Conjoined Wings: Aviation’s Pullers, Parasites and Piggybacks

By Terry Dunn

By lashing two or more aircraft together, the combined machines can often accomplish things that neither vehicle is capable of on its own.

Even in the early pioneering days of aviation, engineers recognized the potential for teamwork among aircraft. By lashing two or more aircraft together, the combined machines could often accomplish things that neither vehicle was capable of on its own. It’s the wild blue yonder’s embodiment of the whole being greater than the sum of its parts.

This concept of mating together different aircraft is one that has persevered even as the art of aeronautics has constantly evolved. Whether powered by a rotary engine spewing castor oil or a space-bound rocket engine, the advantages of airborne teamwork are equally valid.

Here are a few examples of aircraft that were joined together to work together.

USS Akron and Curtiss Sparrowhawk

The USS Akron was a rigid airship that was used by the US Navy from 1931 to 1933. The huge 785 foot-long aircraft was unique in many ways. Its design was the result of a collaboration between the Goodyear Tire and Rubber Corporation and Luftschiffbau Zeppelin of Germany. Zeppelin sent engineering advisors to Goodyear in Ohio. The German engineers incorporated new design and construction ideas that they had been unable to explore in their home country. You can read about many of those innovations here.

THE SPARROWHAWKS ENGAGED A HOOK ONTO A TRAPEZE MECHANISM THAT WOULD RAISE OR LOWER THE BIPLANE INTO THE AIRSHIP’S HANGAR.

The most novel aspect of the Akron’s design was that it could accommodate up to five Curtiss Sparrowhawk biplane fighters in an internal hangar. The Sparrowhawks could be launched and recovered while the Akron was in flight. A large hook located above the fighter’s upper wing engaged a trapeze structure on the airship. The trapeze could then be raised or lowered to move the biplane into or out of the hangar bay.

The original concept for the Akron had the airship’s crew on the lookout for enemy ships while the Sparrowhawks warded off attacks by enemy aircraft. The policy soon transformed to have the airship acting as a home base while the fighters ranged out in search of the enemy’s navy. The effectiveness of this strategy was never tested in wartime. Less than two years after the Akron’s commissioning, it was lost in a storm off the coast of New Jersey, killing 73 servicemen. The same fate befell the Akron’s sister ship, the USS Macon, off the California coast in 1935. Thankfully, only two men perished in the crash of the Macon.

Zveno

Around the same time that the US Navy was working on the Akron and Macon, engineers in Russia were developing a very interesting variation on the mothership theme, the Zveno. Rather than a rigid airship, the core of the Zveno was the Tupolev TB-1 bomber. Two Tupolev I-4 fighters were situated atop the wing on either side of the fuselage. The engines of all three aircraft were used for takeoff, yet the fighters could pull fuel from the bomber’s large tanks. Once in flight, the parasite aircraft could depart on their own and return to dock with the TB-1.

THE ZVENO WAS AN ODD, BUT SUCCESSFUL RUSSIAN DESIGN THAT ATTACHED AS MANY AS FIVE FIGHTERS TO A SINGLE BOMBER.
Funny thing about airplanes: once they are flying, they can carry more weight than what is practical for takeoff.

Subsequent iterations of the Zveno utilized the even larger TB-3 Bomber and up to five parasites. Two atop the wings, two below, and one perched above the fuselage. A later version of the Zveno carrying two bomb-laden Polikarpov I-16 fighters was used during WWII.

Short Mayo Composite

Funny thing about airplanes: once they are flying, they can carry more weight than what is practical for takeoff. This limitation has been the catalyst for numerous aircraft pairings. In the example of the Short Mayo Composite, two airplanes were combined piggy-back style in order to give the top-mounted aircraft the desired range and payload capacity.

THE SHORT S.20 MERCURY NEEDED HELP GETTING AIRBORNE, SO IT RODE PIGGYBACK ON THE MORE POWERFUL S.21 MAIA.

The bottom aircraft was the Short S.21 Maia, an adaptation of the existing C-Class flying boats already produced by Short Brothers. The upper aircraft was a new, smaller design designated the Short S.20 Mercury. The Idea was that the Mercury would be loaded down with fuel and payload (mail) greater than what it could safely takeoff with. The larger and more powerful S.21 would get airborne with the S.20 riding piggyback. Once at an altitude and speed where the S.20 could safely fly on its own, it detached from the S.21 and began a long solo journey of up to 6000 miles.

Like so many lives and machines, the course of the Mayo Composite was drastically altered by the onset of WWII. The Maia fell victim to German bombs, and Mercury was cut up for scrap metal. The rapid advancement of aviation technology during the war negated any effort to resume development of the project at war’s end.

Mistel

The piggyback arrangement surfaced again during WW2 in the German Luftwaffe. What began as a concept to carry assault gliders into battle morphed into a weapon that released explosives-packed, unmanned bombers onto large targets. A single-engine fighter such as a Messerschmitt Me 109, or Focke-Wulf Fw 190 sat atop a twin-engine Junkers Ju 88 bomber. The bomber’s cockpit and occupants were replaced with a 3800lb shaped-charge warhead that was designed to punch through reinforced concrete or even a battleship hull. The fighter pilot lined up on the target and released itself from the Ju 88. A 3-axis autopilot kept the unmanned bomber on course until impact.

MISTEL PAIRED A SINGLE-ENGINE FIGHTER WITH A TWIN ENGINE BOMBER. THE UNMANNED BOMBER WAS PACKED WITH EXPLOSIVES AND RELEASED NEAR THE TARGET.

The Luftwaffe called their ungainly weapon “Mistel” (mistletoe). A few missions were carried out before the war ended. They were first used against the D-Day invasion fleet off the coast of France. It is unclear whether the Mistels scored any hits. Although if they did, their victim was the French battleship Courbet, which had been grounded in shallow water as a decoy. Mistels were later used on the eastern front in an attempt to knock out bridges over the Oder River. Those attacks were ineffective.

The Japanese had a different marriage of aircraft intended for bombing. A Mitsubishi G4M “Betty” bomber was modified to carry a Yokosuka MXY7 “Ohka” winged bomb. What is unique about the Ohka is that it was rocket propelled…and had a cockpit. The kamikaze pilot within was expected to guide his flying bomb into a US warship (preferably an aircraft carrier). Seven ships were hit by Ohkas. None were aircraft carriers, and not all of the damaged ships were sunk.

Tug and Glider

There are several ways to get a glider airborne. One of the most common is to pull it up with a rope attached to a powered airplane. Civilian sport gliders can be towed by a wide variety of airplanes. A particularly popular glider tug is the Piper Pawnee, a simple and rugged aircraft that was actually designed for crop dusting.

WWII was the zenith of glider use by armed forces. Both the Axis and the Allies used gliders for assaults and resupply missions – some successful, some less so. Perhaps the most unusual glider of the war was Germany’s Messerschmitt Me 321 Gigant. As the name suggests, the Gigant was a huge aircraft, capable of carrying nearly 25 tons of payload. The aircraft used to pull the Me 321 were no less unique.

The 4-engine Junkers Ju 90 was originally used as a tug, but it lacked sufficient power for the job. The Luftwaffe then began using three twin-engine Messerschmitt Bf 110 fighters simultaneously. A successful tow (especially a long cross-county tow as intended for the Gigant) requires coordination between the pilots of each aircraft. This is difficult enough with just one tow plane. With three tow planes, it was downright dangerous.

GETTING THE HUGE ME 321 GLIDER IN THE AIR TOOK A LOT OF POWER. THE HEINKEL HE 111Z WAS AN ABSTRACT VARIANT OF THE HE111 BOMBER THAT WAS CREATED SPECIFICALLY FOR THE JOB.

The next proposed tow plane was the Heinkel He 111Z Zwilling, an aircraft created specifically for the challenges of pulling the Gigant. The He 111Z was a 5-engine conglomeration that resulted from joining the wings of two twin-engine He 111 bombers – with a fifth engine thrown in for good measure. Even so, the Zwilling was not powerful enough to pull a fully-loaded Me 321 skyward. Underwing rockets were needed at takeoff to get the odd couple airborne. Even after adding 6 engines to the Me 321 itself (creating the Me 323), a fully-loaded Gigant still required the pull of a Zwilling and/or rockets.

B-29 and the F-85

The infancy of the atomic age was witness to the introduction of bombers that could deliver their apocalyptic loads over great distances. The problem was that bombers needed protection and the first-generation jet fighters were notoriously short-legged. One proposed solution was for the bombers to carry their escorts with them. This idea spawned the pudgy McDonnell F-85 Goblin.

THE PUDGY F-85 GOBLIN REPRESENTED AN ATTEMPT TO FIT A FIGHTER INTO THE BOMB BAY OF THE BOMBER IT WOULD PROTECT. (MUSEUM OF THE US AIR FORCE PHOTO)

The Goblin was little more than a cockpit and wings wrapped around a jet engine. With its wing folded, it could fit inside the huge B-36 Peacemaker bomber. During a mission, the F-85 could be lowered out of the B-36 and launched to fight off attackers. As with the USS Akron and the Zveno, the B-36 mothership was to recover its tiny pugilist and carry it home.

Launch and recovery tests of the Goblin began in 1948, with a modified B-29 standing in for the B-36. The F-85 proved to be a nimble and speedy jet, but returning to its roost was problematic. The Goblin used a hook and trapeze system not unlike that on the Akron and its Sparrowhawks. Unfortunately, the system just wasn’t practical in the jet age. Turbulence under the B-29 made hookups difficult for the best test pilots of the day. After a series of near misses and bent airplanes, the program was cancelled. Other parasite fighter ideas were fleshed out (including attaching F-84 Thunderjets to the wingtips of a B-29) but none were ever implemented.

A TRAPEZE SIMILAR TO THOSE USED ON THE AKRON WERE INSTALLED ON A B-29 TO CAPTURE THE F-85. TURBULENCE BELOW THE BOMBER MADE RECOVERY OF THE GOBLIN VERY DIFFICULT AND DANGEROUS.

The X-Planes

After the close of WWII, the US began a series of vigorous programs that sought to better our understanding of supersonic flight and beyond. The rocket-powered X-planes that stemmed from these projects often operated at high altitude, but they couldn’t carry enough fuel to get there themselves. Early X-planes, such as the supersonic X-1 (of Chuck Yeager fame) were schlepped to altitude in the belly of a B-29. Later ships, like the Mach 6+ X-15, left the ground while tucked under the wing of a B-52. The X-planes provided many valuable lessons in how to fly faster and higher. Some X-15 pilots even flew high enough to earn astronaut wings. Yet, none of those accomplishments would have been possible without the jump starts provided by their lumbering beasts of burden.

X-PLANES DIDN’T HAVE ENOUGH FUEL TO CLIMB TO ALTITUDE ON THEIR OWN, SO THEY WERE CARRIED UP BY B-29 OR B-52 BOMBERS. HERE, A B-52 CARRIES A X-15 BELOW ITS STARBOARD WING. (NASA PHOTO)

SR-71 and the D-21

The Lockheed SR-71 Blackbird was a machine decades ahead of its time. This raven-colored, titanium ship could fly faster and higher than any aircraft or missile sent to intercept it. Even at the end of its 34-year career, it had no peers. Given that the SR-71 was basically impervious to any attempts to stop it, a smaller unmanned drone was built in the Blackbird’s image.

If SR-71s could mate, their offspring would surely resemble the D-21. The ramjet-powered drone had a 19ft wingspan. Like the X-planes, the D-21 needed a boost to get going. The D-21 nuzzled between the vertical stabilizers of the M-21, a modified version of the A-12, which was an interceptor variant of the SR-71.

THE D-21 WAS AN UNMANNED DRONE THAT CLOSELY RESEMBLED THE SR-71 VARIANT THAT WAS USED TO LAUNCH IT. THE JOB WAS LATER GIVEN TO THE B-52.

The D-21 could reach speeds above Mach 3 and travel for 3000 miles – taking high resolution photos all the while. After completing its objectives, the D-21 would then perform high altitude hara-kiri, jettisoning its photographic viscera (to be recovered via a mid-air snatch or ocean pickup) and then self-destructing.

Although test flights of the D-21 had mixed results, the program forged ahead. On the fourth test launch, the D-21 and Blackbird collided, destroying both aircraft and killing an M-21 crewman. Thereafter, D-21’s were launched from B-52s, with no improvement to the drone’s success rate. Eventually, four operational missions were flown over China, but all failed for various reasons.

SCA and the Space Shuttle

NASA’S 747 SHUTTLE CARRIER AIRCRAFT WERE USED TO TRANSPORT THE SPACE SHUTTLES BACK TO KENNEDY SPACE CENTER. THEY ALSO DELIVERED THE RETIRED ORBITERS TO MUSEUMS AROUND THE COUNTRY.

Perhaps the most recognizable set of conjoined aircraft is the Space Shuttle orbiters and the Boeing 747s that were used to transport them. Rather than give you a history of the 747 Shuttle Carrier Aircraft (SCA), I’ll provide the opinion of former SCA pilot, Jeff Moultrie.

“The SCA is a highly modified B-747-100. Although the structure of the aircraft is quite different than the original Boeing airliner, the cockpit is essentially the same. The handling characteristics are remarkably similar to the original airplane when unmated (no Shuttle). When we are carrying a mated shuttle, the additional weight and drag causes a very degraded climb performance along with an accompanying slower throttle response. Another big difference is the 250 KIAS [Knots Indicated Air Speed] speed restriction placed on the aircraft because of the modified structure and tail section. Although flying the airplane takes some getting used to, it is very predictable and a joy to fly.

WhiteKnightTwo and SpaceShipTwo

Burt Rutan has achieved Elvis-like status in aviation circles for his portfolio of composite-built-aircraft. Although his creations are often abstract in appearance, they invariably get the job done. That certainly holds true for WhiteKnightTwo (WK2), the aircraft that Rutan’s Scaled Composites designed for Virgin Galactic to air-launch their sightseeing spacecraft SpaceShipTwo (SS2) (also a Rutan design).

WHITE KNIGHT TWO AND SPACESHIP TWO ARE PROOF THAT THERE IS STILL A PLACE FOR TEAMWORK WITH MODERN AIRCRAFT – AND SPACECRAFT. (VIRGIN GALACTIC PHOTO)

With a 140ft wingspan, WK2 is the largest all-composite aircraft ever built. By comparison, SS2 attached to the center of the wing seems rather ordinary. Both WK2 and SS2 are larger developments of the pathfinders for privatized spaceflight, WhiteKnight and SpaceShipOne. Hopefully, the recent fatal crash of SS2, will not stall Virgin Galactic’s spacefaring mission for long.

More to Come

Although the advent of aerial refueling (itself a merging of aircraft) negated the advantages of some types of conjoined vehicles, it remains clear that aerial teamwork is still a valid concept. As the performance goals for many new aircraft are now moving away from pure speed and altitude to improved efficiency, it will be interesting to see how engineers utilize conjoined aircraft along the way.

Terry spent 15 years as an engineer at the Johnson Space Center. He is now a freelance writer living in Lubbock, Texas. Follow Terry on Twitter: @weirdflight