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Definition of Roll Coupling: Knife Edge

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Old 04-06-2006, 10:41 AM
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jigeye
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Default Definition of Roll Coupling: Knife Edge

What is the defnition of "roll coupling" with regards to doing a Knife Edge manuever? I assume this means the wings was to roll back to the level position while doing the manuever, is this correct?
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Old 04-06-2006, 10:50 AM
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Default RE: Definition of Roll Coupling: Knife Edge

I always thought it meant you had premixed rudder and elevator Or aileron on your box. for the desired manuever

I am probably wrong but thats what I thought.

A little help on this subject would be great
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Old 04-06-2006, 11:00 AM
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Default RE: Definition of Roll Coupling: Knife Edge

With an airplane, if you give aileron your plane rolls, if you give elevator your plane pitches.

If you try to give rudder, rarely you will get a pure yaw. Almost always, your plane will roll also: this is the roll coupling. You give rudder, you get yaw AND roll, whereas your desired response would be a yaw only. There is also the pitch coupling - when you give rudder , your plane will yaw, roll and pitch!
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Old 04-06-2006, 01:46 PM
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Default RE: Definition of Roll Coupling: Knife Edge

Well written, stek79. Note that the plane can roll "with" or "against" the rudder input, depending on many things, not the least of which are dihedral angle and wing height.
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Old 04-06-2006, 04:41 PM
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Default RE: Definition of Roll Coupling: Knife Edge

Thanks Stek79 and Mesae, Thats what I love about this site,,,,,, ask,,,,, and you shall be answered...
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Old 04-06-2006, 04:44 PM
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Default RE: Definition of Roll Coupling: Knife Edge

You will read comments which use the words adverse and proverse --roll couple
as noted these are simply words describing which way the rudder application affects the model
ADVERSE - just like the word is typically used --means against -
left rudder rolls the plane right
PROVERSE - the rudder causes rolls in same direction as it is moved.
On a good aerobatic setup --there is a hint of adverse roll -just a hint
We put this feature into most of our aerobatic designs -- so that when doing an extended slow roll -- more rudder could be added --without causing the roll to speed up
The roll couple will also change from design to design and may also only occur with high deflections
a secondary effect is that pitch is affected on hard rudder application - so---it is very possible to have a setup where the rudder application reacts as follows:
yaw then roll then pitch- it gets spooky .
got it?
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Old 04-06-2006, 04:57 PM
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Default RE: Definition of Roll Coupling: Knife Edge

Does a SIG Fourstar 60 have a lot of roll coupling?
The wings were clipped one rib bay each side.
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Old 04-06-2006, 06:29 PM
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Default RE: Definition of Roll Coupling: Knife Edge


Quote:
ORIGINAL: jigeye

Does a SIG Fourstar 60 have a lot of roll coupling?
The wings were clipped one rib bay each side.
I would guess that it has less yaw-roll coupling than a high wing trainer but more than a competition pattern plane.
Yaw-roll coupling is not nessesarily a bad thing. Trainers have a lot of it on purpose, it's a major contributer to a trainer's stability. Planes like the Fourstars are designed to be a step up from trainers so they likely still have a little yaw-roll coupling.
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Old 04-08-2006, 07:32 AM
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Default RE: Definition of Roll Coupling: Knife Edge

Quote:
ORIGINAL: dick Hanson

On a good aerobatic setup --there is a hint of adverse roll -just a hint
We put this feature into most of our aerobatic designs -- so that when doing an extended slow roll -- more rudder could be added --without causing the roll to speed up
Very interesting Dick, I didn't think about that feature!

What are the design parameters that can lead to such a behaviour?
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Old 04-08-2006, 07:34 AM
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Default RE: Definition of Roll Coupling: Knife Edge

Quote:
ORIGINAL: Waldopepperaxel

Thanks Stek79 and Mesae, Thats what I love about this site,,,,,, ask,,,,, and you shall be answered...
You're welcome!!!
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Old 04-08-2006, 10:43 AM
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Default RE: Definition of Roll Coupling: Knife Edge

Roll coupling is just another name for lateral stability. It is the tendency to roll away from a sideslip. In normal flight this is generally desirable since when a wing drops, the airplane slips toward the low wing and the roll response tends to lift the low wing back to level flight. As others have described, it is roll in response to yaw.

In case of knife-edge flight, the airplane is in a constant yaw (sideslip) and any roll response must be countered by control input or the airplane would roll back level.

As a general rule, for aerobatic flight, stability about all three axis should be a minimum, whereas for normal sport flying a greater degree of stability results in easier and more relaxing flight. The desired degree of stability is very much a personal preference and can be designed in to be almost anything the designer wants.
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Old 04-08-2006, 11:47 AM
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Default RE: Definition of Roll Coupling: Knife Edge

Good old "secondary effects of controls" [8D]
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Old 04-08-2006, 12:58 PM
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Default RE: Definition of Roll Coupling: Knife Edge


Quote:
ORIGINAL: stek79
...What are the design parameters that can lead to such a behaviour?

With models, it's almost always done by trial and error. If a certain model has too little or too much yaw stability, the next version will be similar, but the wing position and/or the dihedral angle will be adjusted very slightly. The math required design an airplane from scratch with guaranteed neutral yaw stability goes way beyond anything that can easily be presented in this forum. There are many factors to consider, such as lateral fuselage area distribution, vertical stabilizer area distribution (rolling moment due to vertical assymetry), wing planform and washout distribution, downwash interaction with the horizontal tail, incidence angles, spiral slipstream effect, and the list goes on.

UNLESS the plane is similar to a Yak 54 or some pattern planes: mid wing, zero thrust angle and zero wing and tail incidence, and a fuselage that is nearly symmetrical vertically about the longitudinal axis. In short, any vertical assymetry can cause undesired (or desired) roll or pitch coupling with yaw.

In general, if it is a low wing model, neutral stability will require some geometric dihedral, and so forth.
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Old 04-08-2006, 11:10 PM
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Default RE: Definition of Roll Coupling: Knife Edge

Stek - you can get a very good aerobatic setup with a setup like a ZLIN 526 or a CAP- and have the roll couple extremely easy to control -or almost eliminate
We actually liked those setups better than the "all in a line , flying flounder" approach so common today--
There is no secret to how to do it
Take a Zlin 526 ASF - build it to scale except for airfoils -use thin sym sections - and keep weight to absolute minimums

Be prepared to saw the wing in center - to change dihedral - typically you need to reduce it some to where the knife edge flight is quite neutral in roll couple - and then upon application of more rudder than needed for smoth level knife edge - the model starts an adverse roll.
The ZLIN does absolutely spot on axial slow rolls, setup right
Some positive elevator mix is usually needed to get pitch couple right.
You can also use a rudder /aileron mix in the TX but I like to keep that stuff at a minimum
The beauty of these non "all on center line models" --is that you can use the high thrust line to get some terrific pitching , tumbling maneuvers going
The Conical Lomcovak for example - using the high thrust line to an advantage - can produce wild end over end tumbles.
We learned to setup planes for these characteristics long before the mix features in radios hit the market .
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Old 04-09-2006, 09:47 PM
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Default RE: Definition of Roll Coupling: Knife Edge


Quote:
ORIGINAL: dick Hanson
... We learned to setup planes for these characteristics long before the mix features in radios hit the market .

AMEN to not going nuts with the mixing! Better to make a good plane from the start. I use mixing only to get the throws the way I want them (though I have fiddled a bit with spoilerons, flaperons and tailerons (for anti-torque rolling)). I have NEVER used a snap roll switch/button. And I decided I don't like spoilerons or flaperons on the planes I have tried those with.

And I didn't mean to imply that the all zero-zero mid wing setup was the best overall. Just presenting a baseline. I got my first Cap back in the mid '80s after everybody on Oahu had already had one and crased it due to it's "horrible" snap roll characteristics. I flew it for years, then sold it to a local when I got transferred away from Hickam. It was a wonderful plane that did utterly fantastic snaps--any fraction. Liked the Cap so much I bought a big Cap 10. (lost it to an disconnected elevator linkage ) I could do precise quarter snaps, 3/4, 1 1/4, 1 3/4, etc. inside or outside from any attitude. Fantastic knife-edge also. Liked it so much I bought another. Flew it for several years then sold it too when I got transferred again. I still have a soft spot for them. I've never had a Zlin, but I'd try one for sure, just to have something different.
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Old 04-12-2006, 02:05 PM
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Default RE: Definition of Roll Coupling: Knife Edge

I wonder if roll coupling might have an alternate definition, perhaps related to an aerobatic mile-high club.
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Old 11-08-2017, 06:38 AM
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Old 11-08-2017, 06:42 AM
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Quote:
Originally Posted by LouW View Post
Roll coupling is just another name for lateral stability. It is the tendency to roll away from a sideslip. In normal flight this is generally desirable since when a wing drops, the airplane slips toward the low wing and the roll response tends to lift the low wing back to level flight. As others have described, it is roll in response to yaw.

In case of knife-edge flight, the airplane is in a constant yaw (sideslip) and any roll response must be countered by control input or the airplane would roll back level.

As a general rule, for aerobatic flight, stability about all three axis should be a minimum, whereas for normal sport flying a greater degree of stability results in easier and more relaxing flight. The desired degree of stability is very much a personal preference and can be designed in to be almost anything the designer wants.
It's a form of torsional instability, not lateral instability.

Cheers!
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