Here we go again.....

I'm afraid so...

Puffmac: Your explanation was just great!!!!!

 

One more question: I got a book for christmas. It said that the AoA increases (before ditching) when we reduce thrust....is it normal?

Puffmac: Your explanation was just great!!!!!

 

One more question: I got a book for christmas. It said that the AoA increases (before ditching) when we reduce thrust....is it normal?

 

Glad my answer was of use!

 

As for the AoA question, I can't think off the top of my head as to why a reduction in throttle would cause a pitch up reaction. What is the name of the book and what type of aircraft is it referring to?

It's in french: " Les bases du pilotage - Jean Zilio".

You can also find it in English!

 

By the way why is weight = lift during climb????

 

If you reduce speed, you reduce lift so you increase AoA, no?

It's in french: " Les bases du pilotage - Jean Zilio".

You can also find it in English!

 

By the way why is weight = lift during climb????

 

If you reduce speed, you reduce lift so you increase AoA, no?

Lift > Weight during the climb, if Lift = Weight the aircraft will neither descend nor climb.

 

 

Lift > Weight during the climb, if Lift = Weight the aircraft will neither descend nor climb.

 

Nope, that's wrong. Lift = weight in a steady (constant VS) climb or descent as well as in level flight.  Lift > weight means that the aircraft will accelerate upwards (increasing VS).

 

Actually, and to further confuse people, Lift < Weight when a powered aircraft climbs.  That's because the aircraft is pitched up, so the engine thrust carries some of the weight.

 

 

If you reduce speed, you reduce lift so you increase AoA, no?

 

Yes that's correct - in some cases at least.   If you reduce speed and keep the pitch attitude of the aircraft constant (ie. hold the nose up), lift is reduced so the aircraft accelerates vertically downwards. This means that the airflow hits the wing more from below, ie. at a higher AOA, so that the lift increases again.  Eventually  a steady descent is achieved, at a slower forward speed and higher AOA so that lift=weight again.

 

This is a potentially dangerous situation because the AOA cannot be increased indefinitely, eventually the wing will stall and lose most of its lift (and the aircraft then accelerates downwards FAST!)  Most aircraft are designed so that the nose drops when the speed is reduced (by making the wing lose more lift than the tailplane), thus keeping the speed high and the AOA low.  So on most aircraft this will only happen if the nose is deliberately kept high by the pilot or automatic systems (including Airbus FBW, which is why it needs all those protections).

Nope, that's wrong. Lift = weight in a steady (constant VS) climb or descent as well as in level flight.  Lift > weight means that the aircraft will accelerate upwards (increasing VS).

 

Actually, and to further confuse people, Lift < Weight when a powered aircraft climbs.  That's because the aircraft is pitched up, so the engine thrust carries some of the weight.

 

I stand corrected. 

Thanks a lot,

 

So our AoA increases. Why? Because we slow down and we lose lift. But why don't we stall? Because at a certain AoA/speed - before stall - the plane will pitch down. Am I right?

But why don't we stall?

 

Well, at cruise speeds AOA is usually about 5 degrees and most wings stall at about 12 degrees (ball park figures here - there is a lot of variation between different wing profiles).  So if you slow down a bit and hold the nose up you won't stall.  Slow down a bit more while holding the nose up and you still won't stall.  Slow down even more and insist on holding the nose up - BAM! 

 

If you let the plane pitch down (or push the nose down yourself) you'll generally be OK, assuming the ground isn't too near.

 

One can't be specific because the pitch-down response to slowing down- longitudinal stability in tech-speak - varies from aircraft to aircraft and depends critically on the position of the CG.  In some designs and under certain conditions the response to nearing the stall may be to pitch UP, which is generally fatal.  The early jet fighters such as the F-100 and the F-104 were (in)famous for this, and led to the invention of the stick-pushers that are installed in modern transport aircraft.

AF330, what plane are you talking about?

 

A C172 may not stall at all, but airliners usually would, if no countermeasure were taken and, without countermeasure, some of them might even enter a "deep stall".

 

So here is your next topic ( ): "What is a deep stall?"  

Ha ha!

Thanks!

 

If you could give the answer for a Cessna and an Airbus 320, it would be great!

 

But if we just put thrust to Idle and if we don't touch the F-CTL's can we stall? Why?

Well, at cruise speeds AOA is usually about 5 degrees and most wings stall at about 12 degrees (ball park figures here - there is a lot of variation between different wing profiles). So if you slow down a bit and hold the nose up you won't stall. Slow down a bit more while holding the nose up and you still won't stall. Slow down even more and insist on holding the nose up - BAM!

 

If you let the plane pitch down (or push the nose down yourself) you'll generally be OK, assuming the ground isn't too near.

But here, you pull on the stick. What if you don't pull on the stick? It depends? It can stall (if our AoA before thrust reduction was already high), isn't it? It can also not stall if the plane starts falling and pitches down directly....am I right?

 

 

What if you don't pull on the stick?

 

Read my second paragraph.  In most cases,the aircraft won't stall if the power is cut without touching the stick. But as always "it depends" and if the natural pitch-down response is insufficient (eg: CG too far aft, or as you say you're already too slow) then you CAN stall.

 

On an Airbus the FBW will keep the nose high even if you don't pull the stick.  However in Normal Law the control laws will push the nose down as the stall AOA is neared (alpha prot).

Ok, thanks!

 

So olli tends to say that an Airbus/Boeing will stall...

 

Ok, I am in an A320 - Alternate law.

But when you slow down, you lose lift, so your AoA increases. So if my AoA is of 8° and I reduce thrust, I will have more chances to stall. If my AoA is of 4° and I reduce thrust, I will have less chances to stall. If my AoA is near Stall AoA before thrust reduction, I will have more chances to stall if I reduce thrust. If my AoA is far stall AoA before thrust reduction, then I can stall when I reduce thrust but my plane can also start pitching down and take back lift.

 

Do you all agree with me?

 

Thanks