RPM

[Dana]

I know this has been discussed before, there's mention of 3400 for the GP conversion, but maybe the Mosler is different with its long intake manifold: What max rpm should I be targeting for a Mosler 40 (1083cc)? The Mosler specs say 40HP at 3200 rpm (though some say 38HP is more realistic). The manual also lists 3600 as "redline".

On my Fisher 404, with a prop labeled 56x24 (but pitch measured at 27.5" from the back face angle at 80% radius) I get 3000 rpm static, and 3200 rpm at 60 knots. Frank at Performance Propeller says I should be looking for at least 3200 static and 3600 WOT. The builder of my plane tried other props that let the engine rev higher and noted:

For best performance I use the 56x24 prop. There are two spare props, a 55-24 & a 58-24. The 55x24 is a little weak, you could use it in a pinch. The 58-24 is also a little weak. In testing I got 150-200 more rpm with it but less performance so I checked it out and found it's a 58-22 or close to it. Had it been a 24 pitch I believe it would have helped a lot at my 4200 ft elevation, again you could use it in a pinch.
Caution be aware these small 4 cycle engines do not like heavy props. They hurt performance & in some cases the motor. Lightweight wood with a hi aspect ratio is best. The 58-22/24 Peery prop is a good example of what ou need.
Mosler recommends a 54-24 prop. Been there done that. May be OK for an ultralight, but not for this 404, lost 1/2 the climb & 10 MPH cruise.

The 55x24 I measured at 20.7" pitch and the 58x24 that he thought was really 22" I measured at 25.7"

Any thoughts?

Dana

 

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