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Mass Positioning

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Mass Positioning

Old 06-14-2005, 11:07 PM
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Default Mass Positioning

Hi all,
This is not technically a aerodynamics question, but I think this is the right place for it. I got Andy Lennon's book a while back and I remember somewhere he mentioned that the mass should be concentrated around the CG of the main wing. I guess that means all the servos are in the "cabin" with the battery etc.... Obviously the components are moved around to balance correctly, but in general they are close to the CG. The most obvious exception is all of the profiles, 3D machines and even many large scale models (cubs) have servos mounted very close to their control surface with short, strong linkages.
A plane can be balanced with all of the electronics mounted relatively close to the CofG OR,....moving servos to the tail, and moving the battery etc.,further forward to balance out the servos. Both ways will give you the proper CG, but the mass is distributed differently.
I think the reasoning is that it would be more stable with all of the mass concentrated at the center. I guess having more mass in the tail would give it more reisitance to moving in the opposite direction. (less stable?) If it starts going up, it would take more force to move it back down again??!?[] The plane would be like those wooden swing things. ("teeter totter?".."see saw") There would only be a small force required on either end to start the tail or nose moving in any direction. I may be WAY off, but I would like to know more about this.
Thanks in advance.
Old 06-14-2005, 11:36 PM
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Default RE: Mass Positioning

The reason for keeping the heavy bits near the centre of gravity is to minimise the moments of inertia about the airplane axes. The easiest way to visualize moment of inertia is to consider the tight rope walker at a circus. He carries a long pole to help him balance. Without the pole it would be much harder to balance because once he started to topple he would fall over too fast to be able to correct his fall. With the pole, he topples slowly with lots of time to correct the fall.

The long pole increases the tight rope walker's moment of inertia. To get a high MOI you need to concentrate weight a long distance from the axis of rotation (the rope in the case of the walker). The bigger the distance, the smaller the weight required for the same MOI.

Of course, if we want an airplane which is responsive to the controls and have a snappy response about all three axes of rotation then we need to keep it's MOI low, and this means concentrating it's weight close to the centre of gravity.

In nature there is no such thing as a free lunch. We have to make many design compromises. Servos close to the control surfaces provide us with short, stiff control rods which minimize free play and dynamic nasties such as flutter, but they also contribute to increasing MOI and consequently degrade the airplane's snappy response. Increased MOI also makes the airplane less sensitive to external disturbances, such as gusts, but requires more powerfull controls when a gross correction is needed.

Note that stability is not involved here. An airplane with a very low MOI can have a fast response, even too fast for a human to control, but still be stable. On the other hand, a big ponderous airplane with a high MOI can be unstable and still able to be flown successfully by a human, although often with some difficulty.

Hope this helps.
Old 06-14-2005, 11:39 PM
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Default RE: Mass Positioning

The question you are asking about is known as the "Polar Moment Of Inertia".

The ability to accelerate around an axis of rotation quickly requires a low polar moment. For an airplane this makes the controls quicker, like the ability to start and stop a roll.

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