Chris,

 

You got me a little confused but I quickly recovered.  All references I have seen used MAC (Mean Aerodynamic Chord) as the reference for CG.  Indeed, the diagram you included does exactly that.  H bar C is displayed as the distance from the forward edge of the MAC, further supporting the use of specifying the CG range as a percent of MAC (MAC%).

 

Sticking strictly to the CG questions I posed and some comments from LMLers concerning the CG, I present the following list:

 

1.  The Lancair 235/320/360 POH gives 2 definitions for the CG range: 

II. Limitations page 10 -- 15% to 29% MAC

VI. Weight & Balance page 3 -- 15% to 20% MAC 

 

2.  The MAC for flaps -7 and flaps 0 are the same because the TE of the chord doesn't change much (less than .1 inch).

 

3. Measurements on my 320:

a. The leading edge of the root chord is at FS15.75

b. The root chord is 48.5"

c. The distance from the root chord to the out board leading edge wingtip is 114.25"

d. The taper distance is 3.5" (from tip to line perpendicular to root chord at the leading edge of the root chord).

e. The tip chord  is 28" where this chord is parallel to the root chord.

f.  The TE root chord to the tip chord in e is 115.4" but to the actual TE tip is another 6.25".  The wing only up to the tip chord that is parallel to the root chord was used in prelim calcs.

g. Using the calculator at: http://www.nasascale.org/howtos/cg-calculator.htm   for 15% results in a MAC of 39.17", 52" from the root and the CG at 7.47" aft at the root chord.  7.47+ FS15.75 = FS23.22

h. For 20% the CG is at 9.43".  9.43 + FS15.75 = FS25.18

i.  For 29% the CG is at 12.95". 12.95 + FS15.75 = FS28.7

 

4. Since this calculator only considers tapered wings with parallel chords, perhaps you have a more sophisticated calculator.  If you need other measurements, let me know.  The measurements I took were from points plumb bobbed to the floor with chalk lines used and a proper rectangle constructed.

 

5. Assuming that the swept out wing tip changes the forward CG limit a bit back to the 24.5 (1.28 inches), that will still not explain the narrow CG from the calculator or logic. The calculations show a range of only 2" and that makes sense to me because the published span is only 5% and 2"/.05 equals 40" for the MAC - close enough to 39.17 inches.  Using this sort of logic and the POH where a 15% MAC results in a CG at 24.5 then 24.5-FS15.75 = 8.75/.15 = 58" for a MAC longer than the longest wing chord and the span of 5.8" - Huh?  5.8/.05=116 which would be a really BIG MAC (no pun intended).

 

HELP!

 

Scott Krueger

IO320

 

In other words, I am still suspicious of Lancair's published CG reference. 

 

Bill,

We definitely have too many MACs out there:  mean aerodynamic centers vs. chords.  Both start out as integrals which can degenerate to averages for simple geometry, like Hershey bar wings.  The combination of washout and sweep make the calculation of the mean aerodynamic center a bit more challenging.

CG is a mass property and only moves if you burn fuel, move something in the plane or get up to use the rest room (not in a Lancair of course).  All forces, including moments, about the CG must vanish for steady unaccelerated flight.  The neutral point is a parameter important to stability and best be safely behind the CG otherwise you'll have one of those Wright Flyer experiences.

 

Chris Zavatson

N91CZ

360std

www.N91CZ.com

 

---

From: Bill Bradburry <bbradburry@bellsouth.net>
To: lml@lancaironline.net
Sent: Thu, July 15, 2010 4:08:18 PM
Subject: [LML] Re: Small tail, MK II tail, CG range

Unless we are talking about MAC and cheese, or the Mickey D kind of MAC, the aircraft MAC is the Mean Aerodynamic Chord.  This MAC is the width of the wing when measured through the center of the wing in the forward-aft direction.  On a plane like a Piper, this is just the width of the wing.  With a more complicated wing design like the Lancair it is the average of this measurement.  That is where the word “Mean” comes from.  This measurement has nothing to do with the “neutral point”.  It really just describes how effectively wide the wing is.  The CG (Center of Gravity) is the point around which the airplane balances (or would balance) if it is sitting on its wheels.  (Maybe that is a “neutral point”?)  This CG is calculated when the plane is motionless on the ground and on scales.  It is not the CG that the plane is operating with when it is in flight because the horizontal stabilizer is usually designed to place a down force on the plane, which will have the effect of moving the CG backward in cruise.  That is why the CG is specified to be in the front 25% of the wing width (MAC) in the specs.

 

When we determine thru the Weight and Balance calculations, the CG, we have no idea what the CG of the plane will be in flight because as the angle of attack moves the Center of Lift forward and aft, and the horizontal stabilizer adds and removes loads, we have no way of calculating or knowing how these forces are moving.  Hopefully the aircraft designer did all this when he specified the CG range that we should keep the plane in when it is on the ground and on its wheels.  I suggest we stay inside these recommendations.

 

Bill B