Trivia Testers : Variable Sweepback

Variable sweepback

Which aircraft was the first to have variable-sweep wings?

  1. The Bell X-5
  2. The F-111
  3. The MiG-23
  4. The Messerschmitt P.1101

Answer: D. The P.1101 was to be a test bed aircraft used to study the effects of swept back wings on high speed flight but was not completed before the end of WWII. As early as 1935, German aeronautical engineers began studying the effects of swept back wings on high speed flight. Among them was the noted Alexander Lippisch, who filed a patent on a variable sweep wing in 1942. Dr. Waldemar Voigt, Lippisch, and others started designing a single-seat fighter interceptor, known as Project P.1101, backed by Messerschmitt. Due to extensive commitments to the Me 262, work on the 1101 went slowly. That is, until 1944, when the Luftwaffe commander issued a requirement for a single-seat, single engine, turbojet fighter with a higher speed than the Me 262. Messerschmitt entered the P.1101 design, got the contract, and started with design and production of a prototype at Oberammergau. The wings could be set to two different positions, forward position, with a relatively small 10-degree sweepback, and a rearward position with wings at a much sharper angle. However, they could only be moved while the aircraft was not in flight. Sometime either before or during WWII, someone figured out that the critical Mach number (or whatever they might have called it then) was related to the sweepback angle (actually, its cosine).

Before the prototype could be completed, however, US forces discovered the Oberammergau facility and under supervision of the chief engineer at Bell Aircraft,the P.1101 was transported to the USA, where Bell designed a better mechanism for adjusting the sweep of the wings, in flight. In 1949 a variable sweep wing aircraft, the Bell X-5, was designed based on the P 1101. The Bell X-5 made its first flight on June 20, 1951, and became the first jet-propelled aircraft ever to fly with in-flight variable-swept-wing capability, and formed the basis of modern variable-sweep wing aircraft. It was designed to sweep its wings from 20 degrees, to 45, and to 60, to see what Mach drag did on variable swept wings.

The Bell X-5 first flew on June 20, 1951. The F-111 “Raven”, which first flew on December 21, 1964, had angles from 16 degrees (full forward) to 72.5 degrees (full aft). The MiG-23 “Flogger”, which first flew in April 1967, had variable-sweep wings at 19, 48, or 75 degrees.

» View the Bell X-5
» View the P.1101
» View the F-111 and MiG-23

Hot and Cold Running Water
Just how cold and how hot can rain get?

  1. from -40C up to 100C
  2. from about -5C to about 40C
  3. from 0C to 23C
  4. from 5C to 30C

First, how cold can rain be? Well, let’s get one thing straight: it ain’t that much below freezing. Why? Because freezing rain is not the same thing as supercooled water. In situations involving freezing rain, precipitation that was originally snow melts, becoming rain aloft, and is above freezing as it falls. At some lower altitude is a (usually) shallow layer of air, which is below freezing. The raindrops fall through it so fast that their temperature doesn’t drop below freezing, primarily due to water’s high specific heat. The drops then hit a surface which is below freezing (like your airplane) where the water does turn into ice because it comes into contact (however violently) with that freezing surface. A significant percentage splashes off the ice at first, but a bit of it freezes on contact. Over time, that ice builds up and the water that splashed off finds a lower surface on which to freeze. There are freezing rain situations where the air temperature is about -5 C, but not much colder than that because if the temperature is very cold, the cold layer is likely to be quite deep. If so, the raindrops do freeze on the way down and instead become sleet.

Now, in the cloud itself, the water is supercooled. These are very small droplets and, in stratoform clouds, they are being lifted slowly (on the order of cm/sec). This is a perfect situation for supercooling. Nevertheless, at -40 C (also -40 F) all supercooled water freezes spontaneously. So in that sense, the answer to how cold can liquid water be when you fly into it is: -40 degrees. But those supercooled droplets never fall as rain. Why? Well, they’re about a million times smaller than a raindrop, and they just won’t fall, and because in a cloud, the saturation vapor pressure with respect to ice is less than the saturation vapor pressure with respect to water—in order for droplets to form from pure water, the saturation must be around 120%, not just 100%—so the process (named the Bergeron process, incidentally) causes snowflakes to grow via sublimation at the expense of the supercooled water droplets, and it is the snowflakes which fall out of the cloud. Of course, if you fly through the super-cooled droplets, they will freeze onto the plane. So the difference between supercooled cloud droplets and freezing rain is academic. As far as pilots are concerned, the effect is the same: you get iced.

How warm can rain get? In theory, it could be almost 100 C! Such a theoretical case would be a bit extreme, but it’s conceivable. If there were a situation such as the one described in the July 8, 2002 Trivia Tester (“Fire and Rain”) where pyrocumulus or “forest fire” clouds (formed by moisture released from vegetation during a forest fire and driven up by the heat) were mentioned, and they managed to release rain which fell through the superheated air, the raindrops could become quite hot. However, in all other more realistic scenarios, it’s much less than that. In India, in April, it gets well into the 100-110 F range before the monsoon breaks, and after the rains arrive it cools down… all the way into the 90’s! (It is possible in the tropics, where evaporative cooling is minimized, that the rainfall could be that warm.) But this has actually not been considered important enough to record accurately. The best answer is choice B.

The Fine Print
What is Rule 240?

  1. the collective set of ASTM and OSHA rules that dictate just how wide (or narrow) airline seats may be, depending on the latest biometric population data
  2. the area rule formula discovered by NACA’s Dr. Richard Whitcomb, proven during the development of the F-102A, that supersonic flight is best acheived with a “Coke bottle” constriction in the cross-sectional area of the mid-fuselage
  3. the little-known rule governing airlines and what compensation they are obligated to provide to ticketed passengers denied a seat for other than “force majeure” events beyond their control (such as weather)
  4. the rule dividing the Atomic Energy Commission’s Nevada site into numbered areas for the purpose of tracking fallout from nuclear tests…in one of which, starting in about 1954, the Lockheed/CIA “Skunk Works” testing was begun that led to the U-2. (We now refer to it as Area 51.)

Answer: C. That stuff about Area 51 is actually true, but there weren’t 240 of them. The other two supposed associations with the number 240 are pure hogwash. Rule 240 applies only within the United States (and an airline often has its own particular version, based on their “conditions of carriage” with the Department of Transportation). It covers you in the event of a delay (which generally must be more than two hours) that is the airline’s fault, such as mechanical problems, a delay in arrival of the airplane or its crew–or a cancellation. It only covers ticketed passengers who arrive on time, needless to say. Most airlines (except, perhaps, America West) are fairly publicity-conscious and are generally responsive. Among the possible benefits of Rule 240 are hotel accomodations (though usually only if the delay is more than four hours and occurs between 10 PM and 6 AM), meals (though sometimes that’s only for first class), a brief (three-minute) phone call, etc., when things get out of hand. More commonly, they will arrange a flight for you, on another airline. Note that because the computer systems of different airlines don’t all communicate with one another, don’t count on your e-ticket being endorsed from one airline to another. A paper ticket might mean a little more money and/or time at the ticket counter, but if you’re traveling during busy holiday periods, bad weather, or if there are labor problems or security delays, it just might be worth it. Obviously, it helps here to have only carry-on bags and not be encumbered by previously checked baggage. And the first thing you should do if your flight is cancelled is not immediately storm the ticket counter, for re-booking. Instead, proceed quickly to the nearest telephone and either book another flight on the same airline, or on another carrier flying to your destination. Then go stand in line. (Unlike the others, you’ll already have your flight arranged, as well as your reservation, for endoresement to the new carrier.) Before airline deregulation in the late 1970s, it used to be a Federal requirement. By the way, don’t get too snippy or supercilious with this new-found knowledge. First, not all airlines have a Rule 240. For those that do, most won’t volunteer this information, so it is up to you to call them on it. Do stand up for your rights, but…do it tactfully!