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- Blohm und Voss BV 40
In the winter of 1943, it was quite evident that the Luftwaffe could not prevent the Allied bombers from carrying out the punishment missions where and whenever they wanted. The powerful American four-engine bombers, specialized in daylight accurate attacks, operated in self-defensive formations, which allowed them to support each other using the concentrated crossed fire of their heavy machine guns. Statistically, this was lethal to the target of 1.53 square meters represented by an Fw 190 in a frontal view at a distance of 1,000 yards.
Theoretically, the solution consisted in reducing the frontal area of the fighter. In practice this meant eliminating the engine and redesigning the airplane as an armoured glider. Germany had much experience in the construction of sailplanes and numerous projects of Gleiterjägern (glider fighters) emerged during the last two years of war.
By placing the pilot in prone position, they could manufacture an airplane with a frontal area of just 0.5 square meters, but the change required numerous studies. With the widespread use of dive bombers during the first months of World War Two, the crews of Stukas began to falter. After making a certain number of missions, some of them experienced a number of black-out phenomena and other fainted, losing control of their aircraft. According to studies by the Luftwaffe Aviation Medicine Branch (LFM), this was caused by insufficient blood pressure in the brain, consequence of the g-forces that tended to accumulate blood in the legs. Experiments with centrifuges reaching 14 g in 1936 showed that a pilot lying in prone position could better retain consciousness that in a seated position, as the heart was at the same height than the brain.
To gain some experience, they used a single seat glider Akaflieg FS-17 and a specially modified DFS Kranich II to place a second pilot in the nose, in prone position.
However, upon reaching a certain level of g, the structural strength of the gliders began to fail. Therefore, to continue experimenting, the company Akaflieg built the Berlin B9, a small twin engine experimental aircraft capable of supporting up to 22 g in flight. In 1943 the project scientists were able to prove that a pilot in prone position could recover in a few seconds after a dive at 8.5 g. During the summer, this information was used to design the Blohm und Voss P.186, an armoured Gleitjäger designed to perform Rammstoss attacks in accordance with the RLM specification of August 19, 1943.
The rammer should be towed by a Bf 109 G and released near the stream bomber.
But it was found out that, being slower than the bombers, the Gleitjäger could only hit against their tail surfaces by diving from great altitude to gain speed. However, the towing plane lacked the power to reach the appropriate flight level in time for the interception. A frontal attack would exceed the maximum 14 g bearable by the pilot, so the only possibility left was to laterally attack the bomber formation, diving from about 1,000 m above the stream.
LFM subsequent investigations showed that the tolerance of the pilot to impacts in prone position was quite limited and the P.186 was cancelled on October 30, 1943. At that time the demand for flying boats, the main aeronautical product by Blohm und Voss was virtually non-existent and to save the P.186 contract, Dr. Ing. Richard Vogt proposed its amendment as a heavy armoured fighter glider. The new design called BV 40 (December 13, 1943) received authorization from the RLM to be built on a pre-series of twelve aircraft. The armoured cockpit,
virtually invulnerable to the impact of US 12.7 mm machine guns, weighed 300 kg and was made of welded steel plates. The pilot in prone position was frontally protected by a 120 mm thick windscreen. The rest of the airframe was built in wood/plywood.
The BV 40 took off from a detachable twin wheels trolley towed by a Bf 109 G, using a 30 m long rope, reaching 10,000 m in 25 minutes. The launch took place 1,200 m ahead of the bomber stream and 500 m over its flight level. With a dive angle of 20 degrees, the BV 40 reached an attack speed of 475 kph. The armament initially proposed consisted of a MK 108/30 cannon installed under the port wing root and a towed explosive paravane bomb of 2 kg called Gerät Schlinge under the starboard wing.
At a distance of 400 m from the target, the pilot made a first attack with the gun and then he used the momentum built during the dive to gain altitude for a second attack with the Gerät Schlinge. To that purpose, the paravane detached from its position under the wing, being linked to the plane by a 20 m cable and kept in stable flight 7 m under the fighter thanks to its special aerodynamic design. The pilot should just fly over one of the 'boxes' until the bomb collided with one of the planes.
The system did not work properly during testing and the Technical Office of the Luftwaffe (TAL) suggested doubling the firepower by installing a second MK 108 instead of the paravane. The manufacturer, on the other side, proposed converting the BV 40 into a rocket fighter by installing a HWK 109-509B bi-fuel rocket engine that would provide enough speed to make attacks from behind the bombers and then flee from the escort fighters. Its armament could later be replaced by air-to-air rockets of the R4M type. After the attack, the plane should land on a ventral folding skid, reducing its speed to 118 kph using flaps.
The BV 40 was very heavy and slow manoeuvring. At 900 kph in a dive it would have been impossible to use the ailerons owing to flutter. The Flieger-Stabsingenieur Tilenius proposed using the BV 40 as air-to-air bomber equipped with four AB 250 submunition containers. But the idea was rejected in favour of the Me 262 of the Kommando Stamp that used SD 500 bombs and AB 500 containers with acoustic fuses for these attacks.
An antiship version was also planned, armed with four BT-700 torpedo-bombs that would be towed by a He 177 bomber to the proximity of an Allied fleet. The prone position allowed the pilot to withstand up to 14 g, when recovering its flight level after a dive attack, to throw the bombs as close as possible to the warship. It was considered that the BV 40 could use a little DFS pulsejet for the return flight.
