Re: LittleSquare finally earns its wings!

From: <dgbj_at_aol.com>
Date: Mon, 4 Dec 2006 07:19:00 EST

"I'll leave all that to anyone else who wants to have a go at it."
 
Bill,
 
I'll have a go at it myself. I fly indoor models in a school gym that has a
sloping ceiling with beams and air ducts, but it is within the range that
you mention. I fly with 2 gram motors. For the last few years I have taught
kids classes and I fly the kind of models that we use in the class. These
differ from what you fly, but the principles are the same. I have a torque
meter and have torque tests going back to 1979.
 
The ceiling height and torque curve limit the amount of rubber that can be
carried by limiting the amount of energy that can be used in the flight
trajectory. The shape of the flight trajectory is directly related to the shape of
the torque curve, mediated by the propeller efficiency. The best duration
will occur when you use all the energy available under the torque curve.
Otherwise you are carrying motor weight that does not contribute to and detracts
from duration. The problem of motor design is one of finding that limiting
weight and distributing that weight between length and cross section. Cross
section is determined by the torque required to fly level. That and energy to
reach the ceiling determine weight and motor length.
 
Assuming the propeller efficiency was constant, the best flight would occur
when the torque to fly level occurred when there was 1/4 of the motors total
energy remaining in the motor. Under that assumption, 1/4 of the energy is
used in the climb, 1/2 of the energy is used to maintain altitude during climb
and 1/4 is used in the descent. This is true no matter what shape the
torque curve has. As an example, a sample of Tan II that I tested had this point
at 47% of breaking turns. Propeller efficiency is not constant. It is
highest at the beginning and drops to zero at the end of the flight. This biases
the cruise torque point toward the high end. My airplanes seem to do best
when the cruise torque is around 53-55% of maximum turns. The exact value
depends on the distribution of propeller efficiency over turns during flight. It
is actually at the 25% point on the energy delivered by the propeller to the
plane curve.
 
Something in your post caught my eye, but was overlooked in my response:
 
"The last motor used was 2g x 17" wound to 2080 turns and .9 in-oz and
backed off to 1760 turns and .30 in-oz. You're giving up a lot of potential time
if you don't teach them to wind to maximum and back off to an appropriate
launch torque. My models will usually barely maintain cruise if wound to 500
turns."
 
I estimate from the parameters that you gave that 2,080 turns is about 94%
of breaking turns. 500 turns is only 23% of breaking turns. Winding to 54%
of breaking turns would produce much more than cruise torque with this motor.
That is another indication that the motor cross section is too great. I
estimated this motor as 0.084 wide. The ratio of torques at 23% and 54% is 0.70
and the cross sections vary with the 2/3 power of torque, so I suggest
reducing motor width by 0.79 to 0.066". The length will go up to 21.6" to keep
the same weight, but you have been hitting the ceiling, so you may want to try
shortening the motor until it just clears the ceiling. Or try a half motor
and extrapolate from the maximum height it gives.
 
Gary Hinze


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Received on Mon Dec 04 2006 - 04:19:34 CET