Re: LittleSquare finally earns its wings!

From: <dgbj_at_aol.com>
Date: Sun, 3 Dec 2006 20:19:59 EST

Bill,
 
"Are we talking about thrustline or are we talking about AOA of the wing?"
 
I'm talking about both. Both affect the performance of the plane.
 
"My view on thrustline is that it needs to be in the direction the model is
flying."
 
That would be ideal, but it is not always possible, because of the relative
positions of the aerodynamic force, the CG and the thrustline.
 
"The only way to adjust the AOA is by shifting the CG."
 
No. That is true in glide. But in powered flight you have a third force,
thrust (and a torque), and a different balancing of forces that will move the
AOA to accommodate the balance of the three forces. In pitch stable glide,
the AOA will shift to move the CP to coincide with the CG. If the AOA is too
high, the CP will be behind the CG and a moment will rotate the AOA downward
toward trim AOA, and vice versa. Adding a thrust moment disturbs this
balance and requires a different trim AOA to make it balance. I am not familiar
with the configuration of the LittleSquare, but most indoor configurations
(Pennypland, Ministick) have CG above thrustline. That would imply that the
thrust produces an up pitch moment. To accommodate that the plane would fly at a
higher AOA to produce a countering down moment. Moving the CG could fix
this, but only for one value of thrust. Same for changing wing incidence. That
would spoil it for every other value of thrust. Changing the location of
thrust is difficult. The motor hangs below the fuselage stick, not between the
 wingposts. Changing the angle of thrust may work; it produces a correction
proportional to the disturbance, downthrust to counter the upward moment.
This puts the prop at an angle to the direction of flight, which produces
additional moments from the prop. As usual, we are seeking an optimum balance
between a number of competing variables. How do you find the best combination?
 Trials.
 
"Neil - 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 think you've just told us the reason for the short flight times
you're experiencing."
 
Backing off 10% from maximum (breaking) turns backs off 30% from available
energy and converts 4,000 ft-lb/lb rubber into 2,800 ft-lb/lb rubber. Backing
off 15% from maximum turns backs off 40% from available energy, converting
4,000 ft-lb/lb rubber into 2,400 ft-lb/lb rubber. Backing off 20% from
maximum turns backs off 45% from available energy, converting 4,000 ft-lb/lb rubber
into 2,200 ft-lb/lb rubber. I think this last case approximates what you
have described. You are carrying a lot of rubber weight and not using its
energy potential. When you back off on turns, you are sacrificing energy on the
highest energy density part of the torque curve. If you need less torque, I
suggest thinner rubber, which also permits more turns per inch, and a longer
motor for even more turns for the same weight of rubber. And wind to at
least 95% of breaking turns. So what if the motors don't last long. Rubber is
fuel. Burn it.
 
Gary Hinze


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Received on Sun Dec 03 2006 - 17:27:52 CET