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I was wondering what are the reasons for not using tilted propellers fixed to ailerons/elevators/elevons? Or even on wings with no control surfaces at all? Wouldn't the 2nd way of thrust vectoring greatly simplify aircraft design?
There is a huge variety of VTOL aircrafts, but all seem to have puller propellers in frontal wing area with ailerons/elevators separate.
It isn't done because a moving control surfaces is easier to design and build than an engine mount that rotates.
Plus the associated structure needed to accommodate the thrust, p-factor, and gyroscopic loads is going to be a lot heavier than ailerons, spoilers, moving tail plane, etc.
Another issue is if one loses one or both engines, your roll and pitch control disappear as well.
Another way to think about it is to think of an airplane's control in flight as a lever centered around the center of lift. One side of the lever will be the weight of the plane, often increased due to the weight of the engine, and the other side is the control surface (usually the tail). For instance, putting downward pressure on the tail will pivot the nose up about the center of lift.
Since you are exerting control with the long lever/arm of the fuselage, it doesn't take much force to effect a change. If you use centrally located engines to do the same work, it will require more power because you lose the help of the "lever"
At high flight-speeds, which most modern aircraft fly, propellers are inefficient and noisy, thus quite useless. Supersonic propellers, with propeller tips moving faster than Mach 1 are just a nuisance!
If you plan to rotate the entire engine and motor for instant pitch control you will have huge problems due to inertia and gyroscopic effects. And you'll need a lot of power, that better would be used for other purposes.
The latest way of designing aircraft is without any movable control surfaces at all, to simplify the aircraft as much as possible, and definitely no movable engines or propellers.
A straight wing's center of lift is around 25% from the leading edge, thus all needed infrastructure is forward, while hanging a moveable engine near the trailing edge of the wing needs a lot of infrastructure in that area, adding a lot of weight to the wing. Hanging engines on a wing's leading edge, as the norm is, is just the best solution from many points of view, and even is the best/simplest way of improving the wing's bending manner under load.
If it would be possible all kinds of moveable surfaces (or motors) should be mounted anywhere but on the wing's trailing edge, possibly for the exception of flaps and spoilers. And there you have many swept-wing fighters, that just have control surfaces in the tail (sometimes both the vertical and horizontal tail surfaces do everything: they control roll, pitch, and yaw. and sometimes they are just two, which simplifies things a lot.
The simpler the wing is, the less it costs, and the less it weighs. So preferably, prop-engines are mounted on the fuselage's nose, and preferably as tractors. If you insist on using pushers, you might need very long drive shafts (like those used in the LearFan design), another costly, and often heavy problem item (not much better if you mount the propeller in the nose and have the engine in the rear).
Pusher props are very susceptible to damage, due to dust and stones (and other loose items) being sucked up during power operation. Sadly, well known from the world of Burt Rutan's early designs (and their clones).
In your own example, the wing is slightly swept forward, which means the center of lift is forward of 25% mean wing chord, and as the engines in most cases are the heaviest item in the aircraft, you'd need to mount the passenger cabin and cockpit far forward, like in a Rutan Long-Eze, a popular home-built.
Mounting the engines in front of the wing improves the propeller efficiency, and increases their service life, and often makes less noise, as the air passing through the propeller disc is less disturbed than behind the wing where the air passing over the wing has higher velocity than the air flowing under the wing.
Making the engines movable adds a lot of weight further as there is needed a lot of heavy and expensive machinery to move the engine, and longer fuel hoses, longer cables, longer hydraulic lines, and so on, while a jet engine with a moveable exhaust is complex as well, but very, very, costly, as you need to use exotic materials in the exhaust design, to survive the thermal stress.
Having movable engines/motors are always tricky. The coming tiltrotor-equipped Bell aircraft, that will complement the Osprey, saves a lot of weight by having engines that don't tilt, just the rotors tilt in this new aircraft.
