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Prop pitch

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Jeffrey Dill

Joined: 23 Jul 2007
Posts: 52
Location: Pleasant view, TN

Posted: Sat Nov 27, 2010 4:23 am Post subject: Prop pitch

You guys probably read why I changed my GSC prop on a different thread. Now I want to know how much pitch is too much? I just read a comment on a thread of similar title from Chris: 



"... when I reached about 12 deg, it overloaded the engine and had very poor performance. 11-11.5 was my happy spot."



 My question now is how much pitch is too much? I want a cruise prop because I operate from long runways and climb performance is not critical, so I tried to match the pitch of my previous prop. The max RPM I saw with the old one was about 6500 on my 503. I liked it because I didn't have to worry about exceeding redline RPM, had plenty of climb performance and cruise speeds approaching the the yellow arc. 



     Now I have installed an identical prop, but new. I did an engine run and saw 6500 RPM at full throttle.  At that point I double-checked the torque, saftied the bolts and flew. In flight, the max RPM that I saw was maybe 6100. I still had plenty of performance in both climb and cruise. The engine was not running hot or anything (OAT approx 4C/40F). I am not seeing the "very poor performance" that Chris mentions. But the full throttle RPM was so much less than Rotax red-line of 6800 that I am worried about unintended consequences.



     I will probably reduce the pitch some more, unless somebody smarter than me tells me there is no problem operating it this way.

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Jeff Dill
Model 2

Tom Jones

Joined: 12 Mar 2006
Posts: 752
Location: Ellensburg, WA

Posted: Sat Nov 27, 2010 5:43 am Post subject: Re: Prop pitch

Jeffrey Dill wrote:

 	  Quote: 	
		  Now I have installed an identical prop, but new. I did an engine run and saw 6500 RPM at full throttle. At that point I double-checked the torque, saftied the bolts and flew. In flight, the max RPM that I saw was maybe 6100. I still had plenty of performance in both climb and cruise. The engine was not running hot or anything (OAT approx 4C/40F). I am not seeing the "very poor performance" that Chris mentions. But the full throttle RPM was so much less than Rotax red-line of 6800 that I am worried about unintended consequences. 	




Jeffrey, my advice is to recheck the pitch to be sure a blade didn't slip after you checked the static RPM.  RPM at full throttle in level flight should be higher than RPM at static full throttle.



If you find the pitch is okay then check both static RPM at full throttle and max RPM in level flight full throttle again.  Your static and in flight RPM's seem to be reversed.



For what its worth here's my number's with a 503 and a 72 inch warp two blade prop, Classic 4.  Static 6200, level flight full throttle 6600.  On a 40 degree day at 2000 msl I get close to 800 feet per minute climb and 85 mph cruise at 6000 rpm.

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Tom Jones
Classic IV
503 Rotax, 72 inch Two blade Warp
Ellensburg, WA

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Sun Nov 28, 2010 11:32 am Post subject: Re: Prop pitch

Getting the right prop load on a 2-cycle engine will help prevent seizures, make it run efficiently, and prevent the temptation of making mixture adjustments. This is a great webinar to watch to do that: http://www.eaavideo.org/video.aspx?v=626943072001



Mike Perkins

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Jeffrey Dill

Joined: 23 Jul 2007
Posts: 52
Location: Pleasant view, TN

Posted: Mon Nov 29, 2010 2:10 pm Post subject: Re: Prop pitch

Thanks Tom, you are always good for reliable info. Mike, Your answer addresses my main concern, overloading the engine. This is an outstanding video. It is the best education I have received in the 6 years of owning a two stroke. That is 6 years and never having an engine with more than 40 hours on it. To some extent my learning curve has been blessed by that. The video needs about an hour to get through properly and I am half through now, but it is great. 



I took off about a 1 1/2 degrees from my prop and ran it up to just short of 6600 RPM at full throttle. Then I went to fly, and once again, I saw much less during takeoff roll, maybe 6100. In cruise at wide open throttle and level I saw a higher RPM, 6400. So, at least the static versus cruise RPMs make more sense this time. I don't know why I twice have gotten a higher RPM right after installation test than later flight test. Different conditions of some type, but not sure what yet. During that initial runup, I wait for minimum 200 CHT before going above 3000. This time I was careful to increase by 1000 RPM so as to avoid thermal shock, or other anomaly associated with insufficient temp somewhere. Still a mystery to me why it tests at 6600 and later I can only get 6100 static. 



Can't wait to see the second half of the presentation, but gotta put it down for now.

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Jeff Dill
Model 2

Jeffrey Dill

Joined: 23 Jul 2007
Posts: 52
Location: Pleasant view, TN

Posted: Wed Dec 01, 2010 6:03 am Post subject: Re: Prop pitch

I have now seen the whole video and have learned a lot. While my temps have all been below max, I would say my EGTs were below what I used to see with my old prop, correlating with the higher pitch and consequent higher engine load. The video suggests that such high load is seen on CHT. If I have it right, detonation (a very very bad thing that can fail your engine in seconds) can occur in two cases. Under high loading, probably more extreme than mine currently and seen on CHT, you can get pinging the same way you might by trying to start a car up hill in 5th gear. The other way to get it is by underloading , such as descent in the 5000 rpm range where EGTs get up toward 1200. This is happening because of relatively high rpm for the mixture, which translates to a lean mixture, which further translates to detonation. As I type this I wonder if I have that correct. Is detonation a consequence of that lean mixture, or just extremely high temps? That point is academic because either way metal is melted and engines can fail within seconds. 



My goal is to match Tom's numbers by further reducing my pitch. Said another way, my goal is to match Rotax numbers for normal operation. My take-away from this is that adjustable pitch propellers are not so much to give us the option to choose climb or cruise prop, rather, it is to provide a way to properly match the prop to the engine, gearbox, and airplane.



As is pointed out in this video, there are ways to get more power out of the engine with aftermarket devices, but these improve one regime at the expense of another. The engine might provide more power for takeoff, but blow up in cruise. My plan is to stick to the Rotax parameters as closely as I can. They have given this a lot of thought. Now that I said that, it was pointed out in the video that some of the guidelines cater to the weakest link, such as changing spark plugs at 25 hours. They assure us that if we run the engine right, it will not build up carbon or foul plugs. He did say he runs premium car gas, despite Rotax ok for regular.

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Jeff Dill
Model 2

mikeperkins

Joined: 22 May 2007
Posts: 123

Posted: Wed Dec 01, 2010 9:04 am Post subject: Re: Prop pitch

Hi Jeff, 



Glad you were able to watch the rest of the webinar video. In all my years flying behind a 2-cycle, it is the single-most valuable source I've found for setting up a prop. Your last post indicates some scientific thinking on your part. And your take-away is exactly right. It is something that I had not thought about consciously myself, but the goal is to match the engine's needs for reliable performance, not optimizing for cruise or climb or some other thing. I like that.   



Regarding detonation, you won't experience it if things are set up more or less correctly. I see 1160 - 1180 in cruise, but I don't worry if I see 1200 for a short time.



EGTs are funny gauges. Westach's are calibrated at 75 degrees ambient. If the gauge in the cockpit is colder, the EGT reads higher by the difference. So at 20 degrees, if the gauge reads 1000, the EGT probe is actually 1055. Conversely, at 95 degrees ambient, a 1000 EGT is only 980. There are compensated EGT gauges, but they're more expensive and usually digital.  



I used to change the plugs religiously in my 532 at 25 hours or less. At 25 hours, they always "read" very normal and showed no insulator carbon build-up or electrode wear whatsoever - both could cause misfire. I now believe the thing to do is to just keep an eye on them.



At one time, I was tempted to add the mixture-adjust modification to the carbs. I even considered at one time trying an in-flight adjustable prop in the spirit of being "experimental" to eek out some more performance. But after watching the webinar, I feel that either of them would be asking for trouble. 



When I built my panel, I put the EGT in front of me, just to the left of the ASI, because I'd heard so much about the importance of EGT in 2-cycle engines. In retrospect, think that was the right thing to do. It certainly keeps my focus on the right engine parameter. I now have an ambient-compensated dual digital EGT with a bright red warning light set for 1220. Even still, the video taught me to not reduce power if the EGT gets excessive, but rather to load up the engine by climbing until the power can be reduced slowly - very good to know. 



Mike

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