Simviation Forums

can someone give me the 411 on stalling and that such?
My flight instructor asked me the other day why a airplane stalls.  I replied with,  "A airplane stalls when the thrust is not fast enough to keep the airplane afloat"  He stared at me with a half strangled look and told me to look it up online and tell him why they really stall.  :(

btw I am on my 3 hour of flying


spirit

Here, read this. http://travel.howstuffworks.com/airplane.htm
Stuff that everyone should know before getting their hands on an aircraft, IMO.

Wait a second, I just skimmed over that aritcle and it doesn't mention stalls. . Still some good info in there though. I'll try my best to explain it. It all has to do with angle of attack. Lets walk through a controlled stall shall we? flying along at 3000' MSL, you pull the power back to flight idle. Maintain altitude for as long as you can. keep the nose up. do whatever you can to keep from descending. Soon you find yourself descending anyway, and your airspeed is falling all the time. Notice that the slower you go, the quicker you descend. pretty soon you hit that magic number, 56KIAS (in a 1968 C172E anyway) or so and the stall horn goes off. the planes shudders a little, the nose goes over and you increase power and gradually return to level flight. No what happened when the stall horn went off? The wings of the airplane no longer had enough air flowing over them to create lift, so the plane stopped flying and started falling. This would be a lot easier to understand if I had some pictures to show you. I suggest you do an internet search, or try the AOPA site for more info, but I hope my drawn out explanation at least helped. Remember, it is important that you really understand this, don't just quote me, or a book, but really understand it. You will be a safer pilot for it. good luck with your training, I'm about to solo myself.

thanks for adding that info  :)
I had just looked all over there and could not find anything talking about stalls  ;D

You have made it a little clearer I still need to find some more info on it so I will be doing some searches.

another question,   What makes that turbulence you feel when you level out, after going through a 45 degree "or more" bank?

spirit

There are key words your instructor is looking for, the main one being "critical angle of attack"  The critical AoA is the angle at which the air flowing over the top of the wing seperates from the surface in a manner that causes an abrupt decrease in lift.  Air flows in much the same way as water does... if you let the water run out of a faucit and place your fingertip in the stream even at 90 degrees you will notice that it "bends" the stream as it goes around your fingertip.  This is just what air does over the top of the wing.  But, at a certain point the air doesn't have enough energy to make as drastic of a change in direction.  Thus it begins to get turbulent and seperates causing the stall.

AoA as you may know now is the angle between the chord of the wing (drawn from trailing edge to leading edge and the relative wind.  

Thrust only has an indirect relationship with the stall... you can be going mach 2 in your Cessnan 150 and still cause the wings to stall.  (it'll be a very spectacular event to watch from the ground as the wing vaporizes... but theoretically possible)  The faster you go (straight and level) the lower the AoA... the slower you go the higher the AoA is needed to maintain level flight.  Also, the heavier you are the greater the AoA needs to be and vise verse.  This applies to turning because you now have centrifugal force pulling the plane to the outside and are continuously changing direction/momentum.   That's why you will need to add power/airspeed in steep turns.

Next time he asks you say that the wing stalls when the actual angle of attack exceeds the critical angle of attack for that configuration.  Also being able to draw out a sketch of the coefficient of lift graph helps to understand this as well and will really impress your instructor.


Hope it helps and my lack of sleep today hasn't made everything I just said nonsense. :p

The critical AoA is the angle at which the air flowing over the top of the wing seperates from the surface in a manner that causes an abrupt decrease in lift

That's the magical phrase when you're talking about stalls, use it to impress your CFI.  The only thing that causes a stall is the angle of attack passing that critical point where the airflow over the top of the wing seperates and it no longer creates lift, exactly what Boss_BlueAngels said.



A coefficient of drag graph, similar to lift, but it illustates the same thing (as AoA increases after a certain point, drag increases alot)...

This technical stuff about critical angle of attack is all very well in theory. This might impress your flight instructor & be what he/she wants to hear but it's perfectly possible for an aircraft to fly under control & without stalling when the the wing is way beyond what would nomally be regarded as a critical angle of attack. Watch any air display by an aerobatic pilot in something like an Extra or Pitts Special & you will see what I mean. So far I've seen no mention of power to weight ratio.

This technical stuff about critical angle of attack is all very well in theory. This might impress your flight instructor & be what he/she wants to hear but it's perfectly possible for an aircraft to fly under control & without stalling when the the wing is way beyond what would nomally be regarded as a critical angle of attack. Watch any air display by an aerobatic pilot in something like an Extra or Pitts Special & you will see what I mean. So far I've seen no mention of power to weight ratio.

The critical angle is relative to the air-flow across the wing.

Make that 3000agl

He could be flying over the Dead Sea, or Death Valley. o_O

He could be flying over the Dead Sea, or Death Valley. o_O

Yup.. and then his MSL would be less than 3000 (if his AGL was 3000)

Ya still want at least 3000 feet between you and stuff other than air when practicing stalls  ;)

Yes, but if he was at 3000 MSL, he would be at 3000+ AGL, so it would be all right.

Your best reference is airspeed.  If air is ramming the pitot tube fast enough to register above stall speed, then you're flying.

Exactly. Let the airspeed drop below a certain point & you will fall out of the sky whatever angle the wing is at. This will also depend on the aerofoil shape. Most of these things seem to be based on a conventional "lifting" aerofoil section & although the same basic principles apply there are many other types each with their own aerodynamic characteristics.

[quote]The critical angle is relative to the air-flow across the wing.

Some high performance aircraft can do virtually unlimited vertical climbing manoeuvres or hover like a helicopter. They will continue climbing under full control all the time sufficient power is applied so the angle of attack of the wing does not come into it.

Exactly.. If a plane is hanging on it's thrust (or it's prop) it wouldn't matter if the wings fell off (as far as lift goes). They could have stalled long ago.

Let's say a jet fighter has a stall speed of 150kias. If it has enough thrust, it could very well climb straight up at 149.9kias with the wings fully stalled acting ONLY as stabilizing control surfaces.

Exactly.. If a plane is hanging on it's thrust (or it's prop) it wouldn't matter if the wings fell off (as far as lift goes). They could have stalled long ago.

Landing could be a tad on the tricky side...

Let's say a jet fighter has a stall speed of 150kias.

But a stall speed can only be quote for a certain parameter, such as the two most quoted examples, the clean stall in S&L, power off, flight, and "dirty", ie configured (gear and flap), power off.

Any change in an aircrafts velocity will vary the speed at which the wing stalls. But as you said yourself earlier, the angle of attack of the wing is measured with respect to the relative airflow over the wing, hence it is quite possible to fly at all manner of attitudes without stalling. I think some of the confusion is often caused by diagrams where "AoA/alpha" is always drawn relative to the horizontal.

Right... 3000' AGL. I was at the end of a 12 hour shift, mistakes were made. . Mobius posted the type of drawings I was thinking of. It was an illustration like that in a book that really helped me understand stalls. Everyone else explained it much better than I did.