Weight and balance is important for any airplane, but it's especially critical in a large supersonic aircraft for reasons that might surprise you. Veteran airman Phil Rowe talks about some W&B lessons learned from his days flying in the B-58 "Hustler," America's first supersonic bomber.
July 23, 1997
|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
If you enjoy Phil's writing on AVweb, check out his
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
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
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
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.