|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 beginning, 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 Sensenich 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 an 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 limited since the computer could be sensing an 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 discrepancies 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 cruise 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 decided 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 21 deg in order to get up to the desired 2270 static RPM. OK so I proved that my engine would turn "a" prop to 2270 if I lower the pitch enough. That doesn't mean that I'm getting good thrust for takeoff. I could have lowered the pitch so much that all the power is going into parasitic drag. I had a brilliant idea. I took two bathroom scales (300lbs each) and hung them on a Sea Container wall next to my hangar. I put a 2x4 to bridge the 2 scales. I took the nose cone off my airplane which leaves a flat bulkhead on the front. Then put the nose of the airplane up against the 2 scales. I took a 3rd scale and pushed on the back of the airplane. The back and front scales agreed, at least up to the 60 lbs I could push by hand.
| So I stared it up and took data up to full throttle. At 2270 RPM I got 608 lbs thrust!!! Then I wondered "how much did my old 70x86 give". I bolted it on and tested it. I got 360lbs at the full throttle 1946 RPM. So the IVO gave me a 69% increase. Man that would really help. I could not fly the IVO for 2 reasons. First I have heard rumors of them coming apart in flight on pushers and second because I have no prop extension for that prop and it is too close to the cowl.
I'm talking to Sensenich about making a new prop based on this data but have not heard back yet. They said so far that they think I need a variable pitch prop. I've thought about that but I can not afford the roughly $13,000 for a Hoffman or the roughly 50lbs extra at the tail. The guy that lent me the IVO said that they make an inflight adjustable model too and even have a control box that makes it constant speed. I told him I was worried about these rummors. He said I should call them and ask about any problems with pushers. I did. I found that the only problem they had was with a guy that didn't torque the prop per instruction on a Lyc IO-360 and it came apart. They found that the problem was due to the torque pulse on the direct drive big aircraft engines. They now don't reccomend their props for direct drive aircraft engines. He said however that with my Subaru operating at 3000 to 5400 RPM, a 6 cylinder and going thru a PSRU that their prop would be fine. He also said that their prop onlyu weights 25.6 lbs and costs $2200 for flight adjustable and $2500 for constant speed. Now that sounds pretty interesting! I also called Velocity since it was one of their airplanes that had the problem. They concurred with IVO. They also said that despite the IVO warning, there are still some direct drive aircraft engines flying with the IVO prop and have not had the prolems, presumably because they are torqued right. I hear that Nat has heard the same info and thinks that the IVO may be good for my engine. I still need to check that out.
While I'm thinking about that, there is still a data point that I need to get. My friend Phil Johnson that has the same engine and PSRU as me, designed his own prop. It is a 3 bladed 68x90 pitch prop. He hasn't flown but gets static of 2270 RPM and 500 lbs of thrust. Why would he get so much more RPM on his engine with more pitch than I do at 89" pitch??? Well I know he has a better intake system and exhaust system but could that make that big a difference? What a tremendous friend he is. He has agreed to shipp his prop here to Ca. all the way from Eastern Canada!!! That should be here in a couple of weeks and will determine once and for all if my engine is up to his standards or not or how much short on power I am. I'm thinking that if I can turn the same RPM as he does the the takeoff should be at lewast 69% shorter. I also don't think the cruise end will change more than the static end since it is a 90" pitch and so far all the props I've tried do not show any more change at the high end than they do at the staitc end. If so then a fixed pitch light weight prop would be the KISS way to go. If not then I may check out more advise on the IVO and perhaps go that way. At least I'm seeing some ligght at the end of the tunnel.