Prop for Rotax 912 100hp

[buckaroo]

Just wondering what people's opinions are on this. I was thinking ideally a 70" 2 blade(most people seem to run 68" 3 blade or 72" 2 blade on rotax), but ground clearance may become an issue.

I'm now thinking 63" or 66" 3 or 2 blade, but I am worried 3 will be slower and 2 would not transfer all power. Though I see Robin had a 65" 2 blade on his world record plane.(also used a 912)

Prop speed would be 2000-2400 rpm and on a Sonerai II.

P.S. When prop manufacturers quote HP ratings, are they saying that is the most efficient range or is it actually a structural limitation? Will a 60 HP prop break if used on a 100 HP engine?

 

[M61A!]

Just wondering what people's opinions are on this. I was thinking ideally a 70" 2 blade(most people seem to run 68" 3 blade or 72" 2 blade on rotax), but ground clearance may become an issue.

I'm now thinking 63" or 66" 3 or 2 blade, but I am worried 3 will be slower and 2 would not transfer all power. Though I see Robin had a 65" 2 blade on his world record plane.(also used a 912)

Prop speed would be 2000-2400 rpm and on a Sonerai II.

P.S. When prop manufacturers quote HP ratings, are they saying that is the most efficient range or is it actually a structural limitation? Will a 60 HP prop break if used on a 100 HP engine?

If you read Robin's information about his later aircraft, he switched from a two blade to a three blade constant speed (airmaster I think). Apparently the was a very small speed trade off for improved smoothness.
If you intend using a ground adjustable prop there are many options. I am currently using a two blade 72" Sensenich on an 80hp 912 on a CH601 and it was a significant speed improvement over the three blade warp drive, but the warp drive was smoother. I recently tried a demo prop of an E-Prop, and if you were going for ground adjustable and not in flight adjustable, the performance was very good, extremely quiet and smooth as well. They are also about the lightest prop you will find for a 912.
Just remember to pitch (ground adjustable) for about 5500-5600 rpm at WOT at your usual cruise altitude.

 

[buckaroo]

Indeed, I saw pictures of his three blade. Though I didn't read anything about diameter. 72" would be a no brainer if the ground clearance was there, but the Sonerai is designed for 60".

Eprop looks interesting. They have a 63" 3 blade for $1800 that looks just right. NRprop has a 2 blade ground adjustable 66" for $450. Not much to lose, so I might try that, and if it disappoints go with the Eprop.

 

[M61A!]

Indeed, I saw pictures of his three blade. Though I didn't read anything about diameter. 72" would be a no brainer if the ground clearance was there, but the Sonerai is designed for 60".

Eprop looks interesting. They have a 63" 3 blade for $1800 that looks just right. NRprop has a 2 blade ground adjustable 66" for $450. Not much to lose, so I might try that, and if it disappoints go with the Eprop.

You should measure up and see what is the biggest prop that will fit. My understanding is that it isn't the Sonerai that is the limiting factor, it's the VW engine.
The E-Prop site has a prop calculator that you can select by speed, engine and maximum diameter.

Just had a look at NR props as I'd never heard of them. They look reasonable. Apparently some are rebranded as Cool props.....I've seen those on a few aircraft.

 

[buckaroo]

Right, NR/Kool/Cool/Luga are all the same to my knowledge. I believe some trike company sells kool. Aeromomentum sells Luga on their website.(for a substantial, but understandable, mark up)

I post the following for people's general information:
I do think 65" is about the upper limit. FAA demands 7" clearance for tri gear and 9" for tail dragger. My finite element analysis says under 800 lb on each leg it will displace absolutely 10 cm/3.9", which is about 2.5" in the vertical direction. An educated guess tells me the tire might deform 1.25" vertically under that same load. So on my design that would bring a 66" blade within 6.75" of the ground.

 

[Robin Austin]

Indeed, I saw pictures of his three blade. Though I didn't read anything about diameter. 72" would be a no brainer if the ground clearance was there, but the Sonerai is designed for 60".

Eprop looks interesting. They have a 63" 3 blade for $1800 that looks just right. NRprop has a 2 blade ground adjustable 66" for $450. Not much to lose, so I might try that, and if it disappoints go with the Eprop.

Hi folks.
Just stumbled across this thread.
Just to confirm, both aircrafts had 65-66” props.
2 blade on SGS and 3 blade on SRS.
If you use a 66” 2 blade, make sure there is enough blade area to handle 100 hp at approx 2400 rpm.
Cheers

 

[vhbill]

Right, NR/Kool/Cool/Luga are all the same to my knowledge. I believe some trike company sells kool. Aeromomentum sells Luga on their website.(for a substantial, but understandable, mark up)

I post the following for people's general information:
I do think 65" is about the upper limit. FAA demands 7" clearance for tri gear and 9" for tail dragger. My finite element analysis says under 800 lb on each leg it will displace absolutely 10 cm/3.9", which is about 2.5" in the vertical direction. An educated guess tells me the tire might deform 1.25" vertically under that same load. So on my design that would bring a 66" blade within 6.75" of the ground.

View attachment 15436

That's a nice drawing.

Will the engine need to be further forward to keep the CofG within limits?

A 64" 3 blade prop should give you 17" ground clearance when the aircraft is parked on the ground and fitted with modified Grove landing gear.
The Grove gear legs may need to be bent a bit more to improve prop ground clearance.

The prop clearance is a lot less (about 12") when in the level flight attitude.

The angle of attack of the wing with the tail wheel on the ground should be about 12 deg using the modified Grove gear legs Closer to the stall angle of attack would be better, but that would have needed longer gear legs or in my case, gear legs bent further.