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What Point Control Surfaces Ineffective  
User currently offlineSean1234 From United States of America, joined Aug 2000, 411 posts, RR: 0
Posted (8 years 4 months 3 weeks 4 days 19 hours ago) and read 2861 times:

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?

12 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (8 years 4 months 3 weeks 4 days 18 hours ago) and read 2849 times:

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



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 2, posted (8 years 4 months 3 weeks 4 days 17 hours ago) and read 2833 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineBobster2 From , joined Dec 1969, posts, RR:
Reply 3, posted (8 years 4 months 3 weeks 4 days 14 hours ago) and read 2807 times:

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]

User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 4, posted (8 years 4 months 3 weeks 4 days 13 hours ago) and read 2796 times:

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."
User currently offlineKlaus From Germany, joined Jul 2001, 21521 posts, RR: 53
Reply 5, posted (8 years 4 months 3 weeks 3 days 16 hours ago) and read 2730 times:

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!


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 6, posted (8 years 4 months 3 weeks 3 days 15 hours ago) and read 2716 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 7, posted (8 years 4 months 3 weeks 13 hours ago) and read 2647 times:

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.


User currently offlineWoodreau From United States of America, joined Sep 2001, 1053 posts, RR: 7
Reply 8, posted (8 years 4 months 2 weeks 3 days 2 hours ago) and read 2557 times:

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.


Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 9, posted (8 years 4 months 2 weeks 3 days 1 hour ago) and read 2549 times:

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



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineLiedetectors From United States of America, joined Jul 2005, 360 posts, RR: 0
Reply 10, posted (8 years 4 months 2 weeks 3 days ago) and read 2541 times:

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.


If it was said by us, then it must be true.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 11, posted (8 years 4 months 2 weeks 3 days ago) and read 2540 times:

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.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 12, posted (8 years 4 months 2 weeks 2 days 21 hours ago) and read 2515 times:

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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