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hgmckay
Joined: 23 May 2006
Posts: 397
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 Posted: Sat Jul 15, 2006 6:53 pm Post subject: Initial Setting of Prop Pitch |
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Gentlemen: I will soon be attaching the prop to my Allegro 2000 (built from the Kit) for the first time. Since I have never done this before I need some advice and guidance. Here is what I have:
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Plane – Allegro 2000
Engine – Rotax 912UL
Gear Box Ratio – 2.2727/1
Prop – WoodComp (3 blade),
Type – Klassic 160/3/R
Diameter – 1600 mm/ 63 inches
Max prop rpm – 2700 rpm
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The WoodComp instruction booklet (second page) gives a general description of setting the prop pitch. I have a Warp Drive Propeller protractor to set the pitch in degrees. I need to know what would be the optimum degree of pitch for good cruise speed. There are various opinions (11°, 13° 15.5°, I understand Bob Griffin operates his Allegro at prop pitch of 20.5°) concerning this, so I would appreciate input from those of you with experience in this area.
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The WoodComp instructions also state that during the test run the rpm (I assume this is the engine rpm) at full throttle must be 6% less than maximum engine allowed rpm (i.e. 5452 rpm if max allowed rpm is 5800). This would give a propeller rpm of 2399. This is well below the max of 2700 rpm. At the max engine rpm of 5800 the prop rpm would be 2552 rpm, still well below the max prop rpm of 2700. Since the carbs on the Rotax engine are spring loaded and set at the factory at full opening (full throttle), I assume this setting gives the 5800 max rpm. Am I correct? If so, how do I set the throttle to never exceed the 5452 rpm (6% less than max rpm) or is this number just a conservative number to assure never exceeding the engine rpm, and propeller rpm? I can set the throttle stop on the Allegro at the max throttle opening easy enough, but don’t know how to get the 5452 rpm setting. I am somewhat confused on all of this.
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If some one has the time I would appreciate a step-by-step procedure on how to do this for the Allegro and, Rotax Engine.
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Hugh McKay
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Sun Jul 16, 2006 5:10 am Post subject: Initial Setting of Prop Pitch |
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Hugh,
We have the same engine/prop combination so the numbers I give you
should be pretty close and certainly good enough as a starting point.
You do NOT ADJUST carburetor throttle stops for cruise rpm settings.
The carburetors should be adjusted so that wide open throttle position
gives you wide open throttle valve in carbs. Don't mess with the carbs.
Our prop is set for best cruise at low altitudes (5,000' DA or less)
since that is where we spend most of our flight time. This gives us
5,500 rpm (max continuous rpm) at wide open throttle in straight and
level flight. To achieve this, our static rpm on the ground at wide
open throttle is around 4900 rpm in calm air. If you do this check with
a head wind the rpm will be higher than this. At this prop setting we
are sacrificing some climb rate but it is still good enough for us.
We have not adjusted our prop since we are happy with the performance
as it is currently set which is the way we got it when we bought the
demo airplane. If you want better climb performance I would still limit
the max. straight and level flight rpm at wide open throttle to 5800
rpm. This will improve the climb performance but reduce the cruise
speed at low DA and increase fuel consumption at a given airspeed.
Typically prop pitch is measured at a point 75% of prop radius from the
prop shaft. On our prop, that is at 23 5/8" from the center or 7 7/8"
from the tip. At this location, I think our prop is set at about 23.5
degrees. I say "I think" only because this is what it calculates out to
be at 80% prop efficiency. We don't have a prop protractor so I don't
really know.
I would set it at this and then tie the airplane securely with chocks
and warm up the engine thoroughly and do a full rpm static test to see
what rpm you are getting in calm air (or close to it) at wide open
throttle. If you are getting somewhere between 4900-5100 rpm, this is
good enough for a test flight. I would not attempt to fly it with less
than about 4,800 rpm static or more than 5200 rpm. Once you are in the
air you can do wide-open-throttle tests to see what rpm you get. This
is the ultimate test and the only one that matters in my opinion. You
have to decide whether you want max climb, max cruise, or some
compromise between the two. Please note that a one degree change in
prop angle makes a pretty big difference in performance and max. rpm. I
would adjust by no more than 1/2 degree at a time and make sure all
three blades are as close to identical as possible. This is important.
I hope this helps, Let me know if you have more questions.
Thom in Buffalo
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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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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Sun Jul 16, 2006 5:15 am Post subject: Initial Setting of Prop Pitch |
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Hugh,
I forgot to mention that the prop manufacturer's max rpm rating is a
safety limitation they set for this prop regardless of what engine it
is used on. As long as their limiting rpm is greater than the
engine/gearbox will produce, you can ignore that 2,700 rpm number. It
has no bearing on prop adjustment with our engine/prop combination.
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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