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Aviation History
1955
1955 - 1590.PDF
"Flight" photograph TUG AND GLIDER Some Notes on Aero-towing By PETER W. BROOKS THE increasing popularity of gliding and the growing useof towing by aeroplane as a means of starting soaring flightssuggest that a few notes on the technique of aero-towing may be of interest to power pilots who may be called upon togive a tow at short notice. At one time aero-towing was regarded as a rather hazardousbusiness which could be undertaken only by experienced pilots. Since the war, however, the attitude to this method of launchinghas changed completely and it is now generally considered to be the easiest and safest method of getting a glider into the air—provided a few simple precautions are observed. Aero-tows are now given to glider pilots at an early stage of their training andseveral aeroplane pilots with less than 100 hours in their log- books have proved themselves able to fly a tug reliably. Wheneither pilot is new to aero-towing, however, it is desirable for the other pilot to have had some previous experience of this typeof flying. Most gliding clubs insist that, with a pilot new to aero- towing at one end of the cable, the pilot at the other end shouldhave previously made at least five tows. The change in attitude in recent years results partly from thegreat fund of experience accumulated during large-scale towing of military gliders during the war and partly from the improvedequipment now available. Towing of sporting sailplanes is normally done with a nylon rope 200ft long and of fin circum-ference, with a breaking strength of 1,000 lb. This kind of tow-rope is a great improvement over the wire cable or hemp ropepreviously used. The elasticity of nylon is sufficient to eliminate most of the violent jerking of tug and sailplane which is liable tooccur when slack is taken up suddenly while on tow. The unpleasant "tweeks," which were so disconcerting when towingwith non-elastic ropes, are thus avoided. The successive phases of an aero-tow, from the tug pilot'spoint of view, can be most conveniently defined as follows: (1) The hook-on; (2) the take-off; (3) towing flight; (4) the release;(5) cable-drop and landing. We may now discuss each of these phases of an aero-tow asthey affect a pilot flying an Auster Autocrat or a de Havilland Tiger Moth, the two types most commonly used in this countryand those on which the writer has had most experience. The Hook-on. The tug should be positioned about 50 yardsup-wind of the sailplane and lined-up to give the maximum avail- able run for take-off. The tow-rope should be laid out on theground between tug and glider but should be attached to the glider only when the latter is ready to take off. While theglider pilot is getting ready, the glider should be left with one wing-tip resting on the ground. The signal that all is set isgiven by somebody at one wing tip levelling the glider's wings. This person will later run with the glider during the take-offuntil there is sufficient speed for its pilot to have lateral control. While the glider's preparations for flight are being completed,the tug pilot should perform his take-off "vital actions." He should if possible, have a rear-view mirror which will enablehim to see straight behind him. (The compass-mirror on the Autocrat and the instructor's mirror on the Tiger Moth are bothadequate for this purpose if properly adjusted.) Only when the glider's wings are seen to be level and the glider's wing-tipholder signals "take up slack" (by waving an arm backwards and forwards across his body) should the tug pilot start to taxi slowlyforward while looking back over his shoulder. If a tug "hooker- on" and marshaller is available, he should stand in front of thetug to port and repeat, for the benefit of the tug pilot, the signals from the glider's wing-tip. A final check should always be made by the tug pilot at thispoint. He must make sure, by looking carefully, that the sail- plane's dive-brakes are closed. This is important, because mosttug/glider combinations have a very poor rate of climb and with the glider's dive-brakes open may not, indeed, take-off at all.Should a take-off be started with the brakes open, or should these come open accidentally, the take-off should be abandonedimmediately. If this becomes necessary at an early stage, the tug's throttle should simply be closed. If it occurs during the laterstages, the tug pilot should jettison the glider by means of his quick-release and continue his take-off alone. The Take-off. As soon as the tow-rope is taut, if all is ready,the glider's wing-tip holder will signal "all out" (by waving an arm backwards and forwards above his head). The tug pilotshould then face forwards and fully open the throttle, without allowing the tug to stop rolling—so as to make the most of themomentum already gained. At the same time, a final check should be made that the aircraft is correctly lined-up and that all is clearin front. When the throttle is fully open, the tug will start accelerating slowly and there will be a tendency for the tail tolift with the pull of the tow-rope. This should not be resisted, because it reduces the drag of the tug and helps acceleration.Care should be taken, however, not to allow the tail to rise fully to the take-off position too soon; otherwise there will be a riskof nosing-over should the tow-rope break. Initial acceleration is disconcertingly slow. Under averageconditions, with the Autocrat or Tiger Moth, the ground run will be between 400 and 500 yd, depending on the type of glider, theaerodrome surface, the wind and the temperature. This compares with a run of about 200 yd for these aircraft flying light. Thedistance to clear 50ft, with a single-seater or high-performance two-seater sailplane, will be between 800 and 1,200 yd underaverage conditions. As speed increases, the glider will unstick and fly along behindthe tug a few feet off the ground. Acceleration then becomes more rapid and, at a speed of not less than 50 m.p.h., the tugshould be allowed to leave the ground, although it must be held down so as to continue its acceleration to towing speed—60m.p.h. (52 kt) for the Tiger Moth and 65 m.p.h. for the Autocrat. Once the correct speed has been reached, the climb can be startedwith the engine still at full throttle. Towing Flight. The only thing about aero-towing whichrequires extra concentration from the tug pilot is the need to hold a steady speed at a lower figure than the normal operating range.Thus, the speed of the Autocrat must be held as nearly as possible constant at 65 m.p.h., that of the Tiger at 60 m.p.h. Lower speedsshould be avoided, because of the risk of engine overheating; and speeds above 60/65 m.p.h. will over-stress the glider. Inpractice, all the average pilot will be able to achieve when towing in rough air—and the air is usually rough when soaring condi-tions are good—is a speed within perhaps 5 m.p.h. of the target figure. This should, however, be maintained as the mean. Themore constant the speed, the smoother the ride and the easier conditions will be for the glider pilot. The majority of aero-tows consist of a straightforward climbwith the object of positioning the glider in useful lift where it can release and start soaring. Often the glider pilot will indicatebefore take-off a particular area in which he wishes to be released; sometimes, during competitions, there will be a designatedrelease area; usually, a limiting height for the tow is laid down and an extra charge made for a climb above, say 2,000ft. If amaximum towing height is fixed during a competition, the tug
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