Jerry Hey wrote:

 It is time for me to decide on the drive ratios for my chain psru.
 Basically, if I want to avoid special orders (and considerable expense)I
 have two choices. 2:1 or 2.21:1.  The 2:1 would have max cruise (prop at
 2750) at 5500 eng . rpm.  The 2.21:1 would achieve the same at  6077
 rpm.   Of course, the engine would be making more power in the latter
 case.  With the extra power, comes additional heat generation.  I think
 the guys who want to run the 2.8 gear box at over 7000 rpm may face
 formitible  cooling issues.  A lot of radiator core and a lot of air
 will be required and that adds up to a lot of drag.  May be  sound and
 fury signifying nothing as far as practical, high speed cruising is
 concerned.   But that is beside the point of this note which is my drive.

 It seems to me  the 2.21 ratio would be just about perfect.  But I have
 never heard of any one claim that they could actually cruise at 6000
 rpm.  Even the planes in the Everett Hatch RACE, ran less than 6000 rpm,
 I believe.

 I would like to think it would be possible to set the throttle at 6077
 rpm and just leave it there.

 The Star Mazdas operate in this RPM range ( 6000 to 6800) and seem to
 survive very well.

 > I will appreciate anyone's commentary.    Jerry

>From Ken Welter

I run a 2.95 on my coot and when I take off with a full load of fuel
100 gallons I take off at 7000 rpm climb out at 6800 and cruise at
6200 to 6500  rpm depending on alt.

Forget everything you know about piston engines.
It is very interesting that the rotor bearing loads are a minimum
at 5900 RPM. In other words they are higher at RPM's both lower and higher.
5900 RPM looks like a good  cruise RPM.

Paul Lamar
 
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