Dive bomber


Dive bomber
An SBD Dauntless drops its bomb. The dive brakes are extended and are visible behind the wings.

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 reduces the distance the bomb has to fall, which is the primary factor in determining the accuracy of the drop. Additionally, as the bomb's motion is primarily vertical, the complex parabolic trajectory is reduced to one that is much straighter and easy to calculate - even by eye. The rapid vertical motion of the aircraft also aids it in avoiding fire from anti-aircraft artillery, although diving to low altitude offsets this advantage as it brings the aircraft into range of smaller weapons.

A true dive bomber dives at a steep angle, normally between 45 and 90 degrees, and thus requires a very short pull-up after dropping its bombs. This demands an aircraft of extremely strong construction, and generally limited the class to light bomber designs with ordinance loads in the range of 1,000 lbs. This type of aircraft was most widely used before and during World War II; its use fell into decline shortly afterwards. The most famous example is the Junkers Ju 87 Stuka which was widely used during the opening stages of the war; during pullout, the forces were so great that the crew would often suffer G-LOC, and the controls were automated to avoid loss of the aircraft. Another famous design of the war is the Douglas SBD Dauntless, whose actions during the Battle of Midway changed the course of the Pacific War in favour of the US over a period of minutes.

It is also possible to bomb from a much shallower dive angle, which is sometimes described as dive bombing, but more generally known as glide bombing. Shallower diving angles reduces the benefits in terms of accuracy, but still serves as an aid in keeping the target visible during the bomb run and helping avoiding anti-aircraft fire. The Junkers Ju 88 was widely employed in glide bombing and was equipped with special bombsights operated by the pilot for this task. Likewise, the Heinkel He 177 is often mentioned as having its development upset by the demand for it to dive bomb, although this too was an example of glide bombing. Contrast glide bombing with the glide bomb, where the aircraft remains level and the bomb glides towards its target. Attachments for this sort of bombing were fitted to examples of the famous Norden bombsight, but in practice this concept proved unworkable.

Contents

Technique

Horizontal bombing

When released from an aircraft, a bomb carries with it the aircraft's velocity. In the case of a bomber flying horizontally, the bomb will initially be travelling forward only. This forward motion is opposed by the drag of the air, so the forward motion slows over time. Additionally, gravity provides a constant force on the bomb, accelerating it downward. The combination of these two forces, drag and gravity, results in a pseudo-parabolic trajectory of some complexity. For aiming purposes, the key calculation needed from this trajectory is the distance the bomb will travel forward while it falls, a distance known as the "range". The bomber's task is to fly along a line to the target until it reaches this distance from the target, and drop the bombs at that instant.

In the past, aircraft did not have navigation systems that could direct an aircraft towards an arbitrary point in space.[Notes 1] Instead, navigation was carried out in relation to objects on the ground - whether they be visual indications or radio beacons. For bombing, after calculating the range, and knowing the altitude, simple trigonometry could be used to calculate the angle between the current location of the aircraft and the point on the ground where the bombs would impact if dropped at that instant. By setting the bombsight to that angle, the "range angle", the aircraft simply had to approach the target and drop its bombs when the target appeared lined up with angle set in the sights.

As the trajectory of the bomb is complex, solving the range and range angle calculation is also a complex problem. This is normally accomplished by looking up data measured on a bombing range and reduced into table form. Any changes in speed, direction or altitude required all of this to be looked up again. In order to reduce this workload, mechanical calculators were increasingly common during World War II, including the famous Norden bombsight and the less well known British Mark XIV bomb sight and German Lotfernrohr 7. Even with these calculators, accuracy was often poor due to inaccuracies in the ballistics of individual bombs, wind measurements, or setup errors. Moreover, the need to fly in a straight line toward the target made it easy for anti-aircraft artillery to aim at the bomber, which demanded that the aircraft fly higher to avoid fire, and thereby magnified any errors in setup. In spite of enormous efforts, accuracies for horizontal bombing throughout the war was generally measured in thousands of yards.

Dive bombing

Consider the same aircraft, now travelling vertically instead of horizontally. In this case there is no horizontal velocity when the bomb is dropped, so the force of gravity simply increases the speed along the existing vertical trajectory. The bomb will travel in a straight line between release and impact, eliminating all of the complex calculation and setup required in the bombsight. Instead, the aircraft can simple point itself directly at the target and release the bombs, the only source of error being the effects of winds after release. For bombs, which are well streamlined and relatively dense, wind has a very small effect, and the bomb is likely to fall within its lethal radius of the target.

Diving perfectly vertically is by no means simple, especially when you consider the forces generated when the aircraft has to return to horizontal flight after the drop. But more generally, as the aircraft tilts further from the horizontal, the horizontal component of its own airspeed is reduced, which reduces the range. At some point, for a given altitude and dive angle, the trajectory so closely matches a straight line that bomb sighting becomes a trivial exercise and a straight line sight is all that is needed. Differences in the path due to the ballistics of different bombs can be accounted for by selecting a standardized bombing altitude and then adjusting the dive angle slightly for these different cases.

In these examples, accuracy of the drop is primarily a function of the accuracy of the pilot or bombardier's ability to accurately sight the target. This is aided by the fact that the aircraft is pointed towards it, making sighting over the nose dramatically easier. In addition, the target continues to approach as the bomber dives, allowing the aim to be progressively adjusted over time. In comparison, if a horizontal bomber notices that it is off the line directly over the target when the range angle is reached, there is nothing they can do - turning to the angle that would correct this would also change the groundspeed of the aircraft (at least in the presence of wind) and thereby change the range as well.

For this reason, dive bombing was the only method of providing the accuracy needed to attack high-value point targets like bridges and ships. They were a common feature of most naval air services, and many land-based air forces as well.

On the negative side, optimizing an aircraft for near-vertical dives came at the expense of performance. In addition, a dive bomber was highly vulnerable to ground fire as it dived towards its target. Dive brakes were employed on many designs. These created drag which slowed the aircraft somewhat in order to increase accuracy and to prevent speeds which could damage the structures of the plane. These were almost exclusive to dive bombers, though the air brakes fitted to modern aircraft are often of a similar design.

History

World War I and inter-war period

The Aichi D1A2, a carrier-borne dive bomber.

The first recorded use of dive bombing was an ad-hoc solution by British Royal Flying Corps pilots during World War I. In 1917 and 1918, they practised the technique at the Orford Ness Bombing Range, but the aircraft of the day were generally too frail to be able to withstand the acceleration generated when pulling out of the dive after releasing the bomb load.

The first combat dive bombing attack took place in early 1919 when United States Marine Corps pilot Lt. L. H. Sanderson mounted a carbine barrel in front of the windshield of his Curtiss JN-4 (an unarmed training craft) as an improvised bomb sight that was lined up with the long axis of his plane, loaded a bomb in a canvas bag that was attached to the plane's belly, and launched a single-handed raid in support of a USMC unit that had been trapped by Haitian Cacos rebels. Sanderson's "Jenny" almost disintegrated when he pulled it into a steep climb after releasing his ordnance, but the bomb had hit its target precisely and the raids were repeated. During 1920 Sanderson familiarized aviators of USMC units at the Atlantic coast with the dive bombing technique.[1] Dive bombing was also used during the United States occupation of Nicaragua.[2]

Interwar era

As planes grew in strength and load capability, the technique became more valuable. By the early 1930s, the technique was clearly favoured in tactical doctrine, notably against targets that would otherwise be too small to hit with level bombers. In the 1920s the US Navy ordered the first custom dive bomber aircraft, the Curtiss F8C Hell-Diver biplane (not to be confused with the later SB2C Helldiver). The Imperial Japanese Navy followed by ordering the Heinkel He 50 in 1931, which they developed into their own Aichi D1A. Numerous examples followed, including the US SBC, Japanese Aichi D3A and others. Because navies operated from aircraft carriers or small airfields, they had "smaller numbers of aircraft available for any one attack, and each aircraft was often unable to carry more than a few bombs per plane."[3] They also were frequently required to attack smaller-sized or moving targets, such as ships. The combination of a small bomb load and the need for accuracy made dive bombing techniques a requirement for naval airplanes.[4]

Land-based forces proved generally less interested in the dive bomber role. Accurate bombardment of point targets at long distances did not appear to be a military requirement, and at shorter ranges artillery could already fill any demand. Development concentrated primary on ground attack aircraft, which were intended to attack primarily with guns and cannon against infantry and light armour. Examples include the Fairey Battle, Henschel Hs 129 and Ilyushin Il-2. However, Luftwaffe experience in Spain demonstrated the value of the dive bombing technique, repeatedly attacking high value targets and causing damage to the enemy out of proportion to the size of the force. This led to the famed Junkers Ju 87, and in turn, to many other air forces starting dive bomber efforts of their own.

One notable holdout was the US Army Air Corps (USAAC). In the 1930s a new generation of bombsights like the Norden were being introduced, which suggested that level bombers could attain accuracies somewhat similar that those of the dive bombers. Although the accuracy would not be as great, an aircraft flying horizontally would not be subject to the great stresses of diving, and could be built to hold a dramatically greater warload. Thus, any loss of accuracy could be made up by carrying more bombs, increasing the chance that one would hit. However, this aircraft would also be able to fly at high altitude throughout the attack, greatly increasing its odds of surviving. This thinking led to a great debate in military aviation circles. The US Navy, developing the Norden, made plans for most of its new aircraft to be able to level bomb. However, they also continued development of dive bombers and torpedo bombers as the best method of attacking ships was not clear at that time. The USAAC confidentially predicted that the Norden would allow it to attack ships with ease, and designed their strategy around long-range bombers like the Boeing B-17 Flying Fortress which would be able to counter any seaborne attack at long range. Work on dive bombers and attack aircraft was scaled back dramatically.

World War II

The only major force not to deploy a dedicated dive bomber were the inventors of the tactic, the British. The Royal Navy attempted to introduce their own on several occasions, but were never able to do so due to various reasons, not the least of which was political interference by the RAF.[citation needed] They only produced hybrid aircraft: the Blackburn Skua, a dive bomber/fighter that was used for a short time and in small numbers, and the Fairey Barracuda, a dive bomber/torpedo bomber.

European theater

Stukas during action, Eastern Front

In the early 1930s, Ernst Udet visited the U.S. and was able to purchase four F8Cs and ship them to Germany, where they caused a minor revolution. The dive bombing technique would allow a much smaller Luftwaffe to operate effectively in the tactical role. Soon they had sent out contracts for their own dive bomber designs, resulting in the gull-winged Junkers Ju 87 Stuka (a contraction of Sturzkampfflugzeug, literally "dive-combat-air-plane").

When it was introduced in 1936, the Stuka was the most advanced dive bomber in the world. Using it as "aerial artillery" solved a major problem in the concept of Blitzkrieg—how to attack dug-in defensive positions. Normally this would require slow-moving artillery to be used, making the fast moving armoured forces wait for it to catch up. Close coordination between the ground forces and Stukas could achieve the same ends, on the move.

This was proven to great effect during the invasion of Poland and the Low Countries. In one particular example, the British Expeditionary Force set up strong defensive positions on the west bank of the Oise River just in front of the rapidly advancing German armour. Attacks by Stukas quickly broke the defence, and combat engineers were able to force a crossing long before the artillery arrived. Another important example was the massive aerial attacks in 13 May 1940 against strong French defence positions at Sedan in the Battle of France, which allowed the German forces a fast and (for the Allies) unexpected breakthrough through the French lines, eventually leading to the German advance to the Channel and the cutting off of large parts of the Allied army.

Despite its success in the French campaign, the Stuka soon showed its weaknesses in the Battle of Britain where great numbers of Stukas were lost due to its inappropriate use as a tactical bomber. In this case the lack of air superiority meant that the slow-moving aircraft was also at great risk to attack by fighters. This had not been the case in earlier battles, where the Luftwaffe maintained air superiority throughout.

The Stuka was the only widely used dedicated tactical dive bomber to be deployed against both naval and land targets, particularly with regard to the latter in the anti-armor role. Stukas also had 7.92mm machine guns or 20mm cannons mounted in the wings, with some modified to have 37mm cannons mounted below the wings for anti-tank work. With the loss of Luftwaffe air superiority in the east they became vulnerable to the Red Army Air Force fighters, and from 1943 had begun conversion to the more conventional cannon attack tactics.

The Royal Navy's Barracuda's made several attacks on the German battleship Tirpitz in 1944. The first (Operation Tungsten) was the most successful and put the ship out of action for 2-3 months.

Pacific theater

United States Navy Curtiss SB2C Helldiver dive bomber

Both the Imperial Japanese Navy (IJN) and the U.S. Navy invested considerable effort on dive bombers. Japan started the war with one of the best designs, the Imperial Navy's carrier-borne Aichi D3A (Val). But as the war progressed, the design quickly became outdated, in part due to its fixed main landing gear (a shortcoming shared by the Stuka). Later, when the IJN was no longer on the offensive, the more advanced Yokosuka D4Y Suisei entered service. By then, Japan's industry was straining under the relentless submarine warfare campaign being waged by the US Navy, which interdicted the raw materials needed by Japanese factories.

The U.S. fielded the Douglas SBD Dauntless, which was similar to the D3A in performance. The Dauntless was replaced with the faster, but more complex Curtiss SB2C Helldiver. As was usual with US war industry during WWII, both airplanes were built in large numbers.

The IJN's dive bombing moment of success was during their Indian Ocean Raids in April 1942. Japanese carriers launched strikes against the British navy's battle squadrons stationed near Ceylon and India, and Vals succeeded in sinking the Royal Navy heavy cruisers HMS Cornwall and HMS Dorsetshire, and the aircraft carrier HMS Hermes along with her escort destroyer HMS Vampire.[5]

The most famous example of successful naval dive-bombing attacks took place in the decisive Battle of Midway in June 1942 when American Dauntlesses scored fatal hits on three separate first-line Japanese aircraft carriers within a six minute timespan. Within hours the Imperial Japanese Navy had lost several years worth of combat experienced naval airmen, which until the end of the war in 1945 would never be replaced.

Decline

After the war, the dive bomber class quickly disappeared. Anti-aircraft warfare had improved as had the speed and effectiveness of fighter aircraft against the vulnerable, slow-flying dive bombers. At the same time the quality of various computing bomb sights allowed for much better accuracy from smaller dive angles, and the sights could be fitted to almost any plane, especially fighter aircraft, much improving the effectiveness of ground-attack aircraft. Although the aircraft could still "dive" on their targets to some degree, they were no longer optimized for steep diving attacks at the expense of other capabilities as the dive bombers of old. As these same aircraft were capable of many other missions as well, they were no longer considered to be dive bombers.

After pioneering efforts in World War II by both the Nazi-era Luftwaffe with the Fritz X, and the USAAF with the Azon controlled-trajectory bombs, today's smart bombs are used for precision bombing. Bombs can be dropped many miles from the target at high altitudes, placing the aircraft at little risk. The bomb then guides itself onto the target through a number of means, which can include laser designation, onboard GPS, radar, infrared, television guidance, and inertial wind-correction. Bomb sights continue to supply several "toss bombing" modes, a sort of reverse dive bombing where an aircraft releases its bomb while steeply pulling up from low level. Shallow, 45° or less dive bombing attacks are still used to deliver gravity bombs when they are employed.

References

Notes
  1. ^ There are some instances of this during WWII, see Battle of the Beams and GEE, in which case the bomb aiming points were calculated prior to the mission and the bombers were able to fly directly to them.
Citations
  1. ^ Nowarra Heinz J: Gezielter Sturz. Die Geschichte der Sturzkampfbomber aus aller Welt. p. 8. Motorbuch Verlag Stuttgart 1982. ISBN 3-87943-844-7
  2. ^ Wray R. Johnson, "Airpower and Restraint in Small Wars", Aerospace Power Journal, http://www.airpower.maxwell.af.mil/airchronicles/apj/apj01/fal01/johnson.html 
  3. ^ Brown p. 13
  4. ^ Brown p. 13
  5. ^ Brown p. 60, 61
Bibliography
  • Brown, David. Warship Losses of World War Two. Arms and Armour, London, Great Britain, 1990. ISBN 0-85368-802-8.

External links


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