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Re: conclusion to weight and balance discussion
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Robert E Cook
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Oct 28, 2009 15:37 PST
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Rob,
With a Wing Chord of 43" and an allowable range of 8.6" that gives a 20%
range in CG..........
In my humble opinion this is too big of an allowable range.........
Without relating it to MAC it is meaningless, but.........
It is more normal to see a CG range something like.........25% ~ 33% MAC
This is only an 8% range........
It seems that on the Moth they are allowing for something like 20% ~ 40%
Mac.........again in my opinion this is far too much range........
Using the firewall as the datum requires that:
A) You built the fuselage dimensionally perfect
B) You built the wings and located them dimensionally perfectly with
respect to the fuselage.
MAC for swept wings is not so hard to figure......then use a datum on the
wing(s).........then dimensional errors don't matter....
For instance, on the Celebrity, there are 3 possible wing locations shown
on the plans with respect the intended engine weight. So using the
firewall as a datum would no work.......but using the wings themselves
will work.
It is a shame that the designers don't give a real W&B analysis for your
aircraft, instead of min and max sample problems.
Cookie
On Wed, 28 Oct 2009 09:04:05 -0400 Rob Brooke <rbro-@tampabay.rr.com>
writes:
Hello, Cookie,
The wing chord as measured on my rib pattern is 43 1/2 inches (3' 7
1/2"). The wings, both upper and lower, are swept back 6°. As it turns
out, the ranges fore and aft of the loaded CG are contained in the Tiger
Moth construction manual, but in disguised form. Whoever wrote the manual
ran two illustrative W&B calculations, one with a 150 lb. pilot (rear
seat) and one with a 240 lb. pilot. The front seat is empty and there is
a full fuel load in both upper and main tanks. The CG for the 150 lb.
pilot is the magic 29.93" and for the 240 lb. pilot, the magic 38.69".
nowhere is it stated that these are the CG limits for the airplane but,
according to Gene Hanson, they are. Kinda lets the builder know he has to
gain or lose weight, but only if he knew these were the limits, which he
doesn't from the manual alone.
I understand your comments about the datum and how using the firewall is
perhaps unusual. However, in the case of a plane with swept back wings,
it does not serve simply to say, "Use the leading edge of the wing as a
datum.". The firewall should work fine. A vertical line dropped to the
main LG hubs would work fine. Anything works fine if you know how to do
the calculation.
As to the specification of the CG range in hundredths of an inch, the
real problem is not the hundredths of an inch, it is that the CG range
(loaded) was not really specified as such, only a couple of example W&B
calculations. The hundredths of an inch are the outcome of the example
calculations. If you wish to take issue, take issue with the fact that
the CG range is not specified in the construction manual!
About little Johnny and his 50,000,002 years, I think that little Johnny
is one bright kid and should be praised for his acumen rather than
scorned for superfluous accuracy.
Rob Brooke
Tiger Moth #45
:
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<HTML><HEAD><TITLE></TITLE>
<META http-equiv=3DContent-Type content=3Dtext/html;charset=3DISO-8859-1=
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<BODY text=3D#000000 bgColor=3D#ffffff>
<DIV>Rob,</DIV>
<DIV> </DIV>
<DIV>With a Wing Chord of 43" and an allowable range of 8.6" that gives =
a 20% =
range in CG..........</DIV>
<DIV> </DIV>
<DIV>In my humble opinion this is too big of an allowable =
range.........</DIV>
<DIV> </DIV>
<DIV>Without relating it to MAC it is meaningless, but.........</DIV>
<DIV> </DIV>
<DIV>It is more normal to see a CG range something like.........25% ~ 33=
% =
MAC</DIV>
<DIV> </DIV>
<DIV>This is only an 8% range........</DIV>
<DIV> </DIV>
<DIV>It seems that on the Moth they are allowing for something like 20% =
~ =
40% Mac.........again in my opinion this is far too much =
range........</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Using the firewall as the datum requires that:</DIV>
<DIV> </DIV>
<DIV>A) You built the fuselage dimensionally perfect</DIV>
<DIV> </DIV>
<DIV>B) You built the wings and located them dimensionally perfectly wit=
h =
respect to the fuselage.</DIV>
<DIV> </DIV>
<DIV>MAC for swept wings is not so hard to figure......then use a datum =
on the =
wing(s).........then dimensional errors don't matter....</DIV>
<DIV> </DIV>
<DIV>For instance, on the Celebrity, there are 3 possible wing locations=
shown =
on the plans with respect the intended engine weight. So using the=
=
firewall as a datum would no work.......but using the wings themselves w=
ill =
work.</DIV>
<DIV> </DIV>
<DIV>It is a shame that the designers don't give a real W&B analysis=
=
for your aircraft, instead of min and max sample problems.</D=
IV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Cookie</DIV>
<DIV> </DIV>
<DIV>On Wed, 28 Oct 2009 09:04:05 -0400 Rob Brooke <<A =
href=3D"mailto:rbro-@tampabay.rr.com">rbro-@tampabay.rr.com</A>> =
=
writes:</DIV>
<BLOCKQUOTE dir=3Dltr =
style=3D"PADDING-LEFT: 10px; MARGIN-LEFT: 10px; BORDER-LEFT: #000000 2px=
solid">
<DIV>Hello, Cookie,<BR><BR>The wing chord as measured on my rib patter=
n is 43 =
1/2 inches (3' 7 1/2"). The wings, both upper and lower, are swept bac=
k 6=B0. As =
it turns out, the ranges fore and aft of the loaded CG <U>are</U> cont=
ained in =
the Tiger Moth construction manual, but in disguised form. Whoever wro=
te the =
manual ran two illustrative W&B calculations, one with a 150 lb. p=
ilot =
(rear seat) and one with a 240 lb. pilot. The front seat is empty and =
there is =
a full fuel load in both upper and main tanks. The CG for the 150 lb. =
pilot is =
the magic 29.93" and for the 240 lb. pilot, the magic 38.69". nowhere =
is it =
stated that these are the CG limits for the airplane but, according to=
Gene =
Hanson, they are. Kinda lets the builder know he has to gain or lose w=
eight, =
but only if he knew these were the limits, which he doesn't from the m=
anual =
alone.<BR><BR>I understand your comments about the datum and how using=
the =
firewall is perhaps unusual. However, in the case of a plane with swep=
t back =
wings, it does not serve simply to say, "Use the leading edge of the w=
ing as a =
datum.". The firewall should work fine. A vertical line dropped to the=
main LG =
hubs would work fine. Anything works fine if you know how to do the =
calculation.<BR><BR>As to the specification of the CG range in hundred=
ths of =
an inch, the real problem is not the hundredths of an inch, it is that=
the CG =
range (loaded) was not really specified as such, only a couple of exam=
ple =
W&B calculations. The hundredths of an inch are the outcome of the=
example =
calculations. If you wish to take issue, take issue with the fact that=
the CG =
range is <U>not specified</U> in the construction manual!<BR><BR>About=
little =
Johnny and his 50,000,002 years, I think that little Johnny is one bri=
ght kid =
and should be praised for his acumen rather than scorned for superfluo=
us =
accuracy.<BR><BR>Rob Brooke<BR>Tiger Moth #45 </DIV><PRE>:
Find a Fishnet member near you at <A href=3D"http://www.frappr.com/fishe=
rflyingproduct">http://www.frappr.com/fisherflyingproduct</A>
Check out the Fishnet Hangar at...
<A href=3D"http://www.lairdlind.com/fishnet.htm">http://www.lairdlind.co=
m/fishnet.htm</A>
Submit a picture of your Fishnet project to Laird Lind =
lai-@airmail.net
For subscription issues, Log In or Register at <A href=3D"http://lists.t=
opica.com/lists/Fishnet">http://lists.topica.com/lists/Fishnet</A> or co=
ntact Scott Stewart dakota-@att.net</PRE><PRE>--^^------------------=
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