May 26, 2008
The Wright Brothers were successful because they combined two vital elements of airplane design: control and stability. Wilbur and Orville achieved control through a pioneering design that evolved into what's used in almost all fixed-wing aircraft today -- a system that makes use of the stability designed into the airplane. Controllability cannot exist without some measure of stability. Stability, in turn, is affected by several factors. One variable even changes over the course of a single flight -- the location of the center of gravity (CG).
When was the last time you computed aircraft weight and balance? What practical effect does knowing the CG location have on your flight planning? What happens to your airplane's stability as you burn off fuel in flight?
Computing the Load
By computing the CG location, you can predict how the airplane will handle. In some airplanes, handling can vary greatly with variations in CG location. If the CG is outside design limits, the airplane may not be controllable at all. How does CG location affect control, even within the certified envelope?
The further forward the CG, the greater its tendency to straighten the airplane out if disturbed by turbulence or control movement. Moving the CG forward increases stability. This is normally a good thing (especially for instrument flight), but even within the CG envelope, a forward CG has some adverse effects on performance, including:
- The need of additional elevator force -- and therefore more speed -- to raise the nose for takeoff. This means it'll take a longer runway to get up to control-force speed.
- For a given airspeed, a greater control deflection to hold a pitch attitude. Greater control deflection increases aerodynamic drag, reducing performance.
- In most flight regimes, increased downward force on the tail to resist the nose's tendency to drop. This results in increased drag and, indirectly, flight at a higher angle of attack for a given speed, both of which reduce performance even further.
- Reduced cruise speed for a given power setting and airplane weight, for the same reasons.
- Increased power (and fuel burn) necessary to achieve a given cruise speed.
- The need for additional up-elevator to flare for landing.
Note that all these effects happen even when the CG is within certified limits. If the airplane is loaded outside the limits on the forward edge of the allowable loading envelope, the airplane may be so stable that even full control deflection isn't enough to overcome the nose-down tendency. The airplane, in effect, becomes too stable to fly. A forward-CG loading that is controllable in flight, with ample airflow over the elevator, may become uncontrollably nose-heavy as the airplane is slowed and control effectiveness is lost. The nose-heavy airplane may be more likely to "mush" into the ground short of the runway, or land hard on the nosewheel when control effectiveness is lost in the flare.
What's typically nose-heavy? Some airplane designs are naturally nose-heavy. Turbocharged aircraft are particularly nose-heavy (from the weight of the extra turbo equipment ahead of the firewall), especially if there are no occupants or baggage in the airplane's aft cabin. Some fairly short airframes also end up nose-heavy without rear-seat passengers or baggage.
As CG goes aft, there is less distance between the CG and the center of lift, and the airplane becomes less stable. In the extreme, modern fighter jets are designed to be completely unstable for maximum maneuverability, depending on computer-driven controls to "create" stability for aircraft control. Although reduced stability increases maneuverability, a rearward CG also induces these effects:
- A tendency to nose up prematurely on takeoff, and to pitch up excessively in response to the "normal" pilot inputs for takeoff. This makes the airplane more likely to stall, and increases drag to reduce initial climb performance.
- If disturbed by turbulence, the airplane will not return to stable flight, but may "hunt in pitch" and require more active control input by the pilot. The aft-CG airplane is a much higher-workload aircraft to fly precisely -- an unstable balancing act.
- When slowed for landing, it may require nose-down elevator to avoid a pitch-up tendency. If "normal" control inputs are applied, the airplane will be more likely to increase angle of attack and land short, or stall.
- The tail-heavy airplane will, however, trim out at a lower angle of attack in cruise, and so for a given power setting, it'll fly a little faster than the same airplane loaded at a further-forward CG.
If CG is aft of the airplane's certified loading envelope, the airplane may be so unstable it cannot be safely flown. The effect would be more pronounced at slower speeds, such as landing, when reduced air flow makes the elevator less effective.
What's typically tail-heavy? Airplanes with large aft baggage areas and long-body airplanes with seats near the back of the cabin are most commonly loaded near (or beyond) their aft CG limit.
Fuel Burn and CG
We all learned to compute CG location as part of our initial pilot training. But how many of us were taught to compute weight and balance not only for the takeoff condition, but for the anticipated landing condition as well? Many airplanes may be loaded within limits for takeoff, only to go out of the CG envelope after some amount of fuel is burned out of the tanks. I surprised renters of a Cessna 172 I flew early in my instructor career by showing them the airplane was safely within limits at nearly fuel with two people in the back seats, but that after burning about half of the fuel in flight the CG was drifting dangerously aft of the aft limit.
Computed CG location for a Cessna 172S with two standard occupants up front, a pair of 150-pound passengers in the rear seats, and baggage for a weekend trip, at (1) full fuel, (2) half tanks and (3) a zero-fuel condition. Note the CG gets closer to the aft limit as fuel is burned, going out of limits well within the fueled range of the aircraft.
Since most airplanes carry their fuel in the forward part of the wing it's most common for CG to translate aft with fuel burn. Individual airplane design and optional auxiliary fuel tanks can complicate this rule.
Here's an exercise: Using weight-and-balance data for an airplane you regularly fly, with a given passenger and baggage load, compute CG location at full fuel, half fuel and zero fuel. See where the CG goes with fuel burn, and whether it'll go beyond the aft limit as loaded while there's still fuel in the tanks. If so, you've now established a shorter aircraft range (including reserves) before you need to land for fuel to maintain controllability.
Knowing Your Limits
If you're inside but near the forward CG limit, the airplane will take more runway to take off and a firm hand to get into a climb attitude. But the airplane will be more stable in turbulence, giving your passengers (and you) a smoother ride. Take advantage of a forward CG and plan fuel stops and your load to be near the forward limit when you fly in rough air or near mountains. If the air is smooth, you might plan for a rearward-but-within-limits CG, for a faster cruise speed. Either way, account for the CG effect of fuel burn and ensure you'll still be safely within the envelope at the completion of your trip, including flight to an alternate airport if needed.
Some pilots like "stable" airplanes, especially for instrument flight. Others like "maneuverable" aircraft. The "stable" types consider more maneuverable airplanes to be "squirrelly," while the "maneuverable" crowd says stable airplanes "fly like a truck." Whatever your preference, the way the airplane handles is in large part a function of its CG.
Thomas P. Turner's Leading Edge columns are collected here.