Battery position for stability?

[pburress]

I have a 35" cub (Herr breezy day park flyer) and it has a tendancy, if I try to manhandle it and turn too fast and too slow, to go into wingtip stall/tailspin:confused:. I've made sure the airplane balances front-to-back and have done lots of experimenting with the balance. I can usually avoid this situation by not taking any risks ank keeping the speed up but it sure seems touchy! I understand that all cubs exhibit this to some degree and I'm willing to deal with it.

Here's the question: Would it affect stability if I placed the battery (7 cell 700mah nimh 2/3 aaa) higher in the fuselage, such as directly below the wing? Currenty the battery sits in the bottom of the fuse directly behind the firewall. It seems to me that it would help in at least some ways to have this weight closer to the center of the plane's axis... What do you think?

Thanks,
Phil

[Sparky Paul]

Raising the battery won't have much effect.
It's your plane, you can do what you want, but the problem is more due to too much control surface movement and a c.g. right on the aft limit.

[Gary Warner]

Check the wing closely for wash-in. Sounds more like a tip stall situation.

My understanding of vertical cg is that a higher location will reduce the dihedral effect, making it less stable. Then again, if you turn using the rudder, the induced slip might cause a pendulum effect that would be greater if the vertical cg was lower. It could possibly cause an overshoot of the roll rate and a roll that would require more than normal opposite input to reverse. Couple this with low airspeed and things get worse.

[Ollie]

Lowering the CG provides "pendulum" stability. The paraglider configuration is an extreme example. In the paraglider case, the longitudinal stability is so great that no tail is required and the lateral instability of anhedral is completely overcome. You would have to lower the CG by a wing span or two in order to get that much pendulum stability.

[Gary Warner]

Originally posted by Ollie
Lowering the CG provides "pendulum" stability.

True. But what about uncoordinated flight, like a rudder used for roll on the dihedral effect? Will the side forces ( the roll axis) have a greater mechanical advantage over the center of lift in a slip if the vertical cg is located lower?

[Ollie]

Gary,

I don't know the answer to your question but think the answer lies in the arrangement of all the force vectors, their directions and magnitudes.


The desirability of or undesirability of something like pendulum stability depends on the purpose of the aircraft. It might be good for a trainer and bad for an acrobatic model.