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Mig 21 Range: 1,470 km (910 mi; 790 nmi) at 10,000 m (32,800 ft) with 2 × K-13A missiles and 800 l (180 imp gal; 210 US gal) (Wikipeda)

J-7 Combat radius: 850 km (459 nmi, 528 mi) (air superiority, two AAMs and three drop tanks)

JF-17 Combat radius 1 350 km

The J-7 and JF-17 are developments of the Mig 21, why is their range greater?

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No. Their range limitation stems from their original concept: relatively cheap simple lightweight agile fighter (MiG-21) or interceptor (Mirage III).

There are plenty of long(er)-range aircraft with delta wing (if that's what you mean). Concorde, for one. B-58. Eurofighter.

For a given class of aircraft, range is primarily a function of fuel quantity and therefore (almost directly) of the maximum takeoff weight. JF-17 is 35% heavier and so has significantly greater range. (It also helps that it and its engine are two generations newer). J-7 is practically the same as MiG-21 and has very similar performance.

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  • $\begingroup$ Uh range is most certainly a function of wing configuration. - Since range is based on CL/CD, which of itself is a function of the aspect ratio of a wing. - So YES this is (again) a wrong answer. Other design parameters might've been more important and driving the wing design. But the configuration DOES influence range, to a very very large extend. $\endgroup$ – paul23 Jun 12 at 13:00
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    $\begingroup$ Everything depends on everything. But for a given class of aircraft, esp. of the same generation, CL/CD (and most other dimensionless characteristics) will be very similar. For these aircraft in particular, there are many overriding requirements that determine the wing shape, such as speed and manoeuvrability. Moreover, the combat radius for such aircraft is not necessarily determined at the best CL/CD: this would be too slow. The end result is that if you want greater range and/or payload, you'll inevitably need a larger aircraft. (Or a leap to the next generation, or often both). $\endgroup$ – Zeus Jun 12 at 13:18
  • $\begingroup$ As I said: other design parameters are considered more important (maneuverability/high maximum wing loading & velocity are prime examples). But range is a direct consequence of the glide ratio. - which is the lift to drag ration (CL/CD). -- Saying "aircraft don't change in CL/CD of the same generation" is a non statement: this is something that is just picked by the designers fairly early on. It can be chosen as one wishes. Picking the glide ratio is done way earlier in the design process than the engines or fuel; as such design range is already chosen at the start. $\endgroup$ – paul23 Jun 12 at 13:24
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    $\begingroup$ @paul23: No, this is not a wrong answer, however, your assertion that L/D is a function of the aspect ratio is at least flawed if not outright wrong. Given the requirement for supersonic performance, other non-delta aircraft with natural stability and the same size achieve very similar range performance, so the main proposition of this answer is correct. $\endgroup$ – Peter Kämpf Jun 12 at 13:53
  • $\begingroup$ @PeterKämpf Again the original question never assumed supersonic to be the case: of course if we make the the driving parameter a delta wing is probably best. - But that's again a choice. It might be a more important parameter (or driving one) than distance or glide performance - however that is still a conscious choice and not something that is an effect. - The point I wish to make is that the reason fightercraft have limited range is based on design choices, supersonic cruise speed is such a choice. $\endgroup$ – paul23 Jun 12 at 14:00
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According to your own data you provide, the MiG-21 has the longer range of the 3...

As to the J7 having shorter range, I'd not be surprised if the engine the Chinese use is less efficient than the Soviet one it was copied from (and it is indeed an almost carbon copy of an early model MiG-21), and the same'd be true for many other of the aircraft's systems (my guess would be for example that the MiG would have thinner metal and flush rivets, rather than thicker metal and round headed rivets on the more primitively built J7, making it lighter and more aerodynamically efficient).

The JF-17 is a more modern development of the J7 concept with improved materials, engines, aerodynamics, giving overall better performance.

Mirage III and MiG-21 were both designed for a very similar mission: short range rapid response interception of incoming enemy strikes (and especially bomber strikes) with air to air missiles only (guns being added pretty much as an afterthought, neither aircraft was designed with the agility needed for a dogfighter). They didn't need long range, the idea was to get up to altitude as quickly as possible, launch missiles at the approaching enemies, then get down again to refuel and rearm and do it all over again. Later variants were given increased range and agility to allow them more versatility, but that was the original concept behind both aircraft.

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  • $\begingroup$ MiG-21 wasn't such a pure interceptor as Mirage III and had guns from the beginning (and no radar). On some modifications in the early 60s the guns were removed, but all later modifications had them again. $\endgroup$ – Zeus Jun 12 at 12:56
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    $\begingroup$ Caution: Combat radius is not range. Normally, combat radius = range / 3. This makes the range of the J-7 and JF-17 a lot bigger. $\endgroup$ – Peter Kämpf Jun 12 at 13:56
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    $\begingroup$ @Peter, certainly, except that the figures in the OP are not compatible: different configurations, different conditions, different versions. For what it's worth, MiG-21 and J-7 are practically the same. $\endgroup$ – Zeus Jun 13 at 1:47

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