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Bruce,
Here's an easy way to think about it:
To simplify, first take the wing's pitching-moment and CG-balancing moment out, by assuming the plane has its CG is at 25% or whatever, and assime that the section is symmetrical (zero pitching moment).
Then:
Now imagine the wing is lifting enough to balance its weight. The horizontal tail will go to zero lift,AOA/tail lift coeff Cl... because it has a long arm and it wants to pivot about the CG. Its purpose is just to correct small deviations of the aircraft AOA/lift coefficient (Cl) required, which the designer/builder selects depending on what the current cruise speed and weight.
The wing is designed to be put on the fuselage usually at the cruise AOA / Cl , and that's where the horizontal tail is put on the airframe at zero-lift... zero AOA if symmetrical section.
When you want the plane to fly outside of this 'design trim/AOA/Cl/V', then the h-tail cranks the wing to the requried wing-Cl by changing the h-tail shape with an elevator deflection, which moves its CL = zero to a different tail-AOA for its airfoil section (see Abott&Doenhoff, Theory of Wing Sections, pps.195 & 196 shows how this works). For example, to go slower, the h-tail will then go downward until its Cl was again zero, and at that point is where the wing would have its AOA/Cl cranked up to the desired Cl, a higher Cl requried for flying slower at the same weight.
Throwing in the wing or Cg pitching moment just make the h-tail go to a Cl to balance that moment, instead of to zero.
N'est ce pas?
Terrence
L235/320 N211AL
Airworthy at last! Not flown yet.
----- Original Message ----- From: "Bruce Gray" <Bruce@glasair.org>
To: <lml@lancaironline.net>
Sent: Monday, February 23, 2009 07:00
Subject: [LML] Re: FW: 360 in a small-tail LNC2?
If I recall correctly, it's the difference between the wing and the tail
angle of incidence (called decalage I believe) that will determine the
stability and speed of the airplane. More decalage, more stability, slower
airplane.
Bruce
www.Glasair.org
-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
LHenney
Sent: Sunday, February 22, 2009 5:36 PM
To: lml@lancaironline.net
Subject: [LML] FW: 360 in a small-tail LNC2?
Tom,
I had occasion to visit my CG limits for a son's science project. Similarly
I was assessing elevator bob weight position relative to the horizontal tail
at race speeds and comparing airspeed data.
In my aircraft, your premise would be wrong. That is, I have to move the CG
forward to flatten out the elevators to the horizontal tail (which also
increases speed (yes it's hard to believe)). As opposed to changing CG, one
might verify horizontal tail incidence. Or more precisely all Lancair 320/
360 I've flown with have this same affect (bob weights a little high at
least minimally).
Regarding your CG comments, serious pursuit of empty CG before any
additional flight would be my recommendation (imho). The phrase " way to far
forward I think" has me squirming.
Larry Henney
PS: In my estimation, several 320/ 360 builders took the tail plane template
and transferred it to an incidence guide. The subsequent mistake was
mounting one's smart level atop the guide and setting the tail plane at the
requisite 1/4 to 1/2 degree nose down. The mistake is missing the fact that
the incidence guide off the blue print was already 1/2 degree nose down.
Thus reading 0 deg on the smart level should have had the tail at the 1/2
degree nose down angle. Many are actually 1 deg nose down. This is not a
problem. It just increases stability and costs 2-3 kts.
Just my 2 cents.
-----Original Message-----
From: Tom McNerney [mailto:dudewanarace@yahoo.com]
Sent: Friday, February 20, 2009 12:51 PM
To: lml@lancaironline.net
Subject: RE:360 in a small-tail LNC2?
Works great. I have a 400! I'm sure the rest of small tail group would
agree. Might have to move your center of gravity back. (I need to) Only
thing I can say is that slow flight with full flaps isn't the most
comfortable, but now that I am aware of how it handles, doesn't bother me a
bit. My CG is way to far forward I think, so that probably makes the slow
flight a little different.
On a side note.. I turned around and looked at the tail the other day
while indicating 200KTS. I could see the elevator counter weight, or almost
all of it. I didn't expect to see that at 200. Maybe 100.. haha This
leads me to believe that I have a forward center of gravity. The big engine
would explain that. Once I get the plane on some scales, the only thing I
can think of is to move the battery. My battery is behind the passenger
seat.
Has anyone mounted a battery behind the baggage compartment? Advice?
Thanks
Tom
www.N54SG.com
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