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Twist change - Hartzell

von_flyer

Well Known Member
Advertiser
I have a Hartzell HC-C2YR-1BF on my RV-7. I have heard that by increasing the twist of the blades they would be more efficient at the speeds we fly. I was told the stock blades are optimized for 120mph forward velocity.

Does anyone have data that supports this modification, parameters, angles, etc. or shows performance differences?
 
Short answer: I know of no such empirical data.
Longer: A. I have used two programs to identify 'best match' between existing propeller choices and airframe; there are more. This is not a simple process. People like Cato and the Prince folks make their living doing it.
B. While I have not asked the Hartzell folks what their optimum design speed for that propeller is, i sincerely doubt it is 120 - or it would likely perform better in climb [comparing to the non-BF] than at cruise - which anecdotal data suggests is the actual case.
C. Changing the pitch 'gate' would be an interesting experiment. Since every propeller is the result of serial compromises, you might discover that some change optimizes for your individual airplane, beyond the manufacturer's specification. It will require you to disassemble the propeller; might want to have at least a static balance rig available.
D. Pretty sure my local FSDO would not sign off on a Phase 1 test of this type, unless I could give them a comparative analysis of the original design's stress analysis, and the resulting deltae in forces from the proposed alterations...
E. I'd like to see your measured test results. Every bit we can learn moves us forward.
 
Twist

Many years ago I had a conversation with a RV6 owner about this. I'm pretty sure it was Tracy Saylor. He was based in Santa Paula at the time. IF I have the name correct this person told me that twisting the blades on the Hartzell had made a significant improvement in top speed. I won't mention speed but I only recall two other RV's that were in that speed range. Both were Rv4's and of course one was Dave Anders.
Tracy's airplane was sold and was destroyed in a ground event when the airplane was tied down or maybe just parked.
This was probably way before the availability of the new generation Hartzell props.
Having said all this, in the RV speed range for SARL racing a Catto prop turning 3000 r/m would have a slight advantage over a constant speed. Airplanes that are considerably faster than the typical RV, the advantage goes to the constant speed.
My recollection is that in the early days of the RV4 some testing was done that confirms this.
 
I can't comment on this particular prop. I do have experience with several props on my Jodel, and the difference can be pronounced.

Last September I wrote this post in the RV-4 section, where I give my take on the theory of why a prop can be optimised for a certain speed. Guess it is applicable to this discussion:

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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
 
Twist

Hans- An excellent explanation. One important additional point is that the Hartzell Constant Speed does not have a good airfoil shape near the hub. This may very well be why an outstanding fixed pitch design will outperform the Hartzell slightly in the speed range of 200-250 statute. However, as you explain, when you get into the Glasair III, Lancair etc speed range the fixed pitch is no longer practical because the takeoff and climb performance would not be acceptable.
The F1 and faster biplane racers don't care that much about takeoff performance and run very high R/M's. One of the best examples being Tom Aberle's course speeds of near 260 with the Paul Lipp's designed, Catto built four blade fixed pitch. Straightaway speeds with that airplane are close to 300.
 
Props on Reno Sport class racers are frequently massaged in various ways for this different application. Some folks have seen useful speed gains with no other changes.
 
I'm curious about why the prop would need to be disassembled. Fixed pitch aluminum props are repitched with a pair of giant 'breaker bars' used like wrenches on each blade; wouldn't the same apply to a c/s prop?
 
I'm curious about why the prop would need to be disassembled. Fixed pitch aluminum props are repitched with a pair of giant 'breaker bars' used like wrenches on each blade; wouldn't the same apply to a c/s prop?

Because you would be applying all that bending pressure against the control mechanism of the prop, probably damaging it.
 
I get that, but in order to change the *twist*, you'd need a pair of 'wrenches' so the area closer to the root wouldn't be moved as the sections progressively along the blade are twisted. That should eliminate any stress in the pitch change mechanism in the hub.
 
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