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Vance and Emily Jaqua wrote:
On Fri, 02 Jul 1999 09:40:56 Paul wrote:
Jeff Spitzer wrote:
At 11:14 PM 7/1/99 -0700, you wrote:
Oh good another data point. Let's see 300 HP with 800 pounds of thrust.
4.8% efficient at 10 feet per second. Do you have a curve
showing the thrust as a function of aircraft velocity or
did Paul Moller consider that amount of thrust constant with speed?
Of course you realize that you're number of 4.8% or whatever has
practically no meaning. It's amazing how jets manage to fly given that
they have zero propulsive efficiency when they release the brakes. There
is a proper way to calculate static efficiency. I don't recall the proper
formula right now but maybe one of the other fan experts on here can
provide it.
Jeff Spitzer
Of course I know that Jeff. It is just the start of the curve.
We need the thrust as a function of aircraft speed. Do you have it?
This way of calculating duct and fan eff. is very valid indeed.
If thrust is constant with speed than the method is quite
simple and very valid.
The same method is used for calculating the eff. of props.
They all have zero eff. at static so no thrust HP is being generated.
Work and HP are being done but they are merely heating the air.
The static thrust number is useful however for calculating
the initial accelleration.
Paul Lamar
The classic expression for propulsive efficiency
leaves you in the lurch when you get to static,
because everything becomes zero - hate those kinds
of expressions - obviously some kinds of static
thrust makers are better than others, and you can be
sure that the helicopter guys have expressions that
work there.
I can imagine a pounds of static thrust per HP ratio as a
good measure. It will be necessary to know engine torgue
and RPM of course to determine HP. See the ACRE NL for a
drawing of a simple hydrulic torque monitor built into a
motor mount. PL
For any given level of "disk" loading
(hp/sq ft) there is a theoretical maximum value
for thrust per hp. This corresponds to a perfect
conversion of input horsepower to stream power
for a column of air of that cross section. Dividing your
actual thust per hp by this theoretical value in
the usual way gives you an efficiency for your
envelope. This is a measure of how good a job your
propeller/fan/pumping device is doing.
For small
area, high loading devices like jets, this form of
efficiency is also lower than big slow moving
helicopter blades. But at least they are not all
zero. ( I have begun calling this factor "jet
efficiency" of the system maybe "pump efficiency"
would be a more descriptive term) - Vance
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