Understanding the Winch Launch

by John Gibson from Sailplane and Gliding

Feb-Mar 1987 - p 28
  1. Getting Into the Climb
  2. Safety Considerations
  3. Speed Control
  4. Structural Loads
  5. Placard Problems

The increasing adoption of more powerful winches, and renewed interest in winching generally has revived many old arguments about launch tactics, which range from an initial gentle climb to 200 ft, a relic of ancient times, to the highly dangerous tail scraping heave into a steep climb straight off the ground, an affront to good airmanship. Winch drivers do not always properly understand how to keep the launch under control, either. Pilots have always tended to switch to panic mode when the speed goes over the placard limit. This article explores the rotation into the climb, the safety aspects of its profile. and the effects of exceeding the placard speed with the aid of a few simple examples. It expands on parts af the wider survey given in my article in the August 1985 (Sailplane and Gliding)issue, p170, and should be read in conjunction with it.

Getting into the climb

The basic technique for safe entry to the climb is simply stated. After lifting off with sufficient nose down control for level night if possible, the nose should be allowed to rise over a period of a few seconds provided the airspeed is increasing or has reached the climb speed, never too steep for a safe recovery at any instant and reaching the climb attitude at a safe height with minimum wastage of field length. It cannot be emphasised too strongly that in a normal launch the nose wants to rise and only needs to be allowed to do so. This is so even when the initial nose up moment from the cable pull has been surpress ed. Many glider pilots probably assume that aircraft in a steep climb have to be kept there with the stick back, but that is quite wrong. For other aircraft types, the stick is pushed forward in the climb.

Any aircraft trimmed for level 1g flight and held at constant angle of attack will start a loop if the speed is increased. If the increase is 41.4% the g value will increase to 2 in the initial level flight. The extra lift produces a pitch rate to which is added and extra g's worth in the vertical where gravity no longer opposes the lift and a further g's worth in inverted flight where gravity assists. If a steady climb angle is required the lift must be reduced to half the weight at 60o or zero at 90o, the thrust party or wholly replacing it in supporting the weight.

Similarly a glider held in level flight at the lift off speed will go into a climb without a further aft stick movement simply because the speed is increased by the winch. It will settle into a steady climb when the resultant of the lift and drag forces equals the resultant of the gravity and cable pull forces, amounting to the equivalent of some 2g's or so (Figs 1 and 2). Pulling against the cable load is the primary reason why a glider in a winch launch climb needs the stick to be basically somewhat aft rather than forward. It is flying normally and would stall in the normal way if the stick is held fully back.

Additional stick inputs are required resulting from the cable moments. The initial nose up moment should be opposed by a nose down stick input. As the nose rises this moment reduces, usually becoming nose down in a steep climb as shown in Fig 3, so the stick has to be backed off during the rotation and may need to become nose up or even on the stop. The pitch rate in the rotation, 15o/sec or so, creates an increase in the tailplane angle of attack only counteracted by considerable up elevator. The pilot's job is to regulate the natural rotation over the required timespan into the full dimb angle by balancing all the forces with the stick. As the height increases further, the increasing cable angle automatically brings the nose round again towards level flight at the top.

Some gliders are known to require the stick to be held fully forward at the start of the rotation. If the manual says so, believe it! However, the glider cannot leave the ground until it has flying speed, and it will acquire additional speed at the rate of 19 kt/sec for a pull equal to its weight. Such a pull will overpower the elevator on many glid- ers, but then the reducing nose down cable moment and higher tail power with speed permits the nose to be held to a moderate attitude if desked. Proper winch control can provide an optimun acceleration. If the pull is weak the speed will increase slowly but the nose can be held down. Gliders will not normally climb excessively steeply if the pilot does not wish it. Once in the climb the stick is unlikely to be much forward of neutral, but excessively aft CG may cause this and pilots who feel uncomfortable with the behaviour are advised to carry cockpit ballast as well as to check the weighing.

If the nose has been held down while the speed icreases rapidly, all the needed climb lift will be provided by moving the stick back, but initially it will have been more nose dawn and ends up in the same position, of course. In prac- tice the lift will usually come from a mixture of speed increase and some stick pull. If the nose will not rise at all without being pulled up there is not enough speed for a normal climb. Obviously the rotation is governed by a quite complex summation of effects, but the task boils down simply to stopping the nose from coming up too quickly.

Safety Considerations

Stalling in the rotation is sometimes feared, probably because pilots cannot distinguish the angle of attack from the attitude, but they cannot in normal flight either and rely on flying at the correct speed. The launch is no different as can be seen from Fig 2. The other vital safety requirement is the ability to recover from a cable break at a low height. Decelerations of 14 kt/sec in a 45o climb and lO kts/sec at 30o make it obvious that the nose must be lowered very quickly indeed to get rid of the large "gravaational drag". Aerodynamic drag is worth only 1/2 to 1 kt/sec. Very high pitch rates can be generated at zero or negative g at 40 kts airspeed and attitudes bet- ween 45o and level flight, eg some 40o to 50o/sec. Inverse with speed, they are only two thirds as large at 60 kts. This is no time for half measures and the stick should be on the forward stop. It is profoundly important that the pilot should not be confused by the ensuing physical sensation. because the glider cannot be in a stall with the stick fully forward and the nose rotating downwards.

When the cable breaks, the surplus lift starts to pitch the glider up, and the pilot takes time to react to get the nose down. Assuming a sim- plified manoeuvre in which the climb is continued at the initial angle for 1 sec followed by an immediate low g pitchover, the approximate height gained and the speed over the top for a range af starting speeds, climb angles and pitch over g are given in Fig 4. These take only 2 or 3 sec altogether and anything more leisurely reduces the speed even more. So long as zero g or less is maintained the glider is not yet stalled, but it is at this point ttrat things can go seriously wrong. Failure to lower the nose to well below the normal flying attitude to pick up speed is some- times terminally compounded by the decision to attempt a circuit. The correct action is to get the nose well down, keep the left hand off the not- the-going-in-to-land lever until there is enough speed and then only if it is needed, and land ahead if there is room, but it does not assure safety if the height at the cable break is too low because the lost speed cannot all be regained.

At best, a descent to the cable break height in a mirror image of the ballistic dimb will have lost some 5 kt due to aerodynamic drag. Even a pilot prepared to put himself in a steep climb at 15 ft will probably recognise that a 45o dive at this height is not a brilliant idea, and the necessity to round out reduces the speed recovery still further. The deceleration due to a wind gradient may equal the gravity induced acceleration and cut off the recovery completely. Although a wind gradient increases the speed`in a slow launch while the glider is still attached to the cable in a shallow climb, it may increase the ballistic speed loss at the top of the recovery pitchover after a cable break near the ground, by steepening the fiight path because n partially blows across it instead of into it. As the glider pitches over into the wind the gradient is less and imparts less speed increase.

All this can be avoided by controlling the rota- tion so that the full climb is reached with a suffi- cient safety height cushion. This does not need a gentle, climb to this height. Just a continuous steepening over a few seconds rather than all at once. After ensuring a safe speed to initiate rota- tion the pilot only needs to look for the increase in indicated airspeed rather than the exact knots. If the speed stops increasing or drops at any point the rotation is stopped or reversed accordingly. A combination of speed and climb attitude unsafe for cable break recovery cannot occur and there is no risk of stalling. The stall speed of the glider determines the safe limits, obviously, a 25kt stall and appropriate launch speed greatly scaling down the "Oops!" factor. The safety height for a steep climb cannot be judged with any great accuracy but 150 ft is ample while 1OO ft (two wing spans) is sufficient given a good speed. If a break does occur the speed cannot be judged from the attitude and it is essential to keep an eye on the trend of the ASI.

Back to the Top

Speed Control

The pilot has no control over the power the winch driver gives him, of course. The driver has to understand the launch and how the winch should be controlled accordingly. No winch should be operated without the required informa- tion about glider limits being known by the driver. They are easy to determine. With an rpm governed diesel, selection of an initial rpm appropriate to the wind conditions will ensure that the placard limit will not be exceeded even if the pilot does not climb, and this rpm selection may by good design also be suitable for the climb. The torque depends on the error between actual and demanded rpm and the maximum can be generated by much less than 'full throttle", so a should be advanced at a rate which matches the required take-off acceleration and climb rotation pattern. If the launch is too fast the driver should react to a tail-wagging glider by a reasonably gradual throttle reduction, because a sharp one with instant cut off the fuel and drop the cable off the glider. Too low an rpm selection will also cause the cable to drop off, regardless of the power available.

The speed of a torque governed winch is controlled by the pilot. It will accelerate to maximum rpm if the pilot fails to climb. Because the cable load will not match the selected torque. The driver should then reduce power to limit the rpm, or set the rpm limiter if one is fitted. A high torque giving fast acceleration may result in an over- speed during the rotation into the climb. Experi- ence has shown that excessive acceleration can be avoided by first selecting a lower initial torque before increasing it to the climb value as the glider nose rises. At the other extreme, a reduc- tion in torque setting or too low a selection will not cause the cable to fall off as it always tries to pull at the seiected value and a nonnal speed can be sustained although the climb angle will be low. A low speed can only occur if too steep a climb is persisted with but the cable will not fall off.

Structure Loads

Despite talk of 70o climbs, in general not much more than 45 to 55o is possible in the early part of the launch. The wing lift and bending loads are determined almost exclusively by the climb angle, controlled by the pilot but limited by the weak link strength and winch torque. The lowest wing lift to break a given weak link occurs at the start of the launch. The maximum possible wing lift occurs at the top of the launch, but here the pilot cannot control the load with any accuracy because small changes in attitude produce large changes in lift.

The wing stress at a given lift is higher than in free flight because the bending relief afforded by the wing mass is only the 1 g due to gravity with- out the additional inertial force from the g man- oeuvre. With a typical weak link strength of less than twice the glider weight the lift cannot exceed three times the weight, and the wing stress is then never more than the 4g free flight case. Such a lift requires a launch speed of more than 1.75 times the stall speed and will give a grossly steep initial climb or needs pole bending with high winch power at the top, though many winches could not pull this load. Pulling the speed down (if this can be done) reduces the maximum possible loads, 1.5 times the stall being enough for any sensible launch. Use of a weak link and launch speeds no higher than required give large safety margins.

There is little uniformity in placard speed limits. It seems that designers simply select a value adequate for the launch and for which formal certification calculations are presented. Speed variations after the drag, but having only a slight effect on the lift and cable load for a given climb profile and insignificant effect on the wing bend- ing stresses. The major effect of speed on gliders is associated with large tail loads and wing tor- sion, and it is nd surprising that pilots feel that the launch placard speed is a danger limit. The system of ioads is shown in Fig 5 for a glider in the level flight launch stressing attitude.

Lift acts through the fixed aerodynamic centre near the 25% wing chord position. There is also a pitching moment at the aerodynamic centre which is independent of lift, is almost invariably nose down and increases with the square of the speed. (The lift and moment can be combined into a lift force at the wandering centre of pres- sure, which may be more familiar to many but does not help in this discussion.) Zero or upward tail lift is required to balance the lift of the glider because the CG is at or aft of the aerodynamic centre in all tailed gliders. A downward tail lift is needed to counteract the basic pitching moment. In normal flight the tota1 tail lift is the sum of these two and whether it is up or down depends on the strength of the basic pitching moment, the CG position and the wing lift. In the launch, another downward tail lift is needed to balance the nose down pull of the cable when this passes ahead of the CG.

Of the seven significant forces and moments which apply loads to the glider structure in the winch launch, five depend on the weight and climbing attitude. These are the gavity force, the cable pul, the wing lift. and the two, tail lift com- ponents which balance the oftset of the wing lift and the cabie pull from the CG. The remaining two depend on the speed. These are the basic pitching moment and the tail lift to balance it, and only these are larger than normal when the placard is exeeeded. Values typical of a single- seater glider are illustrated in Figs 6 and 7. Obvi- wsly on otner gliders they will vary widely with size and speed but the relative proportions will be similar. The wing lift is represented by the g value which would result in free flight, though in the launch this is replaced by the resultant of gravity and cable pull.

Fig 6 shows the torsion at the root of each wing. The basic nose down moment increases to nearty 1OO lbs ft at Vd. Added to this is a moment equal to the iift times the distance from the aerodynamic centre to the shear centre. near enough at the spar, giving the maximum torsion of about 11001bs ft at -1.5g. In the winch launch the net moment at typical lift values lies in the shaded area in the speed range from the assumed placard limit Vw to 1Okt above it. Even when weil above ths placard speed the load is small compared with the maximum torsion moments to which the wing is designed. What- ever the wing torison is at any speed, it is reduced by pulling up into the launch climb.

Fig 7 shows the tail loads. The download to balance the basic pitching moment reaches 200 lbs at Vd. The upload to balance wing lift is zero at the 25% chord forward CG and is added for various g at aft CG. The design loads are provided by the addition of sudden control inputs at Vm and Vd. The tail trim loads lie in the shaded area for the same launch conditions as above. This represents the winch launch case if the cable pull acts through the CG. The load changes by 65 lbs due to CG variation but only 18 lbs as a result of the overspeed. At the aft CG the overspeed can actually reduce the tail load to zero. Generally these loads are trivial compared with the total design loads. If a normal climb attitude is maintained, the cable pull and the added tail load to balance it will also be normal, in which case the effect of the overspeed is still only the extra 181bs. It is possible to generate a higher tail load by pulling back harder and making the climb steeper, but even at Vm it cannot exceed the design manoeuvre load. With typical glider dimensions this would break a weak link of well over twice the glider weight in any case.


Placard Problems

There is nothing to be gained from excessive speed. It reduces efficiency and launch perfor- mance by increasiog the cable drag and reduc- ing the effect of the winch "driving away from the glider" in a wind (see my first article). lnitial over- speed will occur from time to time, but living in terrar af it is counterproductive and liable to lead to accidents due to precipitate action. Pilots who cannot bring themselves to allow the speed to go 1 kt over the placard need not do so but should at least abandon the launch without panic. Even so, avoiding the flying cable chute and sorting out the unexpected landing is physically a higher risk than continuing. If overspeed is experienced, the normal launch profile can be begun without any serlous effect on the loads. It is obviously prudent to be gentle with the stick and to maintain a less steep climb if it is gusty, as cable breaks are then more likely because gusts can temporarily increase the lift above normal limits. Although full rudder is designed for at Vm there is no point in applying larger loads than are necessary. and fishtailing with half rudder amply signals the winch driver to slow down. If the speed remains much too high then reiease can take place at leisure well away from the ground.

The winch placard limit as usually interpreted is an anomaly, for in no other flight phase is the glider expected to operate close to the limit throughout. For all practical purposes it is lift loads which will break the glider, not speed loads, yet the OSTlV and earlier BCAR/E requlrements set no upper limit on cable strength and do not require a weak link. Only a minimum strength of 500kg is recommended for "the cable or the weak link, where employed", and the manufacturer is left to define its maximum strength and the limit speed. The speed is the least cnntrollable element in the launch. Apart from the vagaries of the operation, gusts can take the speed well over a law Vw before the cable can be released, which is rather pointless anyway if the gust increase has already vanished again. In fect, OSTlV has no launch gust requirement at all.

Many glidenrs have a limit around 1.75 times the stall speed, at which they cannot be dam- aged even with urbreakable cabie, as well as a specified weak link strength of less twice the weight. This is like manufacturers having their cake and eating it. Pilots get egg on their face. To be oonsistent with the needs of normal winch operations, it would be more logical to require:

  • the glider to be certified for a specified weak link, for example with a strength of twice the unballasted weight.
  • the launch placard to be set at the speed where limit loads are reached in a extreme gust, or Vm if less,
  • a normal launching speed range to be recommended.

Then we could get on with, the routine of winch launching, unworried by needlessly tight restric- tions. Even without this. no pilot should feel there is any mystery about the launch. Though great alertness is necessary, it is a simple operation in practice. It just needs to be understood.

Prepared for WWW by L.Bogan
Nov 1998 - Bluenose Soaring Club