After a fair amount of experimenting with props over the years, I will respectfully disagree with you, Jeff, and agree with Tommy. ?But I will also state that the statement about longer props equals better climb is only true within limits until you reach a point where the engine HP gets used up by the tip drag as I'll discuss further down in this post. ?
One specific instance that comes to mind was a propeller I had on my KR for a number of years from Performance Propellers. ?It performed reasonably well, but after checking and finding that my Tach was reading 200 rpm high, I knew that with a bit less pitch, I should be able to turn the engine up another 200 rpm improving both climb and cruise performance. ?I pulled the prop off and sent it back to the manufacturer to be repitched to a lower pitch to allow it to turn another 200 rpm. ?When I got the prop back and flew it, I knew without even measuring it that he had cut the tips off the prop rather than repitching it. ?It took a lot more RPM to do anything, but the takeoff and climb performance was seriously off. ?When I called the manufacturer, he confirmed that he decided to be lazy and cut 2" of diameter off the prop rather than repitching it as I had requested (and paid) him to do. ?For use on my plane, the prop was essentially ruined. On an Avid Flyer that I built using a VW engine, I simply could not get it to perform at our 7000' elevation here in Los Alamos. ?I returned the prop to Ed Sterba for some adjustment to the pitch. ?When I got it back, it really didn't perform any differently, nor did the engine turn up any better. ?I called Ed and discussed the issue with him and we formulated a plan. ?The issue was that the prop was simply too long for the engine as the tip drag was using up all the horsepower rather than generating thrust. ?I took a ruler and marked the prop tips in 1/8" increments. ?I would take the plane out for a test ride, then come back and saw 1/8" off both prop tips, reducing the diameter of the prop by 1/4" increments. ?With each pass, the engine turned up more, with some gain in performace through about 3 or 4 iterations. ?The next iteration I saw more rpm, but no change in performance. ?Then the next iteration the engine again gained rpm, but the take off and climb performace was showing a definite decrease. ?I would have loved to have continued cutting on the prop to see how much more it would drop off, but since I couldn't add back onto the prop, decided to stop there. I also built and fly a SuperCub. ?It's pretty common knowledge and has been demonstrated over and over that if you want a 160 HP SuperCub to get off the ground short, you take off the stock 74 x 58 McCauley prop and install an 84 x 43 McCauley prop. ?The rpms are about the same, but the plane will get off the ground much quicker (roughly half the distance!) and climb significantly better. ?It's pretty obvious that the longer prop pulls better. ?The thrust difference is quantifyable by pulling static against a scale. ?A number of the SuperCub guys have done just that to prove it out. ?Craig Catto is in the process of develping an STC to use his long 82 - 84" props on certificated SuperCubs. ?However, everything with a prop is a compromise. ?To get that super take off and climb performance out of a SuperCub, the tip drag is high enough at cruise speeds that the SuperCubs lose roughly 10 mph off the top end of their cruise. ?Why not go to a 93" prop? ?The tip speed is high enough that the tips are creating so much drag that all of the HP gets used up just driving the tips around in a circle, so the thrust drops off. ? >From Valley Engineering (Culver Props): ?The efficiency of a propeller is >reduced as the tip speed approaches the speed of sound. ?Beyond 80% of the >speed of sound, further increases in RPM has little effect on thrust. ?Thus, >it is important to keep tip speeds below 75%-80% of Mach. Bottom line, you tune your prop for the performace you want. ?Everything on a prop is a compromise. ?Within reasonable limits a lower pitch, longer prop will provide more thrust for initial take off and climb. -Jeff Scott Los Alamos, NM <http://jeffsplanes.com> > ----- Original Message ----- > From: schmleff . via KRnet > Sent: 05/09/14 11:39 AM > To: tommy waymack, KRnet > Subject: Re: KR> Fw: Re: Propellers > > Not to start an argument, but I have to disagree ; ) > > How a prop works on a particular airplane is based on the amount of > air moved (past the cowl, not smashing into it) and the velocity of > that air. I have never been able to find a real reference to the > ?large diameter prop=better climb? theory nor have I seen it work in > real life. > > On Pete?s KR, the Cloudcars 52x54 out climbs the 56x52 Sterba. Lots of > factors there, but to sum up, the smaller diameter allows the engine > to spin up more and make more power. The increased pitch makes up for > the lesser disk area. > > On my SI, I have tried about 2 dozen prop variations from a 54x42 down > to a 47.75x50. Climb performance between the two are about the same, > but the top speed is radically more with the later. I?m not saying > that a prop that small would work on a KR since the frontal area is > greater. I would be curious to hear from someone that has experimented > to find just how small of a prop is still effective on a KR. > > I recently put up some prop, climb and speed data on my blog here: > http://schmleff.blogspot.com/2014/04/skye-racer-history-part-1.html > > Jeff Lange > Race 64 - Skye Racer > Blog:?http://schmleff.blogspot.com > Youtube Channel:?https://www.youtube.com/schmleff > > On May 9, 2014, at 10:47 AM, tommy waymack via KRnet > <krnet at list.krnet.org> wrote: >
