FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1964
1964 - 1184.PDF
RIGHT international supplement, 23 April 1964 density, high-value cargo. Passengers and motor cars come into this category with mean values as follows: Passenger Motor car Weight Ib. 170 2.100 Areaallocated ft!7 110 Accept- able fare d/mile 20 60 Clearly then, in terms of revenue per lb weight carried or per sq ft deck area occupied, the passenger gives the greater return. However, conditions may exist where a car-carrying capacity is required and the optimum hovercraft to accom- modate the same load would be larger than the passenger equivalent and could lead to radical changes in design, as will be discussed later. Operational Characteristics The construction of present-day hovercraft has provoked some expres- sion of doubt from conventional ship- builders. The one sidewall type operat- ing is constructed principally of glass- fibre reinforced polyester resin with a marine plywood passenger deck. The other amphibious high-speed types are built up from 18 s.w.g. or thinner high- strength aluminium alloy. The light weight, low inertia and flexible make-up of the craft result in low impact loadings and permit the use of thin plating. Structural loading conditions arise in the main from high-speed impact with the sea; the impact load I increases according to the relationship Vlw where W = displacement weight; hw — wave height; V = craft speed relative to the water; Ai> = wavelength. Waves may contact the craft at any point over the bottom, inducing vertical and often rotational accelerations to the craft. Far more severe to the local struc- ture and a detriment to passenger comfort are impacts on the bow and beam area. Loads incurred when float- ing on the water with no air cushion are less than impact loads on high-speed craft. A background of manufacturing ex- perience on flying-boats and seaplanes enables the constructors to contemplate using aircraft-type structures and materials for hovercraft, but the high- stress levels permitted in these struc- tures make it necessary to expend con- siderable design effort to check each component of the structure. Common sense reasoning was used TO produce loading cases on the first generation of craft which operated with generally satisfactory results. Impact pressure measurements have not so far been entirely satisfactory, and the development of this technique is essential. At present, much useful data is obtained from measurement of craft accelerations when impacting waves, and from an examination of the dents in the craft. In the interests of passenger comfort, and of minimum resistance, it is desirable to prevent the hull making any contact with the water. The early concept of hovering craft and the first working models relied upon the hover gap, or daylight beneath the craft, to avoid impact. If craft rolling and pitching is ignored the mean hover- gap requires to be approximately half the wave height, crest to trough, to just avoid contact. In keeping power re- quirements to a manageable level, pro- ject designs for craft to operate over waves in excess of 4ft became larger than the square/cube law of structure size/weight would permit in practical terms. This state of affairs was changed completely by the demonstration that proofed fabric extensions fitted to the air curtain nozzles in the metal hull could be made sufficiently flexible to deflect readily when encountering waves and yet have a sufficient "life" to be an engineering proposition. The important contribution made by the flexible nozzle extension, or "skirts" to improving hovercraft wave-riding capability can- not be over-emphasized. On the debit side, skirt contact with the water when negotiating waves creates additional drag and the permanent increase in Fig 2: Wave characteristics Air-Cushion Vehicles craft frontal area increases air resistance. Irrespective of whether flexible skirts are fitted, a hovercraft aligns itself with the mean water surface slope. It follows that the hovercraft designer requires some knowledge of wave shape, height, length, and velocity and the contribu- tion of wind speed, fetch, and topo- graphical factors. Fig 2 shows the wave shape envelope of height and length derived using ex- pressions developed by Darbyshire for wind-generated waves. From personal observation it is clear that succeeding waves in a sea have varying heights and that occasionally waves considerably higher than the average do arise. The ordinate scale displays units of significant wave height; for a given sample of recordings the highest one-third are taken and the average of this batch is referred to as the significant wave height. The numerical value so derived is a good indication of a wave height frequently encountered but certainly higher waves must be anticipated. The following table will be of interest. Wave heightparameter H, or Hs/. H\t(\ in 10 average H_i_(l in 100 average) Hmax (lon* Period») Relationship withsignificant wave height 1.0 1.27 1.6 1.67 2.4 Percentage occurrenceof smaller waves 87 96 99 99.5 99.999 It is important to realize the limita- tions of the data derived from the method referred to above. No account sa- I 5- V»INO SPtEOtKNOTS) SO AVERAGE ROUGH SEA ( R E F. 6 ) COASTAL WATERS OCEANIC WATERS (REF.5 ) 100 200 WAVE LENGTH (FEET) 57
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
