Propping to Avoid Engine Overspeed

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ClaudeR

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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.
Makes sense. Thanks.
 

ClaudeR

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So I tried a three blade IVO prop that I had. Cut it down to 61" and tried max pitch. No joy; still too much RPM. Sounds like I need to buy a new propeller, probably 3 or 4 blade with wider blades (to soak up the power since I'm diameter limited).

Thanks for all the help from everyone whom replied.

Claude
 

BBerson

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The previous owner may have lived at high altitude. At high altitude you need a bigger engine or supercharger or only get 75% of the rated hp cruising at 7500. Can't use a throttle stop if you want to climb high. Some airplanes the pilot limits the manifold pressure at sea level. Get the full boost at altitude with full throttle without exceeding the engine limits.
 

rv7charlie

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My 1st stop would be at the prop mfgr(s), to find out whether the prop in question is in its proper environment, and pitched correctly, on that HP engine at that rpm and with that reduction ratio. If the prop(s) are being used with the wrong engine setup, there's the answer. If the prop(s) is/are correct for the engine HP/reduction ratio, then something else is likely the issue. Tach?
 

Dan Thomas

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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.).
You lose 15% of the horsepower, and it shows up real bad in the takeoff and climb. It ends up being much more than a 15% performance loss.

When I was an instructor we taught reduced-power takeoffs so the students could see how a small power loss could kill them. We'd go to full throttle, and then I'd pull the throttle back to get a 100-RPM reduction. That's on a Lycoming in a 172. The student was always amazed at how slowly the airplane accelerated and how much runway it ate up, and then how poor the climb was. Just 100 RPM less than the usual 2400 or so in the takeoff roll.

With a fixed-pitch prop, a throttle reduction not only costs RPM but also torque, and since RPM X torque is horsepower, the losses get pretty big.
 

Dan Thomas

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So I tried a three blade IVO prop that I had. Cut it down to 61" and tried max pitch. No joy; still too much RPM. Sounds like I need to buy a new propeller, probably 3 or 4 blade with wider blades (to soak up the power since I'm diameter limited).

Thanks for all the help from everyone whom replied.

Claude
Maybe you need a new tachometer.
 

pylon500

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You say you have limited diameter space, I know the Hornet (Challenger clone), but a thought occurs, which way around is the reduction setup?
If it is 'upwards' and you can't fit more than 61"ø, then you're gonna need more blades!
I would also suggest having a look at the Warp Drive Prop pitch calculator, as your reduction may be too much, and you're not getting the prop up to an effective speed before the engine over revs.
I have an aircraft with a Hirth F23 (50hp), which had a 3 blade Warp prop, pitched to the correct max ground static rpm, and I had terrible take off performance, and not particularly good cruise at high rpm.
This aircraft is fairly small and low, with trike gear and limited prop clearance.
The belt reduction was the supplied 2.85:1 that would typically turn a 68"~70"ø prop and the correct tip speeds on your typical ultralight.
I was limited to around 62"ø, and after a quick calculation realised I was running around Mach0.6 at the tips.
I machined up a new 'poly V' pulley to get down to almost 2:1, and trimmed the blades to 'HP' spec to get a decent rpm/pitch combination, then actually had to go down to a 2 blade prop to allow the rpm up where it should be.
I went from around 200fpm climb to almost 500fpm.
orig_pulley.png
small_pulley.png
blade_trim.png
blade_trim2.png
3_small_blades.png
2_small_blades.png
For those scratching their heads, it's a modified Corby Starlet.
 
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ClaudeR

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The previous owner may have lived at high altitude. At high altitude you need a bigger engine or supercharger or only get 75% of the rated hp cruising at 7500. Can't use a throttle stop if you want to climb high. Some airplanes the pilot limits the manifold pressure at sea level. Get the full boost at altitude with full throttle without exceeding the engine limits.
Previous owner was near San Diego, CA, so pretty close to sea level.

Good thought, though. :)
 

ClaudeR

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You lose 15% of the horsepower, and it shows up real bad in the takeoff and climb. It ends up being much more than a 15% performance loss.

When I was an instructor we taught reduced-power takeoffs so the students could see how a small power loss could kill them. We'd go to full throttle, and then I'd pull the throttle back to get a 100-RPM reduction. That's on a Lycoming in a 172. The student was always amazed at how slowly the airplane accelerated and how much runway it ate up, and then how poor the climb was. Just 100 RPM less than the usual 2400 or so in the takeoff roll.

With a fixed-pitch prop, a throttle reduction not only costs RPM but also torque, and since RPM X torque is horsepower, the losses get pretty big.
That's an interesting training idea. Once I get my plane flying I'll have to remember to try that.

Most of my training was at sea level, so that training could be eye opening and help to understand higher elevation take off performance. I like it. :)
 

ClaudeR

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Maybe you need a new tachometer.
I've tried four different ones * and think that the current one is correct.

Thanks.

* Original Westach needle, which jumped around at higher RPM; MGL Stratomaster Maxi Single E1, which also jumped around at higher RPM; MGL Blaze EMS-2 which is currently installed and stable at high RPMs; and a TinyTach clone (ie: works by wrapping a wire around a spark plug wire) which is stable at high RPMs and also is consistent with the MGL EMS-2.
 

BJC

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we taught reduced-power takeoffs so the students could see how a small power loss could kill them.

That's an interesting training idea.
Dan:

Never heard of or experienced that. Would be a real eye-opener back in the 7AC. It would be be instructive for any student, but especially one flying with a FP prop.


BJC
 

ClaudeR

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You say you have limited diameter space, I know the Hornet (Challenger clone), but a thought occurs, which way around is the reduction setup?
If it is 'upwards' and you can't fit more than 61"ø, then you're gonna need more blades!
I would also suggest having a look at the Warp Drive Prop pitch calculator, as your reduction may be too much, and you're not getting the prop up to an effective speed before the engine over revs.
I have an aircraft with a Hirth F23 (50hp), which had a 3 blade Warp prop, pitched to the correct max ground static rpm, and I had terrible take off performance, and not particularly good cruise at high rpm.
This aircraft is fairly small and low, with trike gear and limited prop clearance.
The belt reduction was the supplied 2.85:1 that would typically turn a 68"~70"ø prop and the correct tip speeds on your typical ultralight.
I was limited to around 62"ø, and after a quick calculation realised I was running around Mach0.6 at the tips.
I machined up a new 'poly V' pulley to get down to almost 2:1, and trimmed the blades to 'HP' spec to get a decent rpm/pitch combination, then actually had to go down to a 2 blade prop to allow the rpm up where it should be.
I went from around 200fpm climb to almost 500fpm.
View attachment 128701
View attachment 128702
View attachment 128703
View attachment 128704
View attachment 128705
View attachment 128706
For those scratching their heads, it's a modified Corby Starlet.
My engine is inverted and the prop reduction system sticks up above the engine.
Hornet Prop & Reduction Drive.jpg
 

ClaudeR

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FYI current status is that yesterday I talked with Bob at Competition Aircraft and ordered an Ultra-Prop 2. His calculations showed 3 blades and 60 " with 10 degree pitch blocks would be correct, although he's sending some additional pitch blocks incase I need to adjust.
 

ClaudeR

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Maybe you have the wrong gear ratio. A small diameter prop needs higher rpm.
It's the stock reduction system that came from the Hornet factory. The actual ratio is not marked or documented that I could find, but I measured it myself by removing a plug from each cylinder, using a plastic straw (so I wouldn't damage anything in the engine) and counting engine and propeller rotations. Once done I came up with a ratio of approximately 2.27/1 (engine to propeller).

Also, the RPM that I am aiming for (5,800 static full throttle on the ground) is what is called out in the operating and assembly manual for the plane. The engine red line is actually 6,500, but the Hornet factory wants 5,800 on the ground which (according to them) should give 6,000 full throttle level in flight and 6,200 full throttle and descending.

Claude
 

ClaudeR

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I had some success yesterday and I wanted to pass on what I did and what I learned.

I contacted Bob at Competition Aircraft and at his recommendation purchased and installed a 3 blade 60" Ultra-Prop II. He provided various pitch blocks for me to try, but his calculations were that 10 degree would be correct. I installed the prop and still had way too much prop speed (I stopped advancing the throttle when I saw 6,500 RPM (which is the redline for my 2706 engine according to the manufacture, Hirth) on the MGL Blaze EMS-2 tach, but more throttle was available). I tried 12 degree pitch blocks next, but still had the same problem and, again, stopped advancing the throttle . I contacted Bob, and he was surprised as his calculations where then showing that my engine was putting out over 80 HP (from a 65 HP engine). Something wasn't right, but he sent me 13 and 14 degree pitch blocks to try.

I installed the 14 degree blocks, and this time when I went from about 3/4 throttle to full the engine would sputter and slow down, like it was suddenly becoming way too rich and slobbering with fuel.

I was very frustrated.

One thought was that the tachometer might be wrong, but I had three different ones that were in agreement. One issue is that each had adjustments that could be made, but did I have the wrong settings?

For the MGL EMS-2 there is a setting for number of pulses per revolution in the RPM area. My engine is old and no longer in production (a mid-90's Hirth 2706), and I could not find a document anywhere that gave the answer. I posted some questions in the Hirth and MGL areas on Facebook and got some information that it could be 1 or 2 or 6 pulses/rev. On the Hirth area I also received some technical information explaining why it might be 2 or 6 depending on which wires from the engine were being used for the tach.

I also went back and re-checked the reduction drive and engine to verify the ratio. I came up with 2.28:1. This was by using a pastic straw in a spark plug hole to follow the engine piston travel and physically counting the number of engine and propeller revolutions as I rotated the engine by hand.

I ordered a laser tachometer from Amazon, and was able to use it on the prop (it came with reflective sticky tape, but I found that I had better luck just aiming it at the prop). Now that I was confident that I had actual propeller RPM, I was then able to determine the true engine RPM by dividing the propeller RPM reading by 3 (since I was using a three blade prop) and then multiplying this by 2.28 (the reduction ratio). I found that this matched the EMS-2 gauge when it was set to 2 pulses per revolution. I set the spark-plug-wire tach so that it gave me a matching RPM (ie: spark plug fires twice per revolution).

I set the prop back to the recommended 10 degrees and tried again. Now I was seeing a maximum RPM at full throttle (I was actually able to get to full throttle now) of 5,500 RPM. Much better, but still just a little low from my 5,800 goal. Also, the engine was not acting over rich and slobbering at full throttle. I then changed the pitch to 9 degrees and tried again. Now full throttle gave 5,680 RPM. Bob had mentioned that his Ultra-Prop IIs would have a slightly larger increase between static-on-the-ground and in-flight RPMs than some other props, so, at least for now, I think I'll leave the 9 degree pitch blocks installed.

One other thing is that I THINK that when the higher prop pitch (14 degree) was used and the engine would slobber and slow down at full throttle that what was actually happening was that the propeller blades were stalling and increasing the prop drag tremendously (thus slowing down the engine at full throttle). That's just a guess on my part, but it would fit what I observed in that with a lower pitch there was no problem with full throttle slobbering and slowing down.

So, thanks everyone for all your help! I think I am finally over this hump and able to move on after a battle of more than a month.
 

ClaudeR

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Even 4000 sounds low to me for a 532. I don't know the specs for the 532, but I would have thought it would be up closer to 6k (isn't that close to what a 582 would be?).

Good luck. :)
 
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