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I have had a great time working through my 40 hr Phase 1 testing. The performance of my Zenith CH750 STOL / Corvair 2.7 l is OK - not spectacular compared to my training in a C172 (but totally different plane). My Vy is right between 45 & 50 indicated - I rotate at 40 and climb out at 50 @ 500 fpm - good performance. My prop is a Warp Drive 2-Blade 68" ground adjustable that I set for a static rpm of 2850. Full power is close to 3000 rpm and I usually cruise at 2700. Cruise spped is right around 70. When I pull the power back it is seriously pulling back 25% to 33% of full throttle to get to 2700. Today I loaded up a 60# sandbag in the right seat and did some t/o and landings. That was OK. I loaded up another 60# (120# total) and my climb started to get a little scary - a little more close to the high tension wires and trees than I liked. In fact turning into the good stiff wind we had was the only thing that made me climb. I had another sand bag to make it 180# total but I wasn't that crazy. My plane empty is 850#, full fuel it is 1000# even. Max design weight is 1430#. I should be able to carry 430# of meat. I understand the basic concepts well but I'm no aerospace engineer. I am I getting what I should out of my prop? Engine?
I have been very interested in your discussion and wondered if you have completed any additional testing? I will be installing the 3.0 in my 750 and since I will be flying from an airfield at 5,837' MSL, I am very interested in climb performance.
Gents, for any aspirated engine without a turbocharger the loss of horse power is 3% for every 1000 feet of elevation above sea level. Thus at 6000 MSL and with a 120 HP rated engine (HP ratings are always expressed at sea level performance) you can expect (6 x 3% = 18%) an 18% loss of HP or 120 X .82 = 98 HP! For a 750 Zenith series product that is basically at the edge of reasonable HP requirements, especially near LSA gross weight, and you haven’t even taken off yet! I would recommend at your elevation to think about purchasing a fuel injected and turbocharged engine rather than a Corvair conversion. ( Viking Engine can probably help you on this issue) Incidentally you should also look up Kock charts on the web. This will allow you to better understand your T/ and Landing performance at any pressure altitude. Very handy indeed but not related to HP but a reflection of density altitude at any temperature and starting altitude.
Azalea Aviation is currently developing a turbo version of their 120 HP engine which works quite well at any altitude. The turbo is not yet available for sale but the owner Bill Clapp has one installed in his Saberwing and has had excellent performance with it.
I did not mean your post was a sales pitch, just the Viking guy's posts. Oddly enough, I think he removed his posts, which were quite a put down on Corvairs.
First of all I really don't appreciate the sales pitches. I'm looking for people with my same engine (the one I chose) and any qualified knowledge on it. I picked the corvair because it is simple to work on and easy to understand - I would say elegant.
I built it, I know it well and I can fix it (as described below).
My real problem turned out to be 1) A pulled head stud on the #6 cylinder intake valve and 2) A little too lean of mixture.
1) The front lower head stud on the #6 cylinder is also the pivot for the intake rocker. Mine was pulled - barely holding but not allowing the intake valve to open. I'm not sure if this was my fault or previous defect. I've re torqued the heads several times after break-in but have never gone over 30#. I wrote a previous post on how I replaced another stud with a time-sert and it turns out that it is the exact opposite stud - the upper rear on #1. I don't know if this has any significance? I'm not sure when it happened either. I had one dead or mostly dead cylinder. The good news is I don't see any other issues and now I know my ROC/5!
2) I was not getting the 50 rpm boost with leaning on the MA-3. This was a very easy adjustment.
The timing was confirmed to be right between 30 & 32. The mark always jumps around a little bit so I'd say its impossible to say down to a degree.
Differential compression was retested and all cylinders were 76/80 with only crankcase noise (no intake / exhaust / head).
NOW I have a good 2700 static RPM with the prop set at 7 degrees at the tips in relation to the top of the engine being level. Performance is now very acceptable. Better climb and cruise. Without hooking it up to a dyno, these seem to be good performance numbers for this set-up.
SOOO, back to the question:
Prop load is a function of pitch and diameter and is proportional to rpm.
My 2.7 l Corvair is making about 80 hp at 2700 rpm with 68" diameter prop at 7 degree tip angle.
It's getting closer to its rated hp at 3200 rpm (100 hp)
Is it better to run a 66" prop at higher rpm and / or pitch - say 3000 rpm and 8% pitch to get higher on the hp curve?
Is this even achievable - there always has to be trade offs
Tip speeds - 2700 @ 68" and 3000 @ 66" are very comparable - so no problem there.
I have a email in to Warp Drive to answer these questions, soI will keep you posted
A couple anecdotes:
I have gotten good at spinning the prop on a cold corvair engine and feeling the compression strokes on each cylinder. each one at 120 degrees from each other on 2 full rotations. You should practice this because it is very helpful (would have been very helpful before) in feeling any differences between the 6 cylinders. A little art, a little magic,
I had an old 70's Larson tri hull boat with a 100 hp Johnson. I borrowed a friends prop to try out - more pitch and more speed, right? It was too much pitch for my motor - it could not get up into enough rpms to develop enough power to even get out of the hole. It just struggled at low rpm chopping too much water.
****There is no perfect pitch / diameter / rpm but there is a perfect BALANCE. That's my goal.
Response from Warp Drive:
You could certainly shorten the propeller blades and keep the pitch setting the same to allow the engine RPM to turn up slightly higher. There typically isn't much of a change as the STOL airplanes usually like the larger diameter props to overcome their drag. By turning the static up to 3,000 or 3,100 RPM, you would also need to watch the RPM in the air so it does not over-rev. A 66" propeller would still give good take-off and climb performance and is generally the size we use for a faster cruise speed in the air.
Let me know if you have any additional questions. If you wanted to try this, you could cut the blades down or we could cut your blades and rebalance them on our scales like a new set.
Thanks for posting an update of your progress and congratulations on getting to the bottom of the power issue. I will be assembling my 3.0 in a couple of weeks and hopefully mounted on my 750 STOL shortly afterwards. Since I haven't run my engine yet, I haven't much I could add to your issue (sorry) except what I have read in the manuals. I checked my engine installation manual section on prop choices it suggested that the best overall performance for a 750 with the 2700 would probably be found when utilizing a 66" prop. I can't say for sure but it does appear that you are on the right track.