Weekend Wings #20: Inflatable Aircraft

It may come as a surprise to readers to learn that inflatable aircraft are not a new idea. Indeed, they’ve been with us for almost eighty years!

The first mention of an inflatable aircraft is in a patent filed by Taylor McDaniel on August 13th, 1930. It was granted in 1933 as US patent 1,905,298. (This link and the following one are, in my experience, best viewed using the Microsoft Internet Explorer browser – Firefox and others seem to have trouble with the Quicktime viewer used by the US Patent Office.)

McDaniel described his invention as follows:

The invention is preferably embodied in a glider, although the same may be embodied in any type of heavier than air machine, such as an airplane and also in a lighter than air machine, such as a dirigible. In constructing a glider in accordance with my invention, the air-foil or sustaining plane has a stiffening frame made of inflated tubes. These tubes are secured to the fuselage, and means is preferably provided to exert a longitudinal pull upon the inflated tubes, upon the increase of load upon the tubes. … The arrangement is such that the horizontal control or stabilizing of the glider is effected by flexing or warping the trailing edge of the air-foil or sustaining plane by the manipulation of the usual control stick.

. . .

A particular advantage of the glider is that the major portion of the framework of the same is constructed entirely of inflated tubes, which is extremely light, resilient within limits, and not liable to break, as would be the case if constructed of wood. The frequent breakage of the ordinary glider, when making a landing is one of the principal objections to its use. Further, by constructing the major portion of the framework of the glider of inflated tubes, such tubes may be deflated and the glider collapsed for shipment in a relatively small space, the ordinary Wooden frame glider when shipped occupying a considerable space.

McDaniel proceeded to build his inflatable glider, and demonstrate it before US Army officers. No pictures are available, as far as I’m aware, except for a newsreel from 1931 that’s offered for sale by www.buyoutfootage.com (and which I can’t afford to buy for unpaid blogging purposes, I’m afraid!) In their listing for this newsreel (item UE31005, about a third of the way down the page at the link) they describe the footage as follows:

INVENTOR CRASHES RUBBER GLIDER TO PROVE SAFETY VALUE

WASHINGTON, D.C. – Government aviation experts watch successful tests of odd wreck-proof flying device. Taylor McDaniel, inventor, acts as ground crew for pilot Joe Bergling, who takes machine into air and deliberately nose dives it. First bounce shows no injury to man or vehicle.

So, despite the paucity of information, it seems that McDaniel’s inflatable glider really worked. However, it was never put into production apart from the prototype, as far as I’m aware. If any reader can shed more light on this aircraft, I’d be grateful.

During the same decade the Soviet Union experimented with glider aircraft on a large scale. According to Victor Suvorov, on Page 113 of his book ‘Icebreaker: Who Started The Second World War?‘:

. . . the parachute psychosis of the 1930’s was also accompanied by a glider psychosis. Soviet glider pilots and their gliders were well up to world standards, and indeed higher. By the beginning of the Second World War, out of eighteen world gliding records, thirteen were held by the Soviet Union.

The best builders of Soviet military aircraft were sometimes deflected from their main work in order to make glider planes. Even Sergei Korolev, who was later to create the first sputnik, was set to work on developing gliders, which he did with great success. If builders of war planes and ballistic missiles were put to work on making gliders, the purpose was obviously not simply to win world records. Had Stalin been interested in breaking records, why did he not put the best minds to work on creating new racing bicycles?

That Soviet gliding was heading in a military direction is beyond dispute. Even before Hitler came to power, the Soviet Union had seen the creation of the first airborne cargo glider in the world, the 6-63, made by the plane builder Boris Dmitriyevich Urlapov. Heavy gliders were invented which were capable of lifting a freight-carrying vehicle. P. Gorokhovsky even created an inflatable rubber glider; after they had been used behind enemy lines, they could be loaded onto a transport aircraft and returned to their own territory to be used again.

This passing reference to Gorokhovsky’s inflatable glider set me searching through online aviation archives. Again, very little material is available: but the British Flight magazine, in its issue of June 27th, 1935, page 692, has the following report:

It is credibly reported that a Soviet factory is producing a glider which can be carried in a suitcase. It is made of rubberised fabric, and the structural members are inflatable tubes of this material!

In its issue of January 2nd, 1936, on page 9, Flight displayed the following picture and caption concerning the Soviet inflatable glider (click for a larger view):

Again, I’ve been able to find nothing more about this aircraft from online sources. If any reader can provide more information, I’d be grateful.

World War II brought an end to any experimentation in the field. Military aviation wasn’t ready to fool around with outlandish ideas like inflatable aircraft (although the US Navy used blimps extensively for coastal and convoy patrol, with considerable success).

After World War II, interest was revived. Many pilots had been lost because they’d been shot down behind enemy lines, and couldn’t be rescued in time. This problem recurred in the Korean War. Inquiring minds wondered whether some sort of aircraft could be developed that could be air-dropped to them and set up on the spot. The pilots could then fly themselves to safety. In Britain and the USA, these ideas were followed up.

The Goodyear company, makers of the famous blimps, developed what they called the Inflatoplane. They gave their initial prototype the internal project code of GA-33. The wings, tail unit and cockpit enclosure were constructed of a material Goodyear called Airmat. This consisted of two walls of rubberized fabric connected by nylon cords, with air pumped between them. The fuselage was of single-layer rubberized fabric.

The prototype GA-33 made its first flight at Olathe, Kansas, on May 28th, 1957. Piloted by Goodyear’s test pilot, Richard Ulm, it flew in a severe thunderstorm – which must have been an interesting experience!

It proved to have a performance basically similar to the Piper J-3 Cub. After further testing, it was developed into two models. Both had open cockpits, as opposed to the closed cockpit of the GA-33.

The single-seat GA-468 model had a two-stroke Nelson engine delivering 40hp. It was a little under 20 feet long, with a wingspan of 22 feet. It carried 20 gallons of fuel and could lift a payload of 240 pounds. Its cruising speed was 60 mph, and it could travel for 390 miles or remain aloft for over six hours. Its maximum altitude was about 6,000 feet.

The two-seat GA-466 model was almost the same length, but its wingspan was 28 feet. It had a 60hp McCulloch engine to cater for the extra load. On 18 gallons of fuel, it could cruise at 55 mph and cover 275 miles, or remain aloft for almost five and a half hours. It had only a single wheel beneath the fuselage, to save weight.

The Inflatoplane was an ingenious piece of work. The aircraft was packed into a container about the size of a cabin trunk, rolled up into a compact bundle. It was designed to be air-dropped by parachute in this container, then pulled out for erection.

It would be wheeled into position and unrolled.

It would be inflated either by a motor-driven air-pump, or by a cylinder of compressed air.

Once inflated, the propeller would be attached to the engine and the aircraft prepared for flight.

The small engine would then be started by hand, somewhat like early outboard motors, using a pull-cord. The engine drove the propeller for powered flight, and also a small air pump that continuously fed air to the aircraft, keeping it inflated to a pressure of about 25 pounds per square inch (psi). At that pressure, the wings would keep their shape under most conditions – even when men walked on them!

Inevitably, leaks occurred, but the pump proved able to keep the aircraft (literally) in shape for extended periods, even when bullet holes were deliberately shot in the fuselage and wings. One Inflatoplane reportedly flew for over an hour with a dozen .38 Special bullet holes in it.

The cockpit was very basic, but had all essential instruments for visual flight.

The aircraft could land on conventional wheels:

or on a hydro-ski, permitting operation from water, snow and ice (of course, being full of air, with the engine and air-pump well above the water, it wouldn’t sink):

A dozen Inflatoplane prototypes were built, and tested by all US services, Army, Navy and Air Force. Testing continued until the early 1970’s, driven by the imperative need to rescue pilots shot down in the Vietnam conflict. However, there were numerous drawbacks to the idea of dropping an inflatable aircraft to a pilot. For a start, he would need a flat area in which to take off – and the jungles and mountains of Vietnam offered few, if any, such places. Second, he would need to be uninjured, and capable of unpacking, deploying and flying the aircraft. Experience showed that the violence of ejection from a fast-moving jet aircraft frequently left aircrew injured, to such an extent that they might not be able to do so. Finally, there was the time factor. It would take several hours to deploy an aircraft carrying the Inflatoplane, get it to the downed pilot, drop the container, wait for him to erect the aircraft, and then fly home. In much less time, a helicopter rescue team could be on the scene and pick him up directly.

Testing also showed that the aircraft was prone to structural movement in the air (probably inevitable when there was no rigid structure as such). This caused the death of one test pilot in the early 1970’s. As an observer, Jesse Shannon, reported in 2005:

I was one of the two army mechanics attending the Goodyear School on this aircraft. I was stationed at Ft. Rucker Alabama with the U.S. Army Aviation Test Board. I witnessed the crash at Wingfoot Lake, Acron Ohio. “Pug” Wallace was the Army Aviator killed. What happened was that the aircraft was in a descending turn when one of the control cables under the wing came off the pulley and got wedged in the pulley bracket. This locked the stick and the turn just kept on getting tighter until one of the wings folded up over the prop and got chopped up. With the wings flapping because of loss of air, one of the aluminum wing tip skids hit the pilot alongside the head (marks on his helmet proved that). I saw Wallace come out over the nose of the aircraft and fall into the shallow lake. His cute never opened. When we got to him, he was bleeding from the ears and nose. The other Army mechanic was Sam Hess and the other Army Aviator was Lt. Elton. “Pug” and I were having a few beers the night before. He left a wife and at least one child. The project was canceled. There was a two place InflatoPlane that we all would take a ride in after the class graduation. It never got to that.

Given those realities and difficulties, the project was finally cancelled in 1973. A few Inflatoplanes were presented to museums, including the US Army Aviation Museum in Fort Rucker, Alabama; the Patuxent River Naval Air Museum in Patuxent, Maryland; the Franklin Institute in Philadelphia, Pennsylvania; and the Smithsonian Institution in Washington, D.C. The aircraft below is on display in the Patuxent River Naval Air Museum.

At about the same time that Goodyear thought up the Inflatoplane, the M. L. Aviation Company in Britain developed its Utility Mark I inflatable aircraft (the link is to a PDF file, so if you don’t already have it, you’ll need Adobe Acrobat Reader to view it). This was originally conceived as a reconnaissance and communications aircraft for company-size formations of the British Army. It would be towed behind a vehicle, its undercarriage and underslung ‘gondola’ serving as a trailer, and inflated as needed using an electrical or engine-driven pump. It was rather too large and heavy to be air-dropped in a rescue role, like the Inflatoplane.

Development began in 1955, under a contract from the Ministry of Supply. The first M.L. Utility Mark I had a 35hp. engine, upgraded to a Walter Mikron III engine, derated to 38½hp, in the second prototype. The engine was mounted in the rear of an underslung wooden gondola, in pusher configuration. The pilot and passenger sat in the front of the gondola.

The second prototype cruised at 50 mph, with a maximum speed of 58 mph. Its endurance at cruising speed was 2½ hours. It could carry a total of 400 pounds, typically comprising a pilot and passenger. The wingspan was 35 feet, with a length of just over 23 feet, and it stood 10½ feet tall. The wing was inflated to only ½ psi pressure. An electrical air pump in the gondola could be used as needed to reinflate it.

The first prototype was tested in 1956, and the second in 1957. The latter reached altitudes of about 700 feet, and was stable enough for its purpose.

However, the advent of helicopters to support ground formations meant that the need for the Utility disappeared, and it was not developed further.

This marked the end of development of fully-inflatable aircraft, as far as I’ve been able to discover: but it also marked the start of something rather interesting, that has continued to this day. The use of a conventional airframe with inflatable wings has been the subject of much research, and is today under consideration not only for Earth-bound aircraft, particularly small remotely operated vehicles, but even for aircraft to fly on other planets!

The first inflatable-wing project to arouse interest was produced by Daniel Perkins, an engineer at the Royal Aircraft Establishment at Cardington, UK. He was trying to produce a human-powered aircraft, part of the worldwide interest in such things in the 1950’s and 1960’s. In the mid-1950’s he produced his first inflatable wing. It spanned 27 feet, weighed only 38 pounds, and had a wing area of 250 square feet. He also tried to produce fully pneumatic airframes, although these were less successful.

His most promising effort, named the Reluctant Phoenix, first flew in an old airship hangar on July 18th, 1966. It was limited to short hops, due to its high power requirement (all provided by human muscle), but nevertheless made a total of 97 ground-effect flights in the hangar (i.e. flying just above the ground, on the ‘cushion’ of air produced by forward motion, but unable to climb further). Its longest flight was 420 feet, at a maximum altitude of less than 2 feet above the ground. All flights were conducted inside the 800-foot-long airship hangar.

The aircraft was a delta flying-wing with a wingspan of 31 feet and an empty weight of 39 pounds. The envelope of the wing was made of polyurethane-coated nylon fabric. Reluctant Phoenix could be folded away and transported in the back of a small station wagon. It also survived many crashes without requiring repairs, as the aircraft merely bounced when it hit the ground – shades of Taylor McDaniel in the 1930’s!

Perkins gave up after Reluctant Phoenix, and after his death the aircraft passed to another enthusiast, Frederick To. He immediately recognized the potential of the design, and set about producing an updated successor. His Phoenix used an inflatable wing, the prototype of which (with a 20-foot span) is shown below.

The size of the aircraft grew too large for the inflatable wing to support its own weight, so plywood ‘riblets’ were added. The aircraft was tested and flown in London’s Docklands in March 1982, with the longest flights lasting about 20 seconds.

In the 1970’s, ILC Dover, Inc. developed the Apteron unmanned aerial vehicle (UAV) with inflatable wings.

It had a 5.1-foot wingspan and weighed 7 pounds. It was successfully test-flown using a ½ hp motor, controlled remotely via elevons on the trailing edge of the wing. It was not developed into a production model.

Tests were also conducted for the US Air Force to see whether inflatable fins could be used on conventional bombs to improve their glide range and guidance, whilst minimizing their radar signature and overall size. The Air Force Research Laboratory at Eglin AFB, Florida, conducted the Weapons Integration and Design Technology Program in the 1970’s and 1980’s. Inflatable stabilization fin prototypes were produced for 250-pound and 500-pound bombs, and were deployed at altitudes ranging from 300 to 25,000 feet and at up to low supersonic speeds. A carbon dioxide gas generator inflated them in less than 150 milliseconds to a pressure of 200 psi.

ILC Dover continued to experiment with inflatable wings during the 1980’s and 1990’s. One version was built to test whether a UAV could be made small enough to fit inside a cannon shell. It was intended to be fired from long range, and deployed in mid-air to conduct reconnaissance missions, reporting back to its control station by radio. An ingenious stowage system for inflatable wings was developed and successfully tested in the laboratory. The two pictures below show the wing system in folded and inflated form, without a fuselage.

The next two pictures show how the wings would be folded inside a cylindrical fuselage, which in turn would fit into an artillery shell. The wings are folded in the first picture, and extended in the second. A 10:1 deployed-to-packed-volume ratio was achieved, which is pretty remarkable.

This concept was not pursued further at the time, although it’s rumored that it may be attracting renewed interest from certain sectors today.

Another concept pursued during the 1980’s and 1990’s was the possibility of ‘rigidizing’ inflatable wings. This could be accomplished in two ways. One would be to coat the fibers of the wing material with an ultra-violet-sensitive material, which would harden upon exposure to ultra-violet rays from sunlight. The wing would be inflated using air pressure, and would then harden over several minutes, becoming stiff enough that inflation was no longer required to allow it to maintain its shape. The second approach was to coat the wing fibers with a similar material, but one that would be hardened by another chemical reagent, released with the gas into the wing. Both approaches were tested in various laboratory experiments, and both worked. Investigations are continuing.

The use of inflatable wings for UAV’s became more and more interesting to more and more companies with the increasing use of UAV’s by all branches of the US armed forces. The same is probably true in Europe and elsewhere, although I’ve not been able to find many sources of information online about their efforts.

In 2001, NASA’s research center at Dryden flight-tested the I2000 micro-UAV, which was launched from a larger UAV ‘mother ship’.

After being dropped by the ‘mother ship’, the I2000 deployed two inflatable wings, with a combined span of over five feet. They were inflated from a pressurized nitrogen tank on board the I2000. Inflation took only about a second.

After inflation, the I2000 successfully glided down to earth under remote control. Three successful tests were conducted, proving that the basic concept was sound.

Such wings are under consideration for numerous UAV projects, but most of the details are shrouded in secrecy, so there’s little information in the public domain. One that has been discussed in some detail is the so-called ‘Loitering Electronic Warfare Killer‘ study program. The link is to a subscription-only resource, but the LEWK is described as follows:

An even more ambitious UAV-based SEAD program, however, is already underway: the Loitering Electronic Warfare Killer (LEWK) advanced-technology concept demonstrator (ACTD), a program involving all four US military services (with the Air Force serving as the lead). Begun in 2001, the LEWK ACTD seeks to develop a UAV with the ability to deliver precision-guided munitions – in this case, BLU-108 sensor-fuzed weapons – and provide a jamming capability to augment the EA-6B Prowler, all at a unit cost of about $100,000. For the ACTD, the LEWK will be deployed from a CH-53 helicopter, but the plan is to eventually get the UAV certified on a fighter aircraft. The idea is to have LEWKs, pre-programmed with target points as determined by the enemy’s electronic order of battle, carried into a threat zone by a manned aircraft and released. The LEWKs would then fly in close to their targets for stand-in jamming and fly pre-programmed egress routes to a recovery point upon completion of the mission. In addition, the LEWK would provide an additional capability to strike time-critical targets that may pop up in the area. “If there’s a time-critical target out there and we can meet the rules of engagement by employing the BLU-108s, we’ll do that,” said Col John Wilcox, US Air Force.

Again, the benefit to employing a UAV for this type of mission is the aircraft’s persistence. “Putting a LEWK on a fighter that goes in at 500 knots and dropping it allows the LEWK to use all its fuel on station, rather than traveling to and from the site of interest,” Wilcox said. The flight from Mazar-e-Sharif, for example, would take an average UAV three hours. The LEWK, Wilcox pointed out, gets there more quickly and can loiter longer.

But the LEWK ACTD is even more ambitious. Although the initial flights under the program have focused on controlling a single LEWK, Wilcox said, “We plan to have a swarm of LEWKs. We want to have one pilot and 50 LEWKs.” Employing a swarm of UAVs, though, presents some challenges. Deploying the swarm requires a lot more carriage capacity than a fighter possesses, so a rack has been developed that could carry 18 LEWKs, with designs in place for a rack that could carry as many as 24. Controlling the swarm poses yet another challenge. The pilot, he said, could be anywhere – in a ground-control station, in an EA-6B – so long as he’s got the laptop-based control system and a datalink. But Wilcox explained that the swarm concept is still a little way off. “We’re going to crawl before we walk and walk before we run,” he said. “After we get through one guy controlling one LEWK, we’ll probably go to one guy controlling two LEWKs, then one guy controlling six LEWKs and see where we go from there.” This isn’t so different from the way manned aircraft are handled, Wilcox noted. “A lot of times manned platforms abort, and we have to retask other fighters and bombers to pick up their targets. It would be the same thing with LEWKs,” he said.

Clearly, if a fighter aircraft is to carry so many LEWK’s, their size must be as small as possible commensurate with their mission. One early LEWK prototype, developed by SAIC, is shown below Notice the similarity of its wings to those tested by NASA on the I2000 UAV, as described above. Also, note that on a production vehicle, the undercarriage would not be necessary: it would be air-launched, and wouldn’t need to land in the conventional sense.

Inflatable wings and control surfaces would be of great value on such a UAV, to reduce its size before deployment. It’s understood that several studies are under way to achieve precisely that, but no further information has been released.

One area where detailed information is available is for an aircraft to fly on Mars. The University of Kentucky has been working with ILC Dover for the past six years to achieve this objective. The ‘Big Blue’ project has passed through several stages of development, including the deployment and test flight of an inflatable-wing aircraft, with ‘rigidizing’ wings, at altitudes well in excess of 90,000 feet. The culminating flight is scheduled for later this month, when all the elements will be put together and tested.

This fascinating project may be sent to Mars in the 2010-2011 time frame, if all goes well. The video below is long (about half an hour), but gives an excellent overview of the development of the Big Blue program through its various stages. Interesting viewing, if you have the time.

Finally, let’s note that inflatable vehicles are already in space, and may form an important element of future space exploration. The Bigelow Aerospace Company has developed inflatable spacecraft, that are sent into orbit aboard conventional launch vehicles, then inflated to provide habitats for astronauts. Two such vehicles are already in orbit around the Earth. Genesis II is illustrated below.

They are protected against micro-meteorite strikes by ballistic fabric, the same as is used in bulletproof vests. There’s been talk of an inflatable hotel in orbit, to be serviced by private spacecraft. This isn’t a pipe-dream. With inflatable vehicles already successfully tested in orbit, and the growing competition to launch tourists into space, who knows where this might end up?

There’s even talk of an inflatable surface habitat for use on the Moon or Mars, when astronauts need a surface base for a short-term exploration mission. Whilst these wouldn’t be ‘inflatable aircraft’ as such, the technology already developed for inflatable aircraft, wings and orbital habitats would be crucial to their success.

So, inflatable aircraft go back much further than we’d expect, and are at the forefront of current developments in UAV technology. It’ll be interesting to see how they develop in future.

Peter