Why 2700 RPM?

schmookeeg

schmookeeg

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Mike Brannigan
Hello,

Musing and don't really have any references or ideas on this.

How did we arrive at 2700 RPM as the cool max-rpm for most non-geared aircraft engines?

I assume it's some efficiency blend between prop blade length and displacement?

Like does a shorter prop at 3000 RPM pay dividends? Or longer one at 2400?

Sorta curious how the math got worked out for this magic number. :D

Slow Monday, I know.

I searched and found nothing with 2700 in the title, but if I missed an earlier thread, feel free to link-n-spank for my poor efforts. :eek:
 
The longer the prop, the better the efficiency (in basic terms), but the higher ground clearance the aircraft would need.

I would also assume the lower the rpm, the better efficiency and reliability. Unlike car engines that may rev to 6-7k rpms but don't normally operate on the high end, airplane engines run 75% power and rpm continuously for long periods of time.
 
I don't remember where I read this, so I may be completely off. I thought that the RPM limit was partly influenced by how fast the tips of the propeller moved at higher RPMs.
 
SkyChaser said:
I don't remember where I read this, so I may be completely off. I thought that the RPM limit was partly influenced by how fast the tips of the propeller moved at higher RPMs.
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Definitely want to keep the tips subsonic, but that gets partially resolved by having a larger diameter prop.
 
SkyChaser said:
I don't remember where I read this, so I may be completely off. I thought that the RPM limit was partly influenced by how fast the tips of the propeller moved at higher RPMs.
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True…supersonic airflow over the tips decreases prop efficiency. But that math could happen at various combinations of rpm/prop length.
 
Taking our 182 for example, we have an 84" 2-blade prop, with a max speed of 2700 rpm. This is about Mach 0.88 at sea level, which is close to peak efficiency, but also very loud. Much above this speed and you're losing efficiency and turning more fuel to noise.
 
Jeff Oslick said:
Taking our 182 for example, we have an 84" 2-blade prop, with a max speed of 2700 rpm. This is about Mach 0.88 at sea level, which is close to peak efficiency, but also very loud. Much above this speed and you're losing efficiency and turning more fuel to noise.
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Keep in mind that’s .88M tip speed , but the airflow speed increases over the prop just like over a wing.
 
MauleSkinner said:
Wouldn’t that be a smaller diameter that resolves it?
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Well, I was operating under the assumption that the engine wouldn't be able to turn a larger prop faster than 2700 to keep the tips subsonic. Going to a smaller prop allows the RPM to increase, which increases tip speeds. As you mentioned, there would be various combos of prop size and rpm that would keep the tip speeds in check.
 
I'm guessing the max engine speed for most direct drive airplane engines has more to do with the propellers that are expected to be attached to it than the engine itself. Some piston helicopters have similar engines that run continuously at higher engine speeds than where the redline is set for one in an airplane.
 
SoonerAviator said:
Well, I was operating under the assumption that the engine wouldn't be able to turn a larger prop faster than 2700 to keep the tips subsonic. Going to a smaller prop allows the RPM to increase, which increases tip speeds. As you mentioned, there would be various combos of prop size and rpm that would keep the tip speeds in check.
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Usually a longer prop on the same airplane will have a lower pitch so it can still make rpm.
 
schmookeeg said:
How did we arrive at 2700 RPM as the cool max-rpm for most non-geared aircraft engines?
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In general, it is due to prop tip speed and efficiency. Most driven airfoils have the same limitations like helicopter main and tail rotor blades. For example, the majority of tail rotor blades operate at 2200 rpm.
schmookeeg said:
I assume it's some efficiency blend between prop blade length and displacement?
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Matching the prop to engine output is the secondary reason. However, in most setups to match max engine output to max prop speed requires a reduction gearbox to keep the tips in clean air vs super sonic. The masters of this magic are the unlimited Reno racers.
schmookeeg said:
Like does a shorter prop at 3000 RPM pay dividends? Or longer one at 2400?
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It depends. The reason for the GO engines was they ran best at 3000 rpm but no prop would and give a marketable aircraft. There are several books out there on the Reno racers that give a better description of this matching process.
 
Jeff Oslick said:
Taking our 182 for example, we have an 84" 2-blade prop, with a max speed of 2700 rpm. This is about Mach 0.88 at sea level, which is close to peak efficiency, but also very loud. Much above this speed and you're losing efficiency and turning more fuel to noise.
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Right, I have a 78" diameter 3-blade prop at 2700. So one of us is giving something up in the efficiency game? :)
 
Putting prop size aside for a sec, what’s the max rpm one of these engines can withstand without self destruction before TBO?

Another way to understand my question is, if you put a smaller prop on it and zinged it up to 3000, smaller prop yet 4000, etc., how soon would you have to duck launched parts...y’know, coming from together?
 
455 Bravo Uniform said:
Putting prop size aside for a sec, what’s the max rpm one of these engines can withstand without self destruction before TBO?

Another way to understand my question is, if you put a smaller prop on it and zinged it up to 3000, smaller prop yet 4000, etc., how soon would you have to duck launched parts...y’know, coming from together?
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I forget what rpm Steve Wittman said he ran on his racers, but he also said he flew with his hand on the mag switch in case things came apart. I want to say it wasn’t over 4000.
 
Seneca III props turn at 2800 for takeoff. Max 5 minutes there, then you have to pull it back to 2600.
 
idahoflier said:
It appears to me the engine manufacturer determines what the red line is and it doesn't appear to be related to the propeller, at least for Lycomings...

https://www.lycoming.com/sites/default/files/SB369S Engine Inspection after Overspeed.pdf
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The engine is designed for propeller speeds. There is little sense designing it for higher RPMs, as it would add cost and weight.

The prop and engine are closely related. They're not independent of each other at all. Larger props must turn slower, meaning an engine has to produce its hp at a lower RPM. That is difficult, since hp is a function of torque times RPM. You end up with a larger, heavier engine and more vibration. All direct-drive engines are a compromise.

Prop RPM and tip speed affect thrust enormously. At 2700 your 182 will take off and climb. Reduce that to 1350, half the RPM, and you might have only a quarter of the hp or less, enough for a fast taxi.
 
455 Bravo Uniform said:
Putting prop size aside for a sec, what’s the max rpm one of these engines can withstand without self destruction before TBO?

Another way to understand my question is, if you put a smaller prop on it and zinged it up to 3000, smaller prop yet 4000, etc., how soon would you have to duck launched parts...y’know, coming from together?
Click to expand...

It seems like you're asking two separate questions. The first being what the max engine speed is that the engine could operate at and still make it to TBO. The geared engines and helicopter engines may provide some insight for that question. The second question seems to be "what is maximum engine speed for shorter time periods?" For that, I'd point to the guys at Reno. A friend of mine raced in the formula one class for a while and they would run the o-200 at 4,000+ for each race. The CHT and oil temp would be at redline during each race but the engines seemed to remain fairly reliable.
 
mondtster said:
A friend of mine raced in the formula one class for a while and they would run the o-200 at 4,000+ for each race. The CHT and oil temp would be at redline during each race but the engines seemed to remain fairly reliable.
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Up to 4400 according to one racer I talked to. More hp that way. Shorter props. The engines don't get anywhere near TBO. They often have a steel cable around the engine, anchored to the firewall, in case the prop's centrifugal forces bust off a blade. The imbalance would instantly tear the engine off its mounts.
 
Dan Thomas said:
Up to 4400 according to one racer I talked to. More hp that way. Shorter props. The engines don't get anywhere near TBO. They often have a steel cable around the engine, anchored to the firewall, in case the prop's centrifugal forces bust off a blade. The imbalance would instantly tear the engine off its mounts.
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I think my friend was running 4200ish. They required wood props after a metal blade or two were shed. The safety cables were also a requirement.

I don't know how long he'd run the engine before they'd refresh them but I know it was longer than most people would expect.
 
The wasp jr engines I’ve flown fit this conversation nicely. The direct drive swung a smaller two blade. When they wanted to get more low speed thrust a larger diameter and wider chord three blade prop would have done nicely but the engine turned to fast for the big prop. A planetary reduction drive fixed that problem. The engine ran the same rpm to make the same power but big ass prop was turning slower. It was surprising how much better the geared motor with the big prop performed on the bottom side of the speed range.
 
RyanB said:
What happens if you exceed 5 minutes?
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I don't know. Maybe the ghost of old man Piper will come and smack me in the back of the head.

I never get close to the 5-minute limit. After the gear is up and I'm safely climbing away, I pull it back to 2600.
 
Most radials and v motors have required inspections when the rpm/time limit is exceeded. Same thing for the military versions where they are set up with “war emergency” limitations. Break the witness wire on the control and it required the inspection cycle prior to next flight.
 
schmookeeg said:
Hello,

Musing and don't really have any references or ideas on this.

How did we arrive at 2700 RPM as the cool max-rpm for most non-geared aircraft engines?

I assume it's some efficiency blend between prop blade length and displacement?

Like does a shorter prop at 3000 RPM pay dividends? Or longer one at 2400?

Sorta curious how the math got worked out for this magic number. :D

Slow Monday, I know.

I searched and found nothing with 2700 in the title, but if I missed an earlier thread, feel free to link-n-spank for my poor efforts. :eek:
Click to expand...

One night Orville and Wilbur was drinking a beer and discussing prop rpm. After the 6th beer they settle on 2700.
 
I wasn't around when those decisions were being made, but from talking to the old timers who were around at that time (or at least shortly thereafter), it seems like more than anything it was a systems issue, and basically came about to the best compromise all around. You want the prop to be as big of a diameter as possible, but if it's too big it hits the ground, and small planes roughly are the size they're going to be. So that gives you a rough range for what the prop diameter is going to be, and then that dictates where your max prop RPMs can be without running into mach issues with the tips.

From there, gearboxes are considered undesirable as they increase weight and reduce reliability (in general - yes the IGSO series were pretty reliable, but they were heavy). These engines also start to run into some harmonic issues in the 2900 RPM range. Lycoming and Continental handled this differently. The Lycoming TIO/TIGO-541 had an extra set of balance weights on the crank throws as I recall. Of course the gearbox in the TIGO-541 was known for issues, and the Duke/BE56 had small diameter props to allow for that 2900 RPM engine (although there was also a ground clearance issue there from the landing gear) and so the overall setup wasn't as efficient as it could/should have been. If you look at the front of those planes, the props have a pretty large area for which they are blowing air to the engine cowling rather than the open air behind the plane.

The Continental GTSIO-520 had a harmonic damper, more like what you find in an automotive application. These were known to have their issues as well.

Incidentally, those harmonic issues are why I understand that there are "5 minute" ratings in that 2800-2850 RPM range more than anything an were also probably part of the determining factor for 2700 being the normal limit. Something about horizontally opposed 6-cylinders in that range, at least at that size. I'm not sure if 4 cylinders have those issues, but usually it seemed to make more sense to get into a 6-cylinder by the time you were trying to get too much power out of a 4. Cloud Nine's 310 had IO-520-E engines that had a 5 minute rating at 2850 RPM, and then 2700 RPM continuous.

Note that some engines had different RPM ratings in that range. 2400, 2500, 2575, and I think some are 2600. In those cases as far as I can tell, the lower RPM was generally chosen to derate an engine (such as the 540s in 182s or Aztecs), but I think in some cases it also had to do with wanting bigger props for efficiency in an aircraft and some form of noise reduction.

So, like most things in engineering, there's more than one reason why a value is chosen.
 
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Six-cylinder engines, and above, don't have the problem with harmonic resonance that 4 cylinder and fewer engines do. With fours, because of valve overlap, the engine doesn't produce power throughout the rotation, so the propeller has to supply power to the engine to keep it rotating. Everything has an elastic constant, and thus acts like a spring, so you get harmonic vibrations.

In contrast, six-cylinder engines produce power the entire rotation, so the propeller never supplies energy to the engine. Thus, the propeller doesn't act like a spring, and you don't get harmonic vibrations.

That's why the old 6-cylinder Cessnas always seem to run smoother than the 4-cylinder Cessnas.
 
Crashnburn said:
Six-cylinder engines, and above, don't have the problem with harmonic resonance that 4 cylinder and fewer engines do. With fours, because of valve overlap, the engine doesn't produce power throughout the rotation, so the propeller has to supply power to the engine to keep it rotating. Everything has an elastic constant, and thus acts like a spring, so you get harmonic vibrations.

In contrast, six-cylinder engines produce power the entire rotation, so the propeller never supplies energy to the engine. Thus, the propeller doesn't act like a spring, and you don't get harmonic vibrations.
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This is not at all accurate.

So much so, that I just don't even know where to begin. So I won't.
 
Crashnburn said:
Six-cylinder engines, and above, don't have the problem with harmonic resonance that 4 cylinder and fewer engines do. With fours, because of valve overlap, the engine doesn't produce power throughout the rotation, so the propeller has to supply power to the engine to keep it rotating. Everything has an elastic constant, and thus acts like a spring, so you get harmonic vibrations.

In contrast, six-cylinder engines produce power the entire rotation, so the propeller never supplies energy to the engine. Thus, the propeller doesn't act like a spring, and you don't get harmonic vibrations.

That's why the old 6-cylinder Cessnas always seem to run smoother than the 4-cylinder Cessnas.
Click to expand...
 
Crashnburn said:
Six-cylinder engines, and above, don't have the problem with harmonic resonance that 4 cylinder and fewer engines do. With fours, because of valve overlap, the engine doesn't produce power throughout the rotation, so the propeller has to supply power to the engine to keep it rotating. Everything has an elastic constant, and thus acts like a spring, so you get harmonic vibrations.

In contrast, six-cylinder engines produce power the entire rotation, so the propeller never supplies energy to the engine. Thus, the propeller doesn't act like a spring, and you don't get harmonic vibrations.

That's why the old 6-cylinder Cessnas always seem to run smoother than the 4-cylinder Cessnas.
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Ted said:
This is not at all accurate.

So much so, that I just don't even know where to begin. So I won't.
Click to expand...

Tarheelpilot said:
Would you do it for me?
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Author's entire argument was coupled to the idea that a six cylinder aircraft engine continuously produces power for every degree of crankshaft rotation. This is an internal combustion engine, not a ****ing Tesla.

A four-cylinder aircraft engine will fire with every 180 degrees of crankshaft rotation. A six-cylinder aircraft engine will fire with every 120 degrees of rotation. Neither engine fires continuously, and both engines store kinetic energy from one power event so that they can make it to the next power event.

If what the author was saying was true, a six cylinder engine wouldn't even need a starter. You'd just flip the engine "on" and it'd magically start generating power. Real world..we need a starter...as the starter is what does all the work that the kinetic energy does once running.
 
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Let me explain it simply. Theoretically, a cylinder produces power for 90 degrees of rotation, every other rotation. But the exhaust valves open before bottom dead center (BDC). Say the exhaust valve opens 10 degrees BDC.

Thus, a four-cylinder engine cannot produce power the entire 360 degrees of rotation, as 4 * 80 is 320 degrees. Energy to keep the engine turning has to come from somewhere. That somewhere is the propeller.

With a 6-cylinder engine, the power stroke of the next cylinder to fire starts before the exhaust valve of the currently firing cylinder opens. That means the engine is continuously supplying power to the propeller, so it never has to supply power to the engine to keep the engine turning.
 
Someone has read too many terms without understanding, well, any of them.

 
Crashnburn said:
Let me explain it simply. Theoretically, a cylinder produces power for 90 degrees of rotation, every other rotation. But the exhaust valves open before bottom dead center (BDC). Say the exhaust valve opens 10 degrees BDC.
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Sigh. did you read anything I wrote? Each individual cylinder only produces power ONCE every 720 degrees of crankshaft rotation. If you have six cylinders, that means you are getting power every 120 degrees, and if you have four cylinders, every 180 degrees.

As to the mumbling about exhaust valves, I don't see how that is relevant to what you're saying at all. The exhaust valve opens just before BDC because the cam lobe doesn't instantly open the valve (it is a ramp, and takes time to open). If you didn't start opening the valve until BDC, you'd waste energy compressing your exhaust.

Crashnburn said:
With a 6-cylinder engine, the power stroke of the next cylinder to fire starts before the exhaust valve of the currently firing cylinder opens. That means the engine is continuously supplying power to the propeller, so it never has to supply power to the engine to keep the engine turning.
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The engine is not continuously supplying power. There are pulses of power every 120 degrees.

Both four-cylinder and six-cylinder aircraft engines are horizontally opposed engines and have pistons moving in opposing directions on opposite sides of the engine. Both give us great primary and secondary balance.

The biggest reason a six cylinder aircraft engine feels smoother is because there are more cylinders firing in a given moment of time vs its four cylinder cousin at the same RPM. Less pulsey, but make no mistake, the power still comes in pulses and is not continuous.

Signed,

A software engineer that probably made mistakes. Wait for Ted for the real thoughts.
 
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