That 'Ol Black Magic:  Propellers

September 2006

What size of prop should I put on my Fly Baby?

Seems like that should be a very simple question to answer, shouldn't it?

But it isn't.  While prop manufacture, itself, is surprisingly straightforward, prop design isn't.  And, by extension, finding the right prop for your Fly Baby isn't either.

Nomenclature

First, let's get some definitions out of the way.  Props are identified by their diameter and their pitch...as in "72/48".

Diameter is easy enough...just the distance from tip to tip.  The pitch is a bit more complex, but still pretty simple:  It is the distance the prop would move forward in one revolution, assuming it has perfect "bite" into the air.  Imagine the prop as a screw going into wood.  If the screw goes 50" into the wood with one rotation, that's a 50-inch pitch.

So what's the problem?

If both have the same diameter and pitch, one propeller should be the same as another, right?

Take a look at this drawing, illustrating three 72x48 props:

All three props are the "same," from the marketing point of view.  But obviously, they won't generate the same thrust!

And hence, here is where the difficulty lies:  The pitch definition assumes the prop has perfect "bite" into the air, but in the real-world, these three props will actually grab the air quite differently.  The middle prop will probably grab so MUCH air that the engine will lug down and won't produce maximum power.  And the one on the right will probably be less effective than the typical ceiling fan.

The Ugly Truth

The fact is, there is no propeller specification that establishes how efficient a given propeller is...that is, how well it produces engine power to thrust.

Without a specification...comparing propellers between manufacturers is meaningless.  If the owner of Fly Baby N100A replaces his 70x43 prop carved by Joe Smith with a 70x43 built by Ed Jones, the chances that the airplane will perform the same are very small.  Smith's may have broad blades, but Jones may have a skinnier design intended to allow the engine to turn up to full RPM.

Take a look at the Propeller section of the Fly Baby owner's survey.  You will see a surprising variation in the propeller specifications for the same engine.  This indicates how the props vary between manufacturers.

Picking a Prop

So, how do you pick a prop?  If someone else's Fly Baby is running a certified prop and your Fly Baby has the same engine, you should be able to run the same model of propeller.

Most of the time, though, you'll end up going with one of the propeller makers that specialize in the homebuilt market.  Again, if a friend has the same Fly Baby, you can probably order the same model propeller, as long as it's from the same prop maker.

Otherwise, though, it's actually pretty easy:  You don't have to know what the propeller specifications should be.  Instead, just tell the propmaker what you have, and want you want.  "I've got a Bowers Fly Baby mounting an A65 with a tapered crankshaft, and I'd like a climb prop."

Most prop carvers have built units for Fly Babies before, so should have a good idea of the proper diameter and pitch for their particular blade shape(s).  In most cases, wooden prop makers are able to do some amount of adjusting even after they deliver the prop.  They'll send you the prop, and have you test fly it.  If it's not letting you climb fast enough or cruise is too slow, they can then re-shape the blades accordingly.

Make sure you discuss the ability to make changes as you discuss the prop with the carver.

The Sensenich web page includes a lot of great technical information on propellers.



Ron Wanttaja

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