lml@lancaironline.net Mailing List Archive

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

From:	"Wolfgang" <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

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