Propeller speed and efficiency

[Glenn_K]

Forgive me if this has been hashed out before, but a search of the forum did not easily reveal the info I'm looking for.

I'd like to know what is the correct way to determine what PSRU gear ratio is needed. Based on web research, to get the most efficiency and least noise you want the prop tips moving between .88 and .92 mach. So, to obtain that with a 68" prop, you would need to have a 2:1 ratio at 7000 engine rpm (using the calculator at https://www.pponk.com/HTML%20PAGES/propcalc.html ). Since the lowest ratio Rotax makes in a C Box is 2.68:1, it would be very difficult to get anywhere near that speed.
However, on the Powerfin site, they say that a Kolb gets it's highest efficiency running their F blades x4 on a 503 with a 3.47 ratio. If they're using a 68" prop at 5500 engine rpm, that would put tip speed at 320 mph.
Can you guys please help me understand this so I can figure out what ratio would be best to use for my engine with a 68" Powerfin with F blades x3?

 

[scottessex]

Wecome to the propeller voodoo land....
I would spin a 54" prop with a 2-1 but not a 68". That might be fine on a powered parachute, but your prop pitch would be so shallow that your top speed would be low. Spinning a large prop slower makes more thrust. Ask around here what people are using and on what engines.
I have a 60" 3 blade on a 530 arctic cat, 7000rpm max 2.58 rotax gearbox. very good performance.
2713 prop rpm at max power. Cruise is 6000 engine rpm. 2325 prop rpm.

 

[Glenn_K]

Ok Scott. That makes sense. I'm glad there are those like you who are smarter than me out there that can help with stuff like this.
I have 2.68:1 C Box and a brand new 3:1 C box that I have for my new engine. I've been thinking that I'd get better performance from the 2.68:1. But it seems the 3:1 might be better. And it's brand new!

I'd love to hear what other's of you smart people have to say about this.

 

[mark treidel]

Hi Glenn,
Unless I missed it, I'm left guessing what engine you are using to push that prop.
That's a crucial (X) factor you have to figure in the equation (HP). Your altitude also affects the pitch setting as you know but as a rule of thumb, you have to set the pitch (statically) at about 300 RPM's below red line so that in flight, you will hit the red line and not over rev the engine.
Due to you higher altitude, I (my opinion) would opt for the higher 2.68 gears in the C box. This will afford better in flight performance and economy. First, tell us what engine and we will re-address the issue.

 

[Aussie_Paul]

Prop efficiency graph...

Prop efficiency graph...

Here we go...

Aussie Paul.

 

[GyroRon]

Glenn, as I recall, Todd and I decided not to use a 3-1 ratio on the yamahas, because it was a 3 cylinder engine. That may not be a factor at all, but we wanted to use one of the odd numbered ratios. I think I built mine with the 2.68 to 1 at first ( could have been the 3 to 1 ) and then later I swapped out the gears for 3.47 to 1 and really didn't notice much change in performance but did seem to have less noise and it was " smoother "

 

[mark treidel]

Glenn,
Ron was correct. There seems to be a resonance that occurs w/the 3-1 ratio at near cruise rpm's.
I had that same issue running them on a 582 blue head on my old trike w/the 68' W/D.
Went to the 2.68 and had no more problems. If you have enough HP, the higher gears are preferable; if not, a lower range will help you achieve the higher prop rpms but at the cost of higher engine rpm's (premature engine wear) and fuel efficiency. Remember, everything you read is based on Sea Level calculations and you must consider your operational altitude based on your geographic location.
It is indeed, a balancing act.

 

[Glenn_K]

Alright, I guess I have to let the cat out of the bag.
After going through 5 Rotax engines, I've decided to convert the Super Monarch to the Yamaha 3 cylinder engine.
Thanks again, everyone, for the help.
So, based on the chart that Paul posted, the 3:1 would be optimal for my prop at sea level. But it doesn't take into account my altitude (~4500). If I'm understanding this, I would need more prop rpm to get back to the middle of the efficiency range at this altitude. So, the 2.68:1 would be better.
And based on what Mark is saying, the Yamaha has enough power available to do that. So I would be able to run a higher pitch for better performance and lower the noise level at the same time, right?
So Ron, did you run that low of a ratio because you're near sea level? You had a bigger prop on the Dominator, too, didn't you?

 

[PW_Plack]

Why would altitude be a factor for a normally aspirated engine? In my experience with several types of piston aircraft, the decline in power with increasing altitude appeared to be offset very closely by the decreasing load on the prop of the thinner air. Full-throttle RPM didn't seem to vary with altitude.

The speed of sound can affect the mach number at a given RPM, and decreases with decreasing temperature, but not dramatically. In the conditions typical of gyro flight, we probably stay within about a 5% range. If you're anywhere near the center of the "efficiency zone" identified on the chart, you'd remain in the zone at all temps where we normally would fly.

The 3:1 ratio would seem close to perfect for an engine making max HP at 8000 RPM, corresponding to a prop RPM of 2667, near the center of the efficiency zone with a 68" prop. The 2.68 gears would hold the engine at 6700 RPM for a prop RPM of 2500, which would limit horsepower, but A Genesis 120 will still make about 100 HP at 7000 RPM, which is a bunch for a single-place machine.

 

[mark treidel]

Something else you failed to mention is the brand of prop you are using.
Each has its own unique abilities and faults.
I would speak directly to the prop mfgr. for advice on matching it to your engine choice for the best advice. They are the experts...I speak only from my own experiences.

 

[scottessex]

I think that Brooke Beetler is running a 3.47 on his yamaha 3 cyl. That thing climbs like a wildcat!

 

[Aussie_Paul]

If I remember correctly, from research I did a few years ago, gearboxes of any kind should not have exact ratios like 2 to 1, 3 to 1 etc.

Aussie Paul.

 

[Glenn_K]

If I remember correctly, from research I did a few years ago, gearboxes of any kind should not have exact ratios like 2 to 1, 3 to 1 etc.

Aussie Paul.

For what reason?

 

[Jason O]

I think that if you are in an aircraft that you will fly at 70 knots or slower most of the time, you are better off with a slower larger diameter propeller. At these speeds, running out of pitch is less of a concern.

 

[Aussie_Paul]

For what reason?

Glenn I think it was the fact that the same teeth on the pinion and gear always met and caused a problem of vibration and/or harmonics.

Aussie Paul.

 

[bryancobb]

Ouch!

Ouch!

... After going through 5 Rotax engines, I've decided to convert the Super Monarch to the Yamaha 3 cylinder engine...

If that is true and not exaggerated, I would suggest adjusting the torque on the main nut on your cyclic stick!

Rotax has hundreds of thousands of 2-stroke units flying all over the world with probably well over a million flight hours of problem solving under their belt.

There MAY be a few dozen, or maybe even a few hundred Yamaha's in the air and they have not been flight tested by the manufacturer. It may very well be a great aviation engine, but you need to become somewhat of an EXPERT on the particular engine you fly. Even if you do wind up with a Yamaha, you may have a bad experience with it too if you don't "tank-up" on "THIS ENGINE know-how" before aviating. That would have probably made your Rotax experience much better.

I have almost 500 hours in the air with Rotax's supplying the "oomph." Not a single failure except for a disintegrating muffler. (knock-on-wood) 150 of that was in front of a non-aviation, single-ignition, points & fan 503 and an unreliable junk cogbelt re-drive (circa 1983, still no forced landing. I changed the $60 belt every 25 hours. My partner flew one to 28 hours on this airplane and ""ZZZzzzZzZzZZZpppp!"" Stripped cog teeth! Put 'er down! ). More than 100 of it was in the Mini-500 that most feel was the worst possible place for it.

The whole time, I followed Rotax's literature down to the tiniest detail. The fellas at the airport laughed at me when I disassembled my engine every 50 hours to de-carbon, not because it was logical or because I wanted to, but because the manufacturer SAID TO. Then they kept tick marks on the hangar wall showing every time they had to make a forced landing... :noidea: The same group of people laughed at me for changing jets often. They thought I was insane for running 1175 EGT's! They said, "just do like we do and shoot for 1000-1050 and all that jet-changing won't be required."... :noidea: I would curiously stroll-up as they were disassembling and diagnosing. The inside of every one I saw looked like the inside of a chimney and all coked-up inside.

My 50 hour teardowns gave me an opportunity twice per year to assess my jetting, see if my oil was the right kind or if a change was needed, inspect my pistons for scuffing or other things like out-of-balance because of different size oil solids stalactites under the crowns , look at how hot the wrist-pin needles had gotten, and see if the crank bearings were staying cool and getting enough oil. AND A BIGGIE - a chance to "caulk-up" any leaks in the crankcase separation joint that makes them unexpectedly lean out. WHY WOULD SOMEONE NOT WANT TO DO THAT? DID YOU DO IT? ROTAX SAYS TO. HOW CAN YOU CRITICIZE ROTAX'S IF YOU CHOOSE NOT TO ADHERE TO THEIR MAINTENANCE INSTRUCTIONS?

 

[NoWingsAttached]

There is a pretty straight-forward formula to figure out tip mach speeds. Once you hit mach, the shock wave will cause you lose all thrust, so you must throttle back and regain air on the blades.

Well before mach tip speed you will find you are getting into laminar air-flow seperation, where the air is lifting away from the blade surface and you are losing efficiency dramatically to the point that you are just wasting gas and engine life by getting the prop turning faster than it's efficient range. Here's an example of my testing.

Warp Drive, three-blade, 68", pitch-tuned to allow a 150hp engine to turn it up to 3300 PRPM an no more. I think it was around 5 or 6 degrees.

Mach tip speed was determined to be ~3550 PRPM on paper, with an air speed not to exceed 115mph (TAS is a critical factor in determining mach tip speeds).

At 2850 PRPM (most popular target we throw around in bull sessions) I saw something like 475 lbs thrust.

At 3000 PRPM the dynomometer indicated exactly 500 lbs. thrust. Now, please bear in mind this is a calibrated precision instrument, not some goofy pressure-gauge setup or a fish scale, nor a Rube Goldberg shade-tree deal.

I pushed it to 3300 PRPM and it indicated just a tiny increase in thrust - 520 lbs!

Conclusion: So...why bother? Anything past 3 grand on a 68" Warp is wasted fuel and engine life.

Since all props have been tested to show fairly equal efficiency ratings, despite very different designs etc., you can probably take this experiment to the bank.

How do you get more thrust out of a like-size prop? GEARING! Turning it slower and having the grunt from the engine to turn it with as much pitch as possible within the geared prop speed range where engine and prop are most efficient as a package. Finding THAT out will take more time than I have at the moment, but Daryl at Warp tells me it is anywhere from 2400 to just shy of 3k...which bears out my test results.

 

[Glenn_K]

With all due respect, Bryan, this thread is not about my engine decision. And your rant is exactly the reason I didn't want to mention it on this forum in the first place. I'd prefer to keep this thread discussion to my propeller/gear ratio questions.

 

[Glenn_K]

Glenn I think it was the fact that the same teeth on the pinion and gear always met and caused a problem of vibration and/or harmonics.

Aussie Paul.

If that is the case, and it's a known issue, then why would Rotax sell gearboxes with 3:1 and 4:1 ratios at all?

Thank you Paul Plack and Greg for your wisdom. So it seems that I should be fine with either ratio, since either would have me well inside the efficiency range. All things being similar, I think I'll start with the 3:1 box, since it's brand new. Ron A told me I should try both to see which I prefer, and I may well do that.
Thank you all. I think I have a better understanding of this now.
:hail:

 

[NoWingsAttached]

If that is true and not exaggerated, I would suggest adjusting the torque on the main nut on your cyclic stick!

Rotax has hundreds of thousands of 2-stroke units flying all over the world with probably well over a million flight hours of problem solving under their belt.

There MAY be a few dozen, or maybe even a few hundred Yamaha's in the air and they have not been flight tested by the manufacturer. It may very well be a great aviation engine, but you need to become somewhat of an EXPERT on the particular engine you fly. Even if you do wind up with a Yamaha, you may have a bad experience with it too if you don't "tank-up" on "THIS ENGINE know-how" before aviating. That would have probably made your Rotax experience much better.

I have almost 500 hours in the air with Rotax's supplying the "oomph." Not a single failure except for a disintegrating muffler. (knock-on-wood) 150 of that was in front of a non-aviation, single-ignition, points & fan 503 and an unreliable junk cogbelt re-drive (circa 1983, still no forced landing. I changed the $60 belt every 25 hours. My partner flew one to 28 hours on this airplane and ""ZZZzzzZzZzZZZpppp!"" Stripped cog teeth! Put 'er down! ). More than 100 of it was in the Mini-500 that most feel was the worst possible place for it.

The whole time, I followed Rotax's literature down to the tiniest detail. The fellas at the airport laughed at me when I disassembled my engine every 50 hours to de-carbon, not because it was logical or because I wanted to, but because the manufacturer SAID TO. Then they kept tick marks on the hangar wall showing every time they had to make a forced landing... :noidea: The same group of people laughed at me for changing jets often. They thought I was insane for running 1175 EGT's! They said, "just do like we do and shoot for 1000-1050 and all that jet-changing won't be required."... :noidea: I would curiously stroll-up as they were disassembling and diagnosing. The inside of every one I saw looked like the inside of a chimney and all coked-up inside.

My 50 hour teardowns gave me an opportunity twice per year to assess my jetting, see if my oil was the right kind or if a change was needed, inspect my pistons for scuffing or other things like out-of-balance because of different size oil solids stalactites under the crowns , look at how hot the wrist-pin needles had gotten, and see if the crank bearings were staying cool and getting enough oil. AND A BIGGIE - a chance to "caulk-up" any leaks in the crankcase separation joint that makes them unexpectedly lean out. WHY WOULD SOMEONE NOT WANT TO DO THAT? DID YOU DO IT? ROTAX SAYS TO. HOW CAN YOU CRITICIZE ROTAX'S IF YOU CHOOSE NOT TO ADHERE TO THEIR MAINTENANCE INSTRUCTIONS?

My thoughts: Why waste your time tearing down a 2-cycle and rebuilding it/cleaning the thing out every 25 to 50 hours (like a couple of months or three of flying for me during the best flying season) because you have to keep the rings from seizing?

If you have a running engine and are happy with it, keep it and enjoy flying it. But when it comes time for a change and/or upgrade go 4-stroke and fuhgeddaboutit. With the technological advances that have been made in 4-stroke experimental aviation engineering in the past 5 years there is little to no reason to waste your time and money on replacing 2-cycle engines with 2-cycle engines anymore.

When I see the prices of Smoketax engines and parts, I have to gag.

And when I compare fuel burn rates of them to Yamaha YG3 or 4, I have to laugh. A 65 HP 2-cycle burns just as much fuel as a 120 or 150 hp Yamaha. Go figure. Oh, and you dont' have to add all that super expensive oil to the gas every time, either. I swear I can get 3.5 GPH at 60 mph from a YG4, although it's more fun to tune the prop to a 4.0 GPH at 60 mph.

Here's the argument for a YG4: Initial cost is comparable to Rotax. Fuel burn is horrendous for 2xRotax. Parts are horrendous for any Rotax. Reliability is not even in the same class: 2 stroke vs 4 is a moot point in any discussion. HP:Weight is w-a-a-a-y favoring Yamaha, not even a horse race, there isn't an engine on the market today that can beat a YG4, let alone with the 12-year track record and long-term future of this engine.

I can't think of a single good reason to buy a new Rotax, with the exception that you just want the lightest gyro possible in order to meet part 103 rules.