operating from altitude

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StRaNgEdAyS

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Joined
Oct 20, 2003
Messages
816
Location
Northern NSW Australia
There is a bit of a discussion going on on a mailing list that does have some interest for me concerning taking off from higher altitude strips considering that I live some 800M AMSL (2600').

The main stream of the discussion is that in order to overcome the problems of lifting off from a higher altitude strip, all one would need to do is increase the power of the engine, and possibly increase the wingspan a little.

I am not so sure. I was under the impression that the change in density of the air not only affects the amount of lift generated by the airfoils, it also has an impact on the level of control authority in ground effect. Is this so? I am sure I have read an accident report where an aircraft carrying only a moderate load tripped over the boundary fence on take off due to being unable to rotate out of ground effect due to a combination of altitude and heat.

Any comments?
 

orion

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Mar 2, 2003
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5,800
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Western Washington
To provide a proper answer here I think we'd need about an hour's worth of lecture. But let me see if I can simplify this.

An airplane, in order to fly, understands only one thing, dynamic pressure. Regardless of what the air does, the aircraft only understands the amount of mass of air that is traveling accross its surfaces. Mathematically, dynamic pressure is expressed as:

Q = .5 x (rho) x (Velocity)squared

where rho is the air density.

In an aircraft, dynamic pressure is simply the INDICATED airspeed. The pitot system and airspeed instrumentation will give you the correct indication of performance, regardless of density altitude. If you fly at INDICATED airspeed, you should be able to stay out of trouble.

Now, there is also a secondary effect, but that would require more explanation than what I wish to spend time on here. In short though, this effect results in a non-intuitive shift in the best rate and angle of climb speeds - the speeds actually decrease a bit. This means that when you are climbing out on a hot day, if you have clearance issues you will have to actually slow down a hair to get the most performance out of your airplane.

But when doing so, keep in mind that your INDICATED stall speed is still the same!!!! By slowing down you may get to a better rate of climb but you are also closer to stalling. As such, to operate here requires a good familiarity with your airplane and very precise speed control.

Personally though, I would not recommend this unless it is absolutley necessary. I prefer more power in my airplane so that this is not an issue. If I'm flying an airplane with less power than necessary to get the needed amount of performance, it is then probably better to stay on the ground until the weather cools off.

As far as the rotation is concerned, ground effect is only a result of the airplane's geometry and the wing's proximity to the ground, not density altitude. When the wing nears the runway, the surface reflects the wing's downwash, directing it more horizontal. This reduces the angle of attack of the flow over the horizontal tail, thus decreasing the amount of lift it generates.

At higher density altitude, the horizontal, like the wing, is affected by the air's density and thus the surface's capability to develop lift is reduced. However, this too just boils down to "Q" and indicated airspeed.
 
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