In the course of the air war, more and more importance was attached to the flight altitude of combat vehicles, and in 1943, when a variant based on the Bf.109F was proposed as an intermediate solution to the high-performance fighter program, the main emphasis was on the altitude characteristics of the aircraft. Under the designation Bf.109H, this project met the requirements for the first stage of the super-fighter project. This phase was known as an urgent program and included the modernization of an already existing fighter. The second stage of the program provided for the creation of a super-high-altitude fighter and was planned before the end of 1944.

According to the initial design, the Bf.109H was a Bf.109F airframe with a DB-601 high-altitude engine and an additional direct insert in the wing consoles, bringing the total span up to 12 m. However, the Technical Department constantly raised the bar by increasing the ceiling from 13 000 to 15,000 m. This required major changes to the Bf.109H project. In parallel with this work, the Me.209-II was created, which was supposed to replace the Bf.109, and on April 23, 1943, the designer Messerschmitt proposed a high-altitude fighter based on the Me.209-II instead of the Bf.109H. This Project 1091a provided for the use of either a special high-altitude engine DB-628A, or DB-603U (DB-603E version with a TKL-15 turbocharger). According to the calculations, the P.1091a was supposed to have a ceiling of 17,500 meters. But at the same time, it was obvious that the new Messerschmitt fighter would be able to go into production only at the end of 1944, and the Luftwaffe needed a high-altitude fighter as soon as possible. Messerschmitt was instructed to return to the Bf.109H project, but taking into account the decisions on the P.1091a, including the DB-628 engine.

To test the engine installation on the Me.209-II, the Bf.109G-5 airframe (# 16281) was redesigned for the DB-628 mockup, the aircraft was called the Bf.109-V49. After bench tests, the DB-628 was installed on the Bf.109G-3 airframe (# 15338). The aircraft, designated Bf 109-V50, flew for the first time on May 18, 1943 under the control of weather vane Captain Wendel. Despite the design close to the DB-605A, the DB-628A with a two-stage supercharger and an intercooler turned out to be larger and heavier by 170 kg. The aircraft was equipped with a tunnel spinner and a propeller with wide blades. At the same time, the bow was lengthened by 0.75 m, which required the installation of a counterweight in the tail section. On the Bf.109H, for the same purpose, the wing was moved 0.25 m forward, and the tail area was increased.

After a short test in Augsburg, the Bf.109-V50 was transferred to the Daimler-Benz test center in Echterdingen, south of Stuttgart, where test pilots conducted two 30-hour test programs, during which an altitude of 15,500 m was reached. It was calculated that on takeoff the serial DB-628 engine will have a power of 1490 hp, at an altitude of 2000 m 1580 hp, at an altitude of 6400 m 1480 hp, at an altitude of 9600 m 1350 hp and at an altitude of 12 000 m 1130 hp. While the Bf 109-V50 with the DB-628 was being tested at Daimler-Benz, this engine was installed on another Bf 109G-5, which also received an increased keel and direct inserts into the wing. The aircraft flew in June 1943 under the designation Bf 109H-V54 (DV + JB). At the same time, work began on converting several Bf.109F-4 gliders into pre-production Bf.109H-0.

In addition to inserting straight sections into the wing, landing gear struts were also moved to the ends of these sections. The elongated stabilizer received struts. The rest of the Bf 109H-0 were similar to the Bf 109F-4 / Z, retaining the DB-601E-1 engine with the GM-1 afterburner system. Armament consisted of one MG-151/20 cannon and two 7.9 mm MG-17 machine guns. On these machines, it was mainly planned to investigate the problems of flights at high altitudes and equipment modifications. Bf 109H-1, which were intended for military trials, used the Bf 109G-5 airframe with a pressurized cabin and a DB-605A engine with a GM-1 afterburner system. In addition to equipment like the Bf.109H-0, the installation of Rb 20/30, 50/30 or 75/30 cameras was provided.

Several Bf.109H-1s were delivered at the beginning of 1944 for military trials to a unit near Paris. Although the flight data of the fighter was considered sufficient, and the ceiling reached 14,500 m, during a dive at a speed of 730 km / h, wing flutter began. A series of dive tests of the Bf.109H-1 was carried out in Augsburg by weather-captain Wendel, and on April 14, 1944, after a dive from an altitude of 5000 m, at an altitude of 3000 m at a speed of 800 km / h, wing flutter began. After Wendel took over the handle, the left wing fell off with part of the fuselage.

After several more test flights, further work on the Bf.109H was discontinued, as they stopped at the Ta.152H. By this time, several more high-altitude fighter projects were developed. On December 22, 1943, another prototype Bf 109H-V55 (# 15709 DV + JC) was flown. Tests of the Bf.109-V54 showed serious stability problems, which were mainly related to the contours of the nose section of the fuselage. As a result, the Bf.109H-V55 (originally it was made under the designation Me.209-V6, but then was used in the Bf.109H program, since the Me.209 program was discontinued), although it retained the lengthened nose of the fuselage, received a DB- 605B with supercharger from DB-603 and bonnet modeled on Bf.109G. The aircraft also received lengthened wingtips modeled on the high-altitude version of the Me.209, which increased the span to 13.25 m, received extended slats and ailerons, and a new keel. It was planned to install the DB-603G engine on the aircraft, but taking into account the delays in the work on the latter, it was decided to install the Jumo-213E-1 with the GM-1 system on the serial version of the Bf.109H. It was planned to manufacture this aircraft in versions of the Bf.109H-2 heavy fighter with three 30 mm MK-103 or MK-108 cannons and two MG-151/20; a light fighter Bf 109H-3 with one 30 mm MK-108 cannon and two 13 mm MG-131 machine guns and a Bf 109H-4 reconnaissance aircraft with one Rb 50/30 or 75/30 camera. None of these options were accepted, and all work ended on the Bf 109H-V55, which was destroyed after the Allied air raid on Augsburg on 25 February 1944. Its predecessor with the DB-628 engine (Bf 109H-V54) was destroyed half a year later on August 14, 1944 after a raid on the Daimler-Benz test center in Echterdingen.

Photo	Description
	The Bf 109H-1
	DB 605A/B

Bibliography

• "Aviation of Luftwaffe" /Viktor Shunkov/
• "Encyclopedia of military engineering" /Aerospace Publising/
• "The air war" /№ 61/