An idiot’s guide to aircraft design – Part 4: Manoeuvre Performance

When we think of a fighter aircraft we think of its high manoeuvrability. Even today, this exciting and romantic trait is still highly desirable. We look at the best ‘turners and burners’ in service today and the science behind it. 

A missile needs to be placed into the right section of sky to kill its target, and a fighter aircraft must also have a decent chance of dodging enemy missiles. High manoeuvrability also gives the fighter a greater opportunity to evade enemy sensors or eyes. Even when missiles can be told the position of their target not just through their own limited ‘vision’ by via the direction the pilot is pointing her head, or sensors both on and off the launcher aircraft, manoeuvrability is still valuable. High manoeuvrability is expensive though both in terms of the g-force it generates, and the demands it will put on the design of the aeroplane. The g-force, is a measurement of the type of force that causes a perception of weight. On Earth normal gravity gives us 1G conditions, and that’s what the human body is best at dealing with. A hard manoeuvring fighter can reach 9G, though greater G is possible, 9G is the effective limit of what the body can withstand repeatedly while performing the tasks required of a fighter pilot. At 9G a 100kg pilot would feel and move as if he weighed 900kg.

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Over to Jim Smith for more:  “For significant parts of the flight envelope, manoeuvre performance may be limited by the structural design of the aircraft, which is likely to be constrained to no more than 9g. This is due to the limitations of the human pilot, even supported by a ‘g-suit’. One key manoeuvre parameter is the instantaneous turn rate (the ability to suddenly pull a turn from level flight)which fundamentally depends on wing loading (how much weight each square of metre is supporting) and usable lift coefficient (in simple terms, how much lift is available to the aircraft). The significance is that this is a measure of how rapidly energy may be traded against turn rate to temporarily point the nose to the aircraft, for example to gain a firing opportunity, or to evade a threat such as a surface-to-air missile. Since supersonic combat aircraft have relatively low lift curve slopes*, due to sweep, and low aspect ratio wings, a number of the following may be used to provide a short-term increase in turn rate: Thrust-vectoring (the mechanical steering of the jet exhaust) provides a powerful way of nose-pointing, particularly at relatively low speeds; A delta wing with sharp leading edges will generate a leading edge vortex, which will increase both lift and drag; A leading-edge root extension (LERX) or strake may be added to a lower sweep wing to mimic the vortex flows generated by a delta and increase lift; Higher thrust-to-weight ratio may be required to overcome the drag at high incidence – particularly if the turn is to be sustained, rather than allowing energy to bleed off; Finally, unstable configurations are preferred, as these maximise the effect of controls. European and Chinese aircraft favour the use of a destabilising canard, while US aircraft generally do not, preferring closely-coupled tailed near-delta configurations.

*Lift curve slope is the amount of lift you get for a given angle between the wing and the airflow. Low lift curve slope means this is less than usual.

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 A leading-edge root extension (LERX) – the curving surface joining the front of the wing to the main body of the aircraft- may be added to a lower sweep wing to mimic the vortex flows generated by a delta and increase lift.

For instantaneous turn rate  the aircraft may be either structurally limited to 9g, or aerodynamically limited by the lift available, dependent on the maximum possible wing lift (known as CLmax), speed, density and wing loading. Except for that area of the flight envelope where the aircraft is capable of delivering a sustained turn at 9g, energy and speed will reduce, and the rate of reduction will depend on Thrust to Weight ratio (high T/W reduces decay rate), and lift dependent drag (high lift dependent drag increases decay rate). 

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European and Chinese aircraft favour the use of a destabilising canard, while US aircraft generally do not, preferring closely-coupled tailed near-delta configurations. The Russian approach largely uses closely-coupled tailed near-deltas, but sometimes includes canards and in the near future , with Su-57,  will include adjustable leading–edge vortex controllers (LEVCONs)

At altitude, at some point, an instantaneous turn rate of 9g will no longer be achievable because the wing will have reached maximum available lift, Clmax. Above this altitude, the turn rate available will depend on wing loading and Cl max, and the bleed off in energy will depend on T/W, and lift-dependent drag as indicated above. Thrust vectoring may assist in generating a rapid pitch response, as will an unstable configuration with an advanced flight control system.

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At transonic and supersonic speeds, wave drag will become an additional factor, with high wave drag increasing the speed decay rate.

From all this, we can extract the following pointers for good instantaneous turn rate:

– Low wing loading (the ‘wing loading’ is how much weight each square metre of wing is supporting)

– High max lift coefficient

– Thrust Vectoring

– Unstable designs with advanced Flight Control Systems

And for lower bleed-off in energy

– Low lift-dependent drag

– High Thrust to Weight

– Low wave drag if transonic or supersonic

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The close coupled Euro-canards, Typhoon, Rafale and Gripen are likely to be very good; F-22 is also good due to high T/W, thrust vectoring, and wing area; Su-35 likely to be pretty good too – big wing, reasonable aspect ratio, canards, and thrust vectoring. F-35 will perhaps have more energy bleed off due to its higher wave drag, lower T/W, and higher wing loading

Best current aircraft: Difficult to assess and likely to vary dependent on Mach number and altitude, but suggest Typhoon and Rafale, with perhaps Gripen, F-22 and Su-35 also very good.

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Sustained turn rate for part of the flight envelope will be limited to the best that can be achieved with a ‘g-suit’ equipped human pilot. Reaching those levels may influence wing design, through wing loading, aspect ratio and sweep, unless these are constrained by other requirements. Thrust-to-weight ratio will possibly also be influenced by the turn rates required, as sustained turning flight is a high drag situation.

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However, the area of the flight envelope in which the aircraft will be capable of sustaining 9g will be substantially less than the area in which it can generate an instantaneous 9g turn rate. To generate and sustain a high turn rate, the aircraft will be relying on the extra energy available – as we have seen ((T-D)/W) x V, but with the wing at high lift.

For good sustained turn rate, we need:

– Low lift-dependent drag, and hence a higher aspect ratio

– Low wave drag if transonic or supersonic – noting this is likely to drive to low aspect ratio and high sweep

– High Thrust to Weight

– Low wing loading

The highly-optimised close-coupled Euro-canards are likely to be the best current aircraft; the Su-35 has higher aspect ratio but potentially higher wing loading. I suspect F-22 will be competitive, but F-35 is likely to have lower sustained turn rate, as it has higher wing loading. In considering the F-22 and F-35, one should remember that the operating concept for both is likely to avoid the close-in turning fight typical of within-visual-range air combat. 

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Flight at high alpha: Flight at a high angle of attack where the nose is high relative to the direction of flight. Twin tails are better at maintaining controllability at high alpha.

 

Flight at high-alpha seems to me to be a contentious requirement that, in general should not be a design driver. At high incidence, a combat aircraft is likely to be at low, or very low speed. While, given powerful control effectors, this may minimise turn radius and allow rapid change in nose pointing angle, the loss of energy may make surviving a missile engagement very unlikely, and re-joining combat difficult. 

However, given the convergence of structural limits, and the limitation of airshow performances to subsonic speeds, high-alpha performance remains one way of impressing the tax-payers. With unstable aircraft, thrust vectoring and a host of other aerodynamic gizmos, the Su-35 is probably champion at this. But many of today’s aircraft have at least equally high thrust to weight ratios, and similar aerodynamic and structural performance at low altitude and subsonic speeds. Personal experience of displays by the F-22, F-35, Typhoon, Rafale, Su-27, Su-35 and even the less capable Super Hornet show that all of these can put on a jolly good airshow performance.

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On turning performance, and generally awesome airshow characteristics, the canard-equipped, thrust-vectoring Su-35 gets my vote for high alpha performance. On more general manoeuvre performance, all the other aircraft mentioned are very capable, with Typhoon, Rafale and F-22 all benefitting from high thrust to weight ratio, clean aerodynamic design and sophisticated flight control systems.

Click here for part one.

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Have a look at How to kill a RaptorAn Idiot’s Guide to Chinese Flankers, the 10 worst British military aircraftThe 10 worst French aircraft,  Su-35 versus Typhoon10 Best fighters of World War II top WVR and BVR fighters of today, an interview with a Super Hornet pilot and a Pacifist’s Guide to Warplanes. Was the Spitfire overrated? Want something more bizarre? The Top Ten fictional aircraft is a fascinating read, as is The Strange Story and The Planet Satellite. The Fashion Versus Aircraft Camo is also a real cracker. Those interested in the Cold Way should read A pilot’s guide to flying and fighting in the Lightning. Those feeling less belligerent may enjoy A pilot’s farewell to the Airbus A340. Looking for something more humorous? Have a look at this F-35 satire and ‘Werner Herzog’s Guide to pusher bi-planes or the Ten most boring aircraft. In the mood for something more offensive? Try the NSFW 10 best looking American airplanes, or the same but for Canadians. 

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