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I got this email by accident and it looks as if he's calculating the weight for a small plane. If so, I don't want to know what he's hauling!

My dad had a Piper Cherokee and I know he didn't make it this complicated (if I'm correct) and he's an aeronautical engineer.

"I was a little off on the engine fund: we charge ourselves $65/hour, not $165.

I worked up one CG scenario with 370 pounds in the front seats, set to full aft position, 200 pounds in the 3rd and 4th seats and 100 pounds baggage and full fuel.  The takeoff weight is then 3294, with CG of 84.18 (limit at that weight is 84.8).  After burning 360 pounds of fuel, the airplane lands at 2934 pounds with a CG of 85.31.  Limit at that weight is
85.6.

Any other scenario?  It just takes me a minute.  You need the “fireplug” moment limits versus weight chart, which I can’t send to you yet because I can’t get my scanner to talk to my new computer.  I will try to send you a picture of it.

The Fuselage Stations are as follows:

Pilot and front PX:  85 to 89 inches, depending on where the seats are set
3rd & 4th seats: 121 to 135 inches
Baggage area: 150 inches
Fuel tank: 75 inches.

Just multiply weights in each location times the arm, and add them all up.  Add the airplane empty data of 2191.64 pounds with a moment of 172741.96.  Divide the total moment by the total weight to get the CG.  The limits on aft CG are 84.4 inches at 3400 pounds, and 85.7 at 3000 pounds or less.

Chuck"

????

GreggO

he is ensuring that the moment arm is within the CG limit of the aircraft for a given weight, to ensure aircraft stability.
It is interesting that in some aircraft you can take off within CG limits but as fuel,is burned off go out of limits (or vice versa)
Most of my experience has been calculating CG in therms of Mean Aerodynamic Chord (MAC)
The inches he referenced were to the Reference Datum, which is often outside the aircraft structure.  For example, the X axis Ref Datum for a C-130 is forward of the aircraft (the radar bulkhead is FS 93 inches) and the Z axis Ref Datum is below ground.

USAF Form F AT&T the link
http://www.dtic.mil/whs/directives/infomgt/forms/eforms/dd0365-4.pdf (http://www.dtic.mil/whs/directives/infomgt/forms/eforms/dd0365-4.pdf)

Uhhhhh,.......        :confused:

I know that makes sense to you, but for the rest of us what does it mean?               :dunno:

Thanks,


G

Let me explain...

(http://i.imgur.com/EKQtuHL.jpg)

If the center of gravity is too far aft you run out of elevator effectiveness, you can't raise the nose.  Also, aircraft tend to be unstable in that condition.  Weight too far forward is just as bad.

no, it is PFM  :naughty:

Stogie, when I saw you responded I started to laugh. You did not disappoint!

GreggO

Now, that's something a Red Bank High class of '74 graduate can understand.                                                        :biggrin:


G

Here's what happens when the load shifts and things go out of balance

http://www.theguardian.com/world/video/2013/may/01/747-cargo-plane-crash-bagram-airbase-video