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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  • Those speeds are easily archieveable with most FPV racing frames.

  • T3


    This is a neat design.  I like the idea of slimming down the avionics package to minimize its cross sectional area in horizontal flight.  Not sure what I'd use this for as I do not fly much recreationally, but it looks fun.

    However I am a bit apprehensive about your marketing choices.  I myself do military oriented aircraft work, and I am not comfortable flying something with "missile" in the tagline.  I also feel that flying one of these in the park would be much more likely for the police to be called on me than a standard quad (because it looks somewhat like a missile.)  I feel like this product would be bad for the quadrotor's already beleaguered reputation.

  • Hi Gary,

    We've considered doing that. Depending on the actual role, biasing the wing area can make a lot of sense. A high wing area and AR makes a lot of sense for something that needs a long loiter time at low power but not necessarily to hover. The small footprint though is advantageous for higher top speed and maneuverability. Smaller wings means less aerodynamic damping.

    Your thoughts on the larger props also makes a lot of sense. There exists a big tradeoff between prop diameter, wing area (and therefore drag and top speed), and power draw. There is no reason the wings must have equal areas, that just simplified our tooling at this stage.

    You are also right in pointing out the speed effects on the quad motors. The faster you go, the less authority you get from the motors' variable thrust. The control redundancy from the surfaces makes sure you have effective control from zero speed all the way to the top speed.

    Lastly, I am a big proponent of safe hobbyist flight - I'm registered with the AMA and FAA. No hobby is worth risking injury or property loss. I've been so busy with other things that I've been away from DIY Drones for a while, but I'm glad to see as a community you care so much about safety and the health of the hobby. That's the right tone to have and I'll stand with you all day long.

  • Hi Richard,

    Certainly flying horizontal in the + configuration helps, effectively dedicating 2 of your stabilizers as wings.

    But wouldn't at least an asymmetrical configuration with smaller vertical surfaces versus larger Horizontal surfaces be more functional?

    Also, the bigger the propellers you use the more efficient it will be, I suggest you might consider using a design that will let you use as big a props as possible.

    Of course you actually have two modes of operation hover and forward flight , so you also need motors with a wider than normal efficient power range.

    That said, you really might want to be careful about making comparisons with and analogies to military hardware.

    That is a very touchy subject here and with the public in general (and our regulatory bodies) and as some of the others here have indicated, no good can come of it and you will piss off some of the people who could be really helpful to you.

    I understand the attraction, but I strongly suggest you find a more acceptable non military oriented way of presenting yourself.

    The cruise missile which you illustrate and which is most similar in design and flight envelope to your concept has pop out wings with much greater surface area (and lift and drag) than the vertical surfaces which do not provide lift.

    The missiles that are symmetrical are very high performance high turn rate intercept missiles which generally operate without any regard to up and down (gravity) and work fine till they run out of their massive thrust fuel load.

    Yours looks cool, but for functional optimization I am sure a relatively big horizontal flying surfaces versus small vertical stabilization would be considerably more efficient and faster.

    The extra vertical airfoil mass is just drag and wastes power.

    Vertical stabilizers can simply be entirely a small control surface since lift is just wasted drag on them.

    And horizontal flight in an X mode simply effectively wastes half the lift of each wing(let).

    Good to know you are actually going to use control surfaces, the more like a fixed wing plane you make it the less effective using quad motor speed offset becomes for control.

    Best Regards,


  • Hi Gary,

    Ours also uses wings with control surfaces. And like all VTOL aircraft, we too have a high power-to-weight, and we're pushing that higher to expand our envelope. The carbon airframe shown in the image there shows the locations for the surfaces but they were not yet installed for that picture. We can also fly in the X or + configurations. I've tended to fly in the + configuration like you're pointing out.

    I think we may have to agree to disagree on the design philosophy, but thanks for your input. There are many ways to skin this cat.

  • Hi Richard, Note the Pogo is using two horizontal control surface containing wings and had an enormously powerful engine (it could still barely hover and like it's counterparts had a horrible time landing) even with the counter rotating props.

    While I totally agree with Robert that current FPV racing frames are an aerodynamic travesty, I do not think the general approach you are taking is the answer.



  • Hi Rob,

    Very cool video! You can really hear the main rotor grabbing at the air.

    It is true there are some ways to push the limits of conventional helicopters top speeds and one of them is to minimize the airframe blockage and therefore drag which that video clearly shows. That same reduction in drag is part of what we're doing. I completely agree with you about the brick and "starfish" body designs. There are some racing quads coming out that are starting to challenge that notion, but we want to push it further. Just streamlined airframes can make a lot of difference.

    This post was mostly pointing out the usefulness of transitioned horizontal flight for speed - something most people have never considered. The push to a single large rotor does wonders for efficiency that you gain by reducing the helicopter's disk loading. I have no doubt that with the higher onboard power we could get this design to push beyond 100-150 km/hr. We're still running around on 7.4V and just starting to push the envelope outside our early flight testing. Put us on an equal power-to-weight footing and I think you'll see something really impressive. We just need the backing to get there.

  • I actually think this is neat, and something the FPV racers should be looking at.  I get tired watching these really lazy brick designs.  Even worse are the "starfish" bodies which actually make things worse.

    But you're going to have to work a lot harder to make the claim about these things being faster than helicopters.  I'm already doing 130 km/h with a <2kg helicopter, autonomously.  And the record for RC helis is 275km/h. :)

  • Hi earthpatrol,

    What we are bringing to market is not a weapon. It is a collection of toys and hobby racers. The design takes several cues from the military world in regard to its horizontal mode of flight, but there is nothing onboard that is remotely weaponized. The aircraft is simply a platform that we think could be fun for hobbyists. The smallest weighs 20g. The largest right now weighs 250g, inching up to the minimums for FAA registration. They pose minimal threat to manned aircraft or people, particularly when flown safely under the guidelines proposed by the FAA and AMA for hobbyists.

    The complaint you have about its potential for military use is one that could be levied against any aircraft on this website, particularly the ones touting autonomous waypoint navigation.

  • What part of cruise missile and SAM's images makes it past the moderators? Pretty blatant military references that allude to other uses for this design. Simply invert the design, give the missile part the ability to decouple from the lifting device and viola, at worst you've got yourself a gravity driven projectile. Add some control surfaces and some propulsion and you've got a guided projectile. Pretty sure this is way more of a DIYD TOS violation than the book being peddled here a few days ago. -1 to DIYD moderators.

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