Balancing the Aerodrome
Maintaining Steady Horizontal Flight
Samuel Pierpont Langley understood that achieving and sustaining flight required a lightweight structure, rigid plane surfaces and a powerplant that could generate enough thrust to achieve lift by overcoming the forces of gravity and drag.
STRUCTURE
Langley utilized hollow steel tubing to create Aerodrome No. 5’s structural frame, which consisted of a double mid-rod running longitudinally from tip to tail, vertical guy post extensions, and a central cage frame that contained the aircraft’s mechanical systems. Air and water are both fluids and the forces acting on bodies moving through them are similar. Many components of the aerodrome were named after their nautical counterparts, such as the bowsprit, hull, outrigger, and tail rudder.
SUSTAINING SURFACES
Mechanical flight requires a delicate balance between gravity and lift. As the size of sustaining surfaces, or wing area, increases, so does their weight. Few nineteenth century construction materials were sufficiently light, strong, and rigid for use in aircraft wing construction. Langley’s solution to this structural challenge was a tandem wing design, with two pairs of equal-sized wings. The tandem wings allowed for a shorter wingspan, lighter overall wing weight, lower bending stresses, and a stiffer airfoil while also producing lower drag and a high lift coefficient at low speeds. Altogether, Aerodrome No. 5’s wings provided 68.8 square feet of sustaining surface at a weight of just 4.29 lbs. Even with Langley’s innovative wing design, the aerodrome’s wings underwent significant deformation in flight. Langley countered this with a system of piano wire guys strung between the bowsprit, guy posts, and wing ribs to maintain the airfoil’s shape.
LONGITUDINAL STABILITY
The force of gravity acts downward on an aircraft proportional to its mass. An aircraft’s center of gravity is the point about which it would balance if it were possible to suspend it at that point. An airfoil (wing) in motion creates a pressure differential that generates lift. The center of pressure is the point on the aircraft at which all lift forces acting on the aircraft are considered concentrated. Langley hypothesized that stable flight occurs when the center of pressure (C.P.) is aligned with the center of gravity (C.G.).
ACHIEVING BALANCE
Langley was able to adjust Aerodrome No. 5’s center of pressure without significantly altering the center of gravity using wing clamps. The clamps allowed the wings to be adjusted along the mid-rod to fine tune the longitudinal stability. To minimize the shifting of the center of gravity during flight, Langley located propulsion elements that changed in weight, such as the water and gasoline reservoirs, at the aircraft’s estimated center of gravity. To fine-tune longitudinal stability, Aerodrome No. 5’s forward float could be shifted along the bowsprit.
AUTOMATIC EQUILIBRIUM
Langley believed that a pilot would be unable to manually control an airplane when faced with variable wind currents. Therefore, he designed his aerodrome with a “tail-rudder” designed to provide automatic equilibrium in flight. The tail-rudder was attached to the frame by a flat wooden spring. As the aerodrome pitched downwards into a headwind, the tail-rudder lagged, exposing the top of the horizontal stabilizer to the moving air. The force of the wind on the tail-rudder created a moment that returned the aerodrome to a wings level position.
LATERAL STABILITY
Langley chose a dihedral configuration (upward angle) for his aerodrome wings after observing the wing position of soaring birds. After observing the response of different models on his whirling table, he decided on a 9 degree dihedral angle for Aerodrome No. 5.
Dihedral wings provide significant lateral stability in turbulent air. When wind gusts cause longitudinal roll, the physics of dihedral wings help an airplane return to wings level (level flight). As the aircraft banks in response to turbulence, it slips as well, changing its orientation to the relative wind.
As the aerodrome slips the low wing meets the relative wind at a higher angle of attack, generating more lift on the low wing and creating a rolling moment that returns the aircraft to wings level.
LATERAL STABILITY
For lateral stability, Langley located the center of gravity just below the center of pressure, which reduced the tendency for the craft to roll. Aerodrome No. 5 carried approximately 75% of its weight within the central hull of the aircraft with the remaining weight equally balanced on each side. To keep the aerodrome on a straight course, Langley utilized counter-rotating wooden propellers to balance the effects of torque on the aircraft. Aerodrome No. 5’s propellers were constructed with an axial twist, a technique still used today to produce uniform thrust along the length of the propeller blade and reduce drag.