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Old 09-29-2018, 10:07 AM
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Lufthans Lufthans is offline
Join Date: Dec 2017
Location: Hilversum, The Netherlands
Posts: 58

A common misconception is that the constant speed feature of a prop will make everything ok. It doesn't.

Let me elaborate.

A prop is a rotating wing. No news there. And like any wing, it can operate with its angle of attack in a certain range.

But what is the angle of attack of a prop?

How the air hits the airfoil of the prop depends on two factors: the forward speed of the aircraft and the rotation of the prop. The forward speed is the same for the entire prop, the "rotational speed" depends on the distance to the hub of the prop: at the hub itself it is zero, at the tips, the rotation of the prop is in the .7 to .8 Mach numbers.

If we want an efficient prop, we want each station along its span to be at optimum angle of attack (and so lowest drag). If we vector the forward and rotational speed together, we find that we will need quite some washout - with the prop flattening out towards the tip.

Now comes the trick: Imagine the forward speed difference between a fast and a slow aircraft and you'll see that on a fast aircraft, the tips need to accommodate for much more forward speed, and will thus be less flat than on a slow aircraft, whereas near the hub, the difference is much less pronounced. And thus washout for fast versus slow aircraft are quite different.

Note that turning the whole blade (as you do with a constant speed) will not alter the washout! A constant speed prop therefore really is only optimised for one particular speed.

Luckily, the relatively thick airfoils of a prop accept quite a range of angle of attacks, so ill-matched props WILL work. However, while simply adding more pitch will indeed slow down the engine, but not necessarily produce more thrust.

So far the theory.

In real life - On my Jodel, I went from an ill-matched Whirlwind to a well-matched MT, and needed 3 fewer inches of MAP to obtain the same speed, and gained nearly 500 fpm climb rate in the process.

So to get back to your question: yes, it will probably work, however you're likely to leave a fair bit of performance on the table when going with a prop that is meant for a much slower aircraft.

I'd consult with the prop manufacturer if I were you. Or take the engine and sell on the prop to save for a good one.

Happy building!

Hans Teijgeler
Hilversum, The Netherlands

Preparing to build a Rocket
PH-BRR Bowers Fly Baby; Imported and upgraded
PH-MGA Jodel DR1050; Built, with Subaru EJ25
PH-SUM RV-4; Imported and upgraded. Sold my share :-(
PH-EIL RV-4; Imported and upgraded for friend. Sadly crashed
PH-ERD Robin DR300; Built with Subaru EZ30 for friend.
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