July 23, 1997 A Matter of Balance

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by	Philip A. Rowe

About the Author ... Phil Rowe is a retired USAF navigator and R&D engineer, and now does freelance writing, mostly about his own flying experience in 33 types and models of military aircraft, from props to jets. Phil served in a variety of aircrew positions, as: celestial navigator, radar navigator and bombardier, electronic warfare officer, flight engineer and photo reconnaissance systems operator. He also served as flight test engineer on three projects. Favorite airplanes include the RF-4C, B-58A, B-52D and a few light planes - including sailplanes. If you enjoy Phil's writing on AVweb, check out his web site for more stories about aviation, travel, camping, and more.

High performance airplanes, those which fly at both very high speeds in combat and at modest speeds for takeoffs and landings, pose special problems of balance. Failure to manage balance can be catastrophic, while properly maintaining balance enhances the airplane's performance and saves fuel.

The wing (and sometimes the fueslage too) provides the necessary lift to sustain flight. But the center of lift (CL) along the wing's surface is not stationary. Typically, the faster the plane goes the farther aft the CL shifts. And conversely, as the plane slows down the CL moves forward. This shift can be quite large on aircraft with wide speed ranges.

The center of gravity (CG) moves too. As the plane consumes fuel or drops its payload (e.g., bombs), there is usually a shift of the CG.

Imagine a line through the airplane's wing, running fore and aft parallel to the nose-to-tail axis of the plane. This line is called the mean aerodynamic chord (MAC) and approximates the measure of the wing's lifting surfaces. The foward end of that line is referred to as the "0% MAC" point, and the aft end is the "100% MAC" point. CG and CL are always somewhere in between those extremes.

An airplane flies because of the balance between the forces of lift, gravity, thrust and drag. It climbs, falls, accelerates or decelerates as those forces become unbalanced. More thrust will, not surprisingly, accelerate the plane. You get the idea.

But there is another kind of balance that's just as important: the balance between CG and CL. That balance determines the stability of the plane in the pitch axis. The plane can pitch up and pitch down or remain stable depending upon the forces of gravity, lift and the effect of the elevators. There are other factors too, of course, but will not be included here.

Your author flew in B-58's, America's first supersonic bomber. That plane required constant monitoring and control of its CG, because it flew over such a wide speed range. At traffic pattern speeds the CG was usually kept in the 27% to 29% MAC range, but at Mach 2 the CL shifted aft and required a comparable aft CG shift to minimize drag due to elevon position. (The B-58 used control surfaces called "elevons" which combined the function of elevators and ailerons.) Minimizing drag to due up elevon position was important at 1200 miles per hour!

The position of the CG was usually controlled by tranfering fuel fore or aft. And as fuel was consumed the amount available to use as a counter-balance diminished.

On a typical mission that included a Mach 2 segment, fuel would be shifted aft to about 34% MAC to streamline the elevons and reduce fuel-consuming drag. But before you could slow down to subsonic cruise you had to move fuel forward again to maintain a positive static margin. If you didn't, you faced the possibility of a negative static margin and an uncontrollable airplane.

Fuel and CG balance kept the crew busy, especially the fellow in the back seat who served as a combination flight engineer and defensive systems operator.

Failure to properly monitor and control the CG, according to the flight conditions, cost the Air Force several B-58 crashes and one in the B-1 two decades later.

Add to the shifts of CL and CG during normal cruise the complications of dropping bombs (some as heavy as 7000 pounds) and you have one more ingredient to worry about.

Balance is not only important, it's crucial to safe flight. Think about it.