Propping to Avoid Engine Overspeed

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ClaudeR

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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
 

wsimpso1

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I would talk to the maker of the prop and see if your prop can "get there from where you are" or if you need longer and/or wider and/or more prop blades to "get there".

Billski
 
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ClaudeR

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That's 65 horsepower, you need a bigger prop per powerfin pg. 16.
Thanks for the reply and document. I see what you mean. Looks like it would take a 65" diameter for a two blade, but I don't have the clearance for that (pusher, similar to a Challenger design), but the 3 blade and 61" diameter might work. I'll need to measure the clearance to be sure.

I have a three blade Ivo prop that I might be able to use if I cut the blades down a bit. I'll do some checking.
 

ClaudeR

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The static RPM will be lower than the RPM in flight. 5800 on the ground is going to turn into over 6K in flight.
Yes, that makes sense. That's one of the reasons that I am concerned about being able to turn over 6,500 RPM on the ground.

Per the Hornet Assembly Manual, "Hornets are propped for 5800 RPM on the ground, 6000 in level flight, and, 6200 in dive at VNE." I suspect that I did not receive the factory recommended propeller with mine.
 

One Sky Dog

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Horsepower is absorbed by propeller diameter and disk solidarity. You need more blades if you cannot swing a larger diameter. Adding blades increases lift and drag. Drag governs your prop speed lift increases prop thrust.

Pitch sets where you are in the power band. It is a trade off with a range climb settings give 10% better climb rate or max speed settings yield 5% increase in top speed.
 

ClaudeR

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Horsepower is absorbed by propeller diameter and disk solidarity. You need more blades if you cannot swing a larger diameter. Adding blades increases lift and drag. Drag governs your prop speed lift increases prop thrust.

Pitch sets where you are in the power band. It is a trade off with a range climb settings give 10% better climb rate or max speed settings yield 5% increase in top speed.
Thank you. That makes sense.
 

ClaudeR

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Set the pitch higher. Should be able to get static 5800.
If overpowered, you might want to cruise at a lower rpm with more pitch.
Thanks for the thought. I've already increased the pitch multiple times and probably close to 10 degrees. Still able to overspeed the engine easily.
 

BBerson

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Thanks for the thought. I've already increased the pitch multiple times and probably close to 10 degrees. Still able to overspeed the engine easily.
Need more than 10°. Try 20° and 30°. The prop has low rpm of 2555 rpm at 5800 engine. (divide 5800 by 2.27)
So you need more pitch. A bigger prop may be more optimal, I can’t say. But more pitch will give that prop 5800 static.
 

ClaudeR

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Need more than 10°. Try 20° and 30°. The prop has low rpm of 2555 rpm at 5800 engine. (divide 5800 by 2.27)
So you need more pitch. A bigger prop may be more optimal, I can’t say. But more pitch will give that prop 5800 static.
Sorry. What I meant was that I've probably already increased it 10 degrees more than what it originally was. At this time I'm measuring the pitch at the tip of 18.5 degrees. I've been trying to increase the pitch by about 2 degrees each time and now the increases are not changing the max RPM that I can see (still too high), and it's also sounding like the prop may be stalling due to excess pitch.

I did find the Ivo 3-blade prop that I have and it's parts, so hopefully tomorrow I can take it out to the airport and see if it might possibly work. Pretty sure I'll need to trim some length off of the blades to allow it to fit without cutting off the aft portion of the fuselage (pusher design).

Thanks for your help.
 

wsimpso1

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If continued pitch increase does not change over revs vs mp, you have to small a prop. More radius and/or more blade area (more blade chord or more blades) is probably neede.

Billski
 

ClaudeR

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This might be a stupid question, but what is wrong with putting a forward stop on the throttle?
In my opinion, the only stupid question is either the one that's not asked or the one that is asked over and over and over again. :)

The problem that I see with a throttle stop is, amongst a few other things, that you are artificially limiting the power output of the engine. In my case, I have a 65 HP engine pushing an 1,100 pound (at gross weight) airplane. So, theoretically, a power-to-weight ratio of about 16.9 pounds per HP. If we limit the throttle so we can only get, say, 55 HP from the engine, we are now looking at a power-to-weight ratio of about 20 pounds per HP. I'm no where's close to an engineer, but that looks to me like about a 15% performance loss. Probably varies depending on what we are talking about (ie: top speed, cruise speed, climb rate, etc.).

Another issue that I see, that may be limited to two stroke engines (which this is), is that the Bing carburetor on this engine (a pair of Bing 54's), have three ranges of mixture ratios (ie: how rich/lean): idle, mid, and high/full. My understanding is that the mid range is the leanest of the three. So, if we limit the throttle so we can not get out of the mid range in the carb, I would think there's at least a possibility that we would be running too lean of a mixture when we are trying to get the most out of the plane and the engine such as climb.

I'm a rookie on this, so please don't take this as gospel, but it's just based on my understanding.

Anyway, thanks for your idea. :)
 

TFF

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If you limit the throttle, you limit the horsepower. Full horsepower is when you get maximum manifold pressure at maximum rpm. With too small a prop, if you limit the rpm because it’s easy to spin the prop, it might only take 40 hp to spin it. You might have a 65 hp engine, but you can’t use it. Same with too much prop. If it’s so big, the engine struggles to only 40 hp, you are still at a loss of performance.

All fixed pitch combinations play with this. A climb prop is easy to spin but overspeeds when trying to fly fast. A cruise prop is just slightly too much for max rpm so it’s always trying to get the plane a little faster until it can’t. You pitch a prop to the flavor you want the most and put up with the rest.
 
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