You may have heard that center of gravity position affects your aircraft's performance - but do you know why? When we were flight instructing, the explanation usually involved lots of white-board diagrams - with lots of bad dry-erase art. So, to help rid the world of bad CFI art, we've released our first CFI tool - which interactively shows you how CG location affects performance.
First, please forward this page to your instructor. The tool is free to use and will make their life a little bit easier. We really appreciate you passing it on!
The relationship between CG location and aircraft performance is really quite simple - by moving the CG fore and aft, you change the amount of lift and tail down force your aircraft needs for stable flight. More lift = more drag = less performance. Simple. So why are lift and CG location related?
It's All About Balance and Leverage
Your aircraft balances on its center of gravity, or CG. This is where the main downward force acts on an aircraft. Your lift acts upward from the - big surprise - center of lift, which is aft of your CG. These two forces act against each other, and if left unchecked, would rotate the nose straight down. This rotation force is called "torque."
Your elevator balances this torque by providing another downward force, called the tail-down force, by generating torque in the opposite direction. It's essentially a second wing that acts upside-down, generating lift downward. The amount of tail-down force your elevator needs to maintain stable flight depends on two factors: aircraft weight and CG location.
How Much Torque?
Calculating torque is simple: force X distance = torque. A large amount of force at a small distance (aircraft weight vs. lift) can be balanced by a small amount of force at a large distance (aircraft tail-down force vs. lift). As the CG moves forward and away from the center of lift, the torque increases. To counter it, your elevator must generate more tail-down force.
The diagram below illustrates this. Drag the orange weight arrow left and right and you'll see how tail down force, lift and drag change. You can see the full diagram here.
Doesn't Lift Equal Weight?
When you started your training, you learned about the four forces of flight: lift, weight, drag and thrust. Tail-down force wasn't one of them, and you learned that lift equals weight. Not true.
Your instructor wasn't lying - it's just more simple to think of tail-down force and weight acting together. However, lift must balance all of the downward forces acting on your airplane - weight and tail-down force. Essentially, lift = weight + tail-down force.
What About Drag?
The more lift you produce, the more induced drag you produce. By moving your aircraft's CG forward, you require both more tail-down force and lift. To generate it, you increase your angle of attack, which increases your induced drag. Now you need more thrust to overcome drag, and your performance decreases.
Before You Send Your Passengers Into The Tailcone...
On many aircraft, you will notice a performance difference between forward and aft CGs. However, you need to keep the CG within your aircraft's envelope. If the CG moves too far forward, you can exceed your elevator's ability to balance the aircraft in certain flight conditions. If your CG moves too far aft, the aircraft may become unstable. Keep it in limits, and remember, the CG will move as you fly and burn fuel. Make sure you take off in limits and land in limits.
