Author Topic: Knife edge coupling VS CoG position  (Read 3885 times)

Shahid

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Knife edge coupling VS CoG position
« on: 27, February 2013, 10:25:47 PM »
Alright folks,


Penning a tutorial for RCM&E and I can't for the life of me remember how the CoG position affects pitch in knife edge?


Is it moving the CoG forward reduces pitching up in KE or vice versa?


Cheers!


Been a while since I've done any trimming so my mind is blank  :o

Offline Malcolm Harris

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Re: Knife edge coupling VS CoG position
« Reply #1 on: 28, February 2013, 08:51:33 AM »
Moving CG forward makes the model tend to go towards the canopy.
 
Cheers,
 
Malcolm

Offline A Brightman

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Re: Knife edge coupling VS CoG position
« Reply #2 on: 28, February 2013, 01:02:07 PM »
Shahid.      Moving the c of g forward will send the model towards the canopy in knife edge. Up elevator or wing/tail incidence changes would be required for level flight,giving the change in knife edge.
 
Many people have difficulty in controlling ailerons and rudder in unusual positions,it is difficult,but after many hours of practice,sometimes many years, it becomes instinctive. Many years ago I developed a simple, easy to remember method of rudder and aileron control that works well.
If you want any details for an article contact me.
Example-- In knife edge to keep the wings vertical with aileron.--LOOK AT THE TOP WING WHEN FLYING AWAY FROM YOU.--
                                                                                    --LOOK AT THE BOTTOM WING WHEN FLYING TOWARDS YOU.--
 Do not think about left or right control, just look at the opproprate wing and move the stick in the direction the wing requires to go.
 
Arthur Brightman.

Offline SimonC

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Re: Knife edge coupling VS CoG position
« Reply #3 on: 28, February 2013, 01:30:08 PM »
Whilst I would agree in general, I think in practice it is all a bit more complicated and will vary from model to model. I have a Kyosho Osmose that I just about got the coupling trimmed out reasonably with different mixing for left and right, then I moved the cg back and it had a different effect on left and right (don't remember which way round, and I still haven't got it all sorted yet). I do know that if you move the cg back then you need less rudder to hold KE and so the mixing will always change.
 
Moving the cg seems to affect ALL mixings and couplings so I think the message should be to get the cg sorted to your own prefernce first and THEN trim out all coupling with mixing.
 
Simon

Offline Malcolm Harris

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Re: Knife edge coupling VS CoG position
« Reply #4 on: 28, February 2013, 02:11:46 PM »
For the last couple of years I have been using the Brian Hebert trimming method known as Triangulation Trimming, or at least my own variation of it. Having been building and trimming pattern models for over 30 years I was a bit skeptical until I tried it but the bottom line is for me it works.
 
My Asyuler has no mixing other than a couple of percent down elevator with low throttle to correct the vertical down lines. It holds dead straight indefinite knife edge on both sides with the application of rudder alone and goes absolutely straight up too.
 
The basis of this trimming method is to set the CG ONLY by the knife edge performance and no other criteria. Once this is correct you move on to correcting the vertical climb by addition of wing incidence and only then worry about the correct "feel" with adjustments to throws, expo, differential etc. You end up with a model balancing far further forward than we are used to, carrying much more wing incidence, probably less expo and more elevator throw. Trimmed like this the model stays on line much better during snaps and spins and generally feels more "locked".
 
The other benefit is during integrated rolling manoeuvres where rudder/elevator mixing usually causes problems at some point in the figure.
 
Cheers,
 
Malcolm

Shahid

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Re: Knife edge coupling VS CoG position
« Reply #5 on: 01, March 2013, 12:52:53 AM »
Thanks all for responses!

Offline TomLaird

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Re: Knife edge coupling VS CoG position
« Reply #6 on: 09, September 2013, 11:02:58 PM »
Hi,
This is a subject close to my heart just now. Although I know HOW to correct it, (i.e. move the c of g from the previous posts made here) does anyone have an explanation as to why the c of g has an effect when in a knife edge?
thanks
Tom


Offline A Brightman

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Re: Knife edge coupling VS CoG position
« Reply #7 on: 10, September 2013, 07:34:24 PM »
Tom, My thoughts.
Moving the CG forward will result in the aircraft losing height and will require more lift to fly level.
To obtain the increased lift, up elevator trim or more wing/tail incidence is required.
Gravity is a vertical component acting downwards, so in knife edge the forward CG has little or no effect because the wing is vertical.
In knife edge the lift is generated by the  fuselage and not the wing,but the increased wing/tail incidence or up elevator trim now gives the required effect, sending it more to the canopy.
Arthur Brightman.

Shahid

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Re: Knife edge coupling VS CoG position
« Reply #8 on: 13, September 2013, 11:49:13 PM »
Correct! That is a main factor. Though there are other things at play which I will explain when I'm back from abroad

Offline Peter Jenkins

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Re: Knife edge coupling VS CoG position
« Reply #9 on: 30, January 2014, 07:39:49 PM »
Are you back yet Shahid?  I was looking forward to hearing about the other things.


Peter