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Old Nov 05, 2012, 07:36 AM
small_rcer is offline
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Canada, NS, Halifax
Joined Dec 2005
283 Posts
See your picture 4854 and Pic 4859

In the first picture you can taper the thickness of the fuselage side foam to improve airflow to the prop.. Also use some 3/32 or 2 mm balsa and sheet in the bottom area under the motor to improve airflow. If you make the profile of this area curved to avoid the sharp change of direction in the airflow it will reduce prop noise. Look at pic 4859 and make the bottom of fuse curve up to the prop.

Also you can shape the trailing edge to provide a cutout for the propeller as done on the RiteWing flying wings. See www.ritewingrc.com

By relocating the LG by 2.5 inches the CG should be very close. Measure from the target CG position of the plane to the approx center of mass of the LG as it is now. Multiply the distance by the weight of the LG. Write the number down.

Then multiply the distance of the new location of the LG to the target CG by the weight of the LG. Write this 2nd number down.

What you now have are a torque arm calculation for the current position and new position of the LG. The difference between these two numbers is the inch oz or gram centimeters of torque change, caused by moving the LG. Knowing this number you can test the effect of the relocation.

To test this without moving the existing LG, you calculate what weigh placed ahead of the planned CG has the same effect. So I will assume you have a number of 100. That is 10 oz at 10 inches or 5 oz at 20 inches.

Next place weights on the nose of the plane to get the plane CG where you want it. Measure the distance from this weight to the target CG position. Multiply by the weight. In my example, if this number is more than 100, moving the LG will not be enough to get the CG correct. If this number is less than 100, then moving the LG will probably get you close to the CG. Then the battery or other equipment can adjust the CG to the final position.

What you have effectively done is the old school yard teeter totter calculation. Heavy person short distance, light person long distance. By calculating the existing versus the target you can get a pretty close approximation.

If this process is old hat to you, I apologize. If not try it out and see what you get. It is the process I use for equipment relocation.
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