Lecture 14 - Forces on Aircraft

Main Ideas

• Aerostats and aerodynes stay aloft in different ways:
  • Aerostats rely on buoyancy.
  • Aerodynes, such as airplanes and helicopters, rely on aerodynamic forces produced by relative motion between air and lifting surfaces.
• A lifting surface set at an angle of attack can generate lift. For an airplane wing, lift depends on airspeed. The lecture gives the relation:

L = ½ρV²SCₗ

Forces on an Airplane

The four main forces acting on an airplane are:

• Weight, acting downward through the aircraft’s center of gravity.
• Lift, generated mainly by the wings and acting upward.
• Drag, resisting the aircraft’s forward motion.
• Thrust, produced by the engines to oppose drag and propel the airplane.

In steady, level flight:

• Lift balances weight: L = W.
• Thrust balances drag: T = D.

The lecture gives a typical airplane lift-to-drag ratio of roughly 8–10. In level flight, this means the required thrust is approximately 10–12.5% of the aircraft’s weight.

Forces on a Helicopter

A helicopter’s rotating blades act like wings, but their rotation generates upward force directly. When hovering, the rotor’s thrust must balance the helicopter’s entire weight.

The main rotor also produces torque that would make the helicopter spin around its vertical axis. A tail rotor generates a sideways force whose moment opposes that torque.

To move forward, the rotor’s thrust direction tilts:

• The vertical component supports the helicopter’s weight.
• The horizontal component moves it forward.
• Because only part of the tilted thrust supports the weight, total thrust must be greater than the helicopter’s weight. The lecture gives an illustrative figure of about 125% of weight when moving forward.

Why Fixed-Wing Aircraft Are Used for Long-Distance Travel

• Airplanes are more thrust- and fuel-efficient in the comparison presented. They need thrust mainly to overcome drag, rather than to support their full weight.
• Airplanes need sufficient forward speed to generate lift, so they require a runway and airport facilities.
• Helicopters can take off and land without a runway, including from places such as rooftops or stadiums. However, their rotors must generate thrust to support the aircraft, making them substantially less efficient for carrying the same payload.
• The same distinction applies to multirotors and quadrotors:
  • They are suitable for simpler or shorter-duration uses, such as surveillance.
  • Their continuous thrust requirement makes them less suitable than fixed-wing aircraft for long-endurance flights.

The practical takeaway is that fixed-wing aircraft are generally preferred for long-distance or long-duration flights, while helicopters and multirotors offer more flexible takeoff and landing.

Speaker and Source

The lecture is presented by one unnamed lecturer or instructor. No other speakers are identifiable in the subtitles.