How does aircraft balance affect its performance
As mentioned in other articles I wrote,
Weight and Balance is one of the most important topics taught during the early stages of the aviation career, but why is it so important? Basically, the weight and balance of the airplane will directly affect its performance until at some point, the airplane won’t be able to fly.
Moreover, if the airplane is loaded over its limits it won’t be possible to calculate its performance. Airplane manufacturers provide performance charts and aerodynamic characteristics in the Pilot Operating Handbook (POH). Those performance charts are generated based on the airplane’s maximum gross weight (maximum allowable total weight) and center of gravity (CG) location within the specified limits, although sometimes manufacturers also provide performance charts for other than maximum gross weight, allowing pilots to calculate performance characteristics when the airplane is loaded differently (never in excess of maximum gross weight) than its maximum gross weight.
According to 14 CFR 91.9 “no person may operate an aircraft without complying with the operating limitations.”. Loading an airplane over its maximum gross weight or its CG limits it’s illegal, but more importantly, dangerous due to the deteriorated performance.
An unbalanced condition occurs when the CG exceeds its afterward or forward position limits, and like the overweight condition, it adversely affects airplane performance and stability. An airplane can still be unbalanced even when loaded under its maximum gross weight.
The CG is the balance point of the airplane, and where all the airplane axis passes through (the airplane moves around the CG). In other words, if we hang the airplane with a rope/string attached to the CG, the airplane would be hanging balanced in the air. If this is still unclear for you imagine a seesaw, the fulcrum is the CG when there is no person on any of the two extremes of the seesaw. Now if we sit a person in one of the extremes, the CG position will move closer to the heavier extreme and the seesaw will be unbalanced making the lighter side goes upward. The seesaw can be balanced again if we put someone else (same weight) in the other extreme. The same happens in an airplane, as we load the airplane we manage the load to keep the CG within the limits (balanced).
Weight and balance calculations are done with the corresponding airplane POH and it’s something we are not going to cover today. Although I would like to analyze the performance changes that occur when an airplane is unbalanced:
As mentioned, there are two unbalanced conditions, when the CG exceeds the forward limit, and when it exceeds the afterward limit.
Forward CG position
Afterward CG position
Longer take off/landing roll
Higher Stall Speed
Easier/Better Stall Recovery characteristics
Slower cruise Speed
Reduced elevator and rudder authority
Lower Stall speed
Harder/Worse Stall Recovery characteristics
Faster Cruise Speed
Forward CG Characteristics
Longer take off/landing roll: The nose of the airplane is heavier, it wants to remain on the ground. A greater airspeed is required to generate the required downward force from the tail to overcome the heavy nose.
Higher Stall Speed: In order to fly, an airplane upward forces should equal its downward forces. In steady level flight, the lift should equal the aircraft weight plus the downward force from the tail which is acting towards the ground like weight.
When an airplane is nose heavy, a pilot should use more nose-up trim than normal to keep the nose leveled, by doing so he is basically generating a greater downward force from the tail and increasing the wing load.
A stall occurs when an airplane reaches its maximum AoA (between 16°-18°), this is a design factor and doesn’t change with the aircraft weight or CG position. What changes with the weight or CG position is the stall speed (unaccelerated).
Since the wing load is increased, any flight condition will require greater lift than in normal conditions. To understand this, imagine an airplane with a maximum gross weight of 2300 lbs. Let’s say that this airplane is generating 2300 lbs of lift when flying at its maximum AoA and right over its Stall speed. Now imagine this same airplane but with the nose-heavy, needing to generate an extra 300 lbs of downward force from the tail. In the same fly condition, we would have a 300 lbs lack of lift. The AoA of attack can’t be increased to generate those 300 lbs of lift, we would stall, therefore the only possibility is to increase the airplane airspeed. As you can see, the stall speed still occurring when the airplane reaches its maximum AoA but at a greater speed.
Easier/Better Stall Recovery characteristics: This one is straight forward. When an airplane stalls, it will easily drop the nose helping a pilot to recover the normal flight by reducing the AoA.
Slower cruise Speed: This one is directly connected to the higher stall speed explanation. If all flight conditions will require a greater lift due to the increased downward force from the tail, each flight condition will also produce more drag slowing down the airplane.
Afterward CG Characteristics
Reduced elevator and rudder authority: As mentioned, an airplane moves around its CG by generating forces with the surface controls(aileron/rudder/elevator). When we utilize any surface control the total force (momentum) generated is the multiplication of the applied force times the arm from the CG to the surface control.
When the CG position is afterward, the arm between the CG and the rudder or elevator is shorter, so it’s the momentum generated, making them less effective.
Lower Stall speed: This is the opposite of a forward CG. We must be trimming the nose down to keep the nose leveled. Therefore the downward force from the tail is less than in normal conditions normal.
Any flight condition will require a lower amount of lift than in normal conditions. In the same scenario described for a forward CG now with an afterward CG an airplane flying at its maximum gross weight, maximum AoA, and right over the stall speed is producing 2300 pounds. Now imagine that the downward force is 300 lbs less than in normal conditions, the wing load is 2300 lbs — 300 lbs, the airplane can fly slower at its maximum AoA because it needs to generate 2000 lbs instead of 2300 lbs in normal conditions.
Harder/Worse Stall Recovery characteristics: This one is straightforward as well. When an airplane stalls, its tail will be heavy making it harder to nose down to recover the normal flight by reducing the AoA.
Faster Cruise Speed: Opposite to the forward CG a lower AoA is required for any flight condition since less lift is required. A lower AoA equals a reduced drag which lets the airplane fly faster than in normal conditions.
In conclusion, an AFT CG will make an airplane has a faster cruise speed and a lower stall speed, but it would be harder to recover from the stall.
An FWD CG will make an airplane has a slower cruise flight and a higher stall speed, but easier to recover from the stall.
Even within the limits, always try to move the CG towards the middle of the envelope, which will better keep the performance and stability characteristics of the airplane.
