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.
I'm thinking this is a static rpm on the ground. Is this correct?
woops. Somehow my brain locked in on static RPM after reading Pauls first thread. Yes the 5150-5200 figure is for static run-up. Next flight I will check max rpm in level flight and compare with Rogers recommendations.
Static rpm can be different for different props. I found that out a few years ago during a big multi prop research project. Static is only good to get you in the ballpark for a first flight. It needs to be checked at WOT in level flight AT YOUR AVERAGE CRUISE ALTITUDE. It does no good to set it for 1K above sea level if you always fly at 8-9k ft.
5150-5200 rpm is may put you up around or above 5800 in WOT in level flight. Your high altitude will play in here so get back with us.
If you need a better climb or cruise prop then you might be good, but if you can get up around 5700-5800 at WOT in level flight then you are giving up some cruise and fuel. Decide what's more important to you.
Roger Lee LSRM-A & Rotax Instructor & Rotax IRC Tucson, AZ Ryan Airfield (KRYN) 520-349-7056 Cell
With my flight adjustable prop....5700-5800RPM at WOT is my best climb (for a limited time of course)--full fine. AND I make sure I throttle back before leveling out so that I don't overspeed the engine!
At 5800RPM WOT in straight and level flight (at normally flown altitude)and throttling back to between 5100-5500 RPM, this would obtain my best cruising speeds.
If I want to use my prop without adjusting it ( using it like a fixed pitch prop) I set at 5600-5650 at WOT flat and level at my normal operating altitude, and throttle back between 5100-5500.
Manifold pressure is of little importance.
Have I got this straight Roger??
Does anyone else have any suggestions?
Ron Lalonde
601HDS, 912UL
Debert, Nova Scotia
Canada
I have lost track of the number of conversations i have overhead basically saying..."What is better RPM or torque ?"
The problem is that it is a nonsense question. Humans like a simple "one answer fits all answer"
It is a bit like saying what is the best way to calculate 12 is 2X6=12 better than 6X2=12.
Without torque there is NO power, without RPM there is No Power, 0X6=0 and 6x0=0
zero is zero.
How this relates to prop and "best climb" it that the Best climb can only be had at the highest power. So lowering the rpm from 5800 to "best torque" looses power and therefore looses climb rate. It is simple maths.
Therefore the "best climb" will be at a pitch setting that allows maximum power (5800 rpm) anything lower costs climb rate.
Where it gets complicated is NOT where best power or climb rate is, but when you do not have a variable pitch prop. A variable pitch prop allow you to utilise all the available power at each and every stage, be it take off, Climb, cruise.
With a fixed pitch prop you are stuck with having a compromise on Take Off a compromise on Climb and a Compromise on cruise. the "right answer" is that there is "no right answer". It depends on your plane, what height you live and take off at, how much you personally prefer Climb performance vs Cruise. Ie do you do long flights at average height (5000 ft) and take off from a long runway ? The best solution for you will be different from someone living at sea level with a short field.
Given that there is No right answer my suggestions would be ...
1) Buy a VP prop
2) Buy a VP prop
3) Buy a Vp prop
If you definitely cannot do 1-3 then go with someone whom has set up many fixed props (that's Roger) as his experience will likely get you closer to the best start point.
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