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jkuehn(at)mountaintime.my
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 Posted: Wed Jul 26, 2006 7:16 pm Post subject: Static RPM - 912UL |
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I had my 912 UL set at 5000 static, and I noticed some oil burn and
sooty spark plugs. I called Lockwood, the USA Rotax importer, and their
very capable tech support told me to run this engine at 5500 RPM
static. Anything less, he said, was like pulling a huge trailer with a
Volkswagen. I was lugging the engine, he said. At 4500 you are only
getting maybe 60 horsepower. I set the prop flatter, to achieve the
5500 static RPM and the difference in take-off power is dramatic. It
jumps right off. I think you have to just be attentive to throttle
setting in cruise. You may need that power sometime! If you set your
static RPM that low you are cheating yourself out of horsepower, and
this could even be a safety issue. I am flying a Sky Ranger which is
not very "clean" so I do not have the over speed problem. I think your
airplane is perhaps cleaner, and therefore you can overspeed the
engine. There are always trade-offs! Mine maxes out in cruise at just
about 5800, and climbs at about 60 MPH at 5500, with the nose nice and high!
Jack
sonny wrote:
| Quote: |
Hello,
I have a Pulsar XP with a Rotax 912UL. When I first bought the plane it had a GSC 2-blade prop but I switched it to a 3-blade carbon fiber prop (made by the same company as Kiev props). After installing the new prop, I initially set the static RPM for 5200 like the old prop, however on takeoff I nearly over sped the engine when I started to level out. I believe it was set at 22 degrees or so (it was a long time ago). So I tried several different settings to get the max in-flight RPM of about 5700.
OK, here's the downside: My static RPM is about 4400. During takeoff it still accelerates quickly...the RPMs are usually around 4800 - 4900 on climb out. I would much rather prefer the RPMs to be in the higher range to maximize takeoff power, but so far I haven't noticed a problem with the lower RPMs.
My questions are, do any of you have your static RPMs set this low? Does anyone think it's a problem that they're that low? I'm getting ready to move from sea-level to an airport that's at about 4000 ft. Should I set the static to be around 4800 RPMs and be careful not to over speed the engine at cruise? I don't like this idea as it's easy to get distracted, and Rotax recommends tearing the engine down in the event of an over speed. Any recommendations or advice is welcome. Thanks.
Best regards,
Sonny W.
Pulsar XP N912SS
.
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Thu Jul 27, 2006 3:21 am Post subject: Static RPM - 912UL |
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Sonny,
It is all a compromise. Think of a fixed pitch (or ground adjustable)
prop sort of like a single speed transmission in your car. If your
drive in low gear (low pitch) you can accelerate and climb hills like
crazy but your top speed is very limited. If you drive off in top gear,
the engine will lug and you will accelerate and climb hills slowly. In
some intermediate gear you will have a compromise between acceleration
and cruise speed. If you are driving a dune buggy then the top speed is
not important but if you are driving a faster car on the Autobahn, then
cruise speed is important. Same with airplanes with fixed pitch props.
The "proper" static rpm depends mostly on two things, one of which is
how fast your airplane is. The other is whether you want the best
cruise performance, best climb performance or a compromise between the
two. Lockwood's flat statement about static rpm of 5,500 rpm is
nonsense and does not take into account either of these two very
important variables.
I fly an Allegro 2000 with the 912UL (80 hp) engine and three blade
Woodcomp composite prop. Our prop is set for best cruise (5,500 rpm
with wide open throttle in straight and level flight) at density
altitudes below about 7,000'. Under these conditions our Allegro
reaches a top speed of 126 mph TAS which is about 15-16 mph over our
normal cruise speed at 75% power. WIth the prop set like this it gets
about 4,950 rpm static (no wind, which is important) and has decent
climb performance. If we reduced the pitch so that the static rpm was a
little higher, the climb would improve but we would not be able to fly
with wide open throttle in straight and level flight without exceeding
the 5,500 rpm max. cruise rpm and our normal cruise speed of 110 mph
would require a little higher rpm and a bit more fuel consumption.
However, we would be able to get 5,500 rpm at higher altitudes.
Your airplane is a bit faster than the Allegro and a lot faster than a
SkyRanger. The faster the airplane, the lower the static rpm (higher
the pitch) must be to reach best cruise speed in S&L flight. So I think
if you set your static rpm for initial flight at around 4,900-5,000 rpm
(no wind) then this will be a compromise setting that is good enough
for the first flight. If you ultimately want the prop to be set at best
cruise at low DAs then adjust the prop so that you get 5,500 rpm with
wide open throttle at low DA. Since you are moving to a higher
elevation airport, the best cruise pitch will probably be just a little
less (higher static rpm) than at sea level. If you want better climb
then the static rpm no more than 100 rpm for each test until you find
the ideal pitch for your most common flight profile. Good luck.
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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NYTerminat(at)aol.com
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| Posted: Thu Jul 27, 2006 1:23 pm Post subject: Static RPM - 912UL |
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Sound like you would be a great candidate for an in flight adjustable pitch prop.
Bob Spudis
N701ZX
CH701/912S
do not archive
In a message dated 7/27/2006 7:22:33 A.M. Eastern Daylight Time, jtriddle(at)adelphia.net writes:
| Quote: |
the engine will lug and you will accelerate and climb hills slowly. In
some intermediate gear you will have a compromise between acceleration
and cruise speed. If you are driving a dune buggy then the top speed is
not important but if you are driving a faster car on the Autobahn, then
cruise speed is important. Same with airplanes with fixed pitch props.
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ami(at)MCFADYEAN.FREESERV
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| Posted: Thu Jul 27, 2006 2:19 pm Post subject: Static RPM - 912UL |
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My climb out rpm is 4800. How can this be "lugging" the engine when it
develops maximum torque and is at its maximum efficiency at hits point?!
But this is with the ULS engine that has a surfeit of power. If your
airframe is a bit marginal on 80hp then this may not work for you.
By happy coincidence, the prop setting that allows this also results in
5800rpm at Vne (Europa, 172mph), but who wants to hold Vne for more than 5
minutes.
Another factor to consider is balanced field performance (i.e. being able to
take off in a similar distance to the distance it takes to land). What's the
point of not being able to land back where you took off from? Or worse, vice
versa!
Duncan McF.
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lostpilot28
Joined: 26 Jul 2006
Posts: 7
Location: Southern California
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| Posted: Thu Jul 27, 2006 2:19 pm Post subject: Static RPM - 912UL |
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I agree...now if I could just find one that was inexpensive.
Regards,
Sonny W.
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dave(at)cfisher.com
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| Posted: Thu Jul 27, 2006 2:57 pm Post subject: Static RPM - 912UL |
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Duncan,
At 4800 rpm are you WOT ? If so you are infact lugging your engine.
http://www.rotec.com/engines/4sf.htm
How do you figure this ? <<develops maximum torque and is at its maximum
efficiency at hits point?!
Dave
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Fri Jul 28, 2006 4:27 am Post subject: Static RPM - 912UL |
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Sonny,
I had a GSC in-flight adjustable prop on my old Titan Tornado and it
was great. I believe the cost is around $1,400. It is mechanically
adjusted with a vernier control just like that of a constant speed
prop, but without the governor. In my opinion it is better than the IVO
electric adjustable in common use in the USA. If you are interested,
there website is
http://www.ultralightprops.com/gta_inflight_adjustable_props/
adjustprops.htm
I don't have one on the Allegro because we want to keep it legal S-LSA.
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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jkuehn(at)mountaintime.my
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| Posted: Fri Jul 28, 2006 7:35 am Post subject: Static RPM - 912UL |
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Thom,
I had not seen the GSC props. They look really good, and the in-flight
adjustable looks simple and elegant. I am not altogether happy with my
Ivo three blade for a couple of reasons: first there is no good
provision for leading edge protection, the stainless steel tape they
recommend does not stay put for long. The prop itself does not appear
very presicely made and I can't get away from the feeling that I would
have a smoother running setup with something else. The adjustment
mechanism is easy and effective, but I would trade that for higher
quality, and I am tempted by the in-flight adjustable feature for all
the reasons we have heard in these pages. There is another good looking
prop that is relatively new. Check out:
http://www.tampabayaerosport.com/propellers.html
<http://www.tampabayaerosport.com/propellers.html>
Does anyone have experience with this setup?
Jack
Thom Riddle wrote:
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Fri Jul 28, 2006 9:59 am Post subject: Re: Static RPM - 912UL |
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Jack,
One (main) reason I liked the GSC vs. all the electric operated props is that it is mechanical and you tell by the number of turns on the knob exactly where you are in pitch. There is no gauge nor need for one. The electric props, at least the ones I've seen, have brush wear issues and you cannot tell exactly where you are in the pitch range.
Also, the prop is very well built and rugged. Mine had the Maple blades (light weight) with a leading edge insert of some sort of black material that was harder than all the rocks and stones that hit. With the pusher configuration, the main gear tends to pick up stones and throw them into the prop. My prop showed no damage after 6 years and three hundred hours of use. If you prefer, they can also fit Warp Drive blades to their prop hub.
I know it sounds like I'm selling the GSC but I'm just pilot who has had very good results with the product.
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_________________
Thom Riddle
Buffalo, NY (9G0)
Don't worry about old age... it doesn't last very long.
- Anonymous |
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