Angle of attack

[Bill Greenwood]

Bill we are not talking about alieron stall, any more than we are talking about deploying air brakes. We are talking about the differences in stall angles with and without flaps. We did not specify, but obviously we have to mean in level flight, not banking or pulling gs, so that the only difference is the flaps.

As I said, I don't have my book handy now, but will look at it next time I am home, and I could certainly be wrong. Glad we did not wager a bottle of Dom Perignon!

And I am just Bill. My Dad was "Mr. Greenwood"

And now I probably won't even be able to make a normal landing again.

[WLIU]

If I can offer some points, I will suggest that a lot depends on what type of flap you have on your aircraft.

All flaps, and drooping ailerons change the camber of the wing. So the Coefficient of Lift (CL) curve changes.

Split flaps are mostly drag but do move the max lift to a lower speed.

Fowler flaps first increase the wing area as they are extended, and then they increase wing camber with further deployment. This is a big change to the lift characteristics of the wing. Max lift moves to a lower speed first because the wing area increases, and hence the wing loading decreases, then it moves to an even lower speed as the wing camber is changed.

Do not confuse the change in the Lift-Drag curve of the wing with a change to the critical angle of attack. For each new wing configuration, caused by a change in the flap position, you have a new Lift-Drag curve that describes the wing. That new graph may or may not move the max lift point in relation to the AOA axis of the graph.

Now a fully functional AOA display is run by an air data computer that has flap position as an input and changes its calculation of AOA as the flap position changes.

The AOA boxes discussed above apparently provide AOA with zero flaps deployed, and maybe AOA with full flaps deployed. So most users are relying on the no flap calibration to keep them out of trouble in the landing approach. The assumption, a valid one, is that when flaps are extended, the installed indicator calculates the same or a higher AOA than the wing is actually flying, which gives them more margin between what the pilot thinks is the AOA and the actual AOA. Make sense?

Fly safe,

Wes
N78PS

[Bill Berson]

Bill we are not talking about alieron stall, any more than we are talking about deploying air brakes. We are talking about the differences in stall angles with and without flaps. .

Kershner's Student pilot manual has a graph to explain flap stall angle. He says: " increasing the angle of attack in the flapped condition would reach its critical angle of attack (stall) well before the clean wing would have problems".

It is true most pilots don't know or think much about this and survive, but some do not. Langewiesche does devote three pages to AOA indicators (page 75). He says the main benefit is training, since most pilots hardly think about angle of attack. His entire book is actually an attempt to explain angle of attack in detail.

Wes,
Yes, each flap is different. Fancy flaps with slats may allow much more angle of attack.

[Bill Greenwood]

I don't know about this Langewiesche guy anyway. Have any of us seen his birth certificate?
His name sounds foreign, faintly German, maybe he was a Luftwaffe plant to confuse our aviators back then. He sure ain't no home grown simple bike mechanic like our real Amerkin Wright boys.

I have it on good authority that he was the aerodynamics expert at Cirrus in charge of making their planes handle so well at the critical stall point. That's why they ended up with parachutes.

[Ron Blum]

There are so many statements that need to be "adjusted" to be correct in this post.

1. Stall AOA is constant (except in high altitude airplanes where the stall AOA decreases slightly with Mach number) for EACH configuration. In other words, each flap setting has a different stall angle of attack.

2. Aircraft (or body or fuselage) stall AOA does DECREASE with increasing flap deflection - because actual airfoil camber is increased, actual chord line changes, zero lift angle decreases (more negative), etc. But, yes, CL is higher and stall speed is lower with increasing flap deflection (to a point).

3. It was also mentioned that with some flaps, the wing area is increased, too. Although this is true, the wing area (or chord length) used for calculations is not changed, and therefore, the wing CL is "falsely" larger (i.e. not really an apples to apples comparison).

4. All AOA devices measure local AOA (which is not necessarily aircraft AOA). In other words, if AOA devices are mounted on the left and right wing tips, they will measure different values as the airplane is rolling.

5. Be aware of AOA devices that measure/display a simple (two port) differential pressure. This is NOT AOA. Here's why. Think of a simple airfoil in a wind tunnel. Put the airfoil at a near stall angle of attack (say 12 degrees). With no airflow, the differential pressure between the top and bottom is zero. Increase airspeed to 30 knots, and the differential pressure goes to 1" of water. Increase airspeed to 60 knots, and the differential pressure goes to 4" of water. Increase airspeed to 120 knots, and the differential pressure goes to 16" of water. The AOA hasn't changed.

6. Certified airplanes (part 23 & 25) are no longer allowed to use a stall warning that is only visual. Piper did years ago, but the regulations have been carified to prevent this. The reason for this is what has been stated in this forum - the pilot should be looking outside ... that is why horns and stick shakers are used.

7. As for "real" pilots being able to fly by the seat of their pants, this may be true ... in some airplanes, when G-loads aren't introduced. This also ASSUMES that the airplane will give you a warning before it stalls. Newer airplanes/airfoils which are higher performance may not be designed to do this ... especially those with laminar flow wings.

Questions, comments and criticisms are greatly apprciated.

PS. Most certificated airplanes that use AOA for stall warning use normalized AOA, and it is also incorporated into the airspeed tape, but they still have a warning horn or stick shaker (non-visual warning)

[Bill Berson]

Well said Ron,
I suppose ice on the leading edge would be classified as "new configuration"

[Bob Dingley]

I don't know about this Langewiesche guy anyway. Have any of us seen his birth certificate?
His name sounds foreign, faintly German, maybe he was a Luftwaffe plant to confuse our aviators back then. He sure ain't no home grown simple bike mechanic like our real Amerkin Wright boys.

I have it on good authority that he was the aerodynamics expert at Cirrus in charge of making their planes handle so well at the critical stall point. That's why they ended up with parachutes.

Bill, the plot is deeper than even you can imagine.

I read a re-print of an old-old news item from the Portland (Maine) Press Herald from WW2. My old stomping grounds. The article reported that a brand new Vought Sikorsky F4U made a dead stick landing near the boardwalk at Old Orchard Beach. The paper interviewed the test pilot, Wolfgang Langewiesche. He said that he departed Bridgeport, CT and was putting some time on this new bird when the engine quit, so he put it on the beach in front of that joint that sells those great lobster rolls. They also interviewed a nearby clam digger crew that witnessed the whole thing. They say he had the optimum angle of attack all the way down. No damage.

The clam diggers further opined that "he swiped the plane and was headed for those French islands off the Maine coast. You know, St.. Pierre and Miquilon." They were certain that he intended to give it to any Vichy authorities there." After all, he had an accent!" It may have even been a birdcage model.

Bob

[steveinindy]

7. As for "real" pilots being able to fly by the seat of their pants, this may be true ... in some airplanes, when G-loads aren't introduced. This also ASSUMES that the airplane will give you a warning before it stalls. Newer airplanes/airfoils which are higher performance may not be designed to do this ... especially those with laminar flow wings

Would mass balancing the ailerons or elevator to avoid flutter negatively effect the "feel" that so many of the older pilots talk about relying upon? I would wager a guess that they would be less likely to clatter or vibrate in disrupted airflow.

PS. Most certificated airplanes that use AOA for stall warning use normalized AOA, and it is also incorporated into the airspeed tape, but they still have a warning horn or stick shaker (non-visual warning)

Would this be why we see the certificated planes using the "vane-type" AoA systems rather than the ones you see listed in Aircraft Spruce?

[Matt Gonitzke]

Would mass balancing the ailerons or elevator to avoid flutter negatively effect the "feel" that so many of the older pilots talk about relying upon? I would wager a guess that they would be less likely to clatter or vibrate in disrupted airflow.

My glider has an all-flying V-tail that for obvious reasons is mass-balanced, and it has much better feel than many other aircraft I have flown. Thermals will shake the stick a little bit, which is something I have not really noticed in other aircraft.

[steveinindy]

My glider has an all-flying V-tail that for obvious reasons is mass-balanced, and it has much better feel than many other aircraft I have flown. Thermals will shake the stick a little bit, which is something I have not really noticed in other aircraft.

Interesting.

I have debated a trimmable horizontal stabilizer for my design to do away with the need for trim tabs on the elevators at least. I am also going with a mass balanced set of ailerons and rudder. Such is one of the advantages (and issues!) that comes with a clean sheet design.