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Aviation History
1964
1964 - 3204.PDF
fUGHT International supplement, 27 August 1964 Air-Cushion Vehicle* and which frequently resulted in an attachment of the water to the nozzle itself, sometimes building up a bow wave of startling proportions. Provided sufficient force was available to drive the nozzle through this condition, the bow wave would collapse and very little water deformation would be seen at high speeds. The results of these tests were of particular interest to those who have suffered bitterly with craft having abnormally high drag at low speeds, previously thought to be a feature of craft having low hoverheight combined with high cushion pressure. CC-2 Tests at RAE Mr I. L. Keiller, of the RAE Bedford, had something to say on control prob- lems on the CC-2/001, which has been extensively tested at that establish- ment. Mr Keiller began by explain- ing the control system. The most noticeable feature of the control responses were with the weakness of the forces available, particularly in yaw, and of substantial secondary effects. Thus, whilst the control system was basically simple in concept, it was much more complex in practice. For example, it was found that yaw moments could be obtained by the use of the roll vanes (due to loss of thrust from the fixed vanes on one side) and even some yaw effect from the restricter flaps in the transverse jets. One of the major considerations to be borne in mind when driving a free-flying ACV over land was that, superimposed on the tendency of a craft to take up wind speed, was its tendency to slide down to the bottom of all dips and valleys on the ground surface. The largest thrust force available being in the forward direc- tion, the only way of dealing with a sig- nificant slope or wind was by heading up into it. A further consequence of the largest thrust force being in the forward direction was that the most effective braking procedure was to make a turn of 180° and apply full forward thrust. The fixed aft-inclined vanes in the side jets caused the craft normally to travel forwards, and to counteract this ten- dency and achieve static hover such a large part of the available control force had to be used that control became impossible. Notwithstanding all his foregoing remarks, the speaker was anxious to assure his listeners that there was a sur- prising ability for the craft to arrive at its destination and that there had been no handling accidents either in a con- fined hangar or on the airfield, at air speeds up to 25kt. Mr Keiller went on to describe a series of test results obtained with the craft tethered in a hangar. The hangar was sufficiently large that circulation effects could be discounted, and hover- height was measured simultaneously by two cameras at right angles. The mea- sured control forces confirmed drivers' subjective opinions that the control forces were very limited. The craft has since been modified to give improved hovering performance and there has been substantial redesign of the con- trols. A further test programme is to be undertaken. Research at Swansea Mr D. L. Hughes gave some account of the research programme at the Uni- versity College of Swansea. This had been directed principally at the analysis of flow pattern in single-jet, two-dimen- sional models, under various conditions of split and over-fed jets. Mr Hughes' talk was illustrated by some very beau- tiful colour film sequences of a water model with the flow patterns indicated by different coloured streams. In addi- tion to two-dimensional testing, a free- flying, man-carrying craft is being built in the laboratories. Plans are also well in hand for towing tests with completely submerged models, using water as the operating fluid in peripheral-jet configurations. APPLICATION OF SIMULATION TECHNIQUES For some time now, said Mr A. G. Barnes, BAC (Warton) have been con- cerned with the design of a high-per- formance military GEM. His paper was intended to comment on one par- ticular aspect of the design studies made for this vehicle. I quote: "Producing a new aircraft is a com- plex, costly, long-term process, and design mistakes can be serious—more serious than in other branches of engineering. Consequently, elaborate methods are used at the design stage to try to ensure that the final machine will meet the claims of the original specifi- cation. Such methods include wind- tunnel tests, computer studies for aero- dynamic and structural design, and possibly flight-simulation tests, to opti- mize the pilot/airframe combination. "It is not surprising that an aircraft manufacturer setting out to design a GEM will apply much the same design process that he uses successfully on air- craft to the GEM. Naturally, the capital and operating costs of these design facilities are high, and are diffi- cult to justify purely for the design of current GEMs. Nevertheless, if you have such facilities, it is common sense to use them and this is what we have done. As the design aims of GEMs be- come more ambitious, so will the need for advanced design techniques of this type increase. "GEMs are a happy hunting ground for the modern aerodynamicist: fan design, duct design, power/weight opti- mization, dynamic stability and control. Particularly in these last two areas has simulation a contribution to make. Potential Uses of Simulation "To define more closely the areas where simulations can help to optimize design, an analogy with aircraft design appears. With high-performance GEMs, as with high-performance air- craft, dynamic stability can be a serious problem. Moreover, it is often difficult to define what levels of stability are acceptable. There are many examples of pilots/drivers being able to control cer- tain types of instabilities—helicopters, VTOL aircraft, motor-cycles, etc—but there are just as many examples of serious accidents due to the pilot/driver being unable to control other types of instabilities. "Closely allied to the stability prob- lem is that of control. Controls that are either too sensitive or too insensitive can cause unfavourable comments, and under certain circumstances, can result in pilot-induced instabilities. Also, the need to achieve a good compromise between stability and manoeuvrability has long been a problem in aircraft design. Because GEMs have both low power-weight ratios and no ground contact, the provision of adequate con- trol power is invariably a problem; if this is coupled with poor stability, the problem is intensified. "Nor can we overlook the complexity of the GEM driver's task. He has the possibility of direct control inputs into five degrees of freedom—which com- pares with the two inputs of the car driver, and the four inputs of the pilot of an aircraft. To design a control layout to meet this possibility is in itself a difficult exercise, and one which can make or mar the final vehicle. "Simulation, then, allows us to study some of these problems before the hard- ware stage. Furthermore, in such studies we do not have to make wild approxi- mations to describe how the pilot reacts to a given stimulus—we close the loop and he is part of it. The difficulty, oi course, is to ensure that we close the right loop. Problems with GEM Simulation "The loop we are closing is shown on Fig 1. The driver's control movements are picked-off electrically and become 29
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