articles - stol and thrust
STOL
STOL is an acronym for Short Take-Off and Landing, used in the aircraft industry to describe airplanes with very low runway requirements. Famous STOL aircraft include the Fieseler Fi 156, de Havilland Beaver, Pilatus PC-6, Piper Cub, PZL Wilga, and Westland Lysander.
Runway length requirement is a function of the square of the minimum flying speed (stall speed), and most design effort is spent on reducing this number. For takeoff, large power(weight ratios and low drag help the plane to accelerate for flight. The landing run is minimized by strong brakes and spoilers (less common).
Overall STOL performance is set by the length of runway needed to land or take off, whichever is longer.
Of equal importance to short ground run is the ability to clear obstacles, such as trees, on both take off and landing. For takeoff, large power/weight ratios and low drag result in a high rate of climb required to clear obstacles. For landing high drag allows the airplane to descend steeply to the runway with out building excess speed resulting in a longer ground run. Drag is increased by use of flaps (devices on the wings) and slips (causing the airplane to fly somewhat sideways).
Normally, a STOL plane will have a large wing for its weight. These wings often use aerodynamic devices like flaps, slats, and vortex generators. Typically, designing an airplane for excellent STOL performance reduces maximum speed, but does not reduce payload lifting ability.
Most STOL planes either land in the countryside or on normal airport runways. A STOLPORT is an airport designed with STOL operations in mind, normally having a short single runway. These are not common but can be found, for example, at London City Airport, London, England.
Thrust
Thrust is a reaction force described quantitatively by Newton's Second Law when a system expels or accelerates mass in one direction to propel a vehicle in the opposite direction. Total force is equal and opposite to the mass m times the acceleration a experienced by that mass:
F=m.a
Examples:
1. An aircraft generates forward thrust when the spinning propellers blow air, or eject expanding gases from a jet engine to the back of the aircraft. The forward thrust is proportional to the (mass of the air) multiplied by (average velocity of the air stream).
2. Similarly, a ship generates forward thrust (or reverse thrust) when the propellers are turned to accelerate water backwards (or forwards) .... the resulting thrust pushes the ship in the equal and opposite direction to the sum of the momentum change in the water flowing through the propeller.
3. A rocket (and all mass attached to it) is propelled forward by a thrust force equal to, and opposite of, the time-rate of momentum change experienced by the exhaust mass accelerating out from the combustion chamber through the rocket nozzle. This is the exhaust velocity with respect to the rocket, times the time-rate at which the mass is expelled. Of course, for a launch the thrust at lift-off should be more than the weight, and with a fair margin, because a "slow launch" would be very inefficient.
Examples:
- each of the three Space shuttle main engines can produce a thrust of 1.8 MN, and each of its two Solid Rocket Boosters 14.7 MN, together 34.8 MN. Compare with the mass at lift-off of 2,040,000 kg, hence a weight of 20.0 MN.
- the simplified Aid for EVA Rescue (SAFER) has 24 thrusters of 3.56 N each.