The aerodynamicists Harry Fraser-Mitchell and Barry Pegram were called in and concluded that the tailplane had stalled because of its increased negative angle of attack due to the airflow aft of the wing being deflected downwards – downwash. This is normally present behind a wing but the effect increases as the flaps are extended. In this case the amount of downwash due to full flap had caused the tailplane to stall. But what had suddenly triggered it? Again they had a possible explanation. Initially the undercarriage had been down which would have shed a wake onto the flaps and reduced their effectiveness and consequently the amount of downwash. When the undercarriage was retracted this wake was removed increasing the downwash sufficiently to cause the tailplane to stall. The subsequent lowering of the undercarriage restored the wake, reduced the downwash and un-stalled the tailplane, allowing pitch control to be regained.
    Subsequent flight tests proved this to be the case. With the undercarriage down there was no tailplane stalling while at the same conditions it could be provoked simply be retracting the undercarriage. Extending the undercarriage would then restore pitch control. We now had an understanding of what had happened and also a more accurate name for the phenomenon – tailplane stalling – which is what it was known as from then on. This still left the question of what to do about it.
    The solution turned out to be very simple. The parallel chord Hawk flaps had a full span vane supported on a number of brackets protruding from the leading edges of the flaps. Removing a section of the vane would reduce the effectiveness of the flaps and hence reduce the associated downwash. The section between the two outboard brackets, about one sixth of the span, was therefore lopped off as it was the most heavily loaded aerodynamically. This ‘cut-back flap vane’ was trialled to see if it was enough to resolve the problem; it was! The reduced flap effectiveness had a consequent effect on landing performance, requiring slightly higher approach and landing speeds, but this was still comfortably within the landing performance required by the Ministry of Defence (MoD) Hawk contract. The ‘cut-back flap vane’ thus became the standard for all Hawk T Mk 1s and export Mk 50s and 60s.
    At the end of the 1970s the US Navy published its requirement for a new pilot training system, known as the VTXTS, and BAe, as we had become, teamed with the Douglas Aircraft Company (DAC) to propose a system based on the Hawk. The Navy requirements meant that the Hawk would have to be modified for aircraft carrier take-offs and landings. Strengthening the Hawk for this task meant a significant increase in weight which, together with a particular carrier landing approach requirement, known as a ‘glide slope transfer manoeuvre’ was causing some concern.
    By this time the Hawk was being developed as a ground attack aircraft carrying increasingly heavy external store loads. The airframe itself was also being further developed to incorporate a suite of more modern avionics, to become known as the 100 Series, with a consequent increase in aircraft weight. A means was therefore needed to mitigate the impact of these on approach and landing speeds. We needed the full span flap vane back!
    Were this to be done on the back of the T-45TS contract, as the VTXTS had by then been named, BAe would have had to pay royalties to the US Navy for sales to other customers of any aircraft incorporating the solution. It was decided therefore that BAe would privately fund the development so that they would retain the rights to it and could hence use it royalty-free on any future Hawk variants.
    While the aerodynamicists had ideas of potential solutions they needed a more precise understanding of the magnitude of the downwash and its impact on the tailplane. Previously, to understand airflow behaviour on the wing, we had used ‘tufts’, lengths of parachute cord about 12 – 15 cm. long anchored at one end to the wing surface with adhesive tape (yes, it did work!) and filmed from cameras mounted in the rear cockpit looking outboard.
    Tufting the side of the rear fuselage and underside of the tailplane was no problem but how were we going to film them? There was no place to mount cameras on the aircraft that would give an adequate view. We had used a chase aircraft from which to film weapons separation trials on many occasions so the question was asked: could we do the same to film the tufts? The difference was that weapons trials took place in a stable flight regime at fairly high speeds whereas tailplane stalling occurred with the test aircraft not entirely under the pilot’s control in pitch and with both aircraft flying at a speed not much above the stall. After a bit of careful thought by Flight Test, the Test Pilots and Aerodynamics, it was decided that it was feasible but might take a bit of getting right.
    A full-span flap vane was therefore fitted to Hawk T Mk 1 XX338 and the appropriate areas of the fuselage side and tailplane were tufted. For the test flight XX338 was flown by Jim Hawkins accompanied by chase aircraft XX154 flown by Andy Jones with me and a 16mm ciné camera in the rear seat. The objective was to set the two aircraft up, with undercarriage down and flaps fully down, in formation where the camera would have a clear view of both the fuselage side and underside of the tailplane of XX338. For safety’s sake this couldn’t be too close, given the expected behaviour of XX338, but too distant and the tuft movements would not be clear in the film. Once ready, Jim retracted the undercarriage of XX338 and then Andy’s job became one of holding formation on another aircraft with a stalled tailplane by flying one that was at not much above stalling speed itself, while keeping it as stable as possible. This would then give me a chance of keeping the back end of XX338 not just in the camera frame but steady enough to be able to see what was happening to the tufts. The quality of the film we produced was testament to Andy’s flying skills.
    The aerodynamicists now had the more precise understanding that they needed and Barry Pegram devised some ‘tailplane vanes’. These were small, almost triangular surfaces the long axis of which ran parallel to the axis of the fuselage, with the apex forward, and with curved outer edges. They projected normal to the fuselage surface at a position where the vane trailing edge lined up with the leading edge of the tailplane when it was in the fully leading edge down position. In flight it was expected that at higher speeds the tailplane vanes would have no effect but at low speeds the shed vortex from the outboard edge would energise the airflow passing under the tailplane and enable it to remain attached to the under-surface, thus preventing the tailplane stall. Flight tests subsequently proved this to be the case and tailplane vanes became a feature not just of the T-45A Goshawk (as the US Navy’s official designation became) but of all 100 and 200 series Hawks.
    Having been successful in securing the US Navy contract we were, under the terms of that contract, required to provide verification of all aspects of the aircraft’s design to the US Navy Project Team, although this did not affect BAe’s rights to the non-T-45 specific design aspects, including the tailplane vanes. This verification had to be presented at both a Preliminary Design Review (PDR) and a Critical Design Review (CDR). By the time of the PDR, I had moved to the Design Department at Kingston as Project Designer T-45 and as such was involved in both these Reviews.
    Prior to the PDR the USN Project Team had been made aware of the incorporation of the tailplane vanes into the T-45A design and why. A letter was subsequently received from the US Navy acknowledging this and stating that, for the US Navy contract, these were designated Side Mounted, Unit horizontal tail, Root Fins - SMURFs! Given that in those days the Smurfs were popular childrens’ TV cartoon characters this designation produced a bit of bemusement within BAe; until someone noticed the date on the letter – 1st April - Yeah, right!
    At the subsequent PDR the US Navy Project team was given a more comprehensive explanation of the tailplane vanes and updated on progress to date. At the end of this a very unhappy US Navy Project Manager had a sense of humour failure and demanded to know why we were referring to tailplane vanes and not using the official SMURF designation - Oops!
    A copy of the letter was quickly produced and the date and its implications pointed out. The misunderstanding was simply explained. You have to remember that in those days (before every desk had a computer on it) letters were hand-written and then passed to a typing pool for production of the formal version. This letter had been written in the final days of March and then passed to the typing pool. There it had been typed up on 1st April and dated with that date, something that had not been noticed when it was returned to the writer for signature and despatch. Fortunately the Project Manager now saw the funny side of it and, as we promised to correct the designation, the normal very good relationships were restored.
    At the CDR we had the final design information and results from both high-speed wind tunnel and full envelope flight tests to present. The format of the overhead-projector slides (this was pre Powerpoint) for the Reviews was to have a title bar at the top flanked on one side by the T-45 project logo and on the other by either the BAe or DAC logo, dependent upon which company had design responsibility for the particular aspect. For the ‘SMURFs’ presentation we decided to introduce a little levity by replacing the BAe logo with the face of one of the well-known little blue men – a grumpy one for the explanation of the initial issue and a happy cheering one for the slides presenting the solution. Our DAC colleagues were decidedly worried about the reaction from the US Navy but as there was not enough time to produce alternatives we had to run with what we had. As it turned out the fears were unfounded as the presentation was the trigger for smiles and a couple of quiet sniggers from the US Navy Project Team.
    Editor’s Note: the Phantom Dive name came, I believe, from McDonnel’s F-4 Phantom which at one time also experienced tailplane stalling. The SMURFS were devised by Barry Pegram who, because of his beard, became known as Papa Smurf.