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I have ordered a Foxbat A22LS and whilst waiting for it to arrive I have been wondering how other owners have pitched their props. I will have the standard 3 blade Kiev, probably factory set to 5200 rpm static. Has anyone changed their pitch and found a different sweet spot for their numbers (take off roll, cruise, max continuous speed)?

 

I will have the big tyre option and long range tanks so will have a be bit more drag if this makes much of a difference.

 

Cheers.

 

Paul

 

 

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Fly it and adjust to your tastes...

 

Above 4 or 5 thousand I generally fly WOT, and it sits just under max continuous rpm (5500), then slowly drops as I climb due to less oxygen.

 

This gives me about 5650 max down low, flat/level. I feel setting it up to 5800 max is a bit too fine pitched for me.

 

There will be differences to winter and summer running depending on how hot/cold it gets.

 

A nice cold, high oxygen morning and you will feel it.

 

Don't be scared to rev it. 5200 to 5500 and it's like a turbine.

 

If you monitor all egt's you will see how out of balance they are at lower revs, then they all come together over 5000...

 

NOTE* I'm talking about the carby version...

 

 

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Paul:

 

When my Legend was delivered, the factory had set the prop pitch (Woodcomp) at around 4800 static RPM. I flew it like that for most of the first year. Later on I put a new Bolly on and set it the same as the factory prop. Then Rotax put out a service bulletin (Letter?) stating that the minimum was 5200 or 5300 (I forget which). So I decided to re-pitch my prop (Now Bolly).

 

I expected that I would have to use higher RPM to get the same cruise speed and that I would use more fuel. In fact, I do have to use higher RPM to get the same cruise speed, but I still use the same amount of fuel! Takeoff and climb performance is better with the finer pitch but I was surprised that the fuel consumption remained the same (or nearly so).

 

I asked a few people and they said that props are optimised for a certain forward speed and RPM range. If you set the prop too steep, it ends up working against itself somehow. The aerodynamics are fairly complex apparently.

 

In my experience, at cruise RPM, a Rotax uses a certain amount of fuel per hour, regardless of what it's towing through the sky. What varies is the resultant airspeed and that is related to the amount of drag the airframe generates. High camber STOL wings generate lots of lift, but also lots of drag and so those airplanes fly slower than ones with lower camber wings. Airplanes with two struts have more drag than those with one, which have more drag than those with none. Airplanes with wire braced wings have more drag again and cloth wings are draggier than metal or composite wings etc etc.

 

 

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  • 1 year later...
Paul:

................................................ In my experience, at cruise RPM, a Rotax uses a certain amount of fuel per hour, regardless of what it's towing through the sky. What varies is the resultant airspeed and that is related to the amount of drag the airframe generates. High camber STOL wings generate lots of lift, but also lots of drag and so those airplanes fly slower than ones with lower camber wings. Airplanes with two struts have more drag than those with one, which have more drag than those with none. Airplanes with wire braced wings have more drag again and cloth wings are draggier than metal or composite wings etc etc.

Just been having this conversation with the Savannah boys - They dont believe my cruise fuel consumption of sub 13 l/hr at 100 knots, 4800-5000 rpm. My prop is set advantage climb, so I dont get my Zephyrs best cruise speed But because the pitch i fin-ish the engine is lightly loaded (doesn't require as much fuel) at cruise. I can do this cause my aircraft is considerably less draggy/cleaner than the Savannah but still has a stall speed similar to a Foxbat.

 

 

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 The prop is like a wing . It has one most efficient angle of attack. (Best L/D ratio). Either side of it, you develop less power. Power equals fuel flow (more or less). If you fine the pitch off you use more revs  but have less torque which allows the engine to spin more freely.. That's the simple explanation  of prop pitch ..

 

      Now, when using a piston engine, friction rises as the squared of revs so run the engine slower to reduce engine friction, to  get maximum range. That is if you have two combinations of RPM and Manifold pressure  that can apply,  run the one with lower revs and if possible with a supercharger use full throttle at the correct altitude. where WOT gives you the MP you need.. That way you are not reducing it through the carb throttle to later compress it  (boost it) in the supercharger. Nev

 

 

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