RogerBW's Blog

Tailsitters 28 March 2014

One of the desiderata of an air defence system is to put defending fighters close to the high-value targets. That way they don't get decoyed away by diversionary attacks, giving the enemy bombers a clear run, because they're dedicated to protecting a specific target; nor do they need massive endurance (adding to weight), if they don't need to make long-distance flights.

However, an air base is a big thing, what with runways, maintenance facilities, and so on. (And those runways are vulnerable to being bombed.) A VTOL fighter can in theory operate out of a forward base that's little larger than the fighter itself, with a travelling maintenance unit that moves between multiple fighter stations.

So the next problem is to design a VTOL fighter. It needs to be able to go upwards, and sideways. The approach that's generally worked is some sort of thrust vectoring, but this turns out to be remarkably difficult. Having dedicated lift engines is easy to design, but horribly weight-inefficient. Another approach is to have an aircraft that stands on its tail and takes off vertically.

During the latter days of the Second World War, several projects along these lines were tried in Germany. None of them ever got as far as a full prototype. They all looked fairly odd.

Heinkel came up with the Wespe (Wasp), a coleopter design (i.e. with a ducted fan surrounding the core fuselage). It was intended to use a turboprop engine, but this wasn't available, so the Lerche (Lark) was designed round a pair of DB605 piston engines, driving a pair of counter-rotating propellers. Both aircraft were intended to be armed with a pair of 30mm cannon, the Lerche also with four Ruhrstahl X-4 wire-guided air-to-air missiles.

(Yes, a wire-guided MCLOS air-to-air missile, spin-stabilised at 60rpm. Which the pilot would have had to guide (requiring him to keep his eyes on both missile and target) while also flying the aircraft. A triumph of hope over practicality, one feels.)

That wasn't barking enough for Focke-Wulf, who came up with the Triebfl├╝gel (thrust-wing). A fairly conventional fuselage is surrounded by a massive rotor-propeller assembly with three wings/blades, each of which has a ramjet on the end (and a rocket to get them up to speed when taking off). The ramjet keeps the rotor spinning; the blades are tilted to vary the amount of thrust. Torque on the fuselage only comes from friction through the rotor bearing, and this can be counteracted by control inputs.

The Triebfl├╝gel was also intended to be armed with two 30mm cannon. Concerns were raised over landing; the pilot would have to look over his shoulder at the ground (and through the rotor). No good solution was ever found for this, though in any event the project never got as far as a prototype.

After the war, the US military had another use for VTOL fighters: they could be placed aboard ships, in the manner of a CAM ship but without the need to ditch the fighter afterwards, thus providing a convoy with airborne defence without the need to attach an aircraft carrier. It was envisioned that small warships would operate these aircraft from a dedicated protective housing.

Two prototypes were developed, Convair's delta-wing XFY "Pogo" and Lockheed's more conventional XFV "Salmon". Both had dual counter-rotating propellers on the nose, and a tail adapted to support the weight of the aircraft on the ground. The Pogo was flown more extensively, but both had the same problems: landing was particularly challenging even on dry land in perfect conditions, meaning that only the best pilots would be able to fly the things effectively and so they couldn't be put aboard lots of ships, and as propeller-driven fighters in the dawning jet age they'd be at a terminal disadvantage against any likely opponent.

The obvious solution to the latter problem was to do the same thing with a jet aircraft, though the Ryan X-13 Vertijet was actually developed under an entirely different requirement, to build a jet fighter that could operate from a submarine. Its main advantage, other than being jet powered, was that it had reaction control jets to allow manoeuvre at low speed. It was explicitly a research aircraft, never intended to be the direct ancestor of a fighter, and in the hands of Ryan's test pilots it did an excellent job in that role. However, the Air Force was forced to admit that there really was no operational requirement, particularly as autonomous missiles became more effective, and development was discontinued in 1957.

But in the end, landing was the insurmountable problem with all these aircraft that got as far as prototypes. Takeoff is easy: just increase the power and there you are. Transition to horizontal flight can be a bit tricky without reaction jets, but if the aircraft's already climbing fast it's just a matter of gently pushing the stick forward, and going back to the vertical is managed in the same way (leaving the aircraft in a climb). But for landing, the pilot's lying on his back, looking either over his shoulder or in a mirror, adjusting power to control his descent rate with one hand while keeping the aircraft vertical with the other. If the concept were to be resurrected, one could probably do something with cameras and radar altimeters; but the tailsitter is a pure VTOL aircraft, not V/STOL as is currently preferred, and modern aircraft need so much maintenance infrastructure that they get based together anyway.

If you use the X-Plane or FlightGear flight simulators, you can have some of this experience for yourself; both Pogo and Vertijet have been modelled there.

An alternative approach, the "nose-sitter", has never been tried in manned aircraft as far as I know; it would require the pilot to come to a full stop while moving between horizontal and vertical flight, and leave him hanging from his seat while on the ground. However, at least one company has built a prototype unmanned version.

  1. Posted by John Dallman at 11:41am on 29 March 2014

    The Focke-Wulf always seemed to have a different parasitic weight problem. It has three wings, one of which ends up not supplying any lift. Two wings solves that problem, although it requires bigger engines, and means you have to cope with no dihedral on the wings.

  2. Posted by RogerBW at 05:14pm on 30 March 2014

    As I understand it, the whole wing assembly was constantly spinning, and providing thrust directly backwards along the long axis of the aircraft, so the thing would have flown like an aircraft with no wing lift at all. The only choices the pilot had were how much fuel to feed to the rams, and how much to tilt the wings to convert their rotary motion into forward thrust. The thing would be in a constant nose-up attitude so that some of that thrust would keep it in the air; that's why the nose guns were to be angled downwards. I'm not aware of any intention to use the rams for direct thrust; I don't think it would have been able to fly fast enough for that.

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