Why is RPM Redline on a plane so much lower than a Car engine?

SixPapaCharlie said:
I really have nothing to add here.
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My guess: propellers don't really work well past 2700 rpm anyway (tips go supersonic), so there is no need to rev past these numbers. Note that faster turning aircraft engines have reduction boxes to bring prop speed back down to these numbers.

Car engines are not limited by this.
 
Among the other factors, care engines are (almost) never expected to operate at near red-line for extended periods. That is why race car drivers carry a couple of spare engines around with them.
 
SixPapaCharlie said:
I really have nothing to add here.
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For the same reason helicopter main rotors turn ~350 RPM. Efficiency and aerodynamics change substantially when traveling at speeds near and over the speed of sound.

Propeller and rotorcraft hubs are under tremendous stresses from centripetal force, the faster they spin, the more stress imposed.
 
Not really. Race car engines are one-offs and don't last anywhere near as long as production engines. Car engines can run at redline indefinitely, as long as the cooling system can keep up (and they are usually oversized, so that's not a problem). It's noisy and the friction losses are significant (technically, that's what defines the red line), so it's not an efficient place to run a car.

You could do this in an airplane, but you'd need a gear reducer to operate the propeller in a subsonic regime. It's been done, but that's more moving parts and more things to break.
 
MAKG1 said:
It's noisy and the friction losses are significant (technically, that's what defines the red line), so it's not an efficient place to run a car.
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Well, that and valve train issues, mean piston speeds, compressive/tensive forces on con rods...
 
Bill Jennings said:
My guess: propellers don't really work well past 2700 rpm anyway (tips go supersonic), so there is no need to rev past these numbers. Note that faster turning aircraft engines have reduction boxes to bring prop speed back down to these numbers.

Car engines are not limited by this.
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WAIT...I am confused. :yes: So the prop speed goes supersonic. What about bigger engines? Doesn't the prop turn faster? If it doesn't than what makes the plane go faster. I figured they went over 2700 rpms???


thanks
 
Redline on my aircraft engine is 5,800RPM. They are not always a lot lower than automotive. Lack of gear reduction, has the prop's RPM range determining engine speed.

It is the maximum RPM for my engine according to the manufacturer ( with a safety margin )

Propeller pitch, redline and available power determine tip speed.
 
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signu127 said:
WAIT...I am confused. :yes: So the prop speed goes supersonic. What about bigger engines? Doesn't the prop turn faster? If it doesn't than what makes the plane go faster. I figured they went over 2700 rpms???


thanks
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Nope. Direct drive engines connected to ~75 inch props all have very similar redlines.

You can make the plane go faster with a coarser pitch and higher displacement or turbo/super charging to compensate. Power is roughly (displacement)*(manifold pressure)*(RPM). So, if you want to make a more powerful engine, you can wind it up, make it bigger, and/or force air/fuel into it, assuming full throttle.
 
signu127 said:
WAIT...I am confused. :yes: So the prop speed goes supersonic. What about bigger engines? Doesn't the prop turn faster? If it doesn't than what makes the plane go faster. I figured they went over 2700 rpms???


thanks
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Nope. Those big props on turboprop airliners are turning much slower than you'd imagine -- they're just taking a massive bite out of the air with each revolution. The 180hp engine in a Cessna 172 would struggle to turn that kind of blade at all.
 
Side note: did the tips on a Bear go super-sonic? Big honkin' props.
 
SixPapaCharlie said:
what happens if the tips go supersonic?
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Lots of bad things. Makes a ton of noise, loss of boundary air flow, loss of thrust at the tip, torsional vibration across the blade changes dramatically.

Basically it's really bad.

If you listen to a Waco bipe, or a Cessna 180, or a Bonanza with an 88" prop on takeoff sometimes you'll hear that 'bbbbrrrrraaaaaaaaaaaaaapppppp' noise as the tips reach the transonic zone. Beyond that lurks - problems. :D
 
Witmo said:
Very noisy, increase in drag, loss of effieciency...to name a few.
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I must research the physics of this. Just because you are going faster than sound, I would assume not much would change. Sound doesn't have weight, drag, density, etc...
 
SixPapaCharlie said:
I must research the physics of this. Just because you are going faster than sound, I would assume not much would change. Sound doesn't have weight, drag, density, etc...
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mother-nature does not like shocks...unless she creates them for her own purposes
 
Correct, it's the prop. I used to fly behind a geared engine The engine turned 3400 at redline (GO-435) since the prop was only going 2200.
 
SixPapaCharlie said:
I must research the physics of this. Just because you are going faster than sound, I would assume not much would change. Sound doesn't have weight, drag, density, etc...
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The air it moves does have density, weight, drag, and so on.

A shock is a discontinuous change in pressure and temperature. When it passes, it can do A LOT of damage. Explosions are, by definition, shocks. Without the shock, you have a burn and no detonation.

FWIW, it's not so much supersonic as transsonic. That's M~0.8 for a straight wing, not M=1. The camber and angle of attack speed the flow up at a point so that it's supersonic at that point, generally pretty close to the chord line on the cambered side. It will usually form turbulent shocks, which will shake the surface, called "Mach buffet." This is not to be trifled with; it can destroy an airplane.

On a 76 inch prop, I get M=0.8 pretty close to 2700 RPM at standard conditions. It's slightly slower in colder conditions.

FYI, you can delay Mach buffet to a higher speed by sweeping the airfoil; this is why modern airliners all have swept wings. Some piston aircraft have the prop tips swept, presumably for this reason.
 
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Someone find a pic of that little Aeronca that used the one-bladed prop. That thing was a crack-up! I think it turned about 120RPM and looked like it was gonna fly apart. No worries on tip speed there. :yesnod:
 
SixPapaCharlie said:
I must research the physics of this. Just because you are going faster than sound, I would assume not much would change. Sound doesn't have weight, drag, density, etc...
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Correct. Subsonic flight and tip speed is straightforward. Supersonic flight is actually even simpler. The transonic transition, however, can easily shake props and planes apart due to shock, flutter, and a few other nasty surprises when air is being compressed that much.

In props, supersonic tips means that some part of the prop will stay transonic & that is what messes things up.
 
docmirror said:
Someone find a pic of that little Aeronca that used the one-bladed prop. That thing was a crack-up! I think it turned about 120RPM and looked like it was gonna fly apart. No worries on tip speed there. :yesnod:
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You mean this? Piper offered it as optional equipment for a time on the J-2 Cub. I think it turned at pretty much a normal rpm for that airplane.

P1040748.jpg


http://www.supercub.org/forum/showthread.php?45392-Everel-single-blade-prop

http://www.ultraligero.net/Cursos/varios/helice_de_una_pala.pdf
 
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Pilawt said:
You mean this? Piper offered it as optional equipment for a time on the J-2 Cub. I think it turned at pretty much a normal rpm for that airplane.
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Yeah, I know, it was tongue in cheek. I think it was also offered on the little Aeronca, but it's been a while.

sigh,,,, so cute.;)
 
My aircraft engine has a redline of 5800 RPM - doesn't seem that slow to me compared to most cars.

But it doesn't have the long stroke that some other aircraft engines have. And it does have a gear reduction.
 
The Republic XF-84H was an experimental turboprop that had a supersonic propeller. The noise it made was awesome and it was probably the noisiest airplane ever built.
See this: http://en.wikipedia.org/wiki/Republic_XF-84H

It is rumored that no test pilot would fly it a second time. Even inside it was way too loud. It could be heard 25 miles away.

Dan
 
Dan Thomas said:
The Republic XF-84H was an experimental turboprop that had a supersonic propeller. The noise it made was awesome and it was probably the noisiest airplane ever built.
See this: http://en.wikipedia.org/wiki/Republic_XF-84H

It is rumored that no test pilot would fly it a second time. Even inside it was way too loud. It could be heard 25 miles away.

Dan
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Bet the neighbors loved that! :no:
 
warthog1984 said:
Correct. Subsonic flight and tip speed is straightforward. Supersonic flight is actually even simpler. The transonic transition, however, can easily shake props and planes apart due to shock, flutter, and a few other nasty surprises when air is being compressed that much.

In props, supersonic tips means that some part of the prop will stay transonic & that is what messes things up.
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Good explanation..:thumbsup:..


To add some further baseline...

My motor turns 4400 RPM's on take off.. It has a 1.43 redrive reduction on it so the 76" diameter prop is turning 3076... On my video you can hear my motor alot more then the prop tips, which were at .92 mach. That is in the transonic region... not supersonic.. If you go to this site, it lets you input various prop lengths and motor rpms and it will display mach numbers for the prop tip speed... http://www.pponk.com/HTML PAGES/propcalc.html



My plane on a low pass...

https://www.youtube.com/watch?v=
 
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That problem with the tips going super sonic is that there is always part of the prop that is moving at EXACTLY Mach 1. So the prop is basically making a continuous sonic boom which kills efficiency, creates vibrations, and all sorts of other nasty stuff. If you want more thrust, you first need a stronger motor to swing a more aggressive air foil at the same speed.


Sent from my iPhone using Tapatalk
 
Tmpendergrass said:
That problem with the tips going super sonic is that there is always part of the prop that is moving at EXACTLY Mach 1. So the prop is basically making a continuous sonic boom which kills efficiency, creates vibrations, and all sorts of other nasty stuff. If you want more thrust, you first need a stronger motor to swing a more aggressive air foil at the same speed.


Sent from my iPhone using Tapatalk
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Once again....

Use the pponk calculator I posted above for deriving at the real tip speed.. 99% of the props are not really supersonic, they are still in the transonic range... Look at my application for instance.. my prop is 76" and I spin it at 3076 rpm.... and the tips are only .92 mach.. It is a LONG way to 1.00 mach to truly be in the supersonic range..
 
Capt. Geoffrey Thorpe said:
My aircraft engine has a redline of 5800 RPM - doesn't seem that slow to me compared to most cars.

But it doesn't have the long stroke that some other aircraft engines have. And it does have a gear reduction.
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Yep, my old Gopher engine had a engine redline of 3400, but the prop was only turning 2200.
 
So does operating right at redline not hurt the engine then?
Also what's the point of the one bladed prop?
 
Propeller tip speed is one important consideration, the other is engine design. The desire to keep propeller RPM low without a gear reduction drive has lead to your typical Lycoming and Continental's design where the cylinders have a lot larger displacement and longer stroke than the typical car engine. A 4 cylinder O-200 has nearly the same total displacement as some smaller automotive V-8 engines!

So for a given RPM the peak velocity of the piston is higher as it has to travel a longer distance per revolution, add to that a large piston, you end up creating higher inertial loads on the connecting rods and crank at lower RPMs than you do in cars.

Large cylinder displacements with long strokes tend to produce more torque at lower RPMs. This concept is the same reason why semi rig diesels tend to have low red lines as well.

A Rotax 912/914 is closer to a car engine design (short stroke, smaller pistons, high RPM), this why it has a high red line and requires a reduction drive to keep prop RPM low.
 
signu127 said:
WAIT...I am confused. :yes: So the prop speed goes supersonic. What about bigger engines? Doesn't the prop turn faster? If it doesn't than what makes the plane go faster. I figured they went over 2700 rpms???


thanks
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More pitch with the higher torque/power levels available to turn it. This is achieved through the greater volume of the engine being able to accept more fuel/air mix at a given RPM; then you get to geared engines where there is a speed reduction between engine RPM and prop RPM.
 
ebykowsky said:
So does operating right at redline not hurt the engine then?
Also what's the point of the one bladed prop?
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No one runs their engines at red line all the time and expects to make TBO.

Blades add drag. The one bladed prop is the most efficient prop.
 
ebykowsky said:
So does operating right at redline not hurt the engine then?
Also what's the point of the one bladed prop?
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Air cooled engines may have temperature and related detonation issues, and some of them have time limits at full throttle. Operate your engine in the green.

Your car can handle it. Though your mileage won't be good above the powerband, and it will be loud.
 
Geico266 said:
No one runs their engines at red line all the time and expects to make TBO.

Blades add drag. The one bladed prop is the most efficient prop.
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Yeah, but I can imagine the side loads on the bearing and output shaft would be tremendous at operating speeds. The prop may be balanced, but the loads imposed by the one side providing thrust would make it extremely hard on the engine.
 
ebykowsky said:
So does operating right at redline not hurt the engine then?
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It depends what you mean by "hurt the engine".

There are many forces and limitations on an operating engine- piston speed and clearance are 2 of the more important ones. Every time the engine turns over, the piston is going:

dead stop -> 100+ MPH forward -> dead stop -> 100+ MPH reverse -> dead stop -> repeat.

Near redline- not a magic number BTW- the pistons are expanding and moving faster than they do at low rpm. Things moving faster with less "wiggle room" are more likely to break, wear out, etc...

So in that sense, operating at high rpms makes an engine more likely to fail / wear out. But, it is not inherently hurting the engine; it simply stresses any weak spots that much harder than low rpm.

Will problems from near-redline operations show up in any given flight? Unlikely. Will sustained high-rpm operations show up during maintenance? Absolutely!

Clear as mud?
 
RBeaubien said:
Yeah, but I can imagine the side loads on the bearing and output shaft would be tremendous at operating speeds. The prop may be balanced, but the loads imposed by the one side providing thrust would make it extremely hard on the engine.
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It's not something that can't be overcome, the main problem though is getting the required surface area to deliver the power to the fluid, that's why one needs multiple blades.
 
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