Ken, in reference to my old Warp at 2600, that's when it would really bite
and use up the torque, so it does make sense that you would use more fuel at
that RPM, too. It's not that it wasn't efficient at lower RPM.
Walter
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On Behalf Of Ken
Sent: Sunday, September 02, 2007 1:53 PM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] More on propeller mach speed
Note that you can't get near these quoted rpm's during takeoff unless
you have a variable pitch prop so the Cessna's for sure are running even
lower tip speed than this. Well except when at 8000 feet at wide open
throttle making 75% of rated power which is an attractive way to run for
them but just not where most spend a lot of time.
Joe - A quick look at the UL260 makes me consider it to be a just barely
80 hp engine for a Rebel. I doubt that it will turn very many fixed
pitch 72" props at 2800rpm for takeoff and still give satisfactoy cruise
but that's just my wild guess. If true though it really isn't even an
80 hp engine on a Rebel. Tom Inglis gets good performance from his 72" 2
blade Warp prop (no blade taper) on his 80 hp. Rotax 912. A good
comparison might be what his prop rpm is at takeoff and cruise and
whether his engine is turning close to max hp. rpm.
Also we were impressed this summer by Jim Cole's 100 hp rotax 912S.
Good performance with a 72" 3 blade Warp (no blade taper) so maybe he
can also tell us the prop rpm and any thoughts about his prop. That is
the same prop that I run. It is cheap, ground adjustable, and has a
good reputation but it doesn't come larger than 72" which means the
larger engines must take the penalty of 3 blades to absorb the power.
Ground adjustable is a huge advantage for a unique engine and blades
that can be shortened is another ace in the hole. Don't know if he would
recommend it for that engine though. Even at my low tip speed it could
easilly handle more power than I have which it will need to do if I bump
the power on floats. The thing is I run about .66M at takeoff and
normally cruise at .62M tip speed according to the calculator. Quiet
and efficient IMO. Walter said that prop performed better for him at
2600+ rpm but my fuel consumption almost doubles if run that hard. I'm
pretty confident that I make about 120 hp during takeoff and climb which
is in the same ballpark as a fixed pitch 0-320 and I seem to have about
the same performance. I cruise slower than those engines though and
seem to enjoy similar fuel consumption and cruise speed as Tom and Jim.
I'm heavier so I do burn a bit more than them but I'd say that the
lower tip speed is working out satisfactorilly for me.
The UL260 engine looks interesting. I don't agree with some of his
statements including much of what he says about diesels though. Anyway
if the engine achieves several hundred hours on a couple of airframes
with a prop that you like then it might be very attractive if the price
is reasonable. Until it has proven reliability and performance though,
personally I'd still favor the rotax. Unless it is certified or
vibration tested with the prop you want, it is still very much an
experiment until it proves itself in service. Interestingly we are
starting to hear about rotax 912's that have been retro fitted with EFI ;)
Ken
Mike Kimball wrote:
I got curious with recent research and discussion and decided to take a
look
at some typical, modern (term used loosely except for the Lancair)
aircraft.
I found the following:
Piper Archer - Takeoff RPM 2700, propeller diameter 76 inches, mach speed
0.815
Cessna 172 - Same as the Archer
Cessna 152 - Takeoff RPM 2750, propeller diameter 69 inches, mach speed
0.756
Cessna 182 - Using a BlackMac 3-blade, scimitar blade design conversion,
takeoff RPM 2600, propeller diameter 80 inches, mach speed 0.826
Lancair 4P - Cont. TSIO-550, Takeoff RPM 2600, 3 or 4 blade, all approved
props are 76 inch diameter, mach speed 0.786
Clearly, the spam-can manufacturers adhear to Ken's "around 0.8 mach"
philosophy. Slow planes versus fast planes doesn't seem to matter with the
Lancair in the same boat. Lot's more thinking to do before I decide
whether
or not to make a change.
Mike
044SR
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