by Colin Downes
Shortly afterwards I made my first acquaintance with Dr Ernst Mach and the effects of compressibility during high-speed flight. At high altitude in a vertical dive the Mustang, with its low drag ratio from the laminar flow wing design, could run into compressibility problems as the aircraft approached its critical Mach number relating to the speed of sound. The speed of sound at sea level is around 760 mph depending on temperature, and reduces with altitude and temperature until at 35,000 feet the equivalent air speed is less than half that at sea level. The airflow passing over parts of the aircraft will diverge or break away as this airflow approaches the speed of sound. Mach numbers are used to denote the percentage of the speed of sound at a certain altitude, with Mach One representing the speed of sound at that altitude. Every aircraft has a critical Mach number when the airflow over the wing reaches compressibility and shock waves cause the airflow to break away resulting in loss of control. The laminar flow design of the Mustang wing endeavoured to delay this process, and sweeping back the attack angle of the wing on jet aircraft achieves the same purpose. The shock waves created at sonic speed results in increased drag with an ineffective response from the flying controls and even reversibility of the controls; and during the early development of jet aircraft this condition of flight became known as the sound barrier. It was some time before engine design produced sufficient thrust and the use of a swept wing enabled the aircraft to push through the sonic barrier into supersonic flight where the flying controls once more responded normally. The shock wave as the aircraft passed through the sound barrier at Mach One produced a sonic boom that passed along the ground following the flight path of the aircraft. The general effect on a subsonic aircraft as it approached its critical Mach number was a violent vibration and sometimes an uncontrollable pitch up or pitch down of the aircraft, with the controls becoming unresponsive or reversible. As the altitude reduced during the dive the air became denser and warmer, and the critical Mach number for the aircraft rose in ratio. The shock waves then disappeared and the airflow resumed a smooth passage over the contour of the airframe, allowing the pilot to regain control of the aircraft. In the process of attempting to control the aircraft near the critical Mach number a high stress loading was possible that could result in popped rivets, stress fractures and even the break-up of the aircraft. In 1946 Geoffrey de Havilland disappeared into the North Sea during high speed flight testing of the swept wing and tailless DH-108. Although developed from the Vampire, the DH-108 was redesigned so that the twin boomed tail became a single vertical fin and rudder with no horizontal stabilizer and elevator. It was not intended to be a fighter replacement to the Vampire but to be used purely as a research aircraft. Three prototypes were produced and all three crashed, with the second aircraft breaking-up in mid-air. Investigations concluded that during transonic flight an undamped oscillation resulted in a violent pitch up causing the aircraft to shed its wings. Subsequent de Havilland designs reverted back to a reduced sweep to the wing and a conventional tail plane. The aerodynamic phenomenon of sonic flight was not fully appreciated by pilots until the jet fighter arrived on the scene because propeller driven fighters usually experienced too much drag to run into compressibility problems. Some fighters such as the Spitfire and Mustang, which possessed low drag design factors, could approach a speed in a vertical dive from high altitude close to that of a subsonic jet fighter in level flight.
My first acquaintance with compressibility problems occurred during an escort while flying at over 30,000 feet. No one on the squadron thought to mention to me the problems associated with the Mustang when dived vertically from high altitude. The squadron was stacked up in three flights of four aircraft to above 30,000 feet, with my flight providing the top cover. I saw nothing when the squadron commander called the bounce on some aircraft 5,000 feet below us, and my flight peeled over into a vertical dive. I was flying in the number four position and some distance out from my section leader. I followed my leader with a half roll onto my back and pulled through to a vertical dive, a manoeuvre some referred to as a split ‘S’, and by others as ‘a split arse’, turn. The airspeed indicator ran rapidly off the clock and the aircraft started to shake with a strong vibration. I looked out at the wings and saw the ailerons fluttering up and down. I pulled back on the stick to pull out of the dive but the elevators failed to respond and I continued vertically downwards. I then attempted to slow down by closing the throttle and moving the propeller to full fine pitch to provide more drag. In order to provide still more drag I attempted to open the radiator flap fully and tried to extend some 10 degrees of flap; but none of these functions had any effect on what appeared to be a vertical terminal dive. As I passed through 25,000 feet there was still no response from the elevators despite pulling as hard as I could. I had by this time lost sight of the other aircraft, and it then struck me that if I continued vertically in this manner there was no possibility of baling out at this speed. Nothing seemed to affect my vertical progress towards terra firma and by now there was an element, if not of terror, certainly of some apprehension of terra firmly within the cockpit. As I passed 20,000 feet the aircraft vibration stopped; the ailerons stopped fluttering; and I felt control coming back to the stick from the elevators. Continuing to pull hard on the stick I pulled out of the dive with my eyeballs somewhere in my flying boots. I was then below 15,000 feet and on reaching 10,000 feet I looked around and found myself alone and somewhat shaken. I started an orbit and seeing nothing I returned somewhat sheepishly to base. It was some time before I fully understood the aerodynamics involved and I had a dressing down from my leader for losing him and not covering his tail during the ensuing mêle. He was an old hand on the squadron and expressed surprise that no one had thought to brief me on the problem of vertical power dives in a Mustang. I replied that I was used to flying a Spitfire at 300 feet and not a Mustang at 30,000 feet, to which he responded, ‘Well, think of it this way: a few moments of practical demonstration are worth a lifetime of theory.’
The missions of No. 122 Wing during the second half of 1944 involved: escorting Bomber Command’s strategic day raids on the industrial targets of the Ruhr and V-1 and V-2 launch sites, tactical support of the allied advance through France, Belgium and Holland and attacks on road, rail and water communications and the marshalling areas. Air supremacy ensured low interception by the Luftwaffe day fighters already diminished by the split of the fighter force between the Eastern Front and the Western Front where the bulk of the available Luftwaffe fighters was engaged in an air battle of attrition with the US 8th Air Force. Bomber Command’s heaviest bombs were the 12,000lb deep penetration bomb and the 12,000 lb ‘Block Buster’. The huge 22,000 lb ‘Grand Slam’ penetration bomb did not become operational until 1945, but I once got a good view of this bomb from the air during the dropping trials. Due to its great size and weight the only aircraft capable of carrying this bomb was the Lancaster bomber. In order to attach the bomb the aircraft was drastically modified by removing the bomb-bay doors, together with the nose and mid-upper gun turrets. I was one of four Mustangs escorting the Lancaster to the bombing range at Martlesham Heath and while up close and slightly below the Lancaster I could appreciate the great size and beautiful proportions of the bomb. Unlike the tin can shaped ‘Block Buster’ it was conventional in design to achieve the momentum to penetrate deeply into the ground before detonation to fracture the foundations of deep shelters. I was level with the Lancaster when it released the bomb and relieved of ten tons of dead weight the Lancaster leapt some 200 – 300 feet above me. In my surprise I missed seeing the massive bomb hit the target area below. I remembered the occasion we lost a Spitfire dive bombing and the crash crew recovered the Merlin engine about thirty feet down in the marsh. I wondered how far down the recovery crew would have to dig to retrieve the ‘Grand Slam’ bomb.
Another task that arose during 1944 involved attacking the V-1 flying bombs launched from coastal sites in Belgium. The V-1, often referred to as the ‘buzz bomb’ or
‘doodlebug’, had a pulse-jet engine and carried a 1-ton warhead. It had a short range of around 150 miles and flew between 350 – 400 mph at below 3,000 feet. To intercept these missiles required the fastest fighters in the RAF and the task fell to the latest Spitfire XIV with a 2,050 hp Rolls Royce Griffon engine and a top speed around 450 mph. The Spitfire XIV, armed with four 20 mm cannon, claimed the majority of the V-1 kills by day with the Mosquito claiming most of the V-1s shot down at night. The AA guns, the new Hawker Tempest and modified Mustang IIIs accounted for most of the rest of the kills claimed, with a few claimed by the first and only Meteor jet squadron just before the end of the war. In order to catch the V-1 flying bomb the Mustang required modification to the Merlin engine to produce its maximum performance at 5,000 feet. The modification to the two-stage, two-speed supercharger increased the manifold boost from the normal 60 – 80 inches, while using the new 130 octane fuel instead of the usual 100 octane gasoline. This placed a greater strain on the engine producing problems of overheating and the occasional engine failure. Coolant leaks were a common problem and under these operating conditions they increased. An engine change normally occurred every 200 hours and with the increased manifold boost this was reduced by half, as the use of the higher octane fuel caused a reduced engine life through valve burning.
Attacking a V-1 was a hazardous and difficult task. Although it flew straight and level it flew at high speed and presented a small target. Also, because it was constructed from sheet metal it was less vulnerable to cannon and machine-gun fire than a manned aircraft. The hazardous aspect of an attack concerned the 2,000 lb of explosive in the flying bomb, for if the pilot fired at a range closer than 300 yards there was a distinct danger of going down with the V-1 if the warhead exploded. The main danger in flying through the debris of an exploding V-1 was from a rear non-deflection attack. However, if the circumstances permitted and they seldom did, a well-executed high rear quarter attack requiring a deflection shot not only presented a greater target area but offered a better prospect of avoiding any explosion. Such an attack became more feasible when the Meteor entered operational service late in 1944. The average burst of fire during an attack was around three seconds as it was rare for a pilot to hold his aim for longer. The Spitfire with the 20 mm cannon shell had an advantage over the Mustang of impact damage in that a burst of three seconds from four 20 mm cannon produced considerably more weight of metal and explosive than the six 0.5 in Browning machine-guns. However, the six 0.5 in Browning machine-guns were more accurate over longer ranges. Tactics evolved to bring down the V-1 that avoided flying into a possible explosion from the warhead by toppling the gyroscope guiding the bomb. These tactics included the fighter flying in formation on the V-1 and the pilot attempting to get the V-1 to roll by placing one wing over that of the V-1 thereby removing the lift from that wing and causing the V-1 to roll; or under the wing to strike physically the flying bomb’s wing tip upwards with the fighter’s wing tip. The V-1 had wings skinned in rolled steel, whereas the fighter’s wing tips were of far weaker light alloy so that actual contact with the V-1 wing was hazardous and several attempts at this manoeuvre resulted in a damaged wing tip or aileron. Consequently, not many pilots adopted this method of downing the V-1, but among those claiming successes with the wing tipping manoeuvre were the gallant and tenacious Polish pilots on the RAF Polish Mustang squadrons. As it was necessary to tip the V-1 wing through 90 degrees in order to trip the gyroscope, there was a likely chance of damaging a wing tip or an aileron in the process. Several Polish pilots landed with a damaged wing after an attack on a buzz bomb in this manner, and on one such attempt the pilot was killed when his Mustang crashed after a jammed aileron caused his aircraft to roll uncontrollably.
By the later part of 1944 the V-1s with their short range operated from mobile launch sites on the Belgian coast. To intercept the V-1s Fighter Command carried out standing patrols called Diver Patrols over the English Channel and to the north of the Dover Strait. Pairs of fighters patrolled at 10,000 feet awaiting instructions from sector control for a vector to intercept the buzz bomb. The fighters then went into a slight dive to gain maximum speed at full throttle and maximum revs to intercept the V-1 that was usually heading for the London area. Around London was a defensive zone for the guns of AA Command, and barrage balloons flew over sensitive areas such as Buckingham Palace, Westminster and the Port of London. While in the dive there was some apprehension whether the engine would give up the chase while gaining slowly on the V-1 at over 450 mph. The tendency was to open fire out of range for fear of flying into the debris should the warhead explode. Firing at 400 yards or more at such a small target from a bucking fighter meant the shells or bullets were sprayed around the V-1. To ensure a hit on the V-1 the pilot needed to be patient to the extent of waiting until he was within the 300 yards range and it was difficult to keep the gun sight pipper on such a small target for more than three seconds; requiring short bursts of fire. As far as the Mustang was concerned the six Browning guns gave a total of 80 rounds per second for 20 seconds before the four outer guns fired out leaving an additional ten seconds of firing for the two inner guns. Thirty seconds is a long time to be firing at a target yet the V-1 often, and maddeningly, continued its burping way undisturbed with no apparent ill effects. Most pilots did not consider it was a fair exchange to sacrifice their fighter for one V-1, although I am sure there were some on the ground that would offer another view.
The Mustang squadrons of No. 122 Wing had served either with 2nd Tactical Air Force in France or with Desert Air Force in Italy, and as such were completely independent squadrons with their own technical, administrative, medical and motor transport services. In addition, each squadron had a small single engine two-seat Auster monoplane, similar to the Piper Cub, for communication duties. The aircraft was intended for use as an airborne taxi, although it spent much of its flying hours on recreational jaunts. The aircraft used 80 octane gasolines suitable for motor vehicles, and for some this was an attractive means to supplement the recreation gasoline allowance. The 80 octane fuels did not contain the colouration to identify the higher 100 octane aviation fuels used by the Mustangs. The colouration enabled detection by the Special Investigation Branch (SIB) units investigating the pilfering of aviation fuel. A more relaxed and informal approach to discipline and the dress code existed on the wing as was the normal case within Fighter Command. Crumpled and stained dress caps in the best operational and Pilot Officer Prune fashion abounded. Pilot Officer Prune was a cartoon character used in the RAF flying training publication, ‘Tee Emm’, to denote anything that resulted in a careless and stupid approach to flying. One issue contained the poem that summed up ten typical examples of Pilot Officer Prune’s approach to flying.
Ten Little Fighter Boys
Ten little Fighter Boys, taking off in line,
One was in coarse pitch, and then there were nine.
Nine little Fighter Boys, climbing ‘through the gate’,
One’s petrol wasn’t on, and then there were eight.
Eight little Fighter Boys, ‘scrambling’ up to Heaven,
One ‘weaver’ didn’t, and then there were seven.
Seven little Fighter Boys, up to all the tricks,
One had a ‘hangover’, and then there were six.
Six little Fighter Boys, milling over Hythe,
One’s pressure wasn’t up, and then there were five.
Five little Fighter Boys, over France’s shore,
One flew reciprocal, and then there were four.
Four little Fighter Boys, joining in the spree,
One’s sight wasn’t on, and then there were three.
Three little Fighter Boys, high up in the blue,
One’s rubber pipe was loose, and then there were two.
Two little Fighter Boys, ‘homing’ out of sun,
Flew straight and level, and then there was one.
One little Fighter Boy, happy to be home,
Beat up dis
persal and then there were none.
Ten little Spitfires, nothing have achieved,