A Simple Method of Measuring Landing and Take-Off Speed

Abstract

T of the most difficult performance characteristics of aircraft to measure are the true landing and take-off speeds. The airplane airspeed indicator is not suited for obtaining these velocities because of the difficulty in calibrating the pitot-static head for position error at angles near the stall, particularly where the angle changes rapidly and also because the presence of the ground generally modifies any free flight calibration. At the present time, many photographic methods involving the use of high speed cameras taking up to 100 photographs per second are used to take pictures of the airplane as it lands on a runway marked off in known distances. The space-time curve is derived knowing the actual camera speed. The velocity of the airplane added to the measured wind velocity gives the landing speed. For accurate work, only electrically driven cameras are satisfactory as spring driven cameras almost always vary too greatly in speed, requiring involved calibration. This method naturally cannot be used with seaplanes. I t is generally unsatisfactory because the piloting technique is very difficult, inasmuch as the pilot must make a minimum speed spot-landing fairly close to the cameras. In some cases, the camera is mounted inside the airplane, taking pictures of the lines through a cabin window as the ship lands. The exact point of contact must be noted by a ground observer. The above are only two of a large number of camera methods of measuring speeds. All involve the use of a fairly large ground crew and none of the camera methods can be more accurate than the measurement of the wind velocity and the atmospheric pressure and temperature, which are used to correct the speeds to sea level air density. The air temperature must be carefully obtained outside the airplane at wing height in a shaded location away from ground radiation and direct sunlight. Computing the results of a camera test is a tedious task and the results are not immediately obtainable. The cost of such testing is very high. In order to obtain greater accuracy, simplicity, and ease of testing, the writer devised a very simple method of measuring velocities near the ground which eliminates all the objections to the camera system. A swiveling pitot-static head is located on the airplane away from slipstream effects and the boundary layer of any part of the ship. The head swivels about the spanwise axis of the airplane, and it may be located on the fuselage nose, above or beneath the wing or any such place regardless of position error due to local flow velocities. The pitot or total pressure lead only is used while the aircraft is in motion. I t is directly connected by a fairly large line (Vs in. inside diameter) to a verysensitive total pressure gage capable of measuring from 1.7 to 5.0 inches of water pressure above static pressure with the desired accuracy. At the instant of landing or taking off (as indicated by a signal light connected to each landing gear shock strut), the total pressure is read on the gage. When the airplane comes to a stop after landing, it is taxied back to the approximate landing or take-off point, headed into the wind and the gage switched over to a line connected with the static openings in the swiveling pitot-static head or opened to cockpit pressure. A second reading is taken on the pressure gage. If the runway has no slope, the second reading may be taken any place on the field. The difference between the pressure on landing and when stopped gives the true impact pressure at landing speed or