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Comparing CG

#1
I get the concept and how to find my CG but I have a question on how to properly evaluate CG.

When I check my CG and I looking for the bottom line of my airfoil to be parallel to the ground or do I want a slight positive angle in reference to level ground?

Does that make sense?
 

quorneng

Well-known member
#3
Technically the C of G is a point within the air frame so it is impossible to balance the plane at it. To over come this the underside of the wing is used. To be absolutely exact the wing should be at the angle on incidence when in flight but you can't measure that either!
Remember there is almost certainly a range to the CofG depending on how you want the plane to fly so with the wing underside level to the ground is 'good enough'.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
For a conventional model the things I use to determine how exact the CG point I am using is is to do a few stalls, (at height). If it drops a wing sharply or becomes extremely unstable or uncontrollable as I get close to stall the plane is normally tail heavy. If it just drops its nose and continues then it is nose heavy! The other thing I look at is the elevator setting for level flight. If the elevator is pointing downwards when in the centre position the plane is tail heavy and if it is pointed slightly upwards it is nose heavy. Mind you the aircraft weight and wing incidence angles can also cause a plane to trim for level flight in a nose up or down attitude.

A canard is best judged for correct CG in a full up elevator/elevon, power off glide. If the plane stalls it is too tail heavy and if it drops its nose and seems to porpoise it is too nose heavy!

A wing is also checked in a full up elevator/elevon glide. If it wanders from side to side and even drops a wing at stall it is too tail heavy whereas if it lifts its nose and then drops in nose again in a rapid series of repetitive motions the wing is actually a little nose heavy!

Just what works for me!

Have fun!
 
#5
Thanks all. It makes sense. My plane exploded yesterday because I was flying low and over confident, buried the nose in the dirt. It was always quite nose heavy based on where the suggested battery placement was.

The good news I had a new fuselage to make some adjustments with. By sight, it seemed good enough this morning but it was still pitching up pretty good with the battery full forward. I was able to trim it level to keep it flying for the day but now I got a good idea of what to gauge it against.
 

FDS

Well-known member
#6
I always balance my plane every flight. Here’s a picture of my balancer, it’s two bits of wood scrap and pencils, from a Flite Test video on balancing. I take it with me, leave it on the ground by the battery box, drop the plane on every battery. That just leaves my bad piloting to cause any crashes!
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