Dynamic Stall

[mohr]

Hi

Someone can say to me how a pilot could induce to his airplane plane in a process of dynamic stall ?. I know it is very diferent
to a "normal (slow) stall maneuver".
I am trying to interpret correctly this phenomenon from the aerodynamic point of view.

Best regards.

 

[MiketheEngineer]

Often called a "High Speed" stall. Regardless of the speed, if the angle of attack exceeds the wings ability - the smooth air flow will break away from the wing and go completely turbulent over the wing - causing a stall.

I have done it a few times - not nice but easy to recover. Just lower the angle of attack and you are back in business.

I think that is what you are asking...

 

[kontiki99]

Power on stall is a very common training maneuver.

In a 172, from memory, get to safe altitude, make clearing turns.
Slow to somewhere around initial take off speed, maybe 60 kts
Apply full power and pull back the yoke until it stalls.

Sometimes you use some flaps so it stalls at a less steep climb angle.

 

[MiketheEngineer]

99 has the maneuver down.

Done it many times in a Cessna 152. Actually was able to fly "backwards" in about a 40 mph head wind.

Full flaps, full power, pitched up at about 45 degrees or so it seemed - probably more like 15 or 20. It just "hung" there and when I referenced the ground I was going backwards.

Of course, just about then the head wind died and I dropped off into full stall and spun it - just for fun. Spins are fun in 152 - more than two is considered illegal and dangerous.

 

[berkshire]

What Konti is describing is regular stall.
Mike hit it when he said a high speed stall.
Imagine flying at say 70 or 75 kts, starting into a steeply banked turn and hauling the stick way back. That aircraft is going to rumble and buffet and may even flip out into an over the top spin entry as the inside wing stalls. Relaxing the pressure on the stick allows the wing to get back to its more normal angle of attack.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[MiketheEngineer]

berk -

Imagine doing that at 1000 mph in an F-15. I am sure some test pilots and newly appointed F-15 drivers have done it... probably only once!!

Must be a lot of fun!!

 

[berkshire]

I hope he can grunt good when he does that, that is going to pull a LOT of gees.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[kontiki99]

This thread makes me realize I have absolutely no idea what a "dynamic" stall is. Is there a static stall?

When I went for the private license, we did turning stalls too.

I fly with some experienced aerobatics pilots. They often remind me you can stall at any airspeed.

Turning increases the wing loading. In a turn the wing lift counters gravity acceleration and centrifugal forces, hence wing loading goes up so stall speed goes up.

From memory, in a 60 degree turn wing is pulling 2gs, stall speed increases by SQRT of 2 to an airspeed about 1.141 x stall speed at 1G.

Of course that is probably nothing compared to a turning stall executed the transonic region of the flight envelope.

So, I have no idea what we're talking about here, but thanks. I will look into for my own edification.

Unfortunately, the recent flooding overwhelmed a levee immersing M01 under 5-6 feet of water, so the community of pilots I see regularly for recreational flying is somewhat dispersed right now.

Not a day goes by without some question popping up, or that I recognize some subject I once felt comfortable with needs review.

 

[rb1957]

i think (FWIW) that there's a low speed stall ... 1g straight and level, flying slow, using AoA to generate lift) untill the airfoil separates from the wing (upper surface) and the wing stalls.

i can see that there is also a high speed stall, belting along like the clappers. this would imply you're in a loop or a turn generating "x"g. the wing is generating lift from velocity (ie we're way about 1g stall speed) and AoA. something like the maximum lift possible is CLmax*qmax*S (qmax = 1/2rho(Vmax)^2) = x*W. increasing the AoA will cause the wing to stall (again) but in quite a different aerodynamic situation. but to my mind this sounds like a pretty dangerous thing to do ... i'm not sure that planes (other than aerobatic) are designed for this type of load. what do the numbers for a 172 say "x" is ??

 

[berkshire]

The one thing related to dynamic stall, that is of more interest to engineers is that of the aircraft flying into a sharp edged gust ( I believe the FAA rates this under part 23 as 26feet per second.)
Quoting RB1957 the maximum lift possible is CLmax*qmax*S (qmax = 1/2rho(Vmax)^2) = x*W.
If the angle of attack is exceeded the wing stalls. If the angle is not exceeded the aircraft can sustain G loads beyond its design case. This is why you have a flight envelope diagram

http://www.access.gpo.gov/ecfr/graphics/pdfs/ec28se91.001.pdf

showing where the wing will stall and at what speed and angle of attack. Worst case senario would be full up elevator at VNE in a lightly loaded aircraft.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[MiketheEngineer]

In all GA aircraft there is a Ne (Never Exceed speed) and I can't remember the lower speed name - but it basically says no abrupt maneuvers above this speed.

 

[rb1957]

@mike, Vmo = max operating, usually limited by control authority

@BE, a typical flight envelop has a low speed stall curve (Va, Vb) and at high speed is limited to the loadfactor specified, whihc usually has a lower AoA ... maybe taking the ultimate load of this covers you exceeding the design spec, another buffer between the real world and the analytical one is the design analysis works up to Vd, but the real world is limited to Vne (something like Vc). if someone wanted to crank it up to CLmax at Vne they'd be generating a lot of gees.

 

[berkshire]

rb,
Some years ago I helped with a load test where we loaded a wing to 11 G which was the calculated max load for full up elevator at VNE. This was a fiberglass wing and the resultant curve looked like a fishing pole. The wing passed that test. Then we got into it with the FAA because the EMDO rep that witnessed the test said " Well what if the actual CL is higher than you have calculated?"
B.E.


The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[rb1957]

good grief ! ... how much control force is needed for that ? at Vd kimit elevator deflection is only a couple degrees ...

 

[kontiki99]

I grabbed these from FAA-H-8083-3A which is available on the www.

I think this is what Mike is talking about ->

VNO—Maximum structural cruising speed,
represented on the airspeed indicator by the
upper limit of the green arc. It is, however,
permissible to exceed VNO and operate in the
caution range (yellow arc) in certain flight
conditions.

VNE—Never-exceed speed, represented by a red
line on the airspeed indicator.

I don't really understand that graph, but I think what berkshire is referring to is wehat Ive seen called ->

VA—The design maneuvering speed.
This is the “rough air” speed and the
maximum speed for abrupt
maneuvers. If during flight, rough air
or severe turbulence is encountered,
reduce the airspeed to maneuvering
speed or less to minimize stress on the
airplane structure. It is important to
consider weight when referencing this
speed. For example, VA may be 100
knots when an airplane is heavily
loaded, but only 90 knots when the
load is light.

Essentially, in moderate to heavy turbulence you slow to this speed, and just maintain attitude. Reducing speed means wing stalls at lower load. Aerodynamic stall is better than breaking wing. My retired pilot friend also calls it turbulence penetration speed.

I believe the King Videos say to reduce Va by 5% for every 10% decrease in the gross wt used to calculate Va in a 172. I need to review that.

 

[berkshire]

RB
The stick force per G was not very high on that aircraft, but I had to go back into my records to look for that, it required a rearward force of 73 lbs at the end of the control column.

Konti
Now you are getting it.

B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[MiketheEngineer]

The worst case of Vne I have heard of occurred to an F-15 pilot. He was cruising along at about 1,000 mph. I don't why or how but there was a life raft in the rear seat. No pilot.

Somehow it activated and slammed the stick forward.

The F-15 came completely unglued. Unfortunately, the pilot did not survive - like "duh" - a completely un-intentional ejection at 1,000 mph.

How they ever figured that out - I don't know - but that was the official cause.