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  #31  
Old 02-09-2012, 05:30 PM
Vac Vac is offline
 
Join Date: Jul 2009
Location: Niceville, Florida
Posts: 522
Default Unload for Control

The basic concept is to simply "unload for control." Perhaps the easiest way to think about the concept is that at zero G, you're simply not asking the wing to do any work (i.e., produce lift). Think back to the basic lift equation that you learned in ground school, the "pounds of lift" side of the equation becomes effectively zero. To establish the condition, you are simply reducing the angle of attack to a very low number. The purpose for neutralizing the rudder and aileron is to simply avoid a spurious input that could impart any yaw. The airplane will continue in a "ballistic" condition and gravity will eventually start to pull it down, then it will accelerate. Once sufficient flying speed is available, it's possible to begin maneuvering again and recover from any subsequent "unusual" attitude. A target G is one way to establish the condition, but a target AOA is even better if you have the proper gauge in your cockpit.

The real trick is to do this before you've lost directional stability, i.e., fall off into a spin, etc. Sometimes, an unload is just momentary and not a complete "knock-it-off and recover" type of maneuver--just enough to get you through whatever maneuver it is you're attempting to do without stalling.

"Unload for Control" was a film that was produced by a young engineer named Burt Rutan, who was part of a crew of folks to help develop handling techniques that could be adopted by F-4 crews to help avoid mishaps. The F-4 has some pretty nasty departure characteristics if you exceed 30 cockpit units of AOA (about 20 degrees of alpha)...and it doesn't recover from a spin nearly as nicely as the RV-4. The F-15 was a much better ride, but it too lost directional stability at high AOA. Even fly-by-wire types suffer excursions...

So that's the bottom line: if you reduce the AOA, the wing can't stall. The zero G condition is just a good seat-of-the pants cue to get you to that low AOA.

Fly safe,

Vac
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Mike Vaccaro
RV-4 2112
Niceville, Florida

Last edited by Vac : 02-10-2012 at 06:10 AM.
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  #32  
Old 02-09-2012, 06:31 PM
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N355DW N355DW is offline
 
Join Date: Oct 2011
Location: Port St Lucie, FL
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Default

Quote:
Originally Posted by RV8R999 View Post
an airfoil only has one lift curve slope with a critical AOA somewhere near the point where the lift curve slope starts to turn negative. This is independent of airspeed. Doesn't matter if you have 1mph or 1000mph the critical AOA will be the same regardless. With any velocity above (or below) zero an AOA exists - by definition. AOA which provides zero lift is the x intercept of this curve... symmetrical airfoils will have zero lift at zero AOA. Any other airfoil will still have lift at zero degrees AOA due to camber and would require negative AOA to achieve a zero lift condition (if positively cambered).
AOA is the angle formed between the cord line and the relative wind, simple as that.

My bad! Hmm, well perhaps I can fly it but not describe it correctly. Even worse I have been instructing incorrectly, so I need to make sure I get this right!

At low air speeds it takes much less G loading than high speeds to get the airflow to leave the wing. And the Pitts I flew did not have completely symmetrical airfoils, but the Extras did. I had assumed that the angle changed as a function of the airspeed, but what you are saying is the critical AOA is the same, only at lower air speeds it is easier to reach this angle with less G. So over the top of the humpty at 25mph when I pull with half a G and buffet, that angle between the chord and the relative wind is exactly the same as when I pull 6 G's at 150 mph and buffet? It's coming back now I hope! The G force results because I am changing the attitude of the aircraft in relation to the direction in which it is traveling, and the higher the airspeed, the higher the force required to change the angle between the chord of the wing and the relative airflow, no?

It was always instinctive for me to unload the wing whenever I felt it get near the critical AOA, either positive or negative. The Extra does have a symmetrical, or very near symmetrical wing.

Thanks for correcting me!
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  #33  
Old 02-09-2012, 08:05 PM
RV8R999 RV8R999 is offline
 
Join Date: Dec 2008
Location: na
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Quote:
Originally Posted by N355DW View Post
My bad! Hmm, well perhaps I can fly it but not describe it correctly. Even worse I have been instructing incorrectly, so I need to make sure I get this right!

At low air speeds it takes much less G loading than high speeds to get the airflow to leave the wing. And the Pitts I flew did not have completely symmetrical airfoils, but the Extras did. I had assumed that the angle changed as a function of the airspeed, but what you are saying is the critical AOA is the same, only at lower air speeds it is easier to reach this angle with less G. So over the top of the humpty at 25mph when I pull with half a G and buffet, that angle between the chord and the relative wind is exactly the same as when I pull 6 G's at 150 mph and buffet? It's coming back now I hope! The G force results because I am changing the attitude of the aircraft in relation to the direction in which it is traveling, and the higher the airspeed, the higher the force required to change the angle between the chord of the wing and the relative airflow, no?

It was always instinctive for me to unload the wing whenever I felt it get near the critical AOA, either positive or negative. The Extra does have a symmetrical, or very near symmetrical wing.

Thanks for correcting me!
I think your statement is essentially correct in that at low airspeeds your stick force per degree of elevator travel will be lower, and so will your pitch rates, also in unloaded or near zero lift flight, your pitch moments are reduced making your elevator control power APPEAR to be more effective as it no longer has to counter the downward pitch moment of the loaded wing. So in the vertical with some airspeed and unloaded, the elevator forces will be lighter, produce less force, but also have less force to counteract the net result is it feels easy to pull through the top.
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