Figuring CG as a percentage

[Robert M]

I have seen it in print SOMEWHERE, but can?t remember where. I was wondering what the ?CG percent? is? In other words, ?the CG is 15 to 30 %?. 15 to 30% of what?

I know Van?s aircraft provides the CG limits for my RV-9 in inches aft of the datum. For grins and giggles, how would I figure that as a percentage?

Thanks,

Robert

 

[TomVal]

I have seen it in print SOMEWHERE, but can?t remember where. I was wondering what the ?CG percent? is? In other words, ?the CG is 15 to 30 %?. 15 to 30% of what?

I know Van?s aircraft provides the CG limits for my RV-9 in inches aft of the datum. For grins and giggles, how would I figure that as a percentage?

Thanks,

Robert

Robert,

This should answer your question.

http://www.flight-mechanic.com/mean-aerodynamic-chord/

Regards,

 

[Mike S]

Percent of the wing chord-------distance from leading edge to trailing edge.

On a simple wing like the RV, this is pretty simple. On tapered and or swept wings------not so simple.

 

[Robert M]

Perfect thanks. Jogged the memory.

 

[cdeerinck]

Another way

I am not intending to disagree with what was written above.

However, in the gliding world, we frequently refer to the CG% as where the CG of a given configuration falls within the allowed limits of the glider.

In other words, 0% is the full forward limit, and 100% is the full aft limit.

Lower numbers are generally "safer" (i.e. harder to accidentally stall when thermalling at low speeds).

For normal practice flying, something in the range of 60% is typical. In contest flying, it might be pushed back for a more experienced pilot into the 80-85% range, as it influences the gliders low-speed handling characteristics.

I doubt any of this applies much to the powered world, but I thought it at least worth sharing.

 

[gyoung]

Typically it's specified as % MAC (mean aerodynamic chord). As noted above, MAC is straight forward for a Hershey bar planform, not so much for others.

 

[Robert M]

I am not intending to disagree with what was written above.

However, in the gliding world, we frequently refer to the CG% as where the CG of a given configuration falls within the allowed limits of the glider.

In other words, 0% is the full forward limit, and 100% is the full aft limit.

Lower numbers are generally "safer" (i.e. harder to accidentally stall when thermalling at low speeds).

For normal practice flying, something in the range of 60% is typical. In contest flying, it might be pushed back for a more experienced pilot into the 80-85% range, as it influences the gliders low-speed handling characteristics.

I doubt any of this applies much to the powered world, but I thought it at least worth sharing.

You are correct about the CG being different. The datum for the RV-9 is 70" forward of the leading. The measured CG "range" for my RV-9 is from 77.95" to 84.84" aft of the datum line. As long as the measured CG stays within the limits of the assigned CG range through all phases of flight - we are good to fly.

Having said that - and bragging about the RV's - my RV-9 can carry both my wife and myself, full fuel to no fuel and 75lbs. of luggage and never be out of CG.