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Back and Forth on Weighty Issues

One of the “rites of passage” of learning to fly is learning to compute the center of gravity -- but how many pilots keep plotting c.g. after their checkride and just how important and how useful is it? You may be surprised...One of the “rites of passage” of learning to fly is learning to compute the center of gravity -- but how many pilots keep plotting c.g. after their checkride and just how important and how useful is it? You may be surprised...

What We Know, What We May Not
We all know that the load distribution is supposed to be inside center of gravity limits. But what if it’s not? And what happens to c.g. position in flight?

To maneuver within design limits, an airplane needs stability and control authority. Stay within c.g. limits and you should have enough of both to fly the way the airplane was designed. Go outside the envelope, though, and your airplane is unpredictable. It might be controllable, and it might get good performance, but it won’t be as controllable or perform as well as it should. It might not be controllable to perform at all. Plus, you won’t know until you’re committed to flight!

Flight Characteristics: FORWARD CG
An airplane loaded forward of c.g. limits will tend to pitch nose down, it will be *very* stable and resist any change in attitude -- and it comes with serious side effects. It will also take more elevator force to raise its pitch attitude. (Translation: You’ll need additional airflow over the elevator to pitch up or to resist the tendency of the nose-heavy airplane to pitch down.) A c.g.-forward airplane will need more speed -- and therefore more runway -- to take off. It may not even have enough control authority to take off at all. It’ll need the elevator deflected more for a given climb attitude, so there’ll be more drag, and climb performance will suffer. In cruise, the increased drag means less speed or more power (and fuel flow) necessary to get the same speed.

A forward c.g. that was controllable in flight might be fatal, on landing. As you slow down on final approach, you might reach the point when even full up elevator won’t keep the nose from pitching into a dive toward the ground.

Flight Characteristics: AFT CG
An airplane with an aft-of-limits c.g. will be very unstable. It will not try to regain attitude if bumped by turbulence, but instead will wobble around, requiring active control. It’ll tend to pitch up toward a stall on takeoff, and might require full down elevator at slow speeds to resist the nose-up tendency ... again, full elevator deflection may not be enough. It’ll trim out at a faster airspeed in cruise (you’ll need to trim nose down, reducing induced drag), but it may be so “squirrelly” that you can’t control it in turbulence at all.

An aft c.g. will attempt to pitch up toward a stall, on landing. The slower you get on final approach, the more elevator deflection it’ll take to hold attitude. Worse, as the airspeed reduces and the nose high pitch tendency increases there will be less elevator travel available to fight it. The aircraft can easily fly itself -- and you -- right into a stall.

COMPLICATIONS
In most airplanes, the center of gravity location doesn’t stay constant during flight. Since the majority of designs put fuel in the leading edge of the wings (fairly far forward), burning fuel in flight tends to move the c.g. aft. Some designs are easy to load properly for takeoff, only to go dangerously out of limits “aft” after a couple hours’ fuel burn.

Defense: Check your “takeoff condition” c.g. just like you learned for your Private checkride, but find the c.g. location as loaded but without any fuel aboard as well. You may find you can’t fly the whole trip as planned and be within limits when you land.

Pushing The Envelope ... Right Up To The Sticky Part
I enjoy occasionally watching cartoons with my young son. Once we were watching “Pinky and the Brain,” the story of two genetically altered mice that make repeated, outlandish and always flawed attempts to take over the world. In one episode the two leapt into a jet aircraft and began to take off. “Are we going to the edge of the envelope?” Pinky asked from the rear seat. “No,” replied the Brain as they began their takeoff roll, “...but we may get as far as the sticky part.”

CAREFUL: The effects of venturing to the edge of the c.g. envelope aren't obvious from the line that represents it in your POH. You know that staying on the inside is good and going outside is bad, but you can get to the “sticky part” of the envelope even within the design's acceptable c.g. range limits.

TRICKS OF THE TRADE: Using The C.G. Range To Your Advantage
If you’re inside but near the forward limit, the airplane will take a hefty tug on takeoff to get into a climb attitude, but watch for the need to ease off on the backpressure once you accelerate and the elevator becomes more effective. The forward-but-within-limits airplane will cruise a little slower (drag from the displaced elevator), but will be more stable in turbulence -- you might even take advantage of a forward c.g. and plan your load near the forward limit next time you fly in rough air or near mountains. On landing, remember you may have to pull with both hands to correctly flare and know that a missed approach or a go-around will require a strong pull on the yoke ... if you haven’t burned enough fuel to move the c.g. aft.

If the air is smooth you might plan for a rearward-but-within-limits c.g., for a faster cruise speed (lowered nose in flight). Caution: The airplane will tend to nose up on takeoff, and (especially if it’s one of those designs that have a big rearward c.g. shift with fuel burn) it will be more likely to stall on landing.

BOTTOM LINE: Reacquaint yourself with center of gravity calculations. If you’re like most, you’ve probably let this critical skill atrophy. Remember to check “landing condition” location as well as obstructions for takeoff. Any takeoff or approach that requires an unusual climb or decent profile may be dangerous if attempted at or near the aircraft's c.g. limits. However, understanding the relationship between c.g. location, stability and control authority, will help you manipulate the performance of your aircraft to best match a given set of conditions -- and you will be a better pilot.

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About This Author:
Tom Turner is a widely published author and regular forum speaker at EAA's Oshkosh/Airventure and American Bonanza Society. Tom holds an M.S. in Aviation Safety with an emphasis on pilot training methods and human factors. He has worked as lead instructor at FlightSafety International, developed and conducted flight test profiles for modified aircraft and authored three books including: Cockpit Resource Management: The Private Pilot's Guide and Instrument Flying Handbook (both from McGraw-Hill). His flight experience currently spans 3000 hours with approximately 1800 logged as an instructor. Tom's certificate currently shows ATP MEL with Commercial/Instrument privileges in SEL airplanes.
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