7 Driving the tow vehicle
7.1 Positioning the tow vehicle. 7.2 Towing Speeds. 7.3 Coordination between driver and pilot. 7.4 Rope retrieval. 7.5 What can go wrong while dragging rope? The driver of the tow vehicle has the responsibility of creating the safest possible transmission of force through the tow rope to the glider. 7.1 Positioning the tow vehicle The launch first of all involves hitching up the rope and waiting, at the side of the runway outside the lights (if any) so as not to obstruct traffic while the glider crew gets ready. If the vehicle is pointed toward the runway, the driver can check for traffic easily and can see the glider and crew out the side window, giving much better visibility than through a mirror. Prudence requires that the driver have definite, unequivocal assurance of the glider's readiness to go, through signals that are easily perceived by the far-away driver, that have unambiguous meaning, and that have been reviewed among the crew, not only at the beginning of the day, but each time the crew change roles. When it is clear to the driver that the glider is ready, then the driver independently confirms that there is no conflicting traffic, pulls out to the runway centerline and begins the tow. I strongly recommend that the tow vehicle not stop on the runway at this point. It unnecessarily delays the launch of a glider that is already ready, it risks conflict with approaching traffic that twenty seconds before was not a factor, and which may be expecting the announced launch to proceed expeditiously. The driver has excellent vision of the launch point and the runway environment when parked at the side, pointing toward the runway. The driver has poor vision when parked in the center of the runway pointed away from the glider. Thus the launch should begin with the driver at the side of the runway and the driver should not stop at the center of the runway to again make sure the glider is really, really ready and the pattern really, really clear. The driver should have placed enough slack in the rope that the glider is not at all affected until after the tow vehicle has completely transitioned from the side to the center of the runway. The driver should simply enter the runway, after confirming it's safe, at 5-10 mph, turn to the center, and begin driving down the centerline at 5-15 mph until the rope pops onto the runway, indicating the slack is out. The driver should immediately begin full-throttle acceleration at this point and not ruduce it until the target speed is reached. By the time the target tow speed is reached, the glider should have completed the transition to a steep climb, and if this appears to be the case, the driver immediately begins to let the speed drift down to the target minimum tow speed. In still air, the best initial speed for the tow vehicle is 10 to 15 miles per hour until the slack is out of the rope, then full-throttle acceleration to the planned maximum speed for the launch. Any head wind component can be subtracted from this recommendation, except that the minimum speed to allow good engine responsiveness by getting it up on the torque curve is about 3 to 5 miles an hour -- walking speed. There are several reason for recommending a rolling start: - It is safest to provide control authority to the pilot as soon as possible, so that the glider is not just a helpless, awkward container being dragged along on the end of a long string. Speed provides control authority. - The long rope has considerable give and a degree of elasticity, so that the glider is not jerked by a rolling start. The rolling start produces powerful, smooth acceleration, and the Blanik gains aileron authority in about ten feet of ground roll. The wing runner generally can take only about three fast steps before falling behind. A standing start more than doubles the ground roll, in my experience, from about 100 feet to about 250 feet. - Auto engines develop little torque at low rpm. A rolling start allows much better engine acceleration and delivers power more quickly to the glider. Any hesitation or pause in the initial acceleration is dangerous to the glider, as when it lifts of the ground it may be very close to stall, and unexpected slack in the rope could precipitate a low- altitude stall. Thus the initial acceleration should simply be foot- to-the-floor unless there is a clear indication to abandon the tow, and then it should be decisively abandoned. It is a great mistake for the driver to let off the gas because of any sense of uncertainty about what may be happening at the other end of the rope, and a greater mistake to resume full-throttle acceleration after doing so. Hesitation at the beginning of the tow always creates difficulty for the glider close to stall and close to the ground; and if acceleration is resumed while the pilot is just beginning to react to the slowing, this may put the glider in a bad position. It is very helpful, in developing student pilot skills, if the tow is driven consistently. This is more challenging that it seems, as the tow driver must adapt to changing wind conditions through the day, the tow is affected by the way the pilot climbs, and the progress of the glider through the stages of climb is difficult to discern. It's important for the pilot and driver to quickly debrief after each tow. "How did it go?" is usually enough. 7.2 Towing Speeds It takes a higher tow speed to get the glider off the ground and to initiate climb; after climb is established, centripetal force begins to develop that accelerates the glider. The development of this force requires power that is felt by the driver as a need for more gas even as the vehicle slows. The end result is a gradual slowing of the tow vehicle. In a no-wind situation with the L-13, it works well for the truck to first accelerate to 55 mph until the glider begins to climb well, then gradually slow to about 40 mph. The wind speed should be subtracted from this; often there is significant shear between 300 and 700 ft agl that requires additional adjustment (slowing). High density altitude will alter true air speed, but doesn't affect the tow vehicle's speedometer. The actual speeds needed also depend on the pilot's style of climb. The earlier the pilot establishes a steep climb, the slower the tow vehicle must go during climb. In our experience, the conventional signals or wagging the tail (rudder) to request a slower tow speed and rocking the wings to request a faster speed are seldom perceived by the driver. If the tow is too slow, the pilot must focus on staying above stall speed until enough altitude is gained for a safe release and landing. The glider needs the fast tow speed while getting off the ground; it needs a slower tow speed after it has transitioned to a steep climb: If the tow speed is slow initially, the glider will be unable to transition safely to a steep climb, or will be too close to stal during the climb; the climb will be inefficient, but more important is the risk of stalling the glider at low altitude and steep bank. It is difficult for the driver to see when the glider has transitioned to the steep part of the climb because the glider is small, distant, and in the rear view mirror. An observer in the tow vehicle who can focus attention only on the glider and identify the succesful transition to steep cimb is helpful. The glider pilot is partly responsible for achieving the best speed: a good, brisk transition into the steep part of the climb as soon as it's safe results in ideal airspeed while the driver slows; a delay in transitioning to the steep part of the climb will result in a slow, shallow climb. If the driver does not slow sufficiently when the glider transitions to the steep part of the climb, the glider will quickly exceed the maximum ground launch airspeed, which risks excessive force on the wings and makes a rope break likely, so the pilot will have to level off instead of climbing. In our experience, this is most likely to happen when there's unexpected wind shear. If there is suddenly 10 kt more headwind, we have found that the pilot may have to fly in a nearly level attitude to avoid exceeding max auto air speed. The climb rate in this situation is surprisingly high, and the altitude is high enough to easily maneuver to a safe landing, but max altitude won't be reached. We have found that sometimes the driver must slow to 30 mph or less when shear is encountered, and it's not possible to know just when to do this unless the pilot and driver discuss the tow with each other after each flight. 7.3 Coordination between driver and pilot So it's important for the driver to review with the pilot the speeds used for the tow, and it's important for the pilot to understand from the driver what is being asked of the tow vehicle. They should debrief each other after each flight. The driver wants to know if the speeds were good: the initial speed and acceleration, the time chosen to slow the tow vehicle and the rate of slowing down. The pilot wants to know where on (or beyond) the runway the tow vehicle ended up, so as to understand whether the drive was hard on the truck; he also wants to know where the rope dropped (whether his crosswind correction was adequate). If the tow vehicle arrives back at the launch point more than 15 minutes after the glider lands, it's pretty safe to assume that the rope fell in an inconvenient location. 7.4 Rope retrieval One of the most important skills for the driver is the technique of retrieving the rope. A great deal of damage can be done by a 600 yard rope being towed by an impatient driver at 30 or 40 miles a hour: it's like a saw blade, and at its end a small round ring of forged steel is bouncing around, hitting random objects. Ten or fifteen miles an hour is much easier on all the equipment. With thought and care, even driving prudently, the driver can retrieve the rope and re-position it very quickly and efficiently. The moment the driver detects that the rope has been released or has broken, the rope should be released from the hitch. This reduces wear on the rope; if the glider has released the rope while still on the ground, the bridle will have been dragging at high speed along the runway until the driver figures out that this has happened; this frays the bridle. The driver, after pulling the release to free the rope from the hitch, immediately turns to see where the rope is falling. To be able to see the flags on the far end of the rope fall, greatly abbreviates the search for the bridle end of the rope. This can be a time-consuming and annoying chore if there has been a rope break, and a crosswind carries the bridle and whatever rope is attached to it off into a field. I can't imaging looking through desert sagebrush for a vagrant bridle. Next, after looking to make sure that it is possible to drive directly to the other end of the rope, the driver should do so. The ideal procedure is to drive beside the rope, or as near as practicable to it, along its length to study what objects, fixtures, and valuables it may have fallen across. With a good drop near the runway, there will be nothing endangered by it. It would sense to check for loops in the rope as one drives next to it, but there never are any. Loops that turn into knots are only produced by people manipulating the rope. It falls in serpentine zigzags that uncoil nicely if the rope is simply towed back from its other (bridle) end without turning any sharp corners. Assuming there are no objects about which one needs to be concerned, the basic task is to drive to the launch bridle. Once there, the tow vehicle is stopped, the bridle is retrieved, coiled up, and put in the back of the vehicle with a few feet of rope. A couple of fist-sized loops are made in the rope, and these loops are put around the tow hitch and the hitch is closed. Then the driver begins to tow the rope carefully back to the launch site. Usually the rope lies off to the side of the runway in the grass, and the ideal situation is to have the rope glide through grass as it's towed back to the launch position. Grass guides the rope, grass is slippery and non-abrasive. As you drive, you will notice that the rope is suspended above the ground for 100 to 200 feet behind the truck, and beyond that point it's down in the grass. If you make gentle curves with the truck, the rope tracks along the path where it lies, without being dragged sideways. This tendency of the rope to follow its own track in grass is your most important tool in keeping the sand-impregnated rope from sawing off valuable objects in the airport environment. Assuming that the rope fell outside the runway lights, the best procedure is to drive approximately in the direction that the rope lies for a hundred yards or so, until the rope behind you is securely tracking in the grass, then gradually turn toward the runway and -- after checking for traffic -- drive inside the runway lights, crossing them just beyond one to avoid snagging any. Drive and halfway onto the paved runway, and then back to the edge after the part of the rope touching the grass has reached the row of lights. Entering the runway environment just past a post will keep the rope from chafing the light post, and driving along the edge will keep the rope in the grass, reducing abrasion on the fibers. Only drag the rope across the pavement when there's no way to make a proper placement without doing so. You want to cross into the runway environment at least by the point at which the tow vehicle begins its run, so that the rope and ring will be inside the lights. It's inconvenient to lay the bridle at the launch point, then drive back to the other end of the rope to hitch up for the next tow, to discover that 400 feet of rope is on the wrong side of the runway lights. It's bad form to have the rope on the wrong side of a runway light. If you do, when you begin the tow, the rope will draw taught like a giant bow string, under the light, and when the frangible nut at the base fractures, the light and post will be launched hundreds of feet into the air, like a large glass-tipped arrow, to land you know not where. I speak from experience. When you reach the launch point, drive across it and release the hitch. The loops of rope you made should slip out easily, the coiled bridle will unwind and drop onto the ground, and the rope is now laid. 7.5 What can go wrong while dragging rope? Glad you asked. Actually, quite a bit. If the rope happens to be draped across large expensive objects after it falls, or objects than can damage the rope, such as airplanes, hangars, chain link fences, culverts, razor wire, or transformers, it will be necessary to put the rope by hand on the good side of such object. It can be quite time consuming do this with 500 or 600 yards of rope, let me tell you. Some of our rules about how to fly in a crosswind seemed more important after a couple of one-hour rope retrievals. And I might mention that it is bad form to simply drag the rope and ground launch bridle off the roof of the factory on which it landed, as pieces of aluminum that may have had some function tend to come along with it. At our airport, the most common inconvenience is for the rope to land on a taxiway. These are marked, at our facility, with blue plastic pipe spaced at about 100-foot intervals rather than lights on metal posts. A mere touch of the rope under tow can saw these off in a second. This frustrates the airport manager and creates small but annoying bills for the rope-towing genius who didn't notice where the rope was going to be, and who didn't expect to be incurring financial liability by helping out his friend the glider pilot. Runway lights are really not a significant problem. Because they are metal, they withstand a little abrasion from the rope very nicely. Because they are attached to their bases with frangible fasteners, it is not good to let the rope make a sharp angle around the post while towing, but a slight bend and a touch is not destructive, and they can be used to protect the plastic posts along the taxiway and other tender objects such as spectators, whose ankles might not like to have rope burns. When the rope falls across difficult terrain or across "interesting" objects, you simply have to use your knowledge of physics, your keen desire not to incur financial liability, and your abundant common sense to plan a rope-retrieval strategy. My experience is that a really bad rope drop can create a lot of genuine fun as long as everyone is patient and we all make the retrieve part of the game. So use good judgment, make the rope your friend, and pleasantly abuse the pilot who fails to bank into the crosswind and thereby makes the retrieve troublesome. He gets to vault the 10-foot chain link fence, talk to the factory owner about an extension ladder or roof access, and carry long lengths of rope across the hedge. ...Unless he has gotten away, and is turning happily in a tall thermal, oblivious to the sufferings of the ground crew. In which case treats are in order at a local establishment after the day's end. Copyright © 2000, 2002 Daniel L. Johnson; all rights reserved.