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From: "Larry Henney" <LHenney@attg.net>
>By adding weight to the tail (however you want to do it) or even
changing the stab angle (like the MD-80), the tail has all the downward
force it needs with neutral stab/elevator aerodynamics.
Yes, this is obviously correct. But..... My stab is glued in place
unlike the MD-80. Thus by adding more weight to the tail would then
require raising the tail with more down elevator. Thus, increasing the
tails induced drag. No?
My point is that in my plane it would seem to have a detrimental affect.
Or by contrast, I might do better by adding weight up front to zero out
the elevators.
I think that there is some misunderstanding of what happens. Imagine that
the plane is trimmed for cruise and is suspended from its main-wing
aerodynamic center (or its main-wing aerodynamic axis....let's say right
through it's red-green position lights) with no airflow in a wind tunnel.
How will the aircraft hang? Well, it had better be designed to hang a little
bit nose-down. In a stall (a spin too!) when no lift is happening, you want
the airplane to hang nose down.
Back to the suspended airplane. In normal cruise the tail stab (being up)
must then pull downward when there is air rushing over it. This induces drag
in relation to how much downward force it has to generate to keep the
main-wing pitch correct. If (in the extreme) the airplane is out of CG
because you put too much weight in the tail--the elevator must try to keep
the tail UP. This is very bad, and the results kill pilots and passengers,
because when the airspeed reduces, or if for any reason the CG goes back
farther (like raising the landing gear or using up fuel or running out of
ammunition like the P-39 Airacobra). The elevator may no longer be able to
keep the tail up.
If (in the extreme) the airplane is out of CG because you put too little
weight in the tail--the elevator must try to keep the tail DOWN. This is
also bad, but only a little, because when the airspeed reduces, the tail
can't be kept DOWN (so it goes UP like our suspended airplane) and thus the
nose pitches DOWN to increase airspeed. This is much better.
It is important to understand that the Lancair tail does not generate lift.
A canard or an aircraft made in World-War One (I suppose there are others)
has a positive lift tail. Modern airplanes of conventional configurations
have "anti-lift" tails. If you can see that the horiz stab normally
generates DOWN force, then you should see that adding weight to the tail
reduces the amount of DOWN force the tail has to generate.
Just keep in mind that the horiz stab is an upside-down airfoil.
BTW, I did a 30 pound shift one day (very unscientific study).
A good documented test would be great. Often casual tests just confuse
things. But be careful. Better to move the weight in flight than to discover
that the optimal cruise CG is fatal on landing.
Regards,
Eric M. Jones
www.PerihelionDesign.com
113 Brentwood Drive
Southbridge MA 01550-2705
Phone (508) 764-2072
Email: emjones@charter.net
"The man who carries a cat by the tail
learns something that can be learned
in no other way."
--Mark Twain
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