Dive bomber
A dive bomber is a bomber aircraft that dives directly at its targets in order to provide greater accuracy for the bomb it drops. Diving towards the target simplifies the bomb's trajectory and allows the pilot to keep visual contact throughout the bomb run. This allows attacks on point targets and ships, which were difficult to attack with conventional level bombers, even en masse.[1]
For other uses, see Dive Bomber (disambiguation).After World War II, the rise of precision-guided munitions and improved anti-aircraft defences—both fixed gunnery positions and fighter interception—led to a fundamental change in dive bombing. New weapons, such as rockets, allowed for better accuracy from smaller dive angles and from greater distances. They could be fitted to almost any aircraft, including fighters, improving their effectiveness without the inherent vulnerabilities of dive bombers, which needed air superiority to operate effectively.
Accuracy[edit]
When released from an aircraft, a bomb carries with it the aircraft's trajectory. In the case of a bomber flying horizontally, the bomb will initially only be travelling forward. This forward motion is opposed by the drag of the air, so the forward motion decreases over time. Additionally, gravity causes the bomb to accelerate after it is dropped. The combination of these two forces, drag and gravity, results in a complex pseudo-parabolic trajectory.
The distance that the bomb moves forward while it falls is known as its range. If the range for a given set of conditions is calculated, simple trigonometry can be used to find the angle between the aircraft and the target. By setting the bombsight to this "range angle", the aircraft can time the drop of its bombs at the instant when the target is lined up in the sight. This was only effective for "area bombing", however, since the path of the bomb is only roughly estimated. Large formations could drop bombs on an area hoping to hit a specific target, but there was no guarantee of success, and huge areas around the target would also be hit. The advantage to this approach, however, was that it is easy to build such an aircraft and fly it at high altitude, keeping it out of range of ground-based defences.
The horizontal bomber was thus ill-suited for tactical bombing, particularly in close support. Attempts at using high-altitude bombing in near-proximity to troops often ended in tragedy, with bombs both hitting their targets and friendly troops indiscriminately. In attacking shipping, the problems of inaccuracy were amplified by the fact that the target could be moving, and could change its direction between the time that the bombs were released and the time that they arrived. Successful strikes on marine vessels by horizontal bombers were extremely rare. An example of this problem can be seen in the attempts to attack the Japanese carriers using B-17s at altitude in the Battle of Midway, with no hits scored. The German battleship Tirpitz was subjected to countless attacks, many while in dock and immobile, but was not sunk until the British brought in enormous 12,000 lb (5,400 kg) Tallboy bombs to ensure that even a near miss would be effective.
An aircraft diving vertically minimises its horizontal velocity component. When the bomb is dropped, the force of gravity simply increases its speed along its nearly vertical trajectory. The bomb travels a virtually straight line between release and impact, eliminating the need for complex calculations. The aircraft simply aims at the target and releases its bombs. The primary source of error is the effect of wind on the bomb's flight path after release. As bombs are streamlined and heavy, wind has only a slight effect on them and the bomb is likely to fall within its lethal radius of the target.
Bomb sighting becomes trivial, requiring only a straight line of sight to the target. This was simplified as the aircraft was pointed directly at the target, making sighting over the nose much easier. Differences in the path of different bombs due to differing ballistics can be corrected by selecting a standardised bombing altitude and then adjusting the dive angle slightly for each case. As the bomber dives, the aim could be continually adjusted. In contrast, when a horizontal bomber veers offline while approaching the bomb release point, turning to the angle that would correct this also changes the speed of the aircraft over the ground (when there is a wind) and thereby changes the range as well.
In the 1930s and early 1940s, dive bombing was the best method for attacking high-value compact targets, like bridges and ships, with accuracy. The forces generated when the aircraft levels out at the bottom of the dive are considerable. The drawback of modifying and strengthening an aircraft for near-vertical dives was the loss of performance. Aside from the greater strength requirements, during normal horizontal flight, aircraft are normally designed to return to fly straight and level, but when put into a dive the changes in forces affecting the aircraft now cause the aircraft to track across the target unless the pilot applies considerable force to keep the nose down, with a corresponding decrease in accuracy. To compensate, many dive bombers were designed to be trimmed out, either through the use of special dive flaps (such as Fairey Youngman flaps) or through changes in tailplane trim that must be readjusted when the dive is completed.
The Vultee Vengeance, which was mostly used by the RAF and RAAF in Burma, was designed to be trimmed for diving, with no lift to distort the dive. The drawback was that it flew nose up in level flight, increasing drag. Failure to re-adjust trim made the aircraft difficult or impossible to pull out of a dive.[9]
A dive bomber was vulnerable to low-level ground fire as it dived towards its target, since it was often headed in a straight line directly towards the defenders. At higher levels, this was less of a problem, as larger AA (anti-aircraft) shells were fused to explode at specific altitudes, which is impossible to determine while the plane is diving. In addition, most higher-altitude gunners and gunnery systems were designed to calculate the lateral movement of a target; while diving, the target appears almost stationary. Also, many AA mounts lacked the ability to fire directly up, so dive bombers were almost never exposed to fire from directly ahead.
Dive brakes were employed on many designs to create drag which slowed the aircraft in its dive and increased accuracy. Air brakes on modern aircraft function in a similar manner in bleeding off excessive speed.[10]
Origins[edit]
It is difficult to establish how dive bombing originated. During World War I, the Royal Flying Corps (RFC) found its biplane two-seat bombers insufficiently accurate in operations on the Western Front. Commanders urged pilots to dive from their cruising altitude to under 500 ft (150 m) to have a better chance of hitting small targets, such as gun emplacements and trenches.[11] As this exposed the aircraft and crew to destructive ground fire in their unprotected open cockpits, few followed this order. Some recorded altitude at the top and bottom of their dive in log books and in squadron records, but not the steepness of the dive. It was certainly not near-vertical, as these early aircraft could not withstand the stresses of a sustained vertical dive.[12]
The Royal Naval Air Service was bombing the Zeppelin sheds in Germany and in occupied Belgium and found it worthwhile to dive onto these sheds to ensure a hit, despite the increased casualties from ground fire. Again, the angle of dive in these attacks was not recorded.[11]
Beginning on 18 June 1918, the Royal Air Force (RAF), successor to the RFC, ordered large numbers of the Sopwith TF.2 Salamander, a single-seat biplane. The "TF" stood for "Trench Fighter", and the aircraft was designed to attack enemy trenches both with Vickers .303 machine guns and with 25 lb (11 kg) bombs. Of the 37 Salamanders produced before the end of October 1918, only two were delivered to France, and the war ended before those saw action.[13] Whether the Salamander counts in more modern parlance as a fighter-bomber or as a dive bomber depends on the definition of "dive". It had armoured protection for the pilot and a fuel system to attack at low level, but lacked dive brakes for a vertical dive.
Heavy casualties resulting from air-to-ground attack on trenches set the minds of senior officers in the newly formed RAF against dive bombing. So not until 1934 did the Air Ministry issue specifications for both land-based and aircraft carrier-based dive bombers. The RAF cancelled its requirement and relegated the Hawker Henley dive bomber to other roles, while the Fleet Air Arm's Blackburn Skua was expected to do double duty: as a fighter when out of reach of land-based fighter support, and as a dive bomber. It had dive brakes that doubled as flaps for carrier landings.[14] The Hawker Henley had a top speed only 50 mph (80 km/h) slower than the Hawker Hurricane fighter from which it was derived. The American and Japanese navies and the Luftwaffe chose vertical dive bombers whose low speed had dire consequences when they encountered modern fighters.[12]
World War I[edit]
The Royal Naval Air Service developed dive bombing as a tactic against Zeppelin hangars and formed and trained a squadron at Manchester for this task. On 8 October 1914, a Sopwith Tabloid with two 50 lb (23 kg) bombs attacked a hangar at Düsseldorf after a dive to 600 ft (180 m). On 14 November 1914, four Avro 504s attacked the Zeppelin factory at Friedrichshafen on Lake Constance, diving from 1,200 ft (370 m) to 500 ft (150 m) to ensure hits. As Zeppelins were tethered close to stores of hydrogen, results were often spectacular.[11]
The first use of dive bombing by the RFC, which had been urging its pilots to drop bombs at heights below 500 ft (150 m) in order to hit within 150 ft (46 m) of the target since February 1915, was later that year. On 27 November 1915, Lieutenant Duncan Grinnell-Milne arrived in his Royal Aircraft Factory B.E.2c over railway marshalling yards near Lys in Northern France, to find the target already crowded by other bombers. He dived from 10,000 ft (3,000 m) to 2,000 ft (610 m) before releasing his 20 lb (9.1 kg) bombs. A few weeks later, Lieutenant Arthur Gould dived to just 100 ft (30 m) to hit buildings near Arras.[11]
The Royal Flying Corps developed strafing with diving aircraft using both machine guns and small bombs as a deliberate tactic. At the Battle of Cambrai on 20 November 1917, 320 Mark IV tanks and 300 aircraft, mostly Sopwith Camels and Airco DH 5s with 20 lb (9.1 kg) bombs, were used to suppress artillery and machine guns. The cost in pilots was very high, with casualties on some days reaching 30 percent.[15] The initial impact at Cambrai was highly successful. The staff officer to the Royal Tank Corps Lieutenant-Colonel J. F. C. Fuller published findings which were later taken up by Heinz Guderian to form the basis for the blitzkrieg tactics of using dive bombers with tanks employed by the Germans in 1939–40.[16]
Second Lieutenant William Henry Brown, a Canadian from British Columbia serving with the RFC and flying a Royal Aircraft Factory S.E.5a, made the first attack on a vessel on 14 March 1918, destroying an ammunition barge on a canal at Bernot near St Quentin, diving to 500 ft (150 m) to release his bombs. He was awarded the Military Cross for this and other exploits.[17] Brown's technique was emulated by other British squadrons. But the heavy casualties to unprotected pilots cast a pall over the results and influenced RAF thinking for 20 years.[12]