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Low-wing aircraft and knife-edge - 4/30/2002 3:04:22 AM   
woodscra


 

Posts: 895
Joined: 12/10/2001
From: frisco, TX, USA
Status: offline 		Would those more knowledgeable than I explain why low-wing planes, such as the cap, have a lot of coupling during knife edge?

Thanks
       Post #: 1

Low-wing aircraft and knife-edge - 4/30/2002 6:44:44 AM   
RCmob


 

Posts: 28
Joined: 4/21/2002
From: St Joe, IL, USA
Status: offline 		Most airplanes could be designed to have no roll, yaw or pitch coupling if desired, but sometimes looks or other things are considered more important than the aerodynamic couplings of an airplane.
The best way to find out about roll and pitch coupling of your airplane is to fly it into a knife edge. ROLL COUPLING: When you apply rudder to hold the knife edge, if the airplane rolls in the same direction you are applying rudder (proverse or positive roll coupling), your airplane has too much dihedral. If the airplane is rolling in the opposite direction of your rudder input (adverse or negative roll coupling), your airplane's wing does not have enough dihedral. PITCH COUPLING: If in the same knife edge, your airplane pitches up, you have positive pitch coupling. If the airplane pitches down, you have negative pitch coupling. The cure to get rid of positive pitch coupling is to move the canopy of your airplane forward, move the stab up in the tail structure or move your CG back. The cure for negative coupling is to move your canopy aft, lower your stab in the tail structure or move your CG forward. A swept back rudder hinge line also helps reduce a negative pitch coupling.
The use of wheel pants an/or slightly dropped/raised ailerons may also slightly change your coupling but at least this last one it is not a recommended solution because your stall performance will also change. Of course, changing any of the things I explained above will also make your airplane fly different. Neutral stability with no coupling comes at a price. You have to fly the airplane now.
Oh! and there are still many other little trick that may change your coupling in a way or another (winglets, retracts, cowls, wingtips, etc,,)

(in reply to woodscra)
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Low-wing aircraft and knife-edge - 4/30/2002 6:55:00 AM   
woodscra


 

Posts: 895
Joined: 12/10/2001
From: frisco, TX, USA
Status: offline 		Thanks rcmob, I appreciate it.

Do mid wing aircraft have less tendency to couple? If so, would you comment on why that is?

Thanks

(in reply to woodscra)
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Low-wing aircraft and knife-edge - 4/30/2002 7:04:16 AM   
RCmob


 

Posts: 28
Joined: 4/21/2002
From: St Joe, IL, USA
Status: offline 		Aerodynamically a low wing design is considered to have an aeroynamic dihedral of approximately -1 degree, a high wing has a +1 degree and a mid wing 0 degrees. This only changes the ROLL COUPLING. Basically what this means is that if you had an airplane with the same shape, moments, CG and tail surfaces, the low wing airplane may need positive dihedral to have no roll coupling while a mid wing may need no dihedral at all and a high wing may need negative dihedral. For PITCH COUPLING, the wing, stab and engine incidences may need to be changed although most times moving the CG forward or aft may be enough to get rid of pitch coupling.

(in reply to woodscra)
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Pitch Coupling - 4/30/2002 8:43:17 PM   
LeonardN


 

Posts: 17
Joined: 3/26/2002
From: Buckingham, UNITED KINGDOM
Status: offline 		Rcmob writes:
>PITCH COUPLING: If in the same knife edge, your airplane >pitches up, you have positive pitch coupling. The cure to get rid >of positive pitch coupling is to

>move the canopy of your airplane forward,
>move the stab up in the tail structure or
>move your CG back.

I have never understood why position of the stab should effect pitch coupling. The other two are new to me. Does anyone know why these changes work?

thanks
Leonard

(in reply to woodscra)
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Re: Pitch Coupling - 4/30/2002 10:49:50 PM   
can773



Posts: 1664
Joined: 1/25/2002
From: Calgary, AB, CANADA
Status: offline 		[QUOTE]Originally posted by LeonardN

I have never understood why position of the stab should effect pitch coupling. The other two are new to me. Does anyone know why these changes work?

thanks
Leonard [/QUOTE]

In all practicallity it doesnt, the rudder hinge line angle has a more pronounced effect on pitch coupling. Angle the rudder hinge line back to reduce pitching to the belly in knife edge. You can also reduce the area of the rudder above the thrust line to help out as well. Playing with stab height does not do a lot. CG probably has the biggest impact of all but also will effect downline and upline tracking (upline to a lesser extent, which is more governed by engine thrust).

Dont forget that sweep back on the LE also has an effect of dihedral.

If you are looking for parameters that have little to no coupling in knife edge look toward the modern 2M pattern ships, they are virtually mix free in all attitudes.


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Low-wing aircraft and knife-edge - 5/1/2002 12:47:58 AM   
woodscra


 

Posts: 895
Joined: 12/10/2001
From: frisco, TX, USA
Status: offline 		This is all great information.
What affects does having a low-wing have on the slow speed characteristics of a plane?
It seems to me that the Extra 330 with a mid-wing would have less roll coupling with its mid wing with zero dihedral. It appears that more pilots choose it for aerobatics. Does the mid-wing design provide benefits at slow speed?

Thanks

(in reply to woodscra)
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Low-wing aircraft and knife-edge - 6/4/2002 8:10:35 AM   
TimDavie


 

Posts: 112
Joined: 12/18/2001
From: tulsa, OK, USA
Status: offline 		Let's raise the bar a bit...

Why does aft CG cause a negative pitch in knife edge?
Why does a high stab cause a negative pitch in knife edge?

Remember guys, none of this pitch stuff happens until you add rudder. Picture a plane in knife edge...rudder becomes and elevator, and the fuse side becomes the lifting body. Now, How much rudder do you have on each side of the thrust line? Most of the aerobatic planes have much more rudder above the thrust line than below...probably 60-70% more. What would happen if elevator halfs were unequal in size?

Does this make any sense?

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Low-wing aircraft and knife-edge - 6/5/2002 1:12:31 AM   
gubbs3



Posts: 1403
Joined: 12/8/2001
From: Coon Rapids, MN, USA
Status: offline 		I have a GP Stik and it has terrible positive roll coupling and positive pitch coupling. What I'm asking is because the Stik has a good deal more rudder above the wing than below, shouldn't it have proverse roll coupling? It doesn't have much more than 1/4 dihedral at each tip either.

(in reply to woodscra)
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Pitch Coupling - 6/11/2002 6:55:02 AM   
GRH



Posts: 217
Joined: 5/31/2002
From: Savannah , GA, USA
Status: offline 		Hi,
I've attached a couple of Word documents that contain some of my thoughts on pitch coupling. Hopefully the attachments will make it through...if not,email me directly and I'll send them to you.

Roll coupling seems to be pretty well understood so I won't belabor the point too much... the major contributors being:

-wing/h-tail dihedral
-wing/fuse position (low/mid/high wing)
-wing sweep (linked directly to wing CL)
-vertical tail height
-rudder/fin geometry

Almost all of these effects are linked directly to the angle of sideslip (beta). The roll coupling linked to the wing and h-tail sweep angles are a strong function of both angle of attack (alpha-CL) and sideslip.

Most of us mentally link the rudder to these couplings because that's the control surface that we use to create the airplane's response...in fact it's the rudder inputs that causes the airplane to sideslip.

One effect that isn't linked to alpha or beta is the rolling moment due to rudder deflection. Typically rudder deflection, in itself, causes a rolling moment opposite from the rudder input because the majority of airplane designs have vertical tails which stick out above the airplanes centerline in an asymmetric fashion.
This effect is often shadowed by the airplane's lateral response to the sideslip angle.

As with most airplane related problems, the ultimate combination of many effects results in the final direction and severity of the coupling that the pilot experiences.

George Hicks
Aerodynamics/ S & C Group
Sino Swearingen Aircraft Corp
[email]ghicks@sj30jet.com[/email]

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George Hicks

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Low-wing aircraft and knife-edge - 6/11/2002 9:22:54 AM   
GRH



Posts: 217
Joined: 5/31/2002
From: Savannah , GA, USA
Status: offline 		I suppose that the attachment didn't work...let's see if this does.

George

Attachments
Click to see the file in new window.Attachment.txt


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George Hicks

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RE: Low-wing aircraft and knife-edge - 1/25/2004 2:48:39 AM   
Jleyland



Posts: 360
Joined: 11/22/2002
From: Doylestown, PA, USA
Status: offline 		Fascinating stuff George. I hope everyone takes the time to read that. (attachment)
-jon

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RE: Low-wing aircraft and knife-edge - 1/25/2004 4:23:34 AM   
combatpigg



Posts: 10133
Joined: 11/22/2003
From: arlington, WA, USA
Status: online 		The most "even handed" plane you will ever fly in K.E. is one that is completely symetrical,[in the top and side view] and one whose fin/rudder area follows design principles for good horizontal stab area. A plane like this is nearly hands off in K.E., you can fly it pretty much from take off till the tank's empty doing flat turns with the elevator. The plane that fits this description isn't much to look at, but I built it to illustrate a point. When a design strays away from this ideal K.E. example, that's when uneven aerodynamic forces get placed here and there on the airframe, and certain gimmicks are used to counterract tendancies to roll out of K.E. If you don't have a good lifting surface to begin with[usefull side area]you're fighting a losing battle. Dihedral and rudder sweep are actually disruptive forces as far as the ideal K.E. planform is concerned. Their purpose is to balance out an out of balance design, but their stabilizing influences are speed dependent. A plane that is designed for ideal K.E. behavior doesn't care so much about the speed it's being flown at.

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Led Zeppelin is NOT "old fogie" music.

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RE: Low-wing aircraft and knife-edge - 1/25/2004 5:16:29 AM   
Jleyland



Posts: 360
Joined: 11/22/2002
From: Doylestown, PA, USA
Status: offline 		Read George's attachment

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RE: Low-wing aircraft and knife-edge - 1/25/2004 5:34:30 AM