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What is Missile Mode Flight?

XQ-139AC Features“Missile Mode Flight” is just what it sounds like – forward flight in a configuration like that of a conventional missile. This is not a revolutionary concept – missiles have flown under these conditions for decades. What is new, however, is the concept of a missile that can change its flight mode. Something that can hover like a helicopter and also fly forward like a missile. Missiles and fixed-wing aircraft in general are limited to this single mode of horizontal flight. Slowing the aircraft down to zero airspeed would mean stalling the wing causing it to stop producing lift and result in the aircraft falling out of control.

Common Missiles
If you needed an aircraft that could hover in place (perhaps for Search-And-Rescue), you instead used a helicopter. Helicopters do a fine job of hovering, but are severely limited in top speed by several factors. The faster a helicopter flies forward, the higher its advancing blade’s tip speed is. Eventually the tip of the rotor blade will reach the speed of sound, Mach 1, causing tremendous drag and therefore fuel burn, pressure forces on the blade structure, and noise. Because helicopter blades spin very fast, the tip of the rotor blade can reach Mach 1 well before the helicopter itself reaches such a speed.

Helicopter Rotor Disk
In fact, the fastest conventional helicopter, The Westland Lynx, can only reach a speed of 201 mph. That’s blazing fast for a car but fixed-wing airplanes have flown faster than that since the 1940’s. A comparable airplane that also uses turbine engines for onboard power, the Piaggio Avanti, has a top speed of 458 mph – more than double that of the fastest conventional helicopter. Even the Eurocopter X3 – the fastest “compound” helicopter (a design that uses propellers to generate extra thrust and speed) has a top speed of just 293 mph. If you want a vehicle that hovers, you would begrudgingly accept a low top speed.

Comparison Chart of Maximum Speed and Onboard Power
Helicopters have to use the majority of their onboard power just to stay in the air – the spinning rotors generate all the lift needed to hover. This requires much more onboard power than a fixed-wing airplane of the same weight. If that power could be used to hover, and then to generate only thrust in forward flight, much higher speeds could be reached. This is the driving theory behind aircraft like the V-22 Osprey and F-35B Lightning II. But these aircraft also need heavy and expensive mechanisms.

V-22 and F-35B
An aircraft that could take-off and hover in a “nose-up” configuration and then pitch over into “missile-mode” would be faster and more efficient than both helicopters and fixed-wing airplanes. This is the new capability the XQ-139 design brings the US Government, Private Companies, and now the consumers and hobbyists. The design requires no complicated mechanisms to transition between hover mode and “missile mode” flight and back again. The wings do not need to be sized for take-off and landing so they are smaller and simpler than a comparable fixed-wing airplane’s wings. That means less weight and less drag which result in longer flight times and higher top speeds.

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