Re: LittleSquare finally earns its wings!
Hi John,
I enjoy and admire your posts, too.
"You appear to assume that a primary requirement of the motor is that the
aeroplane will not touch the ceiling. Surely the flight is not terminated if
the ceiling is touched."
This is a matter of personal choice. I noticed that almost always when an
aeroplane contacts any obstacle, it ends up bouncing off several more
obstacles and doing a wall dive, nose down, top of wing against wall. We have a lot
of obstacles in our gym. I don't like to think of flight as a 3-D pinball
game. I set for myself the task of developing a method for finding the motor
cross section and length that would get the maximum duration without touching
the ceiling. If you have a smooth flat ceiling and the airplane can circle
with the prop tapping the ceiling, go ahead. You can fly a 60 foot ceiling
motor under a 30 foot ceiling and get a lot more time. I don't consider that
flying free. Some of the smaller planes can be made to circle tightly enough
to circle around a light fixture between the beams, tapping the ceiling with
the prop tips. But there is an air drift across the ceiling that limits how
long your luck will hold out. I don't fly competitively. I fly to explore
the scientific principles of flight. So my task was limited to flying below
a specified height.
"You say that the propeller efficiency is highest at the start of the flight.
My propellers are certainly not designed that way."
The important point to understand here is that the propeller efficiency
distribution affects the shape of the curve of energy delivered by the propeller,
making it different from the curve of the energy delivered by the motor.
That affects the decision on motor cross section. Everyone is familiar with
torque curves. Very few have integrated the torque curve to produce the energy
curve. Very, very few have considered what the curve of energy delivered by
the prop looks like. Very, very few have considered the relationship of
those curves to the flight trajectory. I have calculated flight trajectories
from such curves and that seems to support my concept for motor selection. I
tried to start with the familiar and move toward the unfamiliar. If the
propeller efficiency was constant, the cruise torque point is at 47% of that
particular torque curve, fairly typical of Tan II. The 47% will vary with
different shapes of torque curve, but the point always occurs at 25% of the
available energy. Propeller efficiency alters the shape of the curve and moves that
25% energy point. My experience with one propeller indicates that the shift
is to about 54% of turns, implying the prop efficiency was greater at the
higher torque end, at the beginning of the flight. Maybe other props will
produce a different result.
"Your statements on backing-off are not to clear but you would appear to
assume that the shape of a backed–off torque curve is the same as the shape
of part of a non-backed-off torque curve, and that is not so."
No. I understand that the backed off curve is a portion of the full torque
curve. I understand that the rationale of backing off is that you avoid the
torque spike and fly on the "flat" part of the torque curve. What I do not
understand is the tradeoff involved in sacrificing more that half of the
energy available from the motor, while carrying the whole weight. Maybe I am
missing something, but I have never seen any discussion of this. What is gained
by sacrificing energy? Is there a tradeoff with an optimum tradeoff point?
Where is it? How do you find it?
"You have so much to contribute, please slow down a little!"
I don't have time to slow down, I'm not going to live very long, already
losing my eyesight to diabetes. And I don't mind making mistakes, that's how I
learn. By posting on here I get my ideas reviewed. Often, though, the
comments are not very illuminating.
Gary Hinze
[Non-text portions of this message have been removed]
Received on Mon Dec 04 2006 - 18:49:07 CET
This archive was generated by Yannick on Sat Dec 14 2019 - 19:13:44 CET