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Old Apr 22, 2012, 02:18 PM
Registered User
United States, AL
Joined Apr 2007
823 Posts
Question
Trim, cg, and incidence

I've got a hill racer I've been flying a lot lately. The pitch trim Changes significantly with airspeed and I can't figure out why. It's not even that fast, it goes about 110mph level. I've got the cg back about 1 7/8 inches.

The wing-elevator incidence is as close to 0 as I can measure. I tried shimming the rear of the wing a bit with tape anyway with little effect. I've moved the cg around and it makes little difference. The motor axis is perfectly level.

I've had lots of planes that do this, is it something that just has to be lived with?
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Old Apr 22, 2012, 02:29 PM
"The Judge"
sebbe's Avatar
Sweden
Joined Feb 2006
5,788 Posts
The profile of your wing make hugh differance in what speedrange your plane will fly level with 0 degree incidence...

Take a modern F5D, they have to have incidence to negative value to keep levelfligt at 150-200mph with that profile they have, its kind of complicated as it all is mixed together, the profile of wing, weight of plane, incidence angle, shape of wing/plane, chord, and so on..

I have also found out sometimes with some custom made planes I have built, its difficult to make them fly level in all speedrange...

If you have played with incidence/CG, then I guess you have to live with this, you could always do a mix in radio.. but that does not make the plane perfect in all speeds, but helps you to relax while flying maybee..
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Old Apr 22, 2012, 02:32 PM
Sure it'll work
Flyextreme's Avatar
United States, CA, Torrance
Joined Nov 2005
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Maybe program in some reflex for higher speeds....might help.
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Old Apr 22, 2012, 02:42 PM
"The Judge"
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Sweden
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Quote:
Originally Posted by Flyextreme View Post
Maybe program in some reflex for higher speeds....might help.
My english ,, sorry,, what is reflex ?
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Old Apr 22, 2012, 03:06 PM
Sure it'll work
Flyextreme's Avatar
United States, CA, Torrance
Joined Nov 2005
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Quote:
Originally Posted by sebbe View Post
My english ,, sorry,, what is reflex ?
Flaps and/or ailerons slightly up to reduce lift and increase speed.

Airfoils designed to create lift at slower speeds have a tedency to nose up at higher speed.

I'm sure someone can articulate better than I what I am trying to say.

Bill
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Old Apr 22, 2012, 03:22 PM
"The Judge"
sebbe's Avatar
Sweden
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I understand ,, just didnt understand the english word.....

I know what it is,, just havent used it yet..
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Old Dec 12, 2012, 11:34 PM
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Gordysoar's Avatar
USA, KY, Louisville
Joined Oct 2002
700 Posts
"The pitch trim Changes "

Hi Sebbe,
I'm late seeing this post. But the advice didn't make any sense.

The tail directs the nose on all aircraft. Camber and reflex last I checked aren't located in the tail.

Likely your pitch trim changes are due to areo-elasticity. That means your elevator may be flexing under those high speed forces..

It can be pushrod flex or slop, the stab twisting, the fuselage bending, or the servo moving in its mount.

If its the nose , it has to be the tail.
Gordy
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Old Dec 13, 2012, 12:40 AM
Crashing into the sky!
jackosmeister's Avatar
Auckland NZ
Joined Aug 2007
7,291 Posts
^ ummm.. no.

When you increase/decrease camber/reflex of the wing, your effectively changing the decalage in a round about way. So the tail dont move, but the relative angle between tail and wing does.

It could also be a wobbly tail
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Old Dec 13, 2012, 01:28 AM
Sure it'll work
Flyextreme's Avatar
United States, CA, Torrance
Joined Nov 2005
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Quote:
Originally Posted by Gordysoar View Post
Hi Sebbe,
I'm late seeing this post. But the advice didn't make any sense.

The tail directs the nose on all aircraft. Camber and reflex last I checked aren't located in the tail.

Likely your pitch trim changes are due to areo-elasticity. That means your elevator may be flexing under those high speed forces..

It can be pushrod flex or slop, the stab twisting, the fuselage bending, or the servo moving in its mount.

If its the nose , it has to be the tail.
Gordy
So if I camber the wing (flaps) my plane will keep flying level, and the nose won't go up?? Or reflex (spoilerons) the nose won't go down?? Rrrriiiight

Or it could be a wobbly tail
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Old Dec 13, 2012, 05:49 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
Joined Dec 2009
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A change in pitch trim related to air speed (throttle setting) can usually be traced to incorrect motor down/up thrust.

If there is too much down thrust the model will pitch down as throttle increases and vice-versa. Effectively, the propeller has more of an effect as you advance the throttle.

If you trim the model straight-and-level for full-throttle, then you suddenly cut power and the plane balloons up, then you have too much down thrust. This is because when you were flying full-throttle the propeller was pulling the nose down requiring you to add up trim. But then when you took the propeller out of the equation (cut power) all that up trim takes hold and pulls the nose up when you suddenly cut power.

Vice versa if the nose pitches down when you cut power.

Tim
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Old Dec 16, 2012, 07:32 AM
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AndrésMtnez's Avatar
Madrid, Spain
Joined Dec 2004
2,101 Posts
What I do is trimming the plane on a dive with motor off. Once it goes straght down at high speed the plane is trimmed aerodinamically.

Then a repeat the dive, once the plane is flying fast level it, and see what happens when it slows down. It will tell you if the CG is forward/backward, if the nose goes down when it slows down the CG is too far forward.

Then I repeat the level flight at high speed, always with motor off till this point, and hit WOT. If it ballons up the motor incidence should be changed down.

It´s the best way I´ve found to adjust the plane aerodinamically, the CG and the motor incidence without any interference of any parameter with the other ones
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