Navigation.Our study of physics opens with kinematics-the study of motion without considering its causes. (You canįly a kite only with the wind at your back.) A kite usually has no ground speed (the kite is held on the end of a string).īut the kite still has an airspeed that is equal to the wind speed.Or the air moves over the object, the forces are the same. Whether the object moves through the air, Walls of the tunnel.) The airspeed is then the negative of the wind speed In the wind tunnel, the ground speed is zero.Wind tunnel testing is possible and how kites The relationship between airspeed, wind speed, and ground speed explains why Now mechanical or electronic devices provide the information that is radioed See which way the wind was blowing to adjust their takeoff and landing directions. In the old days, a large 'wind sock' was hung near the runway for pilots to Since runways have a fixed length, you want to getĪirborne as fast as possible on takeoff and stopped as soon as possible on landing. This requires a lower ground speed (as we saw in theĪbove example), which means the plane can take off or land in the shortest distance Airplanes always try to take offĪnd land into the wind. Land on different runways on different days. The importance of the relative velocity explains why airplanes take off and Significance of Understanding Relative Velocity So the aircraft will have to travelįaster (and farther) along the ground to achieve liftoff conditions with the = ground speed (120) - wind speed (20) ]. Speed is 20 mph and the ground speed will now be 120 mph [airspeed (100) At liftoff, the airspeed is still 100 mph, the wind Since the wind and aircraft direction are the same, we assignĪ '+' to the wind speed. If the plane took off to the west, it would have a 20 mph tailwind (windĪt your back). At liftoff, the airspeed is 100 mph, the wind speed is -20 mphĪnd the ground speed will be 80 mph [airspeed (100) = ground speed (80) Of 0 and an airspeed of 20 mph [airspeed = ground speed (0) - wind speed While the plane is sitting still on the runway, it has a ground speed Velocity to be in the direction of the aircraft's motion, a headwind is a negative If the airplane takes off going east, it experiencesĪ 20 mph headwind (wind in your face). Now suppose we had a day in which the wind wasīlowing 20 mph towards the west. Suppose we had an airplane that could take off on a windless day at 100 mph But if the wind is blowing in the same direction that theĪircraft is moving, the airspeed will be less than the ground speed. On a perfectly still day, the airspeed is equal to Airspeed is the vector difference between the ground Airspeed cannotīe directly measured from a ground position, but must be computed from the ground The object and the air, which is called the airspeed. The important quantity in the generation of lift is the relative velocity between The velocity of the object measured relative to the ground is called the ground Ground, and updrafts and downdrafts, which occur perpendicular Winds, which occur perpendicular to the flight path but parallel to the And a positive velocity isĭefined to be in the direction of the aircraft's motion. Only velocities along the aircraft's flight path. (A 20 mph wind from the west is differentįrom a 20 mph wind from the east.) The wind has components in all three primaryĭirections (north-south, east-west, and up-down). Notice that the wind speed is a vector quantityĪnd has both a magnitude and a direction. To understand the relationships of wind speed to ground speed to airspeed.įor a reference point picked on the ground, the air moves relative to the reference In this slide, the reference point is fixed to the ground, but it could justĪs easily be fixed to the aircraft. To properly define the velocity, it is necessary to pickĪ fixed reference point and measure velocities relative to the fixed point. Now things getĬonfusing because not only can the object be moved through the air, but theĪir itself can move. Is not aerodynamic lift.] Aerodynamic lift depends on the square of the (like the Harrier) use engine thrust to 'lift' the vehicle. Must move through the air, or air must move past the object. (Hot air balloons 'lift' because of buoyancy forces, notĪerodynamic forces.) To generate lift, an object

A fixed object in a still fluid does not generateĪerodynamic forces. Aerodynamic forces are generated by an object moving One of the most confusing concepts for young aerodynamicists is the relative

Relative motion