Three Greens While the World Watches
The Space Shuttle typically doesn’t extend its landing gear until
- it is about 300 feet above the runway
- it has descended to within approximately 2000 feet above the runway
- it is established on a seven-mile final
- just below Mach 0.9 at 10,000 feet
Answer: A. Although the landing gear is armed at about 2000 feet (at which point the Orbiter is still following about a minus 17 to 19 degree glide slope, just before the pre-flare maneuver begins at about 1750 feet), the gear extension isn’t actually initiated until it is within only a very few hundred feet of the runway (between 200 and 400 feet), and the orbiter is below about 310 knots. This can be as little as 15 seconds before touchdown. (Also, it takes about seven or eight seconds for the gear to fully extend!) Once the gear uplock hook disengage command is given, the main gear trucks are driven down by simple gravity-assisted free-fall, as well as springs, hydraulic actuators, and also aerodynamic forces. (The nose gear extension has an additional pyrotechnic assist.) The tires are rated to landing speeds up to about 225 knots, are inflated with nitrogen to about 300 psi, and the main tires can each support about 123,000 pounds (but only about 45,000 pounds for each nosewheel tire). Incidentally, the nosewheel tires have a life of two landings, and the main wheels, just one.
Making the Most Of It
Which one of the following statements regarding best glide speed is incorrect?
- With an engine failure in a retractable gear airplane, if the gear cannot be retracted, the airspeed should be increased to that above the published best glide speed.
- When flaps are extended, best glide speed should be decreased.
- In a headwind or a downdraft, glide speed should be increased.
- At reduced weights, best glide speed should be decreased.
Answer: A. All aircraft with wings have a best glide speed. So for that matter do certain flying fish, reptiles, birds, and mammals (bats). Some aspects of aerodynamics behind it are however less than intuitive. In this case, best glide should actually be decreased with the gear extended. To be precise, best glide speed actually decreases by the fourth root of the effective increase in “equivalent flat plate area”. Doubling it (which is about what happens extending the gear in a Bonanza) would require decreasing best glide by about 15%. You would also unfortunately glide more steeply and not as far, but that would become your new optimum glide profile, as well. It might seem odd that in order to maintain an optimum glide profile in a headwind, one must speed up (choice C), yet if you “make your own headwind” by adding drag, you must instead slow down. Slowing below the normal best glide speed also holds true for adding full flaps (choice B). As a rule of thumb, when flying against a headwind or when in a downdraft, one should increase glide speed by about half the headwind component or about four to five knots per hundred feet per minute of sink rate. (For tailwinds and updrafts, that would be about one quarter of the tailwind component, and two to three knots per 100 fpm, respectively.) Best glide speed also decreases proportionally with the square root of weight.
Light the Fuse, And Stand Back!
What was the world’s first rocket-powered airplane?
- the Northrop MX-324, United States, July 1944
- the DFS-194, Germany, in August of 1940
- the Ente, also Germany, June 1928
- the flying chair of Wan-Hu, China, ca. 1500
Answer: C. The Ente (for “Duck”) was actually a sailplane, and was the world’s first rocket-powered airplane. Powered by two slow-burning Sander rockets and piloted by Friedrich Stamer, it flew about three-quarters of a mile in about one minute on June 11, 1928. (A rocket powered glider was also flown by Fritz von Opel a few months later.) The Northrop MX-324 (choice A) was the first US rocket-powered military aircraft, first flown on July 5, 1944, by Harry Crosby. The -194 (choice B) was designed by Dr. Alexander M. Lippisch of the DFS (Deutsches Forschungsinstitut für Segelflug, or German Research Institute for Gliding Flight) in the late 1930’s as a rocket propelled research aircraft. It was the world’s first aircraft to be powered by a liquid-fuel rocket engine. Early in January of 1939, Lippisch and his project team transferred from DFS to the Messerschmitt company. It was first flown under rocket power at Peenemunde West in August 1940, and made several flights, reaching 342 mph in level flight. The success of the DFS-194 led to development of the Me-163 Komet rocket fighter. Although rockets had been used in China since the West began counting years in a positive direction, nearly all uses were for warfare or fireworks. However in about the year 1500 according to Chinese legend, an official named Wan-Hu assembled a rocket-powered flying chair. Attached to the chair were two large kites, and fixed to the kites were forty-seven fire-arrow rockets. His success is undocumented.