An idiot’s guide to aircraft design: Introduction
What is the fastest climbing aircraft? And the best at sustained turn? Which is the nastiest to land..and the nicest? And what is it about a particular aircraft’s design that gives it these qualities? Armed with these questions and many more, I went to pester Jim Smith, a man with significant technical roles in the development of the UK’s leading military aviation programmes, to get answers.
Fortunately, his answers were far longer than I expected and instead of a one-off article, we will be sharing a fascinating series.
Over to Jim for the introduction.
Why are aircraft the shape they are?
“This question arises from a discussion between Hush-Kit and I about using specific aircraft as examples to illustrate how various requirements drive aircraft size, shape, wing planform, thrust-to-weight ratio and so on. In best Hush-Kit style, I’ve been asked to look for ‘best and worst’ in a number of categories, for example: High Alpha flight; Acceleration; Sustained turn rate; Instantaneous turn rate; Climb Rate; Agility and manoeuvrability at high and low altitude; nicest and nastiest to land. And of course, the desire is to focus on fighter aircraft.”
“I’ll explore how different parameters drive aircraft design by considering not only fighter aircraft, but also other types of aircraft, noting that individual performance requirements are not the only requirements that drive aircraft configuration design. We’ll start a discussion of requirements for military aircraft, before looking at specific point performance and other requirements that influence their design.
Mission and Point Performance Requirements.”
“In general, even fighter aircraft today are likely to be designed to meet a number of mission profiles, coupled with point performance parameters as constraints. So, requirements might be summarised as “deliver missions 1 through 5, while also achieving stated maximum g-loading, climb rate, sustained and instantaneous turn rates”. Each of the missions will have been matched to realistic operational scenarios, such as an interception from Quick Reaction Alert (QRA); an interception from Combat Air Patrol (CAP); an extended duration patrol; and perhaps also surface strike missions with differing weapons fits.
These mission requirements will generally determine the sensors and crew required (one or two-seat for the majority); the weapons, auxiliary sensor and targeting pods and tanks that may be carried externally; and, in the case of stealthy aircraft, what sensors and weapons must be carried internally.”
“Taken together, these reference missions will allow the ‘payload-range’ of the combat aircraft to be established, and this along with the determination of whether one or two engines is required, will define the size and (largely) the weight of the aircraft.
Point performance requirements will also be set. In some cases, these will be expressed as constraints, rather than targets, and they should also reflect the operating concept of the aircraft, and be coherent with the mission requirements. There is no point specifying a mission requiring extreme range, for example, and at the same time require that the cruise speed of the aircraft be supersonic at low level. If you do this – oh dear – you end up with a TSR2, where low-level supersonic range was apparently an enormous cost driver.”
“One of the reasons that point performance parameters are often set as constraints, is that the turning performance of typical fighter aircraft could easily exceed the tolerance of a human pilot, typically 9g, even with a ‘g-suit’. Consequently, many of today’s fighters are designed structurally to 9g loadings, which effectively means they deliver the same instantaneous turn rate as each other, at least at low to medium altitudes. Sustained turn rates are generally less likely to be structurally limited, and may vary rather more, dependent on wing sweep, aspect ratio, and thrust to weight ratio.
Point performance requirements are likely to include such items as climb rate, field requirements, maximum speed and operating altitude, the structural ‘g’-limit, and possibly others, depending on the role. There will also be plenty of system and reliability and maintainability requirements and targets, but these will not need to be covered here. (For a discussion of BVR combat requirements see my Hush-Kit article here)
The current multi-dimensional approach to combat aircraft requirements has not always been the approach. The Fifties were a period when huge advances had been made in the understanding of aerodynamics and in jet engine design, enabling great leaps in performance. So much so, that the capabilities of the aircraft could outstrip the thinking behind the military requirements being proposed.
The best example of this is the Douglas A-4 Skyhawk. The Navy had in mind a 30,000-lb twin jet attack aircraft, with a 2,000-lb bomb load. What they got from Douglas was the Skyhawk, eventually with an empty weight around 10,000 lb and max TO weight of 24,000lb, able to carry up to 8,000lb of stores, at high subsonic speeds. (Picture)
In looking at specific requirements, I’ll use some of the older point-driven designs to illustrate the configuration trades being made, and indicate which current aircraft may provide best or worst examples. In addition, some civilian operations also require some fairly extreme configurations, so I’ll also consider some of those.
Click here for part one.
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