# What makes the F/A-XX twin-engined tailless fighter controllable with engines out?

There are operational tailless aircraft such as B-2 spirit, X-45. It seems yaw stability/control at slow speed is achieved aerodynamically, using either split elevons (B-2) or adjacent elevons deflected in opposite directions (X-45) Combined with negative wing tip twist angle, both solutions increase drag / relative swept angle, and by the way, yaw stability. Also note both examples have a very small vertical surface ahead of CoG.

(Left, Right)

On the other hand, looking at this picture of a F/A-XX Boeing concept, it seems vertical surface ahead of center of gravity is quite significant, and forward swept trailing edge is in contradiction with aerodynamic stabilisation using split elevons.

One might think this isn't such a big issue since this concept might use permanent vector thrust adjustments on yaw axis.

Question is : Does it fly with no yaw thrust vectoring?

Edit: One other issue affecting yaw stability is forward swept elevons. Their "split brake" efficiency decreases while sideslip angle increases :

(Source)

• I don't really understand what you're asking here, or whether you have multiple questions in one post. Are you asking whether the Boeing concept could fly with one engine, or any of them?
– GdD
Oct 12, 2017 at 11:31
• @GdD edited : does it fly with no yaw thrust vectoring?
– user21228
Oct 12, 2017 at 11:48
• Same as the other two you've showed, brakes on the wing tips. Indeed if the aircraft is too unstable then fly by wire would need to use those brakes a lot, then drag would be a problem. However this picture is only an illustration anyway, so we will see. Oct 12, 2017 at 14:20
• (B-2, X-45A) "both examples have a very small vertical surface ahead of CoG" and (F/A-XX) "it seems vertical surface ahead of center of gravity is quite significant". Forgive me, but I see only a very small vertical surface on the X-45A, and it looks like it's holding up a very tiny AWACS type radome. Can you be more specific about where the vertical surfaces are on the B-2 and the F/A-XX? I'm just not seeing them... Oct 12, 2017 at 20:57
• Please provide sources for your images. Oct 12, 2017 at 20:57

Those flying wings even before fly-by-wire (FBW) came along are controlled in the yaw axis by asymmetrically deploying the wing-tip mounted split brakes.*

(Source)

With the engines closer to the centerline the effect of one engine inoperative (OEI) is not as pronounced as on jet-liners with the engines far out on the wings. Even with the lack of a vertical fin, the effect will be less than if the engines were more outboard.

In the Boeing 727 it was discovered that an engine out during takeoff did not have a noticeable effect on the yaw, to the point such planes with close engines had different certification for the V-speeds to allow extra time for the identification of the failure. As well as glareshield mounted lights to indicate an engine failure in a clearer manner.

As you can see in the pictures you have, the engines are close, like the 727. And therefore the standard yaw control via split brake rudders* is sufficient.

As for the F/A-XX, it seems it's still a concept. The second image is also a single-engine UAV.

* The aileron-like device that is split at the wing tips. By creating asymmetric drag, the plane yaws.

Unlike conventional aircraft, truly "tailless" flying wings cannot use a rudder for lateral control as it was absent, so a set of clamshell-like double split flaps on the trailing edge of the wingtips were used. When aileron control was input, they were deflected up or down as a single unit, just like an aileron. When rudder input was made, the two surfaces on one side opened, top and bottom, creating drag, and yawing the aircraft. By applying input to both rudder pedals, both sets of surfaces were deployed creating drag so that the airspeed or the glide angle could be manipulated.

• The 727 had a vertical tail and was aerodynamically stable. It would have had a much harder time commanding yaw by means of differential thrust than a B737. Oct 12, 2017 at 22:31

These two aircraaft are aerodynamically unstable in yaw. Stability is not provided by differential engine thrust, but by active differential control of the speed brakes. Differential engine thrust is not fast enough and lacks the frequency response.

The situation is analogous to the F16: the airframe itself is aerodynamically unstable and requires constant active control by a feedback loop automatic system (an inner loop) in order to fly. The flight computers must be powered and functional, otherwise the aeroplane falls out of the sky, no matter how many engines are working.

According to the wiki, differential thrust is used for flight path control. This is in addition to flight path control by the elevons/speedbrakes, which is done by superimposing an offset on top of the rapid stabilisation movements: that is how it flies with no yaw thrust vectoring.

• @ymb1 yes, it uses engine thrust for flight path control when available, otherwise it superimposes a control offset on top of the high frequency control signals for stabilisation. Oct 12, 2017 at 22:23