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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 03:18 PST
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Rob,
A couple of questions and a couple of comments:
What is the length of the wing airfoil chord on the "moth".....?
The wings are swept back , right?
The "allowable range" of CG seems like quite a lot........almost 9
inches.......hmmmmm.....
Plans and builder's manuals always seem to give the CG range in weird
ways.......like using the fire wall as a datum.....of course this does
work fine, but it is more "normal" to give CG as % MAC. (Mean
aerodynamic chord), while using the wing leading edge as the
datum..........
Figuring out the MAC is slightly involved when you have sweep-back
wings, and two wings with stagger, and a straight center section between
the wings........but simple geometry can be used..........
It makes sense to design a plane with tandem seating........put the pilot
in the rear seat to "balance" the aircraft, and put the copilot right on
the CG.........this way the balance does not change from solo to dual
flight, only the flying weight changes....also some change during flight
due to fuel burn.........
Funny that they spec out the CG to the 1/100 th of in inch!...........We
have showed in earlier posts that this degree of accuracy is totally
unnecessary!...i.e. errors in the weighing of 10% or more cause little
change in the actual CG calculation......
Like, would it have killed them to spec out the CG..........30" to 38"
aft of datum........?
With such a large range of CG specified, I would be leery of flying at
the extremes.......I would like to see the actual CG fall pretty close to
the center of the allowable range......
Cookie
PS........A story to illustrate what I am talking about......"degree of
accuracy"........
In school the class was studying dinosaurs..........
The teacher asked the class, "When did the dinosaurs roam the Earth?"
Little Johnny quickly raised his hand......."I know, I know"
"Yes, Johnny, when did dinosaurs roam the earth?" the teacher repeated.
"50,000,002 years ago", was Johnny's answer.......
Fifty Million, AND TWO? the teacher asked........"That is an interesting
answer Johnny. How did you come up with that answer?"
"Well", said Johnny....."My textbook says dinosaurs roamed the Earth
fifty million years ago, but I noticed that the book is two years old!"
On Tue, 27 Oct 2009 20:41:12 -0400 Rob Brooke <rbro-@tampabay.rr.com>
writes:
| |
Hi, Bill,
I puzzled over the CG information in the Tiger Moth construction
manual
for quite a while. I couldn't find anywhere in the manual what the
CG
range should be for the plane in flight. I really didn't care what
the
empty CG was supposed to be since the plane doesn't fly empty and
couldn't get off the ground if it tried. I called Gene to ask what
the
CG range is of the loaded and ready-for-flight airplane and got the
following answer: 29.98" to 38.69" aft of the firewall. That puts
the CG
right where you'd expect it - at the forward (passenger) cockpit
and
also provides the answer to the mystery 8" range. It's just a
little
over 8" between the forward and aft limits of the in-flight CG.
Well, all manuals can't be perfect.
Rob Brooke
Tiger Moth #45
:
Find a Fishnet member near you at
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$133,000 mortgage under $679/mo. Compare rates and save!
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Content-Transfer-Encoding: quoted-printable
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML><HEAD>
<META http-equiv=3Dcontent-type content=3Dtext/html;charset=3DISO-8859-1=
<META content=3D"MSHTML 6.00.6000.16915" name=3DGENERATOR></HEAD>
<BODY bottomMargin=3D0 leftMargin=3D3 topMargin=3D0 rightMargin=3D3>
<DIV></DIV>
<DIV>Rob,</DIV>
<DIV> </DIV>
<DIV>A couple of questions and a couple of comments:</DIV>
<DIV> </DIV>
<DIV>What is the length of the wing airfoil chord on the "moth".....?</D=
IV>
<DIV> </DIV>
<DIV>The wings are swept back , right?</DIV>
<DIV> </DIV>
<DIV>The "allowable range" of CG seems like quite a lot........almost 9 =
=
inches.......hmmmmm.....</DIV>
<DIV> </DIV>
<DIV>Plans and builder's manuals always seem to give the CG range in wei=
rd =
ways.......like using the fire wall as a datum.....of course this does w=
ork =
fine, but it is more "normal" to give CG as % MAC. (Mean aero=
dynamic =
chord), while using the wing leading edge as the datum..........</DIV>
<DIV> </DIV>
<DIV>Figuring out the MAC is slightly involved when you have sweep=
-back =
wings, and two wings with stagger, and a straight center section between=
the =
wings........but simple geometry can be used..........</DIV>
<DIV> </DIV>
<DIV>It makes sense to design a plane with tandem seating........put the=
pilot =
in the rear seat to "balance" the aircraft, and put the copilot right on=
the =
CG.........this way the balance does not change from solo to dual flight=
, only =
the flying weight changes....also some change during flight due to fuel =
=
burn.........</DIV>
<DIV> </DIV>
<DIV>Funny that they spec out the CG to the 1/100 th of in inch!........=
...We =
have showed in earlier posts that this degree of accuracy &nbs=
p; is =
totally unnecessary!...i.e. errors in the weighing of 10% or more cause =
little =
change in the actual CG calculation......</DIV>
<DIV> </DIV>
<DIV>Like, would it have killed them to spec out the CG..........30" to =
38" aft =
of datum........?</DIV>
<DIV> </DIV>
<DIV>With such a large range of CG specified, I would be leery of flying=
at the =
extremes.......I would like to see the actual CG fall pretty close to th=
e center =
of the allowable range......</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>Cookie</DIV>
<DIV> </DIV>
<DIV>PS........A story to illustrate what I am talking about......"degre=
e of =
accuracy"........</DIV>
<DIV> </DIV>
<DIV>In school the class was studying dinosaurs..........</DIV>
<DIV> </DIV>
<DIV>The teacher asked the class, "When did the dinosaurs roam the Earth=
?"</DIV>
<DIV> </DIV>
<DIV>Little Johnny quickly raised his hand......."I know, I know"</DIV>
<DIV> </DIV>
<DIV>"Yes, Johnny, when did dinosaurs roam the earth?" the teacher =
repeated.</DIV>
<DIV> </DIV>
<DIV>"50,000,002 years ago", was Johnny's answer.......</DIV>
<DIV> </DIV>
<DIV>Fifty Million, AND <EM>TWO?</EM> the teacher asked<EM>........"</EM=
an interesting answer Johnny. How did you come up with that answer?"</DI=
V>
<DIV> </DIV>
<DIV>"Well", said Johnny....."My textbook says dinosaurs roamed the Eart=
h fifty =
million years ago, but I noticed that the book is <STRONG>two</STRONG> y=
ears =
old!"</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV>On Tue, 27 Oct 2009 20:41:12 -0400 Rob Brooke <<A =
href=3D"mailto:rbro-@tampabay.rr.com">rbro-@tampabay.rr.com</A>> =
=
writes:<BR>> <BR>> Hi, Bill,<BR>> <BR>> I puzzled over the C=
G =
information in the Tiger Moth construction <BR>> manual<BR>> for q=
uite a =
while. I couldn't find anywhere in the manual what the <BR>> CG<BR>&g=
t; range =
should be for the plane in flight. I really didn't care what <BR>> =
the<BR>> empty CG was supposed to be since the plane doesn't fly empt=
y =
and<BR>> couldn't get off the ground if it tried. I called Gene to as=
k what =
<BR>> the<BR>> CG range is of the loaded and ready-for-flight airp=
lane and =
got the<BR>> following answer: 29.98" to 38.69" aft of the firewall. =
That =
puts <BR>> the CG<BR>> right where you'd expect it - at the forwar=
d =
(passenger) cockpit <BR>> and<BR>> also provides the answer to the=
mystery =
8" range. It's just a <BR>> little<BR>> over 8" between the forwar=
d and =
aft limits of the in-flight CG.<BR>> <BR>> Well, all manuals can't=
be =
perfect.<BR>> <BR>> Rob Brooke<BR>> Tiger Moth #45<BR>> <BR>=
<BR>> :<BR>> Find 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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