The Kellett KH-15 helicopter was designed to function as a stability research vehicle. The data that resulted from the flight tests of the KH-15 significantly contributed to the improvement of the flying qualities of present-service helicopters. Kellett Aircraft/Jeff Evans Collection Photo
You can forgive most people if the only thing they remember about the Kellett Aircraft Corp. are the egg-beater designs the company created in the 1940s. Owing to their oval-shaped fuselages and distinctive intermeshing rotor systems that resembled a twin-bladed egg whisk, the sight of these helicopters would certainly stick in peoples memories. But, the company also spent years as an influential autogyro company before it got into helicopter design, and many years as a research firm once its manufacturing days had come to an end. Plus, during its helicopter years, Kellett also created the initial design for one of the worlds first heavy-lift helicopters, the XH-17 (see p.124, Vertical, Dec11-Jan12). So, before it largely faded from the spotlight, the companys pioneering work in intermeshing rotor systems and heavy-lift helicopters made enough of an impact that echoes of it are still heard today.
A Different Beginning
Founded in Horsham, Pa., the Kellett Autogiro Co. (KAC) commenced operation in 1929 under the guidance of W. Wallace Kellett (his brother Roderick was also a key executive in the company). The companys initial focus was the production of autogyros (which, unlike helicopters, use an unpowered rotor system to develop lift, and an engine-powered propeller to develop thrust). Initially, KAC obtained a license from the Pitcairn-Cierva Autogiro Co. to build rotary-wing autogyros, but eventually KACs chief engineer, Richard H. Prewitt, helped design a new series of autogyros for the company, commencing in 1931 with the dual-control, side-by-side, two-seat K-2. The improved K-3 came out in 1932 with, among other things, a more powerful engine and a great story to tell: the Pep Boys, of auto parts fame, gave Admiral Richard E. Byrd a K-3 to take with him on one of his Antarctic expeditions. The K-4, meanwhile, which followed in 1933, had modifications to its wings, landing gear and tail section.
The next leap forward in design for the company was the tandem two-seat, direct-control KD-1 autogyro, which appeared in 1934. The main rotor was now a three-bladed design (instead of four), and was positioned on a pylon in front of the forward seat. Power came from a 225-horsepower Jacobs radial engine that enabled it to cruise at 103 miles an hour and helped give it a range of 361 miles.
The military version that followed in 1935 was based on the KD-1A, and was called the YG-1. It was evaluated and tested at the United States Army Air Corps base at Wright Field in Dayton, Ohio. The YG-1A, with additional radio equipment, followed, and then the YG-1B (later modified into the YG-1C, XR-2 and XR-3) was the production version the military received.
Prewitt later revamped the KD-1 series into the XO/YO-60 in 1942. It was intended to be an observation autogyro for the U.S. Army Air Forces (USAAF), and the large bubble-type cockpit even had clear window panels on the floor. The two-seater O-60 line had jump takeoff capabilities and the upgraded YO used a 300-h.p. Jacobs radial engine (the XR-2 and XR-3 also had the same jump takeoff capabilities and 300-h.p. Jacobs engine).
During this promising time for the company, Wallace Kellett himself had envisioned the autogyro as an ideal observation platform for the U.S. military, but this all changed with the development of Igor Sikorskys VS-300, the first practical helicopter in the U.S., in 1940, plus the U.S. militarys subsequent acceptance of the Sikorsky R-4 and R-5, and its greater interest in helicopters overall. In response, in 1942 Kellett quickly changed direction, and, after 13 years in the autogyro field, turned his companys sights to designing and building a helicopter for both military and civil applications.
Kellett had been aware of German engineer Anton Flettners work overseas with the FI 265 and FI 282 helicopters, which had two-bladed, intermeshing, counter-rotating main rotor systems. And, although up-to-date information on German helicopters was scarce due to the war, KAC chief engineer Prewitt managed to design an intermeshing syncropter helicopter that was somewhat based on the Flettner intermeshing rotor system. Armed with that design, Kellett sent his companys new proposal to USAAFs Experimental Engineering Section on Nov. 11, 1942. USAAF subsequently accepted it on Jan. 7, 1943, and awarded KAC a $1-million US contract on Sept. 11 to build two three-bladed, intermeshing rotary-wing helicopters that would be designated as the XR-8.
To reflect the change in the companys focus, Kellett Autogiro Co. became Kellett Aircraft Corp.
The XR-8 was the first of Kellett Aircrafts egg-beater designs, with its large oval fuselage and twin fabric-covered, three-bladed, intermeshing rotors. The fuselage itself was covered with both fabric and sheet metal, while the two-place, side-by-side-seating cockpit featured a bubble Plexiglas canopy. The military XR-8 used a Franklin, 245-horsepower, inline engine, giving the helicopter a cruising speed of 85 m.p.h.
Kellett Aircrafts chief test pilot, Dave Driskill (the first licensed helicopter pilot in the U.S.), performed the first flight of the XR-8 on Aug. 7, 1944, and immediately determined that the directional control needed changes. Vertical stabilizers were then added to the sides of the tail, plus the main rotors were redesigned so they tilted fore and aft differentially to improve yaw control.
During the next test flight, a month later, far more serious problems arose: one of the intermeshing blades had flapped against and touched another blade. Worried by this incident, USAAF instructed Kellett Aircraft to design a new intermeshing rigid rotor system, and awarded the company a contract for that on May 10, 1945.
In the meantime, work continued on the XR-8s development, and a second prototype (designated the XR-8A) was created, this time with an alternate two-bladed, intermeshing design. It flew in March 1945, but suffered from very severe vibrations. Before long, the two-bladed system was abandoned.
Focus now shifted to a rigid version of the original three-bladed flapping rotor system for the XR-8B. However, the new rigid rotor system proved to be a very expensive option, due to the major engineering required in its development and re-engineering that would be involved in the XR-8s design. As such, it was decided that development of the XR-8 with the flapping rotor system would continue.
The original XR-8 commenced USAAF service trials on Jan. 23, 1946. It was restricted in its flight maneuvers and top speed, and in the end faced unsolvable engineering difficulties that led to the program being canceled by the end of 1946. Today, one of Kelletts XR-8 syncropter helicopters USAAF Serial No. 43-44714 has been preserved and is presently in storage at the Smithsonian National Air and Space Museum in Washington, D.C.
A Companion Design
While the XR-8 never succeeded, there was enough confidence in its initial design and testing to help Kellett Aircraft garner a USAAF contract for a larger helicopter capable of carrying both personnel and cargo. That project, awarded on Oct. 16, 1944, saw the company beat out Sikorsky, Bell and Platt-LePage. The resulting new helicopter, designated the XR-10, was much larger than the XR-8, and was constructed with improvements learned from the XR-8s trials.
The twin-engine syncropter-type XR-10 helicopter had a maximum gross weight over 15,000 pounds (6,800-plus kilograms), and its fuselage was an all-metal, semi-monocoque construction. The design utilized two Continental 525-horsepower radial engines on nacelles attached to the sides of the helicopter. While the egg-shaped aircrafts cruise speed was only about 90 m.p.h., it could fly at twice the altitude of the XR-8. And, the big helicopter (the largest in the U.S. at the time) could also carry 10 fully equipped troops, six littered patients and two attendants, or over 3,500 pounds (1,590 kilograms) of cargo. Chief test pilot Driskill first piloted the XR-10 on April 24, 1947. (Kellett also envisioned a 16-place civil version that he called the KH-2; a mock-up was eventually constructed, but none were ever manufactured).
The XH-10 (USAF had adopted H designations for helicopter model numbers) was accepted by the U.S. Air Force (USAF) and delivered to Wright Field in 1948. The Air Force then began negotiations to purchase 10 XH-10 models, but budget cuts later canceled that contract.
Following that bad news, there was tragic news: Driskill died in a crash on Oct. 3, 1949, when the control system on the XR-10 he was testing failed. This had followed an earlier incident when the XR-10 experienced a similar problem to that of the XR-8 two of the intermeshing main rotor blades had touched.
For a small company with pre-existing financial difficulties, the crash had significant ramifications. Financial difficulties had already forced it to sell off the XR/XH-17 Sky Crane heavy-lift helicopter it was developing for the U.S. military to the Hughes Aircraft Co., now this crash, and the subsequent loss of contracts in its wake, would forever change the companys focus and fortunes.
The End of an Era
In 1950, Kellett Aircraft moved away from aircraft design and manufacturing and started an extensive program to expand its production facilities in order to pursue subcontracts for major aircraft assemblies. The company subsequently received subcontracts with the McDonnell Aircraft Corp. and the Republic Aviation Corp., which didnt fully save it financially, but did help point it in an eventual new direction.
Further significant change came with the passing of company founder Wallace Kellett in 1951, around the same time as the business was about to face restructuring due to bankruptcy proceedings. While Kellett Aircraft did survive, it would never again have the star status it once did.
The revamped Kellett Aircraft now set up a research and development group to carry out advanced applied research work in the rotary-wing field, and continued to receive a variety of small contracts for a number of years. Among the notable contracts was one from the U.S. Navy in 1954 for a variable-stability, one-person helicopter. This single-rotor helicopter used two blade-tip-mounted, hydrogen-peroxide rocket engines, and had two fuel tanks, one adjacent to each side of the aircraft, close to the pilots seat, over a tripod-type landing gear. The purpose of this mini helicopter was to test out a gyro-stabilizing system, and early flight tests indicated above-average dynamic stability.
The success of the XH-15 Stable-Mable resulted in two contracts to improve the stability of two helicopter types in service. The first came in December 1955, and involved producing a simple stabilizing device for the Sikorsky S-55. The second contract, sponsored by the U.S. Army Transportation Corps, dealt with the application of the Kellett stabilizing system on a Piasecki H-21 helicopter (ironically, Frank Piasecki had worked for the Kellett Autogiro Co. many years before).
It is unclear how many more years the Kellett Aircraft Corp. survived, but there is evidence that shows it was still winning military contracts for research work well into the 1960s.
In the end, even though Kelletts autogyros and experimental egg-beater helicopters did not having lasting success, their designs were innovative and influential. And, the intermeshing rotor system concept Kellett Aircraft helped pioneer did eventually realize success Charles Kaman perfected the concept (see p.108, Vertical, April-May 2011), and its still being used today in the stalwart Kaman K-Max.
Bob Petite is an air attack officer with the Alberta Forest Protection Division. He has over 40 years of experience working on wildfires both on the ground and in the air, utilizing air tankers and helicopters.