Propeller Pitch

Category: 912 / 914 Technical Questions 3 months ago

912 ULS in a Zenair CH750

I am doing some experimenting with my prop. I have been flying with it quite coarse and have not been satisfied with the takeoff performance when hot and heavy. I have 50' wires at the end of a 2000' runway, so safely clearing these is my priority. 

I was not able to reach 5800 RPM previously, so we have just taken out some pitch. I can now reach right around 5800 on the takeoff roll and initial climb and have a much more comfortable margin over the wires. I need to back off the throttle a bit to stay below 5800 RPM when I lower the nose a bit and climb out at a higher speed. 

Searching here, I see others have set their props for a WOT cruising RPM and I have also been told that it should be set for a static RPM of 5200-5300. I am well over 5300 RPM static, but I am not sure that is a good guideline for a plane with such high drag. 

Rotax allows (but appears not to encourage) speeds up to 6200 RPM for brief periods. I want my 5800 RPM/100 HP, but I don't want to risk damaging my engine.

Any input from others who have experience with slow/high drag airplanes?





  • Re: Propeller Pitch

    by » 3 months ago

    Hi David

    I have trained on a 750 with the ULS engine and Warp Drive propeller.

    That plane had the airbox installed on the engine but the air filter was IN the cowling ...breathing warm air. I suggested the owner to install the air filter on the cowling side   ( using a NACA scoop(duct)  ) ...  result: 300 rpm more    see attached picture

    I"m mentionning this because I found a picture of your plane on the net and I don't see any scoop . If you do have the NACA ( don't see the other side) , then,, try to get about 5500 on takeoff...but you will be loosing on the cruise speed.

    The ''normal'' prop setting is to have 5500-5600 WOT in level flight at 'usual' altitude.

    You could also change the propeller .. an E-props propeller will give you those missing RPM on takeoff  without sacrificing on the cruise.

    My 2 cents

    26709_2_naca.JPG (You do not have access to download this file.)

    Thank you said by: RotaxOwner Admin

  • Re: Propeller Pitch

    by » 3 months ago

    The Zenair installation is definitely compromised by the lack of a proper airbox, the design of the exhaust and the cowl. I have made some openings in the cowl to bring in more fresh air and let the warm air out the top. 

    The Rotax makes peak power at 5800 RPM, so might as well be able to get there when you need it most - ie taking off over the wires. Getting the prop right is the part of the equation I am working on right now. 

    I backed off my adjustment yesterday because I thought I needed to back off the throttle too much to keep to 5800 RPM. We are still getting dramatically better take off distance and climb with just a 1 degree change from where we originally had it. Cruise speed is now close enough to where it was (we didn't build a Zenair to go fast).


    Thank you said by: RotaxOwner Admin

  • Re: Propeller Pitch

    by » 3 months ago

    What does your propeller manufacturer recommend for static rpm?  I’m operating a Zenith 701 with the 80 hp Rotax, Whirlwind GARW3B prop cut to 68”.  Whirlwind updated their instructions manual just today for this prop and now recommends static rpm be set at 5,200-5,300 rpm.  I set mine at 5,400 rpm last year during install and test flights based on their previous instructions (400-500 rpm below maximum engine rpm). I operate from a 1,000 ft. sod strip with mature trees at both ends, and clear them comfortably at max weight (1,100 lbs) in standard day conditions. Hot humid high DA days I fly only lighter passengers or wait for any headwind-even just a light breeze makes a big difference in these STOL aircraft- if carrying any passenger that weighs over 200 lbs. I weigh about 170 lbs and carried my 240 pound son over the trees with room to spare with just a light headwind, and that included a 2.5 degree upslope on mowed grass.  When I am clear of the trees at Vx I lower the nose to Vy and when I arrive there the rpm has settled in at the 5,500 rpm max continuous power limit.  I don’t have to reduce power to avoid exceeding the 5,800 rpm limit until acceleration is well underway as I level off. Hope this helps. Edit: also have your prop balanced when you are done adjusting it as not only will it reduce wear and tear on the engine and prop you can confirm your tachometer indication against the rpm indication on the prop balancer.  

      Edit again: I spoke with the Whirlwind engineer today and asked him why their manual recommends 5200-5300 static rpm, and he stated it's based on their experience to achieve what their manual calls the “ideal” static prop RPM- one that allows you to accelerate to maximum indicated RPM at WOT, but not beyond.  He acknowledged no prop harm is done by setting the static higher or lower based on individual preference but the tradeoffs between pitch and performance are well documented.

    Thank you said by: RotaxOwner Admin

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