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dave(at)dreamwings.co.za
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 Posted: Wed Feb 22, 2006 10:31 pm Post subject: RPM vs MP |
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I have just bought a new SportStar manufactured by Evektor-Aerotechnik in
the Czech Republic. I have a Rotax 912S engine and a Woodcomp SR-3000 VP
prop on it.
Can any of you operating VP or CS props give me some idea of what RPM vs MP
you are running at to give good economic cruise numbers?
Dave
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Thu Feb 23, 2006 4:31 am Post subject: RPM vs MP |
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This information is given in paragraph 10.1.1.1 in the Rotax 912 Series
Operator's Manual which is available for download at
http://www.rotax-aircraft-engines.com/aircraft/aircraft.nsf/index?
Openpage
It does not matter whose prop you are using or whether it is a manually
adjustable or CS prop. The combination of MP and RPM is what matters.
Thom in Buffalo
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_________________
Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
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dave(at)dreamwings.co.za
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| Posted: Fri Feb 24, 2006 9:02 am Post subject: RPM vs MP |
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Thanks Thom
I've seen that but don't see how it helps. If I climb to 6000ft I get about
23in.Hg MP at full throttle. What I want to know if how do I now set up for
a reasonable cruise. It doesn't seem to make sense that you can run full
throttle and set the RPM to any value you like - at some point the prop will
be too course. I'm looking for some practical tips from owners of VP props
on how they set up MP vs RPM. Am I missing something here?
Dave
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malann(at)bigfoot.com.au
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| Posted: Sat Feb 25, 2006 2:11 am Post subject: RPM vs MP |
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Hi Dave,
I have about 180 hrs on an Esqual. My 912uls has always been sweetest at
around 5200 revs so it stays there around 90% of the time. If I want to go
fast its full throttle(135-140 kts) If I want to dawdle I come back to
22"map giving about 108 kts. Interestingly at 135kts at 6000-7000' it
will only drop about 1-2 kts if I leave the throttle wide open and drop
back to 4800 revs, but engine becomes less smooth. I had my prop balanced
in flight at 5200 and it did not change the feel but it changed the sound
enough that when we flew home we took some time to accept that things were
hunky dory.
Bear in mind that at 23"map you are not producing a lot of power, but
because speed goes up about 1.6% approx/1000' this helps to compensate the
reduction in map at 6000'. I would just pick a revs that the engine feels
sweetest and go for it. Control the speed with manifold pressure.
Malcolm
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ami(at)MCFADYEAN.FREESERV
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| Posted: Sat Feb 25, 2006 2:32 pm Post subject: RPM vs MP |
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Where the engine sounds sweetest has a lot to do with how the carbs are set
up; i.e. the point at which they are nicely in balance. Bear in mind that
the asymmetry of the carb. arms resulting from (non-precision) manufacturing
tolerance means that it is impossible to achieve balance across the useable
rev-range.
There has been alot of debate about lower engine speeds resulting in higher
rates of wear; but nothing on this issue from Rotax themselves. Meanwhile,
some training ultralights in the US are claiming 3000+hours engine life.
It remains that (in the case of the ULS) the engine is most efficient at
4800 rpm (the max torque speed) and falls off the cam at about 3800 rpm.
There seems to be a common 'roughness' between about 4100 and 4400 rpm;
which is coincident with a slight dip and rise in the torque and bsfc curves
respectively, suggesting that the engine is not 'happy' in this band.
DuncanMcF.
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dave(at)dreamwings.co.za
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| Posted: Sun Feb 26, 2006 7:11 am Post subject: RPM vs MP |
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Thanks to all for the input, that is the sort of info I was looking for.
Dave
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