REFLECTOR:Determining AFT CG Limit
Scott Baker
reflector@tvbf.org
Sat, 6 Dec 2003 12:53:32 -0500
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Lack of pitch stability is certainly a clue.
Set out a detailed, conservative flight test program to determine the =
aft cg limit of your aircraft.
Step 1. would be to calculate the loading conditions to achieve a =
rear-most cg (as shown in the POH).
Step 2. would be to load the aircraft so that the cg of the aircraft is =
... say 2" forward of the rear most suggested limit and go fly the =
aircraft. At a safe altitude and at a safe location - enter slow flight =
(75-80 knots) and raise the nose (about 7-10 degrees pitch up attitude) =
into a low power climb (about 16" map or around 1800 rpm) and wait for =
the onset of the canard stall. Try to hold the pitch attitude during =
the stall. If the nose is dropping and (in order to maintain the =
constant climb attitude) you are pulling "nose up" stick force, this is =
good. Land, calculate what it will take to shift the cg aft another =
0.5" and go do it again.
VERY IMPORTANT NOTE> If, on the other hand, at the onset of the stall =
you find the canard rising - and you are pushing on the stick to =
maintain the constant climb attitude - you are in the dreaded "too far =
aft" cg range ... immediately apply full forward elevator and full =
power, lean forward, and take thyself back to the airport and (while =
adding 5 knots to your normal approach speed) ... land and shift some =
weight forward ... and then go do it again.
SB
----- Original Message -----=20
From: Ronnie Brown=20
To: reflector@tvbf.org=20
Sent: Saturday, December 06, 2003 8:34 AM
Subject: REFLECTOR:Determining AFT CG Limit
There's been a lot of discussion the last couple of months about flat =
stall on Velocities and it has even come up in the more recent VG =
conversations.
But I don't recall any discussions on how to determine where the aft =
CG limit is. Just what phenomenon are we looking for when we are on the =
ragged edge of AFT CG other than the dreaded Flat Stall???
On a conventional airplane, the aft CG is determined by the lack of =
pitch stability as the CG is moved rearward.=20
Danny Maher ran a bunch of "trailer" tests and there were accidents =
but beyond that, just how do you determine what the aft CG is (other =
than use the numbers the factory provides).
Ronnie Brown
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<BODY bgColor=3D#ffffff>
<DIV><FONT face=3DArial size=3D2>Lack of pitch stability is certainly a=20
clue.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Set out a detailed, conservative flight =
test=20
program to determine the aft cg limit of your aircraft.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Step 1. would be to calculate the =
loading=20
conditions to achieve a rear-most cg (as shown in the POH).</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>Step 2. would be to load the aircraft =
so that the=20
cg of the aircraft is ... say 2" forward of the rear most suggested =
limit and go=20
fly the aircraft. At a safe altitude and at a safe location - =
enter slow=20
flight (75-80 knots) and raise the nose (about 7-10 degrees pitch up =
attitude)=20
into a low power climb (about 16" map or around 1800 rpm) and wait =
for the=20
onset of the canard stall. Try to hold the pitch attitude during =
the=20
stall. If the nose is dropping and (in order to maintain the =
constant=20
climb attitude) you are pulling "nose up" stick force, this is =
good. Land,=20
calculate what it will take to shift the cg aft another 0.5" =
and go do=20
it again.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>VERY IMPORTANT NOTE> If, on =
the other=20
hand, at the onset of the stall you find the canard <EM>rising</EM> - =
and you=20
are pushing on the stick to maintain the constant climb attitude - you =
are in=20
the dreaded "too far aft" cg range ... immediately apply full forward =
elevator=20
and full power, lean forward, and take thyself back to the airport =
and=20
(while adding 5 knots to your normal approach speed) ... land and shift =
some=20
weight forward ... and then go do it again.</FONT></DIV>
<DIV><FONT face=3DArial size=3D2>SB</FONT></DIV>
<BLOCKQUOTE dir=3Dltr=20
style=3D"PADDING-RIGHT: 0px; PADDING-LEFT: 5px; MARGIN-LEFT: 5px; =
BORDER-LEFT: #000000 2px solid; MARGIN-RIGHT: 0px">
<DIV style=3D"FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV=20
style=3D"BACKGROUND: #e4e4e4; FONT: 10pt arial; font-color: =
black"><B>From:</B>=20
<A title=3Dromott@adelphia.net =
href=3D"mailto:romott@adelphia.net">Ronnie=20
Brown</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>To:</B> <A =
title=3Dreflector@tvbf.org=20
href=3D"mailto:reflector@tvbf.org">reflector@tvbf.org</A> </DIV>
<DIV style=3D"FONT: 10pt arial"><B>Sent:</B> Saturday, December 06, =
2003 8:34=20
AM</DIV>
<DIV style=3D"FONT: 10pt arial"><B>Subject:</B> REFLECTOR:Determining =
AFT CG=20
Limit</DIV>
<DIV><BR></DIV>
<DIV><FONT size=3D2>There's been a lot of discussion the last couple =
of months=20
about flat stall on Velocities and it has even come up in the more =
recent VG=20
conversations.</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>But I don't recall any discussions on how to =
determine where=20
the aft CG limit is. Just what phenomenon are we looking for =
when we are=20
on the ragged edge of AFT CG other than the dreaded Flat =
Stall???</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>On a conventional airplane, the aft CG is =
determined by the=20
lack of pitch stability as the CG is moved =
rearward. </FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Danny Maher ran a bunch of "trailer" tests and =
there were=20
accidents but beyond that, just how do you determine what =
the aft CG=20
is (other than use the numbers the =
factory provides).</FONT></DIV>
<DIV><FONT size=3D2></FONT> </DIV>
<DIV><FONT size=3D2>Ronnie =
Brown</FONT> </DIV></BLOCKQUOTE></BODY></HTML>
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