Are there disadvantages of coaxial rotor on attack helicopters? My question stems from the fact that, although this is a very fierce looking, and formidable aircraft with a very powerful engine, Ka-52 Alligator has not seen many export success. Its export is very limited as only Egypt uses this type of helicopters as a foreign customer.

Are there disadvantages of coaxial rotor on attack helicopters?

The unique configuration of twin coaxial rotors presents both advantages and disadvantages, and the relative weight of each must be considered in light of the aircraft's mission.

I am going to assume the reader has a basic understanding of helicopter controls (cyclic, collective, antitorque).

Disadvantages

Aeromechanical complexity

A coaxial rotor has a much more complex hub than a regular one. It does not just need to invert the rotation direction, it has two sets of cyclic and collective controls that also mimic the effect of a tail rotor.

Since the lower rotor operates in the down-wash of the upper one, its collective pitch needs to be higher to produce the same lift (and thus torque). This is important because a torque imbalance will make the aircraft yaw uncommanded. In fact, this is how yaw is achieved when the pilot steps on the pedals, by letting the rotors generate differential lift, but ensuring their combined total stays the same so the aircraft retains level flight.

The cyclic controls suffer the same effects, that is the least of their problems, see below.

Rotor self-intersection

A single rotor in forward flight will assume a slightly conical shape due to the lifting force on the blades. This cone will also be tilted sideways with respect to the direction of flight because the blades see different airspeeds at different points in their cycle. That is to say, the blades will generate more lift while they advance into the direction of flight, and less while they retreat on the opposite side of the helicopter. The higher the forward speed, the larger will this effect be (In fact, there will usually be an area on the rotor where the blades do not produce lift because the airflow over them is reversed: the forward speed of the helicopter is greater than the local speed of the blade).

In a coaxial design, both rotors turn in opposite directions, so their tilts will be in opposite directions too: the clockwise-turning one will tilt right, and the anticlockwise-turning one will tilt left (the actual locations of maximum bending are not quite as clear cut because of blade inertia, but this serves as a basic explanation).

The resultant problem is pretty obvious: the two rotors can intersect each other. This is a Bad Thing(TM) and will lead to all sorts of issues like rapid unplanned dissasembly of the rest of the rotor blades. A pilot in this situation will most likely make use of the conveniently installed ejection mechanism.

The actual forward speed at which self-intersection will occur is not published (to the best of my knowledge), but is probably somewhere above 250km/h. This is only taking forward flight into account... sudden collective or cyclic inputs at high but not critical airpeeds may have the same effect.

The overall effect is that coaxial rotor helicopters are more limited in their flight envelopes than conventional ones. Although this is not a fatal drawback,  agility is highly valued for attack helicopters.

Yaw Response

This is somewhat speculative, but the lack of a dedicated tail rotor may limit the yaw rate at low forward airspeeds, where most of the yawing toque would come from the tail rotor.
At high speeds the weathervaning effect of the tail becomes dominant anyway, making this a moot point.

Advantages

Simpler tail design

The lack of a tail rotor frees the helicopter from having to transmit torque to it, thus heavily simplifying the boom design. In fact, some coaxial designs make a particular point of exploiting this advantage, having very short boom, like the Ka-25 and Ka-27, which are navy helicopters meant to fit into the tight confines of a shipboard hangar.

Less weakpoints

The loss of a tail rotor is not, like many movies would suggest, an unrecoverable situation. A helicopter can be stabilised with sufficient forward speed if it has enough tail volume.

That being said, losing a tail rotor in a combat situation will most likely result in a mission kill. And it's not just the rotor that's vulnerable, the boom contains the transmission rods for it, so any damage along that structure can be potentially fatal.

For a coaxial helicopter,the loss of the tail will lead to some stability issues, but nothing catastrophic.

Whether the advantages of the design outweigh its drawbacks is ultimately a matter of preference and of the fit between the aircraft and its intended mission. The adoption rate of a design is not necessarily an indicative of its merits, especially when military procurement is involved, as there are countless other factors at play.


Is the F-16 capable of taking off during a nerve gas attack? Imagine a scenario where air bases are attacked with (or indirectly affected by) a nerve agent during a major war. There is also a desperate need to scramble combat aircraft. Can modern combat aircraft, such as the F-16 still take off during such circumstances? What about other typical modern combat aircraft (F-15, F/A-18, F-22, F-35)?

While higher up in the air, I imagine the nerve agent should pose no threat to the pilot. However, it appears that even just suspecting there is nerve agents in the air on the runway should be life threatening to the pilot. Wouldn't the life support system ingest the nerve agent and present it to the pilot through the same system?

I haven't previously read about and couldn't find anything about chemical warfare filters equipped on the F-16's life support system. Does it have any?

However, I glanced at this info card from Cobham on F-16 Support Equipment:
On-Board Oxygen Generating System.

It seems that the F-16 equipped with OBOGS has an on-board emergency oxygen supply. From various forum posts it also seems like the older block C F-16 has emergency oxygen supply. It seems reasonable that all combat aircraft have some sort of emergency oxygen supply, for obvious reasons. Correct me if I'm wrong.

Perhaps naively, it seems like enabling the emergency oxygen supply while taxiing and takeoff for the F-16 would allow it to still get in the air? Is there enough time to do this? Is the emergency oxygen supply stored pre-pressurized or does it require to become pressurized by the running engine? In that case, I suppose this is a futile effort. This is all of course, assuming a rather unlikely scenario where the hangar for the aircraft is assumed safe enough to even start up the aircraft in the first place.

What if the hangar is also unsafe? Can anything be done for the pilot and crew, beyond this? I suppose the crew could wear hazmat suits, but again, what about the pilot? For some aircraft, such as the SR-71, I believe pilots wear pressure suits before getting into the aircraft. However, then I assume the aircraft is then required to support this type of equipment, which I doubt the F-16 does.

Does any nearby nerve agent attack effectively ground all aircraft? It seems like something there should be a solution for.

Source: Pilots Don MOPP Protective Gear - 25th Fighter Squadron, YouTube

If the base is equipped, then yes. The ground/flight personnel don the specialized suits for chemical/biological warfare.

And according to thedrive.com, a suit is also available for the F-35, though it took a decade to design. The delay was due to the F-35's complicated "enhanced man-machine interface".


With positive passenger/bag matching rules, what happens to a bag that does not arrive with the passenger? According to one answer to Why offload bags for missing passengers when flights are allowed to carry unaccompanied non-PAX air-cargo in holds?, the requirement for Positive Passenger/Bag Matching was introduced in 1988 following Pan Am 103. According to another answer to the same question, airlines cannot accept cargo from the general public.

Yet on a number of occasions when I've flown commercially as a passenger (including once to the US in the summer of 2001), for one reason or another my/our bag(s) have ended up somewhere else, or at the very least, not showed up on the conveyor belt at the destination airport along with the other passengers' bags.

What usually happens at that point is a conversation with an airline representative, and possibly one's card issuer or other travel insurance provider; and the bag arrives a day or two later. For the passenger, that's an inconvenience, but thankfully not usually a disaster.

However, what happens to the bag in the interim in such a situation?

Clearly, the bag very likely wasn't on the same plane as the passenger. This could mean one of two things:

The bag was not loaded onto any airplane (so it's still at the origin airport).
The bag was loaded onto some other airplane (so it's somewhere else).

The case of the bag not being loaded onto an airplane is strange, but doesn't seem to violate the requirements of PPBM; it's just sitting in a cargo holding area somewhere. If the bag was actually loaded onto some other airplane, that would violate PPBM, since there's no passenger on that flight accompanying the bag. (That said, I don't know if it would be possible to exploit this as a security weakness. Probably not, at least not reliably.)

Once the bag is found, it needs to get to where the passenger is now. I can see two basic ways to do that:

The bag is moved by ground vehicle (car, truck, boat, ...).
The bag is moved by airplane.

Driving a potentially long distance potentially just to get a single bag from one location to another seems the less reasonable alternative (it could probably easily cost a significant fraction of the profit from that whole flight), but on the other hand, if the bag is loaded onto an airplane, it definitely is a bag travelling without an accompanying passenger and in this case, it's known as such to the airline, thereby seemingly violating the requirements of PPBM.

If it were to be shipped as cargo, that would presumably qualify as "from the general public".

This seems to me like a "damned if you do, damned if you don't" kind of situation. So what really does happen to the bag, from when passenger and bag are separated, until the two are reunited?

For simplicity's sake, let's assume that what's inside the bag is completely innocous.

This is more a guess:

Think about the rationale behind the rule. A terrorist should not be able to send a bomb on a flight while staying at home. With this rule, a terrorist is forced to sacrifice himself by taking the same flight. Hopefully, most terrorists don't like this idea.

If the luggage wasn't put on board, or was put on another flight, the terrorist will notice this at the destination, and not before. Same effect, he had to be willing to sacrifice himself by boarding the aircraft.

So, the rule is not to never let luggage fly alone, it is about not letting it fly when the owner for some reason does  not take his flight.

If the luggage took another flight, it was for sure an error. The barcode scanner mis-read the barcode, the ID of the trays transporting the luggage was not read correctly, a computer error or whatever. Should not happen, but does from time to time.
If the luggage did not fly at all, it might simply have missed the flight due to technical / logistics problems. Or the barcode tag was ripped off etc.


Is an afterburner louder than the same jet engine without it? I would speculate that an afterburner increases the noise because it is nearer to the exhaust. But it certainly causes so many changes in the exhaust flow that it could be less loud as well. It may also depend on the shape of the nozzle.

That an afterburner increases the spacial volume of exhaust gases does not necessarily mean that the sound volume increases as well, because it strongly depends on the level of turbulence.

Does activating an afterburner make a jet engine louder? And why?

This PDF indicates an increase by ~10 dB for an F-8K in afterburner versus the same aircraft in 100% dry thrust.

This PDF indicates smaller increases:

+5 dB for an F-15
+4 dB for F-22 and F-35


What is the lowest-speed bogey a jet fighter can intercept/escort? Jet fighters are made to cruise at a high subsonic speed, even at a supersonic speed (supercruise?). It means that they can be flown comfortably at relatively high speeds.

Jet fighters are often used to intercept aircraft, whatever the reason (assistance, escort from national to international airspace, check an unresponsive aircraft,...).

General aviation aircraft can cruise at a slow speed compared to jet fighter. Moreover, if needing assistance, it may fly even slower. I imagine it is possible that a GA aircraft needing assistance cannot fly at a speed above the stall speed of a jet fighter. I know that some countries dispatch choppers to intercept slow aircraft to handle such situation.

I imagine the military doesn't fly their jets at stall speeds when close to another aircraft (a margin is needed).

Is there a speed below which a jet fighter cannot intercept another aircraft? Otherwise, how do they do it? (I imagine you can orbit above another aircraft but it is less convenient to guide it safely to an airport)

There is nothing stopping a faster jet from flying a zigzag pattern behind an intercept target if the goal is to remain in close contact with it, but a large difference in airspeeds would indeed hamper some tasks like observing the cabin for signs of life or unlawful interference. So the answer is yes, a jet fighter cannot perform all the tasks involved in a typical interception of an unresponsive aircraft if the speed delta is too high.

For reference, the takeoff speeds of 4.5 generation fighters like the F-16 are in the 130 kt (240 km/h) region, while the takeoff speed of something like a C172 is around 60 kt (111 km/h). The exact weapon and fuel load of the fighter will of course have an impact, but fighters scrambled for an intercept are typically light and with an almost clean configuration. I have deliberately used takeoff speeds instead of landing speeds to factor in a margin of safety in regards to control authority; both speeds are among the lowest an aircraft can sustain, but during  close approaches to a potentially hostile aircraft, some maneuverability is required.

Still, 100+ km/h is a substantial speed gap, and has led to issues in the past:

  The Po-2 is also the only biplane credited with a documented jet-kill, as one Lockheed F-94 Starfire was lost while slowing down to 161 km/h (100 mph) – below its stall speed – during an intercept in order to engage the low flying Po-2.


Does the RA (resolution Advisory) from TCAS II take into account the stall margin required? A resolution advisory from TCAS indicates the vertical speed per minute that you must use to avoid conflict with the other traffic. Does this vertical speed given by the RA take into account the stall requirement for the aircraft?

Yes, but not directly. The TCAS is a generic box that doesn't really know anything about the aircraft performance. What it does have is a number of input discretes that allow other aircraft systems to indicate that the aircraft climb performance is limited. Other discretes code the altitude above which the aircraft cannot perform an RA climb. What drives these discretes will be unique to a specific aircraft type and defined during the STC process.

ARINC 735A TCAS Characteristic defines the following:

  Climb Inhibit Discrete Input #1 through #4
  Increased Climb Inhibit Discrete Input #1 through #4
  Performance Limit Discrete Input
  Aircraft Altitude Limit Program Pins

Based on these inputs the TCAS logic will compute an RA that does not require the aircraft to climb when it is not able to safely do so.


How can I recognize Taylorcraft BCS-12D-4-85? I'm perfecting my logbook and can't name the subtype of the aircraft I've flown. Some directories says it was built in 1946 and is a BCS-12D on floats. Others say it is a BCS-12D-4-85 which according to Wikipedia was built in 1949.

Is there a way to know which aircraft I was training in?

That airplane is registered as a 1946 BC12-D per the FAA's N number lookup service.  This indicates that it came from the factory as a BC12-D but then may have been converted later.  Conversions don't change the subtype.

As far as recognizing specific subtypes of old airplanes, good luck!  Many old, popular planes have undergone all kinds of conversions.  Bonanzas built as early as the 1940s can be converted to look like a late model V35B from 1981!  There are even mods to remove the V tail and replace it with a "normal" tail.  My point is that determining subtype from appearances alone may be impossible.

Link below...

https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=N43459


What are the regulations regarding plane spotting in the US? I was told that certain plane spotting activities like taking photos of aircraft taking off and landing could be illegal in the US since 9/11? What are the regulations on this, and what should I be aware of before I set my camera up at the end of the runway?

Generally absent a specific prohibition you can take a photo of anything you can see with your eyes from public property (like a sidewalk). That includes airports, airplanes, bridges, tunnels, police officers, kittens, and unicorns.
(For more in-depth information check out Andrew Kantor's excellent article and Bert Krages' excellent one-page flyer on photographers' rights, or check out http://photography.stackexchange.com - they've covered airport photography, though not in any real depth, and I'm sure they have some street photographers well versed in the legal aspects of taking pictures.)

So what constitutes a "specific prohibition" on photography? Generally a clearly visible sign that says PHOTOGRAPHY PROHIBITED (the old Grumman facility in Bethpage had such signs on all of the fences where there was a decent view of the runway).

Absent such a clear prohibition you're free to take all the photos you want as long as you're on public property while you're doing it (i.e. "not trespassing on airport property"), but be aware that some law enforcement officials may not see it that way, and you may have to have some uncomfortable interactions with them.
In my experience most police officers are well versed in the law, though occasionally you'll find one who isn't. You're more likely to have trouble with a "rent-a-cop" security guard though.

If you want really good photographs (and you're not photographing at a major airport like JFK or O'Hare) you might want to speak to the airport management. At smaller airports someone - either airport personnel or a local pilot - might be able and willing to escort you airside where you can get some really great shots.
It helps in this situation if you have a body of work you can show (either published photos or art prints). The caveat is that many airports will also ask that you carry some kind of insurance while you're photographing on their property (and at larger airline fields that requirement can be substantial).


Is there a regulation that defines when to use QFE or QNH for landing? Is there an EASA regulation that says to use QFE or QNH when landing?

AFAIK there is no EASA regulations forcing a pilot to use one or the other. The default option is to use QNH, which shall be transmitted to all aircraft:

(3) Except  when  it  is  known  that  the  aircraft  has  already  received  the  information  in  a directed transmission, an QNH altimeter setting shall be included in:
(i) the descent clearance, when first cleared to an altitude below the transition level;
(ii) the approach clearance or the clearance to enter the traffic circuit;
(iii) the taxi clearance for departing aircraft.

But there is an option to use QFE as well, and it is commonly used in some places, for example for certain operations in the UK and sometimes for military operations.

(4) A QFE altimeter setting shall be provided to aircraft on request or on a regular basis in accordance with local arrangements.

The "in accordance with local arrangements" part also incidates that EASA has left it up to the individual local authorities to decide whether or not to use QFE - so it is unlikely that EASA has a common rule for this.
Both quotes are from SERA.8015


Why did Lockheed Martin give the F-22 stealth fighter such small fuel tanks? Why did Lockheed Martin give the F-22 stealth fighter such small fuel tanks while the F-35 got much more fuel capacity relative to its size and single jet engine?

The F-22 was developed in the 90's, primarily for use by the United States Air Force. This means it was designed to serve specific mission profiles. The F-35 however is going to replace the F-22 and serve all the air branches of the US Military. This means that it needed a broader mission profile. The last F-22 delivery was in 2016.

For the F-22 Raptor:

  Range: >1,600 nmi (1,841 mi, 2,963 km) with 2 external fuel tanks
  Ferry range: 1,740 nmi (2,000 mi, 3,220 km)
  Combat radius:
     460 nmi (529 mi, 851 km) clean with 100 nmi (115 mi, 185 km) in supercruise
     590 nmi (679 mi, 1,093 km) clean subsonic
  Service ceiling: 65,000 ft (20,000 m)

And for the F-35 Lightning II:

  Range: 1,500 nmi (1,700 mi, 2,800 km) +
  Combat range:
     669 nmi (770 mi, 1,239 km) - Combat radius
     760 nmi (870 mi; 1,410 km) - Combat radius (interdiction mission on internal fuel, for internal air to air configuration)
  Service ceiling: 50,000 ft (15,000 m) +

You can see that the F-22 range (not the combat radius) is very similar to the F-35. The problem with the F-35 is that because it is available in VTOL configuration, it burns a lot more fuel. The VTOL F-35 (F-35B) only has a mission radius of 505nmi. This is because during VTOL operations the F-35 burns an incredible amount of fuel. So much so that a VTOL take-off usually requires mid-air refueling shortly after take-off.

Either way this boils down to a trade-off between how much gas to carry and how much weapons and other systems to stuff into the aircraft. The VTOL versions need much more complicated internal systems that take up a lot of space, so the range and payload suffer. Tanks can be mounted on external hard-points, but then radar cross-section becomes much bigger and maneuverability is decreased.

With mid-air refueling all the aircraft must really do is be able to go from a friendly area where the tanker is, complete the mission, and get back to the tanker, so range isn't as important as versatility.


How much radiation is an airline pilot exposed to in a year? Flying exposes us to more radiation since we are closer to the sun, and there is less atmosphere to protect us.

Consider an airline pilot flying on a routine and frequent basis. How much radiation is absorbed per flight hour, and per year?

The average additional annual dose for flight crew is 2.19 mSv (milli sieverts).  http://www.hps.org/publicinformation/ate/faqs/commercialflights.html

The typical annual dose from background sources is 4 mSv (this varies enormously on the Earth's surface). However, as a rough order of thumb, the annual radiation dose of a flight crew member increases by about 50% compared to sitting on the ground.

For comparison with other sources, having a chest CT scan provides you with a 7 mSv dose over the period of a few minutes.

The maximum permitted dose in one year for a radiation worker under US Federal Law is 50 mSv.

For more comparisons of radiation dose, see https://xkcd.com/radiation/

On a technical point, the premise of your question is incorrect.

Flying exposes us to additional cosmic background radiation because we are at a greater height above the Earth's surface, not because we are closer to the sun. We are protected from cosmic radiation by the presence of the Earth's magnetic field, which extends far into space and is not an issue here except very close to the poles; and more importantly, by the layer of atmosphere above our heads that tends to absorb cosmic rays before they reach the surface. At 40,000 feet, the bulk of the atmosphere by mass is below you.

It is not presently believed that a substantial fraction of these rays are emitted by the sun. For one, you will continue to be irradiated by them at night.

(For interest, if you could see the cosmic background radiation, it would be approximately as bright on the night sky as star light is; for complex reasons the two energies are in rough equipartition.)


Why do we not see propellers on swept wings or jet engines on straight wings? Why are these configurations not observed often?

The only time I see propellers are on straight wings or on the nose, while jet engines are always on swept wings or the tail. Why?

You can find propellers on swept wing aircraft, such as the TU-95, or more recently, the Airbus A400M.

One turbofan powered aircraft that has straight wings is the Cessna Citation

No, it's not common, due largely to the speed at which the aircraft normally operates. Turboprops tend to operate at a lower speed than turbojets or turbofans, where the benefits of swept wings (better performance when approaching Mach 1) aren't evident, while the benefits of straight wings (better performance at lower speeds) comes in handy on the shorter airfields that smaller turboprops operate from. Bump the cruising speed up to Mach .8+, and the swept wing is the optimal design.

In the case of both the TU95 and A400M, they cruise at higher speeds than most turboprop aircraft, hence the employment of a  sweep to the wings.


How do I calculate this question? mass and balance If we start with $Total\ Weight = 2,850\ lbs$ and $CG = 101.67\ in$, then add $400\ lbs\ @\ 97.8 in$, then how much would the Center of Gravity move?

How can you calculate this?

The basic equation is $Weight \cdot Arm = Moment$, which also means that $Arm = \frac{Moment}{Weight}$.That is all you need to work through these problems.

Calculate your starting moment:
$2,850\ lbs \cdot 101.67\ in = 289,759.50\ lbs/in$

After you add 400 lbs, the new Moment is:$(2,850 \cdot 101.67) + (400 \cdot 97.8) = 328,879.50\ lbs/in$

And the new total weight is:$ 2,850 + 400 = 3,250\ lbs $

This produces a new CG of:
$\frac{328,879.50}{3,250} = 101.193\ in$

The difference in CG is:
$101.193 - 101.67 = -0.477\ in$

Or, doing this all in one big equation:
$$
\frac{(2850 \cdot 101.67) + (400 \cdot 97.8)}{(2850 + 400)} - 101.67\ in = -0.477\ in
$$


What would happen if a Russian TU-95 had an engine failure or other emergency close to the United States? Russians often fly the Tu-95 for long distances, in some cases reaching the California coast.

What would happen if they had an engine failure or another emergency Considering the aircraft's age, an emergency is not a very remote possibility.

Would the Tu-95 land in California?
Would it rather try to come back home on 3 engines?

Losing an engine on a 4-engined aircraft is not overly dramatic. In fact the P3 Orion actually intentionally shuts off one engine in flight to conserve fuel- it's more efficient to run three of them at higher power setting.

I doubt that they would put their aircraft in a position that they could not withstand an engine failure. From Kamchatka to California is it around 6000km max one-way; The TU-95 has a range of 15,000km.

As for potential political mess, you can see what happened when a Chinese J-11 Fighter collided mid-air with a US Navy P3 Orion back in 2001 in what is known as the Hainan Island Incident.

A British Airways 747 lost one engine out of Los Angeles a few years ago, and almost made it all the way to London, but diverted to Manchester (although that event did cause a lot of controversy).


Is there an advantage to combining trim function with yoke? SUMMARY: What if the yoke didn't require constant pressure, as is the case with the current "trim" function?
In all cases that I know of, airplanes have had the yoke (which you need to apply constant pressure on) and trim (which you don't) separate. The yoke is used to short inputs, such as a roll or a pitch down, and the trim is used to maintain the aircraft in a specific attitude, given the changing nature of center of lift and gravity.
But I do not understand why it is necessary for them to be separate. Trim can do (almost) everything a yoke can. Would it be a good idea to combine the two?
Some advantages:

simplification
Safety; fast reaction to sudden trim changes (such as part of wing or empennage breaking off)

Some drawbacks:

trim does not center itself, so there's that inconvenience
increase chance of wild maneuvers if left unattended while deflected

I think you misunderstand what the purpose of trim is, which is to change the attitude of static stability.
The yoke (or stick) is the primary flight control. Virtually all changes in attitude will originate here, and any temporary changes also end here. If you take your hand off, the plane returns to the original attitude.
When you want to make a permanent change, you set the new attitude using the yoke (or stick) and then trim until the required pressure goes away. When you take your hand off the yoke (or stick), the plane remains in the new attitude.
Some planes do have an elevator trim control on the yoke (or stick) to make things easier on the pilot, but this doesn’t change how it is used. Some planes also have rudder and aileron trim controls; I’ve never seen those on a yoke (or stick), but I’m sure someone, somewhere has tried it.


Why don't pilots control window shades & seat recliners? Every time before landing or take off, the cabin crews continuously remind passengers to pull up the window shades, straighten their seats and what not. My question is why can't pilots be given control of these things, at a flick of a button they can open / close all window shades and upright all seats. It's 2015 not 1980.

Adding all those actuators and associated wiring will add weight and cost to each flight, but the cabin crew have to be there for safety reasons, so they may as well have something to do.

Besides, how would you feel if your seat suddenly jolted you upright without warning?


What does it mean for N1/N2 to float? The logic of ECU to hold N1 with fuel flow follow the logic:

Idle N2------------N2 set ,N1 floats
min ps3 schedule -----------N1 and N2 float

So what does this mean? And does the float here with the same meaning of the float display on the UPPER ECAM?

Float just means that that value will vary depending on ambient temperature pressure etc. It is not controlled. E.g. on n2 control, if n2 goes down then the control puts in more fuel and if n2 goes up then the control puts in less. But if n1 changes (maybe because ambient temp changed ) the control does nothing. If n1 goes down then so be it.


What distinguishes the Airbus-Sidestick from an ordinary gaming joystick? I don't think the Sidestick in Airbus aircraft is an ordinary gaming joystick that is plugged via USB into the Airbus Computers. I guess it is something very special, because it's one of the most important and critical parts of the aircraft.

What are the differences between an ordinary gaming joystick and an Airbus Sidestick?

First of all, I'd like to make clear that I highly respect Airbus cockpit design and implementation of envelope protection.

Now to the question. The Airbus side stick is a lot stronger than a game joystick and very reliable, but is functionality-wise the same as a gaming joystick, just a stick loaded by a passive spring force sending back position signals. It had to be that way in the 90s when the sticks were designed, although we were already running digital force loops in real time @ 5000 Hz on a Motorola 68020, and had the technology to couple two sticks in software. Did it for Mercedes and for quite a few simulators, but sadly not for Airbus.

So how it could for instance be implemented for the A320 side stick: program the passive forces that are there now for normal operation but always couple the sticks, program a stick pusher for when in alternate law and stalling, and leave the current override button solution in for when the Control Loading System fails. We were part of an aircraft company then and making the system airworthy would not have been a problem.


Really love aviation but have a computer science degree. What kind of jobs could I possibly get which involve aviation? I really want to work in the aviation industry. Is there any way I can put my computer science degree to use in this industry.

I'm not sure if this question comes within the scope of this site the way it is phrased.

Anyway, Aviation industry is highly interdisciplinary and has a lot of scope for computer science engineers like,

Design of aircraft subsystems
Development of programs for design and analysis of aircraft components (CAD, CFD etc).
Instrumentation and testing of aircraft systems
Development of aircraft control systems (e.g fly by wire)
Aircraft testing and telemetry.
Aircraft maintenance, which is becoming software intensive.
Software for Airborne (avionics) and Ground based systems (e.g. Air traffic Management).
Development of aircraft display systems.
Software for communication systems for UAVs.

This list is by no means exhaustive. In most parts of aviation, there is a requirement for computer science engineers.


What is the night effect? I read the night effect takes place in radio navigation, especially VOR and NDB. My questions are:

What is its nature?
Why does it occur?
What are the consequences of it?

Electromagnetic waves propagate actually using different means at the same time:

Space wave (direct line of sight)
Sky wave.
Ground wave (or surface wave)

Space wave vs sky wave vs ground wave

Note: Electromagnetic waves are not entirely understood. The terminology around ground waves varies with authors. Some authors don't make a difference between space and ground wave, they just consider both as the ground wave. This is not the case here.

The sky wave propagation develops at night and extends the range of LF/MF frequencies. The sky wave is the result of reflection in the ionosphere, the upper layers of the atmosphere, at an altitude between 100 and 300 km.

NDB range is increased, but VOR and DME range are less impacted. While range is increased by the sky wave, perturbations appear, changing the relative bearing of the NBD signal, decreasing accuracy.

HF radio transmission (voice) range is also increased, as the effect exists regardless of the modulation or use.

Frequency spectrum overview

To facilitate the understanding, we need to have an idea of the whole radio spectrum of waves and how aviation uses this spectrum.

Sky wave

The sky wave propagation takes benefit of the possibility for EM waves to be reflected in the atmosphere.

Light is an EM wave, and it can be reflected by a smooth surface: Metal, glass and water are good reflectors. This is the same for other radio EM waves. A good mirror for radio waves is a layer of ionized atoms. Atom ionization consists in losing or capturing one of more electrons, and becoming electrically unbalanced.

Natural ionization occurs in the ionosphere under the bombardment of solar rays. This takes place in the F-layer at about 300 km, and in the E-layer at about 100 km. Waves can be reflected by these layers when they are active.

However the D-layer just below blocks waves during the day because of the ionization of nitrogen monoxide $\small NO$), with a maximum below the frequency of 10 MHz. This effect decreases at night when the source of ionization disappears.

The higher layers are ionized more and more longer than the lower ones, and ionization persists more in the higher layers. The combined effects of D, E and F layers leads to

An absorption of LF/MF/HF frequencies by the D-layer at daytime, preventing them to reach the E and F layers able to reflect them.
A reflection at night and twilight when E-layer and above all the F-layer are still active and the D-layer has ceased blocking waves.

Sky wave propagation

Because the sky wave proceeds by "skips", it is more effective at specific distances from the transmitter and is characterized by minimums and maximums spaced along the direction of propagation, the distance between maximums depending on the frequency.

Remarks:

Because low frequencies are the only ones subject to sky wave propagation, over-the-horizon (OTH) radars use them instead of SHF.

Over-the-horizon radar. Source.
There is some controversy about the role of the US HAARP project in creating artificial ionized mirrors to use "sky wave" during the day (manipulating atmosphere is prohibited by international treaties).

As the propagation of NDB frequencies is augmented at night by the sky wave, a NDB can be detected at a greater range by a receiver. This is also true for other mode of communication in the same range of frequency (e.g. voice communication in HF over the oceans).

The sky wave propagation is associated with a slow variation, the signal can disappear and reappear a few seconds later. This is due to the instability of the reflection, the heterogeneity of the atmosphere layers, and the recombination of waves after a multi-path trip and a rotation of the polarization. This fluctuation is globally known as fading.

While sky wave propagation is in general a good thing but, like any other multi-path phenomena, it also has negative effects: It changes the polarization of the waves and the direction of the wavefront. As ADF works by detecting the direction of the signal wavefront, a reflected NDB signal introduces inaccuracy in the ADF indication.

The beacon seem to move, and the bearing is not reliable. Some cross check is necessary to confirm the validity of a bearing.

The ground wave propagation encompasses reflection on ground and obstacles, diffraction on obstacles and horizon, and refraction thanks to current induced in ground.

Because of the refraction and diffraction, this mode of propagation tends to follow Earth curvature, but it mostly affects LF and MF, higher frequencies (e.g. VHF and UHF) do not really take advantage of this propagation.

The ground wave is due to several factors which slow down the wavefront where it is closed to the ground (the wavefront is a moving spheroidal surface around the transmitter where the wave has the same phase, the wave propagates perpendicularly to its wavefront).

Ground wave: Wavefront slowed down by ground

As the wavefront is vertical at a longer distance, communication distance is increased.

However as the ground wave progresses, its slowed down wavefront is less and less vertical and the effect less and less significant.

Electromagnetic waves are composed of two fields: electric and magnetic, which are perpendicular and normal to the direction of propagation. The wave polarization, that is the plan where the electric field oscillates, considerably influences the ground wave attenuation. Vertically polarized waves (NDB and HF voice communication) propagate better than horizontally polarized waves (VOR variable signal).

The ground wave propagation, like the direct wave propagation, occurs any time, day and night.

Space wave, direct line of sight

A word about space wave propagation: While it's true this is a direct line of sight communication, it's also a simplification. Space waves are usually reflected, diffracted and refracted, exactly like visible light. So the space wave propagation happens without direct line of sight too. This is what allows cell phones to work in cities and/or indoor.

There is a lot of attenuation associated when the wave direction of travel is changed, and usually the wave follow different directions at the same time, some recombining at the receiver location after having traveled a different distance, hence arriving with a different phase (multipath propagation).

Multipath has been difficult to deal with in the era of analog transmission. To prevent significant attenuation limit the power of the transmitter and the size of the aerial, critical transmission channels, like terrestrial high-band radio links have been carefully designed with a Fresnel zone clear of any obstacle, limiting communication towers implementation to specific locations.

With digital modulations (and ease of implementation of fast Fourier transform algorithm in modern electronic circuits) space wave communication is now more easy to manage and these constraints are a bit relaxed.


What drives the design decision of where to place the refueling receptacle? What has driven the decision to place the refueling receptacle behind the pilot and to rear of the plane.  Is this simply driven by the change away from drogue and probe and the location decided by the preference of the pilots, the manufacturers or the service, any ideas?

As earlier aircraft appear to of a variety of locations of the receptacle.

I realize the pilot does not have to fly the probe into its receptacle, that is done by the tanker operator.

Usually just a matter of constraints on where the engineer can physically fit the fueling port with the limits that it must be an upper surface, not too near critical components that may be damaged, and have minimal aerodynamic impact. The plane must also deal with the substantial change in center of mass and angle of attack[required lift] during the fueling. All these factors vary with the design goals of each model.

The drogue and probe method is used by the US navy and most international air forces. This is a very adaptable system that can fuel multiple small planes from wing mounted hoses on one tanker or small plane to small plane, and even works with helicopters(using a very long probe); the down side is that maneuvering to couple is more difficult and each drogue design has a narrow speed range. The probe must be in located view of the pilot.

The port and flying boom method is used by the United States air force; it was developed by the strategic air command early in the cold war for strategic bombers. As such the flow rate of the flying boom is an order of magnitude larger than the hose and probe, and it can be used across a very wide speed range without changing parts, with added benefit of no probe to effect drag, radar signal, or to get damaged; the downside is the tanker can only serve one plane at a time, the port must be on an upper surface, and the tanker needs a boom operator.


Is it harmful to operate an engine "over square"? Never Fly Your Engine "Oversquare!" is an old recommendation. This wisdom cautions pilots that selecting a power setting with a manifold pressure in inches higher than the RPM in hundreds is damaging to the engine -- e.g., 2300 RPM dictates using no more than 23 inches manifold pressure.

Pilots are instructed by flight instructors to always reduce the manifold pressure before the RPM on power reductions, and increase the RPM before the manifold pressure on power increases. From this, it seems even short periods of "oversquare" is harmful during takeoff and landing. What about cruise?

Is it harmful to operate an engine for long periods "over square"?

No it's an old pilots' spouse's tale.  Those are just two numbers.  Having said that, Brake Mean Effective Pressure in the cylinder is higher when manifold pressure is high and rpm is low, for a given horsepower output, than the opposite way around, so there is a bit more stress on the cylinder head and piston.  And you do want to avoid that condition at the extremes to avoid really excessive BMEP, which could result in detonation.

The key word is "extremes".  With wide open throttle with the propeller at min RPM, it could be damaging, so it is normal practice to always lead with rpm on the way up and throttle on the way down.

But in the normal operating range it's not a big deal.  Look at a supercharged engine like a R985.  Redline is 36" of manifold pressure at 2200 or 2300 rpm.  It's jugs should be blowing off left and right.  And you cruise it at maybe 28" and 1600.  Oversquare enough?

So on the one hand you do have lower BMEP running higher rpm/lower MP, so somewhat lower stress on the cylinder.  On the other hand, heat and wear from friction is higher because of the higher rpm itself (and very importantly, the higher piston speed), so that tends to favour higher MP/lower rpm, for the same horsepower output.  My own preference in a normally aspirated engine is to cruise at say 2000 rpm and 22" instead of the other way around, because there is more benefit from the lower piston speed (piston rings wear out after traveling a specific distance, so the lower the piston speed the better, within reason) than from marginally lower internal cylinder pressures.


Can composite materials be used on super-sonic aircraft? Can composite materials be used in aircraft that can fly super sonic.

Sure. Almost all the modern supersonic combat aircraft incorporate a significant amount of composites in their construction. For example, in case of Dassault rafale:

  Composite materials are extensively used in the RAFALE and they account for 70% of the wetted area.

though they account for only 28% of the structural weight. This is in line with other aircraft- for example, composites contribute to around 25% of the weight of F-22 Raptor.

Dassault Rafale structural materials; image from indiandefence.com

You can see that the composites are extensively used in the wings, where the skins are made usually with composites, which enables them to be made in a single piece. The composite materials are superior to the metals used in aircraft in a number of performance parameters. The offer better strength to weight ratio and corrosion resistance, among others. According to Dassault,

  The RAFALE composite wing weighs the same as the MIRAGE 2000 metallic wing, but is 10% larger and can sustain a load that is 20% greater.

Composite materials are being used increasingly more and more in case of high performance aircraft- not only in wings (and radomes, where they have been used traditionally) but also in interior load carrying members, which have been usually made from metals. Even in case of Rafale, the wing box is made from carbon fiber composite. Another reason for using composites, atleast in combat aircraft is for stealth.

There are other areas in structural design, where the metals are better, for example, in impact resistance and in high temperature areas, where titanium is extensively used. Still in other places like landing gear, steel is used. The image below shows the use of various materials in F-22 Raptor.

Materials used in F-22 Raptor; image from allstar.fiu.edu

Again, the distribution is similar- composites make up majority of the wetted area while a minority of structural weight. In the end, the use of composites or metals are determined by their location and intended use.


What is the role of a bound vortex? From what I know the term bound vortex comes from potential flow theory and is a part of circulation around a wing.

Does it mean the same as circulation or lift?
Is it a cause of generating lift over a wing? If yes or partially yes, what is its role in that?

I'll use the following diagram for illustration:

Intuitively, a hard surface resists fluid flow via shear stresses; these shear stresses tend to rotate fluid elements (i.e. vorticity). Therefore, we can regard a hard surface as filled with vortex sources.

In actual fact, only the boundary layer contains vorticity, and there is usually no fluid element rotation outside of it. Since the boundary layer does not affect lift [much] in attached flows at high Reynolds number, we can ignore it and call the entire flow irrotational. This leads to the concept of potential flow. Furthermore, we can model the hard surface as a vortex sheet. The vortex sheet is called singularities and does not affect the irrotational assumption because it is a boundary and not part of the flow itself.

We can simplify the picture further. If the body is thin (like a wing), we can model the entire wing as a single vortex sheet (collapsing both upper and bottom surfaces into one). To simplify further, we can break the vortex sheet into individual vortex filaments running parallel to the wing (in the y-direction in the picture). These vortex filaments are called bound vortices. For high aspect ratio wings, we can simplify even further by considering the entire wing as a single filament, which leads to the Lifting Line Theory.

Since a vortex filament cannot end in a fluid, it must shed and end in the far field at infinity. This is a horseshoe vortex; the shed vortex filaments are called trailing vortices. A filament does not only shed at wingtips; in fact, it's shed everywhere along its length.

Image ref: https://history.nasa.gov/SP-367/f53.htm

Circulation is the total vorticity within a given closed loop. If you draw a closed loop from leading edge to trailing edge at a given span location, the total vorticity passing through this loop is the circulation at the given span location. Note that trailing vortices would be parallel to the said loop and will not partake in the circulation.

Lastly to complete the picture, acccording to Kutta-Joukowski Theorem, lift at unit span ($L'$) is directly proportional to the circulation ($\Gamma$) at the location:

$$L'=\rho_\infty V_\infty \Gamma$$


Are there any NACA airfoils that resemble the X-29's wing? As a part of my project, I am trying to perform structural and aerodynamic analysis of a forward swept wing on an aircraft, specifically the Grumman X-29 (pictured below).

I am assuming that information on the airfoil and wing of the X-29 is proprietary information. So, are there any open source airfoils and wing dimensions (NACA or otherwise) that closely resemble the X-29 wing?

From Mason’s Perspective on the X-29:

  The other issue with this airfoil is that every Grumman aerodynamicist felt the need to “improve” it, so that there were many variations. I think the airfoil that was used on the X-29 was known as K-Mod 2. I’m not sure anybody can say with certainty what the actual airfoil coordinates are. Grumman was not good with
  documentation

The airfoil appears on page 9 of that memo:

The reference given for it is Boppe, which I assume to mean the article "Computational Aerodynamic Design: X-29, the Gulfstream Series and a Tactical Fighter" by Charles W. Boppe in SAE Transactions, Vol. 94, Section 6 (1985), pp. 424-468.

Supposedly you can access that article in exchange for signing up with the host service, but I'm not willing to try from my current machine.


Can the ILS be shut down? Can the ILS be manually turned off? Not by the pilots inside the cockpit by simply shutting of the ILS button, but rather by a 3rd party personnel, such as an ATC or a ground crew member.

ATC shuts off ILS on RWY 05L for safety reasons.. or traffic separation reasons.

I am a second year electronics and communications student, so if you would like to use a couple of technical terms, i can understand them, to some extent

The ILS, or precisely the components localizer and glidepath antennas can be shut off, as they are electronic devices that emit radio signals. They can be turned off and on by ATC where required, e.g. having the same ILS frequency for two opposite runways and you only want one to be active at the same time or they can be shut down by maintenance directly at the emitters. ILS are usually not powered by UPS, so they will not be available during power outages.

If equipment is being shut down for maintenance or other reasons, you will see this mentioned in the NOTAM, such as here from Cologne/Bonn Airport, where the glidepath portion of the ILS runway 24 is out of service:

A4932/15 - ILS GP RWY 24 OUT OF SERVICE. 01 OCT 00:00 2015 UNTIL 30 DEC 00:00 2015 ESTIMATED.
CREATED: 27 AUG 13:34 2015

(Source: NOTAM EDDK)


Is there such a a thing as a single European pilot's license? The pilot licensing process in Europe seems to be rather, well, complex... I've heard of JAA, JAR-FCL and now apparently EASA licenses, although EASA's website says otherwise:

  the Agency is not authorised to issue pilot licences and therefore
  there will not be any EASA pilot licence in the future

And on top of that, individual countries can issue their own licenses (or at least ratings) that may not be valid in other countries.

So is there any such thing as a single European (EASA) pilot's license? What I mean by that is a single license that allows a pilot to operate any European registered aircraft in any European country, e.g a UK pilot flying an F-registered (French) aircraft in Germany?

There's some overlap with this question, but the answers there don't cover the practical use of a single pilot's license throughout Europe in the way I described.

EDIT: to clarify some comments, by "Europe" I mean EASA member states (let's keep it simple); and I'm asking about pilot licensing in general regardless of the type of flying. If there's any difference in where private/commercial/ATP license holders are allowed to operate in Europe that would be interesting to know.

Is there ... a single license that allows a pilot to operate any European registered aircraft in any European country,

Yes.

According to the UK CAA
an EASA Part-FCL licence can be used to fly any EASA aircraft in any EASA member country.

I have interpreted your "Europe" as "EASA member" in order to avoid ambiguity about Switzerland, Croatia, Kalingrad, etc.

  Most aircraft in Europe are EASA regardless of state of manufacture or registration.

  The UK currently grants several levels of licence:

  UK National Private Pilot Licence (NPPL)
  UK Private Pilot Licence (PPL)
  EASA Part-FCL Light Aircraft Pilot Licence (LAPL)
  EASA Part-FCL Private Pilot Licence (PPL)
  EASA Part-FCL Commercial Pilot Licence (CPL)
  EASA Part-FCL Airline Transport Pilot Licence (ATPL)


What is the westerly index? The westerly index, appears in a CPL syllabus but I can't find much info on it, I assume it is a measure of how systems move from west to east in the southern hemisphere.
How exactly is it calculated, and what is it's purpose and significance for aviation?

The only mention of it I could find in relation to a CPL is from a New Zealander CPL syllabus (govt.nz; PDF).
Including NZ in the search leads to this definition:

The westerly index is the Trenberth Z1 index (Trenberth 1976), which is the departure of the monthly mean pressure difference between Auckland and Christchurch (near 44°S) from its mean over the 1961–90 period.
— journals.ametsoc.org

From that definition, a pilot needs to be aware of the pressure difference between Auckland and Christchurch.

Others here may be able to expand on its usage (feel free to reuse, anyone).
From an Earth Science perspective, I asked on the sister site: What does a change in the mean pressure across 750 km indicate/signify?


Are ATC towers built in warzones? Take Afghanistan as a for instance; do governments (e.g. USAF/RAF) or NATO construct ATC towers?

I'd imagine so, at the air bases, in order to land/takeoff the vast amount of aircraft.

Who controls the sectors where missions are ongoing? How are the different countries' aircraft coordinated and kept apart?

Yes, sometimes control towers or other ATC facilities are constructed to support air traffic control. If, for example an ATC tower is needed (there was none previously or it was destroyed), a mobile replacement is used.

For mission control Airborne Warning and Control Systems (AWACS) are used:

But these are quite expensive to fly. If possible, mobile surface based radars are brought in to provide a surveillance picture.

Mobile towers, radars and navigation beacon are not solely used in war situations. In natural disasters they have proven to be useful as well:

FAA sends temporary Air Traffic Control tower to Haiti


Is the "PROB" in a TAF only applying to the next group or all following ones? Let's assume I have a TAF that reads:

[...]
PROB40
TEMPO 0815/0822 5000 -SHSN BKN008 OVC015
TEMPO 0822/0909 1000 SN VV003
BECMG 0902/0904 16012KT

Which of the TEMPO / BECMG groups have a probability of 40%? All three because everything that follows PROB40 has a probability of 40%? Just the first TEMPO because PROB40 only means the one following line in the TAF? Or both TEMPO ones, and BECMG is again certain and not just 40% probable?

PROB40 is targeted only on the immediate TEMPO group. If there are several PROB/TEMPOs both keywords must be repeated every time. Also, PROB is never associated with BECMG or FM groups, which indicate permanent change.


What does 'outlook ... VFR' mean? In most weather reports I see "Outlook VFR" and I am not sure what that means and as a pilot what should I expect? What is an "outlook" and what does it mean when it is "VFR"?

In other words, what does 'OTLK... VFR' mean in weather reports?

Outlook VFR is usually found in the so called "Area Forecast". It is the last part of the portion which describes clouds and weather, which could affect VFR operations.

The area forecast for every region is valid for 12 hours, followed by a 6 hour categorical outlook.

There are three types of outlooks, VFR, IFR and MVFR (marginal VFR). As mentioned in Jeppesen's Guided Flight Discovery Instrument/Commercial Manual:

  A ceiling less than 1,000 feet and/or visibility less then 3 miles is
  considered IFR. Marginal VFR areas are those with ceilings from 1,000
  to 3,000 feet and/or visibility between 3 and 5 miles.

Everything "better" (as in higher ceiling and/or visibility) is considered VFR.

Here is an extract from an Area Forecast:

CLDS/WX VALID UNTIL 062100...OTLK VALID 062100-070300
[...]
NRN PEN...BKN010-020 TOP 100. OCNL VIS 3SM BR. BECMG 1214 BKN025
TOP 120. ISOL -SHRA. 18Z BKN040 TOP FL220. WDLY SCT -SHRA/TSRA.
CB TOP FL420. OTLK...VFR TIL 02Z SHRA TSRA.

The first part is an extract from the Heading Section. here you find the time for which the Clouds and Weather part as well as the Outlook part is valid.
The second part consist of the mentioned Clouds and Weather as well as the Outlook. I'm not going to decipher the whole thing now, but here is a short explanation:

You can see the more detailed forecast, valid from the time of loading the Area Forecast to 062100, which means the 6th of the month, 21:00 Zulu time. After that, from 062100-070300 so from the 6th at 21:00 Zulu until the 7th at 03:00 Zulu, the outlook is what counts. In this case it is VFR until the 7th of the month at 02:00 Zulu, where showers and thunderstorms (in connection with rain) are expected.

Conclusion: The outlook gives a general idea what the weather will be like up to 6 hours after the forecast part of the Are Forecast. This can help in flight planning.

For more information about VFR refer to DannyBeckett's detailed answer.


Is it allowed and possible to hold at the intersection of two VOR radials using a single VOR receiver? First of all, is it permitted? I am practicing VOR tracking on my simulator and the missed approach procedure (KLGB VOR 30) takes you out to an intersection off two VOR radials. Since the plate doesn't specify "DME required" I figured yes.

Assuming yes, are there any tricks/protips to doing this successfully?

As I understand it you are asking two questions:

is it permitted to hold at an intersection of two VOR radials with nothing more than a single VOR receiver? and
what are some tips and tricks for doing it?

Answer to question one:
As others have mentioned, it was permitted at one time (and I believe it is still permitted). I'm old enough to have received training on how to do it at a time where it was much more common to fly in GA aircraft with no DME or GPS and sometimes only a single VOR receiver. I spent over an hour researching this question for you in the AIM, Airplane Flying Handbook, Instrument Flying Handbook, and Instrument Procedures Handbook, and online generally, and was not able to find anything saying it was permitted or prohibited. I don't see any problem with it other than it adds a bit of workload to the pilot, but it can be done safely and relatively easily. If I find an definitive answer I'll update my question.
Answer to question two:
Here are some tips and tricks for flying an intersection hold with nothing but a single VOR receiver. This explanation assumes you know how to hold, including the FAA recommended hold entries.
There are two radials, hence the intersection. I'm going to refer to them here as the main radial and the cross radial. (It is possible to approach the fix on the cross radial then switch to the main radial for the hold after executing the hold entry, however, for purposes of this explanation, I'm going to assume that you are approaching the fix on the main radial (i.e. the radial on which the inbound holding course is located).
In the figure below the aircraft is on the main radial and is approaching the cross radial from the West. However, this aircraft could also be approaching from the opposite side of the fix from the East.

So, while you are on the main radial flying towards the fix, you want to spend most of your time monitoring the main radial because it is your primary course navigation, however, you also want to initially tune the cross radial well before reaching it in order to verify, and take a mental note of which side the CDI is deflected. This can be confusing to some pilots because the CDI will be either on the right or the left depending on where the station of the cross radial is located in relation the main radial. (When you fly a G1000 or HSI, the CDI rotates such that it is easy to tell when you are approaching the cross radial or have already passed it, but this is not the case in stand-alone old-school VORs). Whether the CDI of the cross radial is on the left or right also depends on if the pilot tuned in the radial or the reciprocal. For simplicity in this example, lets assume you tune in the cross radial well before reaching the intersection and the CDI is fully deflected to the left.
You then re-tune the main radial (hopefully your NAV radio has a flip-flop button), and center the CDI. You want to be as precise as possible and note the wind correction angle required and bug that heading! It's important to be precise, because you will be flying the bugged heading when you switch to the cross radial.
As you fly towards the holding fix on the main radial you will periodically switch back and forth to the cross radial until you detect the CDI is alive. At this point, you want to watch the CDI to get an idea of how fast it's moving.  I like to take a note of the deflection of the CDI, lets say it is 8 degrees to the left (still haven't reached it), then take a note of the clock to determine how long it takes to fly 1 degree. Let's say that it takes 30 seconds to go from 8 degrees to 7 degrees, if so, then you know you have approximately 3.5 minutes before reaching the holding fix (7 degrees x 30 sec/deg = 210 sec or 3.5 min). You can then switch back to the main radial and continue navigating on it, until about 1 minute before you reach the intersection, at which point you would tune in the cross radial and fly your bugged heading until the cross radial is centered. At which point you would peform the appropriate hold entry.
At the moment you reach the holding fix, you will be flying the bugged heading and the cross radial will be tuned into your VOR. If you are on the main radial on the inbound heading, then it will be a direct entry and you just turn (right turn in the above figure) and fly outbound. On the outbound leg, tune the VOR to the main radial to the inbound course so that you can intercept it when you turn inbound. If you are approaching the holding fix from the opposite direction (from the East in the above figure) such that you are flying the outbound heading of the hold, then you will need to do a teardrop or parallel entry. In this case, I like to do the parallel entry because it requires less math and turning. Since you are already flying the outbound heading and are parallel to the inbound course you don't have to do anything when you reach the holding fix from this direction other than start the timer or take a note of the time and fly out for 1 minute. During this one minute of flying outbound on the parallel entry, you will tune in the main radial to the inbound course, so that when you make your greater than 180 degree turn you can intercept the inbound course.
Whether you do the direct entry or parallel entry, once you are established on the inbound course and tuned into the main radial, you will need to bug in that heading, and promptly tune the VOR to the cross radial, because you've only got a minute or so on the inbound leg to intercept the inbound course, determine the WCA, bug it, then tune the cross radial to identify reaching the holding fix.
Then repeat until you're ready to leave the hold.
This was a long explanation, but it's really not as bad as it sounds. When you are practicing this on the sim, you can try flying 2 minute legs, which gives you more time to intercept inbound, then tune the cross radial. As you get more efficient, you can shorten the legs until you get to 1 min. BTW, in actual IFR, you can ask ATC for a non-standard leg length. I imagine any airplane in this day an age with no DME or GPS that only has a single VOR is probably a very very slow airplane, and ATC can probably accomodate 2 or 3+ minute legs, just ask.


What benefits does Automatic Dependent Surveillance bring to continental ATC over Mode S? In particular, to the extent that there are mandates for it in USA and Europe?

I checked various posts here and official descriptions (e.g., faa.gov), but I can't figure out the following:

For continental (non-oceanic) usage, what benefits does ADS(-B) provide to ATC that is not possible with Mode S SSR?

I came to a dead end at How can ADS-B replace primary radar when FlightRadar24, using ADS-B, is so inaccurate? In particular:

  ADS-B can and will replace a number of secondary radars, but not all of them.

By not all of them, I reckon those in congested areas will not be replaced, say the New York/New Jersey/Philadelphia Airspace. One could say to provide cheap coverage elsewhere where radars can't reach (too low targets), or not enough traffic to sustain the cost, but then for the FAA mandate, for example, outside Class A, B, C, and E (above 10,000 ft), one is not required to have ADS-B. (What is ADS-B and who needs it?)

I understand that there are plenty of beneficial uses for ADS: [Cheap] online tracking that benefits the operators and general public alike, and situational awareness for any GA pilot willing to spend money on ADS-B IN. But my question is about the benefits to ATC, that Mode S can't provide.

Edit: I found one usage (benefit?) in the post Is ADS-B intended to replace other traffic management systems?:

  A more advanced usage of ADS-B will be flight deck based interval management (FIM) where ATC will be able to instruct aircraft to 'follow that plane XX seconds behind for landing on runway YY'.

Technologically, this is cool. But why the complication? Terminal airspace design that utilizes RNP routes, combined with requested times of arrival relayed to each aircraft, can accomplish the same, and is already a thing. Why let the separation be the responsibility of many, versus a few, i.e., bigger chance of something going wrong.

I'm thinking there must be more to it, even if not yet implemented.

ADS-B is more accurate.  SSR calculates the distance to a target based on when it receives a reply from the transponder, which is inherently imprecise, and calculates the azimuth by the sweep, which is also inherently imprecise.  A blip on a radar scope isn't exactly where the plane is but rather the center of an area where the plane is most likely to be.  ADS-B means ATC knows exactly where the plane is, which means they can (eventually) use tighter spacing if needed.

ADS-B is more reliable.  If two aircraft are near each other, their transponder replies may overlap, making both unreadable--and at the time it is most critical for ATC to be able to see both aircraft for separation.  ADS-B messages are transmitted randomly, which does result in occasional errors but means that two aircraft near each other aren't any more likely to step on each other than two far away, which is a substantial reduction in risk.

ADS-B can improve TCAS by not requiring air-to-air interrogations, reducing the total number of mod eS messages being broadcast and therefore the odds of aircraft stepping on each other.

ADS-B updates faster.  SSR relies on a radar slowly sweeping around, which means targets are only updated every 5-10 seconds, and this magnifies the effect of message errors.  This is "solved" in busy terminal areas by having several overlapping SSRs, but that also increases the error rate so it's of limited use.  ADS-B transmits position randomly every second, and even if some overlap, the readable update rate is still far higher than SSR.

ADS-B is cheaper.  You can put simple ADS-B receivers in many places where expensive SSR wouldn't be economically justified, giving ATC much improved coverage, particularly close to the ground and in mountainous areas.  You can also reduce SSR overlap, even if SSR doesn't go away entirely, which also saves money.

ADS-B provides improved situational awareness to pilots.  SSR doesn't provide any traffic data to pilots, and even TCAS isn't all that accurate (which means lots of false alarms) despite being extremely expensive.  ADS-B cheaply shows pilots exactly where the other traffic around them is--even non-participating traffic in an area with TIS-B.

Future ADS-B enhancements will also show aircraft intent.  SSR (and current ADS-B, to be fair) only show a plane's current position, often with a line projecting where it will be in N seconds/minutes if it continues along the same track since the last position.  ADS-B allows an aircraft to indicate what it will do in the (near) future, which is very useful when you have two aircraft near each other and one or both are turning and thus the straight-line projections aren't accurate.

The inaccurate flight paths you see on FR24 et al are due to aircraft not having ADS-B; the jagged tracks are generally from mode S-only aircraft, and the receivers have software that uses multilateration (MLAT) to cooperatively estimate where non-ADSB messages came from.  The FAA has experimented with this tech themselves in mountainous areas, and it's better than having nothing at all, but it's still far less accurate than SSR, much less ADS-B.


What is the problem if the temperature of the oil and heads of cylinders is too high? What is the problem if the temperature of the  oil and heads of cylinders is too high? What's the fault in piston engines?

Piston engines are designed to run within specific temperature ranges. Going above this range might effect the lubrication [hot oil may not provide the right amount of protection to moving parts, If hot enough it could melt seals, or even catch fire.]
the other danger is as metal changes temperature its structural property's change. If a metal gets too hot it becomes softer and may become less resistance to forces. also there is the issue of shock cooling, if a hot engine gets cooled to quickly, it may unevenly cool, and due to the difference in heat expansion could crack or fail.


Is there a benefit to having the motors for a multi-rotor copter on the top vs the bottom of the frame? There is a new quadcopter being launched on Indiegogo that has the quadcopter motors positioned on the top of the propellers as opposed to being positioned on the bottom.

Is there any advantage to this or is it just for looks?

It could be just for looks, but rotor placement has a noticeable effect on the behaviour of the quadcopter.

The higher the rotors are on the frame the higher the center of lift will be (the center of lift for quadcopters is the center of the plane created by the rotors.) A high center of lift means that the frame will be more stable. A low center of lift means the opposite. The reason for that is that the quadcopter can better naturally deal with lateral rotational forces on its frame. The center of lift being above the center of mass means that the frame is "hanging" like a pendulum from it. If you apply a lateral force to it, it will naturally swing back to place and the more you push the "harder" it becomes to move the frame.

A center of lift that is in the same spot as the center of mass of the frame means that there is no pendulum effect. This is like balancing a ball on top of another ball. All lateral forces, either internal like variations on each rotors' force or external like wind perturbations have maximum effect in moving the frame.

Contrary to what you might think, stability is not something you want in a quadcopter. The more unstable the frame, the more agile it becomes. Small variations in rotor force will create large rotational movements. The penalty you pay for that (small, unpredictable external forces also create large rotational movements) you make up with processing power. Processors are now so fast and sensors so sensitive that they easily can compute the external forces and create a response fast enough to keep the frame flying.


Why are there different types of winglets? If all winglets serve the same purpose (to reduce vortex drag), why do different aircraft have different winglets? This question is not trying to get a comparison of the development of winglets/wingtips, with other aircraft components, as the question of @Manu H is.

Winglet design is driven more by marketing than by engineering, so every aircraft brand tries to cultivate their own, supposedly better, shape.

A study by Nita and Scholz (PDF!), Chapter 6 gives a good overview over recent winglet studies. A raked tip performs better, and if winglets could point downwards, they also could do better. As they are, they give only 28% to 38% drag reduction of an equivalent span increase.

Raked wingtip of the Boeing 777-300


No alternate destination when flying from Oakland to Honolulu: is that correct? When you file a flight plan from Oakland to Honolulu
  you put "Glory" as your alternate destination.[1]

Is that the correct way to file the flightplan?

Scanning back a few paragraphs, the airplane in question was a Cessna 402.

[1] B. May, Under His Wing: Adventures in Trusting God, p. 40 (Multnomah Publications, 1991. ISBN 0-930014-94-4.)

Please note while the publication is 1991, the actual copyright is 1979, so we are looking before that date.

There are plenty of alternative airports in Hawaii, but none in-between of course. So what the author is getting at is that you're probably going to be dead if you run into trouble.

While I can't say I know anything much about the Cessna 402, it apparently has a normal range of around 1,273 nm (1,467 mi, 2,360 km), compared to a distance Hawaii-Oakland of 2050nm (2359mi, 3797km). Hence, you're going to have to install additional fuel tanks and fly pretty heavy.

There a few considerations if you get a problem:

You will have to descend since the engine can't generate enough thrust at that cruise altitude. I found a source stating the ceiling for both engines is around 26180 ft compared to 11320 ft for one engine.
The fuel consumption increases in the remaining engine and will be greater than the total for both engines for the same flying conditions.
You're going to have to run the remaining engine hard for the remain of the flight.
If your temporary ferry tanks screw up (or any of the normal ones for that matter as well), you won't make it.
You may damage/break the gear if you have to return to the airport overloaded. "I once saw a Cessna 402 parked on the ramp at Honolulu with the gear punched half way up into the wings from a poorly executed overweight landing." Source

I can't do a calculation on for what portion of the flight you would be 'screwed' but I imagine it might very well be a bit of the journey. Several people seem to be in the opinion it's a pain to fly heavy and difficult to with one engine.


What's going on in this "confusing" TCAS Resolution Advisory video clip? There's a clip on YouTube from a pilot of a 747-200 whose TCAS system issues a "confusing" RA:

What exactly is happening here?

For those who can't watch the video right now, here are some frames:

The TCAS responded to the other aircraft not following the flight profile that was observed earlier. If you watch closely, you will see that the other aircraft started to descend as well after the first resolution advisory is given.
The onboard TCAS responds by issuing the counter-movement to climb immediately.

The only thing that we do not know is what version of TCAS this is supposed to simulate, because newer TCAS systems can interrogate each other and issue resolution advisories not only based on own observed data, but also on data received from the opposite TCAS.

Related Question: How does TCAS work?

The entire situation could have been part of the training scenario, as this was done in a simulator, according to the YouTube description:

  Practising TCAS RA manoeuvers during our MCC course in a Boeing 747-200 full-flight simulator. This one was quite confusing when the TCAS' voice advised to climb just after having asked to descend.


How does the airflow above an airfoil account for the downward turning-action of the airflow below the airfoil? Wikipedia provides an explanation of the lift force in terms of flow deflection and Newton's laws, emphasis mine:

An airfoil generates lift by exerting a downward force on the air as it flows past. According to Newton's third law, the air must exert an equal and opposite (upward) force on the airfoil, which is lift.
The airflow changes direction as it passes the airfoil and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then Newton's third law requires the air to exert an
upward force on the airfoil; thus a reaction force, lift, is generated opposite to the directional change. In the case of an airplane wing, the wing exerts a downward force on the air and the air exerts an upward force on the wing.
The downward turning of the flow is not produced solely by the lower surface of the airfoil, and the air flow above the airfoil accounts for much of the downward-turning action.

In what way does the air flow above the airfoil account for much of the downward-turning action of the air flow below the airfoil?

The answer is buried in the linked Wikipedia article if you read it carefully a few times.
In a nutshell, at least in my interpretation, the "zone of suction" you might call it, the Bernoulli effect low pressure region above the airfoil, induces air outside the zone to accelerate toward it; to "fill the void".  On a zone of suction that is moving, like the top of the wing, it means that a large region of the air stream above the wing wants to accelerate toward the zone, which being below, means that the net result is the a large package of air is induced to move downward in response to the wing's passage below it.
Maybe half or more of the total package of air that is redirected downward is above the wing.  This air is being induced to move by "pressure trickery" as opposed to being shoved down directly by the pressure wave of physical wing's passage like the air below.  A water ski works by deflecting only the fluid below it.  A wing works by deflecting the gasious fluid below, and also inducing the gaseous fluid above to join in by careful management of pressures.
This is partly why disruption of the flow above has a more drastic effect than disruption of the flow below - spoilers go on top instead of underneath.  Air below is just being physically deflected; the air above, for quite some distance, is being "coaxed" to be pulled down toward the low pressure region, and if you disrupt the low pressure region, all of the air that was ready to be induced to move down doesn't have to, and you lose most of your Newtonian force reaction.
The result is that this contraption flies fine with all that junk obstructing the flow under the wing, but if you put it all on top, it would never get off the ground.


What is a land and hold short clearance (LAHSO)? I was reading this question and I want to know what this term means.

  land and hold short clearance

It means that you can land on the runway but must hold short (read: do not cross) before some point on the runway.

AIM 4-3-11 Pilot Responsibilities When Conducting Land and Hold Short Operations (LAHSO)
6. A pilot who accepts a LAHSO clearance should land and exit the runway at the first convenient taxiway (unless directed otherwise) before reaching the hold short point. Otherwise, the pilot must stop and hold at the hold short point.

This is generally issued for smaller planes that don't need the full runway and a crossing runway/taxiway is also being used.
As the pilot you have final say in accepting the clearance, if you feel you will be unable to hold short then you should reply "unable" and request full-length landing clearance.

[same link as above]
3. The pilot-in-command has the final authority to accept or decline any land and hold short clearance. The safety and operation of the aircraft remain the responsibility of the pilot. Pilots are expected to decline a LAHSO clearance if they determine it will compromise safety.


Pitch control of tailless aircraft/flying wings with very wide elevons I'm writing a simulation of a flying wing and I need clarification on some of its behavior.
Specifically, the simulation suggests to me that a flying wing (or any tailless aircraft) with two single elevons along the whole of the two trailing edges is not controllable. If the elevons are deflected to pitch up, for example, it generates negative lift over both wings and the torque (assuming CoG is ahead of CoL) pitches the aircraft up. But this rapidly increases AoA and therefore lift and positive torque, pitching the aircraft back down until it stabilizes at Cl = 0 (steady state AoA depending on elevon deflection) because Cl = 0 is the steady state where no torque exists. However, it also generates no lift and the aircraft naturally crashes.
This result makes sense to me mathematically, but I struggle to observe this in real aircraft.
In a conventional aircraft the elevator is much farther back than the main wing and can balance the torque at a positive sum of the lift vectors. On a tailless delta aircraft with split elevons, the set of elevons towards the wing root have a CoL different from the elevons near the tips, which would allow small effective Cl according to the logic above. But this seems barely controllable to me and only reliable at high speed to compensate for the abysmal Cl.
Am I making a mistake? Is this really how it works?

Specifically, the simulation suggests to me that a flying wing (or any
tailless aircraft) with two single elevons along the whole of the two
trailing edges is not controllable.

This is certainly not true.  Many radio-controlled model airplanes and gliders with an all-wing, chevron-shaped configuration and no tail have this exact configuration of the elevons, which are the sole control surfaces.

For example, in the photo above, the elevons are red.
Google "Zagi flying wing" for many more examples.

If the elevons are deflected to pitch up, for example, it generates
negative lift over both wings and the torque (assuming CoG is ahead of
CoL) pitches the aircraft up.

This is not true either.  When flying a wing like this, an aft stick input generates a nose-up pitch torque that increases the angle-of-attack of the wing.  In the long run, the wing's lift coefficient is increased, not decreased.  There is no visible evidence of even a temporary tendency for the aircraft to "settle" (accelerate downwards) when the elevons are first deflected upwards, before the wing has had time to rotate to a higher angle-of-attack, though in theory this tendency must exist to some slight degree due to a temporary decrease in the wing's lift coefficient (but not to the extent that it actually becomes negative).  This "settling" tendency might be more significant if the aircraft's pitch rotational inertia were greater.
Conceptually, this is no different than the idea that when we are flying a "conventionally" shaped aircraft, and we move the control stick aft, there must some temporary "settling" tendency (downward acceleration) due to the fact that we've increased the downforce (or in some cases, decreased the upforce) generated by the tail, so the net upward lift generated by the aircraft is now less than weight.  Of course, this situation only persists until the aircraft rotates in the nose-up direction enough to increase the angle-of-attack of the wing sufficiently to bring everything back into balance, at a higher angle-of-attack and lower airspeed than were present initially.  In actual practice, this "settling" tendency is almost always so negligible as to be undetectable.
It might help you understand what is going on if you were to consider reflexed airfoils.  In such an airfoil, the trailing edge of the airfoil is permanently "bent" upwards, contributing a nose-up pitch torque, yet the airfoil still creates positive lift over a wide range of angles-of-attack.
For example:

(Source: https://www.diva-portal.org/smash/get/diva2:450812/FULLTEXT01.pdf )
It appears you are getting into trouble by relying on the idea that the Center of Lift is behind the Center of Gravity.  In reality, the Center of Pressure is not fixed, and in steady-state flight, the Center of Pressure (of the whole aircraft including the contribution from the elevons) and the Center of Gravity must coincide.  When we raise the elevons to create a nose-up pitch torque, this pitch torque can be conceptualized as a forward shift of the Center of Pressure.  Then when the wing rotates to a higher angle-of-attack, the Center of Pressure moves back aft until it coincides with the CG again, and the aircraft ends up in steady-state flight at a higher angle-of-attack and higher lift coefficient and lower airspeed than it started with.
A more modern way to think about aerodynamic forces and torques is to view the aircraft as having an Aerodynamic Center that is fixed at the quarter-chord point of the airfoil, plus a pitching moment coefficient that varies according to the shape of the airfoil.  With this convention, the lift vector can be viewed as acting at the Aerodynamic Center, but we also have to consider the effect of the pitching moment coefficent.  Raising the elevons shifts the pitching moment coefficient in the nose-up direction, as well creating some decrease in the lift coefficient.  A symmetrical airfoil has a pitching moment coefficent of zero, and conventionally shaped cambered airfoil with no reflex has a nose-down pitching moment coefficient.


Where can I find registration information of a private aircraft registered in Germany? Is there any way (website, database etc.) where I can get the registration information of a privately-owned general aviation aircraft registered in Germany?

(Germany has pretty extensive privacy laws, but it would seem that aircraft registrations should be a little bit transparent and verifiable...)

I don't think the information is accessible on a public website.

According to the German wikipedia the Luftfahrtbundesamt (LBA, the German Civil Aviation Authorities) cannot publish the information for data protection law reasons.

On their website (in German) they state that they do release information to members of the public at a fee (they love that here).

A little bit of link-clicking later you will find that you have to pay 40 euros. I am not sure whether they can release the name of the owner.

The law (Luftverkehrsgesetz) governing the information in the register can be found here.


Why would short-haul flights be pressurised at a higher cabin pressure? In the article here : https://abcnews.go.com/Health/Healthday/story?id=4507808&page=1 it is claimed that

  Short-haul flights are usually pressurized at 5,000 to 6,000 feet while long-haul flights are closer to 8,000 feet, according to Thibeault, [M.D., medical advisor, IATA].

But why would this be? My understanding is that the cabin pressure is chosen as a balance of:

what the aircraft can withstand without slow structural deformations, so that it is functional for many years
what is most comfortable for the passengers
most fuel efficient- pumping in air requires some fuel.

With regards to comfort, I should think that low pressures take their toll after a few hours (I think I read 3-9 hours, for people developing altitude sickness symptoms at about 8500ft equivalent cabin pressure).

The only guesses I have at why short-haul flights can operate at higher cabin pressure is:

the aircraft body won't have as much structural change with a short-term pressure difference
the plane can 'retain in' air pressure without actively pumping it for some time on the order of hours/ half-hours, so it is not so energetically costly.

For the structural loading, the absolute cabin pressure is not as important as the pressure differential with respect to the outside pressure.

Short-haul flights typically operate at lower altitudes; either because they are operated by turboprop aircraft which have a lower service ceiling, or because there is simply not enough distance to climb to a high altitude and glide back to land. At lower altitudes, the outside air pressure is higher, and thus the cabin pressure can be higher without exceeding the maximum pressure differential the fuselage can handle.

Your other concerns are not really a factor. Only the pressurisation cycle count really matters to aluminium, not the total pressurised time duration. The fuel required for pressurising the aircraft is also not really a factor compared to the energy required to either heat or cool the air for passenger comfort.


How can a VTOL aircraft control yaw while hovering? I watched this video of a F-35B performing a flight-hover-flight transition and was pretty impressed by the fact that it was able not only to control pitch and roll (which could be, I suppose, performed by differential power from engine and lift fan) but also yaw — while hovering (at 3:15 in the video).

How does it do that? Bleed air ducts from the engines on the sides of the aircraft? (something alike to thrusters used to control attitude of spacecraft I suppose).

In case of F-35, the yaw control during hover (and transition to forward flight) is provided by the Three-bearing swivel module (3BSM), which forma a part of the Rolls Royce Lift System. As Lokheed Martin notes:

  Two roll ducts supplied by engine fan air provide roll control. Yaw control is through swivel nozzle yaw. Pitch control is effected via Lift Fan/engine thrust split.

From codeonemagazine

  The three-bearing swivel nozzle, 3BSN, design uses three segments of the engine exhaust duct cut on an angle and joined by two airtight circular bearings. ... The forward and aft segments always stay aligned with one another in the rotational axis. The center segment rotates through 180 degrees relative to them. The third bearing is on the back of the engine aft of the turbine stage and provides the ability to swivel the nozzle thrust axis in yaw at any pitch angle.

Consider the nozzle in VTOL position, like below:

By Tosaka - Made by uploader (ref:見森昭編　『タービン・エンジン』　社団法人日本航空技術協会、2008年3月1日第1版第1刷発行、ISBN 9784902151329, 1), CC BY 3.0, Link

If we hold the aft two segments and rotate only the first one, it is obvious the rear segments will rotate along the engine axis, producing yaw by virtue of its position at the rear. There is a slight associated loss in lift produced, but the Digital Flight Control System can easily compensate for it.

Of course, this method of controlling yaw in hover is specific to F-35. In case of Harrier, the reaction control system had 'puffers' at the tail to provide yaw control.

Reaction control system, showing pipes and nozzles for engine HP bleed air.; image from harrier.org.uk

  In addition to the vectoring engine nozzles, the Harrier also requires a method of controlling its attitude during jet-borne flight, when the normal aerodynamic surfaces are ineffective. To this end, a system of reaction control nozzles in the nose (blowing down), wingtips (blowing up and down) and tail (down and lateral blowing) are fitted to the aircraft.


Can a jet engine benefit from nitrous oxide boost? I just read this wiki article again about the GM-1 boost system. This was used in Nazi planes during WW2. It injected Nitrous Oxide (N2O) into the supercharger of the piston engine. The idea was to get more oxidizer in the engine, allowing for higher fuel-injection and therefore higher engine power temporarily. This could serve to climb faster or take-off better or maintain high altitude.

My original interpretation of the GM-1 system and the related MW-50 system is that they're sorta like afterburners of their era. But that analogy is not perfect. Afterburners use the same jet fuel as used inside the engine, but sprayed outside the back. GM-1 used a different fuel, N2O (actually a monopropellant with more oxidizer built into the chemical than fuel), injected inside the engine.

What I want to know is, can N2O boost work in a jet engine by injecting directly into the combustion chamber (along with increased fuel injection)? Is that a good idea for military jet aircraft (compared to afterburners)? Is it even possible without overheating something?

I'm interested in turbojets and turbofans, and would even consider a centrifugal jet engine.

For the combustor, I say the answer is no, because what is limiting the fuel rate, isn't the amount of oxygen (gas turbine combustors run very lean), but the ability of the turbine materials to withstand a higher gas temperature entering the turbine. Hence, to temporarily increase engine thust, water injection into the combustor was used on early jet engines, to increase thrust during takeoff.

This sounds quite counter intuitive, but the explanation is based on thermodynamics. Naturally, the water evaporates into steam, which then gets heated to the temperature of the combustor exit temperature. This evaporation and heating process of the water and then steam, is drawn from the heat of the combustor, and so reduces the combustor temperature (for the same fuel flow). However, the engine does not need to add more fuel to make up for this cooling, at the same level of thrust, because the energy added to the steam is still available in the turbine, to be released as the combustion gas / steam mixture flows through the turbine, expands and cools. As it cools, it gives up this energy, which is effectively used to power the turbine. The remaining energy is then converted to thrust, as the flow expands through the final propelling nozzle. But now, because the turbine temperature is lower for the same thrust level, the fuel flow can be increased until the maximum turbine inlet temperature is reached, and so now providing more thrust.

So, rather than inject N2O into the main combustor, inject water.

For the afterburner (AB) however, the case is quite different, and I believe a performance benefit could well exist. AB combustion runs much closer to an equivalence ratio of 1 (about 0.96), i.e. burning almost 100% of the fuel level that can occur with complete combustion of all the available air. And unlike the main combustor, there are no real material temperature limits (cool bypass air exits along the inside of the AB duct surface). But, the question is, do you really want to do this, operationally? Is it better to use some of the planes tank capacity for some N2O, or just entirely for fuel? I suspect operationally it is better to be able to use the AB for longer, at a slightly lower thrust level, than shorter, for a higher thrust. I don't believe military aircraft use AB to out run missiles, so pure thrust isn't the only aim. But that's another (next) question....


Is the time spent flying in a Sailplane concidered a valid flight hour for a LAPL / PPL license? I live in the Netherlands.

In order to get a Light aircraft pilot licence (LAPL) license 30 flight hours are demanded, A private pilot licence (PPL) requires to have 45 flight hours.

flying in a Sailplane is lower-priced then a motorized airplane in my country.

So I'm wondering if flying in a Sailplane is concidered a valid flight hour for a LAPL / PPL license.

In the UK, under the CAA (and as a CVn mentions, it may vary across EASA land, regardless of SERA), these are the rules:

  The privileges of your licence will only remain valid if you have
  completed, in the last 24 months, as pilot of an aeroplane or TMG:

At least 12 hours flight time as PIC, including 12 take-offs and landings; and
Refresher training of at least 1 hour of total flight time with an instructor.

That document also explains that:

  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.

So I think that, in the UK, you can't act as PIC on a sailplane and so can't count any hours toward your LAPL

However, TMG flying is very cheap, if you can find one for hire