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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?
If the published charts I've seen are to be believed, you're not going to get 100Hp out of the Corvair until you hit 3100, but that is with good airflow.
You set the prop to get 2700 static. What angle do you have on the Warp Drive? At at what density altitude? I have a 3100 that I'm still trying to dial in. The best I've got out of it has been 2700 at 100ft DA and 9 degrees on the Warp Drive 68". I could hit 5000 rpm if I set the prop to zero pitch, even higher if I took it off completely. And I might not even be able to hit 2500 today, cause it is 95 degrees and dripping wet humid out.
The fact that you're pulling 30% back on the throttle to pull it off the top end would tell me that the throttle is not the limiting factor to your engine's breathing. That is, you could double the size of the carb throat and it wouldn't help any. There are other restrictions before or after the carb that limiting how well the engine can breathe.
If the engine can't breathe now, you'll have even less volumetric efficiency at 3100. VE is the amount of air the cylinder pulls in at each cycle, vs how much it could hold at atmospheric pressure. Because it is pulling air in by creating a vacuum, the VE on a non-blown engine is always less than one, and goes down as RPM goes up. That why the power curve flattens out. The cylinders don't have enough time to suck in enough air to fill the cylinder, and it's hard to make power by igniting a near vacuum.
So, I'd say, with the information given, that you don't have a smooth intake, so you're engine isn't producing enough power. Ask around if there are any racers in your EAA that could have a look at it and offer recommendations to clean it up.
DA yesterday was 1900 ft
I have a MA-3SPA carb and WW stainless intake and the Weseman cowling kit. The cowling kit has a high performance k&n air filter with a good straight in intake
My prop started out at a typical 8 degree but I followed WW's advice and adjusted it until I got at MIN 2800 static rpm. I think it is down to around 4 degrees - see the pic. This might be an issue.
I have been at grad parties all day - thanks for the quick response and i appreciate the advice.
Hi Mark, I would agree with Jan. Your timing should be around 10 deg. static and a total advance of no more than 32 deg. BTDC at or above 2600 rpm. You can set the static advance using a simple ohm meter. Then once that is set then using a strobe timing light you can set the total advance. Remember to tie down the plane since you need at least 2600 rpm and preferably a wide open throttle! When the points just open your TDC number 1 cylinder Mark should be adjacent to the 8-10 degree mark on the accessory case. Your distributor if you purchased it from WW will have numbers written on it. This is the timing you should set assuming you have not adjusted the point gap which needs to be at 0.019 at TDC (when the points are on the top of the lobe). If that gap is below 0.016 then you are out of spec on that. Your prop pitch sounds really low and the warp drives have very narrow blades in my view. I have a 120 HP on my Cruzer with a 2 blade Sensenich ground adjustable prop set at low pitch. Even at this setting I have loads of power. I have flown Buzzy’s transition training 750 STOL in which he has a warp drive and a Continental 0-200 (100HP) and that plane with two adults and full tanks has excellent performance, this is why I think you are not getting all you can out of your Corvair engine. Once you are sure your timing is where it needs to be I would then focus on the carb set up. If your fuel air mixture is too rich in fuel (I recommend using 100 LL by the way) then you are just not going to get all 100 horses out of your engine. If you know your fuel consumption per hour at full throttle you can actually calculate your fuel/air mixture. It should be 1:12 fuel to air. WW’s website goes into this in great detail and is quite helpful. Also check your plug gap too which should be around 0.032”.
i hope this helps.
By the way Mark the likely reason William gave you the rpm advice is to ensure you are 1. At total advance and 2. Best pitch performance. I believe your issue is not enough advance or your carb is not set up for optimal power output. Also the power curve on the Corvair engine peaks out at about 3100 rpm so the closer you can get to that with a reasonable pitch on your prop the better, however your climb rate is really terrible as you reported it for he 750 STOL meaning that you have no where near your rated HP!
WW’s minimum static rpm is 2700.
My 2850cc gets 2750 static rpm with the Warp Drive set to 8.5 degrees. In climb it turns 2925 rpms. My Vx is 53mph and about 800 fpm but is such a high angle that it is uncomfortable. I don’t think flatting the prop to 4 degrees is a very good idea. If you set it back to 8 or 8.5 degrees, then what would be your static rpm?
From my Installation Manual, page 69
”Measured at the tips, start with a setting of 8.5 degrees. This will yield about 2,700 to 2,750 static rpm on a standard 2,700cc Corvair. Going any flatter than 8 degrees will raise the static rpm, but will not provide enough thrust once the airplane achieves 70mph. Very flat prop settings will get you into the air, but will be an uncomfortable trip around the pattern.”
So WW does NOT recommend setting the prop flatter in order to get a static rpm of 2850! And he doesn’t recommend going below 8 degrees.
He and I do not always get alone, but sometimes even I listen to his advice.
Thank you very much for the advice, I appreciate the input.
Dan Weseman emailed me back and said that from dyno testing the 2.7 l is making about 80 hp @2700 rpm. I'm taking the cowling and prop off this weekend to do some inspection after 40 hrs of flight (plugs, differential compression, general inspection). If everything is where it should be, would it be better to run shorter blades on the prop - 66" instead of 68" and a higher pitch to get the desired static rpm? I will not get anywhere close to 2700 rpm with the blades set at 7 degrees. During my first hours of flight testing I did a static check every time I flew (tying down to the big guard posts by the gas pump). I have obsessively checked the timing a dozen times.
You say "the" 2.7. Is "the" 2.7 "YOUR" 2.7. Some random engine making 80Hp does not mean that yours will.
2700 rpm doesn't mean much in itself. Reducing the amount of prop just drops the load on the engine. Taken to the extreme would be to take the prop off completely, and you could quickly get enough rpm to blow the engine up. Right? You want that 2700 with the prop producing between 350 and 400 of thrust with the airplane tied down. At the end of the day, you don't really care about RPM. You care about how much power is converted to thrust.
What is your mixture like? I have found that the power drops off drastically when I move away from best power.
All said these are the big compelling reasons to get the rpms UP:
***Higher on the HP curve - More RPM=More power (to a point!)
***Wider margin against detonation - hence WW's recommendation for Static RPM>2700
***Ignition will be fully advanced and the timing set correctly ( I believe my top number was 25)
With my prop pitch set at 8 degrees, I was barely getting 2400. WW's critical understanding #2 says this is bad. Hence I lowered the pitch down until I got my RPMs up, hence I had very poor performance at max weight
My Big Question is - if my engine is running correctly should I decrease the diameter and increase the pitch to get back up in the "good range". Mark
Both ways of increasing your rpms (flattening the pitch or shortening the prop) reduce the load on the engine. Because the load is less, the engine, which is making a specific amount of power, is able to spin the prop faster, getting you higher on the power curve and thus getting more horsepower. If you want to emphasize climb performance, get the rpm by flattening the pitch. If you want to emphasize cruise speed, make the prop shorter. Also a factor is prop tip speed, but at 3000 rpm the 68 inch prop is not too long.
But I really think you are getting ahead of yourself. What would really be helpful would be to strap on a prop that is well tested on this engine, and see what your engine does with that prop (that is, how fast will it spin it). I would think that the 68 inch warp has been used on many Corvairs. What pitch do others use? Because you started out with the engine making 3000 rpm in full power climb, and your climb was anemic, it sounds like your engine is not producing the horsepower that it should, and if that is the case then nothing you do with the prop will fix your climb problem. Another factor that directly points to your engine not making expected power is that to get that 3000 rpm, you had to drastically flatten the pitch. It seems more likely that the engine is the problem, not the prop.
I do agree that I am getting ahead of myself. I will know a lot more after testing tomorrow afternoon.
BUT, the engine starts like a shot, idles perfect, runs perfect and sounds great. R & L CHT's are always real consistent at 350. EGT's are always real consistent at 1175. Never any burbs or hiccups. I am super pleased with it that way.
Hi Mark, if you do not have a total advance of at least 28 and no more that 32 degrees BTDC, then at higher rpms your power stroke simply does not have enough time to burn all the fuel in the cylinder. Thus another fact to take into consideration is that poor advance assures poor engine output power at high rpms. Thus you need to be sure you have the advance set up correctly. As WW will say set the static with a point gap of 0.019 inches at the highest point on any lobe then rotate the prop slowly while you have an ohm meter at 2000K ohms setting attached to ground and the points. When you reach 8-10 degrees tighten down the distributor nut. You are now ready to mark the balancer wheel for 16 and 32 degrees using a metallic permanent manic marker. The tie down the plane, warm up the engine and using a strobe light confirm at less than 900 rpm that you still have the initial 8-19 degree setting, now with the distributor nut loose enough to rotate the distributor in a counterclockwise direction, move your rpm to wide open throttle (at least 2600 rpm) then read the timing at that setting. Adjust the distributor as needed by simply rotating it until you have 28-32 total degrees of advance. Stop the engine tighten the hold down nut and restart the engine for a final wide open throttle total advance reading. If it is not within range readjust the advance as before until you reach the magic 28-32 degree range. Ok you are now completely done with optimizing the engine power stroke for the Corvair conversion engine. Go fly the plane and see how it performs. If you still don’t think you have enough power then you have several options, 1. You can reduce the prop diameter 2. You can reduce the pitch until you see the takeoff roll under the same conditions decreasing. That’s as far as you can go. If you are still not happy with the power then you need a higher horsepower engine or some nice cooler weather! That’s it. Hope this helps. See flycorvair.com/distributor timing. You can read all I have said there as WW is dead on about this issue. You can also look up fuel mixture and WW tells you all about that too in great detail. Hope this resolved your issue. By the way the prop you have sounds a little on the high diameter side. My 120 HP Corvair has a 63 inch Sensenich and with the total timing set at 28 degrees I am getting 3100 rpm static. This combination works very well here in Florida on a 90 degree day (that’s roughly 2000 ft density altitude).