False Lift
Accident Prevention Program
FAA P-8740-39
Balloon Safety Tips-False Lift, Shear, And Rotors
Balloons respond to various air currents to a greater degree than other aircraft. Except for momentary delays wherein inertia of the balloon mass resists the energy of a new air current, the flight path of a balloon in equilibrium exactly mirrors the direction and velocity of the air current in which it is operating. This simple fact of balloon operation holds true for horizontal (the kind we like to fly in), vertical, and rotary air currents (when Mother Nature flies). This article is about flying with Mother Nature and includes a few tips on what to do when Mother Nature's flying becomes too exciting.
False lift is an aerodynamic phenomenon, which occurs during the initial acceleration of the balloon. A balloon standing in the wind acts as an obstacle to normal airflow causing the wind speed to increase on the surface of the balloon. Changing the wind speed causes a pressure conversion to occur (static pressure decreases and velocity pressure increases) which results in false lift. During launch false lift couples with the balloon's normal lifting forces to cause the balloon to take off. The problem with having false lift is that it quits when the balloon reaches wind speed. Ready or not, false lift dissipates rapidly after takeoff causing the pilot to apply heat constantly in order to gain real lift before acceleration is complete. In a practical sense, false lift does not pose any operating problems until the wind speed exceeds approximately 10 mph. False lift is unavoidable in a fast wind making takeoffs a challenge for the balloon pilot. Simultaneously, the pilot must watch burner operation, fabric, control the crew, assess the balloon's physical readiness for flight and judge the lift. The confusion of burner noise, passenger and crew demands, and the physical jolting caused by the gondola dragging across the ground can quickly overload an unprepared pilot. The pilot should carefully prepare, inspect and rig the balloon prior to inflation to minimize the tasks required during inflation and prior to takeoff. This procedure is necessary to insure that the balloon is safe in the event a takeoff occurs unexpectedly. The pilot must always assume false lift is present during takeoff that the envelope lift is inadequate, and continue heating until balloon acceleration is complete and a positive climb rate is established. During takeoff and acceleration it is not possible to differentiate between real lift and false lift. The best policy is to heat past equilibrium temperature and then vent as necessary to maintain a comfortable rate of climb. False lift is easier to overcome when burner output is high; therefore fuel pressure is the best information available to judge the balloon's ability to overcome false lift. Small burners on big envelopes are obviously a poor choice of equipment for fast wind operation. Obviously the takeoff field is important in fast wind conditions. Obstructions downwind require additional clearance depending on wind speed due to the reduced initial climb rate of the balloon after losing the false lift.