Balancing Act

I whipped out the POH and fumbled around, finally coming up with a number. Then I ran the pencil point across the center of gravity (CG) envelope in the back of the POH and realized I had made a mistake. The point was outside the front edge of the envelope for this brand-new Piper Cherokee 140. I went back and rechecked my calculations and frowned. It still came out saying the CG was out of the front edge of the envelope. While all of this was going on, the examiner was sitting behind his desk, doing paperwork with a knowing look on his face.

Finally, I admitted, "I don't know what I'm doing wrong, but it says that with just you, me and fuel, we're out of the front edge."

He half-grinned and said, "So, can we go flying?"

I thought back to my CFI training and said, "Not legally. What do we do?"

With a triumphant look on his face, he led me out to the Piper Cherokee, flung the door open, dropped his big, airline-style map case in the back and, while strapping it down, said, "This will do it. Let's go."

He had obviously done this to check-ride candidates before. After I had passed the check ride (thankfully!), we sat down, instructor to instructor, and he told me that he always had his candidates jump through the weight-and-balance hoops to teach them a lesson: You can never take something as important as weight and balance for granted because it's often not what you think it is.

The good news is that weight-and-balance problems are run and rerun in groundschool, and it's pretty hard to graduate without, if not mastering CG calculations, at least being familiar with the concepts. Unfortunately, shortly after passing your private-pilot license check ride, both the details and the concepts begin to get a little fuzzy in our minds because precious few people actually do weight-and-balance calculations as a matter of course.

There's a valid reason why most people pretty much ignore CG calculations: 90% of the time, they're flying the same airplane with the same load (two people, fuel and a little baggage), so although they don't know where it is in the CG envelope, they know it's safe because they've flown it so often in that condition. And it usually is safe. But is not checking it safe?

As an airplane starts bulging because so much stuff is being crammed into it, even the dimmest bulb on the tree knows it ought to run a CG check. Intuitively, we know we don't want the CG to go too far aft. We're not exactly sure why, but we know it's not a good thing, so we try to avoid it.

There are lots of situations where you can be out of either side of the envelope, but still far from being either heavy or light, however. This tendency varies greatly from airplane to airplane. So, what makes the difference?

One of the parameters that defines the CG envelope is the mission for which the airplane was designed in the first place. For instance, if an airplane is designed to carry a lot of load and have as wide an envelope as possible, the makers know that everything a user puts in the airplane is likely to shift the CG back. Therefore, they intentionally set it up so that when it's empty, the CG is as far forward as it can possibly be and still have enough tail authority to land it. Or, as was the case with my Cherokee 140 during the check ride, the combination of additional equipment and weight (fuel) pull the CG forward enough that it's out of the envelope.

A lot of heavy-haulers, like Cherokee Sixes, Lances and other true six-place airplanes, sit right on the front of the envelope when empty, so as the seats fill up, the CG shifts back, but not so far that it's out of the envelope. Quite often, when additional equipment, like radios, are added to an airplane that already has a far-forward CG, it's pushed well out of the envelope in certain configurations, such as full-flap landings with only a pilot onboard.

Then why does it matter that we keep an airplane in its envelope? Are there bogeymen standing around the perimeter of the envelope ready to bite us if we put a toe outside the line? The answer to that is a resounding yes, especially if you go out of the back of the envelope where that particular bogeyman can bury you.

There are several interacting aeronautical factors that come into play when considering what happens outside either side of the envelope. One of these is the relationship between the center of lift (where the big "lift" arrow is acting on the wing) and the CG of the airplane.

First, a basic fact of physics: Aerodynamics notwithstanding, a mass rotates around its centroid, which is another way of saying an airplane will always try to rotate around its CG. If the center of lift isn't right on the CG, which it seldom is, then that big lift arrow is going to try to rotate the airplane about the CG. If it's forward of the CG, it will try to pull the nose up. If it's behind the CG, it's just the opposite.

If the airplane expects to maintain controlled flight, the tendency of the lift to rotate the airplane about the CG has to be counterbalanced, and that's what the horizontal tail is for. If the center of lift is ahead of the CG and is trying to pull the nose up, it's trying to push the tail down. So you push a little forward stick, which puts the elevator down and increases the camber of the tail airfoil, generating more lift and balancing the nose-up tendency. If the wing's lift is behind the CG, it's trying to pick up the tail, and some back stick is required, so the tail is lifting down, not up.

What can really complicate this is that as an airplane slows down and the angle of attack goes up, the center of lift moves around, which changes the relationship of the CG and the lift arrow, generating a pitching moment of its own. And then, of course, as fuel burns off, the CG changes position and, depending on the airplane, may try to raise or lower the nose.

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.