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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  • Hi Hans,

    Yeah I remember the quadshot, they really had an uphill battle back then to get the transition right and with side winds in hover.

    They got it to work pretty welll, but winds and gusts landing and taking off were always a problem.

    I think my design might be better from that aspect, as very little side profile is present if you automate weathervaning parallel to the wind.

    A feature which is already available on quadplane in ArduPlane.

    I think quadshot might have actually suffered by their "vertical stabilizers" and light weight in winds and gusts.



  • Gary McCray nice work! Vastly improved. You got the CG right also.

    However "quadshot" from ... 2010 (?) got this down perfect.


  • Hi Richard,

    This concept does certainly embody the seed of a good idea and as you said about agreeing to disagree on design philosophy, it got me to thinking about what possibly a more optimally functional tail sitter quad plane might be like that embodied the concepts I was speaking of.

    I liked it so much I think I'm actually going to build it.

    Behold, the QuadroPlane - All comments welcome:


  • Developer

    Good post!  It's got a lot of people talking at least!

    I think the major problem is going to be the lack of yaw contrrol which is (I hear) also a problem with the similar XCraft PlusOne drone (flight test video here).  I'd highly recommend having at least two moving fins - that will turn it into more of a SingleCopter but it will resolve the yaw control problems I think.

  • Hi Aleksey, It makes a tremendous difference. At 2 cells the battery is nominally at 7.4V. A 4 cell battery doubles the voltage to 14.8V. That allows you to swing the same prop faster as long as you can provide enough current.
  • 4s battery, but does it really matters?

  • Ha! Great classic show. So many interesting vehicles.

  • @Richard,

    Your design reminds me of this classic Thunderbirds 3:


  • Stephen, It's loads of fun to fly at our local AMA field. I don't think I would fly anything in a public park with people around, except maybe a small foamie or one of the smaller 20g quads. Unfortunately, we have to deal with the public sometimes and the best thing we can do is educate them - show them there is no risk or safety hazard by flying things safely, in approved spaces, and being open about what is inside.

    Aleksey, on a 2-cell battery? I find that hard to believe.

    Digital Wings, The design proposal was originally submitted to DARPA for development but did not win a contract. We have no further connection to them.

  • The max perk says that the design is going to DARPA :-)
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