Are fixed wing aircraft with gimbal thrust feasible?

Question

Elon Musk has suggested that he'd look at implementing VTOL electric aircraft with a "gimballed thrust" system similar to the one used in rockets.

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? [Do you have specific plans?] I've been sort of toying with the design for an electric supersonic vertical take-off and landing electric aircraft for a while, I'd love to do it, but I think my mind would explode

What are the challenges to implementing this? Is it technically feasible?

Answer 1

Could you create an aircraft using an jet engine/propeller/ducted fan on a gimbal? Sure you 'could'; it is feasible by the laws of physics. But just because you can does not mean that you should.

Setting up multiple points of lift on a manned aircraft seems like a good idea with a lot of advantages but is a Pandora's Box of problems, the not the least of which is the grave risks of single point failure. Lift and control is totally reliant on balanced lift between multiple engines. Lose one engine and the craft departs from controlled flight and crashes. You better hope you have an ejection seat and/or parachute on board.

Some solutions to these problems come in the form tiltrotor designs like the Agusta Westland 609 and the V-22. But these aircraft have multiple redundancies in their design including a common transmission system between the rotors; in case of a single engine failure, the good engine has enough power to drive both rotors. Tiltrotors can also glide like regular airplanes and utilize control surfaces during this and cruise flight.

As a final note: Be careful in putting too much faith in Musk and other Silicon Valley whiz kids speculating about other design endeavors. The skills required to run a successful software company are often very different from that required to develop other artifacts like aircraft and automobiles, etc. Musk was a genius with PayPal but nearly went bankrupt starting an electric car company and his development work with SpaceX has been punctuated with a series of spectacular failures. He also quietly withdrew from his proposed 'Hyperloop' project, I suspect, because he really didn't understand what he was getting into there.

Answer 2

Sure you could, but why would you want to? There's no inherent benefit to be had in vectoring thrust as opposed to using control surfaces. It doesn't simplify things to any extent, it just makes things complicated in other ways. In order to have effective roll control you'd need to space motors out on the wings which could vector in line with the center of the aircraft, which would be complex and add weight. Also, you couldn't use thrust vectoring unless you had at least 3 engines spaced around - one in the tail for pitch and yaw, 2 in the wings for roll control.

Of course there is safety as well. A loss of power or an actuator failure, even momentarily, would probably mean a very fast, catastrophic loss of the craft and anyone inside it. Imagine a software glitch which pointed the tail engine to the left for a couple of seconds while the airplane was at cruise speed - the airplane would go side on to wind at several hundred miles per hour and most likely break up. On a VTOL approach to land one of the engines loses power, causing the craft to roll and drop the last 100 feet like a rock, destroying the craft and its occupants.

Answer 3

Gimbaled thrust in 'rocket speak', is equivalent to vectored thrust in 'aviation speak'.

Many modern military aircraft use thrust vectoring in order to improve performance, particularly in tight turns, the F22 being an example. The harrier and the F35 also use thrust vectoring to enable VTOL (vertical take off and landing) and/or STOL (short take off and landing) capabilities.

Of course not only jet aircraft use thrust vectoring, nor is thrust vectoring confined only to military applications. The Boeing V22 Osprey uses thrust vectoring, while the XTI Aircraft Company are currently developing a ducted fan VTOL civilian aircraft.

Anybody with any aviation knowledge, be it in aerodynamics or that of piloting a craft, will know that failure is unlikely to result in an uncontrolled event of any kind.

Firstly, in forward flight wings will continue to generate significant amounts of lift. Aircraft do not require engines to remain in flight, only to maintain significant airspeeds of 500mph+. Think of multiple commercial airliners that have previously experienced complete engine failure, only to glide for many miles.

Secondly, for rotary wing VTOL aircraft, a failure in any stage of flight will likely result in autorotation just as for helicopters, allowing for a great deal of control. Assuming the craft also has surfaces providing lift, this acts to provide an additional benefit.

Lastly, the very first 'control surfaces' used by the Wright brothers were warping wings. Current control surfaces (ailerons, flaps, and the like) were originally brought into play because of the inflexibility of aircraft wings used since the early 20th century. We have used the same control surfaces ever since, but they are incredibly aerodynamically inefficient, hence why NASA has been developing alternative flexi-wings (https://www.nasa.gov/press-release/nasa-successfully-tests-shape-changing-wing-for-next-generation-aviation).

Offering a different perspective by controlling direction via engine management as oppose to manipulating airflow may improve efficiency yet further.

Beware of placing too much faith in armchair 'experts' and naysayers of online forums. Usually they have very little real experience or knowledge, let alone any vision of the future.

Answer 4

You won't like this answer but fixed wing VTOL aircraft with gimbals thrust actual fits many rocket/missile type of aircraft, which include:

1) High altitude anti-aircraft missiles, such as RIM-161 SM3 and S400 systems. Fins at high altitude doesn't work that well, so thrust vectoring is the main control authority at high altitude (while the body provides the lift).

2) SpaceX Falcon booster. If you argue missiles are VTO but not VL, consider the Falcon booster? VTOL, yes. Gimbals thrust, yes. Aircraft, absolutely. Fixed wing? Sort of, if you include lift body.

Within the scope of conventional manned airplanes, F35 and Harrier's reaction control is close (although differential thrust not gamble thrust).

The way you asked, "feasibility" misled many previous answers to "practicality".

For feasibility, it's a definitive yes, but for practicality, not many airplanes have done that, because of fuel economy and reliability.

Remember wings and control surfaces have a lift to drag ration greater than one, while thrust vectoring have a "lift to drag ratio" (control force to thrust lost ratio) of roughly one. For example, if your tail needs 10kn of down force, you have two options 1) use a tail plane with L/D=5 and pay for 2kn of drag, 2) divert your thrust upwards for 10kn and and lose a net thrust of 10kn.

This is the reason why thrust vectoring and differential thrust isn't used unless there are other choices, e.g. very slow(F35, Harrier, R73 AA missile) or very high (S300/400, SM3, etc), deep stall (F22, and various thrust vectoring demo and experimental airplanes, e.g. F18 HARV).