Author Topic: Speed stall ? (Read 609 times)

Saintaw · « **on:** November 20, 2002, 09:13:09 AM »

I've been wondering...

When you're flying fast (400kts+) and you're atempting a tight turn (be it with or without full throttle applied) there's this "effect" that kicks in , and one of your wing drops (I have this happen often in Dora or in P51's) as if it no longer had air to hang onto... what is it called ? I figured it must be some kind of stall, but...

Could anyone please point me to a document that explains this, thank you

Shane · « **Reply #1 on:** November 20, 2002, 09:22:24 AM »

it's referred to as an "accelerated stall" where the airflow over one wing is abruptly disrupted. it's differentiated from a snaproll i guess by the speeds involved. - or maybe it's the same?

dunno about any documents that will shed light on it, but it's a documented effect.

flakbait · « **Reply #2 on:** November 20, 2002, 10:16:50 AM »

Yup, it's an accelerated stall. At 400+mph the high-speed airflow over the wing is moving smoothly. When you increase AoA (angle of attack) you also change the airflow, so it's not as smooth. Once you stop altering the AoA, it smooths out again. When you change AoA so fast that the air can't "stick" to the wing, it boils along the trailing edge. Just like when there's not enough airspeed. Change it even faster and you get the air boiling along the entire back-half of the wing. Which is what happens when you get into an accelerated stall. Propwash alters airflow even more, which is why one wing stalls before the other one. When all this happens at high speed, you usually get a hard snap-roll when the wing stalls out.

Or as someone else put it....
"Doc, it hurts when I do this"
"Then don't do that!"

-----------------------
Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High
"For yay did the sky darken, and split open and spew forth fire, and
through the smoke rode the Four Wurgers of the Apocalypse.
And on their canopies was tattooed the number of the Beast, and the
number was 190." Jedi, Verse Five, Capter Two, The Book of Dweeb

Saintaw · « **Reply #3 on:** November 20, 2002, 10:53:13 AM »

So, the thing to do (as well as the obvious "release pressure on stick") would be to chop throttle & avoid banking, correct ?

Flaps help too, but I can manage to get some flaps out at one point if I'm in a P51 or a P38 alone ... that's a big no no for FW190's...

Sparks · « **Reply #4 on:** November 20, 2002, 03:22:01 PM »

Yes its called an accellerated stall.

Basics are:-

1. for any aerofoil (read wing) there is a critical angle of attack (i.e. the angle the wing is to the airflow) that the wing will stall - basically thats where the air can no longer follow the upper surface of the wing and separates. This angle is the same no matter what the speed of the airflow

2. As the angle of attack of an aerofoil increases so the lift increases up to the stall.

3. To get a wing unstalled you HAVE to lower the angle of attack.

So at slow speed you need high angle of attack to create enough lift to hold the aircraft up.

At HIGH speed in a turn you need a high angle of attack to create enough lift to pull the aircraft round the turn (banked aircraft = lift points to the centre of the turn circle ..... more lift = tighter turn)

SOOOOO - in a high speed turn you can get to the critical angle of attack by pulling hard and the wing stalls.

Which wing drops depends on which wing stalls first which depends in RL on control positions, slip, skid etc. I don't think AH is modelled that accurately.

Answer to any stall is to lower the angle of attack i.e. push stick forward. In a high speed turn accelerated stall easing off the back pull on the stick is usually enough.

I have used this as an evasive in desparate situations (I find F6 is real good at this) - in a tight turn with someone on your 6 - yank back on stick - a/c stalls and drops a wing - immediately push forward and recover - achieves a 1/2 or 3/4 snap roll reversal

Hope that helps

Sparks

Sparks · « **Reply #5 on:** November 20, 2002, 03:27:27 PM »

Oh you mentioned Flaps - they work because extending the flaps changes the camber of the aerofoil - you are basicaly changing the aerofoil shape and so it has a different stalling angle - usually increased

.

Sparks

PS. the stall warning horn is the que to a wing drop no matter what the speed

flakbait · « **Reply #6 on:** November 20, 2002, 03:27:42 PM »

Banking has nothing to do with an accelerated stall, except to partially control which way you flip. And even then sometimes it'll still whip the other way. Throttle can be left right where it is, or chopped, it doesn't really matter. Just don't pull so hard on the stick as to cause one and you avoid it entirely. Flaps help by slowing you down from the excess drag plus they generate extra lift. It can delay an accelerated stall long enough to either recover or maneuver for a shot. One "gaming the game" item a few folks know is this: when the aircraft wants to whip into a snap-roll from an accelerated stall, you can add opposite rudder to slow or prevent one from happening. It doesn't always work, but it has saved me from whacking a few trees so far!

-----------------------
Flakbait [Delta6]
Delta Six's Flight School
Put the P-61B in Aces High
"For yay did the sky darken, and split open and spew forth fire, and
through the smoke rode the Four Wurgers of the Apocalypse.
And on their canopies was tattooed the number of the Beast, and the
number was 190." Jedi, Verse Five, Capter Two, The Book of Dweeb

Saw at Work · « **Reply #7 on:** November 21, 2002, 08:27:27 AM »

mhhh me understand... "not pull stick so hard"

Thx for the explanation guys

This even makes sense to me... the 190 has a smaller wingload.

SLO · « **Reply #8 on:** November 21, 2002, 01:06:02 PM »

NO no no no no....keep pulling harder till the dweeb following u over shoots:D

think of it as a super fast vertical split S:eek: ....ina dive

dweeb is GUARANTED to over shoot.

dweeb is gonna think you gonna dive because of roll action...but d9 snaps right back very quickly.

Blue Mako · « **Reply #9 on:** November 21, 2002, 11:57:13 PM »

The pony is especially prone to accelerated stalls because it has a laminar flow wing. This means that the aerofoil is designed to have a smooth ("laminar") airflow over the wing that gives low drag. The down side is that if you try to change AoA too quickly (as already noted) the airflow over the wings separates from the surface and becomes turbulent, causing a sudden loss of lift (ie. an accelerated stall). Because this usually happens assymmetrically (prop planes almost never have the same airflow on each wing) one wing will stall before the other, causing a sudden wing drop and usually a departure into a spin. All clear as mud? Good

There are two ways to avoid an accelerated stall:
1) Don't pull back on the stick so hard
2) Pull back smoothly to the max pressure you want

You will find that you can pull a high g load (and correspondingly high AoA) with the pony and dora at high speeds but only if you are smooth with your control inputs. I quite often play TnB (at high and low speeds) with my pony and rarely have the problem of accelerated stalling by keeping my inputs smooth.