Originally Posted by
RetiredBA/BY
Some thread drift:
The removed section of flap vane on the Hawk cured the strong pitch down with flap, as discovered by Duncan Simpson on its first flight. As the removed section is outboard of the tailplane span, can anyone explain just how that works ?
Below is an excerpt from an aerosociety.com article...
I have a feeling that the the 'Phantom Dive' IIRC did not occur on XX154 (1st Hawk) but on some of the subsequent A/C and came as a nasty surprise,but sorted fairly quickly by removal of the outer flap vane.
https://www.aerosociety.com/media/48...hawk-story.pdf
(1)....Stall behaviour The stall, as first experienced, occurred at a good low speed, but with very little buffet warning. One or other wing dropped suddenly and uncontrollably, though the aircraft did not depart into a spin and recovery was normal, rolling out to controlled flight. But clearly this was not acceptable. Flow visualisation revealed that a sudden ‘leading edge’ type of stall was occurring, originating at about mid-semispan. This was perhaps partly due to an aerodynamic concession, in that for production simplicity (and hence lower cost), the trailing edge flaps had been made with constant chord and section. But they were fitted to a fairly sharply tapered wing (taper ratio 0.34. tip to root) and so the ratio of flap chord to wing chord was at its highest at the outboard end of the flap and required too much of the flow at the local leading edge. Guided by work on the half model, the flap vane on the outer part of the flap was removed to detune it somewhat, at the cost of some maximum lift. Although the initial separation of the flow still occurred at the same point, the flow breakdown at the stall was kept from rapidly spreading towards the outer wing by the judicious positioning of a large fence. Buffet warning was obtained, at the cost of a little more maximum lift by putting triangular section “breaker strips” on the leading edges, inboard to give warning and outboard to give repeatability. As is related later, the outboard end of the flap vane was removed for another reason, and together with the devices above now gave acceptable behaviour, but lost about 5 knots of stalling speed. However, there was enough of a margin in maximum lift coefficient to meet the field performance requirements for the RAF. Clearly there was scope for much more fine tuning and investigation of more refined stall fixes, but there was a tight deadline to meet for the RAF, and this work was left to be done on the later developments.
(2) Phantom Dive ....It was first discovered when recovering from a stall with full flap and undercarriage up. 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. Recovery was straightforward, either by retracting the flap a few degrees, or by extending the undercarriage, but this was not acceptable as an operation, even though the configuration was unlikely to be used normally. It was dubbed the “Phantom Dive”. . 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. 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.