FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1964
1964 - 0477.PDF
292 FUCHT International, 20 February 1964 FLIGHT 8YSTEMS SURVEY Hard Hats For High Places Some Problems in Helmet Design by J. GREGORY* Fig I M.L noise-excluding helmet Fig 2 Mk 2 flying helmet SOME form of headwear, even a reversed cloth cap and goggles,has always been considered an essential part of a pilot orcrew members' personal equipment, but a great deal of diffi- cult and complex development lies behind the modern protective helmet and its many survival features. The environment in modern high-performance aircraft is exacting and can, in emergency, be- come extremely hostile. To provide an effective answer to these conditions a flying helmet- must fulfil three basic requirements. First, it must allow aircrew to carry out their normal flying duties efficiently. Secondly, it must allow continued performance of these duties over the range of normal environment change. Thirdly, it must provide means of survival following failure of the aircraft or associated equipment, whether by accident or enemy action The first requirement is met by providing high-quality intercom and a manually adjustable anti-glare visor. Recent development of miniaturized microphone and telephone receivers and resultant savings in weight and space, combined with new installation tech- niques such as tube-coupling of remote elements, are important accomplishments. To be effective these components must be asso- ciated with efficient sound attenuation for the ears and skull— particularly important in certain specialized applications. The partly or completely tinted visor must be easy to adjust for adequate shielding under all flight conditions and, in certain cases, also give automatic blast protection for the face during ejection. Ease of mounting and sound attenuation make a rigid shell a convenient basis for the helmet (Fig 1). Internal webbing harness systems and shock-absorbing resilient spacers in turn protect the skull against high-frequency impacts during turbulence or buffeting, and also minimize the risk of serious injury in the event of a crash landing. One of the more difficult problems in the design of a rigid- shell helmet is to provide adequate comfort for all wearers within the minimum range of sizes. The second requirement, to permit operation at low and medium altitudes, involves the supply of breathing oxygen or proportioned oxygen/air mixtures depending on altitude, to prevent aircrew suffering the effects of anoxia. Oxygen is metered by pressure- responsive regulators installed in the aircraft or seat and, in certain applications, in miniaturized form mounted on the helmet or clothing. The mask may be a self-contained unit incorporating the necessary breathing valves and attached to the helmet through an adjustable toggle harness-frame, as in Fig 2, or it may be integrally mounted within the chin area of an enclosed helmet with the valves mounted remotely on the shell. Alternatively, the mask is replaced by an equivalent oxygen compartment incorporated as part of an enclosed helmet in conjunction with a permanently closed clear visor. Ostensibly, the last-named arrangement appears preferable in that the face is freed of the contacting encumbrance of the mask, but the apparent gain is offset by other factors. A contacting seal encircling the facial area is now required to divide the oxygen com- partment from the remainder of the helmet space; and this in- evitably leads to a larger swept volume than that of a mask, with the consequent danger of rebreathing "used" air. The visor must remain closed and sealed above 10,000ft to achieve effective control of oxygen concentration, raising problems of misting and reflective glare and, possibly far more important, the psychological effect of total head enclosure. For these and other reasons the integral * Project Engineer, M.L. Aviation Co Ltd. oxygen mask seems to offer the best compromise, together with the psychological and physiological advantages of safe open-visor operation. Such an oxygen supply is quite adequate at heights up to about 40,000ft, but above this height other provisions, described later, are necessary. The third requirement, namely, survival under severe emergency, presents probably the most exacting and difficult design problem of all. Requirements may call for full shock-absorption during a crash landing and also for cushioning of severe buffeting. Provided there is space within the helmet for adequate movement of the head-cradle harness, whether single- or multiple-layer, and for sufficient thickness of resilient energy-absorbing materials near the fronto-temporal and more vulnerable areas of the skull, shock- absorption is excellent. But this degree of protection tends to in- crease helmet size and weight to a point where it is only acceptable for flight at lower speeds and altitudes. Buffeting in high-perform- ance aircraft will impose severe stress on aircrew unless helmet size, and particularly weight, are kept to a level consistent with the lesser impact forces involved. A balance must be found between the two basic requirements. There is an increasing need for protection against blast during ejection at high speed from modem high-performance aircraft, but the open helmet and mask combination gives only limited facial protection and is fairly easily dislodged. In this case, the enclosed helmet offers a far better solution. In both types of helmet it is preferable for the visor to close automatically as part of the ejection sequence, either by means of a g-sensitive device in the visor mechanism or by direct actuation from the seat through a cable incorporating automatic disconnect. The latter method has the added advantage of positively restraining the helmet and mini- mizing the risk of injury from head flailing. Cabin pressure failure at heights above 40,000ft is the main criterion in the design of a high-altitude helmet, because aircrew may be instantly exposed to the dangerous environment of space or semi-space. In such an eventuality the helmet must function as a life-preserving device to allow descent to a safer altitude or, alter- natively, permit completion of the mission under changed con- ditions. Briefly, the basis of the need for additional breathing oxygen is that, if the neutral gases in air at a sea-level pressure of 14.71b/sq in were removed, the oxygen expanding to fill the same volume would be at a pressure slightly under 3.01b/sq in. This pressure is the key to the quantity of oxygen the body needs. Neglecting certain other considerations, a 100 per cent oxygen gas concentration breathed at this pressure is sufficient to sustain life and, taking into account aircrew work levels, can even be reduced to about 2.721b/sq in, which the altitude/pressure curve in Fig 3 shows to be equivalent to 40,000ft Aircrew may breathe normal air from sea-level up to 10,000ft without undue detriment, but lack of oxygen affects general facul- ties above this height and 100 per cent oxygen—or preferably oxygen/air mixtures—are supplied up to 30 000ft where the tran- sition to 100 per cent oxygen is completed. The gas is fed at a slight positive pressure of about 0.051b/sq in to ensure that the breathing mixture is not diluted by inwards leakage into the mask. Above 40,000ft decreasing ambient pressure will cause the gas to thin out to such a density that even 100 per cent oxygen concentration is insufficient. The pressure must, therefore, be gradually increased in proportion with altitude in order to maintain the minimum required density. For example, at 70,000ft oxygen must be sup-
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
