Question about power settings / fuel consumption

[piperboy84]

Can someone please answer the following, my plane has a 180hp (at 2700rpm at sea level in standard temps) and a fixed pitch prop, most of my flying is done below 1500ft where I cruise leaned out at either 2350rpm (65% power) or 2450rpm (75% power), however when I fly cross country at between 6500 to 7500 above msl which according tothe aircraft manufacturer is the optimum cruise altitude, the engine manufacturer says at these altitudes it will only produce at most 75% power flat out, if so does it make sense to run at full throttle at 7500 feet as it is only producing 75% power which is the cruise power setting at sea level, and if I do run at full throttle will my fuel consumption be the same as the 75% at SL or will it be the full throttle consumption. And if running at full throttle (red line) at sea level is bad for the engine, is it still bad to do it at 7500 bearing in mind its only producing 75% power at that altitude.

Hope this makes sense

[Proteus9]

As you climb you'll see your rpm decrease (I suspect, I run a CS prop and have it turning fairly slowly in the cruise). Leaning is always a bone of contention, but some people suggest running a little bit back from wot to let the butterfly aid mixing if you want to run lean of peak. You will want to lean as you go up to go up so you don't end up ridiculously rich.

[Above The Clouds]

You won't reach max rpm at 7500ft that's why it's only producing 75% power with a full throttle setting provided you have leaned it correctly, the reason for it being the optimum altitude is because you will be getting the best TAS versus power and fuel consumption.

You can lean the mixture at any altitude providing you follow the engine manufactors recommendations, on some types you may even have to lean the engine during taxing and certainly if departing from high altitude airports if you don't lean the engine for take-off you won't develop take-off power for that altitude.

[Big Pistons Forever]

You won't reach max rpm at 7500ft that's why it's only producing 75% power with a full throttle setting provided you have leaned it correctly, the reason for it being the optimum altitude is because you will be getting the best TAS versus power and fuel consumption.

You can lean the mixture at any altitude providing you follow the engine manufactors recommendations, on some types you may even have to lean the engine during taxing and certainly if departing from high altitude airports if you don't lean the engine for take-off you won't develop take-off power for that altitude.

The first paragraph is incorrect. In aircraft with fixed pitch props the maximum RPM at full throttle stays approximately the same at any altitude. What changes is the amount of power that any given RPM produces. The higher the altitude the lower the power. The engine however is most efficient at full throttle because the throttle valve is wide open. Therefore to get full open throttle at a cruise power setting you have to fly high. One problem with this is the high RPM tends to make the airplane noisy, but otherwise your typical 4/6 cylinder non turbo charged carburated Continental/Lycoming engine is rated for unlimited operation at full RPM.

For most light aircraft the no wind sweet spot is the 6500 to 8500 altitude range at 65% power (usually around 2400 - 2500 RPM). This gives a good compromise between speed, fuel consumption and cabin noise. Fly higher with a lower power settings with a tailwind and lower with a higher power setting with a headwind.

[Above The Clouds]

Big Pistons Forever

What controls power output is density in a normally aspirated engine not RPM. And cabin noise has no bearing on optimum altitude.

[Big Pistons Forever]

Big Pistons Forever

What controls power output is density in a normally aspirated engine not RPM. And cabin noise has no bearing on optimum altitude.

With a fixed pitch prop the control of the power of the engine is achieved by changing the RPM. What power any RPM setting will give you is mostly but not totally dictated by the density of the surrounding air. (Carb temp and mixture and airspeed also effect power output). It is up to the pilot to go to the POH and find out what RPM he/she must set to achieve the desired power output at any given altitude and temperature.

IMO cabin noise has a significant effect on pilot and especially pax comfort. For instance many Cessna's have an annoying buzz at max RPM but a more pleasant sound at, and much less vibration, in the 2400-2500 RPM band. This may lead to choosing a lower altitude in order to get the desired power output at a lower and hence quieter RPM particularly if you are flying a long trip against a headwind.

Selecting a real world "optimum" altitude should consider all factors effecting the flight. For example on a sunny summer day I will always choose a higher altitude for a mid afternoon flight as compared to an early morning flight, because of the likely hood of low level turbulence in the afternoon.

[piperboy84]

It is up to the pilot to go to the POH and find out what RPM he/she must set to achieve the desired power output at any given altitude and temperature.

In my aircraft's particular case (its a Maule) calling the POH brief is an understatement, there is no power/or performance charts in it

[piperboy84]

So would it be correct to say that at high altitude at less than full rpm (cruise setting the engine will run out of puff and can not deliver any more power even with the throttle not full against the firewall, if so is the excess fuel that will be delivered to the engine by fully opening the throttle (past the point it can deliver power) wasted or is it just not drawn thru,?

[Big Pistons Forever]

Piper boy

In your case it would be worth buying the engine operating manual from Lycoming. It contains a whole section of power charts which will allow you to build your own RPM vs power for a selection of altitudes. It also contains a wealth of operating tips.

Re your second question the fuel will not be drawn through. As ABC pointed out the power output at full throttle will be effected by the density of the ambient air. Less air means less fuel to maintain a constant fuel air ratio and so less fuel flow and of course a lower power output

[AdamFrisch]

Easiest for me is to think about fuel consumption per hp. Most recips burn in the region of 0.45-0.5lbs/hp.

In my aircraft with 260 hp a side, 520 total, this means about 42gal/hr knuckles to the wall at 26in MP. At 18000 ft the output has gone down to about 45% and the total hp available is now just 260-270. Therefore your consumption is now around 21-23gal/hr. You also gain about 2kts of TAS for each 1000ft in altitude, so it's easy to see that cruising high is almost always better, even in a moderate headwind. Obviously one has to take into the account the time to climb and consumption to get there - it might not be worth it on shorter trips. But on most longer trips it is.

A power reduction at high altitude decreases consumption further, so if you're not in a hurry you can cover long distances at low fuel flow. In fact, if you were to regularly cruise at the aircrafts max ceiling at best glide speed IAS (least amount if drag), that's when you'd save the most.

[mm_flynn]

In a fixed pitch aircraft the RPM is an output not an input. It reflects the amount of power being added to the prop and dissipated by the prop in the air.

Because the air is less dense higher up, less power can be dissipated at any given RPM - therefore the RPM will increase with altitude if you maintain a content power output (hence to dissipate 75% at 7000 ft is 2700 rpm and at sea level may be 2350). Fortunately for a given throttle setting power declines at about the same rate as the ability to dissipate power - hence BPs comment about achieving max rpm at most altitudes.

There are only three things that control power output, the fuel burned per hour, the mixture and the engine geometry (which is of course fixed).

The throttle controls the air volume flow (broadly) and the carb meters fuel to match to volume. So at a fixed altitude the throttle controls the amount of fuel and hence the power.

If you change altitude but not throttle setting, the same volume of air will move so the same mass of fuel will flow, however the mixture will be richer and therefore power will be less. This then results in the prop slowing down (so the power dissipated matches the engine power) and this slower pumping reduces air volume flow (and via the carb fuel flow), further reducing power until we are back in balance (engine power out = power dissipated by prop).

You can see therefore, that you must both advance the throttle to increase the volume of airflow and 'lean the engine' to ensure you retain the same fuel flow rate and combustion mixture (and therefore power ) as you climb.

[24Carrot]

As an exercise, I worked through a rather similar example a while back, using performance charts from a C172 POH.

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This C172S has an IO360 180bhp engine, and a fixed prop. Data is from the POH.

The 2,000 ft performance figures for 2380 RPM are interpolated from the 2300 and 2400 data. The point was to match KCAS at different heights.


https://sites.google.com/site/twododecacarrot/home/C172Table.jpg

One would expect wing, propeller and engine performance (specifically nautical miles per US Gallon of fuel) to vary in different ways with the lower air pressure and lower air density at altitude. This analysis chooses 2380 RPM at 2000 ft so that the CAS (Calibrated Air Speed) matches the CAS at 2700 RPM and 10,000ft. In this way the Lift Coefficient and so the wing performance should be the same in both cases, in particular the Lift/Drag ratio. As Drag times Distance gives energy expended, the mpg figures should be the same in each case, if the wing performance is all that matters.

However, there is no obvious reason why energy losses from propeller and engine inefficiencies at altitude should depend on the Lift Coefficient or CAS, and these should generate extra Drag, and make the mpg figures differ.

The table shows that the mpg figures do match! Also, as if by magic, the key propeller coefficient (the Advance Ratio), and one key engine statistic, the BMEP (Brake Mean Effective Pressure) are the same as well. Practically all the other measures vary by the ratio of the air densities.

The conclusion is that range performance depends only on CAS. Even the propeller and engine efficiencies depend only on CAS (or at least their combined efficiencies do). It is not clear to me whether this is some grand principle at work, or "just" clever design by Cessna.
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It pretty much backs up what Big Pistons and mm_flynn have said already.

One point not made so far: for a given CAS, the TAS is higher at altitude, so for a fixed-pitch prop to have the same angle of attack, the RPM has to be higher. RPM and blade length are not affected by air density:).

The table shows this as the "Prop J" aka the "Advance Ratio".