[NSRCA-discussion] Brio 10 & high altitude

[DaveL322 at comcast.net]

APC makes many different props sizes!!!  Only the odd-balls need a size APC doesn't make <G>

With the electric motor, KV is a big factor and I don't think it is measurably effected by altitude, unless the difference in air density in the gap between the stators and magnets if of consequence (doubtful).  The motor "wants" to turn a given RPM for a given input voltage.   The amps will go up as the prop load increases.  Assuming the motor is running in the "sweet" spot, the RPM probably won't increase much as a result of change in altitude.  To generate similar power at altitude, I think increasing the prop size to match watts at sea level would be a pretty effective method.

Dave

-------------- Original message -------------- 
From: Bob Richards <bob at toprudder.com> 

Jeff,

I don't really have any direct knowledge, but I will speculate on what I know from flying full-scale planes.

The electric motor power will not change due to thinner air. However, the same size prop will spin faster due to the less dense air, which will let the motor unload more, which will result in less HP at the prop shaft. This will result in less performance.

At the same time, the plane will have a higher stall speed since it will have to fly faster to produce the same lift in the thinner air.

You will have to increase the prop size to maintain the same load on the motor. You could go up in diameter, or up in pitch, or a combination of both. My gut feeling is that you would need to go up at least one notch in pitch, since the plane will be flying faster. You might need to go up in diameter as well.

Does Dave L repitch E props? :-)

Oh, one other thing, if the outside temp is the same as when you fly at sea level, you will need to pay more attention to cooling air flow around (through) the motor and batteries. The less dense air will not cool as effectively.

Bob R.


Jeff Hill <jh102649 at speakeasy.net> wrote:
I'm getting ready to buy the motor, speed control and props for the 
Brio 10. Any ideas out there about which prop and motor combination 
will work best from 5000 to 7000 ft?

So far, I've reasoned that the motor will have the same power. So, in 
the less-dense air where I fly the rpms should go up or the motor 
should unload. However, it takes the same thrust to pull the Brio 
through its maneuvers unless airframe and lift drag decrease. That's 
as far as I've gotten.

Ideas please, thanks
Jeff Hill 
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From: Bob Richards <bob at toprudder.com>
Subject: Re: [NSRCA-discussion] Brio 10 & high altitude
Date: Tue, 23 May 2006 14:15:25 +0000
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