AIRWORTHINESS - THE STORY BEHIND THE STICKER

(first published in 1996)

There's a lot we take for granted with the latest generation of hang gliders. Tremendous performance, light and safe handling and enormous strength are a far cry from the early days where luffing dives, poor handling and structural failure were the norm.

The drive for ever better performance and handling, reduced weight and ease of transport and set up have to be matched with caution to ensure that critical safety factors are not overlooked. This is the role of glider testing and certification, and with the number of separate testing standards today it is a complex area that even trips up the professional manufacturers on occasion.

It is the intention of these articles to look "behind the scenes" at the various testing standards to help clarify the current situation and also to propose a certain standardisation of some of the tests to simplify the procedures and ultimately reduce costs for manufacturers and pilots alike.

To understand how the current situation exists, a little history lesson is in order. Prior to 1975 there were no glider testing schemes to speak of. In those days hang gliders were designed and built by enthusiasts who had seen the NASA experiments with Rogallo kites or read the articles by their designer, Francis Rogallo. Made at home with polythene, bamboo and picture cord, they were tested by the simple means of flying them. If they didn't fly properly the pilot (if still able!) went on to work on a new design or bought a set of plans from somewhere else. Some of the early pioneers used the expertise they had gained to make hang gliders to sell to others and the professional hang glider manufacturer was born. The sport progressed, but with unacceptably high casualty figures, there was a major risk of hang gliding being made illegal in several countries. National associations were formed to protect members interests, and one of their first priorities was to set standards for hang glider construction as many of the accidents at that time were directly attributable to limitations of the glider. From this situation, four main national standards emerged. They are all broadly similar, but due to specific safety issues in their own countries they differ in small but important respects. The four main testing standards are:

The Germans: Deutscher Hangegleiter Verband (DHV) Certificate: Guteseigel

The Swiss: Schweizerischer Hangegleiter Verband (SHV) or Federation Suisse de Vol Libre (FSVL) Certificate: SHV Typenzeugnisses or Certificat de Type FSVL

The British: British Hang Gliding and Paragliding Association (BHPA) Certificate: BHPA Certificate of Airworthiness.

The United States: United States Hang Gliding Association (USHGA) Certificate: U.S.H.G.M.A (United States Hang Gliding Manufacturers Association) Certificate of Compliance.

The United States Hang Gliding Association (USHGA) delegates the whole certification process to a manufacturers association. The British Standard was originated by a manufacturers association but is now the responsibility of the BHPA. The British and German standards are tested to by the associations. The Swiss have appointed a company called VDT to perform testing for hang gliders similar to the arrangements between AFNOR and Aero-Test for paragliders.

THE SECTIONS OF THE CERTIFICATE

Broadly speaking the Certification Scheme can be divided into five sections:

1. Construction: Minimum standards of finish for the sail, wires and airframe are specified. 2. Flight tests: These tests are normally carried out within the gliders placarded flight limits to ensure safe handling and freedom from dangerous flight characteristics. 3. Load tests: The glider needs to be strong enough for the pilot's weight. Loading it up far in excess of this weight proves the glider is strong enough for its intended purpose. 4. Pitch tests: Pitch stability for a tailless aircraft like a hang glider is particularly critical as the wing must be self-stabilising throughout the flight envelope. 5. Administration: This covers fees required for the testing, documentation required, permitted advertising and other "paper work" issues.

CONSTRUCTION REQUIREMENTS

The materials and methods of construction specified in the airworthiness requirements were arrived at as a result of 20 years experience of hang glider construction, knowledge from other branches of aviation and good engineering practice. All the European tests have mandatory sections on construction requirements and several practices have been banned. The HGMA scheme is different, it puts the onus of responsibility on the manufacturer to ensure that correct materials and construction techniques are used, and to prove through the other tests that the glider is constructed properly. The following section therefore focuses on the European standards.

The SHV and BHPA standards have a lot in common starting with cables. Minimum standards are laid down for the swaging of them and they are subject to a special safety factor of two, which means they must be capable of withstanding twice their maximum rated load. The side wires especially come in for attention as they are the most heavily loaded bits of rigging. The SHV in addition ban turnbuckles in the lower rigging wires. Cables must be stainless or galvanised steel. The BHPA require that speed bars have an internal wire to re-inforce them, and specify at least 19 strands in all flying wires used.

The BHPA were alone in specifying seamless tubing in areas prone to flexing like the leading edges. Seamed tube may now be used only if there is a BHPA approved quality control method in operation. This is because prior to the airworthiness scheme coming into operation there were ten fatalities traced to the use of seamed tubing where failure of the seam proved catastrophic. The DHV on the other hand have taken the view that seamed tubing is like any other product, just needing the right quality control and testing methods applied. The SHV require that the glider be built such that there are no areas where stress concentration may lead to metal fatigue. In the UK in 1994 there was a lot of talk amongst ordinary pilots about why the Moyes Xtralite had two certificates of airworthiness but was refused a BHPA (UK) certificate. The reason was that the Moyes glider used seamed tubing in the leading edges, which was not allowed under UK regulations. The BHPA and Moyes have now agreed a testing regime which allows the use of tested seamed tube. The Xtralite now has a UK Certificate of Airworthiness. But why the differences in certificates that one organisation grants one and another refuses? The causes are political, scientific and historical, but may be bewildering to the ordinary pilot.

The DHV require that all gliders are capable of being fitted with wheels easily and that all dual gliders are supplied with wheels. They also require that the ends of knots on load bearing textile cords (e.g. VB compensator on front upper rigging wire) protrude at least 10 cm and be additionally secured.

These are just a few of the more noteworthy constraints. All the European documents have requirements for the sail, wires and tubing.

FLIGHT TESTS

Any glider needs to have simple pitch, roll and yaw characteristics that ensure that a qualified pilot will have no problems flying or controlling it. Flight testing checks the range of normal flying manoeuvres and ensures that the glider is safe and controllable within those ranges. The tests also ensure that dangerous manoeuvres cannot be accidentally entered within reason. The tests are always done in the least favourable configuration of pilot weight and hang point setting for each test.

THE MANOUEVRES

Take off: All standards require take off without special skill. In addition the BHPA and HGMA have a basic requirement for the glider to take off in a wind of less than 5 m.p.h. on a slope of no more than 5:1 which basically ensures the glider has glide ratio of at least 5:1. The HGMA explicitly state a minimum of 5:1 and the SHV require 4:1. The DHV do not specify a minimum glide angle but require that the glider be capable of foot launch, aerotow and winch launch.

Landing: All standards ask for safe controlled landing without exceptional exertion or skill. The SHV have a requirement for the minimum speed of the glider not to exceed 40 km/h (25 m.p.h.) at maximum pilot weight. This is not the stall speed but the lowest speed at which the glider is manoeuvreable. The DHV and SHV also specify that "landing aids" must not excessively change control forces, or require exceptional strength or skill to land with. The BHPA require that a landing must be possible in less than a 5 m.p.h. wind, stand up, with no part of the glider touching the ground.

Turns: Turn behaviour is very important as well, and tests are carried to ensure that glider will not stabilise in a spin or spiral dive also that the glider is not "too stiff" i.e. excessively difficult to turn.

TURN REVERSAL

The test to ensure the glider is easy to turn involves a turn reversal in a fixed time. The BHPA require the glider to be capable of going from 30 degrees of bank one way to 30 degrees the other way within 5 seconds. The DHV and the SHV ask for the turn reversal to go 45 degrees each way but in the same 5 seconds, the HGMA require this in 4 seconds with minimum pilot weight. Both the SHV and the HGMA further qualify the test. The SHV state that the manoeuvre must be done the minimum speed (this prevents the bar being to get the glider to pass the test). The HGMA reduce the time allowed depending on pilot weight for example 2.67 seconds if the pilot is 1.5 times the minimum weight but with no SHV style speed constraint.

IN FLIGHT PITCH STABILITY

Some pitch testing is done by means of flight-testing. All standards require a trim speed to which the glider will return to "hands off". All standards require that the bar force must consistently increase as the flying speed increases from this trim speed. Any neutral or negative force above the trim speed will lead to the glider failing the test. This is because we all expect to have to pull the bar in to go faster and push out to go slower. A glider that behaved differently to this expectation would be difficult to fly, as well as not pitch stable. The HGMA also ask for evidence that the glider will pull out of a 75 degree dive.

ROLL/YAW STABILITY

All the standards look for stability at the trim position, by requiring that the glider fly straight and level for 10 seconds or a similar period of time in calm air without rolling or yawing away from a steady course. We expect to have to put an input in to change direction, a glider which required constant correction in still air would be impossible to fly in any other type of conditions.

STALL BEHAVIOUR

There are tests to check the stall behaviour of the glider both in turns and from straight and level. All tests check the possibility of getting into a stable spiral dive or spin, the SHV and the DHV forbid tendency to spin or spiral dive. The HGMA and the BHPA check that the glider will recover from either a spin or spiral dive if the glider is allowed to return to trim. The HGMA also require that in a co-ordinated 15 to 20 degree banked turn that the pilot is centred or below centre on the control bar; i.e. not "high siding". The DHV, SHV and the HGMA all ask that after a stall from straight and level flight that the glider not roll more than a specified amount. The BHPA say the roll must not be excessive.

MINIMUM/MAXIMUM SPEEDS

The HGMA ask for a top speed of at least 35 m.p.h., the DHV for at least 55 km/h and the SHV ask for at least 1.5 times the minimum velocity. The BHPA do not specify a minimum top speed. The DHV specify the maximum speed of all hang gliders is 80 km/h, the other associations expect the manufacturer to specify a VNE(Velocity never to exceed).

The BHPA, SHV and DHV flight-test the gliders themselves. The HGMA require film footage to be presented showing the glider doing the tests and passing them. After the flight-tests the DHV grades the glider depending on the pilot skill required. The classes are 1-basic, 2-intermediate, 3-advanced. There is also an "E" category which means special instruction will be required which is awarded in addition to the number category. The other bodies leave it to the manufacturers to specify pilot skill, with the exception that if the glider needed certain special flying procedures the HGMA will require a minimum of Hang IV (advanced) pilot rating.

There is an additional level of testing performed here by pilots wanting to purchase gliders. They will test fly the glider in question for themselves and make their own decision. This is in contrast to load or pitch testing which nearly all pilots take for granted. Good handling sells gliders and some pilots may not regard gliders that have passed all tests in all standards as necessarily good handling as handling and flying behaviour have a very subjective element.

THE LOAD TEST

The load test is a simple concept. It makes sure that the hang glider will not break when flown within its flight envelope. All four schemes expect the glider to withstand 6 times the maximum weight when loaded positively and 3 times the maximum weight when loaded negatively.

The four organisations calculate the maximum weight differently. The BHPA and HGMA use the suspended pilot weight, the DHV use the total weight of the pilot and glider minus half the weight of the glider and the SHV use the total weight of the pilot and glider. This means that a glider will, if tested to the same loads on a rig, have different pilots weights quoted on the certificate under different schemes.

The test involves the glider being mounted on a vehicle test rig and driven along until either a certain speed is reached without the glider breaking or the limit load being achieved. The HGMA and the DHV test to a speed, at which point the load must be equal to or have exceeded the +6G and -3G limits. The BHPA test until the +6 and -3 limits are achieved. The SHV require evidence of the limits being met. The +6G limit is obtained by setting the glider at the highest angle of attack possible without stalling, and the -3G limit by setting at the lowest angle of attack possible without stalling. Note: It is possible for a wing to stall because the angle of attack is too low, wings work quite well upside down and will generate negative lift if so positioned. This negative angle of attack is a long way outside the normal flight envelope.

The DHV and HGMA also require an additional test where the glider is placed at an angle of incidence of -150 degrees. This means that it is driven along backwards with the keel pointing down at a similar angle to that which it would be if it was parked tail into wind on the keel. This tests the integrity of the trailing edge in a "tail slide" situation.

Despite these tests, gliders do still break on occasion. Firstly, "shock loading" can cause failure (when for example a pilot falls into the sail) or when the load is applied through a point other than the hang point. It is important to note here that it is really the responsibility of the pilot to fly the glider within its placarded limits at all times.

THE PITCH TEST

Conventional aircraft have tailplanes. The tailplane controls pitch (i.e. nose up/down attitude) and ensures that an aircraft returns to stable, level flight after any disturbance in the air through which it is flying. Hang gliders have no tailplane therefore the main wing must replace this function in providing pitch stability. This is the classic design challenge of the "flying wing". For hang gliders, this is solved by use of reflex, sweepback, washout and defined tips. It is difficult to calculate the effect each of these has on pitch stability at any given speed, and this is further complicated by the fact that hang gliders are also flex wings (they are controlled by distorting their airframe with the movement of the load they carry). For these reasons theoretical calculations of pitch stability are difficult and practical measurements on a rig are required.

Pitch testing is most easily understood as the bar pressure at different speeds but it is in reality somewhat more complicated than this. Essentially, a glider is required to have a single speed at which no positive or negative bar pressure is required (this is known as the trim speed). To progressively speed up or slow down requires progressively greater pull in/push out.

To understand this more fully requires the introduction of the pitching moment co-efficient or Cm. Positive values of Cm indicate a pitch stable glider and negative values an unstable one. Cm is a dimensionless number, chosen to remove the effects of speed, weight and upright length, but it is dependent on the aerofoil section and angle of attack. The complication for hang gliding arises because a hang glider is unique in modern aviation in allowing aerofoil distortion at different speeds and hence Cm has to checked at differing angles of attack and speed, (a conventional aerofoil is just checked at different angles of attack).

To pass a pitch test, a glider must exceed a minimum value of Cm. The Cm values at different angles of attack are defined by the testing bodies and with a safety margin added form the pitch curve requirements (see diagram). It is important to note that gliders are only tested within certain limits for angle of attack and speed (which includes a safety factor and covers normal stall situations). Outside this envelope Cm values may be unstable and tumbles or tucks could occur. As with the load test, it is the responsibilty of the pilot to keep within the placarded limits.

The BHPA and the HGMA state the minimum Cm requirements in numbers. Both associations put the glider on the back of a truck and drive on a flat surface with the glider at various speeds and angles of incidence (Fig 2). The pitch pressure is measured and the results plotted on a graph. Both bodies have minimum Cm figures for speeds and angles of incidence on this graph. The HGMA minimum Cm figures are bigger than the BHPA ones at low speeds and zero angle of incidence so gliders need to be more pitch positive in this area to pass the HGMA test. The BHPA figures cover a wider range of angles of incidence, particularly at high speeds. It is traditionally at high speeds and low angles of incidence that pitch problems with luffing dives have ocurred but it is at low to medium speeds that tumble resistance from high Cm figures arise.

The mainstay of the SHV test is a drop test, the glider (with a weight equivalent to the pilot) is dropped from a cable car at an angle of incidence of -95 degrees and the resulting dive filmed. The glider must recover with a maximum loss of height of 50 metres. The HGMA need to see film evidence of the glider recovering from a 75-degree dive.

The DHV also vehicle test gliders. They do not specify Cm values in their test document, but state that Cm must get larger as the angle of incidence gets smaller, i.e. the glider gets more pitch positive as the angle of incidence decreases. If the glider does not meet this requirement (i.e. there is a point at which the Cm briefly dips but still remains positive) then it can still pass if the pitching torque is above a DHV specified mininmum for that particular speed.

The main point to make is that all the vehicle tests place the gliders at angles of attack and speeds far outside the limits normally reached. Unlike flight testing, pitch testing cannot be verified as easily or intuitively by the ordinary pilot and pilots may prefer gliders of lower or marginal pitch stability because they are easier to fly faster or may have a slight glide advantage at speed. This is because reflex reduces the efficiency of a wing slightly, but is the main ingredient of pitch stability and hence safety. Luffing dives are a thing of the past but tumbles are still with us, showing that pitch stability is still one of the biggest challenges in hang glider design. The tests that prevent unstable gliders getting onto the market are therefore major live savers.

Technical notes:

All schemes use angle of incidence rather than angle of attack. The zero datum for this is the angle at which the wing produces no lift.

Cm = M/(1/2pV2Swc) where M = pitching moment p = air density V = air speed Sw= wing area c = mean wing chord

If a = angle of incidence

DHV pitch requirement dCm/da < 0 for 0o < a < trim angle andCm > 0 for -15o < a < trim angle

HGMA require at a=0 Cm > 0.05 BHPA require at a=0 Cm > 0.03

ADMINISTRATION

The administration covers the cost of certificates, membership of the association and voting rights in the HGMA, procedures for grounding gliders and permitted use of the associations name in advertising. The is one notable extra in the DHV Gutesiegel. They require that the glider receives a check up after five years (funf jahre check) and thereafter every two years (zwei jahre check). providing infrastructure for this is the responsibility of the manufacturer in Germany or in the case of imported gliders the agent. The HGMA require that each glider is test flown by the manufacturer and the dealer prior to delivery. For all certificates, if the glider manufacturer is not situated in the country of certification then the responsibility for getting the glider through the certificate will rest with the importer and they will also have to liaise with the association on any other matters like safety notices. Administration also deals with the most visible part of this process to us pilots, the C. of A. sticker. This will normally have all the important limits for flying the glider written on it.

ENFORCEMENT

Certification is mandatory in some countries. In Germany all resident pilots must have a Gutesiegel stamped glider, visitors must have a glider approved in their own country. In the UK gliders must have a UK Certificate of Airworthiness or have been registered with the British Association.

(c) 1995 Steve Uzochukwu and Martin Pepper.

Thanks are due to Berndt Schmidtler (DHV), Alain Zoller (FSVL/SHV), Mark Dale(BHPA), Mark West(HGMA), Brian Gourley(Airwave), Darren Arkwright(Solar Wings) and Steve Boalch(Translations).