sean1234
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What Point Control Surfaces Ineffective

Sat Aug 05, 2006 5:17 am

A few questions:


At what altitude are the control surfaces of an aircraft unable to function to effectively manuever an aircraft? What is the point where one risks drifting out into space without proper control authority?
 
FredT
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RE: What Point Control Surfaces Ineffective

Sat Aug 05, 2006 5:36 am

There isn't one point where it happens.

Roughly, the forces generated by an aerodynamic surface is proportional to the dynamic pressure, which is proportional to the density of the air and the square of the velocity. Fly faster, and the same forces are generated even though the density goes down.

You have to draw a line somewhere, but there is no exact physical line above which you are no longer in the atmosphere. It is merely something we define.

Cheers,
/Fred
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Starlionblue
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RE: What Point Control Surfaces Ineffective

Sat Aug 05, 2006 7:13 am

Mr T is correct in that there is no real border. It mostly depends on the aircraft. There are two ways of keeping control while flying very high: higher speed or larger wings/control surfaces:
- High speed: X-15, SR-71. And even the X-15 used attitude jets at high altitudes.
- Larger surfaces: Myasishchev M-55, U-2.
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Bobster2
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RE: What Point Control Surfaces Ineffective

Sat Aug 05, 2006 9:41 am

Reminds me of the great story about Chuck Yeager as told in "The Right Stuff". He got his NF-104 too high for control surfaces to have authority, but the atmosphere was still to dense to allow the use of the thrusters that were intentionally designed to produce very low thrust, and it was around 109,000 feet. He went into a flat spin and bailed out. I'm not sure if the book's explanation is completely correct, it's possible that one of the attitude jets simply failed.

[Edited 2006-08-05 02:57:00]
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Starlionblue
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RE: What Point Control Surfaces Ineffective

Sat Aug 05, 2006 10:37 am

Quoting Bobster2 (Reply 3):
I'm not sure if the book's explanation is completely correct,

The incident did happen. But the book and movie made the events leading to it much more dramatic (if that's possible) than they really were. Yeager's autobiography has a more accurate account.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Klaus
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RE: What Point Control Surfaces Ineffective

Sun Aug 06, 2006 7:56 am

Quoting Sean1234 (Thread starter):
What is the point where one risks drifting out into space without proper control authority?

You wouldn't "drift into space" at any normal speed... escape velocity for earth's gravitation is quite a bit higher than any past, current or planned aircraft with air-breathing engines could ever attain.

So no real risk there... you'd always get down again!
 
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Starlionblue
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RE: What Point Control Surfaces Ineffective

Sun Aug 06, 2006 8:47 am

Quoting Klaus (Reply 5):
Quoting Sean1234 (Thread starter):
What is the point where one risks drifting out into space without proper control authority?

You wouldn't "drift into space" at any normal speed... escape velocity for earth's gravitation is quite a bit higher than any past, current or planned aircraft with air-breathing engines could ever attain.

So no real risk there... you'd always get down again!

Indeed you would. The risk comes with bad attitude control. Let's take SpaceShipOne. Nowhere near fast enough to reach orbital velocity, but still needing the shuttlecock trick to put it in the right attitude for re-entry. Tumbling is a bad thing for heat, structural integrity and crew health reasons.
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N231YE
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RE: What Point Control Surfaces Ineffective

Wed Aug 09, 2006 10:48 am

It depends on the airplane, but most notably is the airplane's CLmax (Maximum angle of attack), VS0 (slowest flying speed with flaps and gears extended [or as pilots call it, "Vee S**t out"]), VS1 (slowest flying speed with everything retracted), or any combination.

I can't be sure about military/space aircraft, but as far as commercial aviation (including General Aviation), the ailerons of sub-sonic aircraft lose their responsiveness as the aircraft reaches stalling speed, either VS0 or VS1, depending on how you define the airplane's condition. As the aircraft reaches that point, the controls become weak, or "mushy" as we pilots call them. This is because the airflow over the wings is separating and becoming turbulent, and smooth air over the wings (besides under them too) is not only needed for lift, but for good control too. You can pretty much picture the ailerons trying to work with air that is "gurgling" over them.

If the aircraft is stalled, it is not flying, similar to an aircraft on the ground, the surfaces won't do much.
 
Woodreau
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RE: What Point Control Surfaces Ineffective

Sun Aug 13, 2006 9:51 pm

One time waiting for a tow in a glider, I discovered I had roll control while still stationary on the ground when there was a constant 10kt headwind. I was able to pick up my wing and balance the glider on the single centerline wheel with just the ailerons. I thought it was pretty neat anyways.
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FredT
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RE: What Point Control Surfaces Ineffective

Sun Aug 13, 2006 10:54 pm

Quoting N231YE (Reply 7):
the ailerons of sub-sonic aircraft lose their responsiveness as the aircraft reaches stalling speed

If the aircraft is well designed, you have aileron authority well below stall speed. Great care is normally taken to ensure this. You normally do not want aircraft so snap over a wing when they stall. Most aircraft can be made to drop a wing in the stall, but you really do a lot to make sure that loss of aileron efficiency is not the first thing to happen in the stall. No different for supersonic aircraft BTW.

Notable exceptions include the Boeing B17, which did lose aileron efficiency when stalling. Wing shape has a lot to do with it. One of the best tricks to get around it is to have the wing twisted by design, so called 'washout', in order to keep the wing tips (where the ailerons are) at a slightly lower AoA than the wing root, or adding roll spoilers. The fairing at the root of the North American P51 wing is another example of an added design feature which helped to retainin roll controll when stalling.

Cheers,
Fred
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liedetectors
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RE: What Point Control Surfaces Ineffective

Sun Aug 13, 2006 11:40 pm

I dont know about an altitude, but I know if you let a T-tailed airplane get into too high of an AOA situation, the wing can block the airflow over the tail, making it ineffective, resulting in an uncontrolable deep stall.
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SlamClick
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RE: What Point Control Surfaces Ineffective

Sun Aug 13, 2006 11:55 pm

Quoting Sean1234 (Thread starter):
At what altitude are the control surfaces of an aircraft unable to function to effectively manuever an aircraft?

At about fifty feet in this picture.

http://www.historylink101.com/ww2photo/fighter-crash.jpg

Note large amount of LEFT rudder, UP elevator and right wing DOWN inputs. I have it on good authority that none of these commands were executed during the remainder of this flight.
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N231YE
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RE: What Point Control Surfaces Ineffective

Mon Aug 14, 2006 2:49 am

Quoting FredT (Reply 9):

That is correct. However, the ailerons' responsiveness does get very "mushy" at high AoA/Low speeds. They do retain control as you stated, but much force and almost extreme control is used to make a turn that could normally be done with a "tap of the yoke" at normal speeds / normal attitude.

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