Why are some engines geared to reduce prop RPM?

Morne

Morne

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Morne
I know that some engines are geared to reduce prop RPM. My question is why? It would seem that with the correct propeller design you could utilize higher engine RPMs.
 
A prop going supersonic creates drag, and noise. An engine can run at a higher RPM which generates more power.

The Rotax 912 runs at 5,500 RPM with a 2.43 - 1 gear box.
 
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The airspeed at the tip of the prop needs to stay "subsonic". Turning the engine faster gets you more power, gearing it down makes it usable by the prop.

If there is some secret where a supersonic prop could work efficiently and effectively (and not ban you from every airport due to the racket) that would be awesome.
 
Geico266 said:
A prop going supersonic creates drag.
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Tip speed is proportional to the length of the prop. Simply reduce the prop length, adding additional blades as necessary. I think that a 4-blade smaller diameter prop running at say 5500rpm would still be lighter than gearing down to a large 2-blade and running at 2750rpm.

Gearboxes have a weight (and reliability?) penalty.
 
Morne said:
Tip speed is proportional to the length of the prop. Simply reduce the prop length, adding additional blades as necessary. I think that a 4-blade smaller diameter prop running at say 5500rpm would still be lighter than gearing down to a large 2-blade and running at 2750rpm.

Gearboxes have a weight (and reliability?) penalty.
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Reduce the prop diameter and you reduce the amount of air you move for a given speed.
 
the biggest portion of the horse power formula is RPM. More = higher out put.

Relationship with torque
For a given torque and speed, the power may be calculated; the relationship between torque in foot-pounds, rotational speed in rpm and horsepower is:


Where P is power, τ is torque, and f is rotations per minute. The constant 5252 comes from (33,000 ft·lbf/min)/(2π rad/rev).

The standard equation relating torque in inch pounds, rotational speed in rpm and horsepower is:


Where P is power, τ is torque, and f is rotations per minute. The constant 63,025 comes from (33,000 ft·lbf/min) × (12 in/ft)/(2π rad/rev).
 
There is a reason they use windmills to generate electricity and not pinwheels. Same goes for props.
 
TMetzinger said:
Reduce the prop diameter and you reduce the amount of air you move for a given speed.
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There is only one prop that will run at sonic speeds know what it is called?
 
Longer propeller blades are more efficient - look at a helicopter for an extreme example.
 
Morne said:
Tip speed is proportional to the length of the prop. Simply reduce the prop length, adding additional blades as necessary. I think that a 4-blade smaller diameter prop running at say 5500rpm would still be lighter than gearing down to a large 2-blade and running at 2750rpm.

Gearboxes have a weight (and reliability?) penalty.
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Increase the number of blades and you increase drag.
 
Actually the bypass fan blades in a high bypass turbofan engine can be thought of as supersonic propellers. They have a completely different aerodynamic shape though.
 
Mainly it comes down to noise and efficiency. A bigger, slower-turning prop will be more efficient.

Take a look at the Cessna 421 (gear 375 HP Continentals) vs. the Beechcraft Duke (direct drive 380 HP Lycomings) for an example of the noise. The Duke is rated at 2900 RPM, cruise at 2500-2750 RPM. The 421 has prop RPMs in cruise in the 1700-1900 RPM range, as I recall (Tony?). End result: The 421 is much, much nicer to be in the cabin of. It's much quieter. The noise (or lack thereof) is one of my favorite parts of the 421.

Sure, you can put smaller props on to solve the sonic issue, but it'll still be loud and high-pitched.

Gearboxes do have a weight and reliability penalty, that is true. However, typically geared engines are still lighter weight than their direct-drive counterparts of the same power. And the reliability issues can be mitigated by a careful pilot.
 
kontiki said:
Actually the bypass fan blades in a high bypass turbofan engine can be thought of as supersonic propellers. They have a completely different aerodynamic shape though.
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The shape of or number of blades, have nothing to do with wither a design is or is not a propeller.

the high bypass fan is of what design family of props?
 
Morne said:
Tip speed is proportional to the length of the prop. Simply reduce the prop length, adding additional blades as necessary. I think that a 4-blade smaller diameter prop running at say 5500rpm would still be lighter than gearing down to a large 2-blade and running at 2750rpm.

Gearboxes have a weight (and reliability?) penalty.
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As others have said a longer prop is more efficient in that it turns engine HP into thrust with the least loss to noise, and turbulence.

A small diameter prop (regardless of how many blades) will have to accelerate the air to a higher speed in order to translate HP into thrust as compared to a large prop. That will require a higher blade angle of attack. Eventually you get to the point where the blade will stall before it has turned all of the available HP into thrust.
 
the high bypass fan is of what design family of props?
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Can't say I've seen it in a design reference or anything like that.

After spending a lot of time around high bypass turbofans and sitting through whatever training, that's the conclusion I arrive at.

Classic supersonic shape (like a flat 4 sided diamond) through the outer portions of the airfoil shape, and they turn really fast to produce the majority of the engine thrust.

When you see a set in a row on a work table during maintenence and take a good look at them, conceptually, there isn't tons of difference.

To be honest, I probably haven't eyeballed a set of fan blades really closely in a while. I'm sure the shapes are all more spohisticated now. Maybe L1011 -RB211-212s? Maybe later, I can't remember every night I spent in a hangar. Who cares?
 
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Ted DuPuis said:
Mainly it comes down to noise and efficiency. A bigger, slower-turning prop will be more efficient.
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The noise issue I understand.

To the guys who mentioned about windmills, ever see the ones that have a vertical axis?
450px-Quebecturbine.JPG


That'd make for a trippy looking plane...
 
kontiki said:
Actually the bypass fan blades in a high bypass turbofan engine can be thought of as supersonic propellers. They have a completely different aerodynamic shape though.
Click to expand...

No they can't, air is slowed below supersonic speeds before it enters the engine.
 
Inverted said:
No they can't, air is slowed below supersonic speeds before it enters the engine.
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Airflow may be slowed to subsonic speeds as it enters, but the tips of the fan blades can be moving at supersonic speed through that air...at least according to Rolls Royce (pdf).
 
I thought I read somewhere that the Tu-95 'Bear' prop tips went supersonic, but a very brief bit of Googling didn't confirm this.
 
Couldn't sleep, bad headcold, So I went searching for reliable data to support my observation.

I found this ->

Quoting A380900 (Thread starter):
I understand also that the tip of the blades cannot go faster than the speed of sound (or can they?).
Fan tips always operate at supersonic speed. With the plane going at 0.8 Mach the fan tips would also be at M=0.8 even if the engine wasn't spinning at all, so it doesn't take much spinning to make the tips supersonic at speed. Fan tip speed on a cruising airliner is typically between 1.5 and 2.0 Mach. The new P&W geared turbofans will be a little slower, but still supersonic, typically between 1.3 and 1.5 Mach. Most of the efficiency gain will be due to a faster spinning LP turbine.

Turboprop planes, however, they keep the propeller tips just below Mach one.

I don't know if a post at http://www.airliners.net/aviation-forums/tech_ops/read.main/291319/ is a credible source of data.

It's an interesting question. At work tomorrow, I may need to walk down to the PP group or the Boeing office and ask those guys what the tip speed is on a 777 engine. Somebody might know.
 
PP=Powerplants
LP=Low Pressure Turbine - Turbines are the spinning blade sections aft of the combustor.
 
Why are some engines geared to reduce prop RPM?
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The engine in my 182 is 470 Cubic Inches, producing 230 Horsepower at 2600 RPM. That's ONLY .48 HP per CI.

For comparison:
The engine in my Toyota 4Runner is 2.7-liter (164 CI), producing 150 horsepower at 4,800 RPM. That's .91 HP per CI.

And:
The engine in my Subaru Forester is 2.5-liter (152 CI) pumps out 165 horsepower at 5,600 RPM. That's 1.08 HP per CI.

Higher RPM allow for more HP/CI. Thus the weight penalty of the gearbox would be considerably offset by the weight reduction of the engine (or increased HP) and the engine would be more fuel efficient running at the higher RPMs.

The question should be:

Why aren't more engines geared to increase engine RPM?
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Complexities, cost, weight, and reliability are the main reasons for no more gears boxes than there are.

Several attempts to use auto conversions with gear boxes have ended badly. A few seem to have the bugs worked out, but I would not fly one.

If memory serves me correctly several war birds used the Allison V-1710. They had several different gear reduction drives for different applications. P51, P38. Others?
 
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Geico266 said:
Several attempts to use auto conversions with gear boxes have ended badly.
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I wasn't attempting to promote the use auto engines in airplanes but rather to illustrate that with higher RPM comes increased HP, efficiency and fuel economy. And those would be the main reasons to gear the prop down.
 
Geico266 said:
If memory serves me correctly several war birds used them. P51 for one. Others?
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My understanding is that most of the big old radials used gearboxes, especially planetary gearsets. The planetary gears were pretty reliable units when treated properly.

To answer Tim's question with a question: Why is it important to get higher power per cubic inch? Your 182's engine installed I will bet is significantly lighter than your Toyota's engine installed.

While the automotive conversions can work (Ben's done a great job with his), most of them don't work well at all.
 
timwinters said:
I wasn't attempting to promote auto engines in airplanes but rather to illustrate that with higher RPM comes increased HP, efficiency and fuel economy. And those would be the main reasons to gear the prop down.
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Certainly not. I was just illustrating the complexity component.
 
timwinters said:
I wasn't attempting to promote the use auto engines in airplanes but rather to illustrate that with higher RPM comes increased HP, efficiency and fuel economy. And those would be the main reasons to gear the prop down.
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Actually, that's not true. Power per cubic inch does not translate into efficiency, at least not fuel efficiency.

Higher RPM results in higher pumping losses. Not just holding a vacuum (since we typically run WOT anyway), but also in friction with rings, bearings, etc. This results in lower efficiency. Keep in mind that the gearbox will have its losses, as well, which will hurt efficiency.

Higher RPM also results in higher EGTs, which can limit how lean you can really run. This also can hurt efficiency.

Fuel economy is largely a function of your BSFC (brake specific fuel consumption). If your BSFC increases, your fuel economy goes down. Check the BSFC of your Toyota vs. your O-470, and I'll bet you'll find the O-470 does better, with lower weight.
 
Back to the OP's question. As stated the increase RPM is to get more horse power from a given size engine. The reason for the gear reduction is more than one reason. Decreased noise is one consideration. Stress on the hub is another. These hubs are designed for a certain amount of stress then things tend to break. I am sure a hub could be designed but that would be very expensive thus the desire to use off the shelf. A third factor is the blade efficiency itself. A blade is nothing more than an airfoil. Its lift is in the horizontal axis and is referred to as thrust but, it is lift. As an airfoil goes super sonic bad things happen. I am sure there are some Lear drivers here that can explain "mach tuck" but suffice to say as the wing approaches supersonic, center of lift moves aft and total lift decreases. A conventional prop blade (air foil) will lose considerable lift (thrust) as it approaches super sonic thus the need for a gear box to slow prop speed. There is a reason you do not see super sonic prop driven aircraft. Could a blade, hub design be developed to work at super sonic speeds? I am sure it could but, why. It would be very expensive and have a limited market. I hope this helps the OP a little.
 
kontiki said:
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Fan tips always operate at supersonic speed. With the plane going at 0.8 Mach the fan tips would also be at M=0.8 even if the engine wasn't spinning at all, so it doesn't take much spinning to make the tips supersonic at speed.
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Error. The inlet of a jet engine is a divergent duct, which slows the incoming air. The airplane is at Mach .8, the fan's tips are considerably less than that if not turning. They might exceed Mach 1 but the duct limits tip vortices and therefore the losses due to those vortices.

Get a look here:
http://ecampus.nmit.ac.nz/moodle/mod/book/print.php?id=50384

The fans of those big engines are geared, too.

566x228_PW1000.jpg


from http://www.volvoaero.com/volvoaero/...ponents/commercial_engines/Pages/pw1000G.aspx

Dan
 
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Tim, to answer your question ask an handfull of pilots about the G0-300. You'll get "better" answers if they've never flown one.

That and with out a doubt there is an impact on the cost of the engine and maintenance along with extra bits to fail anytime you add parts.

Let's also not forget that fuel was cheap when most of our engines were built so efficency was not of paramount concern to TCM or Lyc.
 
Based on a few a small bit of meandering research I'm ready to retract my earlier proposition.

I believe the statement:

Actually the bypass fan blades in a high bypass turbofan engine can be thought of as supersonic propellers. They have a completely different aerodynamic shape though.
Click to expand...

is in fact incorrect.
 
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As far as i know the PW1000G is the only geared high bypass turbofan (hence the G on the designation) that PW makes. I believe RR and GE's engines all have non-geared bypass fans.[/QUOTE]

Could be. There are some smaller engines though, like the Honeywell (Garrett) TFE 731, with geared fans.

TFE731.gif


Dan
 
Skylane81E said:
Let's also not forget that fuel was cheap when most of our engines were built so efficency was not of paramount concern to TCM or Lyc.
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Please learn a bit more about the relative efficiencies of aircraft engines vs. automotive prior to making that statement.

Best economy BSFCs on typical aircraft engines at cruise power are roughly 0.4 lb/hp-hr. That's significantly lower than what you'd get out of the engine in Tim's Toyota.

Diesels will do better - by a bit. Typically mid to high 0.3s.

It is a common misconception that traditional aircraft engines aren't efficient. While some of the larger opposed engines can be difficult to obtain best economy on (the TIO-541 in the Dukes comes to mind), if you put a comparable automotive engine in, I doubt you'd do much better.

Nevermind the fact that the engine in my Mitsubishi with a gearbox would weigh probably a bit more than a TIO-540-J2BD out of a Chieftain.
 
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