Hello,

Can anyone say for sure whether RPM stays the same with elevation or whether it will decrease the higher you go? (mixture being accounted for and leaned at the higher altitude) I seem to recall you'd get max RPM on a Cessna 172 until around 6 or 7000 RPM and then it would decrease but then that is a 4 stroke engine as well.(and it's been many years) Has anyone flown at different elevations and observed a difference in max static RPM?

Thanks,

Chris

RPM is higher in Phoenix, 1000 ASL, than it is in Williams, 7000 ASL....
Motorcycles in Colorado, way higher than that, get pretty sluggish...Old time flathead V8 Fords worked better in Denver with "Denver" heads, higher compression...

Unless you have a turbo power and rpm decrease with an increase of altitude.

If the carb is re-tuned like it should, rpm should pretty much stay the same, or just slightly lower.
(Higher altitude, lean out carb. lower altitude, richen it up again)
Aerodynamic load reduces ~ the same amount as the air oxigen content. However, engine stays cooler and mechanical losses remain the same, so that will be a larger percentage of the lower power output at higher altitudes.

About 300 rpm between sealevel and 5500 ft ASL

Tried a G26 in Kingman, AZ, elevation about 4000 ASL....Same rpm as Williams, 7000 ASL..Haven't tried one in Phoenix....

Send me one of your G26's and I will test it for you.
Would I have to send it back?

Thanks very much for your comments!

Chris

My experience is that RPM remains roughly the same when you go higher, for a given prop, but the thrust is down. Figgers when you give it some thought.

CR

Terry, in which engine, and did you re-tune the engine?
300 rpm in a 150cc is a lot, in the 26cc it is of substantially less importance.

I live and fly at 3000' ASL but when I visit my club's field which is about 200' ASL I have to richen both my G23 and G20 H needles a bit. The first time around the engine ran up just as always and seemed okay, but just after take off died but luckly I got the model down without mishap.

Karol

Brison 2.4 (39cc) same engine, same prop, readjusted carb. Test runs were about a week apart.
Travel time ya know>>>>

Some thoughts, not directly related to the question at hand, that might be interesting to people who fly at higher elevations.

Flying in Mexico City (7,000ft + ASL) this is a subject that comes up quite a lot. A general rule of thumb is that we loose about 20% of power/thrust compared to SL flying. This obviously varies by engine, plane setup. So we set out to reduce weight by 20%, or use larger engines without increasing weight.
We also use more wing incidence and fly nose heavy to help penetration and use higher nitro levels for glow engines (advise from Quique Somanzini at an acrobatic clinic we held here a few years back).
The availability of lightweight gas engines has produced some interesting possibilities for high altitude flying. The typical SL set up at 16lbs for 50cc IMAC planes with a DA 50, for example, has been pretty disappointing. At 100cc OK but not great. So the challenge has been on to reduce weight and or find more power.
In am currently using a 52cc FTL in a 35cc size Aeroworks Extra 260 weighing 14lbs. The FTL is an inexpensive, light weight, engine and turns a Mejzlik 20x10 at 7,000 RPM. The CG is slightly nose heavy and the flight performance is great.
My next project will be an Extreme Flight 50cc Extra 260 (hopefully at 15lbs) with a RCGF 65cc engine that should turn a Mejklik 22x12 at around 6,800 RPM.
Other guys are using BME 100cc engines in 75cc sized IMAC planes, which produce very acceptable performance at about 20lbs.
Obviously I am referring to precision acrobatic (Pattern) setups, but in general all categories and sizes can benefit from these guidelines for altitude flying.

When I crunch the numbers for a 22x10 prop in my prop calculator I get as prop power demand:
at sea level 3171 Watts
at 7000 feet 2474 Watts

That is about in line with the engine power loss.


About leaning the needles, here is the fuel flow chart snatched from the continental manuals:

One can also advance the timing, if available, as the altitude goes up.

Is this because there will be less air to be compressed in the cylinder therefore greatly reeducing the chance of pre-ignition???

Well you CAN advance the timing a bit with altitude, but this practice actually comes from automotive circles. Back when you could set your own timing, it was common practice to add 1 degree of advance per 1000' of altitude above Sea Level, to the manufacturer's specs as it provided better performance and by that I mean better acceleration.

Cranking the timing too far forward and it'll make the engine hard to start as it fires way too early in relation to where the piston is at in it's stroke to the top. Ask any of the old Chevy guys what happens when the timing is too hot????

Then there is the issue with what we used to call "pinging" if the timing was too far advanced and that is actually detonation which is really hard on an engine.

For our purposes, there's not really much of an advantage where we run most of the time at a more or less constant RPM with gradual power changes. To some extent whether or not it might help kind of depends on the ignition. Is it a mag setup? an electronic? what does the advance curve look like? and so on.

In my own experimenting here a 4500'ASL, I don't find there is any advantage to ground RPM readings on my stuff with electronic ignition modules but there does seem to be a bit more heat generated if I check those temps on landing each time I play with the timing. Now these are just anecdotal observations with no specific attempt to show a direct correlation so whether or not it's actually true I can't say. I can say that I don't see any performance increase by advancing the timing, so I leave mine where the manufacturer specs them.

JMHO, YMMV....[8D]

Oh, for what it's worth you don't set the timing up for full scale stuff. If a Continental or Lycoming manual calls for 24 degrees of advance, that's where you set 'em and that's for magneto equiped engines which have either a separate point set or impulse couplings on the mags to retard the timing for starting.

Gosh, I live and fly at 6000'...............

But I wouldn't want anybody to think I was using my "moderator status" to make a point, so I'll just keep my RPM/performance numbers to myself.

Ah come now Mr Moderator we would never think that of you, so how about sharing your experiences with us like we do constantly with you

Karol

Never stopped you in the past.......

CR

FWIW...When we were using ignition on our racing 289 the timing was originally set at 28....
When I experimented with different timing and set it at 22, it ran the same, but cooler...
We left it at 22....

Engines lose about 2 to 3% power per 1000' of altitude. But what is actually important is what is called "Density Altitude". This is true, or actual altitude corrected for temperature and barometric pressure differences, and is a better indicator of air density and consequently the effect it has on engine power.

Higher altitude, higher temperatures, lower barometric pressure, and high humidity all LOWER air density and LOWER engine performance. In all full size aircraft operating manuals, the actual power percentage loss is stated. Due to both the loss of power AND the loss of effectiveness of the propeller and wings due to the thinner air, a take off at about a 7000' density altitude will be twice as long, and the airplane will climb half as fast.

Short of supercharging, the only help is higher static compression ratio, more nitro, or more cubic inches... Even these things will not improve the losses suffered by the prop and wings however.

AV8TOR

Well I was going to mention density altitude, but that sometimes blurs the discussion for folks...

But yeah it makes a HUGE difference when you figure in especially ambient temperatures. I remember the first time I suggested trying to figure density altitude to my buddy who owned an NHRA car. I explained what the deal was and although he was initially unconvinced, I got an old altimeter from the airport so we could get the pressure altitude and we used an inexpensive set of household type guages for temps and humidity. Then we started figuring the altitudes and setup the engine jetting and timing for that figure as opposed to the actual track elevation. When combined with the data we already had on altitiude changes, it worked great. At that time everyone in the pits made fun of us, now you won't find a good racing team anywhere who doesn't do it.

For any of you who might be interested, full scale stuff is rated at Sea Level, 59 degrees F and 29.92 on the barometer. That applies for both the engine and airframe since as AV8TOR mentioned, density alitutde affects the prop and wing performance as well. Any deviation from those numbers and aircraft performance will change.

Ralph, thanks for the tidbit on temps and retarding the timing, gonna have to play with that a bit more....[8D]

in regards to the temps dropping when retarding the timing, the coolant temp decreases, but the exhaust temp increases, works the same in reverse, if you advance the timing it'll move the heat to the coolant from the exhaust. with gasoline you can counteract the hot exhaust with a bit more fuel, but that doesn't work when you work with gaseous fuels (LP and natural gas)

So where does vapor pressure and engine heat rejection fit in to all of this data? Capt,n[X(]