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In a message dated 7/20/2007 7:26:24 P.M. Eastern Daylight Time,
Panzera@Experimental-Aviation.com writes:
From: Rotary
motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of
Lehanover@aol.com
Sent: Thursday, July 19, 2007 11:21
PM
To: Rotary motors in
aircraft
Subject: [FlyRotary]
Re: Thrust measure.
In a message dated
7/19/2007 9:22:55 P.M. Eastern Daylight Time,
Panzera@Experimental-Aviation.com writes:
With all due
respect, what does measuring static thrust do for
us?
Pat
Just guessing here but I suspect that a propeller with huge static
thrust might be better for the crop duster or bush plane than little pencil thin
high speed prop from a formula one plane with little static thrust. More static
thrust comes from disc area just like the helicopter blades.
Look at an airboat prop. 4 big blades. Wide chords. Large diameter.
Works with much turbulence in the inflow. Takes a fully loaded boat off the
grass from a standstill, and right up to 80 MPH.
My ideas may not be
the complete list of information that could be recovered. Mr Lipps would know
much more. Perhaps you could get Mr Lipps to comment? Paul was it? I enjoyed
his talk at Jean this year. Thank you for that
effort.
My
pleasure.
I just did a key-word
search through our current “All Paul Lipps” issue of CONTACT! Magazine, for
the word “thrust”.
www.ContactMagazine.com/backissu.html
Here’s one short
paragraph where Paul mentions static thrust. All other mentions (and formulas)
of thrust are related to dynamic thrust.
SINGLE
BLADE MYTH
One of the
myths that has been propagated in the aviation community, to the point that it
is “gospel”, is that the most efficient prop is a single blade; all higher
number of blades falling further and further short of this paragon. Did you
ever consider that a single-blade prop, developing thrust on only one side of
the plane as it revolves, would cause the engine to cone violently in its
mounts as it is twisted by the prop? The European’s latest turbo-prop
transport, the A400-M, has eight-blade
props!
The Boeing MD-900 helicopter has a five-blade rotor. A popular regional
turbo-prop airliner has a five-blade prop. Hasn’t anybody filled these
plane-makers in on the errors of their ways? In a recent edition of Kitplanes,
the author of an article on props uttered the same fallacy. He maintained that
multiple blades interfere with each other. When I pointed out to him that at
200 mph and 2800 RPM, the blades on my three-blade prop follow three distinct
helical paths through the air, and each blade is 25” ahead of the previous
blade at the same point of rotation, he rather lamely explained that in static
conditions interference occurs. STATIC? Who uses static thrust? Airplanes are
meant to fly, not pull tree-stumps!
I know from several
conversations with Paul that he cares very little about static thrust.
He designs high speed props for fast home builts and racing planes.
None of them have much in the way of static thrust.
Propeller thrust
might be useful in designing propellers.
At a gathering we had
some time back, we had two nearly identical Corvair engines running on
separate test stands. They had completely different props on them, one made
for a Pietenpol, the other for a single place Sonerai. They both spooled up
nicely to 2700 RPM. The Sonerai prop made a lot of wind for sure. The Piet
prop made so much wind one couldn’t hardly stand or breathe behind
it.
Here’s a shot of me,
behind my test rig, running the Sonerai prop at full
tilt.
http://www.experimental-aviation.com/Corvair/Images/WCCC/Event/TuftTest.jpg
If we had thrust
measuring equipment I’m certain it would show us that the other engine/prop
combination was easily making 2x the static thrust I was
making.
Big picture, what
does that really prove?
If you start the test with all of the questions
answered, it is of little
value.
If you have a pile of props you have borrowed,
trying to find one for the Pietenpol, The ones with the highest static thrust
would be the place to start. The Pietenpol is a big load to get rolling. When
you are lined up and add the power, is that static thrust or dynamic thrust?
When the Ag Cat shudders in the top of a turn with 1,200 pounds in
the hopper, do we need the little pencil thin high speed prop or the 12'
Hamilton standard 4 bladed with the huge static thrust?
Could I bolt the
Pietenpol prop to the Sonerai and expect 2x thrust at altitude? No way. I’d
probably climb like crazy but max out at 110 mph.
Yes that is correct.
But you know that. The sum total of all you know from years of exposure provides
that to you. What if you didn't know that?
What does it tell us
about the engines? If we could measure HP we could be sure that they were
making the same power... but what does these static thrust measurements tell
us about HP output?
Very little. If both engine turn the same prop the same RPM, I
suggest that they have the same HP.
We did swap props
however. My friend ran my Sonerai prop on his engine and got the same RPM. I
didn’t put his prop on my engine however. But for me, his ability to swing my
prop on his engine at the same static RPM tells me that the engines were making
identical power. Thrust told us nothing.
It is hard to imagine a prop dyno not
recovering thrust information.
A prop dyno as
opposed to a water brake?
In this case the dyno is for measuring the engine
power by having a propeller absorb the output of the engine, just as a water
brake would on an engine dyno.
I’m having difficulty
seeing the need for knowing static thrust at all, that’s why I asked the
question in the first place.
So if it’s hard for
you to imagine a prop dyno not giving static thrust info, please tell me what
one can gain from knowing this information?
For experiencing propeller
stall.
Ok... I’ve seen
countless little red biplanes hang from their prop at airshows; certainly the
prop is “stalled”, and knowing that the engine/prop combination can produce
thrust to counter the gross weight in order to allow the plane to hover is
useful information to this small group of pilots. But how many of us really
need that info?
Well, those props are actually not stalled at all. Props stall just
like wings. The angle of attack is too high and the flow separates from the
upper surface. Much lift (thrust in this case) is lost, and the engine is
unloaded to some extent, and gains RPM as the load is lost. The propeller will
remain stalled until power is reduced, and or aircraft speed is increased. The
angle of attack of a fixed pitch propeller is a function of relative wind
through the disc. So higher speed of inflow may un-stall the blade and, or,
reducing propeller RPM may un-stall the blade. The slower the inflow (low static
thrust?) and the higher the pitch, the more likely the stall may occur. It feels
like a slipping clutch and has caught many pilots cold. It is counter intuitive
to reduce power on take off and some people will just not do it, and find
themselves landing again with the engine howling madly.
When you build one of those planes that can hover, you really do
need to know how much static thrust your prop will produce. It has to be some
percentage more than the full gross weight of the plane. When you see one
accelerate straight up out of a hover, the static thrust must be way more than
the planes weight.
For comparing one prop to another.
Yes, you can compare
one to another for a given situation, that being making wind while not moving.
But that does that really tell us???
Say for example you
have XYZ prop installed in a plane you have countless hours in. You measure
the static thrust at 350 lbs at 2250 prop RPM. Now you install an ABC prop that your
buddy says should work better for you and you get 280 lbs at 2250... or you
get 385 lbs at 2340 RPM. What do
we now know about prop ABC? Is it better or worse than prop XYZ? Bear in mind, we know everything there is to
already know about XYZ, but what have we learned about ABC now that we know
what each prop produces in the matter of thrust?
In one case you have the same data point from the two props. In the
second case only one. If the reason for the test is static thrust, the XYZ is
"better". So for accelerating from stopped on a short runway, Or a stall
recovery XYZ is the winner.
Lately for making
movies in a side view of blade flexing in single rotor
installations.
Ah... yes, running a
prop on the ground can tell you a lot of things. That’s why I have such a rig.
But measuring thrust
had no part of the revelations from said prop in the video.
Pat
There is of course no such thing as the static in static thrust.
The low pressure area in front of the prop produces flow through the disc. One
reason the larger disc (bigger diameter) produces more thrust. Larger diameter
has more mass moving through the disc. Props designed for very high speeds trade
high velocity of mass for lower velocity and greater mass.
In this case the device to measure thrust was just part of the
engine dyno display that was proposed as a trailer installation for running a
rotary at Sun&Fun and other venues. It was not the sole purpose of the
engine stand.
Lynn E.
Hanover
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