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P2 Course - Module 13d - Tow Systems, Preflight, Launching, Towing Forces

Tow Systems

There are 3 different types of tow systems commonly in use for paragliders. The static line, stationary winch, and pay-out winch. All of them must be able to limit or reduce the tow force, zero the force (hook knife), and be able to gradually increase and decrease the tow force. Under no circumstances should the line be tied to the spool in such a way that if the line were pulled to the end, it wouldn’t automatically separate from the spool. A reversing pulley may be used in some circumstances and should be able to swivel without binding the tow line between the wheel and block. Typically the tow-lines are Spectra/Dyneema which is light and does not stretch. However, for a smoother tow other lines may be used which, while heavier, will stretch and provide a cushion against bumps in the air or tow surface.

Static line systems use a fixed length of line attached to a moving vehicle and requires an extremely skilled Tow-Op. The Tow-Op monitors the tension with a pressure gauge and adjusts the vehicle speed accordingly. It requires a smooth road or body of water. Static line towing is NOT recommended for paragliders.

Stationary winches (Pay-in or Scooter tow) remain at one end of the field or road and spool the line onto a drum to generate tension. The Tow-Op is able to adjust the tension with a hydraulic pump or with the engine throttle during the tow and can pay-out line if necessary. This is a popular method for rough fields and when using pulleys. The altitude attainable is less than a pay-out winch on a moving vehicle but requires less operating distance.

Pay-out winches are mounted on moving vehicles and release line from the drum as the tow progresses. The pressure is moderated by the speed of the vehicle and the hydraulics of the winch itself. It’s the most popular method for maneuvers courses where the winch is mounted on a boat that can travel in large circles around the lake, towing the pilot to altitudes sufficient for maneuvers. Payout winches typically have a “rewind” system for reeling the line back in after a tow.

Preflight

As with any launch you should always have a pre-flight checklist. Remember that the pre-flight is always the pilots responsibility! The standard checklist can be modified for flights originating with a tow.

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Reserve Parachute – Check the handle and pins. Make sure everything is secure.

1 Helmet Strap – Check to make sure it is securely fastened. You should never attach yourself to a glider without your helmet on.

2 Carabiners – Check to make sure that the gates are closed and locked.
3 Harness Buckles – Give them a tug to make sure they are securely buckled.

4 Corners – From the two A risers and two D risers follow the lines up to the canopy to make sure they are clear and there no snags, knots, etc.

Bridles – Check that they are connected to your carabiners properly, attached to the weak-link correctly and will release cleanly.

Hook Knife – Make sure you have a hook knife within reach and easily ex- tractable from its case. You should be able to reach the weak-link with it! They are mandatory on ALL tow flights.

Stirrup & Speed System – Check to see that your speed system is properly attached, routed, and clear of your reserve parachute handle and tow bridles. If you have a foot stirrup check to see that it is clear of your speed system and will be easily accessible after launch. This usually means putting one leg through/behind the stirrup bar.

Top & Turn – Make sure the correct riser is on top for the direction you will be turning after a reverse launch.

Airspace – Check the surrounding airspace to avoid collisions during launch.

Radio – Check for the correct frequency and that you can transmit and receive.

V-shape – Check for V’s between brake lines and rear risers to avoid brake line twists.

Even pressure – Ensure that tension on A-lines are even when doing a forward inflation.

The Launch Director can assist you when preparing for launch, especially when it comes to testing the release mechanism.

Launching

Your first tows will probably be forward launches as it is the easier to see the tow line and there are fewer chances for stepping on it. The same rules apply to a forward on a tow line as a forward off a hill. The glider should be thoroughly checked for line tangles and laid out in a clean, curved shape so that the center cells inflate first. As the Tow-Op increases the tow force you should be prepared to run and follow the tow line. Remember that you will still have to center yourself under the glider while being pulled by the tow line. This may require stepping to the side and steering the glider with the brakes.

In stronger winds, you may choose to do a reverse inflation. The difference between a reverse inflation and a forward on a tow line is that during a reverse you must ALWAYS turn away from your reserve handle, and the tow line must always be routed to the opposite side as your reserve to avoid tangles or accidental deployments. In addition you should never build tension on the line while performing a reverse launch. In conditions appropriate for a reverse the pilot is often required to step toward the glider and away from the tow rig. If the line is already tight, stepping toward the glider will add additional tension to the line and may be sufficient to lift the pilot off the ground before they are ready. An additional risk during a reverse launch is stepping on or over the line before tension has been applied. When the Tow-Op does apply tension the line may catch the pilot behind one or both feet making it difficult to run and pulling the pilot aloft unevenly. When doing a reverse launch make sure the line is clear of your launch path.

Towing Forces

In order to safely perform a tow launch you should understand some risks, and basic forces that will be exerted on you during the tow. The tow system is designed to apply a relatively narrow pressure range to the glider. Too little pressure and the glider descends. Too much and over-tow or lockout can occur.

When the system is working properly the tow operator is able to adjust the tow forces such that the glider gains altitude, without over-towing. Typical pressure for a pilot new to towing is 60-75% of their flying weight. However, there are some situations, depending on the type of tow system used, that can change the tow forces unexpectedly. Talking with the tow operator about your preferences before performing a towed launch can help avoid misunderstandings about tow pressure.

Line dig is the most common and occurs when the line on a pay-out winch has become stuck either between other lines or on part of the tow rig and suddenly stops paying out. The tow system instantly becomes a static tow and tow forces can escalate quickly depending on the speed of the tow vehicle. Use of a properly sized weak link will prevent anything more dangerous than a weak link break from occurring.

As line is spooled onto the drum during a pay-in tow the spool diameter increases and, at a constant RPM, the tow force exerted on the glider will decrease. During a pay-out tow, where the spool diameter decreases, the line force will increase. The tow operator should be aware of the trend and can adjust for either scenario.

When towing in a cross wind without the tow rig properly aligned with the wind the forces exerted on a glider can be higher than if towing directly into the wind. This is because the towing force is directed across the axle rather than at 90o. Part of the resistance from the glider is now fighting against the sideways pull of the tow line. Using a fairlead at the tow rig will allow the line to be presented at a 90o angle to the axle/spool at all times.

During any tow the force exerted by the tow line will pull the pilot ahead of the glider. This results in an increased angle of attack, and an increased wing loading especially at higher line angles. Most paragliders do not turn well at high angles of attack which may prompt the pilot to use more brake input to perform a turn. Because the glider is already at a high angle of attack, more brake may stall the glider. The increased wing loading has also increased your stall speed. Increased stall speed + increased angle of attack + brake is a dangerous combination. For this reason speed assists and light brake inputs are encouraged. 

 
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