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= =0AFrom: Wolfgang <Wolfg= ang@MiCom.net>
To: lm= l@lancaironline.net
Sent: Tue, July 13, 2010 2:51:23 AM
Sub= ject: [LML] Small tail, MK II tail, CG range

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The quest continues.
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=0AI'm checking further into the data on these questions and am coming to q= uestion the need for a larger tail. I'm not sure a larger tail by itself wi= ll solve the problem. After doing some static and in flight measurements, i= t looks like the tail authority is not a big problem, if a problem at all.<= /DIV>=0A

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Static measurements of N31161 have shown "= vanilla" parameters. 2.5=BA incidence between the wing root at full reflex = and the tail and a 1.3=BA washout. Put the flaps at 0=BA and you get an add= itional AoA of 1.8=BA at the root for a total incidence of 4.3=BA . . . . n= ot radical at all.
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What is interesting is = the POH (Dec. 1994 pg. VI-3) gives the CG range as 24.5" to 30.3" aft of th= e rear face of the fire wall and the MAC at 15% to 20%
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. . . 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.
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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 ran= ge of 15% to 30%
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That range is about 1-1/4" forward of the boo= k and fits better with first hand flight experience.
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Any more to the rear and you get negative stability at cruise a= nd a larger tail doesn't help much with that anyway.
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Negative= stability makes pitch control a real chore. As Scott K. has indicated, goi= ng to 0=BA flaps helps under that loading condition.
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Too far forward and landing becomes "interesting". A larger tail= can help there . . . or don't use as much flaps.
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I think understanding these conditions can help everyone.
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. . . The quest continues . . . Comments welcome.<= /DIV>=0A

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Wolfgang
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From: "Wolfg= ang" <Wolfgang@MiCom.net>
Sender: <marv@lancaironline.net>
Subject: Small tail, MK II tail, CG range
Date: Sat, 10 Jul 2010 21:01:11= -0400
To: lml@lancaironline.net
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The LNC2 uses the NLF(1)-0215F a= irfoil. A lot can be found by doing a Google search on that number.
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More detail can be found by going to Google for "NASA Technica= l Paper 1865".
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I have not taken the time t= o reverse engineer the CG range of the LNC2 but let me offer some observati= ons.
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The airfoil used has long been touted= as "the greatest thing since sliced bread" for General Aviation and it def= initely 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 compo= site construction instead of aluminum lets the airfoil show more of it's th= eoretical advantages.
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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 high= er cruise speeds. Along with reflex, the Cm (moment coefficient) goes posit= ive, 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 go= es with most all airfoils at high speed before considering flaps.=0A

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With down flap, the drag bucket will move to h= igher Cl's making slower flight more efficient. And, of course, the Cm goes= negative giving a nose down force requiring up trim.
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. . . and appropriate variations in-between . . .
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So, the rear CG limit is determined by = high speed flight and available control authority,
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and the for= ward CG is determined by low speed / landing flight and available control a= uthority.
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What is becoming cle= ar here is that the center of lift does quite a bit of traveling fore and a= ft which is exaggerated by allowing negative or "cruise" flaps. Since you c= an't shift the CG during flight, you need a large amount of pitch authority= from the tail to cover that range of lift travel.
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You have two choices in the LN= C2, live with the limitations or install a larger tail to give that extra p= itch authority.
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. . . A larger tail area can also help with&nb= sp;abnormal attitude recovery.
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Wolfga= ng

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