Why do aircraft use flaps rather than vectoring the engine or angling the engine?

Elon Musk said he would gimbal the engine like a rocket with the aim of reducing total weight of the aircraft and thus increasing its range.

Aircraft have all these unnecessary things like tails and rudders and elevators - not needed. Just gimbal.. anyway.. gimbal the electric fan. For some weird reason gimbaling motors is normal in rockets and not in aircraft. Why not.— Elon Musk (YouTube)

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    $\begingroup$ It is a lot easier to move a flap or slat than the entire engine, and it works when the engines don't. $\endgroup$
    – Ron Beyer
    Commented Jan 30, 2017 at 17:02
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    $\begingroup$ Imagine landing with one engine out. Elon musk doesn't live in the real world. $\endgroup$ Commented Jan 30, 2017 at 17:18
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    $\begingroup$ @Greg Taylor: In fact, most of us only HAVE one engine, and that's usually at idle during landing - from short final to turning off to taxi, at least. I can't offhand see what use movable engines or thrust vectoring would be, unless you're building a VTOL craft like the Harrier or Osprey. And of course those are military, where things like cost & efficiency are secondary considerations. $\endgroup$
    – jamesqf
    Commented Jan 30, 2017 at 18:20
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    $\begingroup$ "If all you have is a hammer..." $\endgroup$ Commented Jan 30, 2017 at 18:47

1 Answer 1


Historically, flaps and control surfaces were moved manually: The pilot would move a stick, a control column or pedals, and pulleys or pushrods would transfer this force to the control surfaces. To do this with engines was physically impossible.

With the introduction of hydraulics, large enough forces for tilting engines were available for the first time. Some aircraft used them, but this application did not find widespread use because the possible lift increase was smaller than what could be achieved with flaps. The same goes for controlling the aircraft: Directed thrust control forces are much smaller than what was possible already with control surfaces. Also, the responsiveness of tilting a heavy engine is much worse that that of a light control surface. Consequently, moveable engines were mostly used for short take-off and landing (STOL) applications. The Do-29 was one of several experimental aircraft which could pivot the propellers to take off at lower speed. Note that in case of the Do-29 the engines stayed in place; only the propellers and their driveshafts would move. Other designs would tilt their wing and with it the wing-mounted engines. In all those cases this was only done to enable short take-off runs, not to control the aircraft.

Do-29 in the Dornier museum
Do-29 in the Dornier museum (picture source)

Other applications were for special cases like flying boats:

Do-26 with tilted rear engines

The Dornier Do-26 could tilt the rear engines up for operation on water to reduce the impact of spray on the propellers (picture source). Note that this was not for control, but for lifting the props out of the spray area.

Control surfaces are better suited for creating control moments:

  • Control surfaces sit at the ends of the aircraft, so they have a much better leverage. Engines were placed close to the center to minimize the control problems caused by a loss of a single engine.
  • Control surfaces modify the lift force, whereas engines modify thrust, which is in the order of magnitude of drag and, consequently, an order of magnitude smaller than lift.

However, this is only true as long as aerodynamics works in a linear fashion. Which is not the case with separated flow and stalled flight surfaces. To overcome this limitation, the exhaust stream of jet engines is deflected by pivoting the nozzle (or paddles, as done on the X-31). This is much easier to do than to move the whole engine. Also, since the engine stays aligned with the airframe, the intake will continue to work and the frontal area of the airplane does not increase; both important considerations which stand against pivoting the whole engine.

Vectoring nozzles on the Sukhoi 35
Vectoring nozzles on the Sukhoi 35 (picture source)

Generally, aircraft are controlled by changing moments which in turn rotate the aircraft such that the desired flightpath change is effected by the changed aerodynamic forces. There have been attempts at direct force control, however: One Dassault-Dornier Alpha Jet was fitted with moveable wing pylons which could be rotated around a vertical axis. When moved, this would create a side force which enabled the pilot to directly shift the aircraft's flight path sideways. With moment control, you first need to roll the aircraft in order to tilt the lift vector sideways and then roll back shortly before the desired sideways change is complete. This is much harder to get right with moment control than with direct force control, especially for small corrections.

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    $\begingroup$ Vectored nozzles are fine, but the reason people don't gimbal the engine is because it's gyroscopically stabilized by the rotor's rotation. A lot more energy would be required, and the responsiveness would be much lower, for gimballing engines over moving flight control surfaces. $\endgroup$ Commented Jan 30, 2017 at 19:45
  • $\begingroup$ @IwillnotexistIdonotexist: Only when all spools rotate in the same direction. If the HP spool runs in the opposite direction to that of the LP spool, gyro effects will be almost eliminated. Still, when such an engine is rotated quickly (as would be required for control), the loads on the engine bearings will be punishing. $\endgroup$ Commented Jan 30, 2017 at 19:51
  • $\begingroup$ @peter kämpf Elon Musk was suggesting an electric jet and therefore could have contrarotating propellers in the engine? Would there be any problems with this? $\endgroup$
    – SRawes
    Commented Jan 31, 2017 at 18:48
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    $\begingroup$ @SRawes Quite aside from the angular momentum/gyro problem, with current-day engines jutting out forwards from the wing, deflecting it even slightly would induce a massive drag force seeking to increase the angle, eventually shearing the engine off its pylon. As mentioned by Peter, the bearings for all the moving parts in such a system would take a legendary beating, and the additional hydraulics to steer the engine would be heavy and constitute an additional failure point. $\endgroup$ Commented Jan 31, 2017 at 20:13

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