Slingsby Schreder
HP-14

In 1966 Richard Schreder won the US National Soaring Championships at Reno, Nevada, in his new sailplane, the all-metal HP-14. Schreder had designed and built light powered aircraft at Bryan, Ohio, before he competed in his first US National Soaring Championships in 1956, flying a Schweizer 1-23D. By the next year he had built and was flying his own design of sailplane, the HP-7, and he was soon rated one of the top ten pilots in the USA. With his HP-8 he won the Nationals at Bishop, California in 1958, and in the following year broke three world records for speed round triangular courses. As a member of the US team at the 1960 World Championships, held near Cologne, he landed the HP-8 on the eastern side of what was then the Iron Curtain. He spent a night in jail, but was not ill-treated and rejoined the competition a day later. New sailplane designs now began to emerge from Schreder's small factory at the rate of about one per year, each an improvement on the last. The HP-9, HP-10, HP-11 and HP-11A appeared, kits for home constructors being marketed under the liberal American regulations for amateur building of aircraft. A British writer said: 'For simplicity and effectiveness, Dick Schreder's HP-12 is the most superbly engineered aircraft that I have ever seen'. The HP-13 followed. Another commentator remarked on the American's ability 'to knock up a superb metal glider in no time at all, or, more precisely, in six months or a year... The whole process shows a degree of initiative which we might well emulate in the UK.

Advanced details of the HP-14 were published in Sailplane and Gliding in December 1965. It was said that several American amateurs had started construction from kits supplied by Schreder even before he himself had flown the prototype. The anticipated best glide ratio was claimed as over 40: 1, compared with the 36:1 of the 17m Slingsby Dart (still in production) and 38:1 of the German SHK. Schreder's 1966 US Nationals result with the HP-14 was added confirmation of the excellence of his design. Handling was good, the aircraft having no serious vices, although the ailerons were less effective than desirable because of stretching in the cable-driven controls. A significant point in British eyes was that the HP-14 had been designed with a larger wingspan and lower wing loading than Schreder's previous sailplanes. He said lie now recognised that competitions were won or lost not on the best days, when everyone did well., but on the 'scratchy' days of weak thei-nials when the heavy, fast aircraft, so-called 'lead sleds', had to struggle to keep up while the lighter gliders made better progress across-country. Evet-i so, by British stai-idards i.lie HP-14 was not a lightweight. The wing loading of 28kg/M2 (5.75lb/ft2) was 20 per cent more than that of the Dart 17. The trend towards higher loadings with better winc, profiles was becoming accepted, and the new Schreder design looked like a good compromise.

Slingsby had decided to build metal sailplanes. With its simple structure and ease of construction, it seemed they could hardly do better than to undertake the manufacture of the HP-14 under licence using, in the first instance, kits supplied from America. It would give them in the shortest possible time a product that would be competitive in price and performance with Gen-nan, Swiss and Polish sailplanes, and they would gain valuable experience with metal structures. It was anticipated that, after the first few aircraft, all the necessary components would be made in England.

An agreement was reached with Schreder and the first two kits arrived at Kirbymoorside early in 1967, Schreder himself coming too, to advise and supervise the early stages of assembly. At the same time a third kit went to Ken nipp's Southdown Aero Services at Lasham. The intention was to have the two Slingsby aircraft completed in time for the British Nationals at the end of May, Schreder to fly one hors concours and John Williamson the other. The Southdown example was being built for Peter Scott. No Slingsby type number was allocated.

The HP-14 with a span of 16.7m (54.8ft), had a simple tapered wing, using the Wortmann FX 61-163 profile with a slight modification at the trailing edge as explained below. A 16.3 per cent-thick wing, compared with the 18 and even 20 per cent-thick profiles seen in previous Slingsby products, was claimed to be an important advantage made possible only because of the metal structure. The mainspar flanges were cut from 1/4 inch thick 2024 T6 alloy. The large metal plates were rolled to produce a slight chordwise curve conforming to the wing profile. They then required cutting longitudinally to produce the spar flanges, whereupon the metal tended to spring into a curve. Straightening required careful hand work with a ball pane hammer. The pressed ribs were riveted in front and behind to the spar webs. There was a light rear spar of C section. The ailerons were hinged to the top flange of this spar, the flaps to the bottom. The whole wing was skinned in sheet metal with countersunk rivets. The 3.6m(12ft) wide metal sheets for the skin had to be bent to conform to the leading-edge radius. This was done by a band of five or six persons with one edge of the sheet under their feet (protected), pulling the other edge up and over to make the initial bend. Then, with the aid of a long, heavy plank of wood they stood or evenjumped up and down to tighten the bend until it was close enough to the correct radius. Unlikely as it sounds, this worked quite well.

To the British pilots the most unusual feature of the wing was that, instead of parallel ruler-type airbrakes, flaps along the entire trailing edge of the wing inboard of the ailerons were made to hinge down to 90'. For landing, such flaps proved excellent. Having chosen a field, the pilot could lower the flaps fully at a suitable height on the final approach. The increase in drag was very great. Airspeed was maintained by adopting a steep attitude, about 45' nose-down to maintain 50kt. On rounding out, the speed rapidly decayed and a very short landing run was the result. The proviso was that, if the pilot was undershooting, it was dangerous to raise the flaps suddenly to the neutral position because this precipitated a sudden loss of lift and a very high rate of descent. It was shown, however, that between about 70' and full deflection the wing lift coefficient hardly changed, whereas the drag varied considerably, so a badly judged approach could be adjusted by moving the flaps between 70' and 90'. In practice it was easier to land an HP-14 in a small field than almost any other comparable sailplane. By using flaps of this kind the complications of housing airbrakes inside the wing, with all the required cut-outs, seals, rods and bellcranks to drive them, were avoided.

For thermalling, the flaps could be lowered from neutral slightly, allowing tighter turns, and for high speed 'penetrating' dashes through sinking air to the next-thermal they could be raised 10 degrees to reduce drag. They were not interconnected with the ailerons, so these remained in their normal position as the flaps went up or down.

Strictly, the 61-163 profile has a slight concave cusp on the underside at the rear, but to form this accurately while preserving structural simplicity in the flaps and ailerons was difficult. Schreder made the last few per cent of the profile flat underneath. The control surfaces could then be made in the easiest possible manner by twice folding appropriately sized metal sheets and riveting the extreme trailing edges together to form a triangular-section tube, with small riblets at each end. The loss of performance was very slight. The mainspars were joined on the aircraft centreline with two large horizontal pins through inter-digitating aluminium alloy fittings, and the fuselage was attached by four further pins at the main and rear spars. The flaps were driven from the extreme root end by a torque tube carrying a triangular arm extension which engaged inside the root of the flap as the wings were rigged.

The wings were attached at shoulder level to the fuselage, which was very simple in outline, the only double curvature in the skins being in the nose. The rest was a conical metal tube of oval cross-section with pressed formers and flush-riveted skins. The cockpit canopy was also very simple, the most forward portion being a curved sheet of transparent plastic. The rear section, hinged at the side for access, was a simple moulding. There was a large retracting main wheel and a steerable tailwheel linked to the rudder pedals. The control colun-m was orthodox. The flaps were driven by a rack and pinion with a large handle. Several full turns of the handle were required to lower the flaps fully, but all the necessary adjustments during the landing approach could be done within one turn. Because of the very successful Austria and SHK sailplanes, V tails were already familiar, but the standard HP-14 tail areas looked very small by comparison with the surfaces of the SHK. Recovery from inadvertent spins had proved difficult in a few V-tailed gliders, and the effectiveness of the small tail areas in a crosswind take-off also worried some pilots. Schreder was entirely confident of his aircraft, but the first one from Slingsby's after test flights was given much larger tail surfaces. Linkage of the ruddervators with the controls was, supposedly, automatic. For carriage by road the two elements of the V tail, after disconnecting the lower spar flanges from the fuselage cross-frame, were raised to the vertical position and strapped together. The elevator control linkage did not disengage when this was done, so to rig the tail it was necessary only to lower each surface and insert the bottom pins.

The Slingsby aircraft had a powerful hydraulic wheel brake operated by pushing with both feet together on the rudder pedals. Also on the first Slingsby HP-14, for various reasons, the flap rack and pinion drive was not used, a simple lever being adopted. With this arrangement the full 90' flap deflection was not possible, and a 'one-shot' tail parachute was fitted for landing. (The parachute was a standard item of equipment from a Folland Gnat jet aircraft.) This prevented the installation of the steering tailwheel. Schreder himself retained the original arrangement for the second Slingsby example.

The first flight in Yorkshire was made in May 1967, and John Williamson flew the HP-14 for the first time only three days before he was due to use it in the National Championships. Schreder's aircraft arrived even later. After flying Schreder's machine the day before the competition began, Williamson had his flaps hurriedly modified overnight to the standard rack-and pinion drive, and the tail parachute was removed. As a result of this alteration, which required an all-night working session by Geoff Bailey-Woods, there was a desperate rush to get the HP-14 into its front-row position on the starting grid for the first competition take off. The crew were actually still rigging the V tail as the aircraft was being towed out to the launch point. This almost precipitated a serious accident. When the tail surfaces were in the vertical position the controls did not disengage, but as the HP-14 was rolling along, rocking slightly from side to side and being rigged at the same time, one of the surfaces was allowed to move beyond the usual'at rest'vertical alignment. The ruddervator linkage became disengaged and did not reengage as the surface was lowered and locked in flying position. The standard cockpit check just before take off did not reveal the disconnection because the surface moved normally, resting by its own weight against the drive though it was not actually being driven.

Williamson, on tow, had no control and experienced a wild ground loop, finding himself airborne and travelling sideways before skidding for 10m (30ft) along the ground. It was a thoroughly ignominious and dangerous start to the British contest career of the HP-14. Fortunately only slight damage was done, the wingtip skid plate being ripped off and a dent being made in the nose. With the plate pop-riveted on again, the HP-14 was able to fly after all in the first contest task. After this Williamson was very pleased with it. The weather was not particularly good, only four days being scored in the whole Championships. Williamson, by now getting used to the sailplane, won the last day, but placed 10th overall against the wooden opposition. Schreder, hors concours, was placed 18th in the unfamiliar British conditions.

Later in the year Peter Scott took delivery of his own HP-14 from Lasham and flew it competitively for the first time at the London Regionals in August. In this contest he came fourth, being beaten by some wooden sailplanes (including Carr Withall's Skylark 4, Alf Warminger's Dart 17 and a Ka 6E flown by the author). The absolute performances were somewhat obscured because handicapping factors were applied to the aircraft, but the HP-14 did not impress the assembled pilots greatly. The ground crew found it troublesome to rig and christened it the 'Iron Lady'. Scott won the Western Regionals with his HP-14 in June the following year.

Problems now emerged with certification. To permit an experimental aircraft to be flown in competitions by experienced pilots was one thing, but to issue a general type approval was another matter. By the end of 1967 the BGA Technical Committee reported not that the certification was imminent, but that: 'This type is now being developed into a definitive version and we will soon be concerned with the certification'. No one doubted that the HP-14 was structurally sound and strong, but this had to be proved, which meant there had to be a good deal of calculation and paper-work. Schreder himself relied mainly on experience and had almost no figures to show. Both of the HP-14s built from Schreder's kits were sent for sale to the USA, where they were readily accepted under the official 'experimental' category there. The tail surfaces of Williamson's example were replaced by the normal smaller ones.

Slingsby's aircraft division was now extremely busy and perhaps overloaded. Production of the T-53 twoseater and the Canico V-Liner aerial hoarding were proceeding. Tipsy Nipper light aeroplanes were being built and kits for them sold. Replica S.E.5As were being made for filming, the Swallow sailplane was in production, the motorised Capstan was being developed and two special versions of the Standard Class Dart 15, with Wortmann wing profiles, were being built for the next World Championships, scheduled for Poland in June 1968. Alongside there was production of polyester/glass components for bathrooms.

Also for Poland, Slingsby was committed to the production of two advanced sailplanes for the Open Class. As most of the other pilots in this category would be flying large glass sailplanes, the most obvious expedient was to improve the performance of the HP-14 by stretching the span to 18m. This could be achieved by adding three extra rib bays to each tip, extending the metal skins and adding glass-polyester wingtip mouldings. This entailed removal of the aileron mass-balances, which, on Schreder's original, had been attached at the outermost end of the control surface, outboard of the tip skid plate. To improve handling with the larger span, the tail unit was redesigned with a large vertical fin and rudder and an all-moving tailplane. Changes to the rest of the aircraft were kept to the minimum. Schreder actually fitted a pair of the 18m wings to his original HP-14 for this World Championships.

In Britain, unlike some other countries, it was legal to fly in cloud providing the sailplane was suitably certificated. Loss of control in blind flight was a well recognised danger, the usual outcome being an increasingly steep spiral dive with rapidly rising airspeed, very high g forces and serious structural collapse. It had been decided long before that a sailplane approved for cloud flying must have airbrakes which, when fully open, would restrict the airspeed to the maximum permitted in rough air. It would be very difficult to sell the HP-14C on the home market without a cloud-flying airworthiness certificate. The flaps certainly limited the speed once they were fully down, as Williamson proved in his own test flying, but, if the airspeed was already high and rising, the effort required from the pilot to get the flaps down to 90' was too great. If the sailplane was genuinely going out of control in cloud, the flap brakes would be useless.

During the next months Slingsby worked on the HP-14C to surmount this problem. Various expedients were tried, the addition of a simple bungee spring proving very effective in reducing the loads for normal approach and landing. To achieve full flap at high airspeeds a compressed-air system was devised. Th is. relied on a high-pressure air bottle which had to be pumped up before take-off, and pneumatic jacks to drive the flaps down against the aerodynamic resistance. When the flaps were needed in an emergency the pilot could release air to the jacks and the flaps would go fully down at once. Enough pressure was put into the bottle before take-off to allow this to be done twice or thrice during any one flight. Checking the pressure gauge was added to the pre-flight checklist. Unfortunately, forcing the flaps down in this way at high airspeeds proved too much for the light secondary spar structure, and strengthening this member involved more complications.

The HP-14Cs were barely ready in time for Poland, and the fust one out of the workshop was found to be virtually unflyable, unstable fore and aft and so touchy in pitch that it required the pilot's constant attention to prevent dangerous oscillations. It had to be rushed back to the factory for modifications. What had been a splendidly simple and easily maintained sailplane of modest wingspan threatened now to become something of a monster. The nose was lengthened and a geared anti-balance tab was added to the elevator. This increased the stick force per g enough to make the aircraft steady in high-speed flight, although the stick loads could not be fully trimmed out at low speeds. Nick Goodhart, the chosen pilot, made only one cross country flight in it before the first contest day. His comments in print before the competition started were guarded. The HP-14C, he said, had a very reasonable performance, which was faint praise. The ailerons were heavier than he liked, the rate of roll only adequate. With Goodhart's weight in the cockpit the sailplane was.now neutrally stable in pitch. There was, he pointed out, a good deal of development work still to be done. Many adjustments had to be made in Poland in the urgent days just before the competition started. The other British entrant in this class, George Burton, decided not to fly the HP-14C provided for him, competing in a wooden SHK and cording a very creditable 7th. Goodhart struggled on to 16th place, which was very good in the circumstances but not good enough. Schreder came 21st.

It was accepted that the performance of the 18m aircraft in relation to its price was good, but the technical conclusions after this contest made sorrowful reading. Frank Irving wrote: 'The moral for Slingsby's is that the production aircraft must have better finishes and improved details.... compressed air for operating the flaps at high speeds is simply not acceptable'. It was becoming, said team manager Ann Welch, 'sadly obvious that no longer could we 'fly British if we wanted to have any chance of winning'.

As well as some of Schreder's own kits shipped directly or through Slingsby as agent, sales of several HP-14C kits had been made in advance of test flying. One went to Joe Provins in Yorkshire. The small Sydney Soaring Club, a long-standing customer for Slingsby products, and another group in New South Wales ordered and paid for two HP-14C kits. There were long, unexplained delays. Enough parts arrived at last to enable construction to begin, but many necessary components were not delivered. When ordering his kit, Provins had required a written assurance that a BGA type approval and Certificate of Air-worthiness for the sailplane would be forthcoming. It was also the rule in Australia that an imported sailplane could be accepted by the Department of Civil Aviation (as it was then called) only if type approval was granted in the country of origin. Little progress towards certification had been made. Having already been told after the Polish experience that the type was not acceptable, Slingsby were now legally required to bring the HP-14C up to the necessary standard. Using the ex-Goodhart aircraft, Geoff Bailey-Woods was given carte blanche to do whatever was necessary. He removed the compressed-air system, redesigned the trimmer and airbrake drives and installed a drag parachute, carrying out the test flights during October and November 1968.

On Monday 18 November 1968 came the factory fire described in the previous chapter. The HP-14C which had been intended for George Burton to fly in Poland was destroyed. All of the jigs and tooling and parts for several more HP-14s were lost. The newly modified aircraft however, had been derigged and put in its trailer outside the factory on the Saturday afternoon. After flying it during the day, Bailey-Woods found the available space rather scarce with the chairman's yacht and private aeroplane inside, so countermanded the instructions that had been given to the factory crew and left it outside. It therefore survived.

Provins continued work, but still lacked some parts of his kit. The company cut up the remaining HP-14C to obtain the missing components. Provins's aircraft was completed and test flown by Bailey-Woods in June 1969, eventually being accepted by the BGA and registered.

Not so the two in Australia. The subsequent bankruptcy of the company left these customers without many vital parts for wings and tails and no prospect of the rest of the kits ever being delivered. They were never recompensed. The two sailplanes were eventually fitted with Schreder's own design of T-tail, the required materials and parts being bought directly from Ohio. The full 18m-span Slingsby wingtips were retained. Permits to fly were eventually obtained after long bureaucratic procedures.

The New South Wales group's 18m span HP-14T broke up in clear air at 2,400m (8,000ft) during the Australian National Championships on 5 January 1972. The pilot, Jan Coolhaas, who had been flying at a moderate airspeed, saved himself by parachute. He had heard a loud bang, and the HP-14 entered a steepening spiral dive which he could not correct. After official investigation the cause was attributed to wing-aileron flutter which led quickly to fracture of the main fittings, loss of control and complete disintegration of the wing. The fittings that failed, forged by another company under contract to Slingsby, were of a different design from Schreder's and may have been defective. Presumably the deletion of the aileron mass balance, about which Schreder had never been happy, was also partly responsible. The Sydney Soaring Club's HP-14T also experienced flutter on a subsequent occasion, but did not break up. This 18m aircraft was then cut down to the original 16.7m span and the mass balances were restored, after which it proved very satisfactory. This machine and several other HP-14s in the USA and others in Australia, some built from Schreder's kits or, in one case, entirely from scratch, remain in service.

As all of the homebuilders discovered, by the time their sailplanes were finished and approved for flying they were out of date and no longer competitive with glass/plastic aircraft. The original HP-l 4 did not achieve the claimed 40:1 glide ratio. Flight measurements published in the USA showed that a well-built and carefully maintained HP-14T, even with slightly extended wingspan of 17.4m, achieved a best glide ratio of 1:36.3. This figure was exactly the same as best glide measured for the old wooden Skylark 4, although the metal aircraft achieved the figure at an 8kt faster airspeed and was appreciably better than the Skylark at speeds above 70kt. The 15m glass Libelle was better than both of them.