but how does affect existence of multi engines the capabilities of aircrafts?
An aircraft with a long operational life used in harsh environments, and possibly not getting the best maintainence attention on the front line introduces the possibility of engine failure.
when you only have one engine then an engine failure is fatal to the aircraft and possibly also the crew... which will be expensive. Even if single aircraft only increase the loss of aircraft due to engine failure by a few percent leading to perhaps the loss of 2-3 planes over a ten year period the costs in buying new aircraft outweigh any savings in single engine designs.
the odds on both engines failing is a small fraction of the chances of one failing... and more importantly the chance of the engine not being able to be restarted should also be included...
I have read some studies that claim single engine aircraft are worth it because the number lost to engine failure where two engines might have made a difference is negligible.
Of course the aircraft with the worst flight record in the Soviet military was the Yak-38 VSTOL aircraft which in theory is a single engined fighter but with two lift jets.
the Russian military has clearly chosen twin engine conventional take off fighters instead of VSTOLs for a range of reasons... on a deck at sea even just flying your Yak-38 through some hot air from the aircrafts own jet exhaust can make engine thrust plummet and the auto eject system throw the crew out of the aircraft.
the Harrier has killed rather more of its pilots than the Yak ever did because of the auto ejection system on the Yak, but the Harrier is a much more practical aircraft that has been rather more widely produced and deployed in real combat situations.
Even as good as it is however the Harrier would be terribly vulnerable to MANPADS due to its side mounted engine nozzles offering an IR target from a much wider angle than most conventional aircraft.
well thats because doesnt matter the weight of engines.
Of course it does. A heavier engine makes the aircraft heavier and requires more power to move it around the place.
The advantage of two engines instead of one is that the two engines can reduce demands for high power engines in a lighter over all set up.
Going back to the F-16... you pretty much need a 12-13 ton thrust engine to get decent performance which makes it a Flanker class engine. In the MiG-29 it uses two engines that a MiG-21 might have used if it was developed a bit later in the 8 to thrust class each... but the point is that with two 8 ton thrust engines the MiG pilot has 16 tons of thrust when he needs it... a 16 ton thrust engine for the F-16 would be possible, but very expensive and likely heavy.
I want to ask:can be use two engines one above the other? Like they are vertically mounted?
yes, the British Lightning had that arrangement... it was a very good interceptor.
What about having 4 small engines around the single engine? Each of them could turn on for better maneuvers. Or small engines in front. In this the aircraft could do better in dogfights and escape missiles. Even moving angle could be added to those small engines. The engines could be put in x around single engine.
It would be cheaper and simpler to just use a more powerful main engine... for instance in the example of the F-16 then the engine from the F-35 would offer plenty of thrust, or the engine to be fitted to the PAK FA in the 18 ton thrust range would be useful for a single engine light fighter that is heavier than a late model F-16.
Also small engines could be puf in front of aircraft. Mounted in angle so the flame not touch the plane.
Such small engines would not add that much thrust, yet would add weight and complication with wiring and fuel lines all over the place.
If you want to temporarily add thrust then I would suggest a couple of small scramjet engines mounted on the wing pylons taking fuel from the aircraft fuel system... sort of the opposite of an external fuel tank.
On an Su-35 you could fit them on the wing tips... but at the end of the day I would think it would be better off with jammer pods on the wing tips instead.
BTW extra thrust wont help evading AAMs or SAMs... you are much better off with a decent ESM suite and DIRCMs system.