I get a question that seems to be on a number of minds.
Basically it is this : Does the air density really affect the engine output as much as it affects the prop load?
I tried to clean up the question to get a more accurate answer.
Consider an internal combustion -engine on non oxygen bearing fuel-
it also has full internal compenstating temperature and mixture controls -so altitude density does not affect the engines peaked output -except for the density of the air going into the inlet.
Also consider a propellor on this engine - which has NO flywheel effect - being simply a paddle blade shaped device , which loads the engine strictly by the resistance offered by the air density.
It does not pull -it simply beats the air.
Now - lets start with a standard of STP and say that the engine turns the fan at 6000 rpm.
the engine is right on the center of it's max torque .
Now let 's put this contraption in a BIG ballon gondola and slowly ascend to say 10, 000 ft.
All the while - lets record the engine rpm.
does the rpm go up or down?
How much?
is it a straight line curve?
do we have enough info to really know what is happening?

I've read that in the '60s, the old Formula 1 race cars wouldn't go as fast on a long straighaway at altitude (like Mexico City) as they would at sea level. In that case, at least, it seems that the loss of horsepower was greater than the savings in aerodynamic drag. Don't know if there's a direct comparison here, but it's a start on this little thought problem, lol

On that note, dragstrips times are "corrected" for altitude in most bike and car magazines.

Also jetting requirements changen drasticly for our ultralight engines if the elevation changes from where we take off to where we land.


Jetts

OK two responses - but not to the question -
basically, does the load change with or in porportion to the power available?
Come on all you retired wind tunnel guys --

You may want to read (or you may not want to !) this:

http://naca.larc.nasa.gov/reports/1929/naca-report-295/

It's a very old NACA report. I think that IC engines still operate in the same manner back then as they do today though. Specifically, in the conclusion section, on page 10, they show a BHP ratio of about .70 at 10k feet (in table 1), with BHP reduction almost linear in our altitude regime.

Density of air at 10kft = 89% of sea level density, assuming STP. So propeller thrust should go down accordingly, given the same 6000 rpm.

I only gleaned over the report, but what I got was yes, the propeller would be putting out less thrust according to density change. Engine power output would be much less than 89% though. Without looking much at the numbers, I would predict RPMs would still go down from your 6000 rpm at sea level.

I guess thats why superchargers are used for much higher operations. Maybe I'll actually read the report later!

Your question would seem to be quite a tricky one at first reading.

I'm no expert, but it seems to me that we should consider your question " at its extreme".

If, instead of ascending to a mere 10.000 feet, we ascended to say, 100,000 feet, what would we expect?

I would guess that, the air being so thin, that motor would be wheezing out its very last gasps. Where each cylinder-full of air was weighing only a tiny fraction of its sea-level weight, the motor would be able to burn only a very small amount of fuel. Yes, the prop would be offering little resistance, but the engines internal losses would remain constant, so the power available for useful work would be less than was found at sea-level.


If I remember correctly, Piper specify for their PA-28 that, at 6000 feet, the engine is capable of producing only 75% of its rated output. This engine is fitted with a mixture control, but is normally-aspirated.

Springer

Springer - I agree with you - -and used the same approach to the question- take it to it's illogical extreme.
The puzzle tho - is what is the relationship.
What if-----
we put the engine air intake into a magic box - which held a constant STP as we acsended -
The engine would -of course - pick up speed as we went up . (load decreases)
but how much per thousand ft?What kind of an rpm increase curve would we get?
again -what if------ we let the engine breath normally the air as it thinned but put the "fan load " in a magic box -which retained the same air density .
the engine would again loose rpm -- but what would this loss curve look like.
I guess this all sounds silly to those with aero engineering degrees - but I really would like to know the answers .
The why is:
I get plenty of flak from "experts" who doubt my rpm readings on given engine setups - because I run the engines at 4500 ft elevation.
The claim is that I get higher readings because the air is thinner - plus -the use of a tuned exhaust is of greater advantage at higher altitude.
Frankly, I think my readings would be about the same -if I hauled the whole test setup to sea level.

Further - I think my readings would increase.
Maybe I should stick to easier questions ---

Way back when I was taught that with altitude and internal combustion piston motors with superchargers, the power available to the prop drops off with density... but the power required to run the motor and the blower stays the same. When power required to run at all is all the motor can put out, that's the altitude limit, and for such motors, it's around 56,000 feet.
.
Changing from gasoline to a oxygen bearing fuel such as hydrazine, and changing the motor lets you run the thing higher.
I believe NASA has a CO2 type hydrazine motor that runs up to or close to 100,000 feet.

i suppose it could affect it a little bit with the pressure of the air that goes into the carb

Cm on guys -I will even accept formulas --

"Cm on guys -I will even accept formulas --"
.
Zey are eqVations!... Chemists work with forumulas!!!
.
Ok,
"Without supercharger, or with a sea-level supercharger, the indicated horspower is approximately proportional to the density of the in which the engine is flying and hence drops off markedly with increasing altitude. Since the friction and blower power does not drop in proportion to the air density (sigma) the reduction in power with altitude is accentuated. For an airplane engine in which the friction torque Qf is 13 percent of the sea-level torque Qo (which is typical), the variation of full-throttle torque with altitude is given by...
Qalt/Qo=1.13sigma -0.13"
*
"Technical Aerodynamics, K.D. Wood, 3rd edition, 1955

O.K., here's your answer, in easy to understand terms- yes, only more so.

Jetts

Dick, if you get me the magic box I'll try it and let you know. ;-)

What if you took an elctric motor driving a prop at sea level and again at your altitude, would that not answer your question. Pure DC motor would show more RPM's under the least load.
Ed M.

Ah luves them thar formulas!
I don't see any one dis agreeing with my original thought -which was - that the power input logically, decreases faster than the load on the prop .
I will therefor, tell my detractors who doubt my rpm readings - to kindly go p*ss up a rope

Dick,
Dave here.

In following the thread and in applying my full scale experiences here is my two-bits worth:

Horsepower goes down with altitude increases.
Horsepower goes down with barometric pressure decreases.
Horsepower goes down with lower humidty.
Horsepower goes down with temperature increases.

IE. Cold, Wet, low altitudes produce maximum HP.

Airplanes (non-turbo) fly FASTER at elevation due to cooler air and less air friction. They also turn the fixed pitch prop about he same RPM or perhaps a bit slower.

Airliners can not do Mach .78 at 2000 feet elevation, just too much friction on the fuselage. Drag increases non-linearly with increases in velocity.

Bottom line: RPMS are only one indicator of pwer. The better one is thrust delivered (read effective HP).

Have a great day.

Here's my understanding of a simplified explanation of power lapse.

Normally aspirated engines have a power decrease that is roughly linear with the density ratio sigma, where sigma is the ratio of the density at the current altitude to the density at sea level.

So at a given altiude, the total power available is roughly sigma*P0, where P0 is the sea level total power. This total power includes the power "wasted" in keeping the engine running, i.e overcoming bearing friction, running fans and accesories, etc.

Lets say for the sake of argument that a .40 engine puts out 1HP, and uses 0.15HP to keep going. So the total power is 1.15HP

So at 10,000 ft, where the density is 95% of that at SL, the total power available is 1.15*.95=1.09HP. Subtract off the no-load power of .15 HP, and we have a power available of .94 hp, for a 6% loss in power.

At higher altitudes, this becomes more and more of a problem, until eventually, the engine can produce no useful power because the air available only provides enough power to keep it ticking over. When this altitude is exceeded, the engine quits.

Starfire- I agree except that at 10,000 ft, density is a bit less than 95% -
I think that ours at 4500 ft is down aprox 13% .
did I miss something ?
I appreciate the comments on operating losses -
I plain ol forgot about the constant fixed losses!
That is a very quick and efffective rebuttal to the BS about higher readings at higher altitudes.

You're right - I was looking at the wrong column in my standard atmosphere table - looked at viscosity - what good is that? Density at 10000 ft is 74% of SL, giving total power as 1.15*.74=0.85HP, giving a shaft power available of 0.7hp, for a 30% loss.

However, RPM's may in fact increase with altitude, as it akes less torque to turn the prop at a given speed in thinner air. If that torque falls off faster than available power, then rpm would have to increase to compensate. I'm not sure of the realtionship or torque to density, however - will have to consult Mr. McCormick.

BTW, for an all-around excellent discussion of basics of aerodynamics, performance, and basic S+C, that's a great book:

Aerodynamics, Aeronautics, and Flight Mechanics, by Barnes W. McCormick.

After a quick review of propeller theory, it appears that a given propeller/engine combo should in fact turn about the same RPMs as density falls off, provided that the shaft losses are a similar percentage of total power. This is because as long as we stay at moderate reynolds number and mach number, torque required for a given RPM is roughly linear with density.


However, because the shaft losses stay the same at all altitudes, the available power is actually decreasing faster than the density, while the torque is falling off with density, which should result in a prop spinning slower at higher altitudes, if anything.

thanks again - I think I have the concept in grasp now
which is: basically - if there were no fixed losses in the powerplant - the load and the developed power would rise and fall -pretty much the same -- the constant losses in the engine are the "gotcha ".
Close enough for folk music?

Yup, as far as RPMs are concerned.

The following is just further clarification:

But the fact remains that the power that that RPM is putting out has been decreased, resulting in weaker climbs, etc. Also, the plane has to fly faster to achieve the same CL, say the best lift to drag condition.

The faster speed can be taken care of by increasing pitch, which is why that's often recommended. There's no drag cost for this, since drag is decreased just as lift was. But there's nothing than can be done about the available power without changing the displacement of the engine or adding some sort of intake compression (turbocharging, turbocompounding, or supercharging).

Just wanted to make clear that the same RPM at different densities is not the same amount of power.

That was always understood .

Yeah, I figured you were operating from that knowledge.

I've just seen so many modelers who think that RPM is all that matters, so I just had to throw in a quick disclaimer.

Happy flying!

If you ever do haul the test rig to sea level, please share the results...