Aerodynamic modelling saves development costs of Joint Strike Fighter (JSF)
Submitting InstitutionCranfield University
Unit of AssessmentAeronautical, Mechanical, Chemical and Manufacturing Engineering
Summary Impact TypeTechnological
Research Subject Area(s)
Engineering: Aerospace Engineering, Interdisciplinary Engineering
Summary of the impact
Cranfield University has conducted research in jet aerodynamics,
particularly for vertical or short take-off and landing (V/STOL) aircraft
applications, for more than 20 years, with funding from the aerospace
industry, MoD and RCUK, making a major contribution to the continuing
development of the new Joint Strike Fighter aircraft.
The impact of the research has been:
- savings of many £M in development costs of the Lockheed Martin F-35B
(Joint Strike Fighter) by reducing the development time, improving
safety with less restrictive operating conditions, and enabling better
- a series of continuing professional development courses on V/STOL
aircraft design delivered internationally to more than 300 engineers and
The design of jet-lift vertical or short take-off and landing (V/STOL, or
STOVL — short take-off and vertical landing) aircraft introduces
significant aerodynamic complexities. In particular, in vertical flight
the lifting jets point downwards and the entrained air flow induces a
download on the airframe. This download is exacerbated close to the ground
when the lift jets hit the ground and spread out to flow radially. This
so-called wall jet has a number of consequences:
- it increases the entrained flow and, hence, the download on the
- it carries hot engine exhaust gases towards the engine intakes;
- in a relative head-wind the wall jet can separate from the ground and
form an unsteady ground vortex that surrounds the aircraft, carrying hot
gases towards the engine intakes and changing the air-loads on the
Understanding the characteristics of the wall jet and ground vortex — and
how they vary with operating and design conditions — is crucial for the
efficient design and safe operation of jet-lift V/STOL aircraft.
Over the past 20 years Cranfield University has studied jet-induced
flowfields with the aim of understanding and reducing the adverse effects
of these flows on V/STOL aircraft. Our projects have addressed various
flow phenomena, including the development of wall jets from single and
multiple impinging jets [P2], jet/intake interactions [P3, P6], jet mixing
[G1, P6], coannular impinging jets, mean and fluctuating flowfields
associated with the ground vortex [G2, P1, P4], and unsteady fountain
flows [G3, P5].
Our initial studies were aimed at understanding factors that controlled
the location and size of the ground vortex formed when a jet hits the
ground in a cross-flow [P1]. Our results quantified the differences
between impingement on fixed and moving ground planes, confirmed
self-similarity rules for the size of the ground vortex, revealed the
levels of unsteadiness of the ground vortex, and presented initial
findings on multiple jet interactions in cross-flows. This work led us to
more detailed studies of some of the fundamental flow features involved,
starting with the wall jet that feeds the ground vortex.
In the next stage of our research we investigated wall-jet flows produced
by jets hitting the ground [P2]. This showed the influence of initial free-jet conditions on the
development of the wall jet, and the significance of the angle at which
the free jet strikes the ground on the growth of the wall jet. This led to
Cranfield's subsequent studies on the detailed flowfields produced by
multiple impinging jets [P5].
In parallel with our fundamental wall jet studies, we conducted
more-applied research into the mutual interference between free-jet
entrainment flowfields and aircraft intake-induced flows [P3, P6].
More recently Cranfield has used laser-based flow measurement techniques
to investigate unsteady features of the ground vortex [G2, P4] and the
fountain upwash flow produced by multiple impinging jets [G3, P5]. This
work has revealed the stochastic nature of these flowfields.
||Post details and dates
|Dr D Bray
||Lecturer (1988 – 2003);
Senior Lecturer (2003 – date)
|Dr M V Finnis
||Principal Research Fellow (1993 – to date)
|Prof K Knowles
||Senior Lecturer (1991 – 2007);
Professor (2007 – date)
|Dr N J Lawson
||Lecturer (1999 - 2002);
Senior Lecturer (2002 - 2008);
Reader (2008 – to date)
|Dr A J Saddington
||Research Engineer (1993 – 1997); Lecturer (1999 – 2010);
Senior Lecturer (2010 – to date)
References to the research
Evidence of quality — Peer reviewed journal papers
P1* Ground Vortex Formed by Impinging Jets in Cross-flow. K.
Knowles and D. Bray. Journal of Aircraft, 30 (6), pp. 872-878,
P2* Turbulence Measurements in Radial Wall Jets. M. Myszko and K.
Knowles. Experimental Thermal and Fluid Science, 17, pp.
71-78,1998. — Invited paper
P3 Jet/Intake Interference in Short Take off, Vertical Landing
Aircraft. A.J. Saddington and K. Knowles.
Journal of Aircraft, 38 (5), pp. 924-931, 2001.
P4 Particle Image Velocimetry and Laser Doppler Anemometry
Experimental Studies of a Compressible Short Take-off and Vertical
Landing Ground Vortex Flow. N.J. Lawson, J.M. Eyles, and K. Knowles.
Proceedings IMechE Part G: Journal of Aerospace Engineering, 216
(G4), pp. 171-187, 2002.
P5* Flow Measurements in a Short Take-off, Vertical Landing Fountain:
Splayed Jets. A.J. Saddington, K. Knowles, and P.M. Cabritaa.
Journal of Aircraft, 2009, 46 (3), pp. 874-882.
P6 A Review of Out-of-ground-effect Propulsion-induced Interference
on STOVL Aircraft. A.J. Saddington and K. Knowles. Progress in
Aerospace Sciences, 41 (3-4), pp. 175-191. 2005. — Invited
paper Doi: 10.1016/j.paerosci.2005.03.002
* 3 identified references that best indicate the quality of the research
Evidence of quality — underpinning research grants
G1 MoD. Rapid Mixing Technologies for Exhaust Plume Suppression, £25,400,
1995-1997. PI Prof K Knowles
G2 EPSRC (GR/N02504/01). An Experimental Investigation of the
STOVL Ground Vortex Transient Flow, £50,300, 1999-2002.
PI Dr N J Lawson, CI Prof K Knowles
G3 EPSRC (GR/R42894/01). Unsteady Features of Twin-jet STOVL
Ground Effects, £60,300, 2001-2004.
PI Dr A J Saddington, CI Prof K Knowles
Key to papers and grants
a: Rolls-Royce plc
Details of the impact
Cranfield's research on the ground vortex formed when jets hit the ground
in a cross-flow [P1, P4] fed into the development of the Joint Strike
Fighter (JSF) Our results showed that the ground vortex was 20% further
away from the aircraft when a moving ground plane was used, simulating a
rolling vertical landing vs hover in a head wind, simulated with a fixed
ground plane. The wind-tunnel test programme for the JSF jet effects,
involving over 4000 hours in a large multi-national facility, used a fixed
ground plane. This programme alone is estimated to have cost some £5
million; a moving ground plane for such a facility would have cost an
additional £4 million. Our findings allowed the fixed-ground results to be
corrected by engineers in BAE Systems and Lockheed Martin, without the
need for an expensive moving-ground simulation, [C1].
Similarly, BAE Systems used our work on wall-jet development [P2] to
predict safe operating conditions for ground personnel working in the
vicinity of a V/STOL aircraft [C2]. Wall jets, produced by propulsion jets
hitting the ground, grow in thickness radially and with increasing nozzle
height above the ground. Our work showed that the rate of growth with
nozzle height is not linear, contrary to extrapolations from
previously-published data, and that above about 10 nozzle diameters the
growth rate of the wall jet reduces significantly. This allowed much more
accurate predictions of the thickness of the wall jet produced by a
landing aircraft. This, in turn, allows smaller exclusion zones for ground
personnel and, hence, allows a less restrictive operating envelope for the
Our research on jet/intake interactions showed that there was a mutual
aerodynamic interference which would not have been accounted for by
separate powered wind-tunnel model and independent intake tests.
Nevertheless, this effect was shown to be sufficiently small that it did
not require the expense and complexity of fully-powered wind-tunnel models
with simulated jets and intakes to be mounted on force balances [C1].
These results fed into the development of the JSF to provide confidence in
the separate testing of jets and intakes. A combined model, with both
powered jets and powered intakes, would have been impractical and would
have cost more than £1M. Our research saved this expense and gave
designers increased confidence in the validity of their design, mitigating
the risk of an overly restrictive operating envelope for the aircraft.
Overall, an improved, more accurate design process allows for a shortened
flight-test programme. For a complex V/STOL aircraft like the JSF, even a
straightforward flight-test programme costs more than £100M; many times
more if poor design decisions have been taken.
Cranfield's work on co-annular jet flows led to the conclusion that a
gas-driven lift fan, producing a jet with a high-speed outer stream and a
lower-speed inner stream, was not a viable solution for a V/STOL aircraft
because of thrust loss when close to the ground. This allowed government
and industry to drop this as a design possibility, saving many £billions
in wasted development costs, [C3].
The impact of Cranfield's research was enhanced by the delivery of a
series of continuing professional development courses to engineers and
project managers from industry and government agencies. The course,
"Fundamentals of V/STOL Aircraft Design with Joint Strike Fighter
Applications", has so far been delivered by international experts eight
times in the UK and the US, with Cranfield staff delivering lectures on
V/STOL jet effects. The most recent courses ran in Fort Worth, Texas
(2009), home of Lockheed Martin's Joint Strike Fighter engineering and
assembly facility, and in Patuxent River Naval Air Station, Maryland (2010
[C4]), where the Joint Strike Fighter is undergoing flight testing. More
than 300 delegates, primarily from industry and government agencies,
attended these courses, which provided both broad and deep knowledge of
the varied technical aspects of V/STOL jet aircraft design. The courses
raised the competence levels of the design teams and helped them to ensure
the validity of their designs [C1].
Sources to corroborate the impact
C1 Contact: Former Program Manager, Lockheed Martin Skunk Works
C2 Contact: Technologist Advisor, Propulsion Integration, BAE Systems
C3 Contact: Aerodynamics Lead, The Defence Science and Technology
C4 2010 V/STOL course programme:
http://www.paxpartnership.org/index.cfm?action=CL2&Entry=168 — accessed 14/10/13