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Aircraft Crash Worthiness

- question from name withheld

The drive to design a safer, more crash resistant plane has been around almost as long as flight itself. While early airplanes were of course rather flimsy contraptions built of wood, canvas, and wire, their one forgiving quality was very slow takeoff and landing speeds. It is in these periods of flight that accidents are most likely since the aircraft is very close to the ground and the pilot has the smallest margin to compensate for any problems or errors. Since kinetic energy is proportional to the square of the velocity, reducing the takeoff or landing speed by a factor of two will reduce the energy of impact by four times. As a result, aircraft designers have devoted increasingly more effort over the years to reduce these speeds even as aircraft have become progressively heavier and more optimized for high-speed cruising flight. The entire purpose of flaps, slats, and other high-lift devices now fitted to most commercial jetliners is to reduce the aircraft velocity in these critical portions of flight without sacrificing any efficiency in cruise. For more information on how these high lift devices work, see previous questions on slats and flaps.

Unfortunately, these efforts can only go so far. Aircraft continue to increase in weight while their overall size is generally limited by what airports can handle. What this constraint implies is that the size of an aircraft's wings is limited. Thus, as weight grows, the ratio of weight to wing area (W/S) also grows. While this is not necessarily a bad thing since higher W/S ratios are desirable in cruise flight, a high W/S ratio does lead to increased takeoff and landing speeds. So the aircraft designer is faced with a tradeoff between a high W/S for efficiency in cruise flight and a low W/S that is safer during takeoff and landing. As previously indicated, most designers have opted for maximum efficiency in cruise and incorporated a series of high-lift devices to keep landing speeds as low as possible.

Nonetheless, there is another school of thought that says the traditional aircraft design needs to be abandoned altogether and replaced with flying wing or lifting body shapes that could potentially result in much safer and more efficient aircraft. The flying wing dispenses with the typical layout of separate wings and fuselage in favor of a passenger cabin contained within the lifting surface itself. The advantage of this approach is that the central portion of the wing containing the passenger cabin contributes a significant portion of the vehicle's lift and increases the lift-to-drag ratio (the measure of aerodynamic efficiency). Unfortunately, the flying wing also has drawbacks. The biggest of these has been the configuration's lack of stability that only recently has been resolved by the development of advanced computerized flight control systems. In addition, the cylindrical fuselage is an ideal shape for a passenger cabin since it can be pressurized while minimizing structural weight. So even if a flying wing is more aerodynamically efficient, it tends to be heavier than a traditional aircraft designed for the same purpose. In terms of our discussion, the most attractive feature of the flying wing is its enormous wing area that reduces the W/S ratio and reduces takeoff/landing speeds. However, this feature can also be a disadvantage since aircraft with low W/S are more susceptible to gusts and turbulence that make flying more uncomfortable for passengers.

Another variation on this theme is a compromise between the traditional layout and a pure flying wing. Perhaps one of the most vocal champions of this approach has been the Burnelli aircraft company that built and flew a number of "flying fuselage" designs during the 1920s and 1930s. Although similar in concept to the flying wing, Burnelli's designs were closer to the so-called span-loader and featured a distinctly separate fuselage shaped like an airfoil to produce additional lift. Some examples of his designs are pictured below.

Burnelli RB-2, UB-14, and CB-16 aircraft

Burnelli argued that by generating a significant portion of the aircraft's lift, the fuselage could be composed of a greater percentage of the plane's structure. The combination of a shorter, more compact fuselage dictated by the lifting fuselage design along with the increased fuselage structure allows the passenger cabin to become more of a "safety cage." Thus, the Burnelli-style design merges the increased wing area of a pure flying wing with a structurally stronger passenger cabin. As a result, Burnelli argued that his aircraft would be less likely to disintergrate in an accident not only because of reduced takeoff/landing speeds but also due to the stronger fuselage. When the passenger cabin remains intact, the survival rate in an accident tends to go up. But as with the flying wing, there are disadvantages. In addition to the difficulty in employing a cylindrical pressure cabin and potential susceptibility to gusts, the Burnelli concept suffered from high drag and poor pilot visibility. Burnelli's designs never saw any commercial success, although his supporters claim that all variety of current aircraft infringe on his patents. To learn more about Burnelli aircraft, check out Aircrash.org. Although the site provides some interesting historical perspective on Burnelli's work, keep in mind that it has an obvious bias and goes to some rather fanciful extremes to support a vast conspiracy theory that's almost impossible to believe.

To sum up this discussion, improving the survivability and crash-worthiness of aircraft is primarily a matter of reducing the energy imparted to the plane (and passengers) during an impact. The most obvious method of achieving this goal is to somehow reduce aircraft landing and takeoff speeds without significantly degrading efficiency in cruise flight.
- answer by Jeff Scott, 20 January 2002

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