Yes power is a sort of product of rpm and MAP. I have recently done a load
of flight testing for Mark Burton using a VP prop, fuel computer and
manifold pressure gauge to determine the Rotax power curve for different
MAPs and RPMs. As a result Mark has now refined the power display on his
Constant Speed propeller controller so that it gives a good indication of
%age power based on MAP and rpm. Interestingly speed and fuel flow
correlated very well and for a given fuel flow similar speeds were produced
regardless of MAP and rpm (within reasonable limits). I have flown my
aircraft with VP prop for almost 300 hours using a fuel flow gauge rather
than MAP. This worked very well for me as I could see the %age power I was
using at any time. I have to say now I have been flying with a Constant
speed upgrade which also gives me MAP I like the quick response of the MAP
gauge compared to the fuel flow meter. Having the MAP and rpm linked to give
power as %age power means you can throttle up to your desired power setting
quite easily with fewer adjustments. MAP is also cheaper than a fuel
computer so I think I would be tempted to rely more on that in future.
For a fixed pitch propeller operating at a set height the %age power should
be determined from the prop speed alone providing you know the engine power
at full throttle. That is to say if your Rotax 912 and fixed pitch propeller
for example are configured to give 5500 rpm at full throttle in level flight
that's 79hp from memory at sea level. Using a cube rule the hps at rpms can
be calculated.
5500 is 79 hp
5400 is 75 hp
5300 is 71 hp
5200 is 67 hp
5100 is 63 hp
5000 is 59 hp
4900 is 56 hp
etc.
However as the aircraft moves slower at lower power settings the propeller
becomes more heavily loaded (higher MAP) and this simple method can not
actually be used with total accuracy. In practice the lower power settings
will be predicted lower than they actually are.
Jerry
Jerry@ban-bi.com
www.Ban-bi.com or www.avnet.co.uk/touchdown
----- Original Message -----
From: "Kingsley Hurst" <hurstkr@growzone.com.au>
Subject: Re: was RE: Europa-List: G-OWWW first flight .now a debate about
Throttle/power levers.
<hurstkr@growzone.com.au>
>
> All
>
> I'll stand being corrected if I am wrong but if I remember correctly what
I
> learned over 30 years ago when I did my commercial subjects, the power of
an
> engine is proportional to Manifold Pressure (controlled by the throttle
> and/or altitude in the case of a normally aspirated engine) and is also
> proportional to Revs per minute (RPM)
>
> Given that the RPM remains constant, less throttle (MP) = Less Power, more
> throttle (MP) = More Power
> also
> Given that the MP (throttle setting) remains constant, less RPM = Less
> Power, more RPM = More Power
>
> It therefore follows that for a given power setting, there are many
> combinations of MP and RPM that will do the trick.
>
> Also if I remember correctly, for a given power setting, best economy is
> achieved with the combination of maximum MP (allowable) with the minimum
RPM
> for that power setting. This may only be the case for radials ...... I'm
> not sure.
>
> With a normally aspirated engine, if you are cruising at say 8,000 ft on
> full throttle and wish to climb to say 10,000 ft, the only way extra power
> can be attained is to INCREASE the RPM assuming you were already cruising
at
> less than Max RPM and ignoring mixture settings.
>
> I note John Wigney has already covered what Rotax has to say so I would
like
> to make the comment that with a 912 as in my case, I cannot see why a MP
> gauge should be mandatory but it is certainly desirable if one wishes to
get
> the best out of the engine both in performance and economy otherwise one
is
> just guessing.
>
> Since power is proportional to mp AND rpm, what would be the point in
having
> one gauge and not the other?
>
> Just my two bobs worth. I think manifold pressure may be abbreviated as
MAP
> nowadays but I'm sure it was MP once upon a time!!
>
> Regards
>
> Kingsley
>
>
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