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Aviation History
1959
1959 - 0148.PDF
58 FLIGHT, 9 January 1959 Why do Birds Fear Helicopters? By DAVID GUNSTON IN my recent investigations into the rather fascinating subjectof avian reactions to helicopters in flight I have heard of manystrange cases wherein quite large birds have shown every symptom of complete panic at the approach of the machine. Birdsimmediately get into the most literal of flaps when they see a rotor aircraft coming in their direction; they jink, stall, attempt to climb,lose height and flutter away ignominiously. Furthermore, most birds seem terrified of helicopters in the air even when they them-selves are on or close to the ground. Wild geese, rooks, poultry and sparrows alike seem to assume that the helicopter is an enemy, andthey take immediate avoiding action, often running for cover. Similar reactions are within the experience of almost all heli-copter pilots, and since first airing the subject in a Sunday news- paper I have been offered much first-hand evidence of sheerbird-fear in this connection, and an equally overwhelming volume of speculation and theory as to its causes. For an air-minded age so devoted to watching birds this is a fieldof interest tha1: has so far received scanty attention from the scientific ornithologists; but it is worth examining the most likelyreasons. First, the downwash. As a Tiger Moth pilot discovered recentlywhen he flew (or attempted to fly) beneath the Fairey Rotodyne, this may be of extreme turbulence, making normal bird-flightimpossible. Consequently the crazy uncontrolled aerobatics observed in birds as big as eagles and herons may frequently bedue to this cause. In particular, a bird already alarmed by the sudden appearance of a helicopter almost overhead, and instinc-tively trying to climb to safety, would find itself completely out of control, unable to gain height or even retain it, and exhibiting themost ungainly antics in a frantic strain to get away. This was the explanation of the panic of a heron observed recently by BristolAircraft test-pilot Bob Smith as he flew a Sycamore over the London-Yorkshire motorway. And it is a partial explanation ofthe typical reaction of birds caught in mid-air. Another test pilot, "Slim" Sear of Westland Aircraft, enlargedon this point to me: "To further the discussion, a friend of mine said that when he was flying a Sycamore the birds on rising fromthe ground would fly in a clockwise direction; but when he was flying a Whirlwind, they would rise and fly in an anti-clockwisedirection, i.e., the direction of the rotor rotation, though I person- ally have not noticed this effect." If such behaviour does occur, itwould strengthen the evidence we already have on downwash as a cause of bird "panic." In general, it is clear that, by comparison with the fairly narrowair-turbulence wake of a fixed-wing aircraft, the combined wake and downwash of a helicopter, particularly at slow forward speedsor at the hover, creates excessive local turbulence sufficient to disable any bird in flight. But the creature must be caught moreor less immediately under the machine for the downward flow of air to have any pronounced effect. A bird just nearby would notnormally be so affected, though it might still show every sign of panic. However, to be caught in a helicopter downwash is sucha terrifying experience that if frequently experienced it may to some extent influence a bird's future behaviour in like circum- ". . . . their usual nonchalance towards fixed-wing aircraft"—displayed in this pleasing picture by seagulls towards a Hunting Sea Prince T.I ". . . the comparatively slow-moving, earthward-tilting, forward- peering shape of the modern helicopter creates in the bird mind the illusion of predatory attack." An S-55 being used by the U.S.A.F. in an attempt to drive starling flocks from Rhineland vineyards stances. There is, for instance, some evidence that the birdpopulation around an airfield continually used by helicopters learn to .avoid flight in its immediate vicinity: at the Westland field atYeovU they call their local birds "fully certificated pedestrians"! Yet there are obviously factors other than downwash that in-fluence birds in such striking contrast to their usual nonchalance towards fixed-wing aircraft. Some people aver that fear is inducedby the distinctive noise helicopters make, claiming that they have noticed incipient fear directly the sound is heard. Yet there is littleevidence that wild birds are disturbed by ordinary aircraft noise, even jet noise. They have amazingly selective hearing, ignoringthe overhead roar of gas turbines yet detecting the faintest rustle of a twig in the bushes close by. In most if not all of the above casesit could be shown that sight, not hearing, played the first part in setting up fear reactions in the birds' minds. This brings us on to the vexed question of helicopter shape,outline and silhouette. Usually it is the on-coming machine— mostly, though not al-ways, at low altitude— that scares off the rookflocks, or panics the poultry into theirhouses. Slow or high speed here seems tomake little difference, and panic has been noticed from machines flying at 17,000ft or more, where there is no turbulence factor tocontend with. Poultry in their enclosed runs seem particularly affected by the sight of the approaching helicopter. "Slim" Searconfirms this vividly: "We have a small chicken farm at the end of the airfield [at Yeovil] and flying is being carried out practicallyall the time, yet I still have reports from the farmer that periodically the birds panic enough to kill themselves against the wire mesh."Now it is an indisputable fact that all birds in some degree are highly scared of what might be called the predator outline—shortneck, broad wings, long tail—provided always that it moved forward neck-first. It represents the natural enemy, soaring eagleor hovering falcon, and invariably sets up fear reaction and escape mechanism. The "outline" may be just a wooden model, card-board outline, even a harmless bird of the same shape (like a cuckoo), a large aircraft flying very high—or a helicopter flyingquite low. Similar effects have been noted from dirigibles, kites, balloons and early aircraft which were frequently of this generalshape (with engines small or behind the main aerofoils) or un- ashamedly kite-like. There seems little doubt that the revolving rotors of a helicopteradd to the illusion by suggesting hovering hawk wings, and that the comparatively slow-moving, earthward-tilting, forward-peer-ing shape of the modern helicopter creates in the bird mind the illusion of predatory attack. Hence the immediate instinctiveresults. With the growing use of helicopters it will be interesting to seewhether successive bird generations grow out of this fear. Mean- while, it needs the closest study, and I should be glad to hear fromany reader with experience of it, particularly with regard to the direction of rotor movement affecting birds rising from the ground.
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