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tu144d
Topic Author
Posts: 190
Joined: Wed Apr 24, 2002 11:39 pm

Adverse-yawing Moment At High Speeds

Fri Feb 28, 2003 7:46 pm

Today, in my flight mechanics class we were talking about how banking an aircraft causes an adverse yawing moment due to the imbalance of induced drag caused by difference in angle-of attack of the wings. However, my prof mentioned that in supersonic aircraft, this isn't really a problem. Unfortunately, the bell rang and i didn't get a chance to ask why? This is my theory. Since at high speeds, Dynamic pressure is dominant, CL is very low and since Induced drag depends on kCL^2 it is very low. Thus there is little if any imbalance in drag, and thus not enough to create a strong moment about the cg. Is this correct?

-Vik-
 
aeroguy
Posts: 66
Joined: Wed Aug 15, 2001 1:33 am

RE: Adverse-yawing Moment At High Speeds

Sat Mar 01, 2003 3:17 am

I sort of hope your professor didn't explain adverse yaw by saying the wings have different angles of attack. Consider the lift generated by the wings in a left roll. The right aileron is deflected down, the left is deflected up. The extra lift generated by the right wing due to the aileron deflection will give you extra drag on the right wing. The upward deflected left aileron will give even less lift and drag on the left wing, yielding a yawing moment to the right (adverse yaw).

Anyway, the key to answering your question lies in the fact that supersonic aircraft have swept wings. The main concept here is the sweep angle of the wing relative to the incoming air during yaw. Perhaps it will help to draw/visualize a swept wing aircraft from the planform perspective which is yawing to the left. The incoming air is essentially encountering a wing of less sweep on the left side than on the right. (Let that sink in a second). Therefore, the left wing produces more lift (and thus drag) than the right for a given angle of attack. So the resulting yawing moment is nose left (proverse yaw).
 
avioniker
Posts: 1100
Joined: Tue Jan 01, 2002 5:38 am

RE: Adverse-yawing Moment At High Speeds

Sat Mar 01, 2003 4:58 am

Here's a website that'll put it in laymans' terms pretty handily

http://www.dfrc.nasa.gov/trc/ftintro/roll/nasroll.html
One may educate the ignorance from the unknowing but stupid is forever. Boswell; ca: 1533
 
tu144d
Topic Author
Posts: 190
Joined: Wed Apr 24, 2002 11:39 pm

RE: Adverse-yawing Moment At High Speeds

Sat Mar 01, 2003 7:39 am

Thanks for your replies. DOn't worry "Aeroguy"...the angle of attack thing was my mistake, not my profs...Actually when i was posting it at 3AM it didn't make sense to me, but i still wrote it down foolishly. I was confusing it with "weathercock" stability when the winds is coming at at an angle.

-Vikram-
 
tu144d
Topic Author
Posts: 190
Joined: Wed Apr 24, 2002 11:39 pm

RE: Adverse-yawing Moment At High Speeds

Sat Mar 01, 2003 9:41 am

Actually, I have another question with regards to ailerons and turns. Assume a cartersian coordinate system fixed to the aircraft and level flight with free stream velocity in the x direction. Let's say you're making a left turn...the right aileron is deflected down and the left up..Now, since you're increasing the camber of the right aileron it produced more lift in the +y direction. Fine...But you are decreasing the camber of the left one and also giving it negative camber..Is the left aileron still producing positive lift (in the +y direction though less positive than the right aileron) or negative lift (in -y direction) and hence the right and left ailerons act as a couple.

-Vikram-

[Edited 2003-03-01 01:53:23]
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: Adverse-yawing Moment At High Speeds

Sun Mar 02, 2003 9:20 am

Tu144,
depends on the amount of aileron deflection. Deflect the ailerons a lot, and the wing segment where the aileron goes up might end up generating negative lift. Up to that point, it simply generates less positive lift than its downgoing counterpart. Either way, you end up with a force couple. Discontinuity is rarely encountered in the world!

Did that answer your question or did I misunderstand? Late here...  Smile

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.

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