kuffel(at)cyberport.net
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 Posted: Thu Feb 02, 2006 11:24 pm Post subject: No Title |
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DVD asks:
<<don't need the "tube" because the bulkhead fitting is forward of the
nose about 7/8" and should be in undisturbed air?>>
As in most things aeronautic, "not exactly". 7/8" in front of the wing or
nose cone the air is relatively undisturbed *relative to the surface*. In
other words, almost static. Unfortunately, what you need to measure is
air undisturbed relative to the ground (+/- wind of course). The general
rule is to reach such air you should be as far away from the nearest
surface as the radius of curvature of that surface.
This effect caused a very low and nervous first flight for a friend in his
new Mark III. He had mounted a commercial combined static/pitot probe on
the front of his nose cone. It was about 6 " long which is more than
enough for a wing mounting but in front of the much larger radius nose
cone put it in stagnant air. The resulting low pitot pressure had him
thinking he was on the verge of stall all the way around the pattern when
it turns out he was doing over 80 mph.
The good news is you don't have to abide by the "general rule" to get
useful, repeatable airspeed indications. The airplane will stall at the
same indicated airspeed at the same g loading every time. Just don't
expect the airspeed to be accurate or the same error through out its
range. Use a GPS to find out the actual speed of your 1 g stall and then
use the GPS to find out which indicated airspeed is 1.3 times the stall
speed. This indicated speed is then your normal approach speed (says the
FAA).
My friend increased the range of his indicated airspeeds by disconnecting
the static part of the commercial probe and using the interior of the nose
cone as his static source. This worked fine except for the errors in
caused in his altimeter.....
Tom Kuffel
Whitefish, MT
Building Original FireStar
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