makes it more complicated

heat rejection is generally calculated after an engine is calibrated and set up, helps you size a radiator...

Now there is a ranger that knows his onions Best Regards Capt,n

I probably shouldn't mention "True Airspeed" and how THAT relates to all this as well. But suffice to say, when flying a model airplane at a high density altitude, and then later flying at a low density altitude, one can almost "feel" the difference. It is like you are flying against a "cushion" of air at the lower altitudes, and when you reduce the power, it is like you "put on the brakes!" VERY noticeable difference! Same thing in full size planes....

It is because airpeed is, if you will, a "quantity of air molecules hitting the airplane". In thin air, the airplane has to go faster across the ground to "feel" the same number of molecules that it does in thicker air and hence indicate the same airspeed. Therefore, an indicated airspeed of "x" miles per hour is a much higher speed across the ground at higher density altitudes than that at lower. It quickly makes short airstrips really short at high density altitudes, and longer ones "just right" because what is important in take offs and landings as far as distance needed is your actual groundspeed.

This all of course does not take into account the effect of wind, which changes groundspeed as well...

AV8TOR

I calibrate and test engines for a living, I'm fairly new, but trying to learn, I still have several things backwards and only know a couple engines and systems, so I still mainly listen... and putting a turbo engine in my ford ranger( where the ranger in my name comes from, the rest of it is random) in the near future, double the HP is always good...

Pressure drop is roughly 30ft per millibar AMSL. At 3000', pressure drop is 100mb which is substantial. The charge density to the combustion chamber reduces with altitude resulting in power loss. This is offset by the resulting drop in induced drag on the prop. Prop design and diameter would have an effect on the resultant rpm at altitude. All things being equal, there is normally no vast difference in rpms, just significant loss of thrust.
Flew single engine planes and never noticed a significant drop in rpm at altitude once the engine has been leaned out at with rising altitude.

Well, try a takeoff in a normally aspirated light airplane at sea level sometime, note the performance, then go land at a high altitude airport like Leadville, Colorado. (9927 feet) Then try to take off again. You better have a powerful airplane, or a lightly loaded one, or you never will take off! (Or once you do, you won't be able to climb out of ground effect!)

Lots of pilots have crashed because they didn't understand density altitude and it's effect on engines, and airplanes in general. In fact, the detriment to performance is mostly due to the engine losing power with altitude. With a turbocharger you regain a good percentage of your performance at high altitudes, that is lost with normally aspirated engines.

Full scale airplanes normally cruise at 75% power. With a fixed pitch prop, this is a certain rpm, depending on the application. As you go higher and higher, you soon reach a point where full throttle only gives you 75% power. Depending on temperature, this point is usually reached around 7000 feet or so. At any atitude above that, the engine just keeps on losing power and you can't achieve 75% power, even with full throttle, mixture leaned, etc.

A drop of a couple of hundred rpms with a model engine swinging a 16 to 24" prop over 6000 rpms is a good bit of power lost.

AV8TOR

Agreed, most planes are rated to max t/o and ldg altitude due to expected performance shortfall in engine thrust and lift.Its called WAT. However, when comparing rpm figures its rather close wrt msl and at altitude.
What happens is a significant drop in thrust available even as the rpm remains the same.The engine cannot produce its flat rated power band above a specified altitude and temp.

Nice to see a post that relates to the subject of the thread. We've had plenty of thread drift.

CR

Ahhh, but hasn't it been fun even with a few trick questions added in along the way????

Some of us might have gotten the impression that the OP was really wondering about power output changes at higher altitudes when he asked about RPM changes.

I think it's been covered really well, but if all the OP was really interested in is the RPM numbers, that's been answered too along with the why....[8D]