For several years, the CIVL has been committed to measuring hang glider sprog settings at major competitions. The reason for this policy is to inform pilots of their settings, to ensure a minimum of safety and to gather information useful to the entire flying community.
Sprogs, also known as washout struts, are rods connected to the leading edge of a glider by a swivel and a cable so that they can be lifted upward at their trailing end, but have a limit on how far down they can be moved. They rest underneath the glider sail’s top surface and hold a minimum amount of twist or washout in a wing. They operate in the following manner: If the glider’s angle of attack becomes too low, air moving over the top surface pushes the sail down until it in turn pushes down on the sprogs. The sprogs then put a torque force into the glider’s leading edge to pull the nose of the glider back up, thereby increasing the angle of attack. By this action, sprogs are a powerful part of a hang glider’s pitch stability system.
Sprogs settings are thus important for safety. Sprogs set too high can reduce a glider's roll control because the sail on the wing outside of the curve can hit the sprog and reduce roll response. Sprogs set too low can reduce a glider’s pitch stability, as indicated above. Since sprogs are readily adjustable by the pilot, it is reasonable and important that knowledge of correct settings be available to all pilots and officials. The following information provides the official CIVL measuring procedures (what we use at competitions). With this information in hand, a pilot can control her/his sprog settings so that they are in compliance.
Ideally, a glider should be measured in a protected area (hangar or a large room) on a flat surface protected, although outside with a reasonably level area and wind less than 8km/h (5mph) is acceptable. Many measurements at competitions take place in the landing field or on takeoff. The equipment needed is a digital inclinometer and a telescopic stick. These instruments are available fairly cheaply. A protractor with a weighted string through the apex is also suitably accurate. The following steps explain the procedure.
Step 1: Set the glider up totally (with all battens attached), open the undersurface sprog zippers, and pull the VG chord on full. Note: open the zippers before pulling on the VG to prevent damaging the zippers. Also, if the glider’s VG system has been modified, pull the VG to the original factory maximum setting.
Step 2: Level the glider’s base tube. Use the inclinometer to get the base to level, side-to-side within one degree. You can use pads or other items to prop up one side, if necessary.
Step 3: Level the keel. Prop the rear of the keel up on a suitable support. We use an adjustable telescopic stick, but a ladder or other prop can be used, as long as you can alter the height to get a level keel. If the keel is within a degree or two, you can make the measurements, but will have to add or subtract the difference from the sprog angle measurement (see step 5). If you are only doing one or two gliders it is worthwhile to get the keel exactly level. For CIVL officials measuring many gliders at meets, using digital instruments able to take a setting and give an angle relative to that setting, we typically get the keel within a degree of level and let the instrument do the difference calculation. (If this method is used, the measurements all have to be taken facing the same direction as the keel was measured, or an error will be introduced to the measurement. For some gliders, it is not possible to take measurements all the same way because of sprog access. In this case, the keel must be made exactly level before measuring the sprogs.) Note: the support should be placed as close to the lower rear cables as possible to avoid bending the keel, resulting in false readings. In addition, the true measurement of the keel angle with respect to level is at the nose, just behind the noseplate, which eliminates the effect of keel bending due to tight lower cables and the support.
Step 4: Place each sprog in its normal flying position, then put a hand on the leading edge from underneath to support it, and pull down a bit on the rear of the sprog to “set” it. Next, measure its angle with respect to level. This measurement should be done under the support cable as much as possible to eliminate a false reading due to sprog bending.
Note: It is important to hold the sprog down to full cable stretch while measuring without pulling it down too much and changing the angle. It is easy to push it up as well when measuring. A little bit of practice teaches the right “feel” for when the sprog is setting at its correct working position. Generally, you can hold the inclinometer against the sprog with one hand, while holding the leading edge up with the other so you can feel the solid engagement of the sprog.
Also, be aware that many sprogs made of carbon fibre are tapered or have a rough surface. With a taper, we measure at the very rear of the sprog and hold the inclinometer against the sprog at that point, regardless if it isn’t touching the sprog further forward. With a rough sprog, we measure right below the support cable with an inclinometer long enough (20 cm or 6 inches) to span over enough of the roughness to provide an average reading. Finally, note that on some gliders the zippers are placed so that it is difficult to access the sprog without pushing hard on the sail and thus changing the sprog setting. The best way to handle these situations is to place the inclinometer between the sprog and sail, and gently pull the sprog down so that the support cable is tight, while holding the leading edge up with the other hand.
Step 5: Re-measure the keel angle with respect to level behind the noseplate to get the relative setting, then subtract or add that to the sprog measurements (add it if the keel is nose up from level, subtract it if the keel is nose down from level). Finally, record the settings of both inside and outside sprogs, left and right. These angles are your sprog settings and can be compared to the data collected over the years at competitions, or your manufacturer’s published values.
For more information, watch the Sprog setting measurement videos.