Editing Pulsejet (section)

==Design==
[[File:Puls1Motor en.gif|right|thumb|350px|Animation of a pulsejet engine]]

Pulsejet engines are characterized by simplicity, low cost of construction, and high noise levels. While the [[thrust-to-weight ratio]] is excellent, [[thrust specific fuel consumption]] is very poor.  The pulsejet uses the [[Lenoir cycle]], which, lacking an external compressive driver such as the [[Otto cycle]]'s piston, or the [[Brayton cycle]]'s compression turbine, drives compression with [[acoustic resonance]] in a tube. This limits the maximum pre-combustion pressure ratio, to around 1.2 to 1.

The high noise levels usually make them impractical for other than military and other similarly restricted applications.<ref name="ReferenceA">Jan Roskam, Chuan-Tau Edward Lan; ''Airplane aerodynamics and performance'', DARcorporation: 1997, {{ISBN|1-884885-44-6}}, 711 pages</ref> However, pulsejets are used on a large scale as industrial drying systems, and there has been a resurgence in studying these engines for applications such as high-output heating, biomass conversion, and alternative energy systems, as pulsejets can run on almost anything that burns, including particulate fuels such as sawdust or coal powder.

Pulsejets have been used to power experimental helicopters, the engines being attached to the ends of the rotor blades. In providing power to helicopter rotors, pulsejets have the advantage over turbine or piston engines of not producing [[torque]] upon the [[fuselage]] since they don't apply force to the shaft, but push the tips. A helicopter may then be built without a tail rotor and its associated transmission and drive shaft, simplifying the aircraft ([[Helicopter flight controls#Cyclic|cyclic]] and [[Helicopter flight controls#Collective|collective]] control of the main rotor is still necessary). This concept was being considered as early as 1947 when the American Helicopter Company started work on its XA-5 Top Sergeant helicopter prototype powered by pulsejet engines at the rotor tips.<ref>{{cite web|url=http://www.flightglobal.com/FlightPDFArchive/1949/1949%20-%200879.PDF |title=Excerpt of Flight May 12, 1949 |publisher=flightglobal.com |access-date=31 August 2014}}</ref> The XA-5 first flew in January 1949 and was followed by the XA-6 Buck Private with the same pulsejet design. Also in 1949 [[Hiller Helicopters]] built and tested the Hiller Powerblade, the world's first hot-cycle pressure-jet rotor. Hiller switched to tip mounted ramjets but American Helicopter went on to develop the XA-8 under a U.S. Army contract. It first flew in 1952 and was known as the [[American Helicopter XH-26 Jet Jeep|XH-26 Jet Jeep]]. It used XPJ49 pulsejets mounted at the rotor tips.  The XH-26 met all its main design objectives but the Army cancelled the project because of the unacceptable level of noise of the pulsejets and the fact that the drag of the pulsejets at the rotor tips made [[autorotation]] landings very problematic. Rotor-tip propulsion has been claimed to reduce the cost of production of rotary-wing craft to 1/10 of that for conventional powered rotary-wing aircraft.<ref name="ReferenceA"/>

Pulsejets have also been used in both [[Control line|control-line]] and [[Radio-controlled aircraft|radio-controlled model aircraft]]. The speed record for control-line pulsejet-powered model aircraft is greater than 200 miles per hour (322&nbsp;km/h).

The speed of a free-flying radio-controlled pulsejet is limited by the engine's intake design. At around 450&nbsp;km/h (280&nbsp;mph) most valved engines' valve systems stop fully closing owing to ram air pressure, which results in performance loss.

Variable intake geometry lets the engine produce full power at most speeds by optimizing for whatever speed at which the air enters the pulsejet. Valveless designs are not as negatively affected by ram air pressure as other designs, as they were never intended to stop the flow out of the intake, and can significantly increase in power at speed.

Another feature of pulsejet engines is that their thrust can be increased by a specially shaped duct placed behind the engine. The duct acts as an [[Closed wing|annular wing]], which evens out the pulsating thrust, by harnessing aerodynamic forces in the pulsejet exhaust. The duct, typically called an augmentor, can significantly increase the thrust of a pulsejet with no additional fuel consumption. Gains of 100% increases in thrust are possible, resulting in a much higher [[fuel efficiency]]. However, the larger the augmenter duct, the more drag it produces, and it is only effective within specific speed ranges.

=== Wave ===
J-1 is a u-shaped device designed for UAVs with up to 200-lb (90-kg) gross vehicle weight. It weighs 18 lb (8.2 kg) and measures 5.5 x 12.5 x 64 inches (14 x 32 x 163 cm). It can run on fuels such as gasoline, E85 bioethanol, or jet fuel. Its thrust reaches up to {{Convert|55|lbf||abbr=on}}. When fuel ignites, the increased temperature and pressure push hot gasses out of the device, creating thrust. The resulting partial vacuum pulls in fresh air, preparing for the next pulse.<ref name=":02" /> The engine family has been tested at up to {{Convert|200|mph|abbr=on}}.<ref name=":02" />

Wave is working on a second engine, the K-1, with up to {{Convert|220|lbf|abbr=on}} of thrust for up to {{Convert|1000|lb|abbr=on}}. It claims that this will benefit larger commercial applications and a new class of [[VTOL]].