Why always tail heavy?
05-11-2003, 03:18 AM
#1
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Why always tail heavy?
I have started scratch-building and so far I have not been able to determine ahead of time if the plane will be balanced- most of the time it is tail heavy.
Are there any tricks to determine if the plane will balance after adding covering, control hardware, and radio equipment?
Is there a limit to the amount of ballast to add to the nose before it affects performance?
Thanks!
Are there any tricks to determine if the plane will balance after adding covering, control hardware, and radio equipment?
Is there a limit to the amount of ballast to add to the nose before it affects performance?
Thanks!
05-11-2003, 03:45 AM
#2
Why always tail heavy?
Most scale planes end up tail heavy for one reason. and that is thier full size counterparts engines are much heavier as they relate to the rest of the plane than the model engines do.
Especially on a radial engine type plane. If you build a scale plane like a Zero, Stearman, Waco or Corsair. you're sure to have a tail heavy condition before you start pushing the radio, battery and lead up front. The reason they have such a short nose moment is the weight of the full scale engines.
Just try to push everything as far forward as you can before you start adding lead. I had to add 2lbs of lead inside the cowl on my stearman and that's with a 120 4cyl on the nose.
The amount of wieght will start affecting performance if increases the wing loading to an extream limit. If you add 5lbs to the nose and the wing loading is still around 25-30 ozs per square foot, the thing will fly great. Start get in the 35-40oz+ range and you piloting skills will be tested
Good luck
Especially on a radial engine type plane. If you build a scale plane like a Zero, Stearman, Waco or Corsair. you're sure to have a tail heavy condition before you start pushing the radio, battery and lead up front. The reason they have such a short nose moment is the weight of the full scale engines.
Just try to push everything as far forward as you can before you start adding lead. I had to add 2lbs of lead inside the cowl on my stearman and that's with a 120 4cyl on the nose.
The amount of wieght will start affecting performance if increases the wing loading to an extream limit. If you add 5lbs to the nose and the wing loading is still around 25-30 ozs per square foot, the thing will fly great. Start get in the 35-40oz+ range and you piloting skills will be tested
Good luck
05-11-2003, 03:03 PM
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Why always tail heavy?
There is no good way to know except through experience building a variety of airplanes. If you wanted to you could figure out the density of all the wood, figure out the volume of each piece and then figure out the moments, etc. Even if you were deadly accurate, it would probably not even be close for unknown reasons. 
It helps if you give yourself leeway to mount things. For example, you could give yourself a way to mount the servos in the tail if extremely nose heavy or move the servos up front if tail heavy.
I tend to always put the receiver and battery behind the tank and the servos behind that. It's easy to maintain, but doesn't always contribute to resolving balance problems.
One thing to know about adding weight. If you add a bunch of lead to balance the plane, then it's still going to have an affect on the flight due to added mass at the extremities. That means it will be slower to damp. For example, it may take an extra turn to stop a spin, etc.
The short answer is I don't know the answer to your question. Go by your gut. If it looks like it will be tail heavy, then sand the tail as much as possible, substitute lighter wood or whatever to remove as much weight as possible.
It helps if you give yourself leeway to mount things. For example, you could give yourself a way to mount the servos in the tail if extremely nose heavy or move the servos up front if tail heavy.
I tend to always put the receiver and battery behind the tank and the servos behind that. It's easy to maintain, but doesn't always contribute to resolving balance problems.
One thing to know about adding weight. If you add a bunch of lead to balance the plane, then it's still going to have an affect on the flight due to added mass at the extremities. That means it will be slower to damp. For example, it may take an extra turn to stop a spin, etc.
The short answer is I don't know the answer to your question. Go by your gut. If it looks like it will be tail heavy, then sand the tail as much as possible, substitute lighter wood or whatever to remove as much weight as possible.
05-11-2003, 03:50 PM
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Why always tail heavy?
You can calculate it before hand but it's a real mess of work.
You'd have to measure and calculate each and every scrap of balsa, wire, ply and covering to determine the volume of each part and the surface area of the covering. And then you find the center of gravity of each part and do a length x mass calculation for each part to find it's moment arm from the desired center of gravity for the whole model. Then you can move the components around to set it up so it comes out right.
Sounds like a lot of work, doesn't it? That's why we all just go with "That Looks About Right" and compensate from there.
Full sized aircraft have to do all this though. It's many hours of details but it needs to be done. You don't just walk out to a 767 and add 2.75 tons of wheel weights under the control cabin so it'll fly....
You'd have to measure and calculate each and every scrap of balsa, wire, ply and covering to determine the volume of each part and the surface area of the covering. And then you find the center of gravity of each part and do a length x mass calculation for each part to find it's moment arm from the desired center of gravity for the whole model. Then you can move the components around to set it up so it comes out right.
Sounds like a lot of work, doesn't it? That's why we all just go with "That Looks About Right" and compensate from there.
Full sized aircraft have to do all this though. It's many hours of details but it needs to be done. You don't just walk out to a 767 and add 2.75 tons of wheel weights under the control cabin so it'll fly....
05-11-2003, 03:58 PM
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Why always tail heavy?
I've scratched built a number of my own designs and one point I have always gone with is this.....don't determine the exact location of the wing cradle until you have satisfied your desires elsewhere. There's too many things to take in consideration to just cut out the sadle first. Wait until you have determined the exact location of your battery, receiver, servos. What engine your going to use, the style of its tail, landing gear, flaps or no flaps, etc. All this means weight and trying to set a wing back one inch is sort of difficult once your all done with the plane. Of course theres no easy way to do any of this either. It just takes a lot of patience but the end result is well worth the effort.
Jaymom
Jaymom
