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Date: Sun, 18 Jul 2010 13:47:24 -0400
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Subject: 320/360 CG and Pitch Sensitivity
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Some of us on the list are getting hung up on the CG range as a percent of=
 mean aerodynamic chord.  To an aircraft designer, expessing the CG range=
 this way is used for preliminary design purposes only --we want to make=
 sure that we have a usable product that can support real world loading co=
nditions, and also to know where the landing gear needs to be.  (It's bad=
 when the airplane falls on its tail when the pilot throws a suitcase in=
 the back...)

Once the airplane is built and in flight test, we determine the real CG li=
mits by ballasting the airplane to different CGs and doing the following:

1)  Determining the maximum forward CG by determining where tail power bec=
omes insufficient to flare on landing.  This is actually not hard to calcu=
late and simply verify during flight tests.

2)  Determining the aft CG at which the handling qualities become unaccept=
able.  As I mentioned in an earlier posting, this depends on the intended=
 pilot population (is it a trainer or an advanced fighter jet) and its mis=
sion (is it a trainer, a Bonanza-class airplane or an airliner).  This is=
 very difficult to determine analytically, hence the intial reliance on ru=
les of thumb.  (Since you can't calculate it precisely, you might as well=
 use a quick estimate that's just as good.)

Again, rules of thumb (such as a range of 15% - 30% MAC -- this depends on=
 airfoil section and the aft-loaded NLF airfoils are different from the NA=
CA 65-series, for example) are used only to estimate those actual CG limit=
s which are verified in flight test.

Scott Krueger is abolutely right when he says that there are multiple fact=
ors which influence pitch sensitivity, not all of which were mentioned in=
 the CAFE reports.  First and foremost is CG.  Other factors are (in the=
 320 series) the elevator bellcrank arm and the tail size.  And then there=
 are odd airplanes like Dom Crain's, which has the pitch damping of a larg=
e tail but (I'm guessing) the tail power of the small tail.  He likes it.

Here's one suggestion to non-builder owners of 320 and 360 airplanes.  If=
 you think the pitch sensitivity is too high, measure the distance between=
 the elevator axis of rotation and the bolt hole to which the elevator pus=
hrod is attached.  There were two versions of this bellcrank.  Original on=
es had a longer distance and the newer ones were 1 inch shorter.  I have=
 a copy of the Lancair factory newsletter in the hangar addressing this ch=
ange and I'll post it in the next week or so.  The shorter distance gives=
 higher stick forces but less control stick throw.  You will notice the st=
ick force change but probably not the change in stick throw.  You can modi=
fy your bellcrank without replacing it, if you want.  You just drill one=
 hole, although you'll have to remove at least one elevator (maybe both)=
 in order to do it, and you should check the rigging afterwards to make su=
re the neutral position is still where you want it.  Elevator range of mot=
ion will be unaffected but with the elevator in trail the stick might be=
 tilted forward or aft a little.

- Rob Wolf


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<font color=3D'black' size=3D'2' face=3D'arial'>
<div>Some of us on the list are getting hung up on the CG range as a perce=
nt of mean aerodynamic chord.&nbsp; To an aircraft designer, expessing the=
 CG range this way is used for preliminary design purposes only --we want=
 to make sure that we have a usable product that can support real world lo=
ading conditions, and also to know where the landing gear needs to be.&nbs=
p; (It's bad when the airplane falls on its tail when the pilot throws a=
 suitcase in the back...)</div>


<div>&nbsp;</div>


<div>Once the airplane is built and in flight test, we determine the real=
 CG limits by ballasting the airplane to different CGs and doing the follo=
wing:</div>


<div>&nbsp;</div>


<div>1)&nbsp; Determining the maximum forward CG by determining where tail=
 power becomes insufficient to flare on landing.&nbsp; This is actually no=
t hard to calculate and simply verify during flight tests.</div>


<div>&nbsp;</div>


<div>2)&nbsp; Determining the aft CG at which the handling qualities becom=
e unacceptable.&nbsp; As I mentioned in an earlier posting, this depends=
 on the intended pilot population (is it a trainer or an advanced fighter=
 jet) and its mission (is it a trainer, a Bonanza-class airplane or an air=
liner).&nbsp; This is very difficult to determine analytically, hence the=
 intial reliance on rules of thumb.&nbsp; (Since you can't calculate it pr=
ecisely, you might as well use a quick estimate that's just as good.)</div=
>


<div>&nbsp;</div>


<div>Again, rules of thumb (such as a range of 15% - 30% MAC -- this depen=
ds on airfoil section and the aft-loaded NLF airfoils are different from=
 the NACA 65-series, for example) are used only to estimate those actual=
 CG limits which are verified in flight test.</div>


<div>&nbsp;</div>


<div>Scott Krueger is abolutely right when he says that there are multiple=
 factors which influence pitch sensitivity, not all of which were mentione=
d in the CAFE reports.&nbsp; First and foremost is CG.&nbsp; Other factors=
 are (in the 320 series) the elevator bellcrank arm and the tail size.&nbs=
p; And then there are odd airplanes like Dom Crain's, which has the pitch=
 damping of a large tail but (I'm guessing) the tail power of the small ta=
il.&nbsp; He likes it.</div>


<div>&nbsp;</div>


<div>Here's one suggestion to non-builder owners of 320 and 360 airplanes.=
&nbsp; If you think the pitch sensitivity is too high, measure the distanc=
e between the elevator axis of rotation and the bolt hole to which the ele=
vator pushrod is attached.&nbsp; There were two versions of this bellcrank=
.&nbsp; Original ones had a longer distance and the newer ones were 1 inch=
 shorter.&nbsp; I have a copy of the Lancair factory newsletter in the han=
gar addressing this change and I'll post it in the next week or so.&nbsp;=
 The shorter distance gives higher stick forces but less control stick thr=
ow.&nbsp; You will notice the stick force change but probably not the chan=
ge in stick throw.&nbsp; You can modify your bellcrank without replacing=
 it, if you want.&nbsp; You just drill one hole, although you'll have to=
 remove at least one elevator (maybe both) in order to do it, and you shou=
ld check the rigging afterwards to make sure the neutral position is still=
 where you want it.&nbsp; Elevator range of motion will be unaffected but=
 with the elevator in trail the stick might be tilted forward or aft a lit=
tle.</div>


<div>&nbsp;</div>


<div>- Rob Wolf<br>
<br>
</div>


<div style=3D"CLEAR: both"></div>
</font>

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