Dear Barry
The increased fuel flow with altitude is due to the carbs going rich.
Despite the mis-information to the contrary, the Bing constant vacuum carbin
the 912 does not compensate for altitude. The reason the 912 does not need
a mixture control is that it is liquid cooled with small cylinders and can
run at both sides of the stoiciametric AFR without detonation ie it runs
relatively lean at SL and enriches with altitude. Eventually this shows up
as high fuel consumption.
The frequent claims that the CV carbs are "altitude compensating" are untrue
and rather irritating. Why should a CV carb know more about density than
any other carb (it has no aneroide) and no other special magic. Only an
aneroide can feel the density altitude (so can a turbocharger since it feels
absolute pressure ratio) but that's another story.
Hope this is of some interet to you.
Regards.............MARK WILKSCH
-----Original Message-----
From: Tennant <Tennant@t-online.de>
Date: Thursday, August 19, 1999 07:56
Subject: Re: Flying: Fuel consumption in 'Classic' with Rotax
912.
Hi Alan
You have posed a very complex question.
I have faxed you a copy of the Power settings for the Katana which shows
some
interesting traits of the Rotax.
One of the most interesting things is that the relationship between power %
and
fuel flow. For example 75% at 4000 ft will use only 16,3 L/hr but at 6000 ft
it
will use 19,3 L/hr.
I think that the guys with the VP props will all confirm that the RPM does
not
seem to effect fuel cosumption or speed nearly as much as the throttle
setting.
Also, dont worry too much about reving the rotax, it prefers it and in the
Katana is recomended to run at max continuous rpm always!
Best regards
Barry
alan.d.stewart@bt.com schrieb:
>
> Does anyone know the value of engine RPM which would correspond to 75 %
> power in the standard 912 UL fitted to the Europa ?
>
> At the current prop. setting my engine RPM reads just over 5000 static,
4820
> in the climb, and circa 5700 firewalled, S & L. (Figures accurately
measured
> on Rotax Flydat). I do not have the cool air, plenum box mod.
>
> The reason for my question....
>
> http://www.zenithair.com/kit-data/zac-rtx912.html
>
> 'Fuel burn @ 75%: 4.5 US gph'
>
> This corresponds to 3.75 Imp gal/hr or 17 litre/hr.
>
> My long term actuals, range from 15 - 17.8 litre/hr at cruise RPM of
around
> 4800 - 5200, and I was wondering how closely this correlated with Rotax'
> published figures.
>
> It would make sense if 5100 RPM corresponded to approximately 75% power,
as
> this would tie in with the published figures.
>
> I could look at some of the data in the engine manual for this, but was
> wondering what results other people have been getting ?
>
> Many of you appear to have quoted figures of around 11 - 12 litre hour
which
> I could not achieve without throttling back to much lower RPM's.
>
> I'm fortunate to have a large (early design) 84 litre tank in my a/c. This
> gives a safe maximum of around 70 litres useable in main or about 4 1/4
> hours endurance at standard cruise (approximately 450 nautical miles).
>
> Alan Stewart
> ab ASE - Advanced Systems Engineering
> BT Adastral Park. Tel:- +44 (0) 1473 607571 / Fax:- 606845
> mailto:alan.d.stewart@bt.com
>
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