High Altitude Flying

[UWBob]

Ok all you guys in the high country. I have a question…yes again…
I just moved from the beach in good ol’ Florida to a cold, dry, high place called Denver.

Now being from, well almost UP to sea level to a “mile” high, what is that going to do to my aircraft? I have GWS: Slow Stick with the bigger prop on it, Tiger Moth mostly stock, I’m in the process of building a P-51, and last but not least a Zagi 400 not “X”. All my planes flew great in FL, except the P-51 (not built). Is there any thing I need to know or do to get them to fly as good here? I am a pilot, and in a “real” plane and it makes a HUGE difference, your take off is longer and faster, well every thing has to be faster (ground speed), the payload is less, and on a avgas engine there is less power. So with that said I’m going to assume the same thing happens to my “fun” planes, right? Or is it just not as noticeable? Will I have to change props? If so to what? And what about the over-all power? Will the stock motors still work, or do I have to use a higher voltage pack, which will add weight, or cut into my fly time? Do any of you fly around Denver? I live in Englewood, and I don’t know the area at all. Well I guess that’s about it for now… Thanks all!
Bob

[Ollie]

The electric motors won't have a breathing problem like internal combustion engines do. However, since the air is less dense at 5000 feet than at sealevel, it will take a bigger prop in Denver to load the motor for ther same power output and RPM as at sealevel (assuming the same temperature).

The plane will have to fly faster at a given lift coefficient to generate the same lift force it did at sea level (for the same tempersture). For an on-line density altitude calculator, see:
http://wahiduddin.net/calc/calc_da.htm

On cold days in Denver compared with warm days in Florida the increase in density due to colder air will tend to offset the effect of the altitude difference. On very warm days in Denver, the plane will have to fly noticably faster. A 40 degree C lower temperature will have density effect roughly offsetting a 5000 foot increase in altitude.

[RCheroske]

Here's what I do:

When figuring what to power the plane with, I usually fudge the weight of the plane by about 25%. i.e. if I have a 10 oz plane, I'll use 12.5 ozs in my calculations. ( A 10 oz plane that floats at sea level flys more like a lead sled up here)

Electric motors are not affected by altitude, But like Ollie says you might want to prop up one or two sizes.

If you're using a gas engine then I would go up one or two sizes. i.e. a .40 plane will fly fly better with a .50 or.60.

[Dick Huang]

Bob,
You could use Moto-Cal and compute the difference in performance at sea level and 5280 feet altitude. It's free for 30 days.
Dick Huang:D

[vintage1]

I just checked this out on a simple speed 400 geared model, and motocalc gave virtually the same results - apart from a higher stall speed - by going from a 9x6 to a 9x7 prop for example.

So the general rule - add an inch of pitch - seems to hold good.

You may also find the models fly faster - the stall speed was up by a mile per hour, but then drag is down as well, so it all evens out.

Only heavy higg wing loading models should be badly affected.

[BEC]

OK - It's really easy...... add pitch (or go to a lower numerical gear ratio). For Denver, add about 20% pitch.

Having lived at sea level for many years, but also having family in the Denver area (until recently - my late grandfather was a well known community figure in Englewood) and even higher in Santa Fe, NM, I have done quite a bit of empirical testing. Here is the Ezone article I wrote about it:

http://www.ezonemag.com/articles/2001/aug/altitude/altitude.shtml

For a different perspective on the problem, drop in to Park Flyers and ask Doug Binder (CoClimber) what he thinks. He designs and kits models (Mountain Models) in Colorado Springs and always remarks about how lucky we are to have thick air at sea level.....

[KeithK]

Bob,

Did you see this link? http://www.rcgroups.com/forums/showthread.php?s=&threadid=97540

bunch of "seasoned" high altitude flyers will be in your area this weekend.

Keith