Maximum engine size

[Danzer1]

Even at your altitudes, I think you will find the performance of the 100 hp 912 to be just fine in what should be a very light weight plane,

That might depend on the mission and the density altitude at the time. We very often have 8000'+ density altitude which would give that engine max at takeoff 76hp (for 5 minutes). Drop that to max sustained at 5500rpm and it's closer to 70. The 912hp curve drops off even quicker from there. Cruise at 4000 and it's down to 67hp (s.l.) and closer to 51hp at 8000 d.a.

That might work for some.

[Dorsal]

Cruise @ 4000?? My typical cruise is 5200 RPM.

[Danzer1]

Dorsal, That's kind of my point - you can't. The idea when flying from altitude airports, with high d.a.'s and potentially in or above mountainous terrain, is to have as much "get out of trouble" power available as you can on the right side of the curve. To illustrate:

I picked 4000rpm, as in a direct drive you often would cruise at 70 to 75% and in a 912 you really can't.

Using your number of 5200, about 95% or about 2100 prop rpm and approx. 90hp (s.l. ratings). You only have about 5% of the curve left for "get out of trouble" hp.

Compare that to say the UL 350is at 2100, about 63% and about 94hp (again s.l. ratings for comparison). You would still have 1/3 of the power band left - 36hp and 1200rpm of "get out of trouble" power available on the right side of the curve.

More safety margin before running out of engine. And, installed weights compare almost equally for lbs/hp fwf.

[Dorsal]

I think you will find that 5000 rpm is 75% power.

[Danzer1]

Not sure how you figure that - 5800 is max for 5 min and 5500 max continuous. Even if using hp ratings or prop speed at those figures, nothing comes out to 75%. Can you expound on your thinking as to how 5000 rpm can equal 75% power? I'm not following.

[Av8r3400]

Why would you? 5000 rpm is a common cruise rpm that yields 3 - 3-1/2 GPH. Why turn slower?

Danzer1, you need to speak with some folks, like Stick and Rudder, who run these motors in a Kitfox at altitudes like you want, to understand.

[AirFox]

I agree with Larry(Av8r3400) above. I was considering other engine options until I went to Stick & Rudder and experienced what they have. "Been there and done that" advice goes along way when deciding what engine to go with. You will be amazed at what Stick and Rudder planes will do!

Scott

[Dorsal]

Danzer,
Page 5-6 Rotax owners manual, 75% power @ 5000RPM = 68HP (that is 75% of 90hp which is Max continuous).
Given I don't have a IFA prop I take off and climb @ 5200 rpm which is ~75HP, same as I could get at 8000 ft DA at around 5600 rpm.

[Danzer1]

I understand what the plane and the engine can do. I have a choice and an opinion. It is my choice to prefer 30% of overhead to 5% of overhead (from cruise).

Why don't you guys ask them? Their runway is at 1/2 the height, their summer temps don't very often get to 100 deg. They don't have any 14,000 foot peaks, we have many and many passes over 10,000. Matter of fact, most of Idaho including the peaks is under 8,000 feet and the vast majority under 3000 ft!

So starting at 5,500 ft. adding 95 to 100 deg (8000+ d.a.) and having passes and peaks of 10,000 and above - my choice and opinion is go for as much margin as I can.

The thread was asking about an SS7 and Larry you threw in the 100hp 912. That would presumably be a 912ULS. How are you calculating/getting 3-3.5 fuel burn at 5000rpm? When the factory says 5.3?

[Danzer1]

Dorsal,

I think we are talking about a 912ULS (100hp was given), 95hp continuous at 5500. 75% is 71.25 which occurs at 4,125 rpm. In any case, maybe you are reading kw not hp, it still does not compute.

Greg