Re: LittleSquare finally earns its wings!

From: <dgbj_at_aol.com>
Date: Tue, 5 Dec 2006 01:29:55 EST

 
In a message dated 12/4/2006 9:55:40 A.M. Pacific Standard Time,
fred-tellier_at_cogeco.ca writes:

Low ceiling flying with duration models is all about cruise time and using
the turns efficiently. The longer one can spend climbing to the ceiling the
less time spent bouncing around the ceiling and the longer the flight. Even
with VP props on F1D I use a larger motor cross section in low ceilings than
high. The goal is to use up the turns in the cruise and the larger motor give a
little more rpm in the later part of the flight which should and will
increase the time.




Fred,
 
The thinner motor will spend more time climbing to the ceiling than the
thicker motor. It has a "flatter" distribution of torque around the cruise point
for a longer cruise without hitting the ceiling. It permits you to use more
of the torque curve without exceeding the models ability to control torque
roll or hitting the ceiling. This model clearly will fly better with a
thinner motor. It is using a 60 foot ceiling motor in a 30 foot room. It would
fly better with a better prop, too, but contest rules restrict it to
inefficient props. For the given prop type, the best prop is the one that flies level
with the least torque, an easy task.
 
We are speaking here in generalities. Thin and thick are relative terms.
Thinner isn't always better. There is an optimum size. My criteria help find
that size. The optimum motor is the one that takes off from floor level,
climbs to and levels off just below the ceiling and lands with few or no turns
left on the motor. For such to happen, the cruise torque will be somewhere
in the middle of the torque curve, the exact location depending on the
distribution of propeller efficiency during the flight.
 
In short, my rules for selecting motor size and prop pitch:
 
Prop is selected for minimum torque to fly level.
Motor cross section is selected to produce that level flight torque at about
mid turns.
(Motor is wound as close to 100% as you dare)
Motor weight and length are selected for ceiling height.
(This can be estimated (1.) from energy required to reach ceiling divided by
average prop efficiency in climb, (2.) extrapolation from trial flight, in
which the effect of the prop is implicit in the data or (3.) trials with
progressively shorter motors. If the motor weight becomes significantly different
from torque test motor, calculate or re fly level flight torque and
recalculate cross section.)
It is an iterative process. Starting with a close guess makes for a short
iteration.
 
Gary
 
Note: I wrote this last night and was interrupted, just found it in my
saved file.


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Received on Mon Dec 04 2006 - 22:31:10 CET