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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  • 4561385009_b0cd9da786_z.jpgFYI, here's a picture of the XFY-1 Pogo from Convair in the 1960s demonstrating low-alpha forward flight. The major problem they couldn't overcome was the VTOL part as pilots had to look over their shoulder (specifically over the bridge of their nose, sacrificing depth perception). But for a UAV, this problem goes away.

  • Hi Gary,

    You are right that the tail-sitter concept is an old one. It goes back to the Second World War actually. The famous tailsitter program produced several prototypes, the most famous of which was of course, the XFY-1 Pogo. Where I think you are mistaken is the idea that such an aircraft has to fly at such a high angle-of-attack. That is not what was seen in the Pogo program and not what we have seen in our testing either.

    I will add that if you are not interested in the larger aircraft, we have smaller ones that are ideal for backyard 'goofing around' that demonstrates "missile-mode" flight as well. They are great toys and a blast to fly.

  • Cute, far from practical as shown.

    This is really just another tail sitter Vtol which has already been done by memebers of the DIYD multiple times with from two to six propellers.

    The basic concept works fine, in the early days when they were trying this they had a lot of difficulty with transition, from forward to hover, but now even that is pretty well worked out.

    What is not worked out is the high side profile in hover (they tend to get blown all over the place by winds and gusts.

    And a far more advantageous profile for forward flight is one that features two horizontal wings and vertical control surfaces as necessary.

    Will this thing fly, sure, but even in horizontal flight it is going to be at a cripplingly bad aerodynamic angle probably close to 45 degrees which is about what you get with a fast moving quad.

    And control is going to be hampered by the 45 degree aerodynamic ("wings").

    As bad as they are a conventional tail sitter VTOL design is better than this with real horizontal wings and control surfaces.

    The only thing this has going for it is that it looks cool in a somewhat dangerous looking way and calling it missile mode doesn't help either.

    Please note that the longer range "cruise" missiles they refer to actually have horizontal wings with control surfaces, without that they are an inefficient mess.

    Great toy, I'd buy one for the kids for Christmas.

    BTW nobody is saying a fixed wing isn't more efficient than a helicopter, but you will have a lot of difficulty making anybody think this even begins to approach the efficiency of a conventional fixed wing - no way - no how.

    Fast manned helis lose efficiency very quickly as the rotor tip approaches mach 1, but the fact is UAV helis rotors never approach anything remotely like that and the slow rotor versions are really quite efficient (way more efficient than an equivalent multicopter, but not as efficient as an equivalent fixed wing). 

    I think I'll back there $10.00 for 3 stickers level, I can send one to the FAA, I'm just sure they are going to love Quad Rockets.

    Best Regards,


  • All those crazy people claiming that a multirotor would be used to take out a manned aircraft finally have some credibility.  A quad that might be fast enough to successfully target them and in a missile form factor...

    I have seen a lot of crazy kickstarter projects but this one has to be the worst, unless of course the intent is to give the anti-drone losers more ammunition to attack our hobby.

  • Hi Bart,

    I'm our test pilot on the different aircraft and I'd be happy to answer your questions.

    All things being equal (so at the same power-available and power-to-weight), the QuadRKT's design offers a lower drag shape which fundamentally means a higher top speed than either quadrotors or fixed-wing airplanes can offer. By how much? That is something I cannot say yet with certainty - we have been unable to successfully record the flat-out top speed with radar guns. We simply can't get a reliable radar return. We estimate a top speed of the larger airframes to be in the range of 50-60 mph right now just using 2S lipo batteries. I can tell you that with some forensic analysis and proper experimental controls, we were able to determine the small QuadRKT, under rocket boosting, reached a speed of 133 mph. In a climb. The shape is clearly fast - our cameraman expressed his difficulty in keeping it in frame as it shot past him.

    I am in the process of working on two things. First, to integrate a GPS with the data logger to answer once-and-for-all your question of just how much faster we are. And second, to push us to electronics that bring us to the world of 3S lipos and racing-level electronics. This will bring our thrust-to-weight ratio to around 7. 4S lipos (in my opinion) are overkill and a symptom of the inefficiency of current quads and pilots demanding more out of a weak design - but that could push performance even higher if you want it.

    We are pushing not only to break speed records, but to disrupt the wasteful brute-force methods of current quadrotors. This is our message to racers out there: If you want more speed, stop wasting power on lift. Work Smarter, Not Harder.

    Thanks for the question. I'd be happy to answer any others!

  • Exactly how fast and how far can this thing fly and how does this compare to a normal quad (with the same electronics and propellers but without the missile look) ?

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