Safety in aviation means coming down softly.
I once wrote that it would be impossible for a skydiver in a wingsuit to flare and land without a parachute.
THE AERODYNAMIC EFFICIENCY of a wingsuit is so low, and its area so small, that even with all the stored energy of a 100-mph fall it could not simultaneously support a human body and slow to a safe landing speed.
Shortly after I delivered this pronouncement, a 42-year old British stuntman named Gary Connery did it.
Not quite the way I imagined, however. Connery, accepting that he would not be able to land, like a duck, at a fast walk, and certainly not at a standstill, like a pigeon, decided instead to fly at full tilt into a runway-cum-shock absorber consisting of 18,000 large cardboard boxes. The You Tube videos of his arrival – there are several – are impressive. He disappears into the pile – it’s very long, of course, but not that deep – in a cloud of flying debris, like a three-year-old trying to whip cream.
‘hurl student, instructor and aeroplane back into the air’
After a considerable lapse of time, during which associates poke around the outside of the stack like archaeologists searching for the entrance to an ancient tomb, a grinning Connery strides out unscathed.
It was he, by the way, who, as part of the opening ceremonies of the 2012 London Olympics, parachuted over the stadium disguised as the Queen of England. His wig was white, his dress salmon.
Fast forward to July of 2016. Luke Aikins, a professional skydiver, jumped from an aeroplane at 25,000 feet without a parachute. He landed safely, though off centre, in a 30-meter-square net suspended 60 metres above the ground by four cranes.
That stunt seems to have been a dramatic application of the fact, familiar to all pilots, that the aim point is the one from which all the surrounding points appear to spread outward. Reportedly Aikins made a number of practice jumps, opening his chute at 1,000 feet rather than the customary 2,200, in order to test whether he could aim for the net and position himself directly above it. Evidently he developed quite a lot of confidence that he could.
What Aikins’ and Connery’s stunts have in common is the need to dissipate a good deal of energy in a short distance.
Suppose Connery is going 40 metres per second when he hits the boxes. The acceleration of gravity is 9.8 metres per second per second; that means that the speed of an object falling in the Earth’s gravitational field tends to increase by 9.8 metres per second, or about 59 kph, every second. Horizontal deceleration is measured the same way: A deceleration of one G occurs when speed diminishes by 9.8 metres per second every second. So if Connery slows to a halt in two seconds or so, he experiences about 2 Gs. If in one second, 4 Gs; and so on.
Of course, Connery was not decelerating smoothly, but by a series of shocks as he collided with one crushable wall after another. He probably carried an increasing amount of cardboard ahead of him as he moved through the pile of boxes, and that would, I suppose, tend to increase the rate of deceleration. As a lover of knowledge, I certainly hope that he was instrumented with an accelerometer, preferably a recording one. I wonder whether he used some sort of neck brace. Usually people running into something prefer not to lead with their heads.
The terminal velocity of a skydiver is usually reported to be around 56 m/s, although it varies with the jumper’s weight and size. Leaving the parachute behind reduced Aikins’ weight and therefore his freefall velocity a bit. Unlike Connery, and quite understandably, Aikins did not lead with his head: He rolled over onto his back just before reaching the net and struck it in curved supine posture and, I assume, with a sigh of relief.
Aikins’ situation was more similar to that of a bungee jumper. He probably experienced the greatest deceleration near the end of the process, when the net reached its greatest extension. To judge from the video, this must have been about 50 metres below the point at which he entered it. Bungee jumpers are said to experience around 3 Gs peak deceleration, but of course they are not at terminal velocity when the cord becomes tight. I would guess that Aikins may have momentarily felt 5 or 6 Gs.
Like Connery’s boxes and Aikins’ net, the landing gear of an aeroplane must smoothly and comfortably dissipate the vertical velocity with which the aeroplane reaches the ground. The same relationships of speed, distance and rate of deceleration apply. The aeroplane does not arrive at 65 m/s straight downward, fortunately, and so it does not require a shock absorber 50 metres long. Daniel Raymer’s encyclopaedic Aircraft Design: A Conceptual Approach, states that for oleo struts, a stroke of about 20 cm is considered a minimum; that is, for instance, the stroke of a Cherokee’s main gear. It may seem curious, though it is actually quite logical, that gear strokes for large aeroplanes are similar to those for small ones. It is the diameter of the strut that grows with the aeroplane, not its stroke length, because the assumed descent rate onto the runway remains the same.
General aviation landing gears are designed for a 3 G arrival, which most passengers would find memorable and pilots deeply mortifying. Carrier aeroplanes are tougher: 5 to 6 Gs. Landing gears are tested by dropping the aeroplane onto the floor from a certain height. Part 23 requires that the drop height be not less than 9, nor more than 18 inches. The drop test reflects both the design G load and the fact that even when an aeroplane is stalled two feet in the air and dropped in by a novice pilot, the wing is still carrying part of the aeroplane’s weight on the way down.
A few aeroplanes, notably the Thorp T-18, have been designed with rigid landing gear struts, on the assumption that tires act as springs and that angled legs, such as the T-18 has, provide a little bit of flex even if they are made of quite stiff steel tubing. At any rate, the T-18’s A-frame gear is strong enough to handle more than the customary 3G arrival.
Most aeroplanes, however, have some sort of spring built into their gear. The spring spreads out the force of deceleration over time and distance by compressing air, rubber or metal or by stretching a bungee cord. The greater the distance the wheel can travel while arresting the descent of the aeroplane, the gentler the deceleration will be.
A perfect spring returns all the energy put into it in the form of an equal and opposite reaction. This would not be very practical for an aeroplane, since it would mean that after a hard landing the plane would bounce back to the same height as it was dropped from in the first place. (A car with worn-out shock absorbers wallows uncomfortably because its springs rebound without damping.) No real spring is perfect, but landing gears still need some method of dissipating or “damping” all, or at least part, of the energy they absorb.
‘most passengers would find memorable and pilots deeply mortifying’
Damping dissipates, through friction, some of the energy that would otherwise be stored and subsequently released. In an oleo strut, the spring action is supplied by compressed air, the damping by oil being driven through a small hole. A Mooney’s rubber donuts are damped by the internal friction of the rubber. Spring-steel gear legs are damped by the scrubbing of tires on the runway as the flexing gear spreads outward.
Gary Connery’s cardboard-box shock absorber consisted entirely of damping; there was no spring, and no tendency for him to bounce back. Aikins’ net, like a trampoline, had some springback, but, unlike a trampoline, it seems to have been heavily damped by the mutual friction of its woven fibres.
Ideally, a landing gear would be well damped and have little springback. As flight instructors know, however, the spring steel gears on many trainers have plenty of bounce, and are quite ready, after a hard landing, to hurl student, instructor and aeroplane back into the air.