Por que um strake é colocado na fuselagem em frente ao tailplane na aeronave Hawk?

Eles são chamados de canardas de canardas (TCV) ou as barbatanas traseiras montadas na lateral (smurfs). Basicamente, eles são geradores de vórtices para resolver o problema do barramento sob a platina em ângulos altos (negativos).

Durante os testes de vôo do Hawk, descobriu-se que o tailplane sob o boxe da superfície estava causando o pitch incontrolável do nariz para baixo. De The Hawk Story:

It was found that at forward centre of gravity in that configuration, rapid fore-and-aft movement of the control column could induce an uncontrollable nose down pitch, with the nose down attitude and speed increasing quite rapidly. ... It was dubbed the “Phantom Dive”.

Os projetistas fixaram-no inicialmente removendo a palheta externa da aba; no entanto, uma solução melhor foi necessária e isso foi conseguido colocando uma palheta canard na frente do estabilizador. Os TCVs criaram um voretx, que garantiu que as superfícies de controle permanecessem eficazes, evitando a separação. A imagem abaixo mostra a melhoria.

Imagem de The Hawk Story:

A adição de TCVs resolveu o problema do plano da cauda sob o controle da superfície. Do mesmo documento:

With tail and TCV on, there is ample tailplane authority with aft stick (- $\eta_{T}$ ) at all angles of attack in the usable range.

The addition of TCVs was a cheap and effective solution of the problem of tailplane under surface stall, and allowed further development of the wing to try to achieve the low stalling speeds demanded for carrier operation in the US Navy, and for combat versions of the aircraft carrying heavy store loads.

O artigo tem muitos detalhes adicionais sobre como resolver esse problema, e eu sugiro que você passe por isso. De acordo com RAF Aircraft & Guia de Armas , a modificação foi incorporada no Hawk T2.

Você pode ter notado que o tailplane do Hawk é uma superfície em pleno vôo, também chamada de superfície em movimento. Quando o bastão for puxado para trás, a borda de ataque se alinhará com o bastão, formando, com efeito, um extensão de ponta . Isso produzirá um vórtice sobre a superfície inferior da parte interna da cauda e evitará a separação antecipada do fluxo na raiz da cauda da cauda em baixa velocidade.

A próxima pergunta óbvia é: por que não faz parte do próprio avião? Adicioná-lo ao tailplane aumentaria o arrasto e produziria forças de controle não lineares. Desde que foi adicionado mais tarde na vida do projeto, fixando-o à fuselagem permitiu manter as superfícies da cauda e sua atuação inalterada.

Observe que versões anteriores do Hawk (Mk.1, Hawk 50) não tinham esse detalhe. Em testes de voo, descobriu-se que o falcão mergulharia de repente com as abas cheias e se ajustaria. Este foi apelidado de "mergulho fantasma". Cortar as abas resolveu esse comportamento, mas depois, versões mais pesadas do Hawk precisavam de mais sustentação, então uma correção diferente tinha que ser encontrada. De a história de Hawk por Harry Fraser-Mitchell:

It was shown with the half model of the Hawk at Hatfield that high local downwash at the tail, coupled with the very large nose down pitching moment induced by the flap, was causing the tailplane to stall on its lower surface, so that it could no longer provide adequate balancing power. It needed more lift, extended to higher angles of attack.

In the case of the F-4K Phantom aircraft this was achieved by installing a fixed leading edge slot to the tailplane, harking back to demonstrations of such devices by Handley Page on wings in the early Twenties! A fixed slot with its associated drag was not an option on the Hawk although a cambered tailplane was tried on the model with some success. Removal of the outboard vane of the flap reduced the flap pitching moment to such a value that the standard tailplane could cope, so this was the quick solution for the RAF. However, for the US Navy VTX project, (and for later combat versions of the Hawk) the maximum possible lift was required, so that at least the outer flap vane had to be replaced. The dive phenomenon had to be fixed.

An example of the cross-fertilization of knowledge due to the matrix working of the design department now occurred. Barry Pegram, then Section leader of the Fluid Dynamics section of the Aerodynamics Department, had been working on the adoption of leading edge root extensions (LERX) for the “Harrier” wing, and this work had shown that these devices extended the lift of the wing to higher angles of attack by virtue of the non-linear lift developed by the vortex flow they created. He proposed that these should be added to the tailplane of the Hawk model, but this would have had a serious effect on the tailplane hinge moments. The author, who was working with Barry in the V/STOL tunnel at Hatfield, suggested that the ‘tailplane canard vane’ (TCV), as it was called, could be fixed to the fuselage at such a position that it was lined up with the flow at normal conditions, but with its trailing edge adjacent to, with a small clearance, the leading edge of the tailplane at its maximum nose down position.

Experimenting showed that these vanes could be made quite small and they gave a complete cure to the problem in the wind tunnel, with very little drag in the normal flight regime. To prove the concept in flight, some temporary vanes were manufactured which could rapidly be fitted to one of the test aircraft which had flaps with the full vane. First, the aircraft was flown without the TCV to establish the conditions under which the ‘Phantom Dive’ occurred on that particular aircraft. On a later flight, the TCVs were fitted and the aircraft flown again to the identical conditions as before. Despite every attempt by the pilot to instigate the phenomenon, it did not occur – the vanes were a complete success, even though they looked inconspicuously small for such a large effect.