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rjdaugh(at)rapidnet.com
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 Posted: Mon May 08, 2006 6:09 pm Post subject: Thrust - Over Rated? |
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I have been seeing discussion of thrust lately and I think it is nearly
irrelevant.
I have a PowerFin three blade prop on a 912S Series 5/7. When I first set
it up, static RPM was 5800 RPM. And MAN DID IT HAVE THRUST! The
acceleration would make your head snap back as you pushed the throttle in.
Well, it came time for first flight, and boy did it get off the ground fast!
I don't have good numbers, but I am guessing about 150 feet ground roll.
BUT (you had to know that was coming), it would only fly about 65 - 70 mph
without over-reving the engine.
So I put in more pitch on the prop. Got up to 85 mph. But lower static
rpm, less thrust and acceleration, longer ground roll. Added more pitch -
95 mph, and less thrust. More pitch - WOT about 108 mph - cruise at 5500
rpm about 96 mph - static rpm about 4900 rpm. But now ground roll on take
off approaching 500 ft. Spam can territory!
PowerFin said they would shorten the prop 1" each blade. I finally took
them up on it, but it was a tough decision for me.... When I put it back on
the plane, I added a degree and a quarter more pitch. Now I cruise at 106
mph AND off the ground much quicker - maybe 275' solo.
In short, if you want more thrust, all you have to do is sacrifice top
speed. Therefore, thrust is irrelevant.
Indeed, it is more complicated than that. An "efficient" prop slips enough
on take off to get the rpm up so that you can use most of your horse power
to get off the ground, but still has enough pitch so that you can get a
decent top speed. My plane is "over prop'ed" with the three blade PowerFin.
I am saving my nickels for the Sensinich ground adjustable two blade.
I would have written this earlier but was in Montana visiting my new
grandson and didn't have my flame suit with me.
Randy - Everything in aviation is a compromise.
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lcfitt(at)sbcglobal.net
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| Posted: Mon May 08, 2006 7:00 pm Post subject: Thrust - Over Rated? |
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Randy,
No flames here. I have two friends that went to the PowerFin. They had the
exact issues you had and eventually went to the shortened blades. They got
the improvements you did. One is now with the Warp and doesn't know if he
likes it. I think the other still has the Powerfin. They'll both be at the
Cameron Park fly-in.
I guess that is what the poor man's dyno was all about - finding the HP at
the prop.
Lowell
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lcfitt(at)sbcglobal.net
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| Posted: Tue May 09, 2006 5:36 am Post subject: Thrust - Over Rated? |
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Error Message, Regarding the Powerfin, one went to the Ivo, not the Warp
and I am not sure he is happy with it.
Lowell
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rjdaugh(at)rapidnet.com
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| Posted: Tue May 09, 2006 8:26 am Post subject: Thrust - Over Rated? |
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I guess my main message is that to get the use of your horse power, you need
rpm's. Horse power is rpm x torque. If your prop doesn't slip on take off
and you have so much pitch in your prop that you can't get the rpm's up, you
are not going to get good take off performance and good cruise at the same
time.
Mike Gibb's point about the variable pitch prop is right on, but I want to
keep my plane LSA compliant. I seem to be getting older and want to fly a
long time yet. Some props are more efficient than others. I am still
looking.
Randy
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roncarolnikko(at)hotmail.
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| Posted: Tue May 09, 2006 11:51 am Post subject: Thrust - Over Rated? |
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Yes Randy That is excactly the situation I am exploring. To get VW
horsepower you need 3600 rpm. To pull a Kitfox you should have a 68" or
more prop. To stay less than .8 mach at the tip of the prop requires a
redrive. I have an ifa prop, but will not use it on my AW cert for sport
applicability. This means finding the best combination of length and pitch
to give a climb and cruise compromise.
The VW direct drive has been critisized by some as lasy on climbout. For
that reason I am concerned about thrust. Some think I should be concerned
about using the VW. Don't worry I have flameproof shorts on so flame on.
Ron NB Or
[quote]From: "Randy Daughenbaugh" <rjdaugh(at)rapidnet.com>
Reply-To: kitfox-list(at)matronics.com
To: <kitfox-list(at)matronics.com>
Subject: RE: Thrust - Over Rated?
Date: Tue, 9 May 2006 10:24:28 -0600
<rjdaugh(at)rapidnet.com>
I guess my main message is that to get the use of your horse power, you
need
rpm's. Horse power is rpm x torque. If your prop doesn't slip on take off
and you have so much pitch in your prop that you can't get the rpm's up,
you
are not going to get good take off performance and good cruise at the same
time.
Mike Gibb's point about the variable pitch prop is right on, but I want to
keep my plane LSA compliant. I seem to be getting older and want to fly a
long time yet. Some props are more efficient than others. I am still
looking.
Randy
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