This article appeared in the December 1997 issue of Hang Gliding magazine. See also the sidebar to this article Pitch Stability and Center of Mass Location
Roger is a conservative and safety conscious pilot. He bought an HGMA certified glider made by a reputable manufacturer. He re-packs his parachute every six months, replaces his bottom side wires and hang loops every season or two, and has a complete tear down and inspection of his glider done every year, just as his owner’s manual advises. Other pilots he knows have experimented with bending their battens to different shapes, or lowering their reflex bridles to get more speed in aerobatic dives. But Roger would never do that; for safety he leaves every adjustment on his glider in the original factory settings. What Roger doesn’t know would shock him. What Roger doesn’t know is that for years his glider has been re-adjusting itself, and is now so far out of adjustment relative to the original design that it wouldn’t come close to passing the certification pitching moment tests! How could this be?
For gliders that are equipped with them, reflex support bridles, sometimes known as “luff lines,” are a critical component of the glider’s stability system. Their function is to support the trailing edge of the wing at a minimum height relative to the rest of the wing, regardless of the air loads on the sail. They are normally slack when the glider is positively loaded in normal flight, and go tight if the glider unloads or if the sail becomes loaded negatively (from above). When attached to the inboard parts of the sail, they induce reflex in the wing, and when attached outboard, they preserve a minimum level of washout in the swept back outer portion of the wing. In either case, they induce a nose up moment in the wing when engaged, helping it to recover from low or negative angles of attack that may have been induced by turbulence or pilot actions.
On most gliders in normal flight, as long as the pilot is loaded at one “G” or more, the bridles are slack and do not affect the flight characteristics. They are a “passive” system; designed to “kick in” only when the glider enters an unusual flight mode. Because of this, you might fly for hundreds of hours in a variety of different types of air, and never experience the effects of the bridles. If the bridles are not functioning as they should, you may never know it just from flying the glider. The pitch pressures in the control bar may feel completely normal, even when flying as fast as the glider will go. However, if the bridles are significantly out of adjustment, the level of pitch stability at some combinations of angle of attack and airspeed that you don’t normally encounter could be much lower than what was designed into the glider originally
How is it that Roger’s glider re-adjusted itself? It probably did so in several ways; minor stretching of the bottom side wires and seating of the hardware may have allowed a little extra dihedral into the wing, slackening the bridles as the wing “folded upwards” around the axis of the keel. The bridle cables themselves may have stretched slightly. These effects are minor, however, and by themselves would probably not be a problem. Far more significant is the tendency of the sail to shrink over time. As far as we know, this tendency is probably common to all forms of polyester (Dacron) sailcloth. As the drawing illustrates, if the sail shrinks in the spanwise direction, the bridle attachment point moves inwards towards the keel. Since the bridle cable does not also shrink, the trailing edge of the sail is supported at a significantly lower height. For a bridle attached far outboard on the wing, the sail can be lowered as much as five times the amount that the sail shrinks. Outboard bridle locations are a more effective way of adding pitch stability on high aspect ratio flex wings than inboard bridles because with a short root chord reflex is not as effective as the support of a large portion of the swept back part of the wing. However, outboard bridles also magnify the problem of the shrinkage effect in two ways. The more shallow cable angle increases the amount of trailing edge lowering for a given amount of sail shrinkage, and a given percentage of shrinkage translates to a greater amount of shrinkage over the longer span to an outboard bridle location.
In September of this year, we at Wills Wing conducted a vehicle pitch test series on a HPAT 158 with about 400 hours on it that belongs to a local pilot. When we obtained the glider, the bridles were still at their original factory setting, and we did not adjust them. In our first few pitch test runs, we found that the glider had a positive pitching moment at the VG loose setting, though the pitching moment curve had a few areas where it failed to meet HGMA minimum requirements. At the VG tight setting, the situation was dramatically worse – the glider actually had a negative pitching moment at angles of attack near zero lift. (See the pitching moment graph labeled “20 mph VGT 1.”)
A little explanation of pitch testing methods and pitch test graphs is in order. During a pitch test, the glider is mounted on the test rig such that it can be pivoted nose up and nose down. Electronic load cells, pressure transducers and a computer continuously record airspeed, angle of attack, lift, drag, and pitching moment. The entire test run is conducted at a constant speed (in the case of the graphs shown, it is 20 mph). The run starts with the glider at an angle of attack above trim, and as the run progresses, the angle of attack is smoothly reduced to negative 25 degrees or more. In other words, at the end of the run the keel is nose down, at least 25 degrees below the horizon, and the wing has at least a 25 degree angle of attack, negative, to the airflow.
On the graph, the horizontal or “x” axis is the geometric root angle of attack, which on the test vehicle is essentially the same as the keel angle to the horizon, since the airflow is always horizontal. The vertical or “y” axis is the “pitching moment coefficient” or Cm, which is a measure of how strongly the nose wants to pitch up or down. Negative Cm’s mean the nose is trying to pitch down, and positive Cm’s mean it is trying to pitch up. The straight lines on the graph forming a triangular region represent the minimum pitching moment required for HGMA certification in the 20 mph test. The peak of the triangle is at the “zero lift” angle of attack, where the glider is neither lifting positively or negatively. You’ll note that this does not always occur at the geometric zero angle of attack. The other line is the actual pitching moment measured. You’ll note that in the first two graphs, the curve enters the “prohibited region” by a substantial amount, over an extended range of angles of attack above and below zero.
One thing to keep in mind when looking at the graphs is that they do not depict anything about what you feel in terms of pitch bar pressure when you fly the glider. On the test vehicle, airspeed is held constant, while the angle of attack is varied. As a result, the load on the glider varies, becoming greater at higher angles of attack, and lower as the angle of attack is reduced. In normal flight, (if acceleration is kept to a minimum) what remains constant is the load on the glider. In flight, as you slowly pull in the bar from the relatively high angle of attack associated with a 20 mph airspeed, the glider immediately picks up speed to replace the lift lost by lowering the angle of attack. To plot a pitching curve that replicates the pitch pressures you would feel in the control bar in flight, the vehicle driver would need to continuously adjust his speed to maintain one “G” of loading on the glider.
But that’s not what a test vehicle is useful for. Investigating the static stability of the glider in normal flight, as represented by the pitch force in the control bar as a function of flying speed, is something that can be done much more easily and more accurately by just flying the glider. What the test vehicle allows us to do is investigate angles of attack outside the range we can reach in flight, as well as combinations of angles of attack and airspeeds (a low angle of attack with a low airspeed, for example) that we can’t easily achieve or sustain in flight.
The 20 mph pitch test minimum requirement was originally developed in 1978 as a direct response to the problem of turbulence induced pitchovers, or “tumbles.” (We’ll use the word tumble here to refer to any turbulence induced pitch down rotation, or combination of pitch down and roll rotation in which the glider rotates to a past vertical attitude and the pilot experiences a profound loss of control as a result). Tumbles have long been thought to be primarily a low speed phenomenon, in which turbulence induces an initial nose down pitching rotation in the glider. Mathematical analysis done by Gary Valle in 1978 indicated that if a glider had a minimum zero lift pitching moment coefficient of at least 0.05, it should have a strong enough nose up tendency as the angle of attack was lowered to resist the tumbling motion and recover in a nose up direction. The current HGMA requirements include this 0.05 Cmo requirement, and extend the minimum required pitching moment in either direction as defined by the triangular region. Note that at all times when the pitching moment is above the horizontal axis, the glider is trying to pitch nose up. One can then think of the pitch test as a look at what the glider’s pitching moment behavior would be during a low-speed, turbulence-induced pitch down rotation. The area under the pitching moment graph can be thought of as the amount of work the glider is doing in trying to resist the pitch down rotation. The higher the curve, the more nose up tendency at every angle of attack, and the greater likelihood there is that the glider will arrest the nose down rotation and recover nose up
Clearly, the first two graphs shown do not have the desired character. Not only do they not have a strong nose up tendency in the region near zero lift, they actually show that the glider would try to pitch nose down once it entered this area.
After the first pitch test run, we raised the bridle ring (where the bridles attach to the compensator cable at the top of the kingpost) by 1 1/8 inches and made a second run. The graph labeled “20 mph VGT 2” shows the results. There is some improvement, but clearly the results are still very far from satisfactory. Later measurements and calculations showed that the shrinkage of the sail had lowered the sail at the outboard bridles by about six inches from where it is supposed to be. There was no way to come close to compensating for this by raising the bridle ring at the kingpost.
We next shortened the bridle cables themselves, until the bridles re-gained the original “just slack in one G flight at minimum sink airspeed” adjustment. This required shortening the outer bridle cables 1 3/8″ on each side! The third run – depicted on the graph labeled “20 mph VGT 3” – shows that the original certifiable pitching moment curve was regained once the proper bridle adjustment was achieved. We verified in a separate run that the bridles were still slack at slightly less than one G of loading at the minimum pilot weight, and would therefore be properly just slack in flight. We conducted a complete pitch test series at this bridle setting, and found that the glider passed all of the original HGMA pitching moment requirements under which it was certified, and in addition, passed the slightly more stringent current pitch test standards.
This is a good news – bad news story. The good news is that by re-adjusting the bridles to the proper settings, a very satisfactory pitching moment could be recovered. The other good news is that this glider had been flown for hundreds of hours in mid-day, mid-summer thermals with these grossly mal-adjusted bridles, and had never had a tumble, or even any incident that indicated any level of questionable stability to the pilot. (It is interesting to speculate what would have happened if this same glider had been involved in a tumble, and had been tested during an official certification review process as a result. One might imagine that the temptation for the review board to conclude that the reduced pitch stability had caused the tumble would have been irresistible.) The bad news is that this glider was owned by a very knowledgeable pilot with direct local access to the factory and still had not been maintained properly. The other bad news is that the normal methods for adjusting the bridles would not have been adequate to do the job properly. The final bad news is that these two observations indicate the high likelihood that there are many other gliders in the field with the same problem.
Since we first began using reflex support bridles on high aspect planform gliders in 1980, Wills Wing has specified the “just slack in flight” criterion as the only correct final means of checking bridle adjustment on our gliders. We have recognized from the beginning that while various measurements of the bridle settings made on the glider may provide a good starting point for adjustment, they cannot guarantee proper adjustment as the glider changes with age. The “just slack” criterion has a justification that is both simple and powerful. Bridles are more effective the higher they support the sail. Bridles which are tight in flight seriously compromise handling and control response. Therefore, bridles should be as tight as they can be, without being tight. Ergo – “just slack.” What is new during the last couple of years is the realization of how dramatically the glider can alter its dimensions over time, and how far out of adjustment the bridles can go as a result. We have included the information about the effect of sail shrinkage on bridle adjustment in each owner’s manual we have published since May of 1995.
Wills Wing is recommending that all pilots of Wills Wing gliders carefully check their bridles against the proper “just slack” criterion. A Technical Bulletin covering bridle inspection procedures which was first published this past July is posted on our web site (www.www.willswing.com) and is available on request by mail or fax for no charge. Pilots should also consult their owner’s manuals for proper bridle sighting and adjustment procedures. What is not covered in the bulletin or in the owner’s manuals is what we believe is the most convenient method for correcting bridles where the adjustment required is beyond the range provided for by the normal method of raising the bridle ring. We have found that the most effective method to adjust bridles that are grossly out of adjustment due to sail shrinkage is to shim the bridles from below the sail. What is needed is short pieces of tubing which are larger in outside diameter than the hole in the bridle grommet, and smaller in inside diameter than the bridle ball. Pieces of 3/8″ or 10mm batten tubing work well, as will any ½” tubing with .065″ or greater wall thickness. Plastic tubing works fine as there is no significant load on the tubing during the inspection and adjustment process. If you cut pieces to ¼”, ½” and 1″ lengths, you can easily make adjustments in ¼” increments using no more than three shims to obtain up to 2.5″ of adjustment. Wills Wing will provide a supply of these shims to anyone on request through participating Wills Wing dealers. By removing the bridle ball, sliding the tubing shim over the cable below the sail, and re-installing the ball, you can shorten the cable in calibrated amounts. Since it is difficult to sight the bridles accurately in flight unless you have a lot of practice at it, the most accurate way to achieve the “just slack” adjustment is to actually go a little too far, and adjust the bridles to the point of being snug (see the Technical Bulletin for descriptions of how to sight the bridles). Then by backing off ¼ to ½ inch from “snug” you will have “just slack.” Use caution when making these adjustments, because as the bridles become tight, the glider’s handling will deteriorate and the pitch trim will change. Keep in mind that on VG equipped gliders, the “just slack criterion applies to the VG tight setting; at looser VG settings the bridles will be more than “just slack.” Once the proper bridle adjustment is achieved using the shims, the pilot can order through his dealer a custom made replacement bridle set fabricated to the proper dimensions, by specifying the total length of shims used to correct each bridle. Pilots who do not feel comfortable making these adjustments themselves should seek the assistance of their Wills Wing dealer.
Although we at Wills Wing have stressed in our owner’s manuals for years the need to actively maintain proper bridle adjustment, it has become clear to us that this is not being adequately addressed on older gliders in the field. We do not know precisely what the relationship is between pitch stability as measured on a test vehicle and a glider’s likely degree of resistance to tumbling. Gliders with certifiable levels of pitch stability are likely still to be subject to tumbling in the “right” piece of air. At the same time, as our tested glider illustrates, a glider with demonstrably inadequate stability levels relative to the certification minimums may fly for years without an incident. Common sense, however, indicates that it is prudent to maintain one’s glider in the most airworthy condition possible. This cannot be achieved without continual active maintenance of the reflex bridle adjustment.
Note: While we feel that the general information supplied in this article is broadly applicable to most hang gliders, we do not intend to offer specific technical advice regarding any gliders other than Wills Wing models. We encourage pilots of other gliders to seek technical advice from the manufacturer of the glider they fly.