Friday, 20 November 2009

Oxford Landing

After Tuesday's tutorial I took the 205 bus from Marylebone, where tutorials are now held, to City Road, close to Henny's home in Clerkenwell. I popped into the newsagent cum off licence next to the bus stop to get a bottle to accompany the promised shepherd's pie. The range of the wines on offer was small but included a couple of good Aussie brands, Hardys and Oxford Landing: I chose the latter's Shiraz.

On my desk back in Buckingham I have a 5" x 3" plaque of black bakelite, with white lettering embossed on it.

The plaque was originally on the instrument panel of an Airspeed Oxford, the plane built by the company founded by Nevil Shute, a man much more widely known as a novelist than as an aeronautical engineer. The Oxford was the twin-engined aircraft on which I did my advanced flying training at R.A.F. Dalcross (now Inverness Airport) in 1952–1953.

Click here to go to the last paragraph of this post if you are profoundly uninterested in how to land an Oxford.

Flaps are lowered when you are preparing to land your aircraft. You lower them when you are on the downwind leg of the circuit prior to turning onto your final approach and when your Oxford is flying at a speed lower than 120 miles per hour as shown on the Air Speed Indicator. This has the effect of lowering your stalling speed, the speed at which your aircraft starts falling out of the sky.

A way in which you can demonstrate to yourself the principle of flight is to find a large spoon with a hole at the top of the handle through which you loop a piece of string. Holding the string, turn on the tap on your kitchen sink and draw the spoon towards the flowing water with the concave side towards it. Common sense suggests that, if you were to swing the spoon into the water it would be pushed away by the little torrent. You will find that this is not so. The opposite thing happens: the spoon is drawn further into the torrent.

What's happening is that the water is speeding up as it flows over the convexity of the spoon and in so doing the water pressure becomes less than in the flow on either side of it so that the spoon is, as it were, sucked into the torrent. If you understand that air behaves like a liquid, but in gaseous form, and if you realise that the cross-section of the convexity of the spoon is roughly the same as the cross-section of an aircraft's wing, you will begin to see how it is that an aircraft stays airborne.

In aeronautical terminology this suction is the force called lift. The force that propels the aircraft forward, causing the air to flow over the wing is called thrust, while the one that imposes an upper limit to its speed is drag. When the upward force lift is balanced by the downward force weight, and the forward force thrust is balanced by the backward force drag, the aircraft flies at an even speed, neither ascending nor descending.

If you are flying that hypothetical aircraft and you throttle back slightly so that you reduce your thrust, then the decrease in pressure in the flow of air over the top of the wing and your lift reduces as well. The nose of the aircraft drops slightly and you lose height. You can counteract this by pulling your stick (the column between your knees which controls the ailerons on the wing and the elevators on the tailplane), thus bringing the nose up slightly. The air now has to travel slightly further, pressure is decreased, lift is increased and equilibrium is restored. There comes a point, however, beyond which you can no longer safely pull the nose up.

You will have observed how water in a stream behaves as it approaches and then enters a rapid. The flow is smooth to begin with but then as it gets even faster the current breaks up into whorls and eddies. The flow has gone, in science-speak, from laminar to turbulent. Exactly the same thing happens to air flowing over a wing. At the moment that the air current becomes turbulent, the lift generated by the decrease in pressure is suddenly lost and the aircraft falls.

This, in essence, is what happens when you make a landing in an aircraft with two wheels in front and a smaller one under the tail. As you make your approach to the runway or landing field, you start by lowering your flaps which effectively increases the wing area and thus the lift that is generated. As you slowly throttle back, so the aircraft descends; as you gradually lift your nose, so the speed drops off. If you have judged things correctly, your aircraft is very close to the ground with the wheels in front and behind parallel to the ground. At that point you throttle back completely and pull the stick back into your stomach so that the speed falls below the stalling speed and your aircraft drops out of the few inches of sky left beneath it and you feel a tiny bump and you hear a slight squeak, as the rubber on the stationary wheels suddenly begin to rotate rapidly, and you feel a moment of triumph for pulling off a perfect three-point landing.

That is the theory. In practice, this is hard to do. One of the tests we had to do in April 1953 during our final assessment before being awarded our wings was carrying out a spot landing without power, which is to say that I was expected to throttle back fully at the end of the downwind leg of the circuit and then touch the aircraft down within x yards of a spot marked on the runway.

Now, throughout all my previous assessments I had been rated just an average pilot and spot landings are hard enough at the best of times, even if you are an excellent one. Therefore I devised a devious scheme to ensure success. I determined on doing two things. One, I would not throttle back entirely at the end of the downwind leg but leave the engine ticking over slightly above idling, so that when I did throttle back (with the difference in engine noise undetectable, I hoped, to onlookers) speed and so lift would be reduced. Two, I would when I was just in front of the spot raise my flaps so that lift would be further reduced. This is what I in fact did, and it worked. The only trouble was that my approach had been slightly too high, so that when I simultaneously throttled back completely, raised the flaps and pulled the stick hard back, I was several feet above the runway and the aircraft fell to ground with a spine-jarring thud.

This story of my Oxford Landing has a happy ending, though. I did touch down sufficiently close to the spot to pass. And Mr Shute did build his aeroplanes to be sufficiently robust to survive unscathed abuse from pilots such as me.

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