Building a replacement fuselage - how to make sure things will balance when done?

[Mitch G]

Here's the deal, I have a sp400 pylon-type plane that I smacked up such that the fuselage was trashed but the wing is still fine.
So, I want to build a new fuselage out of sticks (ala a Bare Bones fuselage) but I want to make sure things balance OK when I'm done.

The plane in its original form was quite tail heavy. So, I'm thinking I'll follow the same general fuselage form of the original plane, but make the nose longer and maybe even make the tail, a touch shorter. Is that a valid thing to do? Or, am I ignoring some basic aerodynamics that will get me in trouble?

Also, are there any tricks to figuring things out so that when done I will have a fuselage that will generally balance OK by just moving the battery pack around? Or, do I just guess how much the sticks will weigh behind and in front of the CG point and account for the motor and receiver and servos, etc and then hope things work out?

So, basically, I'm looking for hints, tricks of the trade, ideas, words from the wise, whatever on how I should proceed to make sure things work OK.

Thanks,


Mitch

[JRuggiero]

When I do a scratch-build, I use a balance board. This is a piece of shelving lumber with two nails underneath to act as pivots. Put the board on your workbench. If the board doesn't balance by itself (and variations in moisture will affect this), put any old kind of weights on the end of the board to get it into basic balance. Then add your wings and fuselage parts, respecting their locations relative to each other. Shift things around until the board balances. Do this at every stage of construction. Make allowances for paint and covering. Check and recheck. This has worked fine for me for many years. It's an idea originated by Andy Lennon.

Jim R

[davidfee]

Engineers call the mathematical approach to this problem "weight and balance." You can tabulate all of the weights of the parts, multiplied by their distance ahead of (or behind) the desired CG (defined by the wing/tail), to get the torque components. Then you move heavy items around until the torques cancel.

http://www.grc.nasa.gov/WWW/K-12/WindTunnel/Activities/balance_of_forces.html

For a model, it's pretty easy to do this on paper. As Jim said, be sure to account for covering, glue, etc.

How tail heavy was your plane? Basically, I'd say stretch the nose 1/2" and leave the tail where it is. The tail moment is important for stability and control (another engineering catch-phrase. ;)

good luck,
-David (not an enginneer...)

[av8orfliboy]

I agree with Dave regarding the tail length. A short tail will reduce the control authority of the elevator which will equate to higher stall speeds and a more difficult landing, not to mention less maneuverability.

You may want to get an expert opinion on weight and balance as I have never done one on a model, but this may give you an idea. Place the forward most and rear mostends of the aircraft on a scale. Write down the sum of both scales, this is the total weight. Multiply the rear weight by the length of the airplane, this is the moment with respect the the front of the aircraft. Divide the moment by the total weight, this will give you the position of the CG from the front of the aircraft. On most aircraft, this is about 1/4 to 1/3 of the way back on the wing which will provide a reasonable amount of stability. If the CG is too far back, it could be one reason your airplane felt tail heavy before, especially when it is slow. If it is too far forward, you won't be able to hold the nose up during landing.

Good luck,
Pat

[Mitch G]

Thanks for the replies guys.
I had my suspicions about messing with the length of the tail.
So, lengthening the nose will be all I do.
And, I'll tinker around with the ideas given on how to make sure things balance when done.

Thanks again,


Mitch