What are sprogs?
Sprogs are wire braced struts enclosed in the sail that support the outboard trailing edge on high performance flexwing hang gliders to maintain the washout (twist) in the wing. This washout is essential in order to provide pitch stability, especially at low or negative angles of attack that may be encountered due to turbulence. The sprogs perform a similar function to the luff lines and washout rods on kingposted hang gliders. The sprog height can be adjusted, usually by means of a threaded rod at the font end of the sprog. There is a trade-off between stability and performance, with lower settings improving glide performance but reducing pitch stability to potentially dangerous levels.
Checking sprog settings in competitions
At the CIVL plenary meeting in February last year a resolution was passed to start checking sprog angles at FAI Category 1 competitions (World and European Championships). There are concerns that many pilots are competing with sprogs set too low, improving their glide performance but compromising the glider's pitch stability. As a result there isn't a level playing field: pilots that are prepared to accept a lower safety margin gain a competitive advantage.
The initial proposal was submitted by the German DHV, but following discussion this was revised and the following was unanimously agreed.
• From May 2007: mandatory measurement of sprog settings and recording without penalties. Data gathered from this program will be published for the benefit of the CIVL, the manufacturers and the pilots.
• From May 2008: CIVL will institute mandatory measuring of sprog settings with penalties.
• Between these dates CIVL will consult with manufacturers to establish measurement techniques and suitable tolerances.
• CIVL officials at the Pre-Europeans (Greifenburg, June 07) and the Worlds (Big Spring, August 07) will make the measurements.
• CIVL will ensure there is an informed presentation and discussion at the mandatory safety briefing before each meet.
This article discusses some of the issues raised by this proposal. Inevitably it is a personal perspective. I am not an expert in the subject, but it is based upon discussions with a number of knowledgeable people. One of the problems with the whole subject of sprog settings is that you hear at least as many different opinions as the number of people that you talk to, and it's difficult to separate the facts from the opinions.
To put some perspective on this, tumbles in hang gliding competitions are not a frequent occurrence. Last year was worse than usual, but there certainly seem to be fewer parachute deployments than at paragliding competitions. However, even one tumble is one too many and we should be doing what we can to eliminate them. With the gliders that do tumble, one of the problems is that it is often difficult to find out what configuration the glider was in and whether lowered sprogs could have been a contributory factor.
Despite the plans to start measuring sprogs this year, only very limited measuring was done at either Greifenburg or Big Spring and no data has been published. At the Pre-Euros I gather that on the day that they were planning to measure sprogs in the goal field, many of the top pilots landed in another field after making goal and the measuring was abandoned. At the Worlds it was found that measuring sprogs out on the airfield was not possible in any significant wind. Some measuring was done in the hangar, but only for those pilots that requested it. However, the result of this was just measurements of the angle of each of the sprogs relative to the keel - fairly meaningless as no data was available to compare these measurements with. As a result pilots received no indication about how their current settings compared to the limits. Although Aeros now specify sprog settings based upon angle, most other manufacturers still provide data based upon measuring the vertical distance between the keel and a string stretched between the outer battens.
On the rest day at the Worlds a meeting was called to discuss the issue and allow the pilots to voice their opinions to CIVL. Although the meeting was set up by CIVL Jury President Flip Koetsier, Moyes Litespeed designer Gerolf Heinrichs ran much of the meeting after giving a briefing on the technical aspects of pitch stability, which gave some useful background. Wills Wing designer Steve Pearson was also present to give an alternative manufacturer's view.
The general message from the meeting was that most pilots were against the idea of sprog angle measuring. The key points were as follows:
• The DHV have already introduced sprog angle measuring in their competitions, but it was unclear how this worked or where they got their data from. Many of the German pilots were against this measuring.
• Pitch stability is only one of the factors that contributes to a glider tumbling. Two other significant factors are how the glider is flown and the conditions it's flown in. Letting off some VG reduces the chance of tumbling, as does avoiding flying too slowly in turbulent conditions.
• Many pilots felt that addressing the conditions that we fly in, ensuring that tasks are not flown in dangerous conditions, was more important than trying to regulate pitch stability.
• Sprog angles are only one of the factors that affect pitch stability. Other factors including batten profile, VG limiters, internal webs between the upper and lower surface, and keel pocket tension. If sprog angles are regulated some pilots will still be tinkering with these other things, again potentially giving them an advantage over other pilots and reducing safety margins.
• “How low is too low?” is the million-dollar question. There is no hard and fast sprog height below which a glider becomes dangerous as it depends upon the nature of the turbulence that a pilot may encounter and how he flies the glider.
• It is not possible to design a glider such that it will never tumble regardless of what the pilot does and what turbulence he flies in.
• The three certifying bodies (the BHPA, DHV and USHGMA) all have slightly different pitch stability requirements which place the emphasis in different parts of the flight envelope. The differences are small but there might be some advantage to gliders certified to one standard rather than the others.
• Many gliders are now available with a large number of different options which can be mixed and matched in many combinations. The Litespeed RS is available with aluminium or carbon inner leading edges, outer leading edges, sprogs and battens, a choice of sailcloths, a choice of A-frames, and carbon leading-edge insets as an option. Many of these will affect the sprog setting required for a degree of given pitch stability, but it's not clear how significant these are. Small changes can sometimes have quite significant effects - Wills Wing once found that changing the design of the nose cone made the bar pressure go negative at high speed.
• Opinion differs on whether you can achieve sufficient repeatability in the sprog measurements. Some manufacturers apparently find it difficult to get repeatable sprog measurements even in a factory despite doing it every day. This casts doubt on whether CIVL volunteers, who may have little prior experience of sprog measuring, will be able to make accurate measurements, especially on typical launch sites and goal fields which may be uneven and windy. However others report good repeatability and it may be down to what measurement methods are used.
• For some gliders the sprog setting recommended in the manual, the setting required to pass the pitch test and the setting at which they are sent out of the factory are all different, making it difficult for pilots to know where they stand.
• CIVL do not currently have a database of “approved” sprog settings for all the different gliders in a consistent format, and it's not clear what progress they are making on gathering this data.
• Sprog angle is affected by temperature. Aluminium sprogs go up in higher temperatures as the aluminium expands more than the steel cable. This can have a noticeable effect on trim speed at full VG and hence - presumably - also on pitch stability.
• Many of the top pilots feel that they should be able to decide how they wish to set up their gliders and that they have the knowledge to do that safely. The problem is with less experienced pilots who may not have the knowledge to make an informed decision.
• Steve Pearson pointed out that to a large extent the manufacturers are self-regulating. It is in their interests to build gliders that are both safe and competitive, and give pilots the information they need to ensure they are set up in a safe and competitive configuration. If they don't do this and their gliders get a reputation for tumbling they will soon start losing business.
I understand that the current proposal only applies to certified gliders (although it's not clear from the wording of the proposal), presumably because it is impractical to get the required sprog angle data for prototype gliders. However, this potentially gives an advantage to those pilots flying prototypes, and opens up another grey area of what's a prototype and what's a certified glider. Most manufacturers are continually developing their competition gliders and making small changes to the design. Where these are small and the basic design has previously been certified the glider is generally still sold as a certified glider, even though it is not identical to the one that was certified. The various certifying bodies take differing approaches to the recertifying requirements for small changes. If prototypes are exempt from sprog measuring a significant number of pilots would declare that their glider is a prototype, which would negate the aims of this proposal. Also manufacturers use competitions as a proving ground for their new gliders. Banning prototypes from competitions would be unpopular and might hamper development.
The procedures for inspecting gliders have yet to be drawn up. It is essential that this is done in such a way that it is easy for pilots to ensure that they comply with the requirements. With spot checks, if gliders are not all checked at the start of the competition or if there are significant error margins in the measurement techniques, there is a risk that pilots might be penalised for inadvertently contravening the rules, which would be highly undesirable. One suggestion was to seal the sprog adjusters after the initial measurement, so pilots could be confident that if they didn't adjust their sprogs they couldn't be penalised.
Where the proposal goes from here is unclear. There are a lot of significant challenges still to be overcome, but if implemented after appropriate testing and consultation to show that any proposed procedures and limits are workable it has the potential to improve safety and fairness. In addition, implementation of the proposals will potentially mean that better information is available to pilots about the stability of the gliders that we are flying and the effect of different options, etc. This can only be a good thing. If it is implemented too quickly without sufficient understanding of the issues, by both the pilots and competition organisers, it has the potential to cause significant controversy, protests and bad feeling. Since so little testing was carried out last year it seems to me that we are nowhere near ready to start measuring with penalties in 2008 as originally proposed.
by Bruce Kavanagh