Best power vs best economy

[bjornhall]

Let's say the POH power setting tables say 65% power is 2,400 RPM. When leaning to best power or best economy, is it assumed that one ensures RPM remains at 2,400 after leaning? If so, why does one get lower airspeed when leaned to best economy? If not, what RPM are you supposed to set then?

What I have in mind is small piston engines with fixed propellers, but I suppose similar principle applies to heavier stuff.

[411A]

Small airplanes with fixed pitch props...
At a constant RPM, best power mixture delivers more BHP, therefore more speed.
Economy mixture, less BHP, less speed.

Then we look at constant speed props on small (and large) piston powered aircraft.

A glance back to the days of the 4-engine piston-powered airliner will provide a clue about fuel economy.

These aircraft were cruised at (normally) a constant BHP (using BMEP recovery techniques) and the lowest RPM, within engine operating perameters, was used, to enhance engine longivity.

On smaller GA types (with constant speed props), generally speaking, the lowest RPM cruise allowed will normally provide the best fuel economy.

[bjornhall]

Ok, so if you understand you correctly regarding fixed prop: 2,400 RPM @ best power = 65% power; 2,400 RPM @ best economy = < 65% power... Then it makes sense.

[Brian Abraham]

regarding fixed prop: 2,400 RPM @ best power = 65% power; 2,400 RPM @ best economy = < 65% power

Not quite. If at a particular altitude 2,400 RPM equates to 65% power it matters not if you are operating at best economy or best power mixture. To produce a given RPM on a fixed pitch prop at a given altitude takes a given horse power. Whether that horse power is produced at a best power or economy power setting is inmaterial. The TIO-540 at 65% power and 2,400 RPM burns 16 US gallons per hour at best economy and 19 gallons per hour at best power.

[bjornhall]

That is what I used to think, but I can't see how it makes sense... How can you be slower when producing 65% at best economy than when producing 65% at best power?

To first approximation, the following should be true:
- To produce a given RPM on a fixed pitch prop at a given altitude takes a given horse power.
- A given power at a given altitude (and temperature, and so on) should result in a certain cruise speed.

But if I'm not mistaken those two statements together are inconsistent with the cruise speed at a given power setting and RPM depending on the mixture setting. So it appears as if we must look beyond the first approximation...

Might have to dig up some aeronautical engineering textbooks and do the number crunching, unless someone already knows the answers...

[411A]

Cruise speed (POH) is usually calculated (and observised/logged) at best power mixture.
Economy mixture, expect a three knot decrease, not much more, with fixed or constant speed propellors.
However, on some types, the margin is (or can be) seven knots, (approximately).

Depending.

[Mark1234]

I'm with bjornhall here. Let's ignore CSU aircraft as that wasn't the question.

It's simple physics:
For a constant airframe configuration (not varying drag), airspeed and RPM are proportional to the power inpot into the prop. Feed it more torque, it will spin faster, and the plane will move faster. And vice versa. Direct causal relationship.

If you change the engine's power output (black lever OR red lever), the rpm will vary (very slightly). so lean to best econ, and the revs (and airspeed) will drop slightly as the power reduces. Unless 65% lean is with reference to a different max, you're now producing less power - let's say 63%

If you regain that power through moving the black lever to get 2400rpm again, you have to see the same airspeed. But the black lever will be further forward in the lean condition; it's still probably more economical as you're burning the fuel as efficiently as possible, and not flushing fuel out the exhaust.

Remember that for those simple aircraft without EGT etc., one of the recommended ways of leaning is to 'lean to peak rpm, then richen it a bit' (paraphrasing)

[Brian Abraham]

How can you be slower when producing 65% at best economy than when producing 65% at best power

There is a bit of a misunderstanding of physics here. The TSO-540 I referred to earlier produces 350 horse power at rated power. 65% of that is 227.5 horse power. At 227.5 horse power the aircraft will only fly at one given fixed speed (all other things being equal and not talking back side of the power curve). Whether that 227.5 horse power is produced at a best power or best economy mixture setting is completely inmaterial. Horse power is horse power and a fixed horse power produces only one fixed speed. Where the confusion arises is that for a fixed throttle setting as you lean from full rich the power output initially increases to a maximum (RPM increases with a fixed pitch prop) and the point at which the RPM peaks is the best power mixture setting. The RPM then falls away as we lean because power is being lost until we get to the best economy mixture setting. If we plotted horse power against mixture we would come up with a representative chart as follows. For a given throttle setting lets assume that best power mixture equals 100% power. If the mixture is made full rich the power output will drop to 95% and at best economy the power out put will drop to 93%. (Figures taken from the TIO-540 manual).

[bArt2]

The thing is however that the TSO-540 is connected the a constant speed propeller in general and therefore when you lean past best power to best economy, the power will reduce and the blade angle will decrease to keep 2400 Rpm.

The question however was about a fixed propeller. Do you have to adjust to 2400 Rpm again after leaning as power goes down at best economy, therefore rpm decreases a bit?

In general the engines are set to operate at a mixture that is too rich. So what happens according to me is that as you lean to best power Rpm increases a bit (20-30 Rpm i'm guessing) and speed increases a few knots, then as you continue to lean to best economy Rpm goes down a bit again and will be roughly around to initial Rpm again.

So in my view the question is not do you have to increase Rpm at best economy but do you have to decraese Rpm at best power?

I would not change the throttle position after leaning.

Bart

[Brian Abraham]

The thing is however that the TSO-540 is connected the a constant speed propeller in general and therefore when you lean past best power to best economy, the power will reduce and the blade angle will decrease to keep 2400 Rpm

Even with a constant speed prop it matters not whether the 65% is being produced at best power or best economy. The prop will be at the same pitch since you are putting out the same horse power. Lets look at it another way. With a fixed pitch prop you settle into the cruise, full rich mixture, and set 2.400 RPM which the book tells you is 65% power. You then begin to lean and the RPM will start to increase and will reach a peak which will represent approx 68% power. This is the best power point. Continuing to lean to best economy the RPM will fall until a power output of about 64% is being produced. That 1% loss of power will show as a slightly reduced RPM (a bit less than the original 2,400) and loss of a couple of knots airspeed. I'll let you do the maths on your particular aircraft as to the airspeed change with a 1% power reduction.

[bjornhall]

If you leave the throttle and accept the RPM decay, then it is all crystal clear and makes fine sense.

But if you are supposed to increase throttle to regain 2,400 RPM, and that results in the same RPM but slower airspeed, then something is apparently more complicated (prop efficiency varying with airspeed is all I can think of as a reason for how that would work out...).

With a wobbly prop I think it is all much simpler; then you could have the same MP and RPM but lower fuel flow at best economy (or LOP), thus giving you less power, less airspeed, and the governor setting finer prop pitch to maintain RPM despite less power.

[bookworm]

Let's say the POH power setting tables say 65% power is 2,400 RPM. When leaning to best power or best economy, is it assumed that one ensures RPM remains at 2,400 after leaning? If so, why does one get lower airspeed when leaned to best economy?

Are you reporting this as an experimental observation in a fixed pitch prop aircraft? That you:

* start at 2400 rpm and airspeed V
* lean the mixture to best economy
* increase the manifold pressure (i.e. push the throttle forward) until the rpm returns to 2400
* observe an airspeed less than V

?

With a fixed pitch prop at a particular altitude, for a particular aircraft, in a particular configuration/weight, and assuming you're well above minimum power airspeed, any one of of power, airspeed and rpm determines the other two. Hence I would not expect the observation above.

With a constant speed prop the relationship is different. The variable pitch of the prop means that the different combinations are possible. Hence for example the same airspeed can be achieved with several different combinations of engine power and rpm, which have different efficiencies.

[bjornhall]

No, it's not an experimental observation, and I would be a bit surprised if one would actually observe that...

The question was rather prompted by what is in the POH of the C172 I fly; it says airspeed is 3 kts lower when leaned to best economy, than when leaned to "recommended lean" (which is close to best power). But it doesn't say whether or not that is with the same RPM for both cases, thus my confusion...

[bookworm]

I think it means it's 3 knots lower if you do not restore the RPM.

[bjornhall]

Yes, I think you are right. When re-reading the POH carefully and interpreting what it says, it seems you are supposed to start the leaning procedure from "the full rich or maximum RPM mixture", and there is no mentioning of touching the throttle except that "any change in altitude or throttle position will require that peak EGT be redetermined and the mixture be reset".

So I suppose this must be the procedure then:
- Low altitude: Start at full rich, set desired RPM from power setting table, then lean to peak EGT or 50 rich of peak without touching the throttle.
- High altitude: Start at "maximum RPM mixture" (which I take to mean the mixture you used just before TOC), set desired RPM from power setting table, then lean to peak EGT or 50 rich of peak without touching the throttle.

Thanks for getting me sorted on this!

[Brian Abraham]

bjornhall, you may find the following link of interest. John Deakin is the recognised industry guru when it comes to talking engine management.
Pelican's Perch #18:<br>Mixture Magic