When the single seat Vampire Mk 30 with a Goblin engine was upgraded to the Mk 31 with the more powerful RR Nene engine, a pair of additional air intakes were installed behind the canopy on top of the engine. They were known as "Elephants Ears" The wing root intakes were retained.
During Vampire conversion training at No 2 Fighter Operational Training Unit at Williamtown NSW, there were three fatal accidents I think in the very early Fifties when the Vampires concerned went into Mach tuck of sorts but known then as compressibility. The cause was shock waves forming over the air-intakes at around 0.78 mach number and seriously affecting airflow over the tail surfaces. One accident occurred during a practice quarter attack where one Vampire rolls in and attacks another Vampire flying straight and level as a camera gun target. I think the aircraft were around 15,000 ft.
In this case, the attacker dived towards the target aircraft but got into compressibility. The pilot reported on radio that his Vampire was vertical and the control column had no effect. Although the air was thicker at low altitude where the mach number is less, the pilot ran out of altitude to pull out of his dive and went in vertically still trying to pull out. The early Mk 31 Vampires did not have ejection seats and the speed was too great for the pilot to bale out.
Also before or just after this accident, two more Vampire Mk 31 crashed almost in formation while in compressibility. They had been conducting formation aerobatics where the technique was to extend the dive brakes and close the throttle as the aircraft passed through the top of the loop. This was to prevent a dangerous increase in speed during the recovery dive.
However, if I recall correctly, the lead Vampire did not extend its dive brakes and thus quickly accelerated during the recovery from the loop. The second Vampire held good formation but failed to realise the leader had left the dive brakes until too late. Both aircraft entered compressibility during the dive and were vertical at 10,000 ft with insufficient height to pull out.
In each case the control column becomes virtually useless until the aircraft reaches thicker air and the mach number drops off and so do the shock waves over the engine intakes. Again, with no ejection seats there was no hope of baling out. In all cases the pilots were able to transmit what was happening. All the pilots were inexperienced with around 400 hours total flying time each.
Whether these accidents can be put down to mach tuck I don't know because the cause was shock waves over the newly installed intakes but the effect was certainly the same. After these prangs, the Australian test pilot "Blackjack" Walker was tasked to explore the problem with the air intakes.
In his book Black Jack, he explained (edited for brevity): "After we had delivered half a dozen Vampires to the Air Force and they appeared to be functioning quite well from the RAAF station at Williamtown, two Vampires after flying in formation went into the ground in an almost vertical dive. Apparently they never even looked like recovering.. So De Havilland, to whom the matter had been referred, told me. "You, Mr Test Pilot, had better take the next Vampire up and see what happened to those two aeroplanes"
Blackjack Walker continued his story: "Well do I remember that flight. I knew it must have something to do with compressibility. I took the Vampire to well over 40,000 feet and put it into a steep dive. Once the aircraft went over Mach 0.80, the nose got heavier and heavier and the aeroplane kept endeavouring to go past the vertical and the controls were largely ineffective.
So I closed the throttle and put on the dive brakes immediately and this would have been somewhere around 27,000 ft. The aeroplane obviously had to be got out of a very sticky situation. As it got into the lower denser atmosphere, the dive brakes started to slow it up. I was even thinking of throwing the undercarriage out which would probably have destroyed the fairings - but anything to slow it up. Otherwise I would not come back with any answers.
We eventually came out of that dive at about 13,000 ft. It was pretty close and as the characteristics were so different to the English Vampire at high mach numbers it seemed to me it must have something to do with those wretched Elephants Ears on the upper surface of the fuselage.
We redesigned the intakes and placed them on the bottom of the fuselage. I took the aircraft to beyond 40,000 ft an put it into a dive of about 75 degrees. This time instead of ducking its nose down, to my relief once I reached Mach 0.8 the nose began to rise. This was in 1951. After that ejection seats were fitted"
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As a matter of personal interest this scribe flew the Mk 31 Vampires during fighter training. Session No 2 was to take the Vampire to high altitude and carry out what was called a Mach Run, in order to experience the onset of compressibility or mach tuck. Of course we didn't quite know what to expect although we had been briefed about the previous accidents. At this stage the modifications to the intakes had not reached our squadron, so the risk of mishandling was still there. Since we wouldn't have a clue if the first nibble of compressibility was there, an instructor in another Vampire would fly in close formation with us. He would stick on our wing tip as we commenced the dive towards compressibility. As the instructor felt the onset of compressibility and the control column losing effectiveness in his own aircraft he would order the other Vampire to close the throttle and extend the dive brakes before things got too dangerous.
Later, after the modded Vampires arrived on the scene we would delight in climbing to height then rolling inverted and pulling through past the vertical and passing mach 0.75 or thereabouts, the Vampire would pull sharply out of the dive on its own accord without action by the pilot instead of the old problem of bunting over into a lethal situation.
The modded Vampires were fitted with Martin-Baker ejection seats, which was a relief. The only problem with these ejection seats was that tall long legged pilots were in danger of losing their knees during the ejection sequence because of the lack of space in the cockpit. Being vertically challenged (as I was), was thus considered a Good Thing instead of being a normal object of amused contempt
