Weight and balance principles apply to all aircraft regardless of configuration, but the method of control and structural design greatly influence how these principles are evaluated. Aircraft such as weight-shift-control (WSC), powered-parachute (PPC), and amateur-built aircraft differ significantly from conventional airplanes because the pilot often controls attitude by shifting mass or by suspended loading rather than by aerodynamic control surfaces alone. As shown in Figure 1, these aircraft rely on unique structural arrangements that require special consideration of center of gravity (CG) location for safe operation.
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| Figure 1. Here are some examples of weight-shift control aircraft: powered parachutes, gliders, airplanes, hot air balloons, and amateur-built aircraft (from top left, clockwise) |
Unlike traditional fixed-wing airplanes where loading is distributed inside the fuselage, many light recreational aircraft carry the useful load suspended below the lifting surface. This pendulum-type arrangement naturally stabilizes the aircraft but also changes how loading variations affect performance and controllability. Even though these aircraft may tolerate wider loading variation without dramatic CG shifts, improper loading can still reduce stability, alter trim characteristics, and increase structural stress.
In addition, the growing popularity of recreational and experimental aviation has introduced pilots to aircraft categories that operate under different certification, maintenance, and inspection standards. Because pilots and owners are often directly responsible for maintenance and flight preparation, they must have a clear understanding of loading limitations and the relationship between weight, performance, and controllability before flight operations.
Light-sport category aircraft have less restrictive maintenance requirements and may be maintained and inspected by traditionally certificated aircraft maintenance technicians (AMT) or by individuals holding a Repairman: Light Sport certificate, and (in some cases) by their pilots and/or owners.
Weight and Balance
Aircraft such as balloons, powered parachutes, and WSC do not require weight and balance computations because the load is suspended below the lifting mechanism. The CG range in these types of aircraft is such that it is difficult to exceed CG limits. For example, the rear seat position and fuel of a WSC aircraft are as close as possible to the hang point with the aircraft in a suspended attitude. Thus, load variations have little effect on the CG. This also holds true for lighter-than-air aircraft, such as a balloon basket or gondola. While it is difficult to exceed CG limits in these aircraft, pilots should never overload an aircraft, as doing so may cause structural damage and/or failures.
Weight affects performance; therefore, pilots should calculate weight and remain within the manufacturer’s established limits at all times.
Weight-Shift-Control (WSC) Aircraft
The definition for WSC can be found in 14 CFR part 1. A WSC aircraft used for sport and private pilot flying must be registered with an FAA N-number, have an airworthiness certificate, a pilot’s operating handbook (POH), and/or limitations with a weight and loading document aboard.
As mentioned earlier, WSC aircraft are commonly called trikes. These aircraft have few options for loading because they lack places to put useful load items.
The most significant factor affecting the weight and balance of a trike is the weight of the pilot and, if the aircraft has two seats, the weight of the passenger. The trike acts somewhat like a single, main-rotor helicopter because the weight of the aircraft hangs like a pendulum under the wing. Figure 2 shows a two-place trike, in which the mast and the nose strut come together slightly below the wing attach point.
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| Figure 2. CG of a trike |
When the trike is in flight, the weight of the aircraft hangs from the wing attach point. The weight of the engine and fuel is behind this point, the passenger is almost directly below this point, and the pilot is forward of this point. The balance of the aircraft is determined by how all these weights compare. The wing attach point, with respect to the wing keel, is an adjustable location. The attach point is moved slightly forward or slightly aft, depending on the weight of the occupants. For example, if the aircraft is flown by a heavy person, the attach point can be moved farther aft, bringing the wing forward to compensate for the change in CG. Figure 3 shows a close-up of the wing attach point and the small amount of forward and aft movement that is available.
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| Figure 3. Wing attach point |
Similar to airplanes, sailplanes, and powered parachutes, increasing weight creates increases in speed and descent rate. However, the WSC aircraft has a unique characteristic. Adding weight to a WSC aircraft creates more twist in the wing because the outboard leading edges flex more. With less lift at the tips, a nose-up effect is created and the trim speed lowers. Therefore, adding weight can increase speed similar to other aircraft, but reduce the trim speed because of the increased twist unique to the WSC aircraft. Each manufacturer’s make/model has different effects depending on the specific design. For detailed weight and balance information, characteristics, and operating limitations, always reference the specific manufacturer’s manual or POH for the make and model. Figure 4 shows an example of a weight and loading sheet that would be issued with a WSC aircraft. Every aircraft has its own weight and loading data that should come from the manufacturer. The example in Figure 4 comes from Airborne, an Australian company, named Airborne XT WSC aircraft. For additional information, refer to the Weight-Shift Control Aircraft Flying Handbook (FAA-H-8083-5).
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| Figure 5. Weight and loading for WSC aircraft |
Powered Parachutes
Powered parachutes have many of the same characteristics as WSC aircraft when it comes to weight and balance. They have the same limited loading. A powered parachute acts like a pendulum with the weight of the aircraft hanging beneath the inflated wing (parachute). The point at which the inflated wing attaches to the structure of the aircraft is adjustable to compensate for pilots and passengers of varying weights. With a very heavy pilot, the wing attach point would be moved forward to prevent the aircraft from being too nose-heavy. Figure 5 illustrates the structure of a powered parachute and the location of the wing attachment.
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| Figure 5. Powered parachute structure with wing attach points |
A powered parachute used for sport and private flying must be registered with an FAA N-number, have an airworthiness certificate, a POH, and/or limitations with a weight and balance document aboard. The aircraft must be maintained properly by the aircraft owner or other qualified personnel, and the aircraft logbooks must be available for inspection. Always refer to the POH for weight and balance information specific to the powered parachute being flown. For additional information, refer to the Powered Parachute Flying Handbook (FAA-H-8083-29).
Weight and Balance Computations (Amateur-Built Aircraft)
A good weight and balance calculation is the keystone of flight testing an amateur-built aircraft. Accurately determining the aircraft’s takeoff weight and ensuring that the CG is within the aircraft’s design for each flight is critical to conducting a safe flight test The aircraft should be level when weighed, spanwise and fore and aft in accordance with the kit manufacturer’s instructions, and should be in the level flight position. It is highly recommended that the aircraft be weighed in an enclosed area using three calibrated scales. Bathroom scales are not recommended because they are not always accurate.