As y'all can see, this is my first post in this forum. If I post something that's been discussed ad nauseam already, accept my apologies and, please, point me in the right direction. My cursoray search didn't turn up anything suitable.

Here's my question:

I am flying a gyroplane, powered by a Rotax 914 driving a constant speed prop. Reading in the Rotax manual, I find a table giving power settings which are recommended for fuel economy. I did a very careful series of flight tests, measuring fuel flow and airspeed over a large range of RPMs and MAPs. (BTW, I can upload the graphs of IAS as a function of MAP and RPM as well as fuel flow if the disccussion warrants.)

Now, I find that the power settings recommended by the Rotax manual are anything but ideal for my configuration. And that's not surprising, since they only take the engine into account, not the enire aerodynamic efficiency of the aircraft at various speeds, which has a large effect.

It turns out that my most efficient power setting (giving the highest miles-per-gallon or liter/100 km or range) is 4500 rpm at 35 inHg MAP resulting in 20.5 l/h.

My question now is this: what are the limits beyond which I have to worry about too low rpm for too high a manifold pressure? Are there other factors to take into account which preclude certain power settings for cruise (all at or below 5500 rpm, of course, and at or below 35 inHg).

Any input or pointers in the right direction are appreciated.

Thanks, -- Chris.
  • Re: Power setting on 914 UL with c/s prop

    by » 11 years ago

    Hi Chris,
    4500 rpm at 32 inHg is pretty hard on the engine. You’re going over-square and could get into detonation. Have a look at the Preventing detonation video.
    The max MAP at 4800rpm is around 29 inHg according to the 914 ops manual. The only other power setting listed is 4300rpm at 28 inHg so I guess the max at 4500 would be around 28.

  • Re: Power setting on 914 UL with c/s prop

    by » 11 years ago

    OK, after fetting a lot of different opinions from different people today I called up the German Rotax distributor and talked to their chief mechanic. This is the scoop:

    First off, to avoid detonation or pre-ignition you should only use gasoline with a RON of 98. Theoretically 95 could be enough but they have seen a lot of variation in the quality of the gasoline and the detonation margin just isn't big enough on a Rotax 914. That's why they recommend RON 98.

    Operationally, he says, the only area to stay clear of is below power settings of less than 5200 rpm and above 35 inHg. As long as you are at or below 35 inHg you can reduce rpm down to 4300 without having to worry. Also, you need to have at least 5200 rpm static on the ground in order to shove the throttle to the wall.

    Well, that's it right from the horse's mouth ;)

    -- Chris.

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