|Flights 39 thru ???|
|Flt 39 1743 lbs 101.8 5 Feb 05 2.1hrs
I'm in propeller hell! This is my fourth propeller configuration and none have made more than 100RPM difference either static or at high speed cruise. And that is engine RPM prop RPM is that divided by 1.85. OK here are the facts. Phil Johnson suggested that I get data of the power required for level flight and also to get zero thrust data to determine drag vs. airspeed. To do this he told be to take the prop off and measure RPM vs MAP. This is a measure of the power required to turn the engine with no prop (zero thrust). Then put the prop on and fly. I took data of MAP and RPM vs. IAS at 5500' in level flight. I also climbed to 8500' and pulled the power back to the map and RPM from the curve with the prop off. I dove to each steady speed increament from 120 to 170 kias and noted rate of descent. I noted OAT altitude and fuel burned at each point. This allowed me to calc TAS and test gross weight. From these I can calculate flat plate drag and convert ROD into power.
|There is an obvious difference at low speed between the zero thrust data and the level flight data both in terms of power and apparent drag. I also made some gross asumptions from Nat's claim of 216 TMPH. and backed out a drag number. This indicates that Nat's configuration is a fair amount cleaner than mine. That is something I can work on. I know areas that are not optimum; nosewheel hole cover missing, NACA scoop enlargement, oil cooler scoop, and Cowl bulges. The difference in power on to power off is more interesting at the moment. This indicates that the power calulated from the engine MAP and RPM is indicating a higher power than it really takes to push the airplane thru the air at the lower speeds. This is true at lower speed and RPMs. This data has already been adjusted to subtract the power required to turn the engine with no prop and I applied a .85 factor for prop efficiency. At the 130 KTAS point there is 30 hp missing which is a factor of 60%!!!. Something in the propulsion system is seriously amiss. This could be gross propeller inefficiency, or gross engine miss-tuning, or exhaust restrictions. Or more likely some of each. At speeds above 150 KTAS the data agree pretty well, indicating that the propulsion system is working properly. The formulas and data are contained in my Performance spread sheet if yo want a closer look.
After that flight I put prop config 3 back on. That's a Sens. 70x86. I tested the intake system to be sure there were no restrictions there. Again I got 3600 RPM static with a MAP of 30.0". With the engine off the MAP wait 30.2. A 0.2"Hg loss thru the ducts and filter is very good I think so I'm not concerned about that.
Prop Pitch Study 3-13-05
Since I've finished my official test period, I'm going to stop logging flight by flight but discuss topics that I am looking into. I've been trying to properly match my prop and engine since the begining, about a year now. From my first run up I have been unable to get more than 1946-2011 static RPM. I started off with a prop that Sessenich designed based on 230 hp, cruise of 200kts, and peak power at 2919 RPM. They said the same prop that they use for the IO-360 would be right. That was a 2 bladed 70x89. At first I thought I had an engine problem and tried many changes to the computer to be sure that was not a problem. The Subaru computer looks at a bunch of signals that make sense on the car but are not used on the airplane. I made sure these were properly simulated so that no error signals showed up on the Subaru diagnostic read out. In particular the Park and Neutral switches need to be set and there has to be a "vehicle speed" signal. This is normally from the transmission on the car, sensing wheel speed. I at first simulated this using a modified tachometer signal but that made the computer think that the "car" was going 180mph at 1946 RPM (prop). So now I have switched to a injector pulse and get 28 mph at 1946 RPM. Both of these give the same results at static RPM. I haven't tested the latter at high speed cruise. It is possible that the 2649RPM that I see there may be limitted since the computer could be sensing a over speed since it thinks the "car" is going 245mph. I also read out timing data and MAP vs RPM and had a Subaru expert look it over, he thought it was normal. So I feel comfortable that the engine is running normal. Plus the power/speed/drag data I took indicates that there is good correlation, at least at the high speed end. Below 120 kts I get large descrepancies that could be due to propeller inefficiency or poor engine performance. After that I decided to borrow another prop that might get more RPM. This was a Catto 3 bladed prop that was 68x84. I did fly with that but it did not get any more static than the Sensenich. Later I put the Sensenich on and flew it. again the static was the same. It also got the same RPM at high speed sruise topping out at 2649-2700 at about 150 kias at 5500'. Both the static and the HIgh speed end are low. I decided I wanted a spare prop anyway so I told Sensenich what I had found. They said that a 4" pitch change should raise the RPM by 200 RPM. That would put me at 2900 at the high end, where I would like to be. I thought that the static case would come up more since I am operating on the sloping part of the torque curve there. So I bought a 70x86 pitch. It got the same static and cruise RPM as the other 2. So I thought perhaps the first prop was off a bit. I asked Sensenich to re-pitch it to 4" lower than it was. They checked it and said it was within limits of the desired 70x89 and they repitched it to 70x85. I flew that and got the same RPMs at both ends. At my wits end, I decided I had to figure this out myself. I figured the pitch at the 3/4 radius for the 89" pitch is about 28 deg. Changing to 85" pitch changes it to about 26.5 deg. This is less than a 5% change. This is within the measurement range I had been seeing and less than half the amount Sensenich expected. I decied I needed to see what a bigger change in pitch would do. Luckily, I found a guy on my airport that had a ground adjustable IVO 3 bladed prop that I could fit. It was designed for a smaller Subaru but the same gearbox. It was a 66.5" diameter and could be set at pitches from 28deg to 16 deg. I set it to 28 since I thought my big Subaru 6 cylinder would over power it. I was shocked when it would only spin up to 1622 RPM! I had to reduce the pitch to 19 deg to get 4280 RPM