Re: Prop Choice
Dick B,
I think that’s a great question. We can all learn the thought process of how and why a particular prop was installed. It’s a much better question than one that simply provokes unsubstantiated opinions like, “What do you think of the Buzzbat vs. the Zoom-master prop.”
The question promotes learning from actual experience. Thanks for your query.
My aircraft has not flown so I wasn’t sure if my reply would be much help without actual Kitfox performance numbers. However, I decided the question is really more about the decision process than actual performance.
I wrote this two days ago and sat on it before posting. At first, I was reluctant to reply, knowing the popularity of three blades and how some builders are fanatical about them. But these are decisions I made for my airplane, not anyone else. Perhaps the process will help someone.
Everyone makes the “which prop?” decision at some point in their build. Usually long before it will take flight. You have to start somewhere.
For my engine, a 100 hp Corvair six cylinder, I chose a 66 inch Warp Drive non-taper two blade propeller.
Why two blade?
There are dozens of articles published on two-blade vs. three-blade props. .. the latest is in this month’s Aviation Consumer. All of them pretty much agree that, barring ground clearance, a three blade prop is largely cosmetic. Face it, the guys with slide rules got it right decades ago when designing props. Since then it’s been just a few tweaks and twists but the performance is about the same as it was in the 60s. A correctly sized two blade prop will perform just as well as a three blade.
Other reasons are:
A two blade generally has less drag than a three blade prop. A reduction in drag can translate into higher thrust.
A two blade prop costs less.
and;
A two blade prop weighs less.
My selection is but a starting point. The only way to get reliable data on prop performance is to do scores of tests with different pitch settings using multiple props. That is a lengthy process the average builder is not willing spend the time and money researching.
I built my engine under the guidance of William Wynne who has dyno tested more Corvair flight engines than anyone on the planet. He has hung not one or two; but many props on the front of these engines. He has hundreds of engine runs on his dyno test stand. William was quite helpful in choosing a prop for my aircraft based on similar airframes within the same speed range.
The reason for the wide blade is also a result of flight and dyno testing. The wide blade Warp Drive absorbs more of the engine power and produces more thrust than the taper tip when loaded to the same RPM.
Of course, nothing is as good as actual in-flight testing, so I can’t report on that yet. I have promised to publish all the figures, including weights and speeds when I get the aircraft flying. I’m starting to cover the aircraft now, so it won’t be too much longer.
That’s pretty much my thought process of choosing a prop to mount on the Corvair / Kitfox combination. Because the vast majority of builders have a Rotax engine they may have a different choice. However, my selection process demonstrates I did not simply grab a prop off the shelf because that’s what everyone else is using. It was carefully thought out and evaluated using the best test data I could find.
Let's hope it flies as well as predicted.
Regards,
John Pitkin
Greenville, TX
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Originally Posted by HighWing
