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How Does CG Affect Aircraft Performance?

Aleksander Markin

When you load up your airplane with passengers, bags, and fuel, you need to make sure you're within the center of gravity (CG) limits. And you probably know that when you move your CG forward or aft, it affects your airplane's performance. But how? Let's take a look.

Forward Vs. Aft CG

The relationship between aircraft performance and CG location is simple: by moving the CG forward and aft, you change the amount of tail down force and lift you need for stable flight.

If you need more lift, you create more induced drag, and your performance goes down. But why are lift and CG located related?

Your Airplane's Balancing Act

Your airplane balances on its CG, and that location is where the downward force acts on your aircraft.

Your lift acts upward from your wing, at a point called the center of lift. And the center of lift, as long as you are within the aircraft's CG limits, is always aft of your CG.

If you look at the diagram below, you can clearly see that if you didn't have a tail creating its own force, your airplane would nose straight down because of the rotation force called torque.

Your elevator (or stabilator) balances out the torque by creating another downward force, called tail-down force, by generating torque in the opposite direction.

Essentially, your tail is an 'upside down' wing that generates lift downward. The amount of lift it needs to maintain depends on two factors: CG location, and aircraft weight.

How Torque Comes Into Play

The torque calculation is pretty simple: Force X Distance = Torque. So if you have a large amount of force at a small distance, it can be balanced out by a small amount of force at a large distance. You can see it in the diagram below.

So Lift And Weight Don't Equal Each Other?

One of the first things you learn as a pilot is that in level, unaccelerated flight, lift and weight equal each other. But that's not exactly the case.

When you look at the diagram below, what you really see is that weight and tail down force both need to be balanced out by lift. Essentially, lift needs to equal weight and tail down force.

How Does Drag Factor In?

When you create more lift, you create more induced drag as well. So as you can see, when your CG is forward, you need more tail down force, and more lift. You generate that extra lift by increasing your angle of attack, which in turn increases induced drag.

And as always, when you have more drag, you decrease performance.

How Much Difference Does It Really Make?

Before you go throwing everything possible into the baggage compartment, consider this: while having an aft CG definitely improves performance, in most light aircraft, the difference in will be a few extra knots of airspeed, and a little better climb rate.

So if you're taking a long cross country, an aft CG might help you get to your destination a few minutes early, but in most cases, it won't make a huge time difference.


Colin Cutler

Colin is a Boldmethod co-founder, pilot and graphic artist. He's been a flight instructor at the University of North Dakota, an airline pilot on the CRJ-200, and has directed development of numerous commercial and military training systems. You can reach him at colin@boldmethod.com.

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