Weight is a measure of the force of gravity acting upon the mass of the WSC aircraft. Weight consists of everything directly associated with the WSC aircraft in flight: the combined load of the total WSC aircraft (wing, wires, engine, carriage, fuel, oil, people, clothing, helmets, baggage, charts, books, checklists, pencils, handheld global positioning system (GPS), spare clothes, suitcase, etc.).
During gliding flight, weight is broken down into two components. The component that opposes the lift, acting perpendicular to flight/glide path, and the component that opposes the drag and acts in the direction of the flight/glide path. During gliding flight, this component of weight is the weight component providing the forward force which some call thrust for gliding flight.
During gliding, straight, and descending in unaccelerated flight:
Lift (L) and Drag (D) components = Resultant force (RF) = Weight (W)
Total Drag (DT) = Weight component (WD) in the direction of flight
Lift (L) = Weight component (WL) that opposes lift
Similar to airplanes, gliders, and PPC during gliding flight, less lift is required because the resultant force composed of lift and drag provides the force to lift the weight. In other words, in gliding flight, drag helps support the weight. [Figure 2-19]
At a constant air speed, the amount of thrust determines whether an aircraft climbs, flies level, or descends. With the engine idle or shut off, a pilot is descending or gliding down. Maintaining a constant airspeed, when enough thrust is added to produce level flight, the relative wind stream becomes horizontal with the Earth and the AOA remains about the same. As described for the airplane in the Pilot’s Handbook of Aeronautical Knowledge, thrust equals total drag for level flight. [Figure 2-20]
When in straight and level, unaccelerated flight:
Lift (L) = Weight (W)
Thrust = Total Drag (DT)
At a constant airspeed, when excess thrust is added to produce climbing flight, the relative airstream becomes an inclined plane leading upward while AOA remains about the same. The excess thrust determines the climb rate and climb angle of the flightpath. [Figure 2-21]
When in straight and climbing, unaccelerated flight:
Lift (L) = Component of weight that opposes lift
Weight (W) = Resultant force (FR) of lift (L) and excess thrust to climb (TE)
Thrust = Total drag (DT) plus rearward component of weight
Thrust Required for Increases in Speed
Above the lowest total drag airspeed [Figure 2-18], faster speeds (lower angles of attack) for level and climbing flight requires greater thrust because of the increased drag created from the faster speeds.
AOA is the primary control of increasing and decreasing speeds, and increasing thrust generally does not produce higher speeds, but additional thrust is required to maintain level flight at higher speeds.
Ground effect is when the wing is flying close to the ground and there is interference of the ground with the airflow patterns created by the wing. At the same angle of attack, lift increases slightly and the drag decreases significantly. The most apparent indication from ground effect is the unexpected lift given to an aircraft as it flies close to the ground—normally during takeoffs and landings. More details for ground effect aerodynamics are found in the Pilot’s Handbook of Aeronautical Knowledge. Flight characteristics for ground effect are covered in the takeoff and landing chapters.
Center of Gravity (CG)
The CG is the theoretical point of concentrated weight of the aircraft. It is the point within the WSC aircraft about which all the moments trying to rotate it during flight are balanced. The most obvious difference in the CG for a WSC aircraft is the vertical position compared to an airplane, as it is always lower than the wing. The Pilot’s Handbook of Aeronautical Knowledge accurately states the CG is generally in the vertical center of the fuselage. The same is true for the WSC aircraft. However, the WSC wing is higher above the fuselage/carriage and, since most of the weight is centered in the carriage, the CG is well below the wing.
In a two-seat WSC aircraft, the second seat is typically behind the pilot’s seat and the CG is usually located close to the rear passenger seat. Therefore, the CG location does not change significantly with a passenger. Fuel tanks are typically located near the vertical CG so any difference in fuel quantity does not significantly change the CG fore and aft with different fuel quantities.
For level flight, the CG is directly below the wing/carriage attachment point known as the hang point, and the propeller thrust line is typically designed to be near the vertical position of the CG. [Figure 2-22]