Pierre, Your first prop was closer to what you need, but quite honestly, you just don't have enough engine to turn the prop at that density altitude. People get all wrapped around the axle about pitch, but tip drag is also a big issue. You don't say what the displacement of your VW engine might be, but it's pretty clear that you are pushing it's limits. With the Density Altitudes you are looking at, an 80 HP VW isn't going to make more than 60HP. You are going to need a short prop to turn the RPMs you want, so the prop efficiency is going to be off pretty badly on take off. Even with an optimistic number of 75% efficiency during take off, you are only getting at best 45 HP to pull your plane into the air.
When I ran into this problem with a VW powerplant on one of the planes I built, I sent the prop back to have some pitch taken out of it. When I got the prop back, it made no detectable difference in RPM. After a discussion with the prop maker, per his direction I marked off each prop tip with 1/8" increments from the tip. I'd fly the prop, then cut off 1/8" and fly it again. It was a trade off between picking up engine performance at the cost of prop efficiency. With each cut, I got an improvement in RPMs. I kept going until I picked up more rpm, but lost performance due to decreased prop efficiency. That was where the prop stayed for the next 40 or so hours. Ultimately I concluded that while the 2180 VW I had on that plane had sufficient HP for the application, and would be a decent performing aircraft at sea level, the high reving engine and slow airplane combination with marginal power at my high altitude airport just wasn't going to be safe to fly. FWIW, while my KR is a bit lighter than yours, I am significantly heavier than you and fly my KR at 1200# gross based at an airport at 7200' MSL. I operate with density altitudes from 7000 to greater than 10,000'. Even with my hopped up O-200 powerplant, depending on the DA and weight, I never get off in less than 2000' and if heavily loaded on a hot day can take up to 3000' of runway. Additionally, I see a 550 rpm difference between what I turn at full throttle on take off run vs what I turn in cruise when the prop starts to unload. Thinking about it in those terms, your first prop may very well turn up to around 3300 RPM once your plane is airborne and pushed over into cruise. Those that have seen me operate at sea level can attest that my KR is not a ground loving pig and in fact is a pretty good performer under normal conditions. That's the price we pay for living up in the mountains. -Jeff Scott Los Alamos, NM ? ? Hi Guys, It feels as if I've hit a brick wall. I had a 48(pitch) X 52 prop on my KR2S (haven't flown yet). The max revs I got was 2,760rpm static. I then bought a 45(pitch) X 54 prop to see if I can get to 3,000rpm, but I only got to 1,850rpm. Stripped the 2 Stromberg 175 cd carburetors and had them rebuilt - no difference. The main problem with these numbers is that I can only get to 65mph IAS and it takes 1,500 feet to get to that speed. The main reason for wanting 3,000rpm is that the vw max torque is at 3,000rpm and I think I will then jump to heigher rpm and thus more thrust and then higher speeds...... What am I missing? The KR2S has got wing tanks and tri-cycle gear and is 4" wider in the cabin, weight is 876lbs empty, so she is a bit heavier that she should be. (I am 176lbs and 5ft 8in) My altitude is 5,500feet ASL and the temps we had is between 89 degrees and 108 degrees Fahrenheit. (32 degrees - 42 degrees centigrade) Hope somebody can help! Cheers, Pierre
