Flightline: 61/TBD

"user314" (user314) 10/14/2020 at 11:00 • Filed to: flightline, Planelopnik, planelopnik history

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Transcendental 1G (N2704A)

Almost before the first flight of a heavier-than-aircraft, research has looked into combining the vertical takeoff and landing abilities of a helicopter with the long range and high speed of an airplane. One of the concepts identified was the tiltrotor, in which the the propellers, either connected with shaft to the engines or mounted as a unit in nacelles, are able to be tilted through 90°, allowing them to function as helicopter blades for vertical flight and as propellers for horizontal. Brothers Henri and Armand Dufaux began work in 1902, with a patent being awarded in 1904. George Lehberger patented the first recognizably modern tiltrotor in May of 1930, though it was never built. Prior to WW2, the !!!error: Indecipherable SUB-paragraph formatting!!! company designed their !!!error: Indecipherable SUB-paragraph formatting!!! , which would have been a hybrid of tiltrotor and tiltwing designs. A Daimler-Benz DB600 piston engine was to be mounted in the fuselage, and linkages would transfer power to the wingtip-mounted propellers. the wings were split approximately half-way along the span, and they would tilt along with the nacelles holding the propellers. The P.1003 was never built, though it did establish some concepts for future tiltrotors and tiltwings.

Orthograph of the P.1003/1

Concurrently, !!!error: Indecipherable SUB-paragraph formatting!!! designed the Focke-Achgelis Fa.269 went in a different direction, with the rotors facing downwards for VTOL and swiveling backwards 80° for horizontal flight. Focke carried out extensive wind tunnel tests, along with bench tests of the gearboxes and pivoting mechanisms, but the design did not advance beyond a full-scale wooden mockup before Allied bombing ended work.

Fa.269 wind tunnel model

Orthograph of the Fa.269

After WW2, two former employees of !!!error: Indecipherable SUB-paragraph formatting!!! formed Transcendental Aircraft Company to work on a tiltrotor design of their own. The Model 1-G was powered by a !!!error: Indecipherable SUB-paragraph formatting!!! -A flat-four engine which drove two 3-bladed rotors, controlled by a cyclic and collective. Despite heavy damage caused by the rotor blades shattering on a test rig in 1951, funding from the USAF allowed Transcendental to complete the Model 1-G in 1954, and the plane took its first tethered flight on 15 June, with an untethered flight occurring on 6 July. Flight testing continued until a control failure led to a crash on 20 July 1955, injuring the pilot but destroying the 1-G. The tiltrotor completed 23 hours of flight time, and although it did not completely transition from vertical to horizontal flight, tilt angles of up to 75° were explored, with up to 90% of the plane’s lift being provided by the wings, making it the first functional tiltrotor.

Transcendental 1-G in flight.

The success of the Model 1-G allowed Transcendental to refine the design into the Model 2, which flew in 1956. The Model 2 was powered by a Lycoming O-435-23 six-cylinder, and the enclosed cockpit could accommodate two. USAF funding dried up however, with the government becoming more interested in the Bell XV-3. The Model 2 was abandoned, and Transcendental was sold to Republic.

Transcendental Model 2 in flight

The result of a joint program between the USAF and US Army, the Bell XV-3 was the first successful tiltrotor design, and the ultimate ancestor of the V-22 Osprey. Powered by a !!!error: Indecipherable SUB-paragraph formatting!!! piston engine in the fuselage, the XV-3 was fitted with two 3-bladed props during initial testing, but the second example flew with two-bladed props. The first XV-3 was damaged beyond repair during a test flight in 1956 when extreme vibrations caused the pilot to black out. After extensively redesigning and reinforcing the second XV-3 (serial number 54-148), the type restarted test flights in 1958, though vibrations and instability continued to plague the design. As a result of wind tunnel testing, the rotor diameter was reduced and the wing structure was stiffened again. Tests resumed once more, and the XV-3 achieved a full transition from vertical to horizontal flight on 18 December 1958. Flights continued until 1962, after which the XV-3 was subjected to a series of wind tunnel tests with the aim of eliminating a dangerous condition known as pylon whirl , which had led to severe vibration and instabilities throughout the program. These tests ended in April of 1966, when both rotors failed, damaging both the XV-3 and the 40x80 wind tunnel at NASA Ames. The damaged XV-3 was found at AMARG in 1984, and under the leadership of former Bell XV-3 engineer Claude Leibensberger the tiltrotor was partially repaired and placed in storage. In 2005 Bell employees began a two year restoration of 54-148, after which it was transferred to the USAF Museum in Dayton, where it has been on display in the R&D Gallery since 2011.

The 2nd XV-3 on display at the NMUSAF

In 1972, NASA awarded a contract to Bell Helicopter for what would become the XV-15. The XV-15 was designed to simplify the conversion process and improve performance, and as such it was p owered by two !!!error: Indecipherable SUB-paragraph formatting!!! turboshafts in nacelles mounted at the ends of the wings. This eliminated the complex gearing and shafts linking the propellers to the engine as seen in earlier types, though a cross-linking shaft was included in case of an engine failure . The XV-15 also took advantage of improvements in avionics and materials, further improving the transition from vertical to horizontal flight. Flight testing of the XV-15s commenced in 1977, with one being displayed at the 1981 Paris Air Show.
 

Bell XV-15 N702NA at the 1981 Paris Air Show

N702NA was later damaged beyond repair on 20 August 1992 when a bolt slipped out of the collective control system on one pylon, causing that rotor to go to full pitch. The aircraft rolled uncontrollably and impacted upside-down, injuring both crew. The XV-15 was deemed uneconomical to repair, but the cockpit was salvaged and used as a simulator. The second XV-15, N703NA, was used in NASA test flights until 2003, after which it was placed on display at the Smithsonian National Air & Space Museum’s !!!error: Indecipherable SUB-paragraph formatting!!! at !!!error: Indecipherable SUB-paragraph formatting!!! . The data generated by the XV-15 program led to the Bell/Boeing V-22 and Bell/Augu sta BA609.

ttyymmnn > user314 10/14/2020 at 11:33

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I find it interesting that we’ve come full circle on the tilt rotor design. E arly attempts rotated the propellers, then the XV-15 and Osprey went with rotating the whole engine. Now we’re at the Bell V-280 Valor and just rotating the propellers again.

Meanwhile, the Sikorsky SB-1 Defiant, which is a coaxial compound chopper going up against the Valor for the future of medium lift in the US Army, recently hit 232 knots (in a descent) and 211 knots in level flight. Not as fast as the Valor, but the Defiant may offer other benefits. It will be an interesting contest.

facw > user314 10/14/2020 at 11:37

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Hmm, I missed those T ranscendental aircraft:

STOVL/VTOL Aircraft Ranked

user314 > ttyymmnn 10/14/2020 at 11:54

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Pretty sure that’s due to damage being done by the Osprey’s engine exhaust. Not scorching the Navy’s ships is probably worth the complexities of tilting the propshaft rather than the whole nacelle.   

That pic of the Defiant looks like a render, the fuselage is so smooth.

user314 > facw 10/14/2020 at 12:00

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Yeah, despite being the first, they were reduced to an “also-ran” by the fact that the USA/USAF went with the XV-3 instead of the Model 2.

Rock Bottom > user314 10/14/2020 at 12:06

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Funny the topic of tilt-rotors should come up today. I was just talking to an old friend about the XV-3 crash in the 40x80. I worked there until about 6 years ago and a few of The Old Guard were still wandering the halls when I started . They had some pretty interesting tunnel crash stories! The XV-3 that removed its own wings, the AH-56 Cheyenne that cut its own tail off, and the Proprotor Test Rig (PTR) that puked a transmission and ‘hucked the rotor down into the settling chamber... at the exact moment the incident recorder tapes were being changed! A rich history of vertical flight will be lost forever if that place goes away.

Fun fact about both the XV-3 and the XV-15: They were designed specifically to fit in the 40x80. In fact, the XV-15 test program schedule included a number of months in 40x80 to confirm the stability and controls derivatives and aero/vibrational loads before going to flight test. Talking to one of the XV-15 test pilots many years ago, he said it almost made the flight test program... a bit boring!

There’s a fantastic book about the subject called “The History of The XV-15 Tilt Rotor Research Aircraft: From Concept to Flight” by Maisel, Giulianetti, and Dugan. I highly recommend it for any aviation geek. It’s not expensive and probably available on Amazon. Similarly, there’s a substantial (read: dummy thicc) 3 volume work called “Introduction to Autogyros, Helicopters, and Other V/STOL Aircraft” by Frank Harris that is extremely hard to find but likely the most detailed series of books ever written on vertical flight. My copy was published by the NASA Technical Information Service, and so is pretty unusual. Not sure if they have plans to print it externally for educational use, but they really should. It’s a fantastic series if you’re interested in tilty/whirly machines.

I will close with a couple test installation images from the old 40x80 archive. The XV-3 pic is interesting in that it shows the later 2-blade rotors that replaced the early 3-blade design around 1957. The 2-blade was interesting in that it was rigid in-plane, like the later Sikorsky A-B-C and X2 choppers rather than using the normal Bell teetering hub style rotor.

user314 > Rock Bottom 10/14/2020 at 12:25

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Thank you so much for that; always nice to hear from someone who’s actually worked on or with these wonderful machines!

Nice pics too, really give you a feeling for how cavernous that tunnel is!

Rock Bottom > user314 10/14/2020 at 13:20

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Heck, the 40x80 is the little tunnel. The 80x120 next door is the big dog !

This is an NREL wind turbine test from several years back.  This isn’t a warehouse or hangar, this is inside the test section!