Within the Nineteen Forties, American and German helicopter producers started experimenting with mounting tip jets on the ends of helicopter rotor blades.
These methods helped designers eradicate weight and cut back torque on fuselages. Helicopter producers developed tip jet designs for small one- and two-seat fashions and huge sky crane variations that would carry heavy navy and area program payloads.
Tip Jets Allowed Designers To Cut back Helicopter Weights
One of many key targets and challenges of helicopter design has been lowering weight to extend lifting capability. Tail rotor assemblies, gearboxes, clutches, transmissions, and blowers all add to the burden. Designers decided that among the finest methods to scale back helicopter weight can be to maneuver engines or different energy sources out to the ideas of the rotor blades.
German Started Work on Tip Jet Designs in World Battle II
In 1941, German engineer Friedrich von Doblhoff developed a WNF 342 helicopter that might pipe compressed air blended with aviation gasoline to rotor-tip combustion chambers fitted with automotive spark plugs. Its radial piston engine powered a compressor that fed a fuel-air combination by way of three tubes to the tip jets.
The spark plugs ignited the combination, inflicting the blades to show and produce carry. A pusher propeller behind the primary prop offered airflow over the rudder, permitting it to regulate the helicopter’s course throughout flight.
Because the rotor spun with out energy from a central hub, the WNF 342 didn’t create a lot torque on the fuselage. Nevertheless, a number of issues did happen, together with excessive gasoline consumption and the necessity for a particular heat-resistant alloy for the tip jets. The gasoline consumption was so excessive that the Germans determined to solely use the tip jets for takeoffs and landings.
The WNF 342 made its first check flight in 1943. The German Navy had needed to make use of it for remark and anti-submarine missions. Allied bombing slowed its growth, however Van Doblhoff continued engaged on the undertaking till the struggle ended. He then went to the US and continued engaged on helicopter designs.
Hiller Plane Firm Develops A number of Tip Jet Designs
In 1949, the American Hiller Plane Company started researching tip jets for helicopters. Their preliminary designs have been just like the German fashions. Following testing, Hiller discovered that jet engines mounted on the rotor ideas have been extra environment friendly than the earlier designs with combustion chambers.
Hiller labored on tip jet ideas for the subsequent 15 years. Certainly one of their designs was the two-bladed Hiller XHOE-1 Hornet. This helicopter had ramjet engines mounted on the rotor ideas. They weighed 11 kilos every and produced 31 kilos of thrust, sufficient energy for the helicopter’s small, gentle rotor blades.
Whereas the Hiller Hornet did fly, designers encountered a serious drawback with autorotation within the occasion of an engine failure. The ramjets created extreme drag on the rotors, which compelled crews to make fast descents to maintain the rotors from stalling. Throughout an autorotation, the Hornet needed to descend at a charge of about 3000 ft per minute, virtually twice that of different helicopters. Such a fast charge of descent compelled pilots to start their touchdown flare maneuver between 200 and 300 ft, when helicopters sometimes start their flare below 100 ft from the bottom.
US Military Seeks Heavy-Elevate Sky Crane Helicopters
Within the Nineteen Fifties, the US Military introduced it needed a flying crane helicopter able to lifting radar vans and even tanks. Hiller proposed a number of designs they known as the “Aerial Carryall” or “Flying Truck,” with rotors 169 ft lengthy and numerous jet engines on their ideas.
Hiller Plane Firm Plans to Construct World’s Largest Helicopter
The Military didn’t settle for Hiller’s proposals, however in 1965, the corporate started engaged on a tip jet flying crane request from NASA. Its mission can be to recuperate Saturn V booster first phases, weighing as much as 400 tons, for Undertaking Apollo. This required that Hiller design the biggest helicopter ever constructed. It could have a gross weight of 450 tons and a rotor greater than 328 ft in diameter.
Hiller designed it with two or extra tip jet engines on every blade. The mission profile would have the helicopter wait about 466 miles downrange from Cape Kennedy. It could then snag the rocket stage because it descended by parachute. Finally, whereas NASA felt the Hiller Sky Crane would have labored and been cost-effective, it additionally started creating a reusable area shuttle as a substitute of single-use rockets and halted funding for the undertaking.
Hughes Plane Firm Assessments its Personal Sky Crane Helicopter
Hiller was not the one firm that attempted to develop a tip jet rotor system. In 1949, the Hughes Plane Firm designed its personal model of a heavy carry helicopter, the XH-17. It featured lengthy, stilt-like legs and a box-like fuselage. The legs allowed automobiles to drive below the plane and be lifted away. The Military additionally needed it to hold ordinance, personnel, and different provides. Throughout testing, it did show its potential by lifting payloads as heavy as 10,284 kilos.
The XH-17’s most important rotors have been large. Every weighed 5,000 kilos, was 12 inches thick, and was virtually six ft extensive. A pair of modified J35 turbojets mounted on all sides of the fuselage despatched compressed air to the rotor ideas. This air blended with gasoline and ignited, spinning the rotors.
Nevertheless, testing discovered the XH-17 had a variety of simply 39 miles, far under the Military’s requirement of just about 100 miles. Its tip jet-mounted rotor blades additionally produced vibration stresses, which precipitated it to be regularly grounded throughout testing. In 1955, this system ended when the rotors reached the tip of their design lifespan.
