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

[Non Compos Mentis]

So does operating right at redline not hurt the engine then?

Now we're getting into the physics of engine design.

When the typical GA airplane uses a propeller that is most efficient in the 2500-2700 rpm range, it makes sense to design an engine that operates in that same rpm range.

On the other hand, a friend races a Porsche 911 that will not run well below 4000 rpm. On the track, it has a useful range of 5500-8000 rpm.

An engine is an air pump. The intake and exhaust need to be designed for the proper velocity to go with the designed rpm. If the exhaust is too big or too small, efficiency suffers.

Disclaimer to the mechanical engineers: I'm dumbing this down so even an imbicile like me can understand.

Imagine two engines, identical except one has a three inch stroke, the other a six inch stroke.

During one complete revolution of the crank, the piston speed is highest at the 90 degree and 270 degree position, while piston speed comes to a complete stop, and reverses direction ay 0 and 180 degrees.

The piston speed with 6" stroke will be twice the piston speed of the 3" stroke at the same rpm at the 90 and 270 positions. A shorter stroke will allow for higher rpms.

On the other hand, a 6" stroke has twice the leverage of the 3" stroke at the crank centerline. Hence the engine with the 6" stroke has twice the torque as the engine with the 3" stroke at the same RPM.

Torque is easy to figure. What about horsepower? Why not always use a huge stroke and get gobs of torque?

Simple! (sort of)

The more rpms, the more power pulses per minute! A four-cylinder engine, at 2000 rpm, produces 4000 power strokes per minute. crank the rpm up to 6000 rpm, and we now have 12,000 power pulses per minute, producing way more horsepower.

So the engine designer's compromise is always balancing the bore and stroke to produce maximum power, while maximizing efficiency for the job at hand.

Big over-the-road diesel trucks need lots of torque to get heavy loads moving, and top speed is not important. Slow turning engines with a long stroke get the job done. A light-weight Porsche racing car needs a screaming-high redline to maximize how many power pulses he can get over his competitors. small stroke is the right way top go.

Aircraft engines, while old tech, are really highly developed for their specific purpose- swinging a prop at 2700 rpm for hours on end.

 

[mtuomi]

There is no harm to run an engine constantly at "redline", assuming the engine is correctly designed, and the "redline" is correctly set. And of course, assuming there's ample lubrication.

Piston speed, loads on the rods etc really are a non-issue unless there is a fundamental flaw in the engine specification.

When talking about utility engines (such as an aircraft engine, a marine engine or any other engine that is designed to run high loads for a long time), the factors that we* care about when we specify the max allowable continuous rpm for that engine are the thermal loads, mainly the heat transfer from piston head to the skirt. This is the main thing.

You need to estimate/model/measure/calculate/guess the combustion speed. This is tricky on old big-bore carb'd airplane engine. The mixturation is very uneven between cylinders, the air charge changes with air density, and even in best cases we end up with a very poor combustible mixture in the combustion chamber (large volume, low rpm is actually not a very ideal place for combustion to happen).

Then you simply calculate the thermal loads (easy, you really only need the peak cylinder temperature and CA50 value), and make sure there is enough time for the heat transfer from piston head to the skirt to happen so you wont overheat the piston. When you increase your rpm, you increase the combustion events per given time, thus reducing the time available for this thermal "spread" to take place.

The rest, piston clearances, valve springs, rod deformation, inertial loads etc are very simple things to specify and generally are a non-issue.
An engine running at redline is generally in a "steady state", with hardly any wear even during very long runs. Fast rpm changes stress the engine more than constant redline operation.

*I don't work with airplane engines, but the general principles of what I work with do apply to them as well.

 

[Dan Thomas]

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?

Way off. The pistons in a typical Lycoming O-320 average just under 20 MPH at the redline of 2700 RPM. Even at its peak speed a piston is only doing 31 MPH.

The piston's movement, since it is related to the rotation of a crank, can be graphed as a sine wave. It's not a square wave, so it's not an abrupt start-stop function.

An O-320, like many aircraft engines, is certified to operate at redline for the whole 2000 hours. It has no five-minute restriction on full power like some larger engines, and those can operate at somewhat lower power settings for long, long periods. Cooling and fuel consumption issues are the bigger reasons we cruise at 65 or 75% power. An engine run at those power settings could easily go 3500 hours. Some do, typically pipeline patrollers and fish spotters, operating on-condition. And those pipeline guys get paid by the mile, so they're using lots of RPM to get the job done sooner.

Dan

 

[Dan Thomas]

Also what's the point of the one bladed prop?

That single blade was counterweighted and pivoted on the prop hub in such as way as to create an automatic pitch-changing propeller, governed by thrust forces. Ingenious and rather simple. And weird.

Dan

 

[GCA319]

Gearing vs. Direct Drive. Load Factors.

Which is why you'll see higher RPM figures on most geared Rotax engines.

 

[mtuomi]

Way off. The pistons in a typical Lycoming O-320 average just under 20 MPH at the redline of 2700 RPM. Even at its peak speed a piston is only doing 31 MPH.

The piston's movement, since it is related to the rotation of a crank, can be graphed as a sine wave. It's not a square wave, so it's not an abrupt start-stop function.

Correct. Just to give some perspective, the highest production car piston speeds are usually between 50-55mph.
Usually it is measured as average speed, the peak speed isn't that important..

 

[Tarheelpilot]

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...

at those speeds the airflow becomes compressed at the airfoil surface and the pressure wave causes the air to separate from the surface resulting in the loss of efficiency

 

[Skyscraper]

Only semi related fun fact:

When the Ford Motor Co. was developing their Taurus SHO (Super High Output), they tapped Yamaha for the engine design (which originated from a boat motor).

During testing, they ran the motor at 10,000 RPM for 24 hours straight .

 

[Bill]

Only semi related fun fact:

When the Ford Motor Co. was developing their Taurus SHO (Super High Output), they tapped Kawasaki for the engine design (which originated from a boat motor).

During testing, they ran the motor at 10,000 RPM for 24 hours straight .

All well and good, except the engine was from Yamaha.

 

[Skyscraper]

All well and good, except the engine was from Yamaha.

Yep, just edited.

 

[Witmo]

at those speeds the airflow becomes compressed at the airfoil surface and the pressure wave causes the air to separate from the surface resulting in the loss of efficiency

There's also something called wave drag that is a product of the formation of the shock wave.

 

[mtuomi]

All well and good, except the engine was from Yamaha.

Heads were from Yamaha, block was Ford/Cosworth, then all assembled by Yamaha.

 

[Capt. Geoffrey Thorpe]

All well and good, except the engine was from Yamaha.

And the engineers from Japan pronounced Yamaha differently if they were speaking Englinsh (YAM ah ha) than when they were speaking Japanese (yu MAH ha)

 

[Ralph Corsi]

here

 

[Ralph Corsi]

I really have nothing to add here.

IN fact, an aircraft engine could reach much higher RPM than 2,700 RPM or there abouts. Just take off the prop, push up the throttle and watch the RPM. However, the engine was designed to operate at around that speed so oil quantity, oil flow (which aids in cooling) and the fins on the piston were designed to operate normally at the 2,400-2,700 RPM speed. Had the engine been designed to be placed in a car, the ancillary systems would be designed to handle the temperatures and friction generated at those speeds. Also, the engine is designed to make its expected horsepower and torque at around 2,700 RPM let's say for a Lycoming O-360. Could it generate more at 4000 RPM? Probably, but the longevity and reliabillity of the engine is based on the 2,700 RPM limit. Any more RPM than that would degrade the reliability of the engine. Look at is as if the engine were stepped down to keep it running longer and better.

 

[ettsn]

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

We should chip in and buy one of these to base at KSMO...

 

[Dan Thomas]

Side note: did the tips on a Bear go super-sonic? Big honkin' props.

IIRC, it turned at 1300 for takeoff and 950 for cruise. Similar to the turboprops on a Dash-8, for instance.

 

[Dan Thomas]

IN fact, an aircraft engine could reach much higher RPM than 2,700 RPM or there abouts. Just take off the prop, push up the throttle and watch the RPM. However, the engine was designed to operate at around that speed so oil quantity, oil flow (which aids in cooling) and the fins on the piston were designed to operate normally at the 2,400-2,700 RPM speed. Had the engine been designed to be placed in a car, the ancillary systems would be designed to handle the temperatures and friction generated at those speeds. Also, the engine is designed to make its expected horsepower and torque at around 2,700 RPM let's say for a Lycoming O-360. Could it generate more at 4000 RPM? Probably, but the longevity and reliabillity of the engine is based on the 2,700 RPM limit. Any more RPM than that would degrade the reliability of the engine. Look at is as if the engine were stepped down to keep it running longer and better.

Cylinder fins are sized to handle the waste heat, not the RPM. RPM on a direct-drive engine is matched to the propeller's limitations. An O-200 turns at 2750, but the Formula racers put small props on them and run them at 4000 RPM.

 

[genna]

Adding to what others say. Without some sort of clutch/torque converter setup like in the car(and even with one) it would take too long of time to spin an engine up from idle speeds to full power(like on go around). This is not too much of an issue with car engines, but becomes a serious potential safety concern for aircraft engines. Large displacement, long stroke piston engines have near immediate maximum torque and power available with a push of a throttle and RPM

 

[MIFlyer]

what happens if the tips go supersonic?

is it like a "dont' cross the streams" moment?

 

[MAKG1]

is it like a "dont' cross the streams" moment?

Not inherently, but it could be if the prop starts shedding vortices or shock waves. Resonances have a way of becoming catastrophic.

It will make shock waves, which will be L O U D, and not improve thrust over a subsonic speed with the same prop.

 

[PaulS]

Squirrels can't run as fast at altitude.

 

[avongil]

Let me try to answer the question too...

Power = Work/Time

so its a function of time. There are two ways to make power. Big torque, little rev/min or small torque/high rev / min.

That's it end of my answer.



Some real world examples:
So say a small displacement high speed motorcycle 4 banger that does 20,000 RPM. Big power, little displacement. The high rpm is possible because of the low mass of the components.

In airplanes, you want that prop to spin at 2.5K so you really have two methods, gear it and put a lighter engine in or direct drive and have a honking 360 cubic inch monster 4 banger up front. They seem to both work, but simplicity has won in this market.

 

[Art VanDelay]

what happens if the tips go supersonic?

Same as if the beams cross in Ghostbusters !

 

[MBDiagMan]

Answering the original question, it is slow so the prop tip does not exceed the speed of sound.

 

[dmspilot]

The Rotax on an LSA I flew was geared. The engine redline was 5800 and the prop RPM was less than half that, I think it was about about 1/2.4.

The Cessna 175 Skylark had a geared engine but it was unpopular. According to Wikipedia, the main problem was pilots not RTFM and not operating it the way it was supposed to.

 

[tsts4]

I had an overspeed with my IO-540 back in August that resulted in an engine teardown, mag 500 hour inspections (with only 138 hrs on them), a new prop, and 2 new alternators. The amount of overspeed-- just 560 RPM for 11 seconds. The results, a $30K repair bill.

 

[PaulS]

I had an overspeed with my IO-540 back in August that resulted in an engine teardown, mag 500 hour inspections (with only 138 hrs on them), a new prop, and 2 new alternators. The amount of overspeed-- just 560 RPM for 11 seconds. The results, a $30K repair bill.

That is a bummer, what caused the overspeed? And was the engine damaged? I can't imagine 560 rpm damaging the alts, what was wrong with them?

 

[mtuomi]

I had an overspeed with my IO-540 back in August that resulted in an engine teardown, mag 500 hour inspections (with only 138 hrs on them), a new prop, and 2 new alternators. The amount of overspeed-- just 560 RPM for 11 seconds. The results, a $30K repair bill.

Out of curiosity, did insurance pay for it?

 

[tsts4]

That is a bummer, what caused the overspeed? And was the engine damaged? I can't imagine 560 rpm damaging the alts, what was wrong with them?

Prop gov self-destructed right as I applied power on a practice missed approach. Besides the prop gov drive being ruined I had a tappet come apart. My guess is the tappet was already on it's way out and the overspeed just accelerated the process but I'll never know. The alternators exceeded their design speed beyond the overhaul limits, especially the backup one that runs on the vacuum pump pad. They still work but as I have an all electric airplane, I wasn't taking any chances. Same for the prop. Hartzell (and MT) has a handy-dandy chart that you use to determine what to do with the prop based upon overspeed RPM and duration of the ovespeed. My event was easily into the "scrap" zone of the chart. Bottom line was everything that rotated got replaced or torn into.

Out of curiosity, did insurance pay for it?

Yes - all except the prop governor that caused the problem to begin with.

 

[Dan Thomas]

That is a bummer, what caused the overspeed? And was the engine damaged? I can't imagine 560 rpm damaging the alts, what was wrong with them?

The alternator is driven at a speed much faster than the engine.

A 560 RPM engine overspeed is a large overspeed; the manufacturers typically define an overspeed as 10% over redline for more than five seconds or something like that. If that 540 is a 230-hp model that redlines at 2400, 560 over is 23%, and 11 seconds is a long time.

The prop is the thing that suffers soonest. A 23% overspeed places 51% more cenrifugal force on it, and since that prop is one of the most highly loaded items on the airplane, that's asking for trouble. 51% more force on the alternator rotor isn't good, either.

 

[PaulS]

Prop gov self-destructed right as I applied power on a practice missed approach. Besides the prop gov drive being ruined I had a tappet come apart. My guess is the tappet was already on it's way out and the overspeed just accelerated the process but I'll never know. The alternators exceeded their design speed beyond the overhaul limits, especially the backup one that runs on the vacuum pump pad. They still work but as I have an all electric airplane, I wasn't taking any chances. Same for the prop. Hartzell (and MT) has a handy-dandy chart that you use to determine what to do with the prop based upon overspeed RPM and duration of the ovespeed. My event was easily into the "scrap" zone of the chart. Bottom line was everything that rotated got replaced or torn into.



Yes - all except the prop governor that caused the problem to begin with.

The alternator is driven at a speed much faster than the engine.

A 560 RPM engine overspeed is a large overspeed; the manufacturers typically define an overspeed as 10% over redline for more than five seconds or something like that. If that 540 is a 230-hp model that redlines at 2400, 560 over is 23%, and 11 seconds is a long time.

The prop is the thing that suffers soonest. A 23% overspeed places 51% more cenrifugal force on it, and since that prop is one of the most highly loaded items on the airplane, that's asking for trouble. 51% more force on the alternator rotor isn't good, either.

Wow, thanks for answering guys. Seems an electronic control system with a rev limiter would save a lot of aggravation in the case of a governor failure.

 

[Palmpilot]

NECRO-THREAD ALERT!

 

[Hank S]

When your car is running 5000-6000 RPM, are the tires turning that fast, too? Not!

When your airplane is running 2700 RPM, is your propellor turning that fast? Yes!

Effect of RPM on airplanes: example Mooney M20-R (data from a friend's modification).

• From the factory, 2500 RPM redline, 280 hp. Takeoff distance ~1200'.
• With STC, 2700 RPM redline, 310 hp. Takeoff distance ~800'.
• This STC involves replacing the Prop governor and repainting the tachometer, in addition to writing a hefty check and receiving a large pile of paper to put in the aircraft logbooks.

 

[NoHeat]

Side note: did the tips on a Bear go super-sonic? Big honkin' props.

Yes. Extremely noisy. Supposedly they can be heard by US submarine sonar, they are so noisy.

Here's a video.

 

[Checkout_my_Six]

music....or harmonics....is why.

 

[Dan Thomas]

Wow, thanks for answering guys. Seems an electronic control system with a rev limiter would save a lot of aggravation in the case of a governor failure.

It wouldn't help much. If you're at cruise and the prop goes to lowest pitch, it's going to overspeed big time even if the fuel or ignition are cut off.

 

[PaulS]

It wouldn't help much. If you're at cruise and the prop goes to lowest pitch, it's going to overspeed big time even if the fuel or ignition are cut off.

I suppose you are right, like downshifting a manual to first flying down a steep hill, bad stuff will happen.

 

[Zeldman]

Because Clyde Cessna wanted it that way.....

 

[paflyer]

my guess: at what RPM would the tip approach supersonic?

I hate pop quizzes..... that are almost three years old.