REFLECTOR: Aft cg limit

[Al Gietzen]

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of John Dibble
Sent: Wednesday, January 05, 2005 1:13 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: MODIFICATIONS - Aft cg limit

 

 

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. 

 

I agree with this; but I would think that it does make a difference how much
fuel is in the tanks since the CG of the fuel shifts as you pitch up, and
the amount and effect of the shift varies with the amount that is in the
tank. No?  I'd guess that with about half full tanks you can shift the CG by
more than 0.5" by a change in pitch attitude.

 

Al

 

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

 

 

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