The story of the Focke-Wulf Triebflügeljäger, vertical take-off and landing tail-sitter interceptor with three rotating wings that never was

The story of the Focke-Wulf Triebflügeljäger, vertical take-off and landing tail-sitter interceptor with three rotating wings that never was

By Dario Leone
Oct 14 2020
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Each of the three wings was tipped with a combination ramjet and 300kg thrust solid fuel Walter starter-rocket.

The Focke-Wulf Triebflügeljäger (power-wing fighter) was a German concept for an aircraft designed in 1944, during the final phase of World War II as a defence against the ever-increasing Allied bombing raids on central Germany.

As explained by Dan Sharp in his book Secret Projects of the Luftwaffe Volume 1 Jet Fighters 1939-1945, a design for a vertical take-off tail-sitter was first patented on Sep. 10, 1938, by Otto Muck — a prolific Austrian-born inventor — but Focke-Wulf’s Triebflügeljäger married the company engineer Dr Otto Pabst’s work on ramjets to a concept invented by Professor Erich von Holst, with input from the Aerodynamische Versuchsanstalt (AVA, one of the four predecessor organizations of the 1969 founded “German Research and Experimental Institute for Aerospace”) Gottingen-based jet intakes specialist Dr Dietrich Küchemann.

Von Holst, who worked at the Zoological Institute at Gottingen, had entered two model ornithopters for the Second Reichswettbewerb für Saalflugmodelle or Reich Contest for Indoor Flying Models in Breslau on Nov. 20 and Dec. 1, 1940, and won second prize. While one model was intended to recreate the effect of a bird’s two wings flapping, the other was based on the movements of dragonfly’s four wings.

Research around this latter movement formed the basis of a paper published in the Jahrbuch des Deutsches Luftfahrtforschung 1942 by von Holst, Küchemann and K. SoIf which looked at how von Holst’s models could be scaled up to a full-size aircraft – effectively replacing the lapping dragonfly wings with two counter-rotating sets of helicopter blades.

Pabst published a set of calculations detailing how his ramjet engines might perform when attached to the tips of the von Holst/Küchemann aircraft’s helicopter blades in March 1944 and during May Focke-Wulf engineer Heinz von Halem worked on determining the best layout for the aircraft. His efforts focused on a cigar-shaped fuselage with three spinning wings attached either at the nose tip, with the pilot positioned behind them; part way back from the nose with the pilot still positioned behind them; or nearly halfway along the fuselage with the pilot positioned in the nose.

The single-seater aircraft’s basic form was established by Sep. 15, 1944, and a short preliminary description was produced. The aircraft was designed to sit vertically on the ground, resting on four 380 x 150mm outrigger wheels on the ends of its cruciform tailfins and a single 780 x 260mm wheel housed within a fairing on the tip of the fuselage. Maximum fuselage length, with the mainwheel retracted into its housing during flight, was 9.15m.

The story of the Focke-Wulf Triebflügeljäger, vertical take-off and landing tail-sitter interceptor with three rotating wings that never was

Each of the three wings was tipped with a combination ramjet and 300kg thrust solid fuel Walter starter-rocket. Once the rockets had got the wings spinning fast enough the ramjets could be activated and would then take over. Maximum wing diameter was 11.50m and total wing area was 16.5m2 (3 x 5.50m2). Fuel for the ramjets – either gas or liquid – totalling 1,500kg was housed within the fuselage. Armour protection was given as ‘usual fighter armour’ and armament consisted of two MK 103s with 100 rounds each and two MG 151s with 250 rounds each in the aircraft’s nose – one of each on either side of the pressure cabin cockpit.

A second description was produced on Oct. 12, 1944, which altered the aircraft’s dimensions slightly – fuselage length increased to 9.35m while wing diameter was reduced to 11.40m but wing area remained 16.5m2.

According to a description in a British intelligence report. “In this aircraft the normal wing is replaced by three rotating ‘wings’ or vanes each with an athodyd (Lorin duct) at the tip. The ‘pitch’ of the ‘wings’ can be adjusted by the pilot and the maximum peripheral speed which is normally used for climb is 670ft/sec (455mph). It will be appreciated that with such an arrangement the air speed of the athodyds may be considerably greater than that of the fuselage.

“In this way efficient operation of the athodyds can be achieved when the aircraft speed is relatively low and one of the main objections to this form of propulsion is thus overcome. The higher the forward speed of the fuselage the slower the rotation of the ‘wings’ and the smaller the difference between athodyd speed and fuselage speed.

“For take-off the aircraft stands vertically on its tail (in which the wheels are housed) and initial rotation is imparted to the ‘wing’ assembly by three auxiliary Walter rockets incorporated in the athodyds. As soon as the peripheral speed appropriate to athodyd operation has been attained the main units are started up. During this stage the ‘wings’ are in neutral pitch. When they are moved into fine pitch lift is imparted to the aircraft partly due to the rotation of the wings and partly due to the components of athodyd thrust parallel to the fuselage axis.

“After levelling out, the pitch is further coarsened and the speed of rotation correspondingly reduced in order to maintain a constant Mach number of 0.9 at the wing tips. At the maximum designed speed of the aircraft the wings rotate at 220rpm.”

The Triebflügeljäger was claimed to have six key advantages: “1) High efficiency and low fuel consumption. 2) High ceiling. 3) No runway required for take-off or landing. 4) Low weight. 5) Simplicity. 6) Any combustible medium (gas or liquid) which can be vaporised may be used as fuel.”

The project appears to have gone no further after October 1944.

Secret Projects of the Luftwaffe Volume 1 Jet Fighters 1939-1945 is published by Mortons Books and is available to order here. Save 10% on all books with exclusive promotional code ‘AVGEEK10’!

Photo credit: War Thunder

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Dario Leone

Dario Leone

Dario Leone is an aviation, defense and military writer. He is the Founder and Editor of “The Aviation Geek Club” one of the world’s most read military aviation blogs. His writing has appeared in The National Interest and other news media. He has reported from Europe and flown Super Puma and Cougar helicopters with the Swiss Air Force.

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