Prop pitch

[jrthomas]

I am trying to optimize my prop pitch in search of the perfect setting for cruise and climb. I was making about 5400 rpm on climb out, which gave a good climb rate but I could pass redline at full throttle in level flight. 5000 rpm gave about 95mph and 5500 close to 110. By the way, I have a Model 4/912ul with 22" Dessers. I added 1 degree of pitch. It gave a big boost in cruise but now only making about 5000 rpm on climb out. It still climbs good but my concern is am I lugging the engine. I know Rotax says not to overprop. What's the opinion? Should I remove 1/2 degree? James Thomas

[Av8r3400]

What kind of prop?

[jrthomas]

Prop is a Kiev. I flew it yesterday, solo with 1/2 fuel. As best as I can tell, I picked up about 7 mph cruise. I like the improved cruise but I'm concerned about over stressing the engine with too much pitch. With the old pitch setting I've seen as much as 1500 feet per minute climb solo. I didn't have much time yesterday to check the rate of climb. I ran into rain so I cut my flight short. It was pretty hot and humid too.

[kmach]

At wide open throttle, level, at your normal cruise altitude, Aim for 5500 to 5600 rpm.

Climb rpm should be 5300 rpm or above to not lug engine.

[jiott]

I agree with Kevin, although I might go just a little more at WOT level normal cruise-say about 5650 rpm to give just a little better climb, but not overspeed at WOT level cruise.

By the way, there is lots of good discussion on this topic on the Rotax Owner forum website.

[jrthomas]

Thanks to all for your advice. Yesterday I was showing a solid 5600 rpm (maybe a little more) at full throttle in level flight and indicating about 117 mph. The Kiev pulls good, maybe too good only making 5000 rpm on climb out. That was my main concern, that over propping might be unnecessarily stressing my 912. I made a call this morning to the best Rotax man I know.(He's well trained and experienced with the 912's.) His opinion is that it would be a good idea to back off the pitch a little, maybe split the difference and back off the pitch about 1/2 degree. He was concerned as I was that too much pitch is not a good thing. My cruise at 5000 rpm went from about 95 mph at the old pitch (5150 rpm static) to about 102. I like the added speed but not sure 7 mph is worth sacrificing climb out performance. So my plan now is to remove 1/2 degree and see how it does. Thanks again, James Thomas

[efwd]

Well it was sounding like you we're heading in the right direction to me. So, please post what happens when you adjust the .5 degrees. I look forward to hearingthe outcome.
Eddie

[jrthomas]

Thanks Eddie. Will do.

[Wheels]

Hi,
I fly the IV 1200 with a warp drive 3 blade taper tip on an 80 HP. I believe the key to
"lugging" an engine is to know the difference between "loading" and unloading the prop I.E. engine. When your on the ground with the brakes on at run up, you have essentially loaded the engine to the maximum load it will ever have. Once your moving, air is allowed to "feed" the prop and your rpm climbs accordingly. You will be 'unloading' the work the engine has to do as you fly faster. We usually spend a short time in the loaded category, i.e. run up, climb out. Once your straight and level, you are as unloaded as you can get without being in a decent. A condition that can quickly overspeed your bird. Consider the amount of time you actually spend in the loaded category and then at cruise and pitch your prop to the category that brings you the most performance at cruise without "overloading" the prop. It shows up in RPM, and heat.
If you need to unload the prop for high angle departures in mountains or such, your cruise will suffer but that is the price we pay for needing STOL performance over cruise performance. We All Load the prop to the max for a short period of time and as long as we don't live there, the engine is going to perform well and be happy. We have numbers for our gauges for a reason, use them judiciously and be kind to your Rotax. Hope that helped a little.

[jrthomas]

OK. I removed a half degree of pitch. There seems to be a very fine line between too much pitch and not enough. One degree makes a world of difference. One degree too flat and it redlines in level flight and one degree more pitch and it doesn't reach a good rpm on takeoff. I think I have it at a pretty good compromise now. I flew it this morning. It was already hot and humid by around 9:30 but it got off the ground pretty quickly and climb out at 70 mph was around 1000 feet per minute with just me and full gas. Tach was reading about 5200 on climb out.I have seen 1500 fpm on a cool day with the old prop setting. Flying at about 1200 feet altitude and 5100 rpm airspeed was indicating around 105. Flat out was showing around 5700 rpm and airspeed between 115 and 120. GPS usually looks like I may be a few mph faster than my airspeed indicator reads. I'm pretty satisfied with these numbers so I think I'll leave it alone. It's a Model 4/1200, 912ul, long wing but with all the Speedster options and 22 inch Dessers. Do these numbers seem reasonable to y'all? James Thomas

[Wheels]

The numbers look similar to mine. Solo Full tanks 65 MPH Im around 1000 FPM.
My Field elevation is 2160 ft.

I have heard and experienced that the Rotax RPM gauge is about 200 RPM low. Watch that. Find out if you are safe at max indicated RPM. Probably not. Use a strobe or digital RPM on the prop and do the math. I need help, but my friends are smart so they do the math for me.

Hope to see you at the fly in at Kitfox fun day.

[av8rps]

It sounds like you are very close to the optimal setting for a 912 with that prop. My experience with different settings on my IVO IFA on my 912ul Kitfox 4 amphib would completely agree with how you have your Kiev set.

Maybe I missed it, but what is your prop diameter/ length? If it is a 75 inch Kiev those are GREAT numbers.

Worth mentioning, I tell friends flying 912's that if they want better STOL and climb but still want to get somewhere at a reasonable cruise, set prop so that you get 5800 rpm with WOT straight and level. You'll have a much better climb than at 5500-5600 WOT, but you won't notice much drop in cruise when you pull it back to 5500 for max cruise. (Plus, if you want to race your buddies 172 or something, the plane will fly fastest at 5800 :D )

[jrthomas]

My Kiev is a 67.5" 3 blade.

[av8rps]

Ok, that actually makes more sense to me. I've said for a long time that the best prop diameter for overall recreational flying in these planes is between 68 and 72 inches. Smaller is faster at the loss of some climb, and longer provides better STOL at the loss of some cruise.

But the reality is that your 68" Kiev makes for a very good performing Kitfox, as you have a nice cruise with a good climb. And on only 80 hp that is pretty impressive.

[jrthomas]

I flew the last 2 mornings, yesterday with a passenger and close to full fuel and I was impressed that I experienced little if any loss of speed after removing a half degree of pitch. I didn't do the math but I'm sure I was pushing my 1200 lb gross weight limit. I was well aware that my take off run was longer and my rate of climb was less but it still performed very well. I think my pitch is just about a perfect compromise of climb and cruise. No more tweeking for me. Thanks for all the good advice. James Thomas