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VTOL operations need 4-5 times more thrust than a conventional take-off of an aircraft.

If you don't want to install additional lift-engines into a hypothetical aircraft and use thrust vectoring to turn the thrust towards the ground, you naturally end up with largely oversized engines for the cruise flight.

But how bad does the fuel efficiency (thrust per amount of burned fuel) get, when the engines run at their minimal self-sustaining throttle setting during cruise?

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  • $\begingroup$ A thrust/weight ratio around 1 is standard for fighters, so making them VTOL does not result in any larger engines than otherwise! $\endgroup$ – Jan Hudec May 2 '18 at 19:49
  • $\begingroup$ Sorry, I think my question was not precise enough. IMO VTOL operations are not only beneficial for fighters, but also for small to medium sized civil aircraft, such as an equivalent to a rescue helicopter. Normal airliners or private jets usually have thrust to weight ratio of 0.3-0.4. $\endgroup$ – Levi Neuhaus May 3 '18 at 7:07
  • $\begingroup$ In this specific case, there no high thrust needed to sustain a speed well below the speed of sound and without any spectacular maneuvers. $\endgroup$ – Levi Neuhaus May 3 '18 at 7:19
  • $\begingroup$ Efficiency of a jet engine is pretty bad at low throttle settings. Idle rpm is something like 70% of max. rpm. $\endgroup$ – Hobbes May 3 '18 at 12:23
  • $\begingroup$ A VTOL aircraft needs a 1:1 T/W ratio at maximum takeoff weight. Fighters don't usually achieve that, they only get to 1/1 T/W at typical air-air combat loads. $\endgroup$ – Hobbes May 3 '18 at 12:25
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Current jet engines are actually not built for efficiency rather than performance.

If you consider efficiency, you don't build a VTOL aircraft. This is why the F35 has multiple versions with and without VTOL capabilities.

Also, jet engines even during flight have to be able to sustain fast speed change, fast altitude increase and high load factors, all combined with lightweight designs. Therefore, I wouldn't consider takeoff as the most demanding phase of flight for such engines, but I may be mistaken on that point.

All these conditions make it difficult to reach the efficiency of say turbofan or turboprop engines.

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  • $\begingroup$ Note that the term “jet” engine usually includes turbofans. And nobody's making pure turbojets anymore anyway—fighters usually use turbofans with low bypass ratios 1:0.5 to 1:1 these days. $\endgroup$ – Jan Hudec May 2 '18 at 19:55
  • $\begingroup$ @JanHudec, totally agreed $\endgroup$ – BambOo May 2 '18 at 20:15
  • $\begingroup$ @JanHudec the opposition between jet egines and turbofans was more about their usage. No high bypass ratio turbofan would sustain the movements of a fighter aircraft for long $\endgroup$ – BambOo May 2 '18 at 22:36
  • $\begingroup$ There is no opposition between jet engines and turbofans, because turbofans are jet engines. They are not turbojet engines, but they are jet engines as most people understand that term. $\endgroup$ – Jan Hudec May 3 '18 at 10:01
  • $\begingroup$ @JanHudec what do you mean by they are not turbojet engines ? $\endgroup$ – BambOo May 3 '18 at 11:24
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The main problem with installing bigger engines is that they are heavier. And because you need to lift the engine weight too, the thrust/weight ratio grows slower than the installed thrust. The result significantly reduced payload fraction and payload fraction is the major factor in efficiency.

Now fighters normally do have thrust/weight around 1, but they are basically aerodynamic shells wrapped around the engines and some fuel tanks and almost nothing else. Their payload is too small compared to transport aircraft to be practical.

Now vertical take-off is occasionally useful in civilian operations, but with all the airports already built not as often. For the cases where it is we have the rotorcraft and the AugustaWestland AW609 tiltrotor.

Unlike jets, the large rotors have much lower induced power, so they can produce much more static thrust with the same power and thus support hover with reasonably small engines. The price is that their thrust decreases faster with speed, so the top speed is lower.

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Extensive research was done into civilian VTOL operations in the 1960s and 1970s. Those projects were killed by several factors:

  1. high fuel cost (combined with the fuel crisis in the early 1970s)
  2. high capital cost (due to all the extra lift engines needed compared to regular aircraft, there were designs with dozens of lift engines).
  3. noise. The attraction of VTOL is being able to operate near a city center instad of having to drive to a remote airfield with a big runway. But all that thrust necessary for VTOL results in an awesome racket (the Harrier is one of the loudest fighers ever, despite having much lower performance than its contemporaries). Nobody is willing to have that much noise in an urban setting, making the whole exercise pointless.
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https://en.wikipedia.org/wiki/Harrier_Jump_Jet

https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle

Both are twin engine fighters and have similar thrust - one appears to be a litte more aerodynamic than the other. So I would think that fuel efficiency when minimal self-sustaining throttle setting during cruise is used could be similar too.

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    $\begingroup$ The Harrier is a single engine aircraft $\endgroup$ – BambOo May 2 '18 at 19:39
  • $\begingroup$ Ok, comparison can be made to the F16 instead. $\endgroup$ – CrossRoads May 3 '18 at 12:48

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