Plane & Pilot
Sunday, January 1, 2006

Balancing Act


Understanding the center of gravity


Balancing ActSometime back in the dark ages, I was getting ready to take my instrument instructor check ride, and the examiner, who was an actual FAA type from the FAA headquarters, asked me if I had done a weight-and-balance for the flight. Two thoughts flashed through my mind, the first being the obvious question: What has a weight-and-balance calculation got to do with an instrument check ride? The second was a little panicky thinking while I tried to remember how to do the calculations.

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Without even thinking about it, when fighting a nose-up or nose-down tendency, a pilot simply trims the airplane, which actually is asking for more or less lift out of the tail. By driving the trim tab out of line, the elevator is forced up or down, which changes the empennage lift. The next time you’re flying along, look back at your tail. Chances are you’ll see the elevator is either slightly up or down and the trim is holding it there. Also, the horizontal tail is mounted with an angle of incidence that may be positive or negative, again, depending on the airplane, which means it starts out generating lift one way or the other; the position of the elevator just modifies that lift. What happens if the CG is placed out of the envelope?

Okay, think about what the tail is doing if the CG is ahead of the center of lift. The center of lift tries to lift the tail, so you crank in some trim, which displaces the elevator up, driving the tail down. But for a given size of tail, there’s a limit to the amount of force it can generate. Eventually, there’s a forward CG position that, as the airplane slows down for landing and you have it perfectly set up, you discover the stick is against your chest, but you still can’t hold the nose up (at that speed, there’s not enough tail authority to overcome the nose-down pitch). So, you belly-flop onto the ground. It’s ugly, but you’re not likely to hurt yourself or the airplane.

An airplane with a forward CG actually is much more stable in the air because the mass is forward and the tail is loaded. It’s sort of the lawn dart theory of aerodynamics, where the heavy end always winds up out front and wants to stay there.

The opposite is true when the CG is aft, and this is where it can get dangerous. As the CG is placed farther back, the airplane becomes increasingly unstable, with the elevator becoming more and more sensitive. Eventually, the tail can’t quite trim out the out-of-balance configuration and you get a divergent situation where, when the nose is displaced up or down by turbulence, rather than returning to neutral (as is the normal stability pattern), it moves away from neutral. If it’s pitched up, it wants to move even farther up, and when you try to force it down, you can’t help but overshoot because any input you give is greatly magnified. The net result is a divergent oscillation (it gets bigger and bigger) that you can’t damp out, and the airplane usually crashes.

As the CG moves back, all airplanes get lighter in pitch. This is nothing to be alarmed about. In fact, the usual CG envelope has a little margin of error built in to it, but don’t depend on it. If the elevator is getting really light and twitchy, give some thought to redistributing your load. If you can’t do that in the air, land immediately and sort it out. The way you know you’re in trouble is when you’re already in trouble; then it’s too late to do anything about it.

When doing your CG calculations, be sure to remember that the CG isn’t going to stay in one spot during the flight. As fuel is burned, the CG will shift, but it doesn’t always shift the same way in all airplanes. You need to know which way it will travel while fuel is burning so you don’t inadvertently paint yourself into a CG corner. For example, you knowingly load the airplane right to the aft side of the envelope, but fuel is ahead of the CG, so when it burns, the CG shifts back, and guess what, it goes out of the envelope just because you’re burning fuel.

A good exercise is to take the airplane you usually fly and work up some “what-if” scenarios that represent your normal-use configuration and paste the outcomes in the back of the POH for quick reference. For instance, do each of the following extreme scenarios with full and then empty tanks so you know which is the worst-case loading.

• Pilot only
• Pilot only, max baggage in rear
• Pilot, two rear passengers, max baggage
• 200 pounds in each front seat

If any of these go out of the envelope, recalculate them and change the loading to keep it in the envelope. Make a note on the page you put in the POH, writing something like, “Me, two rear pax, full tanks, max baggage is 42 pounds. For 120-pound baggage, fuel limited to 1?2 tanks.”

Don’t assume you’re within the weight-and-balance limits. Prove it. It only takes a few minutes and may be the best few minutes in which you ever invested.





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