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Re: conclusion to weight and balance discussion
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Robert E Cook
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Oct 30, 2009 04:09 PST
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Rob........
Just to continue the discussion..........
We recently had a Christen Eagle (aerobatic biplane) in the shop for
radio work............The Boss made a comment that we had to be very
careful not to change the CG and careful in computing the new weight and
balance, as the range on this aircraft was so small.
For "normal" flight (Utility Category) the CG range is 5-1/2",
..........IF the plane is flown at light weight........as the weight goes
up, the CG range narrows considerably.........
For "Aerobatic flight" the CG range is reduced to a range of
5".............BUT............as the flying weight is increased to gross
weight............the CG range is gradually reduced to ...........ONE
INCH !!!!!!!!!!!!!!!!! (also lower gross weight for acro)
So this is why I question the almost 9" range on the Fisher
Moth..........Yes, different planes, but.........
Cookie
On Thu, 29 Oct 2009 12:37:04 -0400 Rob Brooke <rbro-@tampabay.rr.com>
writes:
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Hi again, Bob,
Just for grins, I ran some numbers based on some quick and dirty
measurements of my plane. Using 30% MAC, I wound up with the
following
measurements based on distance from the firewall:
Leading edge of upper wing - 4"
Leading edge of the lower wing - 29.25"
Avg leading edge ((LE lower-LE upper)÷2) at root - 12.625"
30% MAC from leading edge of wing chord - 13"
set-back of leading edge at wing mid-point at 6° sweepback - 6.898"
12.625"+13"+6.898" = approx 32.5"
This does not take into account the difference in incidence of the
two
wings (upper wing 2° greater), the lift provided by the center
section,
the interaction of the two wings nor the effect of the horizontal
stabilizer/elevator. However, it does provide some comfort that the
CG
location provided by FFP is rational. I have no views as to the 8"
range
of the CG being too large. Since my very loose estimate of the CG
based
on MAC puts the CG nearer the forward edge of FFP's range than the
aft
limit, I would be happy to make a test flight if 32.5" aft of the
firewall was the result of my W&B. Fuel burn will move the CG aft
somewhat, but both tanks are sufficiently close to the CG that the
change would be very modest.
I thank you for suggesting that we look into establishing the CG
through
the use of MAC calculations. I don't think that it is an
inconsequential
task to get a totally accurate CG that way for a biplane, the
effects of
decalage and stagger being really imponderable, but at least I have
been
able to reassure myself that FFP's CG range is not completely off
the wall.
Rob Brooke
Tiger Moth #45
:
Find a Fishnet member near you at
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<DIV></DIV>
<DIV>Rob........</DIV>
<DIV> </DIV>
<DIV>Just to continue the discussion..........</DIV>
<DIV> </DIV>
<DIV>We recently had a Christen Eagle (aerobatic biplane) in the shop fo=
r radio =
work............The Boss made a comment that we had to be very careful n=
ot to =
change the CG and careful in computing the new weight and balance, as th=
e range =
on this aircraft was so small.</DIV>
<DIV> </DIV>
<DIV>For "normal" flight (Utility Category) the CG range is 5-1/2", =
=2E.........<STRONG>IF</STRONG> the plane is flown at light weight......=
..as the =
weight goes up, the CG range <STRONG>narrows</STRONG> =
considerably.........</DIV>
<DIV> </DIV>
<DIV>For "Aerobatic flight" the CG range is reduced to a range of =
5".............BUT............as the flying weight is increased to gross=
=
weight............the CG range is gradually reduced to ...........<STRON=
G>ONE =
INCH</STRONG> !!!!!!!!!!!!!!!!! (also lower gross weig=
ht for =
acro)</DIV>
<DIV> </DIV>
<DIV>So this is why I question the almost 9" range on the Fisher =
Moth..........Yes, different planes, but.........</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Cookie</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>On Thu, 29 Oct 2009 12:37:04 -0400 Rob Brooke <<A =
href=3D"mailto:rbro-@tampabay.rr.com">rbro-@tampabay.rr.com</A>> =
=
writes:<BR>> <BR>> Hi again, Bob,<BR>> <BR>> Just for grins,=
I ran =
some numbers based on some quick and dirty <BR>> measurements of my p=
lane. =
Using 30% MAC, I wound up with the <BR>> following <BR>> measureme=
nts =
based on distance from the firewall:<BR>> <BR>> Leading edge of up=
per =
wing - 4"<BR>> Leading edge of the lower wing - 29.25"<BR>> =
Avg =
leading edge ((LE lower-LE upper)=F72) at root - 12.625"<BR>> 30% MAC=
from =
leading edge of wing chord - 13"<BR>> set-back of leading edge at win=
g =
mid-point at 6=B0 sweepback - 6.898"<BR>> <BR>> 12.625"+13"+6.898"=
=3D approx =
32.5"<BR>> <BR>> This does not take into account the difference in=
=
incidence of the <BR>> two <BR>> wings (upper wing 2=B0 greater), =
the lift =
provided by the center <BR>> section, <BR>> the interaction of the=
two =
wings nor the effect of the horizontal <BR>> stabilizer/elevato=
r. =
However, it does provide some comfort that the <BR>> CG <BR>> loca=
tion =
provided by FFP is rational. I have no views as to the 8" <BR>> range=
=
<BR>> of the CG being too large. Since my very loose estimate of the =
CG =
<BR>> based <BR>> on MAC puts the CG nearer the forward edge of FF=
P's =
range than the <BR>> aft <BR>> limit, I would be happy to make a t=
est =
flight if 32.5" aft of the <BR>> firewall was the result of my W&=
B. Fuel =
burn will move the CG aft <BR>> somewhat, but both tanks are sufficie=
ntly =
close to the CG that the <BR>> change would be very modest.<BR>> <=
BR>> =
I thank you for suggesting that we look into establishing the CG <BR>>=
; =
through <BR>> the use of MAC calculations. I don't think that it is a=
n =
<BR>> inconsequential <BR>> task to get a totally accurate CG that=
way for =
a biplane, the <BR>> effects of <BR>> decalage and stagger being r=
eally =
imponderable, but at least I have <BR>> been <BR>> able to reassur=
e myself =
that FFP's CG range is not completely off <BR>> the wall.<BR>> <BR=
Rob Brooke<BR>> Tiger Moth #45<BR>> <BR>> <BR>> :<BR>> Fi=
nd a =
Fishnet member near you at <BR>> <A =
href=3D"http://www.frappr.com/fisherflyingproduct">http://www.frappr.com=
/fisherflyingproduct</A><BR>> =
Check out the Fishnet Hangar at...<BR>> <A =
href=3D"http://www.lairdlind.com/fishnet.htm">http://www.lairdlind.com/f=
ishnet.htm</A><BR>> =
Submit a picture of your Fishnet project to Laird Lind <BR>> <A =
href=3D"mailto:lai-@airmail.net">lai-@airmail.net</A><BR>> For su=
bscription =
issues, Log In or Register at <BR>> <A =
href=3D"http://lists.topica.com/lists/Fishnet">http://lists.topica.com/l=
ists/Fishnet</A> =
or contact Scott Stewart <BR>> <A =
href=3D"mailto:dakota-@att.net">dakota-@att.net</A><BR>> =
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