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<<<< If Thrust is defined as the force required to pull the airplane through
the air and V is the velocity of the airplane then would not "np" be the
"Propulsion Efficiency" you mention?
Curious readers want to know. >>
Thanks for the help, but no, I'm afraid the V can't be ignored. V at or
near
zero invalidates the basic thrust equations of
T = np * 550 * HP / V
and...
np = ( T * V ) / (HP * 550)
because we would have to divide or multiply by zero. Gee, that means that
np
would be either infinite or zero (percentage or not <LOL>). In fact, the
inversely proportional relationship of T and V represents J, the difference
between the angle of the helix wake field and the freestream. Thrust is
nonlinear with V near zero and np does not apply.>>
Wow. I must have been out to lunch and missed something on this thread. I
don't know what a "hard physics background is" so I must have a "soft
physics" background. I could let it go, but I would like to understand the
subject better. The equation for "np" above certainly sounds like
efficiency to me. I assume that "PE" is defined as the efficiency of the
system installed in the aircraft (if you mounted a flat plate on the
airframe just behind the propeller the propeller might think it was being
very efficient, but most of the thrust it produced was negated by blowing
the air against the flat plate?) and "np" is the efficiency without the
airplane behind it (as measured by putting a load cell on the engine mount,
let's say)? Now that is settled (or do I have it backwards?). And where in
this is what I have heard called "slippage," or the ratio of the distance
the airplane travels to the theoretical distance calculated by the blade
pitch. Oh, and the equation above MUST apply at zero speed - how could it
not - it just says that propulsion efficiency is zero at zero speed. Maybe
my confusion comes from the definition of efficiency in the first place. I
learned that efficiency is the desired "outcome" divided by the "effort" it
takes to get it. The propeller designer wants to blow air - as far as he is
concerned the propeller can be very efficient at zero speed because it will
still blow a lot of air. The user (me) says that the propeller has zero
efficiency because I want the propeller to move the airplane and it isn't
moving. My passenger says that the whole airplane has zero efficiency if he
wants to go north and I am flying east. Since the propeller designer can't
control the drag of the airplane behind it the only real way to compare
props is to look at the thrust the prop produces at a given speed with a
given input power. In all this discussion I don't seem to get much closer
to understanding what that might be. For example, what is the range of
static (zero speed) thrust to horsepower? How much can I expect to lose by
having a smaller 4-blade prop compared to a larger 3-blade? How concerned
should I be concerned about actual tip Mach numbers in cruise? Is the twist
distribution along the blade a big deal? Little deal? What is the limiting
Mach for an wood (MT) prop blade compared to an aluminum one, which I assume
is thinner?
Gary Casey
ES project
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