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Propping to Avoid Engine Overspeed

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ClaudeR

Well-Known Member
Supporting Member
Joined
Jul 22, 2020
Messages
69
Location
KSMX
Hi, all.

I've been working on this problem for a while now and am getting very frustrated and hoping that someone (or plural) here can help me.

I have a Hornet airplane with a Hirth 2706 engine that I'm working on to get it ready to fly after sitting for a long while. The main problem that I'm having now is getting the engine and prop liking each other enough so that the engine doesn't exceed 5,800 RPM full throttle when on the ground. That's the Hornet manufacturers recommendation, although the engine manufacturer (Hirth) has a max RPM for this engine of 6,500 RPM.

My issue is that the engine will easily exceed 6,500 RPM on the ground. And, that is after I have made multiple pitch adjustments to the Warp Drive 2-bladed propeller. At this point the total prop pitch at the tip looks to be 18.5 degrees (27.5 less the hub 9 degrees); this using the Warp Drive propeller protractor. The total prop diameter is approximately 58 1/2 inches, and it's connected to the engine via a unique-to-Hornet chain drive (in oil bath) reduction system that I've measured as having approximately a 2.27:1 ratio.

Also, BTW, the EGT and CHTs are doing fine. If the engine was under propped I would have expected to see the CHTs high and the EGTs low, but that's not how it's looking to me.

As to the RPM gauge (ie: is it providing correct readings) I started with the original Westach gauge, but at the higher RPMs it was fluctuating a lot. I have a MGL Stratomaster Maxi Single E-1 engine monitor that I tried; better, but still a log of upper RPM fluctuations. I then purchased and installed an MGL Blaze EMS-2 engine monitor, and the RPM readings are now much more stable. I also connected a spark-plug wire connected tach (similar to a Tiny Tach I think) to compare and it and the EMS-2 are in agreement. I also used an optical tachometer for model airplanes (ie: have it looking through the prop to give a prop RPM and then adjust by the amount of the reduction ratio to get engine RPM) and it appears to match (the 2-blade setting on the optical tach appeared to be 1/2 of the actual prop RPM, but when multiplied by two it matched the other tachs.

So some questions.

Is it possible that this propeller is too small for this engine?

Is it possible that an over pitched propeller will allow an engine to turn faster than a properly pitched one?

How do you determine the correct propeller for an engine, especially a 2-stroke with a reduction drive?

What am I missing?

Thanks in advance for your help!

Claude
 
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