What CG should I use?

Belial2801

Member
Hi, I’m building a generic Piper Cub (not Flite Test). The CG is approximately at 5.2 cm, but this places the battery about 15 cm from the nose (60 cm fuselage). My question is, should I use those 5.2 cm corresponding to 30-33%, or should I go for 25% for the Center of Gravity? At 25%, I’d need to move the battery about 2 cm forward, but I’m not sure what consequences this might have. I’d like to hear your opinions.

1m wingspan.

550gr aproximately.
 

Tench745

Master member
The typical range for CG on a high-wing conventional tail model is in the 25% to 33% range. If you're anywhere in there, it should fly fine.
As the CG moves toward the forward limit, the plane will recover from a stall quicker, and be more resistant to spins, but the plane will be slower to respond to elevator inputs. Depending on the plane, a CG at the forward limit might mean you won't have enough elevator throw to flare for a landing.
A CG closer to the rear limit will make the aircraft more sensitive to elevator inputs, and thus more able to perform various aerobatic maneuvers.
My personal advice is put it somewhere between the two limits and then push the cg fore or aft to see it feels and tune it to where you like it.
 

Merv

Moderator
Moderator
...or should I go for 25% for the Center of Gravity?...
I agree with Tench.
With any new plane I start with the CG @ 25%, this give a stable flying plane to get it trimmed out. Once trimmed, I will slowly move the CG aft to make the plane fly the way I want it to. I enjoy a plane with a bit of instability. On most planes, I prefer the CG somewhere around 30%.
 
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L Edge

Master member
For any plane that you build, you should start off at the 25% range on the maiden flight.

What happens if the tail area is built is not built to specs.(skewed)
What happens if the thrust line is not proper?
Plane's wing angle is not right or skewed?

And so on? if you end up with a further aft CG, you could end up hovering in the worse case. Then relocate the CG to your flying needs.
 

Piotrsko

Master member
Tis a cub. Cg needs to behind the mains in flight attitude or you cant avoid ground loops. The other commentators are correct as to actual position.