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goodings(at)yorku.ca
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 Posted: Sun Apr 30, 2006 5:39 pm Post subject: RotaxEngines-List Digest: rpm-prop pitch-vne |
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This note is primarily an answer to David Joyce. Thank you, David, for
your reply to my post. You are right; I made a mistake (one of many,
perhaps!). Originally, my prop pitch was 18.75 degrees (not 19.75), and I
increased it to 19.5 degrees, causing a drop in DYNAMIC rpm in climb from
near 6000 at full throttle (of course, I never ran it over 5800) to 5600.
It seems to me that 5600 STATIC rpm should give about 5800 dynamic rpm in
flight; this is what I would like to achieve. I believe my airspeed
indicator is close to correct, checked against the GPS. I have mounted a
pitot tube (under the left wing) which incorporates a static pressure port
underneath, scavenged from a Piper aircraft; it has been very satisfactory
from day 1. This static pressure port gives correct altimeter readings,
as checked against my transponder and encoding altimeter. This avoids a
static pressure port on the side of the fuselage; on some aircraft, there
seems to be great confusion as to just where to place it!
John Goodings, C-FGPJ, CH601HD Zodiac.
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goodings(at)yorku.ca
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| Posted: Mon May 01, 2006 6:16 am Post subject: RotaxEngines-List Digest: rpm-prop pitch-vne |
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I would make one final comment from my point of view about adjusting blade
pitch for maximum rpm. If you set the blade pitch for maximum rpm of 5500
(maximum continuous power allowed by Rotax), you can never achieve full
power at 5800 rpm (maximum power allowed by Rotax for 5 minutes). I
suspect the difference in power between 5500 and 5800 rpm is substantial.
What is the point of having an 80 or 100 HP engine if you can never use
the top 10 or 15% of that power? The one flying school I know about with
75,000 hours on Katanas with 80 HP Rotax 912 engines sets maximum rpm at
5800. You are quite right about calibrating the tachometer with a
phototach. My new Mitchell tach reads correctly, but the first one read
low by about 30%. Since the first Mitchell tach was new, it caused some
anxious moments with respect to overspeeding the engine at full power;
i.e., full power gave a reading on the tach of about 4200 rpm.
John Goodings, C-FGPJ, CH601HD with R912S, Toronto/Waterloo.
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Thom Riddle
Joined: 10 Jan 2006
Posts: 1597
Location: Buffalo, NY, USA (9G0)
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| Posted: Tue May 02, 2006 3:16 am Post subject: RotaxEngines-List Digest: rpm-prop pitch-vne |
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John,
For the sake of accuracy, the 912S produces 100 hp at 5,800 rpm and 95
hp at 5,500 rpm. Pitching the prop for a max of 5,5800 gives you 5.3%
more power. The 912UL is 81 hp at 5,800 and 79 hp at 5,500 or an
increase of only 2.5%. Is it worth it? Yes, if you operate at high
density altitudes, near gross weight, or from short air fields.
Otherwise, maybe not.
Each owner gets to decide for himself. Since we don't operate often in
those conditions, we like the extra security of knowing that the engine
will never go over redline without doing a full throttle descent, which
in our airplane would risk going over Vne as well. Have you read what
Rotax says you are supposed to do if you go over redline? Not worth the
risk in my book when the little extra power will not make a meaningful
difference in our normal ops.
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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