New aircraft

    Quote from Overloaded

    I thought that things had been quite mild until I noticed a gentle bank and very slow rotation and strangely useless ailerons. There wasn't much wind noise either! Solo of course so forward COG and full opposite rudder with down elevator got me out of my fix.

    So if I understand you, it entered the incipient stages of a spin, and you had to recover from that? If the nose didn't drop abruptly, and there was no wind noise, that would imply you were almost stationary in the air -- a very strange state to be in. I've been warned about the problem of a tail-slide messing up the elevator, so I stay away from anything that might lead to that…

    • Official Post

    I could litterally write whole pages about this topic...
    Here is my way of seeing this, as a real life pilot, sim enthusiast and aero-space engineer:

    A stall occurs when the airflow can no longer follow the contor of the airfoil. The reason for that is the angle of attack (only!). The airspeed is only secondary and is not the reason for a stall.

    A stall can occour for each airfoil that an aircraft has: the airfoil of the main wing, horizontal and vertical stabilizer, propeller and in jet engines (compressor stall etc...). And a stall can always happen in two directions: upright or inverted, left or right.

    And here is why:
    If you fly a perfect "zero G" parable (x^2) you will never enter a stall even if the airspeed at the peak reaches almost zero. You won't stall because the airflow always hits the airfoil in a non critical angle at all times and with such a flight path you don't need any lift force what so ever.

    Always remember: Leaving a stall is as easy as bringing the angle of attack back to a non-critical angle.
    After that is achieved you can think about the speed again.

    Sometimes full elevator is not enough to recover: if available use the horizontal stabilizer trim or flaps

    Where the speed comes in:
    If you want to have a flight path that does differ from that of any stone that is thrown (and will hit the earth sooner or later) you need a certain speed to create enough lift to counter the gravitational force. In a straight and level flight the total lift force of the aircraft needs to be exactly the aircrafts weight. So at the angle of attack where the airflow is still attached for the most part and the lift coefficient reaches its peak the aircraft only needs very little amount of airspeed to maintain level flight. This speed is calles the (1g - unaccelerated) "stall speed" of an aircraft. The stall speed is a function of the square root of the aircrafts weight...

    While the main wing creates the largest amount of lift, the vertical stabilizer usually creates a downforce. This down force is needed for a stable aircraft. But this downforce has to be compensated by the main wing as well and also creates drag. So in reality the main wing creates maybe about 1.1 times the aircrafts gravitational force.

    Now, if you want to change your flight path, up down or left or right you need a centripetral force to pull your aircraft around the corner. That force adds to the lift required.
    To maintain level flight in a perfect circle at a given bank angle the airspeed needed to create enough lift increases with 1/cos(bankangle) as Overload posted above.
    -> Any acceleration requires extra amount of lift and creates additional drag

    If you just increase the angle of attack to create more lift you might hit the stall angle again. The stall than happens at a speed higher than the 1g "stall speed" of the aircraft which is usually called "accelerated stall". (I am not a big fan of that name because there is only one stall for me. Either the airflow is attached or not ... and the actual stall speed constantly shifts due to the current acceleration).

    To me there is no real point to keeping a close eye on the airspeed. It does not save you from entering a stall because the critical speed is always different depending on the aircrafts weight, trim and acceleration. The stall speed is by far not fix and therefor cannot be a reference point for safe flight for me. My philosophy: You should rather feel the aircraft and pay close attention to the flight path and attitude of the aircraft. This includes thinking way ahead of the aircraft.

    Some words to the deep stall
    A deep stall is a stall where the main wing and the horizontal stabilizer are fully stalled. In a deep stall the elevator shows nearly no response. A reason for that is that the main wings down wash happens to hit the stabilizer and makes it inefficient. Some deep stalls are not recoverable is what I have read. The F16 is a great example here (it has an emergency button to leave a deep stall).
    When in a deep stall and the aircraft has flaps, moving them should be the first thing to try if the altitude allows it. Raising the flaps can move the downwash significantly and bring the elevator pressure back.
    When the aircraft has a horizontal stabilizer trim that moves the whole stabilizer like in most airliners trimming down can help a lot.
    Other techniques are oscillating the aircrafts pitch with monotonous elevator up and down inputs combined with throttle impulses. Also crossing the controls could help (trying to leave the stall to the side), extending airbrakes to change the momentum, lowering the gear and if practical moving the center of gravity forward (fuel pumps, passengers (last option)).

    Fly safe,
    Jan

    Quote from Jet-Pack

    To me there is no real point to keeping a close eye on the airspeed. It does not save you from entering a stall because the critical speed is always different depending on the aircrafts weight, trim and acceleration. The stall speed is by far not fix and therefor cannot be a reference point for safe flight for me.

    Jan,

    That was a very nice dissertation on the subject! I agree with all of what you said. Airspeed is secondary, and there is no single value you rely on except in very standard conditions, like a normal straight-in approach to the runway with a particular flap setting. In that case I suspect you and most pilots do check the airspeed indicator from time to time, and take note how close you are to the stall speed. I mean, that is kind of the main point in a landing (or at least 3-point landings).

    Also, I was speaking of airspeed as a proxy for power -- if you are approaching a stall, reducing the angle of attack certainly helps but so does adding power. This combination is necessary to avoid losing (as much) altitude in the stall recovery: just dropping the nose will eat up more altitude that doing that and adding power. That was my point. If that is wrong, then maybe they could revise pilot manuals...

    Very nice interchange, I was in a very flat spin where one wing tip had some laminar airflow over part of the chord, the ailerons were in dirty air and were doing nothing. When I got back to the flying club the instructors were mildly horrified, at least it was a group owned plane not one of theirs. I might not have come out of it, spins were definitely prohibited in the flight manual, part of the COA.
    Earlier I was especially referring to the rate of descent creating a higher AoA for a given attitude. I think the test crew of the BAC 111 were so busy that they delayed flight recovery to the point where the fuselage and wing roots were throwing turbulent air up at the T tail and normal recovery was ineffective.
    Slotted and blown flaps add another dimension to stall behaviour, the air can be coaxed into going where it would normally do a U turn. I read that the landing speeds of aeroplanes with blown flaps without the blowing is about 25% higher. Is blowing only in military aircraft?

    Hi Jet-Pack, trimming down as in screwing the horizontal stabiliser leading edge down as in trimming nose up?, perhaps with up elevator to unstall the tail to produce ultimately a tail up force when the unstalled tail had down elevator applied? Would down elevator stall it again.
    Did the 727 test pilot recover from it's one and only deep stall by strongly rocking the wings and letting the nose fall out? Hard to imagine swept wings having that much aileron effectiveness in a stall, had it spoilers or an inboard high speed type aileron? Was the wing less stalled than the horizontal stabiliser?

    Overloaded, you and Jan are fast leaving me behind! LOL

    I should say in full disclosure that all I have ever been is an occasional (weekend) aerobatic pilot. I never put in the time to really refine my skills. And believe it or not, I actually have more fun doing aerobatics in flight sims, especially AFS. The planes handle beautifully (and realistically), and while my "virtual" body might have to take a beating from the G-forces, my real body does not!

    I sure learn a lot from following your discussions, which add a lot to the forum, IMHO. I hope they aren't regarded as tangential or off-point, because it is really great to have people with expertise get into the nitty-gritty of the details.

    Regards,

    Adak47

    • Official Post

    Nose down trim so that the tail section creates less downward force. Even in a full stall the airfoil creates lift and by increasing the aoa of the stabilizer at maybe 30-40° aoa you can still produce more lift. The lift coefficient during stall can get even higher than when the airflow is attached! (The drag that comes with it is extreme of course). That is because the pressure difference on the stalled side still is less then sourrounding air pressure.

    In a real deep stall where the elevator is fully stalled and in the dirty air of the main wing that would not really help, maybe it could create enough extra drag to pull the tail back and therfor stabilize the aircraft.... maybe. But if the elevator trim is full up and you cannot get out of the stall, try neutral trim...

    Remember the Air France A340 Flight 447 that crashed into the atlantic ocean? It was in a "deep stall" (we don't actually know if the elevator showed no response because the pilots only applied full up to neutral, the actual output was never "down") with elevator trim far up (trimmed to the last position where the fly by wire system did still allow up trim). The full back stick did not help either but I think they would not have been able to keep the aircraft in a 10° up altitude with full power at 40° angle of attack if the elevator trim would have been moved back to neutral position.
    Now its easy to blame the pilots but they were in turbulent weather at night, thunderstorms around them, stall and overspeed warning at the same time full power and yet a rapid descent with a nose 10° up. But they never tried to pitch down for at least 10 seconds. And they did not try touch the pitch trim even though the airbus produces a "use man. trim" warning on the PFD when in abnormal law.

    Regards,
    Jan

    Quote from Jet-Pack

    Now its easy to blame the pilots but they were in turbulent weather at night, thunderstorms around them, stall and overspeed warning at the same time full power and yet a rapid descent with a nose 10° up.

    On the flip side, sometimes we overlook the role of outstanding aeronautical engineering and systems design that allows pilots to do what they do in dangerous situations. Take the landing by U.S. Airways Flight 1549 on the Hudson after the engines inhaled a flock of geese: the pilot and copilot did an excellent job managing the crisis, but the fly-by-wire system used in the Airbus A320 deserves a lot of credit for the successful ditching. The story is told in the book by William Langewiesche "Fly by Wire: The Geese, the Glide, the Miracle on the Hudson." With a last name like that, you won't be surprised to hear that William Langewiesche, a journalist, is the son of Wolfgang Langewiesche, author of Stick and Rudder (a book discussed in my posts on wheel landings).

    Cheers,

    Adak47