I have always been curious to know the machinations behind the minds of designers of fighter jets.
Why do fighter jet designers come up with wildly different designs to essentially do the same job ?
I would like to compare 2 most common wing shapes to better understand why aeroplanes are designed the way they are.
I will start off the thread by enumerating some advantages of both.
Delta Wing
Advantages:
1.Higher Instantaneous Turn Rate(ITR)
2.Better for achieving and maintaining supersonic speeds
3.Higher stall angle
4.More lift.
5.Less wing loading.
6.Stronger than conventional wing, relatively simple and cheaper to manufacture
7.More internal volume for fuel
8.Less stress on the wing, longer life, higher availability and cheaper maintenance.
Conventional Wing:
Advantages:
1.Better Sustained Turn Rate(STR)
2.Better at low speeds.
3.Less air drag
Can anyone care to do a comparative analysis of both, I am sure I have missed many points.
For a comparison. Go here.
Aeronaut: The Mirage III/5/50 Family
Ill post the most relevant excerpts here.
Delta Wing Aerodynamics
by ACdre Retd Kaiser Tufail
Sweep angle of the wing leading edge helps delay drag rise with increase in speed. In a swept wing, the velocity of the airflow normal to the leading edge is reduced by a factor of the cosine of the sweep angle, with a corresponding delay in drag rise. High sweep angles are, however, associated with the problem of wing-tip stalling which results due to the airflow drifting span-wise across the wing, causing the tips to stall before the rest of the wing. The result is usually a violent pitch up followed by a spin. Wing fences and notches are a stop-gap solution as they generate a vortex over the wing which virtually arrests the span-wise airflow.
On a swept wing, the torsional stresses during manoeuvring flight are enormous and indeed, dangerous at high Mach numbers. Greater structural strength can only be obtained by paying a greater weight penalty.
There is one way in which sharp sweep angles can be used without a lot of problems: delta wing. The shape is optimum for high speed flight. The extremely broad
chord(average distance between leading and trailing edges) means that a low
thickness-to-chord ratio needed for high speed flight can be achieved. The structure can be made rigid, has sufficient volume for fuel and, there are hardly any practical limits to the angle of sweep.
The low
aspect ratio (square of the wingspan to wing area) of the delta wing gives excellent supersonic performance by presenting a smaller frontal area to the airflow. At lower speeds, however, the poor lift-drag ratio of the low aspect delta planform demands higher angles of incidence to generate the same amount of lift compared to a conventional wing. This causes greater induced drag resulting in speed bleed-off during manoeuvring flight; it also increases take-off and landing distances. It may be worth noting that the Mirage III/5/50 as well as the double-delta winged Draken, held the dubious distinction of having the lowest (read worst) aspect ratio of any fighter to date ie, 1.94 and 1.8 respectively, but this record has now been surpassed, surprisingly, by the very modern Tejas with a ratio of 1.75!
By its very shape, a delta wing has a large area which tends to give a relatively low
wing loading (aircraft weight per unit area of lifting surface ie, the wings). This helps offset its poor sustained turn performance and enables it to turn tightly at low speeds – often below its normal landing speed – especially in descending manoeuvres in which it can trade height for energy.
Woes of Tailless Deltas
To date, only a few tailless delta fighters have been produced besides the Mirage III/5/50. These include the F-102 Delta Dagger, F-106 Delta Dart, J-35 Draken, J-37 Viggen, Mirage 2000 and Tejas.
In a tailless delta, lift augmentation devices like trailing edge flaps cannot be installed for want of space (though in the Viggen, these are cleverly placed on the large fixed canards). Also, upgoing elevons diminish wing lift which needs to be compensated by higher take-off and landing speeds, worsening short-field performance. Many a pilot who ended up in the arrester barrier has ruefully wished for a longer runway when confronted with a take-off emergency.
Two modern tailless delta fighters, the Mirage 2000 and Tejas feature relaxed static stability. A benefit of this design is that it confers an unstable nose-up moment which reduces the pitch-up required for take-off or during manoeuvres; the harm done to wing lift by upgoing elevons is, thus, minimised to a considerable extent. Leading edge flaps/slats on these fighters also add to the total lift when they automatically activate at slow speed, thus allowing lower take-off and landing speeds.
Canards acting as control surfaces work essentially like tailed deltas, except that the ‘tails’ are located at the front. They obviate the need for elevons to change the pitch, hence saving precious main wing lift. Modern delta-winged fighters like Chengdu J-10, Eurofighter, Gripen and Rafale have fully active canard controls.
Some of the older Mirage III/5/50, Cheetah D and Kfir C-II found a partial remedy to their congenital woes through retrofit of small fixed canards, while the Viggen had fixed canards designed from the outset. These canards added to the overall lift in ways similar to the leading edge flaps/slats, except that they remained stuck out even when not needed in high speed flight
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