New X-plane Aurora CRANE announced for DARPA: Our analysis

Hush-Kit has asked me to write an analysis piece about a new programme which has just been announced by Aurora Flight Sciences. The plan is to develop a novel configuration which uses active flow control (AFC) to provide a flight control system for the demonstrator, without using moving control surfaces. The project is under contract to the Defence Advanced Research Projects Agency, DARPA, as part of their Control of Revolutionary Aircraft with Novel Effectors (CRANE) effort.

The technology is of interest as part of the technology path towards extremely low-observable aircraft. Moving control surfaces cannot be incorporated in the design of such aircraft without the introduction of gaps, edges, actuators and support structures, all of which can introduce discontinuities, edges and gaps, increasing the signature, even when the air vehicle is flying straight and level, and control effectors are not being used. The deflection of control surfaces required to manoeuvre stealthy air vehicles can be quite large, and can generate undesirable and detectable radar returns.

F-35 showing extreme elevon deflection, Avalon Air Show 2019

The first phase (Phase 0) will examine mission applications in which AFC technologies would be useful, and the programme will then go on to develop, in Phase 1, a design for an X-plane demonstrator exploiting AFC, and presumably able to demonstrate the benefits of such a technology in relevant mission scenarios, informed by the Phase 0 studies.
The exemplar used in the Aurora press release to illustrate the concept is a wind tunnel model of a joined wing concept, with blended body and a diamond planform. The model has no vertical fin, and no conventional control surfaces are visible.

Attractions of joined wing concepts (according to my 1977 MPhil thesis on the topic) include an inherently strong structure with both forward and aft swept lifting surfaces which, being joined at the tips, results in low structure weight for a given stiffness Other advantages include a wide range of centre of gravity; predictable behaviour at the stall; and low rolling inertia.

A range of active flow control technologies might be used, including circulation control exploiting the Coanda effect, or by the use of air jets or suction to manipulate the boundary layer. The Coanda effect has been described by its inventor, Henri Coanda, as “the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops”.

This technique has been investigated (by Dr R V (Ron) Smith, among others), and shown to be capable of generating large, and controllable, forces which might be used for control and lift augmentation purposes. The diagram below shows the flow around an aerofoil with an air blowing system providing very high lift. The principle has been successfully applied in the development of the MD900 helicopter, where the tail boom is used as a Coanda device, replacing the tail rotor.

Diagram of flow around an Aerofoil with Coanda-effect flap. Source: Flight Handbook edited by Bill Gunston (1962 edition)

In its objective, the Aurora demonstrator, has much in common with the BAE Systems and Cranfield University Demon UAV, which has demonstrated controlled flight without the use of conventional moving control surfaces. The Demon was developed as a demonstrator for the flapless air vehicle integrated industrial research (FLAVIIR) programme. It was used successfully in 2010 in a flight demonstration in which its conventional flight control surfaces were disabled, and control was maintained using a combination of Coanda flow control and a thrust-vectoring nozzle.
The Demon uses a diamond-shaped wing planform, but unlike the Aurora wind tunnel model, hr Demon has a one-piece wing, rather than having joined front and rear wing components.

While BAE have pointed to potential civilian applications for the circulation control systems embodied in Demon, it seems clear to me that the driving interest behind both the DARPA/Aurora and BAE programmes is the reduction in signature of future combat aircraft.

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It is no coincidence that almost all schematic illustrations of future Gen 6 fighters have no vertical tail fins. The technologies being investigated in the CRANE and FLAVIIR programmes are clearly an enabling step to the development of ELO aircraft.

Cue sound clip – Roll Over Beethoven by the ELO. 

-–– Jim Smith

I hope my answers will satisfy you and thank you for your interest in our world of “Chasse B …”,
dedicated methods and reserved for insiders.
Jean Copponnex

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  1. Ron Smith

    Seems like a combination of Jim’s joined wing MPhil thesis and my PhD thesis (A Theoretical and Experimental Study of Circulation Control with reference to Fixed Wing Applications) looks peculiarly prescient …

  2. Brian

    What am I missing? The configuration they are starting with doesn’t look stealthy to start with. It has more edges and junctions which, I think, would contribute to a greater radar return, than a more conventional flying wing.

    • Jim Smith

      That configuration is, I am guessing, there as an attention getter, rather than an indication of the final configuration. I describe it in the article as an exemplar – an example of a novel configuration with low or negative lateral and directional stability that can be used to demonstrate active flow control strategies. The final demonstrator will be shaped by whichever requirement they decide to pursue, but will probably have no vertical fin, and size, platform etc is t.b.d.
      But the eventual intent will to be to evolve towards ELO aircraft, and control systems which avoid moving surfaces.

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