Stalling

[Guest pelorus32]

Here's a little brain teaser. The answers to this are very simple and elegant. The airwork behind this question was conducted in a Tecnam P92 ES but the theoretical reasons behind the answers mean that you would expect similar results in other similar aircraft.

 

The scenario: S&L 3500' trimmed and with revs set for 70 knots IAS. Roll into gentle turn not exceeding 30 degrees. Exert gentle stick pressure to have the speed fall by about 1 knot/second. Don't alter power settings and continue until aircraft stalls. Stall complete at 3800 feet. Repeat in opposite direction.

 

Questions:

 

1. What was the characteristic of the stall when the aircraft stalled? That is what happened in the stall?
    
    
   
2. Was it the same in both directions or different?
    
    
   
3. What is the reason for this?
    
   

 

I was surprised by the answer to 1 and 2 but it was repeatable and assured for me. The reason is really elegant and simple.

 

The scenario was chosen because it roughly approximates the base turn (without the stall of course) scenario.

 

Regards

 

Mike

 

Big Warning: Do NOT assume that your aircraft even your Tecnam will do this. Do NOT bank your life on my experience. Do NOT do this without an instructor if you are unsure.

 

This is placed only as a Brain Teaser.

 

M

 

 

[Guest pelorus32]

Hi Chris,

 

I owe you a reply on this one:

 

Your addendum is interesting. I expected what you report of your recent stalling experience. My understanding is that it's to do with what we lazily call "torque".

 

What I actually got was almost what you report in your first answer. Congratulations:

 

1. As the a/c approached the stall there was a very very slight sensation like water running over your hand. It couldn't be called buffet;
    
    
   
2. Then the a/c briskly rolled level and lowered the nose - returned to S&L flight. At this point I released back pressure to avoid a secondary stall;
    
    
   
3. The response was exactly the same in both directions - the wings always rolled level with no sign of tucking under in a turn to the right;
    
   

 

So why? And here is the trick and the reason that it should come with the warning "don't do this at home folks without having safe altitude and maybe some help along". This was a very specific scenario and the secret to the answer lies in the fact that I gained height in each stall. Because I limited the angle of bank the only sensible way to get a stall was to keep pulling and therefore to generate more lift and therefore to climb. Kermode on pp 239 and 241 of the 9th edition says:

 

On a gliding turn the whole aircraft will move the same distance downwards during one complete turn, but the inner wing, because it is turning on a smaller radius, will have descended on a steeper spiral than the outer wing; therefore the air will come up to meet it at a steeper angle, in other words

 

the inner wing will have a larger angle of attack

 

and so obtain more lift than the outer wing. The extra lift obtained this way may compensate, or more than compensate, the lift obtained by the outer wing due to increase in velocity. (his emphasis)

 

So you've guessed the answer, but to quote Kermode on p 241:

 

In a

 

climbing turn

 

, on the other hand, the inner wing still describes a steeper spiral, but this time it is an

 

upward

 

spiral, so the air comes down to meet the inner wing more than the outer wing, thus

 

reducing the angle of attack on the inner wing

 

. So in this case the outer wing has more lift

 

both

 

because of velocity

 

and

 

because of increase angle, and

 

there is even more necessity of holding off bank than during a normal turn

 

(his emphasis).

 

So there it is - the decreased angle of attack of the inner wing due to the climbing turn leads the outer wing to reliably stall first in this aircraft.

 

More experimenting to come. Don't you love Kermode's sentence structure? BTW: Mechanics of Flight, Kermode A C, 9th edition, nd.

 

Regards

 

Mike