There's been mention lately of ground adjustable props. If you've got an O-200 a CS prop is the cat's meow but for VW.Revmaster-powered KR's I would suggest a variable pitch prop is completely unnecessary. My first KR had a Revmaster 2100 with the Maloof CS prop. The controller never did work very well so most converted (as I did) to a two-position switch which gave either full coarse or full fine pitch. It's been so long since I sold that plane that I can't remember if I found the ability to vary the pitch to be of any value - but it looked cool. The Maloof was most useful for those who flew high in turbocharged Revmasters. It was only on the turbocharged installations that the Maloof failures occured if I recall correctly. There were only one or two failures, but that was enough to doom the Maloof.
I have three props for my current KR - Ken Cottle's KR-1?. When I bought it from Steve it had a Sterba 52 x 52 which would allow me to max out at full throttle in cruise at around 3600 RPM. I was going 170 MPH at that RPM and a lot of the old KR guys who liked to rebuild their engines frequently used to run around at that RPM, but engine temps are through the roof. For reasons of fuel economy and engine longevity I wanted to run full throttle (above 8000 ft.) at a more reasonable RPM. (Steve, BTW, used to cruise this plane at 2900 RPM with the 52 x 52, partial throttle). So I bought a Prince 52 x 54. This gave me a full throttle RPM of 3200 in cruise. I would have kept the Prince on the plane except going through quite a bit of rain on a trip to Mt. Vernon and Naples Florida and back home the leading edges of this prop pretty much dissolved. I ordered a Sterba 52 x 56 (Sterba has good urethane leading edges) which gives me a full throttle RPM of 3100-3200 in cruise above 8 thousand feet. I may get around to sending it back to Ed to put just a tad more pitch in it. 52 x 57 would be just perfect. I'd rather run the engine full throttle at 3000-3100 when cruising above 8000, instead of 3100-3200. I'm usually quite a bit higher than 8K, so the more pitch the better. I get 100 more RPM at 12K than I do at 8K. Why the emphasis on full throttle? Well, the throttle plate in the Ellison is fully open and the engine is breathing unrestricted. As an air pump the engine is at its most efficient when running with a completely open throttle. Below 8000' full throttle would develop more than 75% power with resultant high oil and cylinder head temps and poor economy. Above that altitude full throttle renders less than 75% so for this reason and the reduction in parasitic drag, and for any number of other reasons, up high is the place to be. Flying partial throttle above 8000' is inefficient. Except for takeoff, I really don't want to pull more than 75% power from my engine below 8000 feet. At altitude, 11.5 - 13.5, with my Sterba 52 x 56, I cruise an average of 149 MPH/3.5 GPH using around 50% power. Makes for a happy and long-lasting engine. With my 21 gallons I plan on 500 mile legs stretchable to 600 or 700. I have oxygen, which makes this sort of flying practical. Even down low and using partial throttle, I get more thrust per RPM - more efficiency - with a coarse prop. Engine temps (the VW killer) are dramatically lower with lower RPM. Friction losses (and resultant heat) are minimized. Unless you're based at a 1500' grass strip with obstructions at each end, there's not much reason to use the flat props I see many KR people using and there's no reason at all to use something that's ground adjustable. IMHO. The point of this little prop history is that whether I've got a 52 x 52 turning 3300+ on takeoff or a 52 x 56 turning 2900 on takeoff, my KR accelerates and leaves the ground quickly. The coarser prop is not turning as fast, but it's pushing more air with each revolution. When visiting friends I've taken off at Telluride with full tanks with the 52 x 56 and although I seldom frequent the short grass strips common in many other parts of the country, I've never been anywhere where I was concerned about taking off. Ken built my KR with the normal 20 ft. 8 in. wingspan and the same forces that make it want to float forever when landing are also at work in getting it off the ground. In my opinion there is no reason to consider the complexity, weight, and cost (not to mention the unreliability issues with some of the makers of those props) of a variable pitch prop for VW engines in a KR. Mike KSEE ____________________________________________________________ How Old Men Tighten Skin 63 Year Old Man Shares DIY Skin Tightening Method You Can Do From Home http://thirdpartyoffers.juno.com/TGL3141/54d00ed157f98ed133b9st03vuc -------------- next part -------------- A non-text attachment was scrubbed... Name: borrego.jpg Type: image/jpeg Size: 81531 bytes Desc: not available URL: <http://list.krnet.org/mailman/private/krnet_list.krnet.org/attachments/20150202/0c740df4/attachment.jpg>
