Constant speed prop and max rated power

[ScottG]

Is it always, never, or sometimes true that, from full power and max RPM, if one were to pull back the RPM lever, you could go over max rated power?

Side note- been away from the forums for a while and looking forward to diving back in, thanks in advance for the replies!

 

[Roger Roger]

needs more information. But generally, no. Remember power is proportional to torque x RPM, so even though you'll see a rise in MP (in a piston) or torque (in a turbine) your total power won't generally increase. Also remember that in a piston your MP reading is just that, an MP reading, and while it's the closest proxy we usually have for torque on a piston engine it is not a torque indication like a turbine engine.

 

[ScottG]

I generally understand that, what more information would be needed and I am hung on the "generally no" part. Could there be a scenario i.e. engine and prop configuration or something else where less than max rpm produces more power\thrust?

I suspect the slight increase in torque is more than offset by the drop in rpm but I swear I heard or read somewhere that you can go over rated engine power by pulling the propeller back at full power, further getting the same nasty effects on the engine we were all taught to avoid by pulling power first.

 

[ScottG]

Basically you increase thrust per revolution when you reduce the RPM control but generally the reduced RPM outweighs the increased thrust from the increase blade angle. I guess I am asking is if there is a propeller with a L/D curve where the increased angle of attack and therefore thrust outweighs the reduced RPM?

Or am I out to lunch.....

 

[Fly_Unity]

I would say the answer is "never" on a direct drive engine. You will not increase power by reducing "engine" RPM.

Might be possible on a PT6 turbine engine or a free shaft turbine (if you are temp limited) as the prop angle will not slow down turbine speed. So you may get a better prop efficiency without reducing Ng or fuel flow. But if you are Torque limited, you will need to reduce power (fuel flow) to keep torque within limits and reduce overall power output.

 

[Roger Roger]

I would say the answer is "never" on a direct drive engine. You will not increase power by reducing "engine" RPM.

Might be possible on a PT6 turbine engine or a free shaft turbine (if you are temp limited) as the prop angle will not slow down turbine speed. So you may get a better prop efficiency without reducing Ng or fuel flow. But if you are Torque limited, you will need to reduce power (fuel flow) to keep torque within limits and reduce overall power output.

Or you could fly an engine that's past the 50s technologically that has, say, a torque limiter.

 

[lightspeed]

I'm going with "never."

 

[ScottG]

Well, as to that equation, torque does rise but RPM drops, so as a percentage game the torque get outdone generally. However, after looking at lift coefficient curves and knowing that at the low AoA spectrum, each increase of angle dramatically increases lift(thrust), I just wonder if there is not some circumstance where a bigger bite more than offsets the drop in RPM. I know the equations, its a whole different story when applying the equations to different prop\engine combinations i.e. one engine may drop much less RPM for the same pitch increase than another.

I feel like I had a eureka moment one way or the other while reading on this in the Naval Aviators Handbook, I am about to give up on the google and go check that out.

 

[lightspeed]

To get really specific, you're going to need to clarify some definitions from your OP:
What type of engine are you talking about?
What value are you referencing when you mention "max rated power"? Are you talking horsepower? Since you brought up the question of "lift(thrust)," are you looking for shaft horsepower, thrust horsepower, or something completely different?
With regards to limits, are you asking about design limits (what the engine is physically capable of producing), or operating limits (the maximum authorized value for the engine)?

Power output is almost directly related to fuel (and therefore air) flow. What will happen to the amount of fuel/air in your intended engine when you decrease RPM? In a piston engine it will decrease, in turboprop it may stay the same. Your questions about prop AOA are related to propeller efficiency.

<just completed an aerodynamics university course (pilot level of depth).

 

[ScottG]

Perhaps I mixed some different ideas in an attempt to elicit an answer, but I guess at the end of the day I am looking for max thrust horsepower. I understand a decreasing RPM will drop shaft horsepower. But if a bigger bite might mean a more efficient AoA then couldn't that increase the thrust horsepower if the increase in efficiency more than offsets a weaker engine output; a bigger push forward? I stress the "couldn't that mean" part.

If we aren't hitting L/D max at full RPM but a slightly lower RPM (increase pitch) boosts efficiency at a faster rate than the percent decrease in engine power, we get more thrust. Is that ever possible?

 

[lightspeed]

I understand a decreasing RPM will drop shaft horsepower. But if a bigger bite might mean a more efficient AoA then couldn't that increase the thrust horsepower if the increase in efficiency more than offsets a weaker engine output; a bigger push forward?

I would go farther than "could be," and say absolutely. That combination of RPM/SHP resulting in maximum available THP is going to coincide with the setting for max takeoff power. Why would a designer let you, the pilot, push the engine any harder when all it will do is result in less thrust (while burning more fuel)?

Great discussion BTW!

 

[Roger Roger]

TBH this is pretty far past any practical application. On a normally aspirated piston the answer is going to be no, and a turbo piston or a turbine is going to have power setting tables to abide by so who cares?

 

[ScottG]

I guess I would want to know if there are ways of squeezing extra performance out of an aircraft To me it is practical to know that I can get more thrust by over stressing an engine.

 

[NickH]

Just remember, if you go over square, the engine will immediately detonate, taking out any nearby cities...

 

[CFI A&P]

Is it always, never, or sometimes true that, from full power and max RPM, if one were to pull back the RPM lever, you could go over max rated power?

Side note- been away from the forums for a while and looking forward to diving back in, thanks in advance for the replies!

You're question is more about propeller efficiency than it is engine performance. What you are asking: Is there a propeller airfoil that is more efficient at a lower rpm than 2,700RPM and if so does it create more thrust on a lower RPM setting?

A naturally aspirated piston engine develops max horsepower under max manifold/ redline RPM & appropriate mixture. Say 29", 2,700RPM and leaned for peak in say IO-540 yields 300hp. Any deviation in MP/RPM/FF will yield less.


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