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Old 17th Jun 2004, 10:25   #1 (permalink)
 
Join Date: Jan 2003
Location: SX in SX in UK
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Percentage power vs RPM

I have to confess that I've never understood the relationship between percantage power and RPM.

MY POH talks of '75% power at 2300' but to my simple mind, 0% power = 0 RPM and 100% power = firewalled throttle with a linear progression in between.


Can someone knowledgeable please explain the relationship.
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Old 17th Jun 2004, 11:17   #2 (permalink)
 
Join Date: Aug 2000
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'Fraid it's not quite as simple as you're probably hoping.

The engine manufacturers provide data for power/RPM/MAP at different mixture settings.

For a fixed pitch prop, the aircraft manufacturer can then convert this to Power/RPM/Altitude/IAS graphs for a particular aircraft in level flight.

In most VP aircraft there is a combination of RPM/MAP/Altitude/fuel flow settings that give a particular power setting.

For typical fixed pitch there are some rules of thumb that work quite well. e.g. 2550/2400/2300 = 75/65/55% on a small lycoming.

Take-off and climb in the average fixed pith a/c is somewhat less than 100% power as you won't reach 100% RPM given the lower IAS in the climb. A 'cruise' propeller will just reach 100% RPM at fulll throttle in level flight at a higher speed.
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Old 17th Jun 2004, 11:56   #3 (permalink)
 
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I think that for a fixed pitch prop, the power is the cube of the rpm.

With a VP prop, the question doesn't apply because the governor keeps the rpm constant. The actual shaft horsepower is roughly proportional to fuel flow, assuming the engine is running at or past peak EGT.
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Old 17th Jun 2004, 12:51   #4 (permalink)
 
Join Date: Mar 2003
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(NB: All this assumes we're talking about a fixed-pitch prop with a non-turbo engine)

The important thing to understand is that when the POH talks about "percentage power", they mean a percentage of the engine's maximum rated power. In a non-turbo engine, this power is only achievable at sea level (and even then you'd need to be moving at high airspeed otherwise you won't get the RPM you need to achieve that power).

So, in practice a firewalled throttle will never give you 100% power. The reduced air pressure at flying altitudes will reduce the power output of the engine, so that at 5000 feet (for example), it's probably only possible to get about 75% power at full throttle. The power tables in your POH will give full details.

The other thing, as others have mentioned, is that the relationship between power and RPM is very non-linear. At higher RPM, a small change of RPM corresponds to a significant change of power, whereas at low RPM you need to make larger RPM changes to get the same change of power.
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Old 17th Jun 2004, 19:05   #5 (permalink)
 
Join Date: Mar 2002
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The problem lies also in the RPMs below max power. Because the shape of the fixed pitch prop is designed for a specific range of speeds, the prop rapidly loses efficiency below certain settings. Idle RPM is around 700 but youre not developing any power to speak of, so your 0% power point is somewhere there. This is different again when flying. Your prop may be developing more drag than thrust at settings around 1300-1400 and below. So where is the 0% power point?? Certainly not at 0 RPM!

As you can see from the figures posted earlier, there's a small band of RPM figures at the top end of the scale where you can go from almost 0% to almost 100% RPM. This band is not linear and it also shifts depending on altitude, speed etc.
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