Wolfgang, et al,

<<Any more to the rear and you get negative stability at = cruise and=20 a larger tail doesn't help much with that anyway.>>

A larger tail moves the MAC rearward allowing the CG to move = farther=20 aft while maintaining the same level of stability.

There has been a lot of discussion about Cm. We need = to be=20 careful to distinguish between the Cm for the wing, tail and total=20 aircraft. It is the later that is critical to stability and this = is=20 where the larger tail influences the situation. The large tail = moves the=20 MAC to the rear approx. 1.5 inches. For the same CG, the more = rearward=20 MAC produces a greater restoring force if the plane is disturbed from = level=20 flight. The practical benefit for us is that it allows a = lot more=20 baggage to be thrown the rear of the plane before = suffering stability=20 problems. You pointed out the other benefit of increased control = authority at slow speed with full flaps.

Chris Zavatson

N91CZ

360std

www.N91CZ.com

From: Wolfgang=20 <Wolfgang@MiCom.net>
To: = lml@lancaironline.net
Sent: Tue, July 13, 2010 = 2:51:23=20 AM
Subject: [LML] = Small tail,=20 MK II tail, CG range

The quest continues.

I'm checking further into the data on these questions and am = coming to=20 question the need for a larger tail. I'm not sure a larger tail by = itself will=20 solve the problem. After doing some static and in flight measurements, = it=20 looks like the tail authority is not a big problem, if a problem at = all.

Static measurements of N31161 have shown "vanilla" parameters. = 2.5=BA=20 incidence between the wing root at full reflex and the tail and a = 1.3=BA=20 washout. Put the flaps at 0=BA and you get an additional AoA of 1.8=BA = at the root=20 for a total incidence of 4.3=BA . . . . not radical at all.

What is interesting is the POH (Dec. 1994 pg. VI-3) gives the CG = range as=20 24.5" to 30.3" aft of the rear face of the fire wall and the MAC at = 15% to=20 20%

. . . well . . . no . . . that range is more like a MAC range of = 15% to=20 30% - - - a good range made touchy only by the small size of the air=20 frame.

After going over the plan view kit drawings, I come up with a CG = range of=20 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=20 hand flight experience.

Any more to the rear and you get negative stability at cruise and = a=20 larger tail doesn't help much with that anyway.

Negative stability makes pitch control a real chore. As Scott K. = has=20 indicated, going to 0=BA flaps helps under that loading = condition.

Too far forward and landing becomes "interesting". A larger tail = can help=20 there . . . or don't use as much flaps.

I think understanding these conditions can help everyone.

. . . The quest continues . . . Comments welcome.

Wolfgang

lml@lancaironline.net

From:	"Wolfgang"=20 <Wolfgang@MiCom.net>
Sender:	<marv@lancaironline.net>
Subject:	Small tail, MK II tail, CG=20 range
Date:	Sat, 10 Jul 2010 21:01:11=20 -0400
To:

The LNC2 uses the NLF(1)-0215F airfoil. A lot can be found = by doing=20 a Google search on that number.

More detail can be found by going to Google for "NASA=20 Technical Paper 1865".

I have not taken the time to reverse engineer the CG range = of the=20 LNC2 but let me offer some observations.

The airfoil used has long been touted as "the greatest = thing since=20 sliced bread" for General Aviation and it definitely has some=20 advantages. But it's not new. Compare this airfoil to the P-51 = airfoil=20 and you will see some close similarities. The LNC2 being = composite=20 construction instead of aluminum lets the airfoil show more of = it's=20 theoretical advantages.

It's a laminar shape with a good drag bucket. That bucket = can be=20 made to move to the lower Cl (lift coefficient) ranges with = reflex=20 allowing noticeably lower drag at higher cruise speeds. Along = with=20 reflex, the Cm (moment coefficient) goes positive, the center of = lift of=20 the wing travels forward giving a nose up force requiring down = trim.=20 This is in addition to the usual nose up force that goes with = most all=20 airfoils at high speed before considering flaps.

With down flap, the drag bucket will move to higher Cl's = making=20 slower flight more efficient. And, of course, the Cm goes = negative=20 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=20 available control authority,

and the forward CG is determined by low speed / landing = flight and=20 available control authority.

What is becoming clear here is that the center of lift does = quite a=20 bit of traveling fore and aft which is exaggerated by allowing = negative=20 or "cruise" flaps. Since you can't shift the CG during flight, = you need=20 a large amount of pitch authority from the tail to cover that = range of=20 lift travel.

You have two choices in the LNC2, live with the limitations = or=20 install a larger tail to give that extra pitch authority.

. . . A larger tail area can also help with abnormal=20 attitude recovery.

Wolfgang