Center of Gravity

cenman · Jul 17, 2001, 07:52 AM

I have an Ace Simple Series Me-109 with a speed 400 motor, 1.85:1GB, micro servos, standard Rx, and 8cell 600AEs (flat).

After the first couple of attempts to get it airborne, I noticed a tendency to tip stall to the left once I tried adding any control inputs.
The plans call for the CoG to be about 1" or so back from the leading edge at the root. I have only managed to have the actual CoG to be about 1.5" or so from the leading edge.
Would incorrect CoG (too far aft)make that big of a difference? I assume so. If that is the case, should I add weight to the nose, try changing the AoA of the wing (rubber band mounted), drop the thrust line of the motor down, what?
Should I try to move the CoG forward of the recommended spot?

Any advice would be greatly appreciated. By the way, thanks for reading this long winded post.

C.

Paul · Jul 17, 2001, 08:15 AM

An aft CG will cause the problems you mentioned. The 1/2" your CG is off is a lot for a small plane like that. If your wing angle of incidence and motor thrust line are at the plan specs I wouldn't mess with them. Try rearranging your battery or other components, and if all else fails add weight up front. But whatever you do get that CG right on, or your plane will have a short life.

Paul

Mike Smart · Jul 17, 2001, 08:31 AM

I agree with Paul, half an inch could make a significant difference to a model like this. Your problem also may be lack of power/thrust. What prop are you using?

Mike Smart

PS, how big and how heavy is the model?

[This message has been edited by Mike Smart (edited 07-17-2001).]

ChrisP · Jul 17, 2001, 08:31 AM

This is the 3 step way to trim a model aircraft :

1. Get the C.G. in the right place. This determines the amount of stability the model has.

When this is right :

2. Alter the angle of the elevator or incidences so that the model glides properly. Do not change the C.G. at this stage.

When this is right :

3. Alter the thrustline of the engine so that the model flies properly under power. Do not change the C.G. or incidences any more at this stage.

(Sorry, I couldn't help that !)

[This message has been edited by ChrisP (edited 07-17-2001).]

Bill Glover · Jul 17, 2001, 08:35 AM

Exactly what Paul said. A rearward CG will make the plane unstable, you can't change that by altering thrustline or incidence angles.

Jul 17, 2001, 07:52 AM	#1
cenman Registered User N. Andover MA Joined Sep 2000 157 Posts	Center of Gravity I have an Ace Simple Series Me-109 with a speed 400 motor, 1.85:1GB, micro servos, standard Rx, and 8cell 600AEs (flat). After the first couple of attempts to get it airborne, I noticed a tendency to tip stall to the left once I tried adding any control inputs. The plans call for the CoG to be about 1" or so back from the leading edge at the root. I have only managed to have the actual CoG to be about 1.5" or so from the leading edge. Would incorrect CoG (too far aft)make that big of a difference? I assume so. If that is the case, should I add weight to the nose, try changing the AoA of the wing (rubber band mounted), drop the thrust line of the motor down, what? Should I try to move the CoG forward of the recommended spot? Any advice would be greatly appreciated. By the way, thanks for reading this long winded post. C.

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Jul 17, 2001, 08:15 AM	#2
Paul Registered User United States, CT, Enfield Joined Nov 2000 343 Posts	An aft CG will cause the problems you mentioned. The 1/2" your CG is off is a lot for a small plane like that. If your wing angle of incidence and motor thrust line are at the plan specs I wouldn't mess with them. Try rearranging your battery or other components, and if all else fails add weight up front. But whatever you do get that CG right on, or your plane will have a short life. Paul

Jul 17, 2001, 08:31 AM	#3
Mike Smart Registered User Aylesbury, Buckinghamshire, Great Britain (UK) Joined Jan 2000 2,811 Posts	I agree with Paul, half an inch could make a significant difference to a model like this. Your problem also may be lack of power/thrust. What prop are you using? Mike Smart PS, how big and how heavy is the model? [This message has been edited by Mike Smart (edited 07-17-2001).]

Jul 17, 2001, 08:31 AM	#4
ChrisP Master Weilbach, Germany Joined May 2001 2,414 Posts	This is the 3 step way to trim a model aircraft : 1. Get the C.G. in the right place. This determines the amount of stability the model has. When this is right : 2. Alter the angle of the elevator or incidences so that the model glides properly. Do not change the C.G. at this stage. When this is right : 3. Alter the thrustline of the engine so that the model flies properly under power. Do not change the C.G. or incidences any more at this stage. (Sorry, I couldn't help that !) [This message has been edited by ChrisP (edited 07-17-2001).]

Jul 17, 2001, 08:35 AM	#5
Bill Glover Registered User Bracknell, Berks, UK Joined Nov 2000 10,239 Posts	Exactly what Paul said. A rearward CG will make the plane unstable, you can't change that by altering thrustline or incidence angles.