Setting Prop pitch RPM for best climb

by Paul Hamilton in 912iS Technical Questions 8 years ago

I just test flew my new 912iS Sport. At 6000 foot density altitude, hot day, I get 5700 rpm full throttle. I have to throttle back to max continuous 5500 to climb to 10,000. Does not appear to climb as good as my 912S which I have the prop pitch set to get 5500 rpm full throttle on climb which I set before for best climb (verses 5800 max throttle).

My Sling with a 912iS Sport is set to 5000 RPM full throttle for climb.

I am about to install a new prop and will be pitching the prop for a specific RPM for climb.

Operating at high altitudes, I need best climb performance since I get about 3% reduction per 1000 feet density altitude and my flight profile requires I climb to 10,000 feet.

Based on the torque curves in the Rotax 912iS Sport operators manual page 5-2, I am inclined to set the pitch to get 5000 RPM, which is the best torque since the torque drops off from 5000 to 5500 and further to 5800. However the engine power goes up... :S

Note this is a fixed pitch prop and generally designed for best performance in climb.

Less torque and more engine power, or more torque and less engine power, which will produce the best thrust for climb?

Any thoughts, experience or wisdom would be appreciated.

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Re: Setting Prop pitch RPM for best climb

by Roger Lee » 8 years ago

Roger Lee

128 5.28k 4.98k

Hi Paul,

It's hard at times to compare different planes performance even with the same engine because of different hull designs. Hull design, prop pitch and pilot all play a part in how you think your plane performs and there can be quite a difference. I've always told people to set a ground adjustable prop to be WOT at 5600-5650 at your average altitude since we all sometimes fly lower and higher. Over pitching a prop has absolutely no redeeming qualities. It cost you fuel, climb and cruise performance plus higher engine temps, torque and HP. Owners that are pitched less than 5500 rpm at WOT are not doing themselves any favors. I have set hundreds of props and I usually fly down around 3-4K', but there are times I'm at 10K-12K and you can easily see the effects.

So now begs the question what is the perfect rpm? (ground adjustable props) To me it is a balance between climb, cruise and all the other items I listed above. In a lot of my testing that was 5600-5650 at WOT. There is a big but here though. You may fly heavy like a float plane or have special circumstances like always being at short fields where climb over cruise may be more important. So now it becomes somewhat of a personal choice. Do you tweak more or less pitch? 5700 rpm is a better climb prop over 5500, where 5600 is a better balance point for you. If you tend to always takeoff at high density altitudes, climb and cruise fairly high and or you're heavy then maybe you want a slightly better climb prop, but you will sacrifice some fuel and cruise speed. This is all that personal choice. You are correct that with high altitude you're losing engine performance and any prop overloading makes it much worse. If you flew at 10K most of the time then you may want to set the prop to get 5600-5650 and try that for a while and see if you like it.

I have a Flight Design CT and a Sensenich 3 blade 68" prop. 5600-5650 is a good place for me. I get good low and high altitude BALANCED performance for the plane and engine.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Setting Prop pitch RPM for best climb

by Richard Boslaugh » 8 years ago

Richard Boslaugh

9 122 84

on a Kitfox SS7 with 912iS the Kitfox factory recommends between 5150 and 5200 with a three blade prop. I used 5150 with good performance and acceptable fuel consumption. Then I had the prop off for gear box work and ended up with 5000 when re-installed. I notice the climb performance is not as good. One of these days I will set back to 5150.

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Re: Setting Prop pitch RPM for best climb

by Roger Lee » 8 years ago

Roger Lee

128 5.28k 4.98k

Hi Richard,

If Kitfox recommends 5100 at WOT in flat and level flight you are severly over loading your engine and have lost a lot of performance. If you change to get up around 5600 you will think someone turbo charged your engine. You will take off sooner, climb faster, cruise faster and at reduced throttle settings, have lower engine temps. In the pre mid 2006 engines if you did those things you would crack your crankcase. I have been involved in 3 aircraft MFG's resetting the pitch from 5200 to 5500+ at WOT in level flight.
Another Kitfox with his prop adjusted right would run off and leave you in climb and cruise and do it with about 1.5 gph less. I have reset hundreds of props and not a single person ever has gone back to those low rpms.

Here's the Challenge:

Go out and fly one morning with your prop setting. Land and reset the prop to get around 5600 rpm at WOT in level flight. Now do the same take off test. If I'm wrong you can land and reset your prop right where it was. If I'm right then you'll have a huge smile on your face and can call Kitfox yourself and tell them how much better your performance is.

You have nothing to lose and everything to gain.

The only reason I'm suggesting this is I already know the outcome.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Setting Prop pitch RPM for best climb

by Paul Hamilton » 8 years ago

Paul Hamilton

31 148 343

Based on Rogers "experience", I will set my prop to 5500 at my climb speed and WOT so I can get maximum climb. However this still does not satisfy my curiosity about over pitching since the 912iS sport torque is more, 130 nm at 5000 rpm and declines to 120 nm at 5800 rpm ROTAX operators manual page 5-2. Yes the power goes up at higher rpm but the torque goes down.

Still trying to get my head wrapped around this one.

Again, more tongue more thrust?

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Re: Setting Prop pitch RPM for best climb

by Roger Lee » 8 years ago

Roger Lee

128 5.28k 4.98k

Hi Paul,

You are right about 5500 getting you better climb, but cruise and fuel is going to take a huge hit and then the max WOT rpm in level flight will end up over 5800. This seems like a waist of your engine performance.
Climb rpm set to be around 5000 rpm would put WOT in flat flight up around 5600-5650.

You do have a max torque spec and HP spec, but two things in paly here. You are at high altitude so you lose a lot from the sea level guys. Then depending on the prop you have if you set the pitch too course the engine torque and HP is not enough to turn it EFFICIENTLY.

There is the key. Yes your engine can turn the prop at 5100 RPM WOT in level flight, but it suffers doing it. To get the BEST HP and torque at your high altitudes you need to allow the engine to have enough rpm to unload it and be efficient. Actually that is at all altitudes. The in flight adjustable prop guys get the best choice as their altitude changes, but us ground adjustable people have to pick a balance point that serves all our flight parameters.

More torque can give more thrust, but you need the HP to turn it. each of these have a line that if you cross it you start back downhill from any positive effects. Over do it for either one in the wrong direction cost performance somewhere. This is why a balanced prop pitch is best for a ground adjustable prop. The in flight adjustable prop guys get to adjust for some of this during their flights. We don't get that luxury so we balance between climb and cruise unless you have a special needs for a possible flatter pitch. Like I said there are no redeeming qualities about over pitching a prop to an engine.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Setting Prop pitch RPM for best climb

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