What types of fuel quantity sensors are used in an aircraft? How do they work? Can you explain the different types of fuel quantity sensors used in most recent aircraft?

Are ultrasonic tank sensors used these days? How does an ultrasonic sensor work?

From this PPRune forum thread, the Boeing 777 does use ultrasonic sensors (some 20 per tank)

  This airplane has some 20 sensors per tank. Each of these is effectively a "radio doppler" and works on the same principle. The sensor calculates the height of fluid by measuring the difference in the speed of the sonic signal sent out thru one sensor to another,as against the ACTUAL local speed of sound (which is a preset value) thru standard temp air.This difference is then converted very accurately into height in the tank in mm.Which translates into quantity of fuel AT THAT sensor given the actual specific gravity of fuel at that point.
  - King on a Wing


Can ekranoplans (GEV) be more efficient than traditional airliners? Ekranoplans or Ground Effect Vehicles (GEV) fly low enough to the ground that their wings stay in the ground effect. This increases lift and efficiency.

But GEV fly in the lower atmosphere (literally ground level) which greatly increases drag compared to flying in the stratosphere.

Is it possible that a GEV designed using contemporary composite materials and high bypass engines could be more efficient per cargo weight-mile (or passenger mile) than a tradition airliner? Or are GEV inherently less efficient due to the altitude they fly at?

This question is specific to fuel efficiency over distance. Feasibility, safety, navigation, time, and cost of airframe are not part of the equation.

As some of the comments above (which are really on the money) suggest it's a complicated question to answer because it is a case of "horses for courses" and that is even if we focus purely on straight-forward fuel efficiency. But given the programs that have been attempted (or are still ongoing) and also various studies it seems that ekranoplans seem to work best in two very distinct roles. Either as small, short range puddle-jumpers or as very large trans-oceanic monsters. For the later see also this which may well come to pass.

I guess at some level such utterly different roles is a problem for ekranoplan development. But, yes, in principle, there would certainly appear to be certain, very different niches, for which the ekranoplan is extremely competitive in terms of fuel consumption. Having said that the second niche is arguably also competing against conventional surface sea-freight which of course complicates matters but offers interesting possiblities. Aircraft like the Boeing Pelican (which didn't get past concept stage) was designed not for standard air-cargo pallets but for ISO containers. Now that capability is a real game changer as long as the cost per kg/km fuel economy is as good or better than a conventional air-freighter which I suspect is very doable. At least in principle.


Is it possible that commercial airliners use an EMALS or other form of electric launch? I am doing a big project for school over the summer, and was wondering if anyone can help? I just want a professional opinion on the idea. Is it possible or no?

As a start I suppose the reason of that is a particular configuration where only a short runway could be build, for instance in a highly touristic island!

EMALS uses an electromagnetic linear motor operating on a track, some studies in the 50’s have been done in Europe using the same principle not to launch the train but as a continuous powering system however they were abandoned because of compatibility between countries.

As far as power is concerned, what was theoretically possible for a train could be possible to push or to pull an aircraft, but coming to the practical application, on which part of the aircraft the pushing or the pulling force shall be applied for launching; the landing gear presently available are not designed to accept such shear forces bringing the aircraft at least to V1.

If we consider the EMALS is just to reduce the runway length and is used just to bring an additional takeoff acceleration power, and supposing the location on the aircraft of the applied EMALS forces is solved, we get into another problem, that is the decision of the pilot at any moment before V1 to continue the takeoff or not? Should the pilot decide to cancel the flight, how to coordinate pilots decision with the EMALS disconnection, should it fail to disconnect the consequences could be terrific.

From my point of view this system requires specially designed compatible aircrafts, the economical feasibility can’t be general but specific to particular geographical configurations.

As a project why not? But economically it is hard to justify.


How can I tell if a rental aircraft has been maintained in good condition? When I rent a Cessna (typically old) from the local airport, how concerned should I be that the plane may have been abused at some points over many years (over-load forces, hard landings, over-rev'd engines, etc.)?
Perhaps the question is "are the plane's standard inspection schedules enough to catch such abuse?". Should I be concerned?

From a fatigue perspective, this isn't really that much of a concern.  The beauty of metal structures is if they get "overloaded" one time, but the overload was not past the permanent deformation point, the yield point, of the affected structure so that nothing is permanently bent, the structure is fine, in general.  A hard landing that did hidden damage to the gear box should get detected at the next annual.
If you do a careful walk around and include gentle pokes and prods with your inspection, looking for loose bits or sloppy clearances, and everything is reasonably tight and straight, the chances of the thing letting go on you for no reason, while you are minding your own business sightseeing, are microscopic.  Look for stories of 172s just falling out of the sky because the wings let go without warning, and you will be looking for a long time.
Cessna once tracked down, back in the 70s, the most beat up high time 172 they could find, and located a 12000 hour airframe that had been used for pipeline patrol, to see how it was holding up.  It just required replacement of hinge bearings IIRC.  Corrosion is the bigger concern than overloads, and I might be wary of a 172 that has spent its entire life tied down outside (it's the daily condensation cycles that's the killer for outside parking unless it's in the desert) unless I was confident that the maintenance at the operation was good.  A 172 that is hangered every night will pretty much go forever.
Same with engines. Lycs and Conts are built like air cooled tractor engines.  Slow turning with large clearances and thick, heavy cranks and connecting rods, dual ignition, simple carburetors.  They usually give off warning signs if they are going to blow up on you (for example, an impending main bearing failure typically results in major amounts of oil coming out the breather line and coating the belly as the excess oil spews out the bearing and fills the crankcase with spray). When Lycomings go bad it's usually their weak point, the camshaft lobes, going south, and this puts metal shavings in the oil filter long before engine operation is affected, which should get caught at annual (as the cam lobe wears down eventually the engine will lose power and misfire).
If it starts nicely, runs nicely making normal static RPM at full power, oil pressure comes up smartly, isn't dumping oil everywhere, no crazy vibrations or scary sounds, the chances of the thing blowing up on you are maybe not microscopic, but pretty small.  Be observant of the FBO's maintenance operation; organization, cleanliness etc.
Learn some of the warning signs of engine trouble (like "morning sickness"; rough running that goes away as the engine warms up, a sure sign of a sticking exhaust valve, and which is guaranteed to get worse).
Statistically speaking, if you are flying around in a halfway decently maintained, straight and tight 172, at 3000 ft on a nice day, with gas and oil in the thing, you really have to work at it to kill yourself.


How is aircraft control positively transferred between pilot flying and pilot not-flying? I am watching the video NTSB Animation Flight 1549 Hudson River Landing US Airways  (Youtube) and at 0:36 in the video, after the bird strike, you see the following dialogue:

HOT-1: my aircraft
HOT-2: your aircraft

I was curious to know what the formalities behind this is.  Could HOT-2 also claim the aircraft?

What you're quoting is the verbal portion of an important principle called Positive Exchange of Controls. This is a process of ensuring that someone (and only one) is always in control of the aircraft, and that there is no doubt about which pilot that is.

The best practice, taught to student pilots, is a three-stage handoff procedure:

  Pilot A: You have the flight controls.
  Pilot B: I have the flight controls.
  Pilot A: You have the flight controls.

Pilot A is the Pilot Flying before the handoff, and Pilot B is the Pilot Flying after the handoff. In an emergency situation like the above, the captain (Captain Sullenberger, HOT-1, in this transcript) is responsible for the safety of the aircraft, and can assign control as they determine is best. Skipping the first line of the handoff is a reasonable way of making this happen  quickly, while not giving up much in terms of clarity. The two-stage transfer also can happen whenever someone, generally more experienced or in a teaching role, wants to be in control of the aircraft. As DJClayworth points out, a CFI will commonly take the controls in this fashion if they wish to demonstrate a maneuver - no imminent danger required.

HOT-2 refers to the "hot microphone", or recorded feed from the pilots' headset mics, from the first officer. Any time there's a change in Pilot Flying (if the captain gets up to go to the bathroom, for example), the controls should be exchanged in a manner similar to what we see above. If the FO feels that they should have the controls, they can always ask. There is no standard procedure for refusing the exchange of controls, but generally, if the Pilot Flying does not reply to the request with something along the lines of "You have the controls", then the exchange is incomplete, and the role of Pilot Flying has not changed.


Why do we need a negative derivative of the pitching moment? restoring moment should be enough? I am wondering why I read everywhere, that for static stability (longitudinal) the slope of the lift coefficient has to be negative.
Imagine a curve (cm= coefficient of moment , alpha = angle of attack) like the one on my picture. In the trimmed condition where c_m=0 : when I have a an increase in angle of attack, it should be enough, when c_m is negative, because I will always have a restoring moment. The more negative it is, the stronger the restoring moment.
So also when the slope is increasing, I am still in the negative area like in the red area of the picture with a restoring moment.
What I can imagine: such a slope of cm over alpha for a whole aircraftwill maybe not exist?
Thank you for your comments and thoughts

So let’s assume the aircraft has a $C_{m_a}$ curve as you show. When you start to pull up you will feel nose down force on the yoke because of negative pitching moment. As you further increase the pull, the force required should increase also. However in your picture at some point the negative pitching moment $C_m$ decreases (in magnitude), still negative though. That can cause all unwanted motions in the aircraft because suddenly the force you are applying is becoming too excessive. It can even over-g the aircraft depending on the conditions. You are basically braking the linear response of the aircraft.
Ironically enough , that was the main reason Boeing 737 Max was equipped with MCAS in the first place. The stick force per g was not linear at some corners of the envelope. So they put MCAS to linearize the $C_{m_a}$, initially. Of course then they extend it to low speed region also.
Additionally, Part 25 Airworthiness Standards for big aircrafts requires stick force curve slope to have a stable slope:

§25.175   Demonstration of static longitudinal stability.
…
(1) With the landing gear retracted at high speed, the stick force curve must have a stable slope at all speeds within a range which is the greater of 15 percent of the trim speed plus the resulting free return speed range, or 50 knots plus the resulting free return speed range, above and below the trim speed (except that the speed range need not include speeds less than 1.3 $V_{SR1}$, nor speeds greater than $V_{FC}$ / $M_{FC}$, nor speeds that require a stick force of more than 50 pounds), with—

As for the small aircrafts the Part 23 requirement has a little bit of a leeway:

§23.2145   Stability.
(a) Airplanes not certified for aerobatics must—
…..
(3) Provide stable control force feedback throughout the operating envelope.

The part you are asking is specifically addressed in FAA AC25-7C:

§26
...
There may be no force reversal at any speed that can be obtained, except lower than the minimum for steady, unstalled flight or, higher than the landing gear or wing flap operating limit speed or VFC/MFC, whichever is appropriate for the test configuration.
...
(4) Examples of “local reversals” are given in Figure 26-2. Curves A and C depict a local gradient reversal within the required speed range. Even though it might be argued that the “average gradient” meets the one pound in six knots criterion, the gradient reversals would render these characteristics unacceptable. Curve B depicts a situation in which the gradient reverses, but only outside the required speed range. In addition, Curve B demonstrates a situation in which the local gradient does not always meet the required one pound in six knots, even though the average gradient does.


What are some "FAA approved" simulators for home that I can log time on? I'd assume xplanes on iPhone or Android would not be log-able, but wanted to know if there is a affordable simulator kit for home usage that'd help me logging time for IFR requirements?

Read this answer from AOPA, especially the last 2 paragraphs.
https://www.aopa.org/news-and-media/all-news/2004/november/flight-training-magazine/since-you-asked

Logging sim time
Dear Rod:
I'm a private pilot just a couple of days from taking my instrument checkride. My CFI and I have had discussions about simulator hours. He logs them but does not include them in total time. I've just closed the Federal Aviation Regulations/Aeronautical Information Manual for the umpteenth time and after reading about maximum of 20 here and a credit of 100 there, I'm confused. Maybe you can explain. What is the point of taking advantage of a sim if the hours logged don't count toward my TT?
Thank you,
Mark
Greetings Mark:
Simulator hours can count as training time, but are not considered "flight time" and can't be logged in the flight time (total time) column of your logbook. They can, however, be logged in the "Flight Simulator" column of your logbook. If your logbook doesn't have such a column, then make one using the letters "FS/FTD" which stand for flight simulator/flight training device.
Regarding simulator time counting toward your total time, consider that the commercial certificate allows 50 hours of simulator time to be counted toward the total aeronautical experience requirement of that rating -- see FAR 61.129(i)(1).
Thus, you could meet the commercial requirements of 250 hours of aeronautical experience with 200 hours of flight time and 50 hours of simulator time.
Even if simulator time didn't count toward the total aeronautical experience requirement of a rating, it might still count as training time. For instance, 2.5 hours of appropriate simulator time can be credited toward the total training time required for the private pilot certificate (FAR 61.109 i).
Now, read the federal aviation regulations carefully before running off and trying to log cross-country time in a simulator. The regs specifically state you must meet certain "flight time" requirements in an airplane. You're not getting away with packing a lunch and sitting in front of a simulator for six hours in hopes of logging this as a cross-country flight to meet the private pilot cross-country requirements.
(Yes, I heard of a case where someone actually tried to do this. Perhaps the ruse unraveled when the FAA examiner asked, "Hey, what kind of airplane is a Combat Flight Sim?")
Also keep in mind that you can only log simulator time when there is an appropriately rated instructor providing dual instruction. In other words, solo time in a simulator can't count toward your private or commercial certificate or instrument rating.
And it goes without saying that a simulator must be approved by the FAA for it to be used as a FS or FTD. (Sorry, but Combat Flight Simulator is only approved for shooting down other simulators, not flight training.)
Rod Machado is a flight instructor, author, educator, and speaker. A pilot for 34 years and a CFI for 30, he has flown more than 8,000 hours and owns a Beech A36 Bonanza. Visit his Web site.

FAA regs on simulators
https://www.faa.gov/about/initiatives/nsp/ac/
List of FAA approved simulators, and the aircraft type simulated
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/afx/afs/afs800/afs810/media/FAA_Approved_Airplane_ATDs.pdf


When, exactly, were the DC-8’s airbrakes removed? The DC-8 was originally equipped with airbrakes on the lower rear fuselage, but these were quickly removed.  Wikipedia says that they were removed before the DC-8 entered service:

[...]  Douglas made a massive effort to close the gap with Boeing, using no fewer than ten aircraft for flight testing to achieve FAA certification for the first of the many DC-8 variants in August 1959.  Much needed to be done: the original air brakes on the lower rear fuselage were found ineffective and were deleted as engine thrust reversers had become available...

Emphasis added.
However, according to the CAB report on the crash of United Airlines Flight 859, the first few DC-8-10s, at least, did have airbrakes when they entered service, and at least one was using them as late as July 1961:

When this action failed to straighten the landing roll he knew the aircraft was going to leave the runway.  He said he used the emergency airbrakes to slow the airplane as much as possible before leaving the runway.  [...]
[...]  The second officer said that after touchdown a series of events occurred which he could not place in sequence with any degree of certainty.  He saw ejector indicator lights blinking, heard a foot thumper, saw the captain reach for the emergency airbrake handle, and saw that the aircraft was heading about 30 degrees off the runway.  [...]  [page 4 of the report]

Emphasis added.
And it’s definitely not misusing the term “airbrakes” to refer to the aircraft’s wing spoilers, since it refers to those as “spoilers” elsewhere in the report.
When were the DC-8’s fuselage airbrakes removed?

The DC-8 is equipped with emergency air brake system. This isn't the type of air brakes you might expect on an airplane, but rather on a truck. The captain could pull a handle on the instrument panel to release pressurized air into the hydraulic brake system in the event of hydraulic pressure loss, as explained in this AAIB bulletin. The fuselage air brakes were removed from the first prototype, no DC-8 entered service with them. Although the wing spoilers are not used as speed brakes in flight, reverse thrust is allowed on the inboard engines during descent.


What is this device below the door of a 737 (and other aircraft)? The other day I made a question about the External Power Hatch. Now I would want to know what's that hole below the door.

That is the outflow valve. The outflow valve allows air to leave the pressurized section of the fuselage. The aircraft's pressurization system controls the valve to keep the desired pressure, balancing the air coming in through the bleed system.

Source


What are some examples of pilot error with criminal negligence? I’m having a hard time understanding why pilot error technically can’t be criminal negligence all the time? So I’m curious to find out some examples of this exact scenario, and how investigators determine if ones negligence is punishable by law.

In the US, negligence would imply willfulness. Errors and accidents don’t imply willfulness nor negligence. That’s the difference between a death caused by an automobile traffic accident due to weather and one caused by a drunk driver. The latter is criminally punishable. The willfulness to do something wrong or not do something right is negligence. Making a decision that an average person would consider reasonable should not be prosecuted. Even if it is the wrong one.

The converse of making a reasonable decision in the case of flying would be to willingly fly the aircraft in a manner beyond the pilots capability or legal competency. If the pilot is not certificated and current, or flying the aircraft into a situation for which it is not certificated could be considered as negligent. The pilot had to do so due to an emergency, that would be a different story.

The same is true regarding intent. If your intent was to break a law, rule, or regulation (even if you did not know there was a law, rule, or regulation regarding your actions), that is criminal. A prosecutor or FAA official can even argue for the criminality of breaking a recommendation, best practice, or commonly accepted procedure if your actions led to an accident, injury, or fatality.


What are the criteria for classifying which generation a fighter aircraft belongs to? Current jet fighters have been classified as 4th generation, 4++ generation, and so on, with 5th generation being the latest, i.e F- 22. What is the criteria for such segregation? B-2 is a stealth aircraft but considered 4th generation whereas Mig-35 is classified as 4++.

The Wikipedia Page for Fighter Generations has a relatively comprehensive list on how they derive at the "generation" of the fighter. For example, here is how the Air Force classifies them:

  1: Jet propulsion : F-80, Me 262
  2: Swept wings; range radar; infrared missiles : F-86, MiG-15
  3: Supersonic, pulse radar, Beyond-visual-range missiles : Century Series, F-105, F-4, MiG-17, MiG-21
  4: Pulse-doppler radar; high maneuverability; look-down/shoot-down missiles : F-15, F-16, Mirage 2000, MiG-29.
  4+: High agility; sensor fusion; reduced radar signature : Eurofighter Typhoon, Su-30, F/A-18E/F, Rafale
  4++: Active electronically scanned arrays; continued reduced signatures or “active” (waveform canceling) stealth technology, supercruise : Su-35, F-15SE
  5: All-aspect stealth with internal weapons bays, extreme agility, full-sensor fusion, integrated avionics, some or full supercruise : F-22, F-35,
  6: extreme stealth, morphing capability, smart skins, highly networked, extremely sensitive sensors, optionally manned, directed energy weapons.

Until we have a major leap in technology, it would seem we are capped off at 5th generation fighters for the moment.


May the tower use the runway while an emergency aircraft is inbound? I have a question regarding an aircraft in emergency. Of course it will have  priority for landing but if the position of this aircraft is very far, like for example 20 minutes or more away,
would the tower be allowed to clear other aircraft to land or take off? Is it permissible for the tower to do so?

Is there any law or statement in the civil aviation regulation about such things?

FAA Order JO 7110.65W - ATC Procedures, Chapter 10 outlines how ATC should handle procedures for emergency situations

Right at he very beginning, we see the most important part:

  10-1-1(d)
  Because of the infinite variety of possible emergency situations, specific procedures cannot be prescribed. However, when you believe an emergency exists or is imminent, select and pursue a course of action which appears to be most appropriate under the circumstances and which most nearly conforms to the instructions in this manual.

In other words, they don't dictate what controllers should do in an emergency; they are to use their best judgment about how to handle it.

So, a controller is free to continue using a runway for both arrivals and departures if they believe doing so won't interfere with the emergency aircraft.

In fact, for non-urgent emergencies like gear extension failures, I've heard them hold the emergency aircraft to get as many others in and out as possible before the runway is closed for a gear-up landing.


Could an EFB-based pre-flight check be automated? Since an electronic flight bag (EFB) is basically an iPad with the right app installed, is it possible that some sort of plane-tablet connection could be formed (either wireless or wired) to automate all of most of an EFB pre-flight check? Or does aviation regulation not allow this?

This is a purely hypothetical question since I don't think any EFB app so far supports this, let alone any plane.

Clarification: I'm talking about the preflight checks that ensure that everything in the cockpit is set up correctly for takeoff. I understand that an EFB is an electronic version of the 40 pounds of paper a pilot usually needs in the cockpit. Can this preflight check be done on the EFB? And can this be automated?

An EFB could not be used for this purpose, all your suggestion would do is replicate things that are already done by EICAS (engine indication and crew alert system).  The reason for this is that "receiving", "pre-flight" and "before takeoff" checklists all involve physically moving switches.  The EFB would be unable to perform these actions (e.g. an EFB cannot select flaps 9 degrees).

What an EFB could do, if tied into the aircraft ARINC data bus, is to verify things.  Such an EFB could verify that flaps are indicating a set position and could alert us if it were not.  However, as mentioned, the EICAS already does this, and if I were to advance a thrust lever beyond taxi power I would get a master warning and a takeoff configuration EICAS message as well as an aural warning "TAKEOFF FLAPS" (EMB-145).

You also mention the bulk of paper carried as a reason for this, but this is not relevant to the checklists.  The checklist on many large airplanes is a single sheet of paper, laminated and installed in the airplane.  Some airplanes may have a few sheets, but overall checklists account for very little paperwork and they belong to the airplane, not the pilots.

Where EFBs do help with paperwork are the approach charts, which account for 75% of what I carried on any given flight.  Three 2" binders, full of nothing but approach plates.  An EFB can condense that down to a tablet, and this is where the value of the EFB is.


How can I calculate the effect of winglets on aircraft performance? For a project at university I am having to produce a SIMULINK model of a Nimrod MR2 aircraft. The next step is to increase the endurance of aircraft and a possible route for this is adding winglets to the tips of the wing to increase the efficiency of the wing. However I am struggling to calculate the effect this will have on the performance of the aircraft.

Could anyone offer some help on this topic please?

Basically, there are two ways. One is easy but imprecise, and the other is hard but more precise.

First the easy one: Look at other aircraft and estimate how much winglets affect lift and drag. Make sure to do this at several lift coefficients; winglets help much more at high lift coefficients. Don't forget to account for the added mass of the winglets and their added friction over their surface area. Bonus points for estimating losses due to sideslip.

The hard way is to do a numerical simulation of the wing. Since you want to know what difference the winglets make, you can focus on a comparison between a regular and a winglet-equipped wing and transfer the effect to the full airplane. For the lift and induced drag effects, you will at least need to divide the wing into panels and simultaneously solve the Biot-Savart equations for bound vortices at the quarter chord of each panel plus the trailing free vortices with control points at the three-quarter point where local flow must be parallel with local wing inclination. Make sure you use a method which does better than to assume that all panels lie in one plane!

I have no experience with Simulink - I guess there are libraries around for vortex lattice or even Euler codes, but if you are free to choose your tools, maybe it is worth to take a look at XFLR5. This should work out of the box for your project, and all you need to add is a proper input file.


What is the FMS Bridge Visual 28R approach at SFO? I've read that SFO has a specialized FMS Bridge Visual approach to 28R that is not published but instead is custom tailored for specific operators. What exactly is this approach and how does it work? When is it assigned, and what advantage does it have over other visual approaches to runway 28R?

The FMS Bridge Visual Approach 28R is a version of the Quiet Bridge Visual Approach 28R which is coded with GPS coordinates and can be included in an FMS database for approved operators.  This allows the procedure to be used when the SFO
VOR is out of service, and also gives ATC additional flexibility by allowing them to clear pilots direct to any of the fixes without needing to intercept the radial on the standard arrival.

The pilot must either tell ATC that they can accept it verbally, or include FMS28R ABLE in the remarks of their flight plan so that they know that the flight is capable of flying the approach.


LAPL vs PPL - Which is more suitable? I would really like some help on making the decision as to which license is more suitable for my situation. From reading various forums online, others have had the same questions, but I couldn't find a concrete answer anywhere.

My situation:

No previous flying experience
Would like a license so that I can privately rent on a very recreational basis (I expect to average an hour or two per month after obtaining either license, largely due to cost - the local airport will hire out a 172 for £140 per tacto hour (including fuel and VAT), which equates to around £120 per flying hour for typical usage). Taking friends along for the ride would of course, be ideal.
Desire to keep cost to a minimum - I would like to keep the cost of the training, from zero to license to under 8k (pounds/euros) if possible. This includes training materials, lessons, test/exam fees and landing fees. I believe this is possible, if I manage to stick to the lower end of the average number of hours.
Local school offers full LAPL course (providing the minimum 30 hours) for £5290.
Local school offers full PPL course (providing the minimum 45 hours) for £7450.
Both prices are based on learning in a Cessna 150.
The weather in Uk and part of France I move to is not always great. There is often a lot of cloud, rain and wind.

So based on all of the above, which license would you recommend? The LAPL is cheaper, requires less hours (local school says 30) and is more attainable. It can also be upgraded to a PPL at a later date for instrument / night rating. However, due to the weather, will I get to fly as much with the tighter restrictions, if not, maybe it's better to just fork out the extra time and money and go straight for the PPL?

Apologies for the long winded question, I wanted to provide as much info as possible so that I can get the best answer possible as I am very conflicted at the moment but want to make a start. I hope that the answer isn't 'neither'. Which is what I'm worries about, since I don't want to be spending all of my disposable income on flying, I can't very well go out every week.

Location information: The training will take place in the UK, although I will be moving to France shortly after obtaining either license.

An LAPL allows you to act as PiC, carrying passengers, but only in the type and variant you pass your test on.

  A LAPL for aeroplanes will allow you to act as pilot in command (PIC) on two classes of aircraft: either a single-engine piston aeroplane (land) or touring motor glider (TMG) with a maximum take-off mass of 2000 kg or less, carrying a maximum of 3 passengers, with no more than 4 persons on board.
  source

Whereas a PPL you will be able to fly all variants of SEP (source), with the exception that for things like taildraggers, and more complex aircraft (such as those with retractable undercarrige)  you will need extra training and endorsement.

The only other major difference is the day you pass your receive your PPL license, assuming you have conducted 3 take offs and landings in the prior 90 days, you will be able to take passengers. An LAPL requires you to complete 10 hours solo once receiving your LAPL license before taking passengers.


What is the name of this vintage helicopter trainer? I am curious about the tiny training helicopters that can be seen from the second 0:10 in this vintage video about the Cuatro Vientos helicopter school, near Madrid.

Were they "real" helicopters, capable of flying?

Were they a mass-produced model or an artisan solution? Who made them?

It is a Flettner Bo102 Helitrainer. There were 18 of these produced which were used by six countries.

Source: Airus Helicopter History

Here are some more pictures

It is also known as the Bölkow Bo 102 (Wikipedia).

From Aviastar.org:

  Rolled out in 1959, this Helicopter Trainer was powered by a 40hp ILO piston engine and had a twenty-one foot counterbalanced fibreglass rotor blade. In all eighteen of these HeliTrainers were built operating throughout Europe, training military helicopter pilots. Although unable to fly, they were ideal for the teaching of hovering techniques and were replaced by dual trainer helicopters.

  The Bo-102 Helitrainer, powered by a 40hp Hirth engine driving a simple one-bladed rotor with a counterweight, was developed in land and water based versions, both of which were semi-captive. This feature allowed the helicopter to rise to a height of 0.6m, turn around a vertical axis and dip at up to 6� but prevented it from flying outside these limits.


What is the physical explanation for the formation and position of stagnation points on an object moving in a fluid? I have some questions regarding the stagnation points, both on an airfoil and on a generic object.

What is the physical explanation for the formation of two stagnation points on an object moving in a fluid (e.g. a cylinder)?
Why in potential flow with zero circulation the rear stagnation point is located above the trailing edge of an airfoil (and not below)?

Does the Kutta condition hold even on flatback airfoils like this one? They have a truncated trailing edge, so I think the "infinite velocity" problem would still arise (in theory, of course).
Where is the rear stagnation point located? The image below gives their approximate position, but it's not clear where the rear stagnation point is.

(source)

The forward stagnation point is easy to explain: Imagine the flow path of molecules which approach an airfoil. Some will continue above and some below the airfoil contour. Between them there must be one flow path which will neither go above nor below, but hit the airfoil straight on. The point where it hits the airfoil is the stagnation point.

The rear stagnation point is similarly explained: When the flows from above and below meet, there will be a line dividing both. Continue this line forward until it hits the airfoil: This is where the rear stagnation point is located. A rear stagnation point is only possible in inviscid flow, which exists only in theory. In both points the speed of air relative to the airfoil is zero, and air molecules will never completely arrive at the stagnation point, but slow down on their way into it.

An airfoil of zero circulation and zero camber will have its rear stagnation point right at the trailing edge. Only when you add asymmetry by giving the airfoil positive camber, the flow has to develop an asymmetric pattern which shifts the rear stagnation point away from the trailing edge to satisfy the zero circulation condition. Give the airfoil negative camber, and the rear stagnation point will duly relocate below the trading edge.

The Kutta condition postulates that the rear stagnation point is fixed at the trailing edge. Make the trailing edge blunt and you need to define where precisely your stagnation point will be. Note that your choice of location will influence the circulation at a given angle of attack! The Kutta condition imposes an arbitrarily chosen stagnation point which just helps to explain real flows better. This does not mean that real flows will observe the Kutta condition closely.

If you look for the "real" stagnation point, you should really search for the point where local pressure reaches a maximum, fully knowing that a full deceleration to stagnation will not happen in reality. This can only be outside of the boundary layer, since the energy loss of the boundary layer flow will result in a pressure loss. Now the picture from the second paragraph will help: Find the line separating upper and lower flows and follow it upstream until it hits the airfoil. This procedure will land you straight at the lowest speed section of the boundary layer, and near the airfoil the pressure there is too low. But now move back again and watch how the slow center part of the boundary lower gets accelerated by shear with the neighboring flow. At some point pressure will peak: This is the closest you can come to for a rear stagnation point.

Below I plotted a quick XFOIL calculation of a NACA4418 with a 5% trailing edge gap to get something similar to your airfoil. Note how the $c_p$-value of the viscous flow (solid line) peaks at some point behind the blunt trailing edge. For the outer flow the airfoil looks as if it is slightly longer, the void behind the blunt trailing edge being filled by separated, slow flow. Also note that the upper boundary layer is thicker, shifting the "rear stagnation point" a little up.

XFOIL Plot of a NACA4412 with 5% blunt trailing edge at 4° AoA (own work).


How can I keep my A&P license active without being in aviation? I am no longer in aviation. I do not want my A&P license to go dormant. Are there any ways to keep my A&P license active without being in aviation for over two years?

If you are "not in aviation", then I assume you are not working in/around/on airplanes. You are required to keep currency as per 61.53:

  § 65.83 Recent experience requirements.

  A certificated mechanic may not exercise the privileges of his certificate and rating unless, within the preceding 24 months -

  (a) The Administrator has found that he is able to do that work; or

  (b) He has, for at least 6 months -

  (1) Served as a mechanic under his certificate and rating;

  (2) Technically supervised other mechanics;

  (3) Supervised, in an executive capacity, the maintenance or alteration of aircraft; or

  (4) Been engaged in any combination of paragraph (b) (1), (2), or (3) of this section.

So the only way you can keep your currency (or regain it after you become non-current) without doing anything in Aviation is to demonstrate to the "Administrator" that you are able to do that work. What this entails is not defined in the CFR's, so will be up to your local FSDO.

Otherwise you can take a few hours a week and do mechanic work, for example you may be able to find a flight club and do some fleet maintenance (exchange it for instruction time or something).

tl;dr; You can't keep your license current without working on airplanes or supervising other mechanics. It would be analogous to keeping your pilots license current without ever flying.


Are matches in checked baggage a safety problem? A friend of mine checked in on Singapore Airlines a bag which had a box of matches in it. They are regular matches and not the "strike against anything" type.

My friend subsequently realised that these were banned on that airline, and went back to the baggage check-in location. The baggage check-in staff said that it was not worth the bother to retrieve them, and that security personnel later on would remove the matches if they thought they were a problem.

Presumably there is some chance the personnel later on would miss the matches, but the staff did not seem to bothered by this, which suggested to me that maybe there was no genuine safety problem here.

What's the rationale for this rule existing? Of course in life on the ground matches do not generally spontaneously ignite, but I wondered if maybe different conditions in the cargo hold made matches more likely to ignite on their own?

The reason that matches are banned on most airlines is not that they might accidentally catch fire. Especially not in the cargohold of the plane. Sometimes the cargohold is not even pressurized. and a lot of safety measures are taken to prevent cargo fires, including suppression systems in case this does happen.

Most likely the ban is for carry-on baggage. Most airlines will allow you to take a lighter with you, but only 1.
The reason is that if someone wants to start a fire in the passenger cabin, this is not as easy with a lighter.
A box of matches on the other hand is a lot easier and faster to set on fire.


Is Vertical Speed based on True Airspeed or Groundspeed? I have edited this for clarity

For the same angle of descent or climb, does vertical speed increase if true airspeed is greater or if ground speed is greater (due to a tailwind)?

Imagine a triangle and assume a three degree angle of descent. The hypotenuse represents the flight path. The opposite end represents the amount of descent. The adjacent side represents the ____. Airspeed or ground speed?

If headwind matched true airspeed, I imagine an aircraft would climb and descend in place (like an elevator) without knowing anything about it's ground speed. However, with large tailwinds, ground speed will obviously be much larger. Going back to the triangle concept--the hypotenuse will be very long--which means the aircraft would descend more quickly.

Furthermore, every Captain I've talked with swears rate of descent increases with a tailwind. However, I believe this may be a result of reduced tailwinds at lower altitudes.

Do you have a source or a formula you can cite?
Thanks.

Vertical speed, as measured by the Vertical Speed Indicator (VSI) should be measured entirely by the changing air-pressure at the static port, and should be completely unrelated to airspeed and ground speed.

The static port is a small hole, (about the size of a large needle or tiny nail) in the side of the airplane, with a pressure-sensor inside.  As that sensor measures a decreasing pressure, it indicates that the airplane is climbing.

An increasing pressure indicates the airplane is descending.

Neither measurement should depend on the plane's air or ground speed.  The sensor should still work even if mounted on a hot air balloon (or other stationary craft).

Source: http://www.boldmethod.com/learn-to-fly/aircraft-systems/how-does-a-vsi-work/


Is it dangerous to do a skidding turn to lose altitude without increasing airspeed? I was taking some lessons in a light sport aircraft in South Africa a few years back, my instructor taught me something which I had never seen or heard of before or since. We went through an emergency scenario in which we had to make an emergency landing in a field (not really landing but just going through the motions, of course). I found a place to land and started my descent, but had to loop back around after I passed it. The instructor then told me to give full right rudder with left aileron to quickly descend without gaining airspeed.

Wouldn't this qualify as a recipe for creating a spin and possibly killing myself? Should I actually implement that skidding turn in an emergency to land if need be?

As Ron Beyer pointed out in a comment, your instructor was showing you a forward slip - a pretty standard maneuver for light aircraft.

A spin would involve full rudder and a stalled wing, which usually means pulling the yoke or stick back a lot.  That's why overshooting the base-to-final turn can be dangerous - you likely already have a lot of rudder in and it's tempting to pull back on the yoke to try to tighten up the turn.  An unexpected spin at low altitude is not recommended as recovery tends to be both abrupt and fatal.

As long as you're keeping the yoke more or less centered forward and back and your airspeed up, there should be nothing to worry about.


Is a jet engine's shaft neutral in the direction of thrust? I think I read somewhere that the forces on a jet engine shaft is neutral because the forces at the turbine and compressor cancel out. This means that the axial bearings don't have force on it in direction of thrust!?
Is this true? In that case I think the thrust will be zero because there's noting against which the pressure can push to produce thrust.

Thrust is produced in the compressor; as its blades push the air backwards to compress it, a reaction force pushes the blades forwards. These blades are fixed to the shaft. The shaft has a flange on it which pushes against a similar face on the engine mounting, in an arrangement known as a thrust bearing. This in turn transmits the thrust to the airframe.
A turbofan has extra big blades which push more air backwards, adding to the thrust.
The turbine blades draw work from the exhaust flow which slows it down, so they push against the main thrust and in a simple single-spool design can even require the thrust bearing to face backwards.
Heating the compressed air in between is what makes the thing work at all. Also, the acceleration of air out the back of the combustion chamber creates a thrust reaction against its forward wall. For a pure turbojet, and especially for a centrifugal compressor, this will be greater than the compressor thrust. It might not be as much as one might suppose because the chamber is relatively small.
For supersonic flight, the air intake acts as a shock compressor to generate most of the thrust, while the exhaust nozzle can be made to generate more by expanding high-pressure exhaust against a divergent nozzle. At high enough speeds the spinning bits just get in the way.


Could Henri Farman have steered his plane with ailerons on January 13, 1908? It is known that Henri Farman flew (over) 1 km in a circuit on January 13, 1908, and gained a 50000-franc prize. Orville Wright even witnessed Farman on November 18, 1907, attempting to make the full circle but each time touching the ground before completing the required course. The altitude of the machine dropped while the French pilot attempted some turns. (Farman did not use ailerons till after September 1908.)

The question is: Would the ailerons have made the Farman plane to steer more efficiently without losing altitude, which was critical because, for technical reasons, the plane could not fly higher that what you can see in the picture?

Henry Farman flying on January 13, 1908. The picture appeared in many French publications of the time.

The image below apparently shows that a plane loses altitude while taking turns using its ailerons. Does the airplane also go down if it is steered just with the tail?

I would first check to see if the car in the background was actually pulling him along in 1908, or maybe the aircraft engine was a bit better.

Banking does indeed reduce lift, but not as much as we may think:

If vertical lift at 0 degrees bank was 1000 lbs, a 20 degree bank at the same AOA and speed still produces: $$cosine 20 degrees × 1000 = 940 lbs$$ of vertical lift, easily corrected by slightly increasing the Angle of Attack of the wing.

The reason the plane previously could not make the turn can be explained by the relationship of thrust and drag.  Turning increases drag, which will slow the plane down.  These early aircraft were very draggy and underpowered, barely getting off the ground.

Deflecting the ailerons (really 2 halves of a speed brake) increases drag.  The bank additionally decreases lift.  With no reserve power to add, the plane goes down.
What an accomplishment the first circle must have been at that time!

But "skidding" the plane around the circle, with rudder only, presents the whole side of the plane as a dragging surface, there for is not a better solution.  The "coordinated turn" using  both rudder and ailerons produces the least amount of drag, and remains the recommended aircraft turning method to this day.

But even the best method of turning requires more power (thrust) than straight and level flight.  A few more horse power, and/or a lower drag design wins the prize.

Reducing weight and a better prop design would also be helpful.


What does it take to enter the Red Bull Air Race? Starting from being a newly minted private pilot, what are the steps I would have to accomplish in order to eventually qualify for and compete in the Red Bull Air Race? Assuming I put most of my time into the endeavour and progressed at a reasonably fast pace, how long would it take me?

According to their about page

  New pilots who wish to participate in the World Championship must first fulfil the minimum criteria set by the Red Bull Air Race Committee, which include top achievements in international flying competitions organised by the FAI. They must also be active aerobatic air display pilots. Eligible pilots that prove their skills at the Red Bull Air Race Qualification Camp can achieve the Red Bull Air Race Restricted Superlicence. Once they have the Restricted Superlicence, they then qualify to take part in the Red Bull Air Race World Championship in the Challenger Cup.

So I would suggest if this is something that interests you, you begin with some instruction in aerobatic flying - a discipline which in itself is quite tough. To find the next qualification camp, I would suggest contacting red bull air race directly. It seems they run annual qualifications, most recently held in Texas, USA.

The amount of time it would take is impossible to measure, or even speculate.


Does the A-10 Thunderbolt fly without the GAU-8? Has the A-10 Thunderbolt ever flown without the GAU-8? Can it be done with a ballast in place of the gun? I'm not referring only to the combat role of the A-10, a plane used for training or a ferry flight would also qualify.

Near as I know, the answer to the first question is no. The aircraft requires that a tail stand be fitted to it when the gun, magazine and electromechanical drive mechanism is removed for maintenance.  Operation of the A-10 without the gun would exceed the weight and balance envelope for the aircraft, making flight, as well as ground operations hazardous.

The A-10 has been proposed for use as a converted water bomber to fight forest fires but this balance problem without a counterweight installed in this region would make this very difficult.

http://fireaviation.com/2014/09/25/a-10-warthog-proposed-again-as-an-air-tanker/

If one was going to remove the gun for a ferry flight, it would have to be fitted with some kind of counterweight in its place, which would defeat the original purpose of removing the gun.


What may cause a strong smell of paint thinner like odor and no power to master in a Piper Warrior? I am sitting in the hangar with my Piper Warrior II sitting outside.  The battery is completely flat and the cabin smells like paint thinner.  This is the first time that this has occurred and if anyone has experienced this before please let me know what is going on!  This is a maintenance question and I am concerned leaving the plane in the current condition over night.  This is NOT a 100LL smell.

You've got two unrelated problems: The battery and the smell.

The Battery

Battery electrolyte doesn't really have a "smell" to it - at least not one you're going to notice in the cabin easily. If you open the cell caps on a newly-topped-off flooded battery and sniff you'll get a feeling for what the electrolyte smells like.
If you have a sealed battery you're not going to smell electrolyte unless the battery completely loses its mind and vents a large quantity of electrolyte - you'd definitely notice the mess in the battery box if that happened.  You may notice it in a conventional flooded-cell battery if the battery is overcharged (from high bus voltage, or from trying to charge a flat battery after jump-starting the plane), but again it's not going to be readily apparent in the cabin unless you've lost a large quantity of electrolyte or you're really boiling the battery from an over-charge.

If your battery's cells are full (or it's a sealed battery) and you didn't leave the master switch on then the likely cause of it being flat is that it's old and can't handle the low temperatures of winter anymore (or if it's a new battery it may have developed an internal fault and discharged itself).

If you left the master switch on putting the battery on a charger/conditioner overnight can usually bring it back to good operating condition. You can try that on a flat battery from an unknown cause too: The good charger/conditioners will warn you if they think the battery has an internal fault. If an overnight charge doesn't resolve the issue then you're going to need to replace the battery before further flight (and frankly replacing the battery is cheap insurance against getting stranded somewhere, so it merits consideration anyway).

The Smell

The smell of 100LL changes as it evaporates - lighter aromatic components vaporize faster, and will waft out of the cabin more quickly, leaving the smell from the heavier hydrocarbons lingering.
As you discovered avgas that's been evaporating for a while loses the characteristic benzene/toluene scent we're familiar with from sumping the tanks and has the general odor of turpentine/conventional mineral spirits ("paint thinner").

The first thing to check in the PA28 when you smell something like this in the cabin is the fuel selector valve, as it's a common failure point particularly in cold weather: The packing/tapered plug in the fuel selector valve has shrunk in the cold, and you're leaking fuel around the shaft (which is then soaking into the carpet and evaporating off, giving you the smell in the cabin). Other aircraft may have similar failure modes in their fuel selectors, but I'm a Piper guy so I can really only speak to their systems and associated quirks :-)

Since your shop identified the fuel selector as the problem it's an easy fix: Rebuild or replace the fuel valve and the leak will stop, probably for many years.  (Piper Service Bulletin 355 is relevant to this issue for older Pipers - the O-ring identified in the exploded diagram dries out and shrinks/cracks, allowing avgas to leak around the shaft. There may be something similar in Piper's service documents for the Warrior II, I'm not sure if it uses the same internal valve or not)

Another common failure point would be the O-rings around the primer plunger in aircraft with a mechanical primer (anything before the Warrior III).
This doesn't normally manifest as a lingering smell in the cabin but rather a smell when you operate the primer plunger (usually also accompanied by the plunger being stiff/hard to operate). Much like the fuel selector it tends to manifest when the weather gets cold and things shrink or become brittle, and the cure is to disassemble and appropriately service the plunger mechanism (replace the O-rings and re-lube the mechanism with a fuel-safe lubricant).


Why was the Spitfire's elliptical wing almost uncopied by other aircraft of World War 2? The Spitfire was one of the most successful designs of its day, with flying qualities of a similar standard to the other best designs of the era. In its decade of production from 1936 it grew bigger, stronger and faster.

Yet there seems to be almost no appetite from any of the major aircraft-manufacturing powers to emulate its most iconic feature. In fact, there is only one mass-produced aircraft of that time with an elliptical wing, the American P-47 Thunderbolt. Nothing German or Japanese, and nothing more from the British either.

In a period where every manufacturer was trying to gain every last bit of advantage, it seems odd that a prime design feature attracted so little appetite to copy.

There are good explanations here on the aerodynamics or performance of the elliptical wing. Why is it so rare when it demonstrably works so well?

Source

Interestingly, I couldn't find an answer to this question on the website, but I've found an answer of Peter Kämpf on Quora. He brings forth the same arguments I wanted to mention, so I'll repeat them here.

Elliptical wings are very good - aerodynamically. If you want to minimize induced drag for a given lift requirement, you end up with an elliptical wing.

But a plane is not only aerodynamics.
You also have to consider:

Weight, an elliptical wing is not structurally efficient, and will lead to a higher weight, which leads to higher lift requirements which will lead to more induced drag, even with a very efficient wing.
Controllability, where and how the wing stalls determines if you're able to recover from a stall. Elliptical wings stall tip first, leading to bad stall behavior.
Manufacturability, a fully elliptical wing is very hard to make, with its double curves. This will make the wing more expensive.

If you include these factors, you'll see that you'll end up with a compromise. If you use wing taper (which is somewhat close to the aerodynamic optimal shape) but much easier to make and much lighter you'll see that you'll end up with a better design overall.

An analysis of how the design of a wing changes if you include the structural requirements was done by Jones and can be found here.


Why do ADS-B outages occur without simultaneous RAIM outages? If you check the ADS-B prediction tool and the RAIM prediciton tool on SAPT.FAA.GOV you will see far more ADS-B outages.

What causes ADS-B outages to occur at different times than RAIM outages?  Are ADS-B stations shut down for maintenance?  Is it due to the local weather? or something else?

I know ADS-B systems are reliant on the Global Positioning System to determine location to send in the squitter, so I am assuming that a loss of RAIM is one reason for an ADS-B outage, but what else is causing the additional ADS-B outages not simultaneous to RAIM outages?

RAIM (Receiver Autonomous Integrity Monitoring) is a function within an avaition GPS receiver. It uses redundant measurements for a consistency check to determine the intergrity of the position measurement.

Since RAIM requires redundant measurements, it is only available when there are sufficient measurement sources available. In practice, this mainly boils down to the number of functioning GPS satellites that can be seen from the receiver, and the geometry of the satellites positions relative to the receiver.

For normal RAIM prediction, the RNP integrity limit of 1 or 2 NM is used. This means that the RAIM function needs to be able to verify that the GPS receiver will not produce a lateral position error greater than 1 (or 2) NM with a probability exceeding 10-7 per hour.

The  online predition tool simulates the GPS constellation and calculates whether a GPS RAIM system would be able to reach that integrity limit with the (healthy) satellites in view of the receiver at the time of flight.

For ADS-B, the position transmitted needs to have a certain level of quality in terms of accuracy and integrity. The accuracy of ADS-B position (encoded as NACp) need to be 0.05 NM or below, and the integrity (encoded as NIC) needs to be 0.2NM or below.

To determine the integrity of the ADS-B position, the GPS receiver again uses RAIM. To qualify for ADS-B, not only needs RAIM to be available, it thus needs to determine the integrity at 0.2 NM or less, and at the same time an accuracy of 0.05 NM needs to be achievable.

Since the ADS-B Integrity requirements are much more stringent (0.2 NM vs 1 or 2 NM), the prediction of integrity for ADS-B shows more frequent outages.

Note that the prediction tool only provides a prediction. The actual values during flight may be different.

References:

RAIM prediction requirements FAA AC 20-138D section 5-2.3pdf
ADS-B requirements: 14 CFR 91.227 c) 1) i) & iii)


What is the purpose of these movable panels on the MiG-35? The MiG-35 has movable panels on the LERX (leading edge root extensions). There are a number of images around the web showing these panels deployed during slow flight and retracted otherwise.

When do they have to use these and what does the use of them change in the flight envelope?

The aerodynamic devices in question here were already present on MiG-29KUB which is a parent version of MiG-35: Wikipedia: MiG-35

The devices are in fact, as some have suggested, Krueger flaps, as Russian test pilot Pavel Vlasov describes in this article: The MiG-29KUB: in the Russian and Indian skies for 9 years

  "It is a modern multirole plane with revolutionary fighting capabilities, with an increased payload and a longer list of weapons," said Pavel Vlasov, Honoured Russian Test Pilot, Hero of the Russian Federation, and General Director of the Gromov Flight Research Institute (and Head of the Flying Service at MiG until 2010). He noted at the same time that the MiG-29KUB featured a range of new solutions, such as Krueger flaps, a new flap design, a modern fly-by wire control system, and significantly improved piloting conditions

In addition to calling them Krueger flaps, this article also gives the devices the acronym LEVCON, leading-edge vortex controllers: SlideShare: Fulcrum Reloaded (see page 4)

The purpose of Krueger flaps is to increase the coefficient of lift of the wing, and to improve slow speed handling: Wikipedia: Krueger flap

After facts some speculation: the reason the Krueger flaps are not deployed in some of the pictures with gear extended is probably bacause they are not used in take off configuration, and the pictures most likely have been taken in t/o situation, not while the aircraft is landing


Why would an airplane be approved for ditching? So, the Learjet 60 that I used to fly has the following limitation:

My question, is why would a manufacturer certify an airplane for ditching if it isn't required?  The airplane is most likely going to be totaled anyway, right?

Or does it just mean that since it isn't certified for ditching that we aren't allowed to do it?  Guess no double engine failures over water will be in my near future....

I think that 14 CFR 25.801 might sum this one up. Aircraft that are approved for ditching under 25.801 have to meet several requirements:

(b) Each practicable design measure, compatible with the general characteristics of the airplane, must be taken to minimize the probability that in an emergency landing on water, the behavior of the airplane would cause immediate injury to the occupants or would make it impossible for them to escape.
(c) The probable behavior of the airplane in a water landing must be investigated by model tests [emphasis mine] or by comparison with airplanes of similar configuration for which the ditching characteristics are known. Scoops, flaps, projections, and any other factor likely to affect the hydrodynamic characteristics of the airplane, must be considered.
(d) It must be shown that, under reasonably probable water conditions, the flotation time and trim of the airplane will allow the occupants to leave the airplane and enter the liferafts required by § 25.1415. [emphasis mine] If compliance with this provision is shown by buoyancy and trim computations, appropriate allowances must be made for probable structural damage and leakage. If the airplane has fuel tanks (with fuel jettisoning provisions) that can reasonably be expected to withstand a ditching without leakage, the jettisonable volume of fuel may be considered as buoyancy volume.
(e) Unless the effects of the collapse of external doors and windows are accounted for in the investigation of the probable behavior of the airplane in a water landing (as prescribed in paragraphs (c) and (d) of this section), the external doors and windows must be designed to withstand the probable maximum local pressures.

Basically, the aircraft needs to have its hydrodynamic behavior while ditching investigated, and it must then float long enough for the occupants to escape. It sounds like the Learjet 60, if ditched, is not guaranteed to either a) remain structurally sound, or b) allow the crew and passengers to get out before it slips below the waves.
An aircraft that was certified for ditching, on the other hand, basically means two things:

The crew knows that they have at least a chance of doing it right (and will possibly change how the aircraft is configured or prepared), probably with an actual published checklist available to them; and:
If the crew screws it up, it's their own fault and the manufacturer might not be held liable; the aircraft met certification standards. This would significantly reduce their liability in a crash. It's probably mostly that one, honestly.


How can multiple airplanes prevent simultaneous communication between ATC? If two planes try to talk at the same time, do you hear nothing or both? Let's say ATC clears someone to land and they read back. I wait 4 seconds and then ask them something, how do I know another aircraft won't ask something at the same time?

As you fly more you will get to know the flow of radio transmissions and be able to identify when another exchange has completed. This comes with time and needs to be learned over time, it may be beneficial to listen into things like liveATC in your spare time to get comfortable. The FAA has a nice little brief on it here with some good pointers:

  Listen before you transmit. Many times can get the information you want through ATIS or by monitoring the frequency. Except for a few
  situations where some frequency overlap occurs, if you hear someone
  else talking, attempting to transmit will be futile. You will probably
  jam ("step on") someone else's attempt to transmit, causing a need to
  repeat the call. If you have just changed frequencies, first pause and
  listen to make sure the frequency is clear.

  Think before keying your transmitter. Know what you want to say and, if it is lengthy, (e.g., a flight plan or IFR position report),
  jot it down so you do not waste transmission time trying to remember
  what you need to say.

A solid 1-3 second pause should ensure the channel is free for you to transmit. The nature of transmissions is to keep them short and to the point, even on a busy frequency you will have plenty of time get in what you need and get a response.

As for the multiple transmissions at once, you will know! When two people attempt to transmit at the same time a nasty noise occurs. This is addressed here.


How to design a near vertical take off airplane? How to design a near vertical take off airplane, conceptually?

Say I have an airplane, similar to a Zenith 701. Say that it has a stall speed of 50 mph and a wing area of 60 sq ft, and it weighs 500lbs.

Now I want to make it land at 20 mph with full power and my tail volume is sufficient to keep the nose up at 30 deg. I also have slats and flaps to ensure my wing stalls above 30 deg AOA.

Say my max HP is 100.  I understand, as a general rule for GA aircraft, thrust is 4x100hp=400lbs. Vertical thrust would be 400 lbs x sin 30 deg x 65% prop efficiency for a prop in climb, giving me a vertical thrust component of 400 x .6 x 65% = 160lbs vertical thrust.

Now, I only need to produce 500-160=340 lbs of lift from my wings. So I increase my wing size so that I can fly at 20mph with a lift of 340 lbs.

Is this correct, at least conceptually?

Yes that is conceptually correct. You might have to mount a huge area, since lift is proportional to $V^2$.

Issues to resolve that I can see:

The craft would be relying upon aerodynamic forces to maintain attitude, which will be very ineffective at 20 mph. A secondary pressure relief valve system for attitude control would be advisable.
Landing at 30 degree nose up will be not easy from ground view perspective, and for landing gear design.
Ground effect will change the attitude of the craft if not carefully designed.
A wing that does not stall until 30° will be a tall order, and landing at the brink of stall is not advisable, you'll need some margin.


Does taxiing in London City (LCY) happen on the runway? I recently had a ride to LCY (London City). Awesome, by the way. They have only one runway (09/27) and no apparent taxiways.

(Source: Google Maps)

After landing, how do the aircraft get from the runway to the terminal? Do they just perform a U-turn on the (end of the) the runway and taxi back to the terminals?

To answer the title question, yes.

There are nice timelapse videos for EGLC/LCY on YouTube, if you want to see it in action. (Example.)

As for the actual operation at the airport, below is the official chart and textual data via the UK AIP:

  6 USE OF RUNWAYS

  b) Minimum Runway Occupancy Time - Arriving Aircraft.

  Pilots are reminded that prompt exit from the runway enables ATC to apply minimum spacing on final approach that will achieve maximum runway utilisation and will minimise the occurrence of 'go-arounds'.
  When landing on Runway 09 pilots should commence back-track as soon as practicable and exit via Hold Echo unless otherwise instructed by ATC. Any aircraft that continues landing roll beyond Hold Kilo may infringe the ILS critical area.
  When landing on Runway 27 A318 and CS100 pilots should plan to exit at Hold Delta (after a back-track if required). A318 and CS100 aircraft are not permitted to exit via Holds Charlie, Bravo, or Alpha. All other aircraft types may use any Hold to exit the runway except Hold Echo which shall only be used when specifically instructed by ATC. Pilots should be aware that use of Hold Alpha will increase Runway Occupancy Time.
  Pilots expecting to use the full runway length to stop (e.g. due aircraft weight/meteorological conditions) are requested to inform Thames Radar on first contact.

(Bold emphasis mine.)

The short version (runway 9): backtrack [on the runway] to vacate as soon as possible (example from cockpit) using the specified exits even if other exits may seem to be available, and if you need full runway length, notify the approach controller (Thames Radar).

(If the pilot does need full length, the approach controller needs to know to add extra spacing for the plane behind.)

For the exits mentioned in the text, reference the chart below:


In case of emergency, is it possible (doable) to land a small propeller-driven aircraft on an aircraft carrier? I was watching the Disney movie Planes with my younger brother, and one of the characters/planes perform a emergency landing on an aircraft carrier.

And that got me thinking...

In case of emergency, is it possible (doable) to land a small propeller-driven aircraft, like a Cessna 350 Corvalis on a military aircraft carrier?

A South Vietnamese Air Force Cessna O-1 landed on the USS Midway at the end of the Vietnam War. Fixed-wing VNAF aircraft usually flew to Thailand, and the naval evacuation was carried out by helicopter (with crews sometimes ditching in the water after the flight decks ran out of room), but this O-1 pilot wound up making his first carrier landing, with his family on board and an hour of fuel left.

Wikipedia wrote a paragraph about it and has another couple photos.


Do military pilots use amphetamines or other stimulants? Related to this question: Do military pilots have flight & rest hours regulation?

According to the wikipedia article, amphetamines were commonly given to Allied pilots.

Are they still in use today?

The U.S. Air Force (and Navy) still uses, and has used off and on since WWII,  what are commonly referred to as "Go Pills" for instances when pilots need to be up for long periods of time - in wartime, sometimes longer than the crew rest regulation. (These pills can also be used for altering sleep cycles, along with "no-go" pills; for example, when changing from day to night).

The most common go pill is Dexedrine, which is an amphetamine. You can find some more information in this article.


Why is stabilator used in some small airplanes? What makes it so attractive to use a stabilator in some small airplanes like Piper Cherokee or Seminole? Its primary purpose is to allow the pilot to control a pitch movement with a less force (which I presume better serve big airplanes), and if so, wouldn't a horizontal stabilizer coupled with an elevator be a better choice for the manufacturers, especially given that a stabilator installed in small airplanes typically requires an antiservo tab to prevent overcontrol by the pilot?

Simplicity. Yes, a servo tab is helpful, but most control surfaces should include a trim tab, so this can double as a servo tab. Total part count should still be lower than that for a stabilizer-elevator combination. But there are other advantages: Easier adjustment of stick forces and little loss of stability with free-floating controls.

By hinging the stabilator (or full-flying tail) near the quarter point, its control forces can be kept small and tailored with the servo tab. Also, this helps to keep the hinge moment coefficient derivative over angle of attack $c_{r_\alpha}$ low. The ratio between it and the derivative over deflection angle $\eta$ determines the angle to which the control surface will float, and a small $c_{r_\alpha}$ means that the stability with a floating control surface will be close to the stability with a fixed surface.

Also, balancing masses are easier to place when the full surface moves.


How is fuel removed from a light aircraft if required for weight and balance? With small, light aircraft, weight is always a consideration. Say you had a four-place single engine aircraft that you wanted to take yourself and three of your (non-FAA-standard weight) friends, but also at the time you get to the aircraft, you discover the tanks have been topped up.

Is there a standard way (or best practice) for removing fuel from the tank so you could carry all your intended passengers (provided you don't actually need all the fuel to get to your destination) and stay within the gross weight limit.

Several ways.

First, determine if there really is a weight and balance discrepancy with a proper weight and balance calculation.  If the aircraft is overweight and out of CG, can you simply remove some baggage or other non essential cargo in order to return to the weight and balance envelope?  If so, then do this, perform another weight and balance calculation and if it is within the envelope for safe flight, problem solved.

If the airplane is within gross weight but outside of CG, can you move some payload around to another fuselage station e.g. A fwd baggage compartment, etc so the CG can be moved within limits?  If so, do that, then do another weight and balance calculation.  If you're within the envelope, you're good to go.

If you cannot lose or rearrange any other non essential baggage in the cabin and you're still overweight, first verify that removing said amount of fuel will 1) keep the aircraft within weight and balance limits and 2) will be enough to safely complete the flight with the required onboard reserve, you have two basic option, drain it out, siphon it out or burn it off.

Before beginning this exercise, make sure you have an accurate means to gauge just how much fuel is in the tanks currently.  The tanks are generally placarded with their useable capacities or this will be listed in the Pilot's Operating Handbook for the aircraft in question.  Sometimes takes also have pre-marked indicators inside the tanks where a known quantity of fuel will level with these markings or metal tabs.  Some firms also sell gauges for determining fuel quantity in the tanks; just make sure you have one that's approved by the manufacturer for your make and model airplane.  DO NOT use the cockpit fuel gauges in a light GA aircraft to determine onboard fuel quantity; by regulations these are only required to be accurate when they read as empty (how convienent).

You could drain fuel from the tanks through the sumps, generally located under the wings.  A tool with a rod would be required to open the valve on the sumps and you could use a tube with a funnel on it to collect the fuel into a container.

Siphoning the gas may be easier using a tube through the gravity filler ports on the top of the wings.  One downside is that these filler ports are often on the highest part of the tanks and may require a long hose to siphon the gas.

The third option is simply to burn off the gas by making a flight prior to the arrival of the passengers for the planned trip.  Using the takeoff, climb, cruise and power settings charts in the Pilot's Operating Handbook, you can make an accurate estimation as to how long it will take and what power settings to use to consume the excess fuel prior to loading for the trip.


Why doesn't the plane accelerate/move forward when it's at rest, if the propeller is spinning? I'm having difficulty seeing the difference between a stationary plane and a moving one.

When you start the engine in the C172, the blades on the prop are spinning - but the plane doesn't move at all.
When taxiing out, it feels like the forward motion is being provided by the wheels - like in a car.
When taking off, you can audibly hear the change with the throttle increases.
In cruise, the plane almost looks the same from the inside as when it's on the ground (i.e. prop spinning). Which is where the weird psychological block is for me - the plane feels stationary even though the horizon/backdrop is moving towards you, and i get that fear kick in that it could drop because there's not enough momentum.

I'm trying to understand what the propeller is doing in each of these stages: rest, taxi, take-off, and cruise - as well as how the plane is pushed forward with thrust, if it can just sit there and not move with the prop spinning away.

My question(s): what's happening with the engine, throttle, and propeller that provides the thrust, and/or forward momentum? Does the plane stay stationary when you turn the engine on because there isn't enough RPM to draw it forward, or will it automatically pull forward and the brakes are just being held on? Does the plane accelerate in proportion to the prop RPM? What's different with the prop when stationary, compared to when it's taxiing or taking off?

When you start the engine in the c172, the blades on the prop are
  spinning - but the plane doesn't move at all.

When the propellor is idling its obviously moving air, this you can see, feel, and hear, however its simply not spinning fast enough to provide enough thrust to overcome the force of the plane sitting on the ground. Basically this is comprised of friction air resistance as well as some limited brake pressure that is usually applied. With fresh bearings, on level ground an idling prop may very well pull a plane forward. For the record it is a matter of certification that the brakes on an aircraft can hold it in place at full throttle. It also takes a bit more force to get a plane rolling than it does to keep it rolling. You need to break static friction to get rolling but then only provide enough power to overcome kinetic friction.

Generally speaking static friction is higher than kinetic friction,

(source)

  When taxiing out, it feels like the forward motion is being provided
  by the wheels - like in a car.

The first time you are in small plane you associate it with a car. You
equate that feeling with a car and thus you think intuitively that the tires are pushing you. In a car you could not be able to tell the difference between a front wheel drive and real wheel drive setup the idea is the same here.

  When taking off, you can audibly hear the change with the throttle
  increases.

I don't think there is a question here.

  In cruise, the plane almost looks the same from the inside
  as when it's on the ground (i.e. prop spinning). Which is where the
  weird psychological block is for me - the plane feels stationary even
  though the horizon/backdrop is moving towards you, and i get that fear
  kick in that it could drop because there's not enough momentum.

The plane feels stationary because you are in unaccelerated straight and level flight. In other words the forces are balanced about the aircraft. You feel nothing because there is nothing to feel ("feeling" is really another word for acceleration). This is different than a car, our road system lends its self to speeding up/slowing down often and unless you are on a very straight highway at night when its empty its less likely you are in unaccelerated movement in a car. The twits and turns of the road and the constant passing and slowing down of cars leads to a "feeling" of movement.


What requirements are there to fly into KDCA? In the A/FD for KDCA, it says:

  NOTE: See Special Notices — District of Columbia Ronald Reagan Washington National Airport.

I know that there are special restrictions for flying into DCA, but where is this "Special Notice" that they are referring to, and how can I get permission to fly into DCA under Part 91?

The "special notices" are a section of the A/FD, but those notices paint a very incomplete picture. Both the FAA and the TSA have requirements regarding DCA. The very basics are that you must get authorization from both the FAA and TSA 24 hours in advance, you must have an armed security officer, and you must depart from a gateway airport.
The details are, well...
FAA Requirements

Regulations require an arrival/departure slot to be reserved via the e-CVRS system during most hours, just like other airports designated as high-density airports.
Flights to DCA must originate from within a 1250 mile perimeter (with limited exceptions).

TSA requirements
The TSA requirements are referred to as the DCA Access Standard Security Program (DASSP). An overview is given on this tsa.gov page (scroll down to "DCA Access Standard Security Program").
Details can be found in the Interim Final Rule... because calling the ruling both interim and final won't confuse anyone. Here is a summarization of that document:
Section A. Aircraft Operator Requirements:

Security Coordinators The aircraft operator (company) must have a designated security coordinator who is the point of contact for the TSA and is responsible for implementing the DASSP requirements. This person must undergo a TSA security threat assessment, and a fingerprint-based criminal history records check (CHRC) by the FBI.

DCA Access Standard Security Program (DASSP) TSA will verify the operator's status with the FAA before sending the DASSP document which contains "sensitive security information" which must be implemented, and the operator must notify TSA when all requirements have been implemented.

Flightcrew Members Each flight crew member (does not include flight attendants) must undergo a fingerprint-based CHRC and an FAA record check. Almost any prior restricted airspace violation is an immediate disqualifier.

Flight Approvals Each flight into or out of DCA requires a tentative slot reservation from the FAA (see FAA reqirement #1 above) before applying for TSA authorization. The TSA application must be submitted at least 24 hours prior to aircraft departure. If TSA approves the application, they will transmit this to the FAA which will issue the final slot reservation, and TSA will notify the operator. The TSA only approves up to 48 applications for a given date. The application must include (once per round-trip):

The name, address, and citizen status of each passenger and crewmember (includes flight attendants).
The registration number of the aircraft.
The flight plan.
and... "any other information required by TSA."

Operating Requirements Each flight must:

be operated under instrument flight rules.
not make any intermediate stops between the designated gateway airport and DCA.
have the aircraft, crew, passengers, and property inspected by TSA prior to departure.
have the cockpit door closed and locked for the duration of the flight (if equipped).
have at least one armed security officer who meets the armed security officer requirements.

Costs TSA charges \$15 for each crew member and passenger to conduct the security threat assessment. Additionally, they charge for the screening services at the airport which are "currently estimated at \$296 per round trip into and out of DCA."

Protection of Sensitive Security Infonnation The specific security details described in the DASSP must only be shared on a need-to-know basis.

Other Security Procedures This paragraph basically says TSA can make you do whatever they want... as if that wasn't obvious by this point in the list.

Compliance TSA can perform any inspection or test it wants to verify the operators compliance with DASSP.

Section B. Fixed Base Operator Requirements
That's right, there are two section B's, one right after the other... in case you weren't already convinced the TSA has strong attention to detail.
This section just says there are FBO requirements for gateway airports. Here is a list of Gateway Airports dated March 21, 2013. The list is always subject to change and seems to have more to do with whether there is an FBO at the airport who wants to put up the DASSP more than anything else.
Section C. Armed Security Officer Requirements
Armed security officers must be an active or retired law enforcement officer, or an individual approved by the TSA. This requires a similar background check as the security coordinator and law enforcement training which TSA finds acceptable. For more information, see the Armed Security Officer (ASO) Program.
Flying under Part 91?
At first glance it doesn't seem like there is any rule explicitly prohibiting Part 91 operations in 49 CFR Part 1544, and §1544.1 Applicability of this part. says:

(a) This part prescribes aviation security rules governing the following:
(1) The operations of aircraft operators holding operating certificates under 14 CFR part 119 for scheduled passenger operations, public charter passenger operations, private charter passenger operations; the operations of aircraft operators holding operating certificates under 14 CFR part 119 operating aircraft with a maximum certificated takeoff weight of 12,500 pounds or more; and other aircraft operators adopting and obtaining approval of an aircraft operator security program.

The general definition of an aircraft operator is not limited to an operator with a Part 119 operating certificate. However, the possibility of using part 91 seems to be ruled out by §1544.101 Adoption and implementation. where it enumerates the scenarios TSA is willing to approve, and there are no Part 91 operations in the list.
There are probably a few reasons why Part 91 is not allowed. Most notably:

TSA has specific aircraft operator requirements which an individual operator is unlikely to be able to comply with, and
There are only 48 slots available per day, and therefore preference is given to commercial operators - the explicitly given explanation for why non-airline flights are permitted at all is to permit "operators, their customers, and affected local businesses to recover from the adverse economic impacts brought on by the events of September 11, 2001."

Other fun quotes:

TSA may, at its discretion, require any flight into or out of DCA under this subpart to comply with additional security measures.

and...

In addition, if TSA or the FAMS requires that the aircraft have one or more Federal Air Marshals (FAMs) on board, the aircraft operator must allow the FAM(s) onboard, at no cost to the Federal Government.

and...

TSA may, at its discretion, cancel any or all flight approvals at any time without prior notice to the aircraft operator. For example, if the threat level in the Washington, D.C., area or in the vicinity of any of the gateway airports is set at ORANGE or RED, TSA is likely to cancel any and all flight approvals into and out of DCA.

So, happy flying everyone!


Does the Piper PA-28 Cherokee have a limitation on its maximum lifetime flight hours? Is there a lifetime limit on the (airframe) flying hours of an aircraft (specifically the Piper Cherokee)?

Short answer, not in the United States; as long as the airframe is regularly maintained and passes its mechanical inspections, it can continue to fly. Some reasons this is so (when it might not be true for other models) include:

The Cherokee is non-pressurized. Pressurized aircraft are often rated for a lifetime limit of pressurization cycles (determined by the number of times the aircraft rises above and then descends below the altitude at which the pressurization system kicks in, typically 8000ft MSL for many small craft). This pressurization stresses the airframe in ways that eventually more or less requires the entire fuselage to be replaced in order for the airframe to pass its next mechanical as originally rated. The Cherokee, however, has no system for full-cabin pressurization and so its fuselage is not subjected to these stresses.
The Cherokee (180 or 235) is not rated for aerobatic maneuvers, and so does not typically perform them. Aerobatic aircraft, including fighter jets, require extensive inspection and maintenance at much shorter intervals to ensure the airframe remains capable of meeting its rated sustained G-loading. Coupled with regular testing of those ratings by the pilot, aerobatic aircraft are more quickly retired from service when it becomes impractical to repair or replace overstressed structural members.
The Cherokee is not commonly used in Part 121 or 135 operations involving passengers or air freight. As a four-seat single-engine, its capacity for passengers and cargo is limited, and so its usefulness to major airlines which are subject to Class D(econstruction) inspections every 6 years is sharply limited. Bush airlines like Era Alaska might see some use as more or less an air taxi, but even these outfits prefer somewhat larger planes for more flexibility and longer range.


Does runway center lighting increase runway lifetime? I wonder if the centerline lighting helps add a few more years to the runway, especially with heavy aircraft like A350/A380, in addition of being helpful for landing and takeoff?

My question is based on a recent reading where this question was asked:

  Why did the airport operator BIAL not plan for centre line lighting
  at the time of runway construction?

Update - From what little I garnered, understood and logic applied, it seems/feels that a centre line lightning would make when the airplane lands with more precision than otherwise. This would mean that the aircraft doesn't go all over the place and while some specific parts of the runaway would be more stressed, others wouldn't. I know there are a mixture of narrow and wide-body aircraft but still it should make life of the runaway, at least on the edges more. Also maintenance of the runaway should be a tad easier as perhaps only some specific parts of the runaway would need more maintenance . I may be totally off in my assessment hence asking here.

Update 2 - From the comments then, what is the centreline lightning useful for ? Also if there any pictures which explains it as well would be useful. I know there are lights on left and right of the aircraft for it to know that there is a runaway here. What does centreline lightning bring to the table ?

No, runway centreline lights do not increase runway lifetime, and it is not their purpose to do so.


What was the flight route from New York to Paris that Air France took in 1955? I have a photograph of me and my sister in 1955 or 1956 boarding Air France to Paris. I seem to remember the route was:

New York - Gander
Gander - Shannon
Shannon - Orly

Is that correct? I know the return flight (westbound) was that route. Does anybody know how long it took?

The route was IdleWild (renamed JFK in 1963) or LaGuardia -> Gander, Canada -> Shannon, Ireland -> Paris. The trip took a total of about 18 hours going eastbound with tailwinds, and 24 hours going westbound with headwinds.

Gander was opened in 1938 and continues to be the preferred refuelling stop for aircraft that can't make the hop across the north Atlantic in one go, simply because it is the northeasternmost point on the North American continent where such a facility could be built. For a while it was the largest airport in the world, and was used extensively by the military during WW2 as a stopover point for aircraft being ferried to Europe.

Air France used the route from the beginning of their transatlantic service in 1946 until the late 1950's, when the jet age made it possible to fly the route non-stop.


Why do jet planes with multiple engines have separate throttle levers for each engine? What are the reasons to not just use one throttle lever for all engines?

All multi-engine airplanes, whether jet or prop driven have separate throttle controls for each engine. There are good reasons for this:

If there is a problem with one engine you can separately control it for troubleshooting purposes
If one engine isn't developing the right amount of power you can independently increase the other engine(s) to compensate
If you have a control failure of some kind you can use differential thrust to steer the airplane, this has been done more than once, the example that comes immediately to mind is United 232


Unusual Traffic Ene 68 degrees over Dorset United Kingdom - No strobes, green or red just solid white lights I have spotted a line of traffic 1 minute apart, at least 12 objects, with 3 objects to the left and right of the main line of traffic.  Clear night, no pollution, no noise, whit solid lights only.  I know what satellites look like in the night sky and that is exactly what this looked like in terms of height and speed but there was a stream of them equally distanced apart with 3 random ones to either side of the line of objects.  There was no strobe, the white light of each was showing form the front, side and rear. No green, no red.

I have never seen anything like this, nor have I ever posted anything like this but please if you know what this was let me know.  Can re positioning aircraft because of covid-19 fly with no strobing or colour lights etc?  Is it military?

I filmed this tonight 29th March over southern UK at around 2120hrs.  Many thanks.

Satellites can look like this shortly after a serial deployment from a single launch vehicle.


Is it possible to achieve torque equilibrium in order to fly? I am trying to create a tricopter which will be used for SLAM, but my main problem is that before starting to construct it I am not sure whether or not it will work at all. The problem is that I need a sensor to be rotating on an axis vertical to those of the rotors. Moreover that part will not be on top or below the drone but rotating around its "chassis axis"(blue line on second ppicture), because the sensor must not have anything blocking its view. Is it possible to achieve it? If yes what is the optimum placement of the motors and the moving sensor? Since English is not my native language I understand that some things may not make sense please feel free to ask me anything you don't understand.

If I understand the problem correctly, there will be a camera attached around the perimeter of the copter, which must have an uninterrupted view.

If the camera just swings around a perimeter, there will be two problems:

Trim of the device. There are only three rotors. If the device is in a trimmed hover and the camera swings around, the device needs to be re-trimmed rapidly to prevent roll/pitch.
Reaction yaw. The device starts spinning the other way that the camera is swinging.

Best way is to give the camera a fixed place and make the whole device spin, via gimballing the main rotors or using two vertical propellers. A counter-weight to the camera will put the CoG in the centre.


Why use different airfoils on the same wing of an aircraft? Why different airfoils are used in different portions of the same wing of an aircraft, for example Boeing 737 wing?

Boeing Commercial Airplane Company model 737 airfoils' dat files from here:
http://m-selig.ae.illinois.edu/ads/coord_database.html

BOEING 737 ROOT AIRFOIL (b737a-il),
BOEING 737 MIDSPAN AIRFOIL (b737b-il),
BOEING 737 MIDSPAN AIRFOIL (b737c-il) and
BOEING 737 OUTBOARD AIRFOIL (b737d-il)

Because the wind tunnel has determined what the best airfoil is, at this particular span, over the whole operational range of flight:

Stall speed: docile pre-stall behaviour with outboard wing stalling last to retain roll control.
Landing speed with flaps & slats out: high stall angle, high $C_L$
Climb speed: low drag in high lift conditions.
Cruise speed in clean configuration: low drag, low pitching moment, high drag/lift divergence speeds.

The design goal is the performance of the total wing, wing profile is one of the parameters that can be used to optimise performance, minimise drag, provide good structural characteristics, and provide a large volume for the on-board fuel.

For the wing root, the aerofoil must have a good cantilever ratio (thickness) for torsional and bending stiffness.
For the wing inboard section with flaps extended, aerofoil drag must be low in high lift conditions.
For the wing tip, the aerofoil must have a high maximum $C_L$ and gradual stall characteristics)
For all wing sections, the aerofoil must have a sufficiently high critical Mach number.
For all wing sections, pitching moment coefficient should be low to moderate, to avoid high trim drag, and high torsion at high dynamic pressure.
For all wing sections, aerofoils must not be extremely sensitive to manufacturing tolerances, dirt/bug build-up etc.

So different bits of wing have different design characteristics. The whole wing is a gradually blending unit of different aerofoils.