The growth in the size and weight of airplanes outpaced the ability of sod airfields to accommodate newer designs from the 1930s. Paved runways answered the problem, but soon giant bombers were envisioned that could defeat concrete and asphalt.
Traditionalists went with larger tires, or more tires, in an effort to make the surface area contacting the landing strip bigger to distribute the weight.
But as early as 1939, the flight test and engineering facilities at Wright Field in Dayton, Ohio, explored a different option. A track mechanism, resembling a tank or bulldozer belted arrangement, was envisioned for a twin-engine Douglas A-20 attack bomber.
Proponents of tracked landing gear argued the multiple reasons the concept was promising. It could allow very large aircraft to use conventional runways and it could allow smaller aircraft to use soft, boggy, or sandy earth instead of prepared runways.
Firestone got involved with the process, and successfully demonstrated a track landing gear on a Stearman PT-17 and a Fairchild PT-19. This paved the way for the larger A-20 gear demonstration.
The A-20 track gear weighed about twice as much as conventional landing gear for the bomber, and required an estimated 15% more runway length for takeoff, reported Tony Landis of the Air Force Materiel Command History Office in a 2019 web publication.
By August 1947, the A-20 project had shown tracked gear offered good flotation.
Earlier, in 1943 with World War II still raging, tests were initiated with tracked gear on a Curtiss P-40 fighter. But the necessarily smaller units on a fighter aircraft required side-by-side tracks on each main landing gear strut. These got packed with earth and debris and proved too small to negotiate rough terrain.
After the war, Fairchild built tracked landing gear for a small number of C-82 transports for the Air Force. The Fairchild tracks were steerable and fully retractable, unlike some earlier test aircraft tracks. The C-82 tracks looked viable on sod and some types of mud, but lost out on soft sand.
Maintenance, not surprisingly, was higher on the tracks than on conventional wheeled landing gear. And skis were declared superior for snow operations.
Boeing hedged its bets by subcontracting with Goodyear for tracked main gear and Firestone for tracked nosegear on a B-50 bomber. Testing in 1949 showed the need for high maintenance and, by January 1950, the B-50’s flirtation with tracked gear was over.
One more chance for tracked landing gear came when the Air Force contracted with Convair to equip the giant XB-36 bomber prototype with tracks. For the test, the XB-36 was limited to a top weight of 250,000 pounds. The massive B-36 main gear tracked assemblies used dual tracks on each gear. Each track belt was 16 inches wide. Steel cables, plated in brass, reinforced the rubber belts.
The B-36 tracked gear showed the good and the bad — it added thousands of pounds of dead weight to the bomber, but it featured a maximum average pressure on the ground of 57 pounds per square inch, compared with 156 pounds per square inch for a production four-wheel main gear set up.
Taxi tested in 1950, the huge B-36 tracked gear made its first — and only — flight on March 26. After a circuit of the field, the monster bomber landed, and accounts say it deposited parts of the system along the runway on rollout.
Track gear might have made it onto a jet if a Northrop proposal for equipping the YRB-49 model of the Flying Wing had been given the green light. Ultimately, multi-bogey landing gear spread aircraft weight around multiple conventional tires to make large aircraft feasible.
Other ideas were tried in the 1950s, like the dually mainwheels given an F-84 at Edwards Air Force Base. This allowed the fighter to takeoff at a heavy gross weight. As soon as it lifted its weight off the wheels, the extra wheels dropped from the jet, retarded in forward motion by drag chutes as the experimental F-84 climbed out.
The track mechanism is an interesting concept, and works well with heavy equipment moving on rough terrain. Most of that heavy tracked equipment operates are relatively low speeds, especially when compared to aircraft takeoff and landing speeds. It may not be immediately obvious, but the upper, unloaded track has to move at twice the speed of the equipment it is attached to. The reason for this is that once a track segment is laid on the ground, it does not move until the supported equipment moves past it. So, let’s say that a large airplane has a takeoff speed of 150 kts. Then the upper, unloaded track must be moving at 300 kts, and that makes for a very difficult design challenge to make a reliable, heavy and complex mechanism that is moving at high speed.
I spent almost 30 years involved with flight test at Edwards AFB during and after a military career 1972-1999. I thought I’d heard of every cockamamie idea tried on an airplane but your historical articles show that I didn’t; I dearly enjoy them … keep ’em coming. Thank you.