Sport_Pilot

Some here have been arguing that our engines will maintain the same revs at altitude. But for some reason the full scale aircraft won't.



Read the last line of this link.

http://www.lycoming.textron.com/main...rectDrive.html


While the following two power charts are for constant speed pitched propellers, it does show that you must decrease pitch of the propeller to maintain RPM at altitude. The See Bee cannot maintain the RPM at lower altitudes, but the turbo engine can, but only by keeping the manifold pressure constant.

http://www.republicseabee.com/Files/...operations.pdf

http://www.aeromods.com/performance.htm

I could go into more detail, but I AM SURE many will debate this to death, and so by keeping it short I may save lives.

flyinrain

My Feedback: (25)

I've seen a number of posts here over the years with folks stating they don't get as much power at higher altitudes

Jim Thomerson

Why would anyone who does not have a big jug of nitromethane disagree with you?[8D]

Sport_Pilot

I think the arguement is that although the power is less, because the prop is easier to turn the RPM stays the same. But the prop resistance goes down by the square of the props surface against the thinner air, but engine power goes down by the cube of its displacement of the thinner air.

Motorboy

Thinner air = less oxygen and density in altitude..

Jens Eirik

rcdude7

My Feedback: (264)

I think you are right there. I just don't think some folks relise that despite turning the same R's as the guy at sealevel, they are experiencing decreased performance. The fish scale method of measuring static thust would show the difference, all else being equal of course.

Sport_Pilot

I said that was the arguement, I didn't say it was my arguement. I say that the engine and prop combo really does lose RPM at altitude, but that power is reduced even more than it may appear. Look at the full scale engines. They can easily go over their redline at lower altitudes. But with enough altitude they can't go past the redline with the throttle firewalled.

rcdude7

My Feedback: (264)

With increasing altitude comes a power loss. Without the help of forced induction or nitous oxide, it comes on that much quicker. Even turbine engines will reach a point where they will no longer run at extreme altitude.
Whether prop speed decreases or not, performance will always decay so what does it matter?

sscherin

My Feedback: (1)

Hmm mayby that is why OS put this in it's tuning guide.

http://manuals.hobbico.com/osm/pre-2...structions.pdf

I believe it.. None of my engines will turn the same RPM at 6,100ft here in Colorado that they did in Washington at 800ft.

same motors, mufflers ect.

Maybe this summer I'll take one up Pikes peak. Run tests at 8k and 14k ft

rcdude7

My Feedback: (264)

Would you fly your model at 14K ASL? That would be interesting. I wonder how sluggish it would perform at that alt. both the engine and flight controls.

Rcpilot

My Feedback: (78)

I learned to fly at 6000' altitude. I have flown at 10,410' altitude.

I used to argue this too, but I can now say with confidence:

I've got the experience. I've done it at high altitude.

My engines usually turn the same RPM or maybe just a teeny teeny bit less than the same engine at sea level. My Zenoah G-62 engines will all turn a 22-10 prop in the 7100--7400RPM range--depending on the manufacturer of the prop.

Those same props on those same engines at sea level will turn the same RPM or maybe 100 RPM more. NOT 1,000 RPM or 1,500 RPM.

The difference between high altitude and low altitude is the actual THRUST produced. I'm losing about 15%--20% thrust up here. Same prop. Same RPM. But the air is a lot thinner and the actual thrust I get is significantly less than a guy at sea level. Thats why we use bigger engines up here. It's because that bigger engine will turn a bigger prop. The bigger prop can help to deliver more thrust. If a guy gets enough engine/prop on a plane at this altitude--we can almost get the same thrust as you guys at sea level would with an engine 15% smaller in displacement. You fly an LT-40 with a .46 and a 10-6 or 11-6 prop. You get X number of pounds of thrust out of that combination. We fly an LT-40 with a .60 and a 12-6 prop and we get X+15% thrust out of that prop/engine. It just about makes up for the loss of thrust.

wiz310

Although its true that there is a loss of thrust with altitude, it hardly effects the engines at the altitudes we operate at.
you will see a bigger loss with the temperature change than altitude change at the altitude differences we see.
I anyone is interested in the difference, I will put a couple of examples of density height (pressure height corrected for temperature) up and pressure height up here.
end result of all this is that the difference between sea level and say, three or four thousand feet, will hardly be noticeable for our purposes and hardly worth thinking about.
that is a confused statement. The purpose of a CSU (constant speed unit) is to set your RPM with the propeller control, and the propeller will maintain that RPM......regardless of what power is being provided by the engine. except of course where the Manifold pressure has been reduced manually to a point where the engine will not provide enough thrust to maintain RPM or you reach a height that the engine cannot provide enough thrust to maintain RPM....... this is called service ceiling for the aircraft. but under normal conditions and within the envelope that the aircraft is designed to operate at, (usually between sea level and 20K for normally aspirated engines) the CSU will maintain engine RPM at whatever the control has set.

pe reivers

Good statement RCpilet. The small difference is due to the mechanical friction remaining ~ the same, whilst the engine looses power at altitude. The power to turn the prop and the internal engine power remain in balance, but at elevated altitude, the friction losses are at a higher %level than at sea level.

lucien

I just to test all this, sorta, last week when I finally took my plane around the patch after moving it from sea level (700' in central TX) to 7000' (Santa Fe, NM).
The plane is a larger gasser, motor is a Rotax 503 2cyl 2 stroke, prop is a 68" 3 blade on a 3.47:1 reduction.

I had to lean the motor out a LOT on the high-speed to get the EGT's about where they were at sea level (around 1000F).

The overall performance loss is pretty dramatic, I believe by the numbers it's 70 to 75% of the power at sea level. That'd be less than 40 hp out of my 50hp motor .
But the loss was very noticeable, I went a looong ways down the runway before the stick and pedals were firm enough for me to go ahead and liftoff. Climbout was significantly less than at sea level....

What was interesting was my full throttle RPM ended up being the same as it was when at sea level - about 6350 motor rpm (about 1830 prop rpm). I guess the power loss of the motor was evened out by the easier turning of the prop through the thinner air in this particular case. I was surprised since I was expecting to have to tweak my pitch a bit but turned out not to have to.

I should mention there was a temp/humidity difference. When I flew in TX the temps were in the 80's with lots of humidity, last week the temp was around freezing but with fairly low humidity. I don't know what the density altitude was at the time, but I'd imagine it was close to 7000' due to the temps/humidity.

So basically, the overall performance is significantly less at the higher altitude, a lot of leaning of the high speed mixture was needed to get a correct EGT, and the prop rpm ended up being about the same.

I need to fly longer to see what my typical cruise rpm's to maintain altitude are. I expect that to go up a little bit, but I'll just have to see next time I get some flyable weather at the same time as I have time....

LS

Cyclic Hardover

My Feedback: (3)

I'm at 5000ft here in Albuquerque. Yes there is a healthy overall power loss. Approximately 17%. I calulated one time aroun 3% loss for every 1000ft. Then add to that the DA in the summer time. I have a handful of guys around the country with nearly if not identical setups which confirm most of this. To make it a simple theory, My Jett 90 gets 12600 on a 12/6 and Dub gets in the 14's down in Texas.

Even Flying in the Army all those years. Alaska was really good. During the winter months , the power charts would tell us we coupl d carry much more than the total gross weight of the aircraft. During the summer time around here, you don't even think about carrying a full crew.

During the summer, our field gets pretty busy early in the morning and by noon , the planes get sluggish.

pe reivers

This is one more vote against sport pilots' statement that engines loose <rpm> at altitude. The power loss is due to the lesser density of the air, which BTW also means a lower effective compression ratio, resulting in lower start of combustion pressure.
For model pilots, it is "important" to compare rpm, so the engine and muffler/prop combination can be checked against data received from others. It is a very fortunate fact that rpm values hardly change with altitude, so rpm charts will need no change.
I adapted [link=http://www.mvvs.nl/prop-power-calculator.xls]My calculator[/link](http://www.mvvs.nl/prop-power-calculator.xls )for altitude, and it reflects the power loss which indeed can ve very serious. I have not yet incorporated near-constant mechanical losses, and compression ratio effect losses.
In a plane, there is less power at altitude, and the plane indicated air speed must be higher to lift all that mass off the ground. This has a serious effect on runway length.

wcmorrison

My Feedback: (2)

Well, gee guys, ever heard of pressure density. Normally asperated engines will not generate as much power. Props will turn fasted because of the reduced density. Ergo use a higher pitched prop, fatten up the fuel flow and expect lower power.

Now if you put a super charger or a turbo charger on the apseration system (that's carb for the rest of you), you would still maintain the power but the air density will lead to a higher pitched prop to get the same amount of thrust.

Basically two problems both related to the pressure altitude. As it goes up in altitude (lower pressure) we lose power and it allows props to turn faster but less effective, ergo higher pitch and less power. Temperature also effects pressure density. So at lower altitudes we can have very high pressure densities on a hot day (such as in Phoenix in the summer time).

It also effects our cars, diesel cars are effected the most unless they have a turbo charger. Gassers too, they just don't have as much power though with modern ignition systems controled by computer chips, we do not seem to experience engine knock and other things because the system compensates for that very rapidly.

Cheers,

Chip

pe reivers

Is that based on experience?
If you add pitch, you will load down your engine too much. Please think before you make statements.
We do not use car engines or diesels, just ordinary two strokes that run on gas. They dont have turbo blowers, but are normally aspirated.

Semi Retired Aviator

This is a for information post only.

A jet engine runs at about 25% efficiency at 40,000', i.e., it is producing only 25% of the thrust produced at sea level, but the aircraft is travelling at about 200 mph faster at 40,000 than it is at sea level. That's the positive effect that reduced air density has on the drag on the airframe. The opposite applies to the thrust produced by the engine.

I'm not sure there would be a perceptible difference flying a model engine at sea level or 1000 or so feet above sea level, but there would be if you were flying it at, say, 5000' above sea level.

Kweasel

My Feedback: (29)

All of the fixed pitch GA airplanes I have flown tach a little higher at 8000ft than they do at 1000ft.

Cyclic Hardover

My Feedback: (3)

The thing with us is that our planes always fly better when we go some place else! I love the way a thread like this takes off. it's like everybody's brain cells playing "bumper cars." It's ^%$#& great!

Cyclic Hardover

My Feedback: (3)

Heres on out of left field for one of you smart boys. Is there an altitude where the temperature remains pretty much a constant? I really don't know but just curious when someone mentioned a jet at 40000

Semi Retired Aviator

Yep Cyclic, there is, and it's about -57 degrees celsius. I can get a little less in high latitudes, but from about 40 degrees north through to 40 degrees south, that's about the minimum you see, and it occurs from about FL360 (36,000 feet) and above in what is known as the standard atmosphere which is +15 degrees C at sea level, and assumes a lapse rate of 2 degrees C per thousand feet. +15 degrees to -57 degrees = 72 degrees, divided by 2 degrees (per thousand feet) gives 36,000' but it does vary a little, and one of the variables is surface temperature where the lapse rate starts from.

You sound like a helicopter pilot with a name like Cyclic Hardover??

Sport_Pilot

Wrong! You can go past redline flying level at or near sea level. You have to throttle back to about 3/4 throttle to prevent going over the redline. At some point you can firewall (or is it instument paneled?) the throttle and not go over the red line. Apparently you need to re read the tech links I posted.

pe reivers

Are you realy reading your own quoted web material? It shows that rpm stay the same when going from sea level to about 7000 feet.
Let me recollect the charts where 100% power is specified, for better comparison:
I separated the data with ";"

Climb Data* Altitude
MSL; Engine RPM; Manifold Pressure; % Power; KIAS; KTAS; ROC ; FPM
SL; 2575; 43; 100; 100; 100; 1,350
4,000; 2575; 40; 100; 100; 104; 1,250 (even with lower manifold pressure!)

The plane climbs slower @ altitude, but rpm stay the same

Cruise Data*Altitude
MSL; Engine RPM; Manifold Pressure; %Power; KIAS; KTAS; MPH; TAS
1,500; 2575; 43 100; 160; 162; 186
7,000; 2575; 43; 100; 155; 165; 190

The plane is slightly faster @ Altitude, but rpm stay the same