When we flight our Tecnam P2002, we select the power (mainfold & rpm) following the charts that Rotax inform, we select 26" and 4800 rpm (65%) or 26" and 5000 rpm(75%), when we are climbing up to 5000 feets, at 20º C OAT or less, the mainfold pressure descend, if we continuous ascending, at 7000 or 8000 feets, the mainfold continuous decreasing, normal, the question is: Ascending, when the mainfold go down, out of the normal value respect the rpm, we need reduce the rpm according with the new lower value of the maindfold or continuos ascending without change the rpm?, some time the mainfold reduce the value at 24" or lower, in this case, ascending, we need to reduce the rpm at 4300 (55%) following the Rotax chart?
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Re: Power setting for 912 ULS S3 + Hoffman variablke propeller
by Rob Seaton » 8 years ago
No, you do not have to match the RPM to a lower MAP.
Thank you said by: RAMON VINUALES
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Re: Power setting for 912 ULS S3 + Hoffman variablke propeller
by Bill Hertzel » 8 years ago
The MAP is dropping because the Air pressure is dropping.
The Outside pressure will drop about an inch for every 1000ft of altitude.
This is what your altimeter is measuring.
If the Altimeter setting is 30", the pressure will be ~24" at 6000ft.
Nothing short of adding a turbo is going to get the MAP above 24"s.
A lower MAP is easier on the engine as the combustion pressures are lower.
Increasing the altitude is the same thing as partially closing the throttle.
All you are really doing is lowering the Manifold pressure.
The numbers on the chart are the MAX MAP you should use under sea Level conditions at WOT.
AT Altitude the numbers change.
At ~8000ft only 75% of the atmosphere remains.
Running WOT will never produce more than 75% of Sea Level rated Power.
So, while there is no need to reduce the RPM to match the MAP, it may be to your advantage to increase the prop pitch which will have the effect of reducing the rpm and increasing the MAP. Again this is all at WOT.
As long as you stay under 5500rpm, adjust the pitch for whatever gives the best rate of climb.
In the thin air, the engine will be loafing despite all you can get out of it.
Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.
Thank you said by: RAMON VINUALES
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