"This seems to come quite close to Fred's observation, that in the salt
mine the question is not how to get to the ceiling, but how to get there
with the smallest number of turns used. Sounds to me that this is
precisely the same thing, how to keep the prop efficiency up at the high
pitch setting. Am I right if I speculate from these messages, that a
flaring prop might have a better efficiency at high pitch than a VP? If
so, why?"
The important question for prop design is not about using it to control RPM,
but using it to convert torque energy into altitude energy with the greatest
efficiency. It is not getting there with the smallest number of turns, but
getting there with the smallest energy. If the plane climbs too fast with an
efficient prop, a thinner motor producing less torque is indicated. If the
plane is capable of hitting the ceiling, a motor of lower weight is
indicated. The ceiling, together with the weight of the plane, the weight and
specific energy of the rubber and the prop efficiency, imposes an absolute limit on
how much energy the plane may use. Spoiling the efficiency of the prop to
keep the plane from hitting the ceiling is making it necessary to carry more
weight of rubber than necessary. That extra weight takes away from duration
during the entire flight. The increase in sinking speed times the model weight
times the duration is the energy wasted by the extra weight, and you are
using less of the energy available from the rubber with the spoiled prop.
Gary Hinze
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Received on Mon Mar 19 2007 - 14:05:43 CET