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Author: Steven Pearson Created: 5/13/2009 3:01 PM
Steven has been a managing partner at Wills Wing in charge of product design and engineering for over 30 years. Steven shares his passion for hang gliding with his wife Lisa and daughter Kelsey.

New sailcloth options and recommended configurations 

There are several distinct attributes associated with every fabric style and I think it’s important to consider them independently in order to make the choice that best suits your needs. 
  1. Appearance and fashion
  2. Structural performance (strength, stiffness and weight)
  3. UV stability and longevity
  4. Flight characteristics
  5. Price
Appearance and fashion often drive the marketplace. Pilots seem to let those factors dominate the conversation simply because it’s easier to choose ‘what looks good” when they don’t have easy access to technical details.
 
As an example of why choosing by ‘what looks good’ is a bad idea, consider that white and black polyester fibers look just like the much stronger Technora and PEN fibers. Similarly, UV-modified film is indistinguishable from clear polyester film but lasts more than twice as long in strong UV environments. So, not only can’t you tell from inspection how well a fabric will perform, but it often takes 100 or more hours of airtime for a superior fabric to show its advantage.
 
We introduced fabrics based on high-strength PEN and Technora fiber over a year ago. High strength and stiffness are essential to control twist and prevent trailing edge flutter especially as gliders age. PEN and Technora fabrics offer significant advantages over the popular ‘PX’ polyester-based styles. Our PEN-based fabrics weigh 180 gms/m^2, match the structural performance of PX15T (270 gms/m^2) and exceed the common PX10T style (230 gm/m^2) typically used on trailing edge panels. ODL Technora-based fabrics are even stronger with a 20% weight savings compared to PEN.
 
The lighter weight of these PEN and Technora based fabrics allow us to make the entire sail body from high strength fabric which is important because some of the highest sail loads are in the interior panels at VG tight. In years past we were constrained to using the stronger but heavier PX10T/PX15T fabrics in relatively narrow trailing edge panels because of the weight and handling penalties.

The loads are high throughout the sail body
 
For most of this year we offered two PEN styles (PE10 and UVMPE10 white) and ODL04 Technora. PE10 has a grey polyester film, and UVMPE10 is structurally identical except for UV film and bright-white pigment replacing the grey film. The performance of these fabrics has been extraordinary and we now unconditionally recommend them over PX10T for every application. The only disadvantage of these fabrics is higher price but that is easily justified by their other attributes.
 
The performance of sailcloth depends on the direction that you apply the load. Most fabrics used on hang gliders are stronger on the warp (long axis) than across the fill (width) or the bias (diagonal). I try to orient the sail panels to take advantage properties of the fabric by aligning the warp with the highest expected sail loads whenever possible. The orientation is not critical in lightly loaded panels or when a fabric type is relatively balanced in strength on the warp, fill and bias. High-performance sails are subject to high loads from several directions and it’s especially important to have high strength in the bias and “off-axis” direction. Off-axis strength is provided by the polyester film and the X-ply yarns if the X-angle is wide enough.
 
Earlier this year, I reconfigured our custom-UV-PEN styles by substituting Technora fiber on the diagonal X-ply and increasing the X-angle to improve off-axis performance. These new PEN-Technora hybrids are now in-stock and available with UV film in both grey and white. These new styles replace UVPE10 White. We consider these fabrics to be the all-round best choice for T2s and T2Cs.
 
New UVPT10 White and UVPT10 Grey as seen from UV film side.
Note the black Technora X-ply
 
We will continue to offer PE10 (with grey standard film) as a lower cost option on T2s and U2s configured with Mylar sails. Although PE10 is more expensive than PX styles, the fabric performance is so much better that we are the discontinuing PX05T/PX10T option for these models.
 
PE10 grey with standard film
 
We also offer ODL06 (50% stronger than ODL04) as an extra-price option for T2Cs. ODL06 has Technora on the warp, fill and bias for unequaled structural performance. We don’t offer ODL06 with UV film and we still consider the new UV-PEN-Technora styles the best option for most pilots because they are somewhat more durable.
 
ODL06
 
The UVPT fabrics are highly recommended if you live in an area with high-UV intensity. In northern latitudes, the standard film will last for years and is less expensive. The new UVPT fabrics are included in the price of T2Cs.
 
Altogether, we can rank the fabric attributes in the following order:
  1. Appearance and fashion – up to you
  2. Ranked in order of structural performance (strength, stiffness and weight)
    1. ODL06
    2. UVPT10 (New PEN-Technora hybrids)
    3. PE10
    4. PX10T
    5. Woven sails (V170/205MT/Hydranet)
  3. Ranked in order of higher UV stability and longevity
    1. UVPT10, Woven sails.
    2. PE10, ODL06
    3. PX10T
  4. Ranked in order of nicer flight characteristics
    1. UVPT10, PE10, ODL06 are equivalent
    2. Woven sails
    3. PX10T
  5. Ranked in order of low price
    1. Woven
    2. PX10T
    3. PE10
    4. UVPT10
    5. ODL06

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Part of the test flight for all production gliders at Wills Wing is a pilot full-forward speed run at VD to check pitch pressure and sail quality. The sustained dive speed varies from the mid 30s on a Condor to as much as 80 mph on a T2. New gliders do not flutter at ‘Vd’ dive speed however older sails will invariably begin to flutter at much lower speeds. Fluttering will sometimes progress from a short-intermittent buzz at high speed to a debilitating and persistent beat at much slower gliding speeds with as little as 50 hours of airtime. Fortunately, you should be able to coax as much as 500 hundred flutter-free hours from a sail with just a little preventative maintenance. Curved tip gliders are more susceptible to flutter because there is more unsupported sail area aft of the roach line (defined by a line from the end of the leading edge to the trailing edge at the aft end of the root cord) so they require somewhat more attention and tuning.
 
There are 3 steps to correcting tip flutter.

1. Adjust your batten tension. The batten lever force should become firm about 15 degrees before the latch engages. As you sight up the batten pocket, you should see the lateral sail wrinkles and slack disappear just before the lever clicks in place. Do not tighten the batten so tight that the high point of the batten is visibly pronounced in the sail top surface. Battens from the root to mid-span can be somewhat looser since this area almost never develops flutter and high batten tension in the inboard section of the sail often hurts handling. Make your final adjustments after the glider is completely assembled.
   I check the batten tension every time I fly a glider and make changes as required. New gliders often require tuning on every flight for the first few hours until the sail is fully relaxed from stitch tension and residual creases in new fabric. Also, you may notice that the sail changes with temperature or if it has been set up continuously for a day or more.

2. Balance the leading edge and wand tension. This procedure varies somewhat depending on the model and configuration of your glider but the following principle applies to all Wills Wing curved tip gliders. The spanwise sail tension (preventing the sail from moving forward on the leading edge) should not be supported predominantly by the tip wand. Higher leading edge tension and correspondingly looser wand tension will reduce flutter. 
   You can check the tension on the leading edge sail mount through the outboard sprog zipper. At VG loose, the sail mount strap should be snug and without slack. On newer model T2s with an adjustable sail mount, the tension may be relatively tight at VG loose. The tension in the strap will become tighter at higher VG settings.
Older model sails were mounted to the airframe with an adjustable webbing/tang assembly. If your sail has this configuration, tighten the webbing until it is snug when the glider fully assembled and VG loose. You'll have to remove the clevis pin that secures the tang to the leading edge (with the wings folded) to make adjustments. A reference circle is drawn on the inside of the sail body showing the original tang position. Typically, you will need to tighten the webbing about 1/4 inch. Higher leading edge tension will be more effective at reducing flutter but may make lateral control stiffer and less responsive.
   Later model sails have a fixed length sail anchor which is less likely to stretch and become out-of-tune but cannot be tightened. If your sail has this configuration and the sail strap is too loose then you must shorten the tip wands. The tip wand lever has 3 hole positions where it is bolted to the sail. Remove the lever from the sail and move it to the position closer to the cup which effectively shortens the wand by 3/16 inch. Alternately, trim your tip wand by 3/16 of an inch. Note that the stock tip wand length is 34-5/16 inches.
Later model T2 and T2Cs have an adjustable sail anchor to precisely control leading edge tension. If your sail has this configuration, tighten the leading edge approximately 3/16 of an inch.
 
3. If your sail has already developed flutter then you must correct the problem without delay or the sail will be irreparably damaged. If the tuning changes described above do not solve the problem, install a small-gauge zip-tie between the grommets of the #2 (shortest) batten and tighten it enough to pinch the sail together about 3/8 of an inch. If the sail still flutters, the final remedy is to return it to Wills Wing for repair. In many cases, we can re-cut the trailing edge and extend the service life however it’s still important to adjust the sail as I described above.

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How seals work

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