Can we have an open and honest discussion about RPMs?

Formula 1 racers like the Arnold AR-6 run specially built Continental O-200 engines with very expensive, stubby little fixed-pitch racing props that turn up to 4,400 RPM. They're cranking out more than 150hp at racing speeds of more the 250 mph. And, they are incredibly loud!
 
teejayevans said:
I thought the long pushrods limited rpms? Corvettes (1987) had a redline around 4000. I know my engine seems to run effortlessly at 2600, but 2700 not so much (seems louder and more vibration).
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My LS3 (6.2L pushrod v8) is limited at 7000. Any higher and I have to worry about valve float
 
rtk11 said:
BTW, ever driven a Mazda with a rotary engine? That Wankel engine design is the epitome of small displacement, light weight engines that need to spin to 9,000 rpm to make power.
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I used to race one. We would spin ‘em all the way to 12 grand sometimes. Beyond that the power fell off too much. I think the hp peak was something like 10500.

On the stock engines, the water pump will cavitate somewhere around 8000 and then the engine overheats, so we changed the water pump pulley to slow it down. Had to be careful about letting it run too long at low rpm in hot weather.

I’m surprised not to see Wankels in airplanes. The fuel economy isn’t great but the power to weight ratio is amazing. Plus they already have dual ignitions.
 
Half Fast said:
I used to race one. We would spin ‘em all the way to 12 grand sometimes. Beyond that the power fell off too much. I think the hp peak was something like 10500.

On the stock engines, the water pump will cavitate somewhere around 8000 and then the engine overheats, so we changed the water pump pulley to slow it down. Had to be careful about letting it run too long at low rpm in hot weather.

I’m surprised not to see Wankels in airplanes. The fuel economy isn’t great but the power to weight ratio is amazing. Plus they already have dual ignitions.
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I think reliability is the big issue. I'd take a 4 rotor in my 182 if it would work 99% of the time
 
ktup-flyer said:
I think reliability is the big issue. I'd take a 4 rotor in my 182 if it would work 99% of the time
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??

Wankels have much fewer parts than piston engines, so why wouldn’t reliability be better? Yes, you need gear reduction, but so does a Rotax.
 
ktup-flyer said:
How often do they go though apex seals
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About 150,000 miles in my street RX. Never lost a set in my race car. When they begin to go, oil consumption goes up and the motor smokes a lot, so you know it’s time. It’s not a catastrophic failure like when a piston engine eats a valve or throws a rod, items the Wankel doesn’t have.
 
ktup-flyer said:
How often do they go though apex seals
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Ditto - apex seals is the Wankel’s weak point. Well, this and the fact that it consumes oil to keep those apex seals alive. I would imagine that, instead of checking oil levels before each flight, you may need to plan to land every few hours to check oil levels.

I’m sure Half Fast can attest to this, but ever hear a Wankel at 10,500 RPM? The exhaust noise (literally) shatters glass. Pretty impressive unless you own the car with shattered windows! (Racing Beat is in the next city over, and I’ve visited their shops a few times.)
 
rtk11 said:
Ditto - apex seals is the Wankel’s weak point. Well, this and the fact that it consumes oil to keep those apex seals alive. I would imagine that, instead of checking oil levels before each flight, you may need to plan to land every few hours to check oil levels.

I’m sure Half Fast can attest to this, but ever hear a Wankel at 10,500 RPM? The exhaust noise (literally) shatters glass. Pretty impressive unless you own the car with shattered windows! (Racing Beat is in the next city over, and I’ve visited their shops a few times.)
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The oil has to be mixed into the fuel and that’s why they use so much. The street cars automatically do it in the fuel system.

With the race cars we pre-mixed oiled an fuel just like with a 2-stroke piston engine. Not a big deal.

And they do scream! But that’s why God created mufflers. I always me SCCA noise limits using a SuperTrapp.
 
Half Fast said:
??

Wankels have much fewer parts than piston engines, so why wouldn’t reliability be better? Yes, you need gear reduction, but so does a Rotax.
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more seals to wear....and have to make to get compression. Wankels in UAS had a horrible reliability.
 
Rotary in aircraft will not really happen.
Problem is volume, there just is not enough to cover the r&d costs plus certification.
For experimental there is not enough knowledge of the problems and issues with an engineer willing to put the time and money solving it.
So basically it is a money problem....

Sent from my SM-J737T using Tapatalk
 
If you like....run a Mazda engine in an RV....see how that works out. :lol:
 
SixPapaCharlie said:
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?[/QUOTE}
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tested limits
flux capacitors
 
Checkout_my_Six said:
If you like....run a Mazda engine in an RV....see how that works out. :lol:
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join-the-climb-on-a-record-breaking-flight

The rotaries are pretty neat. I ran an ITA RX-7 in SCCA racing for a while (12A engine). They are not tolerant of overheating events, and as previous poster said, they tend to die long, slow deaths rather than fail catastrophically. It's really difficult to seal that long, narrow combustion chamber with apex seals, side seals and corner seals, with oil seals on top of that. A worn rotary can be very hard to start as it's not generating much compression. Exhaust temps can be really high...1900 to 2000 degrees, as there's a "hot side" of the engine that never gets the cooling effect of an intake stroke, just continual power/exhaust cycles churning out the peripheral ports. Wall thickness of the exhaust header tubing on a rotary is impressive...the header weighs a ton!!
 
SoCal RV Flyer said:
join-the-climb-on-a-record-breaking-flight

The rotaries are pretty neat. I ran an ITA RX-7 in SCCA racing for a while (12A engine). They are not tolerant of overheating events, and as previous poster said, they tend to die long, slow deaths rather than fail catastrophically. It's really difficult to seal that long, narrow combustion chamber with apex seals, side seals and corner seals, with oil seals on top of that. A worn rotary can be very hard to start as it's not generating much compression. Exhaust temps can be really high...1900 to 2000 degrees, as there's a "hot side" of the engine that never gets the cooling effect of an intake stroke, just continual power/exhaust cycles churning out the peripheral ports. Wall thickness of the exhaust header tubing on a rotary is impressive...the header weighs a ton!!
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Same here. SCCA IT-A or IT-7 (marque-only class). Stainless header worked well. Twin tubes past the axle, then a collector and a SuperTrapp. Those twin tubes next to the floor pan made that car roasting hot, though.
 
I remember doing a race in a Showroom Stock Neon in among a bunch of RX-7s. They're so loud you can't hear your own engine!! Also, out at Riverside during a test of Jim Busby's Mazda-Lola T616 (I used to race rc cars with his son, Buzz). That beast was deafening when it blasted by the pits!! Fun times.

1006860.jpg
 
SixPapaCharlie said:
My prop doesn't know what is making it go 2700 RPM.
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That's not the point, it's not about the prop, it's about the engine.

Ever seen the 100hp Rotax, it's small. With a 73 cu. in. displacement, it is 1/3rd the size of the 100hp Continental O-200. It also happens to be liquid cooled. But it needs to spin at a much higher RPM to make that 100hp.

SixPapaCharlie said:
I think I don't understand power.
How does adding power to something that has a limit help?
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You're not adding power, you're making the same amount of power with a smaller engine.
 
Different engines produce peak power at different rpms for a variety of reasons that some have already mentioned. Gearing them allows you to provide the best rpm to the prop while still running the engine at its highest or most efficient rpm. .
 
and run that auto engine at 70-80% HP for very long....and it'll be trashed. Dat's why you don't see auto engines in aircraft.
 
Checkout_my_Six said:
and run that auto engine at 70-80% HP for very long....and it'll be trashed. Dat's why you don't see auto engines in aircraft.
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Myth. For example:

No aviation engine is tested that much or could survive.

The issue with engine conversions is really two fold. 1. They are more complex. 2. There is not a body of knowledge for ea-b builders to readily access.
The lack of point 2, makes the first point even more hazardous.

If you look, corsair and 1/2 VW engines are some of the most popular auto conversions. They are simple and they have built up the requisite knowledge base.

Sent from my SM-J737T using Tapatalk
 
Dave Theisen said:
I know that, but I didn’t want to make him feel inadequate. :)
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I explained to someone yesterday that the MU-2 is 45,000 RPM at the core and 2,000 RPM prop. That surprised this person.
 
The propeller is the primary RPM-limiting item on a direct-drive engine. The engine itself can often handle more revs, as some have mentioned here in regard to racing versions of the O-200. The propeller's operating range is limited by tip speeds and by centrifugal forces. Once the tip speeds get much beyond 600 MPH the drag goes up, much more noise is generated, more power is lost, and the forces trying to pull the prop apart get too large. Remember that those forces multiply by the square of the increase in speed, so a 10% overspeed puts 21% more pull on those blades. A 20% overspeed puts 44% more stress on them. That prop is already the most highly stressed part on the airplane, having to fight thrust bending loads, drag bending loads, center-of-pressure pitch-increasing loads and centrifugal pitch-decreasing loads, and the usual ordinary centrifugal loads.

Centrifugal loads can reach 40 tons or more on each blade. Remember that next time you preflight that prop and find a significant nick in it (nicks create stress risers that can cause cracking), and try to understand what happens if a foot or so of a blade breaks off: the vibration will likely tear the engine off the airplane. What happens, besides a loss of power, when hundreds of pounds leave the nose? Where does the CG end up? Will the airplane glide? Nope.

At 600 MPH tip speed the air over the cambered surface is much faster, same as the air over the top of an airfoil, so it is approaching the speed of sound. It generates shock waves we hear as the snarl of takeoff. Drag is the result, and some operators of engines like the IO-520, redlining at 2850, find they get as good or better takeoff performance if they pull the RPM back a little for takeoff.

A manufacturer like McCauley defines overspeeds in their maintenance and overhaul manuals. Typically, they call for prop removal and NDT if there's an overspeed of 10% for more than five seconds or something like that. They're worried about stress cracking. The engine manufacturer also has overspeed recommendations but they're a bit less stringent.
 
dmspilot said:
That's not the point, it's not about the prop, it's about the engine.

Ever seen the 100hp Rotax, it's small. With a 73 cu. in. displacement, it is 1/3rd the size of the 100hp Continental O-200. It also happens to be liquid cooled. But it needs to spin at a much higher RPM to make that 100hp.



You're not adding power, you're making the same amount of power with a smaller engine.
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Ultimately, it's all about the prop. The whole point of more HP is to be able to swing a bigger prop or one with more pitch to turn it into more thrust.

Bryan, our engines are capable of much more HP than we get out of them. Our direct drive powertrains are limited because you can only swing a prop so fast safely. If you increase the RPM of your engine, you get more power up to a certain point, but you would spin the prop too fast. To keep that from happening, you can add a gear reducer to get the benefit of the higher horsepower and spin the prop at the right RPM. One might ask, what's the point of the increased HP if you limit the prop RPM. What makes the increase in power worth anything is that a higher HP engine can spin a larger prop, which means more thrust.

As @dmspilot said, in the case of the Rotax, you can get a good amount of horsepower out of a tiny engine by running higher RPM. The Rotax makes as much HP as an O-200 (200 cubic inch) running half the RPM, and probably weighs half as much. In our case, with direct drive powertrains, we have to run large displacement engines to achieve the desired amount of power because engine RPM is limited by the prop. Your engine displacement is roughly the same as the V-8 in my crew cab pick-up, but is only rated at half the power because it rotates half as fast.

The 140-150 hp O-300 in the C-172 and the 165-170 hp GO-300 in the C-175 are the same engine. The increase in HP of the GO-300 comes from being able to run it at 3300 RPM and has a gear reducer to keep the prop spinning at 2400 RPM. Same displacement, more HP. What makes that increase in HP effective given equal propeller RPM, is the larger prop that the higher HP C-175 is able to spin.
 
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30 years ago, I had access to Jim Downing's shop in Atlanta. It was THE premier Mazda race shop in the day. Engine reliability was never an issue for them, and they were probably producing 350 hp in race trim on a two rotor engine, including the 24 hour events like Daytona. Those engines would have done very well, detuned to 200 hp for aviation. The big drawback with the rotaries is/was noise and exhaust heat. Jim generally ran inconel exhaust systems to combat the heat issue. Noise? Well...

I'm sure the 3 rotor engines would have been good for 250-300 hp if developed for aviation.

The issue for aviation is the gearbox, which Tracy Crook may (or may not have) solved when he was active. Unfortunately, Tracy has retired, and support for the rotaries is more limited than it was when he was engaged.
 
SixPapaCharlie said:
That can't be true...

My prop doesn't know what is making it go 2700 RPM.
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True. But the thing that makes your prop go 2700 RPM knows how much work it needs to do (i.e. power it needs to produce) in order to get the prop spinning at that speed. A 115hp Rotax (gearbox and radiators included) is smaller an lighter than a 115hp air cooled direct drive engine. There ain't no such thing as a free lunch. In order to get the same hp from a smaller and lighter motor, that motor generally has to spin faster. And because there are factors that limit how fast a prop should spin, you need a to gear that faster engine down on the output side to keep the prop in its most efficient range.

Get into your '72 'Vet and take it for a spin and you'll notice that heavy Chevy 8-banger turning in low 2000's. Then hop into a Porsche built the same year and go the same speed (wheels turning a similar RPM) and you'll see that smaller lighter motor is turning at a much higher RPM than the Chevy was. If the smaller light Porsche could produce the same power as the 'Vet at the same RPM, there would be no reason (other than 'Merican small penis insecurity I suppose) to build the Chevy as big as it is and as heavy as it is.
 
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quite a story there with the wankel, it was supposed to be the next engine, leaving piston engine tech in the dust,

bottom line, as simple as possible has reigned in general aviation, including rpms
 
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1) RPMS is simply wrong. Kind of like knots per hour.

2) The reduction ratio of a ROTAX 912 is actually 2.43 to 1.

3) Some wag on this forum insisted the brake wear on his LSA was so high because of the high idle on his plane versus a non-geared engine. Do the math and you’ll see the prop on a ROTAX is turning roughly the same speed at idle as a non-geared equivalent engine.
 
I know we can get 3 pages out of this thread. Keep em coming!
 
also, way more full dubs out there
 
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