From: Wolfgang
<Wolfgang@MiCom.net>
To: lml@lancaironline.net
Sent: Tue, July 13, 2010 2:51:23
AM
Subject: [LML] Small tail,
MK II tail, CG range
The quest continues.
I'm checking further into the data on these questions and am coming to
question the need for a larger tail. I'm not sure a larger tail by itself will
solve the problem. After doing some static and in flight measurements, it
looks like the tail authority is not a big problem, if a problem at all.
Static measurements of N31161 have shown "vanilla" parameters. 2.5º
incidence between the wing root at full reflex and the tail and a 1.3º
washout. Put the flaps at 0º and you get an additional AoA of 1.8º at the root
for a total incidence of 4.3º . . . . not radical at all.
What is interesting is the POH (Dec. 1994 pg. VI-3) gives the CG range as
24.5" to 30.3" aft of the rear face of the fire wall and the MAC at 15% to
20%
. . . well . . . no . . . that range is more like a MAC range of 15% to
30% - - - a good range made touchy only by the small size of the air
frame.
After going over the plan view kit drawings, I come up with a CG range of
23-1/4" to 29-1/4" for a MAC range of 15% to 30%
That range is about 1-1/4" forward of the book and fits better with first
hand flight experience.
Any more to the rear and you get negative stability at cruise and a
larger tail doesn't help much with that anyway.
Negative stability makes pitch control a real chore. As Scott K. has
indicated, going to 0º flaps helps under that loading condition.
Too far forward and landing becomes "interesting". A larger tail can help
there . . . or don't use as much flaps.
I think understanding these conditions can help everyone.
. . . The quest continues . . . Comments welcome.
Wolfgang
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The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found by doing
a Google search on that number.
More detail can be found by going to Google for "NASA
Technical Paper 1865".
I have not taken the time to reverse engineer the CG range of the
LNC2 but let me offer some observations.
The airfoil used has long been touted as "the greatest thing since
sliced bread" for General Aviation and it definitely has some
advantages. But it's not new. Compare this airfoil to the P-51 airfoil
and you will see some close similarities. The LNC2 being composite
construction instead of aluminum lets the airfoil show more of it's
theoretical advantages.
It's a laminar shape with a good drag bucket. That bucket can be
made to move to the lower Cl (lift coefficient) ranges with reflex
allowing noticeably lower drag at higher cruise speeds. Along with
reflex, the Cm (moment coefficient) goes positive, the center of lift of
the wing travels forward giving a nose up force requiring down trim.
This is in addition to the usual nose up force that goes with most all
airfoils at high speed before considering flaps.
With down flap, the drag bucket will move to higher Cl's making
slower flight more efficient. And, of course, the Cm goes negative
giving a nose down force requiring up trim.
. . . and appropriate variations in-between . . .
So, the rear CG limit is determined by high speed flight and
available control authority,
and the forward CG is determined by low speed / landing flight and
available control authority.
What is becoming clear here is that the center of lift does quite a
bit of traveling fore and aft which is exaggerated by allowing negative
or "cruise" flaps. Since you can't shift the CG during flight, you need
a large amount of pitch authority from the tail to cover that range of
lift travel.
You have two choices in the LNC2, live with the limitations or
install a larger tail to give that extra pitch authority.
. . . A larger tail area can also help with abnormal
attitude recovery.
Wolfgang |