Can we have an open and honest discussion about RPMs?

I used to work for a Mazda shop back in the early 80's. At the time there were still a decent number of RX-3s and RX-4s on the road, and some would occasionally come in with a can of starting fluid sitting in the front seat. As soon as you saw that, you could pretty much diagnose the problem as low compression, and the fix was a rebuilt engine from Mazda Technical Center, which was not something the customer liked hearing. What we would offer them is a compression test for a half hour's labor to confirm this. For a rotary engine, the compression tester was a recording device, way back then it wrote the results on a piece of paper, since we needed to see what the compresson was on each of the three lobes of the rotor. It never failed to confirm the initial diagnosis. One of the guys who was very experienced with rotaries told me it was usually one of the large O rings on the sides of the rotor that failed rather than the apex seals. There are some corner seals that were troublesome as well.

The engines in the RX-7 never gave us much trouble, in the seven years I was there we replaced one, and that was for high oil consumption. One thing that we did see that would disqualify the rotary for aviation or marine use was something we called "body shop syndrome". For those cars that were street driven and then in an accident, if they sat for a few weeks without being run, the engine would not turn over. If someone did something to try to force them to start, then one or more of the apex seals would break as it had stuck to the rotor housing, probably from carbon buildup. The way to get the rotor unstuck was to turn it in the reverse direction from its normal rotation. I could see this happening to a infrequently flown airplane or pleasure boat.
 
The Mazda rotary has flown in a number of homebuilts. One of the issues was the redrive, and another was cooling. Generating a lot of power in a small package means a lot of waste heat as well in that small package, and getting the heat out before it warped the engine's major castings was a problem.
 
SixPapaCharlie said:
So I am watching a Trent Palmer special and he casually throws out that redline on his Rotax is like 5400 RPMs...

Record Scratch... What????

Why is my engine limited to 2700?
I thought RPM limits were to keep prop tips from going super sonic. I read that on POA..

Also if it gets 5000 RPM how come it doesn't fly 400kts.

I am confused about physics and everything I have learned on POA.


So I guess the questions are:
why are the redline limits where they are?
why would doubling the RPMs not add tremendous speed to the plane?
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Airplanes are designed as a total package. Keeping things simple when designing airplanes keeps costs down and improves reliability. Adding gearing to an engine creates a bucketload of extra parts, increases complexity, cost, weight and does nothing for reliability. In most piston engine aircraft you can dispense with the gearbox and keep your goal of keeping things simple which results in many of the benefits that makes both the manufacturer and buyer happy. Some larger piston engines do incorporate gearboxes but the balance is tipped when efficiency vs. initial cost is considered. Turboprops and helicopters wouldn't even be around if it wasn't for gear reduction as they turn in the neiborhood 30,000 RPM, well out of the range of props and rotors. For them a gear box is a necessary evil (ask any helo pilot how he feels when he sees a "chip light".)

In the days when Lycoming, Continental and Franklin where designed (70 yrs. ago?) the theory was to keep things simple and reliable and gearing an engine was usually not chosen when all things were considered. These days, high spinning ROTAX engines with reduction drives are available but suited more toward the LSA market. To keep cost in line, the LSA builders universally opt for the ROTAX. Your still going to need something a little stronger to cart your Cherokee, Bonanza, Baron etc. around. The mainstay of the GA market will continue to be large cube, non-geared, slow turning engines that are matched with large, efficient props that get the job done with a minimum of fuss. The market is the one that ultimately decides what works and so far simplicity has been the answer.
 
Brad Smith said:
In the days when Lycoming, Continental and Franklin where designed (70 yrs. ago?) the theory was to keep things simple and reliable and gearing an engine was usually not chosen when all things were considered. These days, high spinning ROTAX engines with reduction drives are available but suited more toward the LSA market. To keep cost in line, the LSA builders universally opt for the ROTAX. Your still going to need something a little stronger to cart your Cherokee, Bonanza, Baron etc. around. The mainstay of the GA market will continue to be large cube, non-geared, slow turning engines that are matched with large, efficient props that get the job done with a minimum of fuss. The market is the one that ultimately decides what works and so far simplicity has been the answer.
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Yup. The Cessna 421 had geared engines. Ask any owner about issues with the operation, and about overhaul costs. A 421 with runout engines is worth very little.
 
Capt. Geoffrey Thorpe said:
Power = torque * rpm => Faster is, in general, more power that you can deliver to the prop.
To keep the prop speed under control with a fast turning engine you gear it down so prop torque = engine torque* gear ratio - More poop to where it counts.

The limits on an engine are usually a function of piston speeds (or sometimes valve open/close rates) wear in the cylinder bore is pretty much a function of how fast the piston is moving inside it. Typically, the sewing machine / 1930's auto engine derived aircraft engines (Lycoming and Continental respectively) turn at slow crankshaft speeds, but have a long piston stroke where a snowmobile derived engine has a very short piston stroke and runs at higher rpm. The net result is that if you calculate the actual speed at which the piston is moving in a straight line - both end up coming out about the same in terms of feet/minute so both "styles" will last about the same number of hours.

Direct drive is simpler (no gears) but require a metric buttload of cubic inches to get the job done. A geared engine can spin faster and make the same power from a smaller engine - but the weight savings of the smaller engine is partially offset by the weight of the gear reduction.
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This^^^^^^^

Think tuner car vs muscle car.

Sent from my Pixel 2 XL using Tapatalk
 
Dan Thomas said:
Yup. The Cessna 421 had geared engines. Ask any owner about issues with the operation, and about overhaul costs. A 421 with runout engines is worth very little.
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Any twin with runout engines is worth very little. Geared or otherwise.
 
This has been a highly informative thread.
I learned quite alot.

Thank you POA.
 
Can we get back to bacon now?

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The Merlins in modified P-51’s like Strega, Voodoo, and Dago Red (RIP...sniffle) are good examples. Prop RPM is still limited by tip speed but the engines are making 3400 HP or more, compared to mid-teens for a stock Merlin. 120-plus inches MP vs 70. More pitch, etc make use of the additional power to generate more forward motion.
 
About these engine things...

The engines in our airplanes, and most of our cars are four stroke spark ignition engines. They pull in an air and fuel mixture on the first (intake) stroke, then compress that mixture on the second (compression) stroke. Just before the engine reaches the top of the compression stroke, the ignition system provides a spark, and the mixture starts to burn, which causes it to expand, for what we call the power stroke. This is what we are after here, this is what provides the useful work that the engine is going to do for us. Finally, the combustion gases exit the cylinder during the final (exhaust) stroke. There's an animated diagram here, if you're not familiar with how an engine works. Back when I was in middle school they taught this sort of stuff in science class, I don't think they still do. Anyway, that's the basics.

Probably the best way to think about what an internal combustion is doing is to think of it as a pump. If you want to fly faster or haul a bigger load, you need a bigger pump. The more air you can get into the engine, the more fuel you can burn, which provides more power, which will drive a bigger propeller, which produces more thrust. For those of us who want to fly, thrust is what's going to get us off the ground. I'm going to start with the Continental O-200, which has four cylinders, arranged in a horizontally opposed order, and has a total displacement of 200 cubic inches. When we say displacement, that is the amount of air that the engine would move in one complete cycle of two crankshaft revolutions, assuming it is at full throttle and the process is 100 percent efficient. (Remember, a four stroke engine needs two revolutions to complete its entire cycle.) It has a cylinder bore of 4.06 inches and a crankshaft stroke of 3.88 inches. At 2750 RPM, it produces 100 hp, and has a dry weight of 224 lbs. There is a lighter weight version that is closer to 200 lbs but I don't know of any uses for it.

The Rotax 912 S and ULS are also have four cylinders in an horizontally opposed arrangement, but only has 82.6 cubic inches of displacement from its 3.3 inch bore and 2.4 inch stroke. it's also considerably lighter at 140 lbs. Now you'd look at these two and think that the 0-200 would produce way more power than would the 912, and it would if they both ran at the same engine speed. The Rotax turns 5800 rpm. Remember the pump analogy? We can do a quick calculation to see how much air each engine would consume at maximum power. Since it takes two revolutions for a four stroke engine to fill all of its cylinders one time, the formula is RPM/2 * displacement. So,for the O-200: (2750/2) * 200 = 275,000 cubic inches, and for the 912: (5800/2) * 82.6 = 239,400 cubic inches. This is the theoretical amount of air that each engine could consume in one minute at full throttle and maximum power RPM.

The Rotax is still about 12 percent behind the Continental, according to our theoretical air flow calculation. It turns out that with careful design, it is possible to get more air into an engine than the theoretical number would suggest. For an engine without a supercharger or turbocharger, it is possible to get as much as 130% of that theoretical value into the engine. For more on that topic there's a brief introduction to the concepts in Wikipedia's article on volumetric efficiency. The Rotax, being a newer design, is apparently a more efficient engine. It's also much more compact and about 90 pounds lighter than is the 0-200, and has a couple of complications. Propellers don't do well at 5800 rpm, so the Rotax has a propeller speed reduction unit, a gearbox that reduces the propeller speed so something more reasonable. Also, since so many more combustion events happen per minute in a Rotax, the cylinder heads are liquid cooled, which is an added complication. (The cylinders are air cooled.) So now you've added a water pump and coolant. Even after adding these things, the Rotax installation is still somewhere around 80 pounds lighter than is a standard O-200. That's why you see the Rotax in so many LSA's, and why the Skycatcher has such a low useful load.

Back to the pump analogy for a minute. I suggested earlier that if you wanted more power, you needed a bigger pump, which would mean more displacement (bigger engine). We just saw that running the pump at a faster speed works, too. I'm going to add a third option, which is to force more air into the engine. You can add an air pump to your engine. During World War II, designers added superchargers, which are engine driven air pumps, to push more air into their engines. The supercharger did consume some power, but that was more than offset by the added power from the additional available air. Nowadays there are lots of cars, and a few airplanes, that use turbochargers to boost their maximum power. There are a number of Lycoming TIO-540A engines that use turbochargers to boost the manifold pressure to 40 or 42 inches for takeoff. There are also airplanes that use a turbocharger to offset the loss of power that comes with altitude, which is not what we're talking about here.

Power comes from burning fuel, which requires air. You can make the engine bigger, turn it faster, or add forced induction to get more power. Each has its pluses and minuses. I got to fly in front of a Rotax recently, and I was very impressed. It's also nice that it can run on 91 octane E10 mogas. I have a reasonable number of hours flying behind an 0-235 and an O-320, and I preferred the Rotax. At cruise the propeller is turning fairly slowly and it's quiet. YMMV, but if you get a chance to fly in something Rotax powered, by all means take it.
 
SixPapaCharlie said:
So I am watching a Trent Palmer special and he casually throws out that redline on his Rotax is like 5400 RPMs...
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Gotta be like a Harley owner, never talk about HP, always talk displacement. So if your talking to a tuner with a 600 HP 350 chevy be sure and tell him you've got a 320 cubic inch engine in your airplane and your Dad's is 550 cubes. Ignore the fact that the engines in our airplanes turn so damn slow they don't make big power. They'll just assume the 550 makes around 900 HP and not 300.
 
FormerHangie said:
Back when I was in middle school they taught this sort of stuff in science class, I don't think they still do.
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They don't, and it sure shows. Just try finding people under 40 or so that know anything about what goes on under the hood.

Or that know that if we all buy electric cars we're going to need a LOT more powerplants. Big powerplants. WInd and solar will never do it.
 
bnt83 said:
Gotta be like a Harley owner, never talk about HP, always talk displacement. So if your talking to a tuner with a 600 HP 350 chevy be sure and tell him you've got a 320 cubic inch engine in your airplane and your Dad's is 550 cubes. Ignore the fact that the engines in our airplanes turn so damn slow they don't make big power. They'll just assume the 550 makes around 900 HP and not 300.
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Tell 'em your engine will happily run at 75 percent power for 2000 hours! At, just for example, 150 KTAS, which works out to nearly 350,000 statute miles, assuming roughly equal head- and tailwinds.

Horses for courses...
 
Dan Thomas said:
They don't, and it sure shows. Just try finding people under 40 or so that know anything about what goes on under the hood.

Or that know that if we all buy electric cars we're going to need a LOT more powerplants. Big powerplants. WInd and solar will never do it.
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This goes for all hands-on trades. I have talked to several folks in a trade who have said that there are very few younger folks interested in taking over when their generation retires. This isn’t helped by the fact that we’ve become a throwaway society where things aren’t designed to last or be repaired.
 
cowman said:
Now one figure I do have for every car engine I've ever seen that I've NEVER seen given for aircraft engines is torque. With that displacement and RPM curve I would assume they should have a great deal of low end torque but I've never seen an actual figure.
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I had always wondered about the same thing, so awhile back looked it up, and there is a simple calculation to figure out torque.

hp x 5,252 / RPM = torque

So, Lycoming 360 makes 180 hp at 2,700 rpm

180 x 5,252 / 2,700 = 350 ft lbs of torque.

or my favorite: IO520

300 x 5,252 / 2,700 = 583 ft lbs! That's a lot of twist :)
 
Cricket1 said:
or my favorite: IO520

300 x 5,252 / 2,700 = 583 ft lbs! That's a lot of twist :)
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Yup, and all of it goes into that propeller. Further, it's not smooth when the prop gets it. Every time a cylinder fires there's a big push, momentarily and considerably more than 583 foot-pounds.
 
SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?

Meaning if readline is 2700 but I have an engine that can spin 5400 so I gear it down so the prop still limited to 2700.
Seems like you're putting a governer on a hot rod.
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Why does a F1 engine turn 20,000 RPM... Because it wouldn't make 10 horsepower at 2700.
My 447 2 stroke Rotax turns 6800 because it needs rpm to make power.
 
teejayevans said:
But they only have to last a couple of hours. I don’t want to have to overhaul my engine every week.
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My single cylinder YZ250F had a redline of 13,500 which I hit regularly. I have no idea how many hours I put on that thing but I ran it almost every weekend for 3-4 years with no maintenance except for regular oil/clutch changes and it never showed any signs of internal engine wear.
 
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