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Aviation History
1959
1959 - 2211.PDF
198 FLIGHT, 11 September 1959 I2O CUSHION LOADING 5Olb/sq ft ?S Ib /M ft THE HOVER-CRAFT . . . 2O 4O 6O 8O GROSS WEIGHT (tons) (OO The effect of size, hover height and cushion loading on the power requirement of a flat-bottomed hover- craft at 100 m.p.h. increased clearance for the same efficiency. There is thereforea minimum size which will yield an acceptable power-to-weight ratio, for any specified clearance and speed. Speed is important, because the vertical accelerations involvedin attempting to make the body of a vehicle follow surface irregularities increase as the square of the speed. At a given speedthe vertical accelerations are inversely proportional to the square of the length of the irregularity. A motor car may leave the roadif it attempts to go fast over a short irregularity such as a hump- backed bridge, but the driver is quite unaware of the small verticalaccelerations involved in driving (even at a fast pace) up and over the "rolling Downs." For stationary hover, a circular plan-shape is the most efficient;but directly movement is considered a non-circular shape is better, and practical craft will probably have length-to-breadth ratios ofbetween about 2 to 1 and 4 to 1. The power for the lift system is directly proportional to the velocity of the air curtain; but theminimum velocity which can be used is tied to the cushion pressure; and for the craft to remain stable the free-stream stag-nation pressure at the design speed must be less than the cushion pressure. Design studies of hovercraft in their present undeveloped stateseem to show that the payload lies between about 30 per cent and 40 per cent, and may well be 50 per cent for large craft of theorder of 4,000 tons or more. Some tentative performance data of "fiat-bottomed" hovercraftare set out in the accompanying curves, to show the effects of size, hover height and cushion loading on the power-to-weight ratio.It should be noted that the shaft horse-powers specified have nothing in reserve and that the wind speed must be allowed for toobtain ground speed. A five-ton craft with a hover height of lft and a cushion loading of 25 lb/sq ft would have a disposable loadof about a ton, whereas a 100-ton crafe operating at 3ft and with a cushion loading of 50 lb/sq ft would have a disposable load of50 tons. Small hovercraft are not very efficient. If the craft is for amphibious use or use over water, then thedesign must take account of the hump drag which occurs at a speed of 1.9 %/length kt. An approximation for the value of thisdrag in lb for deep-water operation may be obtained by multiply- ing the width of the cushion by the square of the cushion loadingand dividing by sixteen. The component of drag due to wave formation at twice hump speed is about one-sixth of hump drag;at three times hump speed it is about one-eighteenth, and negli- gible in terms of the form-drag at hovercraft speeds. Craft foruse overwater are not, however, likely to have flat bottoms. Developed hovercraft will require little more than enough pro-pulsive thrust to overcome form-drag; therefore, unless extra thrust is installed they will have very poor acceleration, braking,hill-climbing and turning characteristics. The braking (and possibly the turning performance) over water could be greatlyimproved by installing fins and water-brakes, and it is likely that automatic anti-sideways drift systems will be installed, at any ratefor land craft. There is an immense amount of work to be done before thehovercraft emerges as an established mode of transport in the particular fields in which its rather unique characteristics enableit to compete with other forms of vehicle. It is thought that these fields may lie among the following: large and small land vehiclesfor use in undeveloped countries; army vehicles of improved mobility; slow-speed river hovercraft capable of about 25 to 30 kt,and high-speed amphibious river hovercraft of about 50 to 90 kt; hovercraft ferries of about 100 tons for use in sheltered waters;ferries of about 1,000 tons for use in more open waters such as the English Channel; amphibious vehicles for special purposes;amphibious assault-craft; water-jet stabilized platforms and Arctic hovercraft; and—much later—ocean hovercraft of 4,000 tons andupwards. Such a programme of development would require a whole industry. INERTIAL GYROS FROM FERRANTI A NEW laboratory for the production of inertial gyros andaccelerometers at the Crewe Toll, Edinburgh, factory of Ferranti Ltd. was opened by Mr. Aubrey Jones, Minister ofSupply, on Wednesday of last week. Costing £150,000, this extension to the existing laboratory was financed by the M.o.S.and contains special machining and heat-treatment facilities and one of the most advanced "clean" areas in the country formetrology and gyro assembly and testing. Speaking at the ceremony, the Minister said that the newbuilding was an important national investment. The swing to electronics and nuclear physics was impossible without precisionengineering of this kind and, ten years ago, such standards were unknown in Britain. There was "an industrial revolution" in thisfield. As less developed countries set up normal industries, Britain had to keep ahead. The new Ferranti establishment wastherefore important, and in addition it absorbed some of the skilled manpower in Scotland. A floated, single-axis, rate-integrating gyro is now being pro-duced at Crewe Toll under licence from the Kearfott Company in the U.S.A., but a pendulous integrating gyro accelerometerof Ferranti design is also being made. The company expects to supply complete guidance systems at a later date and miniaturegyros will probably also be developed. Ferranti have been work- ing on inertial-quality instruments for 4\ years and producingthem for 1^ years. The new laboratory (additional picture, page 193) providesfor integrated manufacturing and development. Its "clean" area, which is shortly to be brought up to full cleanliness and sealedoff, contains an atmosphere controlled at 45 per cent relative humidity and 72 deg F dry-bulb temperature, ± 1 deg. Incomingair is filtered to exclude particles of more than 1 X 10" in diameter. Nylon boots, caps and overalls are worn and people can enter onlythrough air locks and over particle-removing 30 m.p.h. air blasts. Components are dried and cleaned before being passed into thearea through other air locks. A large conditioning and filtration plant changes the air 16 times each hour through domed ceilinggrilles and collects it without draughts through floor grilles. Lights shine through dust-proof plate-glass panels in the ceiling. Many hours of test-running are required to ensure that each gyro meets stringent specifications for accuracy and this workmust be done with special test tables and recording electronic equipment. The gear must meanwhile be isolated from anyvibration and must not be subjected to rotation greater than 1 sec of arc or translations with accelerations greater than 0.0001 g.Three concrete blocks weighing 40 and 60 tons are mounted in a pit under the laboratory floor and set on rubber or spring dampers.Projecting from the blocks through the suspended laboratory floor is a series of concrete pillars isolated from the floor and protectedby guard rails. On these are mounted a series of Graseby test tables on which the gyros are run.No specific application for the Ferranti-produced inertial gyros was mentioned during the opening ceremony; it was stated only that these components were "essential for inertial navigation systems used in aircraft, missiles and ships." A Ferranti technician assembling components of a Kearfott floated gyro. Motor, gimbal and heated case are at left
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