Cruise power setting

[matthewlai]

Hello!

I am a new student pilot (10 hours), and was just wondering how the cruise power is usually chosen.

From what I have seen, most small planes (172, DA-20, etc) have a listed cruise power of 75%-80%. Is that to save fuel? Or can the engine not handle 100% power all the time?

Since plane rental is by the hour and wet Hobbs at most places, wouldn't it make more sense to cruise at 100% power if range is not a problem?

Thanks!

[peterh337]

It depends on how deep you want to go into it :)

There appears to be a commonly held view that best engine life is achieved around 65% of max rated power.

In cruise, you normally set the desired power setting (or set wide open throttle if you are at such an altitude that the otherwise desired power setting cannot be reached, due to insufficient air available) and then you lean for approximately peak EGT. That gives you the best MPG.

The following notes (http://www.peter2000.co.uk/aviation/engine-management/index.html) apply to adequately instrumented fuel injected engines but you will get the general idea.

But you are right - a cynical b*astard renter doing a short burger run will just fly everywhere at 100% power :) That is the stupidity of renting a plane out "wet" but in practice the schools have no other options because "dry" involves complicated fuel metering and other arrangements which are too much of a hassle to operate.

[Pilot DAR]

Matthew,

Many engines are only permitted to run at 100% power for 5 minutes at a time. Though for some planes full power cruise is possible, there is absolutely no advantage. It's very noisy, hard on everything, and wastes immense amounts of fuel, comapred to a curise at 65% - 75% power. For the extra 3 or more gallons per hour in the 172, you're going a couple of knots faster, why bother...

Cruising headwind downwind should affect your choice of cruise power setting, when considered during flight planning. Other than that I'd fly at 65% to 75%. When a carburetted engine is very well instrumented, there can be some optimum power settings in terms of fuel distridution, but that's getting pretty advanced, and is not critical anyway.

Make your rental agency happy, and don't run the heck out of their plane, waste their gas, and give your passengers a loud flight:ok:

[BackPacker]

Cruise power is just one of the many choices you make as a pilot. Yes, the majority seems to fly around 65%-70%, but there may be overwhelming reasons to set a cruise power lower than that (e.g. if you need best endurance to wait out a rain shower over your destination), or higher than that (e.g. if you need to be home before end of daylight).

Even for a small spamcan, the POH typically lists performance figures for three different cruise settings (55%, 65% and 75%, for instance), for all conceivable altitudes (which is another choice you make as a pilot). So for each of these settings you can work out fuel flow, RPM(/MAP), IAS and TAS at all altitudes.

If you rent an aircraft "wet", there are typically some notes from the owner how they expect you to fly the aircraft. This includes permitted cruise power settings. And the cost calculation of the owner is based on that. So by picking another cruise power setting you essentially break the contract between you and the owner. Even though the setting you chose may be listed in, and approved by the POH.

Or can the engine not handle 100% power all the time?

There are engines that can (such as the Thielert 1.7 and 2.0), but most 1950s style aircraft engines (Lycoming, Continental) can't. Mainly because they don't have the cooling capacity to get rid of all the heat that's being generated at those settings. So running at 100% for a long time may cause the engine to overheat, leading to detonation and physical damage. Heck, in some aircraft (the R2160 for instance), running at 100% in the cruise even leads to an engine overspeed. Which may cause damage by itself, but may also cause the prop tips to approach supersonic speeds, causing a lot of noise.

There are a number of things commonly done to mitigate this specific cooling problem:
- Some engines are specifically limited to 5 minutes at 100%.
- Never lean the engine if it's producing more than approximately 70% power. The extra fuel going into the engine doesn't burn (there's not sufficient oxygen for that) but evaporates, which helps with engine cooling. (But is hard on the wallet.)
- Only climb at Vx and Vy for the shortest duration required, then switch to a "cruise climb" power setting and profile.

[david viewing]

I think this is a really, really good question.

If you are flying a PA-28 a good place to start is the back of the sun visor where you may find a table of power settings. It's worth trying them out to see if the resulting IAS is anything like the figures quoted. In my experience, it isn't!

Some FBO's require you to write down the tach readings as well as the hobbs. Again in my experience, the tach time is always significantly less, even allowing for taxying etc. It's virtually impossible to know what RPM a tach is set for (they usually don't have the RPM written on the front) but they are ofted quoted as '2400' etc. The 172R that I fly would need 2500 RPM continuous for the tach to keep up, which is too hard on the plane and engine as already said.

Many spamcans are quoted as 'cruising 110 kt' or something similar and again in my experience you'd do well to get that at any reasonable power setting in an old airframe. So in real life, it's best to find out what RPM your instructor cruises at, and just do the same. The result should be about 100kt because that makes estimating ETA's really easy!

[UV]

They all cruise at 2350 rpm...end of!

[Ultra long hauler]

To the Rotax 4 strokers out there:

What RPM do you cruise at when in a hurry?

###Ultra Long Hauler###

[Big Pistons Forever]

low power settings are efficient but result in long flight times. High power settings are less efficient but result in shorter flying times. For most piston airplanes 65% power seems to represent the best compromise between efficiency (cost) and speed (time).

The most effective way to reduce the cost of flying where you are paying for the fuel is to learn how to properly lean the engine.

[matthewlai]

Thanks! I see now. Didn't know it's bad for the engine, but that makes sense.

[matthewlai]

By the way, I fly a Champion Citabria (tailwheel!), and the instructor usually cruises at 2500rpm at around 75% power, and full power only gets another 100-200rpm, which I always found a little strange.

[peterh337]

The power absorbed by a prop is proportional to the 3rd power of the RPM, IIRC, which is why the extra 25% power gets you an increase of only 1/3 of 25% of RPM (very roughly).

[Pilot DAR]

Didn't know it's bad for the engine

Matthew,

I commend your asking a "basic" question, that's always good to do if in doubt. It does worry me a bit that you got this far into flying without being given some insight on this.

Another way to think of it is: Do you run full speed everywhere you go? Or do you walk most of the time, and just run when you have a short distance to cover quickly?

Anthing which moves will use much more fuel, and endure much more wear and tear, to go a little bit faster.

[Jan Olieslagers]

What RPM do you cruise at when in a hurry?

I always try to avoid hurries, not only when flying. As a pilot, I have succeeded very well up till now, which mainly goes to show how little flying I have yet done.
That said, I think it depends very much on how long the hurry is expected to take. I might cruise her at 4700-4800 rpm for 15 minutes, but not for an hour, I think. 80 hp Rotax 912UL, that is. If ever I do push the bird, I'll keep more than half an eye on the oil temperature.

[miroc]

The extra fuel going into the engine doesn't burn (there's not sufficient oxygen for that) but evaporates, which helps with engine cooling.The extra fuel does not cool the engine by evaporating. That extra fuel is there to slow down the speed of burning. The peak pressure event happens later when the piston is further away from top dead center and the volume of the burning mass is bigger. That causes lower temperature and pressure in the cylinder.

If you lean the mixture at this moment (full throttle or more than ca 75%), the speed of burning increases and the peak pressure event happens sooner. The piston is still close to the top dead center and the volume of the combustible mass is smaller. Pressures and temperatures are much higher.

This conditions (high pressure and temperature) can cause the explosion of still unburned mass. Explosion is not a continuous burning with a flame front, the chemical reaction is going to happen at once in all the still unburned volume. This is detonation.

Again. The cooling is not the result of evaporating fuel but slower speed of burning.

Regarding engine settings, if I am flying longer legs I like to go high. During climb I let the engine on full throttle, watching the EGTs and leaning gradually to keep EGTs the same as they were just after the take off.

In cruise I lean to -10 to -20 °F below peak on the lean side. If higher than ca 9000 ft I can let the throttle full open. If lower I throttle it down a bit, running lean of peak. I try to find a setting where the fuel flow is reading 7.2 to 7.8 gal/h what represents 60 to 65% power (if lean of peak only!) and the EGTs are more or less the same on all cylinders (-10 to -20 °F lean of peak). This 65% level or less is the recomended setting for long engine life.

If lean of peak the number of US gallons/hour burned multiplied by 15 gives the power output of the engine. You can calculate the corresponding fuel flows like this:
engine max rated power=180 HP, 60% power =108 HP, 108/15=7.2 USgal/h. This engine running lean of peak, sipping 7.2 USgal/h is giving 108HP. It's that easy. 7.8 is 65%, 8.4 is 70%....


Miroc

[Pilot DAR]

If lean of peak

While remembering that some Lycoming engines operating manuals prohibit operation lean of peak. That is an airwortiness limtation....

[miroc]

While remembering that some Lycoming engines operating manuals prohibit operation lean of peak. That is an airwortiness limtation.... I do not know if operating lean of peak would do harm to the airworthiness, but it is safe to do so from the engineer's point of view. I am not a lawyer...

Seriously, one has to know what one is doing when leaning an engine and that's the reason why it is prohibited in the POH. Of course the proper instrumentation like an engine monitor is absolutely necessary. And a fuel injected engine.

Miroc

[piperboy84]

So what is the correct translation of %power to RPM. For example my fixed pitch prop on the the lycoming 0-360 (rated for 180hp at 2700rpm) does not get more than 2500 rpm on take off, so do I assume that I am only getting 166.5 HP at take-off by doing the following calcs:

2500 rpm /2700 rpm = 0.925%

180HP X .925 = 166.5HP

If this is correct and I am calculating this the right way I wish I had bought myself a CS prop !!!

[BackPacker]

2500 rpm /2700 rpm = 0.925%

As Peter pointed out, the relation between RPM and thrust is not linear at all. If you disregard engine efficiency, propellor efficiency, parasite drag and a whole load of other minor factors, thrust is related to the third power of RPM. So in reality, at 2500 RPM, the engine will be producing less than 90% of its rated power.

But honestly, you really need to look in the "performance" section of the POH for the exact numbers. Because things like density altitude also have effects on the engine power output which might not be apparent from the RPM alone. (After all, if the air is less dense, it takes less power to spin the prop at 2700 RPM.)

If this is correct and I am calculating this the right way I wish I had bought myself a CS prop !!!

If your engine has less than about 200 HP and you're not too worried about extreme short field take-off performance, you'll find that a cruise-optimized fixed pitch prop is about as efficient as a CS prop, in the cruise. But fixed pitch props are less complicated in operation, and far less costly in maintenance/overhauls etc.

[piperboy84]

If your engine has less than about 200 HP and you're not too worried about extreme short field take-off performance, you'll find that a cruise-optimized fixed pitch prop is about as efficient as a CS prop, in the cruise. But fixed pitch props are less complicated in operation, and far less costly in maintenance/overhauls etc.

Yes I agree with you, and I have to admit my Maule is a great plane with great performance, I was just having a rant because i have been impressing the sh*t out of non pilot friends and family with my STOL capabilities (450ft TO + Land) from our farm strip. To date I have been the only aircraft to use it UNTIL a friend swanned his smarmy arse in and took off in 230 ft ( In a xwind !!) in his 235hp CS Maule, Now instead of the nods of admiration from dad and the nephew after each TO/landing, they ask if its possible for me to get close the what the "other guy managed". So I,m now faced with the choice of swallowing my pride and tell them "No, now piss off and get a life and stop watching me flying" or I suppose I could go buy a friggin Wilga.