REFLECTOR: MODIFICATIONS - Aft cg limit

[John Dibble]

Douglas Holub wrote:

> I wish there was a way to determine by testing where the aft c.g. limit is
> on a Velocity, the c.g. at which a deep stall occurs.

I believe there is a way.  At least I have done so with my SERG because I wanted to
know for sure where the aft cg limit is for my plane.  The factory numbers are general
ones and I expect they are a bit conservative for liabiity if for no other reason.
The thing is, if you are going to perform stalls where you may be aft of the true cg
limit for your plane, you have to understand the deep stall issue and how to
avoid/deal with it.  Last year Scott B gave an excellent description of how to perform
a stall test and how to be aware of when you are about to enter a deep stall.  Here is
what he wrote:

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

To better understand the aft cg situation, I think it helps to consider the
following.  The moment arms for the rear seats and the fuel tanks are both close to
the aft cg limit.  Therefore when the plane is loaded near aft cg, fuel and weight in
the rear seats have little affect on the cg location.  The only thing that matters is
the weight in the front seats.  Of course weight in the nose matters, but I'm assuming
that does not change.  So as long as you have sufficient weight in the front seats,
you will be within the aft cg limit for your plane.  If you can stall the canard, then
you are within the aft cg limit.  If you can stall the canard when flying solo with no
other weight in the front, then you never have to do an aft cg calculation again,
unless you lose weight or a lighter pilot solos in your plane.

John