| If the prop you are using provides good performance at lower altitudes | I would not increase the pitch. Planes tend to have the most | satisfying performance when speed set by rpm times the pitch is | roughly equal to realized air speeds.
I think the (rpm * pitch) needs to be slightly higher, actually. (But you may be looking at ground rpm rather than the faster unwound rpm in flight.)
| At high altitude you will need a larger diameter prop. Depending on | how marginal the whole system is it might even need less pitch and | even more diameter.
If so, then it would have benefited from that down lower too.
| I would only go to three blades if forced to by lack of ground | clearance and even then doubt if it will solve the problem.
Agreed. As a rule of thumb, the more blades your prop has, the less efficient it is. Some FF guys even use one-blade props. | More rpm may well help.
Alas, you can't just buy RPM at the hobby store. RPM comes with a smaller prop, or more volts, a different motor or a different gear ratio. It's easier to just put on a prop with a higher pitch.
| With any changes make sure your heat dissipation is still adequate or | the motor will cook.
I suspect that the effective cooling the motor will get will actually remain the same at altitude (assuming the same prop), as the plane will have to fly faster, which will move the less dense air across the motor faster.
However, you need more power in less dense air, so the motor will generate more heat. So yes, you've got to be wary of that.
| You really are not all that high so it should be fixable.
Depends on the plane and power system. If it was marginal down lower, it might not work at all up here without changes.
| You say if floats ok so total lift does not sound like it is a | problem. | | If you are not getting much air speed and still are able to control | the plane the control surface areas are ok.
It usually boils down to power system. As an extreme example, if you've got half the air density (which corresponds to roughly 18,000 feet), then you'll need to fly 41% faster to get the same lift and I think you'll need twice as much power to maintain altitude (but you're going twice as fast, so your `mpg' doesn't change.) However, your plane will still be just as maneuverable as it was down lower -- you can still do a 180 degree turn in X seconds -- but you'll need more space to do it, because you're going faster. The added speed cancels out the lowered air density.
As for the `power needed', I'm ignoring effects like engines and jets being more efficient at certain ranges and pressures and such. It really gets to be a whole lot more complicated.
talk about the additional factors somewhat if you want to read it, but they don't really matter much to us.)
Fortunately, the original poster doesn't live at 18k feet, so things aren't quite so bad.