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Aviation History
1955
1955 - 1591.PDF
708 FLIGHT TUG AND GLIDER . . . pilot must signal the glider to release when this height has beenreached. While climbing on tow, the Tiger Moth should be kept at fullthrottle; the Autocrat can be throttled back slightly to maintain 2,300 r.p.m., the maximum permissible continuous rating. Therates of climb of both aircraft will be between 200 and 300ft/min under these conditions. The rich mixture at the wide throttlesettings helps to keep the engines cool at low airspeeds. If a cross-country tow is being made, the engine must bethrottled back slightly at cruising height so as to give the correct towing speed in level flight. To descend on tow, further reduc-tion of power will be necessary, but complete throttling-back should be avoided. The speed must be maintained at the samefigure throughout. Turning on tow presents no problem, particularly if the gliderpilot employs the correct technique and prevents slack developing in the cable. In rough air, the tug's tail may be jerked about agood deal, causing momentary side-slips and skids; these should be firmly corrected with rudder. Rate-^ turns should be usedwhen taking somebody on his first aero-tow, but Rate 1 turns can normally be used; and, when both glider and tug pilot areexperienced, higher rates of turn cause no difficulty within the limits imposed by the low towing speed. An experienced gliderpilot can quite effectively steer the tug in the general direction in which he wants to go. By flying out to one side, he can safelypull the tug's tail in the appropriate direction, thus prompting the tug pilot—provided that long-suffering individual knows what ishappening and does not object too much to interference from the "back-seat" driver. The combination's rate of climb falls off with the increaseddrag during turns, so these should be kept to a minimum con- sistent with achieving the objects of the tow. The glider shouldnot, however, be taken very far downwind of its aerodrome during a normal aero-tow climb to height. If the wind is strong, theglider may not be able to regain the aerodrome should it fail to find lift. In general, and unless the pilot is attracted to aspecific area of lift elsewhere, the glider will release upwind of the aerodrome. Towing is normally done in the "high-tow" position. Thisinvolves the glider flying just above the tug's slipstream and has the advantage that the glider is always clearly visible to the tugpilot, either in his rear-view mirror or by turning his head. This position is assumed automatically by the glider as the tug takes offand is normally the most convenient for short tows such as climbs to 2,000ft. The drawback to the high-tow position is that it isunstable and there is always a certain amount of "hunting" between glider and tug. This is largely absorbed by the elasticityin the nylon tow-rope but is tiring on a long tow. The low-tow position overcomes this difficulty because thecombination is then stable with all tendency to hunting damped out. However, it has the objection that the glider is practicallyinvisible to the tug pilot; only the wing-tips can normally be seen beyond the tug's tailplane. Another complication is that theglider has to descend through the tug's slipstream some time after take-off. Though this presents no difficulty it must be under-taken as a deliberate manoeuvre. The glider should return to the high-tow position before releasing so that the tug pilot can seethe release when it takes place. The low tow has important advantages for cross-country flyingbecause it is less tiring for both pilots and also because the com- bination has less drag in this configuration, giving a better rateof climb and more air miles per gallon. "Low-towing" should, however, only be done after agreement between the two pilots.Otherwise, the tug pilot may assume that the glider has released when it disappears from his field of view into the low position.For the tug pilot, the only effect of the glider going into low tow is a slight nose-up change of trim, associated with a feeling ofgreater steadiness. The Release. Normally, the glider pilot releases his end of thetow-rope leaving the cable attached to the tug. The glider pilot will release: (1) if, at any time, he feels uncomfortable about thetow because of getting out of position, excessive turbulence or any other cause; (2) when he finds lift he wants to use or is overheadat his destination after a cross-country tow; (3) when he has been towed to the maximum height allowed for in the fee paid or thatlaid down for a competition launch; (4) when he is "waved off." When releasing, the glider pilot should ensure that the cable istaut at the moment of operating the release. The tug pilot will then feel a jerk and will know that the tow has been cast off.The increased performance of the tug after the glider's release will normally be obvious to the pilot. This can, however, beoverlooked and the tug pilot must be on his guard to avoid the embarrassment of perhaps a long and pointless continued climbby himself. Immediately after releasing, the glider should turn away sharply so that the separation is obvious to the tug pilot. Thelatter must, in any case, check carefully that the glider is really free before closing his throttle and diving away. The tug pilot's signal to the glider to release, should he want itto do so, is a violent lateral rocking of his wings. As the tug may already be rocking in rough air this rocking must be pro-nounced to avoid any chance of misunderstanding. The glider pilot must obey the signal immediately. In an emergency—particularly engine failure—the tug must be freed of the glider with a minimum of delay. Engine failure while on tow, particu-larly low down, is an emergency which is likely to require the quickest possible action by the pilot. The tug is flying perhapsonly 15 m.p.h. above the stall while the drag of the tug/glider combination is so high that speed will soon be lost; and a stall,and serious accident, will be almost inevitable unless the glider is released at once and the tug's nose pushed well down. The tugpilot should regard his emergency drill, in the event of engine failure, as starting with the immediate jettison of the tow by meansof his quick release. The glider should be "waved-off" in such circumstances only if there is plenty of height available. In the circumstances of a normal release, there is another kindof emergency which must be guarded against. This is the unlikely one of the glider's quick-release failing to function. If thishappens, the tug should itself release the tow above an aero- drome so that the glider can safely make a high approach clearof obstructions in which the cable hanging from its nose might become entangled. The glider pilot will signal to the tug that heis unable to release by pulling out to port and rocking his wings until he is noticed. Cable-drop and Landing. Normally, as soon as the tug pilothas satisfied himself that the glider has released, he will close his throttle, dive away and return as quickly as possible to theaerodrome. The cable-dropping zone is usually sited at the down-wind end of the landing area and the tug pilot should aimto come in high and steeply so as to cross the dropping point at about 150ft. It is very easy to underestimate the extent to whichthe catenary in the tow rope allows its rear end to sag below the tug, particularly during a slow power-on approach. It is thereforea good idea to be deliberately too high and to make the approach as steep as possible. A help in judging the release point correctlyis to put on some engine at the end of the approach glide just before the releasing height is reached. The approach will thenflatten momentarily and the pilot is given more time to operate the cable release before finally closing the throttle and landing,straight ahead, off a further short glide. Aero-towing for the Glider Pilot The glider pilot should not allow the tow-rope to be attachedto his aircraft until he is ready to take off. As soon as the cable has been attached the wings should be levelled and the dive-brakes checked as closed and locked. The wing-tip holder should then check that all is clear and immediately give the "take-upslack" under-arm signal. The "all-out" over-arm signal should be given as soon as thetow-rope is taut so that the tug can start its take-off run without stopping and with the momentum acquired in taxying to pullout the cable. As the glider gathers speed, the pilot will be able to maintain lateral balance with the ailerons and the wing-tipholder must then let go. The glider should be "unstuck" as soon as possible and flown a few feet off the ground behind the tug. Ifthe glider is fitted with jettisonable wheels, it will have to be momentarily taken a few feet higher while these are dropped toavoid risk should they rebound from the ground. As the tug unsticks, the glider should be allowed to climb slowly, maintainingthe same relative position. Once tug and glider are well clear of ground, the glider pilot should hold his position so that the tugappears to be below the horizon by a distance equal to about half its wing span. This is the normal high-tow position. If a low-tow has been agreed, the glider must descend through the tug's slipstream—the turbulence is surprisingly slight—and must thenmaintain a position with the tug an equivalent distance above the horizon. When turning on tow, the glider pilot should deliberately makehis own turns rather flat, using top rudder where necessary and pointing his nose at a tangent to the path of the turn—not directlyat the tug as when flying straight. In this way, the tow-rope will be kept taut and undesirable jerks avoided. The glider pilot'sgolden rule at all times when under tow is to concentrate on main- taining his correct relative position and not concern himself aboutany slack which may develop in the cable. If he holds his position, any slackening and sudden tautening which may occur—and the resultant jerks—will soon damp out. A further reference to the action taken in the unlikely eventof the glider's quick-release failing to operate is perhaps desirable. As already explained, the glider pilot should signal to the tug bypulling out to port and rocking his wings. He will then be released over the centre of the aerodrome and he must follow this with ahigh approach so as to avoid any chance of fouling the cable in an obstruction. As an additional safeguard during the approachand landing, he should continue repeatedly to operate the cable release and should land rather fast so as to compensate for thedrag of the cable on the ground.
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