallel to the :

A: chord line
B: flight path
C: aerofoil section upper surface
D: longitudinal axis

This is a basic question, and I give you the answer from the initial pages of a aerodynamic learning textbook.

The  is presented in next figure:

Where:

L: lift: component of R (resultant force) perpendicular to air stream,
D: drag: component of R parallel with the air stream,
N: Normal Force: component of R perpendicular to chord,
A: Axial Force: component of R parallel with chord,

For further information you can refer to Anderson's Aerodynamic book.


What are the characteristics of the F-16 airframe lifetime? What limits the lifetime on the F-16 airframe? I assume it is material fatigue, but what kind, and where on the airframe?

Moreover, what exactly happens? What will happen if this problem is ignored? Will the wings break off? Will the airframe collapse or break somehow?

What can be done about this problem? Can the airframe be maintained somehow, repaired, replaced? For how long can this be done, indefinitely, or only up to a point where it is financially feasible?

It is difficult to estimate how long a particular airframe might last. Also, different aircraft might last different amounts of time for esoteric reasons. For example, mercury amalgamates with aluminum and can weaken it. Even a small amount of mercury can significantly weaken an aluminum structural member. Therefore, you might have one plane with some mercury contamination that will not last nearly as long as one free from that accidental contamination.

Also, how the aircraft is used will significantly affect its lifetime. A few high-stress maneuvers can weaken an airframe. Conversely, an aircraft that is babied and lightly driven may last for many decades.

Ultimately, there is no way to predict how long a given aircraft will last. The best thing to do is to observe it and when cracks start to appear, then you know there is a problem.

To compensate for fatigued structural members, it may be possible, but it depends entirely on the scope and nature of the fatigue. If the fatigue is limited to a restricted location, then theoretically that area could be reinforced. The problem is that the complexity of the engineering is such that in most cases it will be cheaper and more effective to simply replace the aircraft.

Usually wear is most severe at the wing roots, so a plane that is experiencing fatigue can potentially be improved by replacing the wings, but the cost of this is pretty significant, so normally you would only see this in an air force that could not buy new aircraft for some reason.


What does the T in RVR-T stand for? In the Chart Supplement (A/FD) for CRQ/KCRQ, you will find RVR-T for runway 24.

I was wondering what 'T' stands for?

From page 5-1-7, contractions in the AIM:

  RVRT . . . . . . . . . RVR Touchdown

The full list is:

  RVR . . . . . . . . . . Runway Visual Range
  RVRM . . . . . . . .  RVR Midpoint
  RVRR . . . . . . . . . RVR Rollout
  RVRT . . . . . . . . . RVR Touchdown

On some runways there are multiple RVR sensors and different points along the runway, and these are the names given to identify the different locations.


What types of gear/supplies are "must-haves" for private pilots? Is there certain gear that people consider "must-haves" for a private pilot? Specifically, a new pilot or student pilot.

Get a good headset and a decent mobile device that can run a charting app. I use my Moto4 with Avare.

Download The Airplane Flying Handbook -and- The Pilot's Handbook of Aeronautical Knowledge from the FAA site (both free).  Also download the Airman Certification Standards (ACS) for private pilot airplane.

The only books that I usually recommend people actually purchase are the aircraft manual and a written test study guide from ASA, Jeppesen, Gleim, or King.


Why is the air traffic control profession so commonly perceived as "stressful"? In my experience, almost every time the topic of air traffic control (as a career) comes up in conversation, someone makes a remark like

  Air traffic control - isn't that a very stressful job?

Notwithstanding the fact that this point could be debated, surely there are many other "stressful" jobs which don't receive this kind of response.

My question is this: How did the "stressful" air traffic control job become such a common perception?

Some very spot-on comments, some comments that are totally ignorant of the job. I am a retired ATC, 32 years.  I worked at one of the world's busiest and most complex US facilities (as described by many independent sources).

Have you ever:

Heard a family on the other end of the radio screaming just moments before they were obliterated in the air by a military fighter flying VFR?
Tried in vain to find a way to contact the occupants of Payne Stewart's jet only to watch helplessly as it finally crashes after several hours of flight?
Had 30+ airliners in your sector which happens to be dotted with Level 5 thunderstorms?  The amount of airspace you now have has decreased substantially and the aircraft all need to deviate. They all want to either go in different directions, or they all want to head towards the few clear spots left. You sit there for 2 hours directing aircraft to basically thread a needle at 500mph and pulling out every trick in your book to ensure that there is enough space to ensure all 30+ aircraft (and more, because those aircraft never stop coming) make it through your airspace without encountering the dangers of severe weather or coming into physical contact with another aircraft. Aircraft that are often executing movements that they don't have clearance for, which requires quick and accurate reaction from the controller.
Dealt with one of the times when a corporate jet "flames out" at 43,000 feet and immediately descends through a dozen or more jets flying the same path, requiring the controller to immediately clear all of those aircraft out of the path of that corporate jet.
Had a small twin prop lose all electronics and be unable to navigate properly while in a thick cloud layer.  His portable radio is not working properly, so radio communications must be short and accurate. (Remember, you have another sector full of aircraft that you are also providing services to). Finally, his portable radio fail.  Right before that, you ask the pilot if he has a cell phone. He does.  You give him a phone number and you and he are able to communicate via telephone to finally guide him to a safe landing at Newport News Airport.

I could write a book about many specific events that controllers have experienced.  The "pucker factor" events.

But the real stress is the long term chronic stress that the job presents.
Certainly, there are facilities that are both low traffic and have uncomplicated operations. If you are lucky enough to work at one of these (and willing to accept the low end of the pay scale) then stress is probably much lower and stressful events infrequent.  But for the majority of controllers (who work at a number of very high traffic facilities; air traffic in the US is higher and more complicated than the next 29 countries' air traffic, combined) the stress is a chronic, continuous level (not the big adrenalin-pumping situations) from being attached to your position for several hours, trying to maintain a constant "big picture", maintaining situational awareness, issuing control instructions to ensure that SAFETY is always the first product of your efforts.  Many, if not most, controllers will most likely retire with high blood pressure, diabetes, sleep disorders, etc. that despite best efforts to exercise and live a healthy lifestyle will probably affect a controller for the remainder of their life.

Certainly, other jobs are stressful, and I would never try to compare one as "more stressful" than the other.  The top ones are all stressful.  But those that try to downplay the stress of air traffic control would only need to visit a busy air traffic control facility for a day and they would see and probably feel the stress themselves.

I have a very good friend that is an FBI field agent in Washington, DC.  He has worked on some very high-profile cases.  We were talking one day and he said something about the stress of air traffic control.  I chuckled saying that at least I wasn't being faced by some of the most dangerous people in the country. But his response was that in his job they have detailed plans for what they do.  His raids were in the middle of the night when these people were asleep and unsuspecting.  He had a well-trained team that each had specific responsibilities.  They had plenty of firepower and protective equipment.  He said that most of his operations were quite easy and they experienced little resistance. He did have moments of intensity, but he (who had visited an air traffic facility) did not think the stress he experienced was anywhere near to what he saw at the air traffic control facility.

But I still would not want to be a law enforcement officer, nor a school teacher for that matter!  My personality was a good match for the job of air traffic control.  But now that I am retired, I take blood pressure medicine, am borderline diabetic despite a regular exercise program and am within a healthy weight and eat very healthy.  I also still have sleep apnea and other sleep issues that I see a sleep doctor several times a year for.  All of my present health issues, which are well-controlled, are according to the doctors a direct effect of the stresses of being an air traffic controller.

Air traffic control IS stressful.  Is it more stressful than other jobs? I doubt it. And to argue about whose job is more stressful is a silly exercise. The trick is having a personality that meshes with the job.  If you are in a job that is not a good match, that is stressful.


Why did the 747-8's freighter version make its first flight before the passenger version? I have read that the 747-8F entered service in October 2011, almost eight months ahead of the passenger version (747-8I).

Anyone know why this is?

Thanks

The launch customers for the 747-8 were Cargolux and Nippon Cargo Airlines, both air freight operators who wanted the 747-8F. The first orders for the 747-8I didn't materialize until several months later.

Source: Wikipedia


Aircraft Emergency Evacuation slide inflation system: Why is a mixture of CO2 & N2 used instead of pure N2? I was intrigued knowing from this SE-Aviation post that modern, aircraft Emergency Evacuation-Slide inflation-cylinders are often filled with a CO2+N2 mixture.

See  here:

Which gas is used to fill the evacuation slides of modern aircraft?

Intuitively the reason behind the CO2 addition instead of using simply pure N2 is not very obvious to me. Digging deeper into the technology, Wikipedia reveals that "The CO2 is used to slow down the rate at which the valve expends the gases."

Any idea about the science behind this? Why would CO2 slow down the inflation rate? And why do we want to slow the inflation in the first place? Fabric damage?

Also, what is the CO2 percent in the mix, anyone know?

Let me first give some background information on the art of inflating an evacuation slide. There are two ways to fill them:

Expand gas from a high-pressure bottle: This will cool the gas as it expands, so ice may form at the aspirator outlets where sucked-in air and the expanding gas mix, blocking the flow of outside air into the slide. Also, storing compressed gas needs bulky and heavy steel vessels.
Pyrotechnic gas generators which use an exothermic reaction to create a large amount of hot gas. They are more compact and lighter than a pressure vessel for the same amount of gas produced, but their hot gas could scorch the slide material and will contract when it cools down after the filling process, so a slide filled by a pyrotechnic gas generator will sag after a while.

Because of the disadvantages of both pure stored gas and pure pyrotechnic inflation devices, current state of the art emergency evacuation slide systems use a hybrid inflator, which comprises a stored compressed gas together with a pyrotechnic gas generator. The pyrotechnic gas generator augments the stored compressed gas by providing additional gas as well as heat to counteract the effects of the expansion-induced cooling of the compressed gas as it expands out of the pressure vessel.

Operation of a hybrid inflator

The typical gas generator uses solid ammonium nitrate which is electrically ignited and produces nitrous oxide (N$_2$O) and water (H$_2$O) when heated. Nitrous oxide will exothermically decompose into nitrogen and oxygen when heated above 650°C or 1200°F, so even more energy and gas is produced. Note that the decomposition of two moles of nitrous oxide will produce two moles of nitrogen (N$_2$) plus one mole of oxygen (O$_2$), thus adding 50% more gas volume. This hot mixture of nitrogen, oxygen and steam is then piped into the pressure vessel and mixes with the gas flowing out from it, thereby heating the emitted gas.

The pressure vessel contains gasses like nitrogen or carbon dioxide. Other candidates are argon or helium, but their higher price has held them back. To increase the yield from the pressurized gas, the trick with the nitrous oxide can be repeated here: By storing nitrous oxide in the pressure vessel, again 50% more gas volume can be produced from what is stored, while the exothermic decomposition produces additional heat that offsets, to some extent, the heating of the stored gas.

Nitrous oxide is also attractive because it offers the best manufacturability, storage and handling of the possible decomposable gas source materials. Although classified as an oxidizer, in practice it is generally nontoxic and non-corrosive and is relatively inert up to temperatures of about 200°C. Also, nitrous oxide, as compared to gases such as oxygen, nitrogen and argon, liquefies relatively easily at ambient temperatures - helpful when the gas in the pressure vessel is stored in liquefied form to reduce its bulk.

Heated to its decomposition temperature, pure nitrous oxide will undergo a rapid disassociation chain reaction unless a coolant gas is added to absorb some of the heat generated by the exothermic reaction. Therefore, the mixture in the pressure vessel contains 50% or more of carbon dioxide which has a latent heat of vaporization of approximately 66 Calories per gram. As a three-atomic gas it is better suited for this task than two-atomic alternatives. Thus, it does not serve as a diluent (as aeroalias wrongly states), it absorbs heat and stabilizes the disassociation process.

Why a mixture of nitrogen and carbon dioxide

Now we can answer your question: Nitrogen is contributed both by the pyrotechnic gas generator and by the compressed gas and is produced from the dissociation of nitrous oxide (N$_2$O). Carbon dioxide is added to dampen the exothermic dissociation and absorb some of the heat produced. Without it, the gas generator would experience a runaway chain reaction! Its concentration in the compressed gas is between 50% and 75%, so the concentration in the gas generator mix is around 35% to 50%.

The high speed flow of gas from the pressure vessel is now piped through an aspirator, which sucks in ambient air by using the Venturi effect, and from there into the slide. This contributes up to three quarters of the total gas in the slide, so most of the nitrogen in the slide is from ambient air. In the slide, the concentration of carbon dioxide is diluted to around 10% to 15%.

Aspirator (from BF Goodrich's US Patent 4368009). High-speed gas from the gas generator (red arrows) enters from the angled pipe on the left and pulls ambient air (blue arrows) with it when shooting from the nozzles on the right. Contrary to aeroalia's claims, an aspirator is not just another name for a gas generator, it is an entirely different thing altogether.


Does 14 CFR 25.1310 apply to hydraulic systems? I'm struggling to understand if this particular rule was intended to apply to Electrical Power Generation Systems only or if this rule was also intended to apply to Hydraulic Power Generation Systems.

At first glance I assumed only electrical power generation systems would be applicable.  Then when I go to the EASA AMC for 25.1310 which reads the same it says the following:

  "When alternative or multiplication of systems and equipment is
  provided to meet the requirements of CS 25.1310(a), the segregation
  between circuits should be such as to minimize the risk of a single
  occurrence causing multiple failures of circuits or power supplies of
  the system concerned.  For example, electrical cable bundles or groups
  of hydraulic pipes should be so segregated as to prevent damage to the
  main and alternative systems and power suppliers."

So EASA puts the words "hydraulic tubes" in as part of their example.

Is there some other guidance material out their that would clarify if the reg really applies to 25.1310?

Usually this link below can be very helpful in finding guidance for a particular regulation (e.g. 25.1310). In this case there isn't any guidance for 25.1310.

https://www.faa.gov/aircraft/air_cert/design_approvals/transport/rules_acs_policy/

Next try the preamble for when this regulation was passed or introduced and see if the FAA provided any guidance on their intent for the regulation. The FAA discussed an EWIS (Electrical Wiring Interconnect System) accident which only confuses the intent. However I briefly perused the preamble.

http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFinalRule.nsf/0/81B77634BED9EAA78625738E007123B3?OpenDocument&Highlight=amdt.%2025-123

25.1310 is in Part 25 Subpart F and most of these regulations (e.g. 25.1309) apply to all systems (e.g. hydraulic, electrical, pneumatic, etc.). Best of luck I hope this helps.


Would plain and slotted flaps increase lift substantially in a distributed propulsion application? Would plain and slotted flaps increase lift substantially in a distributed propulsion application for a slow moving aircraft (15mph)?
I understand that:

A classic NACA foil without flaps such as the NACA 650-18, as used on Zenith 701 STOL aircraft produces a maximum lift coefficient (Clmax) of approx. 1.5.

Add a plain non-slotted flap, and the Clmax increases to roughly 2.2.

Add slotted flaps, and the Clmax increases to roughly 2.8.

The maximum Clmax of distributed propulsion is 12.

Given a GA aircraft such as the Zenith 701, with an engine of 100HP, with a stall speed of 45mph, means the entire wing is bathed in air travelling at 45mph when flying at 45mph.

Assume the same given aircraft now has multiple propellers installed along it's entire leading edge in a distributed propulsion arrangement, with multiple engines totalling the same total Hp, flying at roughly 15mph and with an propeller exit speed of 45mph. This also means the entire wing is bathed in air travelling at 45mph. ( assume 100% efficiency, no swirl angle, etc.).

See graph below.
(Solid black graph - Source: Aviationchief.com. Added graphics estimated by the OP.)

As distributed propulsion is just "extra air", and flaps move the lift slope curve up and to the left, I would think the wing would see substantial increase in Clmax if plain flaps or slotted flaps added.

I understand that in distributed propulsion, a wing with a Clmax of 6, on a slow flying aircraft, (15mph), but prop exit speed 300% faster at 45mph, the local AoA would always be under 10 deg.

I would think there would be a substantial increase in lift when using a plain flap and even more when using a slotted flap, but almost no increase in Clmax above 10 deg AOA.

Is this correct thinking?

No, that thinking is incorrect.
The air involved in lift creation is roughly the amount that flows through a disk with a diameter equalling wing span, and your distributed propulsion concept will involve a far smaller amount of air.

The maximum Clmax of distributed propulsion is 12.

In theory maybe. Practical values are half that, at most. Now consider that this lift should ideally be independent of engine health and power settings, or you will plummet from the sky when power fails or you need to throttle back. Distributed propulsion will not save you from empty fuel tanks or batteries, so this scenario must be considered. Besides, the lift coefficients you cite in your question are valid for 2D flow; in reality the outer wing will contribute much less to provide some margin for control and stall behavior.


How does the severity of barotrauma vary with rate of descent? This answer proclaims one way to mitigate barotrauma:

reduce the rates of your descents, which will give your ears a chance to equalize. ATC, however, expects you to descend at between 500 and 1,500FPM, unless you are given "AT PILOTS DISCRETION."

Correct me if I don't write this mathematically correctly, but what's $\dfrac{d(barotrauma)}{d(FPM)}$: the first derivative of barotrauma with respect to FPM?
E.g. how much less painful is 500 compared to 1 500 FPM?

It's purely subjective, but I can tell you from personal experience descending in unpressurized airplanes that a 500 fpm descent is WAY less painful than a 1500 fpm descent if you have something like a blocked sinus passage.  The blockage is usually not 100%, so the pressure rate of change has a huge effect on the pressure differential that is allowed to build up in your head as pressure equalizes, but just not fast enough.

Your head and sinus cavities (or inner ear) are effectively turned into a human Vertical Speed Indicator (which simply measures the rate of air pressure equalization between an aneroid chamber and ambient static through a restrictor, like your blocked sinus), but instead of a VSI pointer on the dial, you have the pain receptors in your skull.  Like the VSI, a 1500 fpm descent will crank your pain receptor dial quite a bit higher than a 500 fpm one will if you have a sinus infection.

The important thing to point out however, is the typical airline passenger never experiences anything more than 500fpm in normal operations, because pressurization control schedules limit the rate of change in cabin pressure to that value.  In any jet you are normally descending anywhere from 1-3000 fpm on an arrival, and even on final on a normal glideslope at, say 130kt, you are descending somewhere around 750 fpm.

It's only in unpressurized airplanes do you generally try to limit climbs and descents to 500 fpm when passengers are on board.


Why is the use of tail-down force so common? Why is the use of tail-down force so prevalent in aircraft design?  Why not use canards to avoid induced drag?

If it were so simple ...

To achieve natural longitudinal stability (the aircraft stays at the trimmed speed, even after disturbances like gusts), the rear lifting surfaces need to have a lower lift per area than the forward surfaces. Why? In a gust, the aircraft's angle of attack changes on all surfaces nearly at the same time. By having less lift per area, the relative lift increase due to an increase in angle of attack is higher for the rear surfaces, so now they gain relatively more lift increase than the forward surfaces. This creates an imbalance which lifts the tail up, until the old angle of attack (and, consequently, the old speed) has been reached again.

A negative tail loading is a sign of high stability. This creates more drag, but allows the pilot to take the hands off the stick, to read his maps, check something or have a cup of tea. But its effect on induced drag can actually be helpful. You might be surprised, so please bear with me!

What is induced drag, anyway? It is the consequence of creating lift over a limited span. The wing creates lift by deflecting air downwards. This happens gradually over the wing's chord, and creates a reaction force orthogonally to the local speed of air. This means the reaction force is pointing up- and slightly backwards. This backwards component is induced drag! The lengthwise distribution of the surfaces which do the downward deflection is of little importance (for a very technical explanation: See here, where they talk about the Treffz plane): If you have a highly loaded canard wing, its downwash will hit the wing eventually, creating a lot of backward-tilted lift there. The canard by itself will not cause so much induced drag, but will mess up the airflow over the wing. Behind the airplane it does not matter if you fly a canard or a conventional configuration, what matters is over how much span you distribute lift creation (for a constant amount of lift at a given speed).

You probably know that an elliptic distribution of circulation over wingspan produces the smallest induced drag. Now imagine that your wing has more a triangular than an elliptic distribution. The downward lift on the tail will reduce this hump of lift in the middle, making the distribution closer to that ideal elliptic distribution behind the whole airplane. The induced drag of the whole airplane is lowered by the tail!

Another explanation: Since the wing creates a strong downwash at its center, the tail flies in an airflow which points slightly downwards. Negative lift there (being approximately orthogonal to local airflow) will actually point slightly forward, so your negatively loaded tail produces a small amount of thrust!

Black: Total force, Blue: Lift, Red: Drag

And if you think that a triangular lift distribution is unusual, please read this NACA report by R. T. Jones. It takes wing weight into account, and this changes the picture how the lift distribution for minimum induced drag should look.


What is this component in an Airbus cockpit? I have noticed that on a lot (all?) Airbus cockpits, there are these "things" (for lack of a better term) under the panel.

I have highlighted them in this picture of an A340 cockpit:

What are they?

Since Airbus planes use a sidestick instead of the traditional center yoke, they use the space where the yoke would be to include a fold out table instead. You can see a laptop on one here. The A380 version also has a keyboard. This is used to interface with the onboard systems and input information.