# Disadvantages of using thrust vectoring only in an aircraft with variable rotation nacelles such as the X-19

Take a quad tilt rotor aircraft such as the curtiss-wright X-19 or even a bi-tilt aircraft such as the V-22.

What would be the advantages or disadvantages of using thrust vectoring only for control of roll, pitch and yaw assuming for a moment that there is no added mechanical complexity in individually tilting each nacelle a variable rotation and assuming each nacelle can tilt 180 degrees from full vertical producing downward thrust (lift) to reverse vertical, essentially sending the aircraft downwards.

In theory, the front two nacelles would control pitch by rotating up or down simultaneously while the back two nacelles would control roll by each nacelle tilting oppositely. Combined all nacelles would be capably of controlling yaw or by changing the thrust being generated on each side of the aircraft, a yaw moment could be generated.

of course there are clear issues in the form of Cost, mechanical complexity, control of the aircraft in an engine out scenario, gyroscopic moments and control generally but purely from a flight mechanics perspective, what are the advantages or disadvantages of removing ailerons and using thrust vectoring to control the flight of such an aircraft.

Currently, it is my belief is it would allow the use of highly complex airfoil geometries that would increase the efficiency of the aircraft in flight as airfoils would no longer be necessary and perhaps increase maneuverability of what i would imagine to be large aircraft but there is no drawbacks other than those previously mentioned however, would thrust vectoring produce less turning moments than ailerons?

I look forward to any and all replies, thank you.

There is one disadvantage: it makes control dependent on engines running.

This disadvantage effectively precludes using it on any aircraft carrying people. Usual requirement for a critical system, which controls are, is that the probability of fatal failure must be estimated to be less than $$10^{-9}$$ per hour of operation. There are simply no propulsion systems that would be as reliable.

Turbine engines themselves have, IIRC, risk of failure on order of $$10^{-4}$$, so for independent failures you can get below $$10^{-9}$$ with double redundancy, but common causes—which means mainly various problems with fuel—are included as well and the risk is not low enough. And even triple redundancy is a lot.

It is my belief is it would allow the use of highly complex airfoil geometries that would increase the efficiency of the aircraft.

The best airfoil is one that adaptively changes shape to meet current requirements (read: flaps or flaperons that run the entire length of the wing).

What would be the advantages of using thrust vectoring only for control of roll, pitch and yaw?

That's how the V-22 works. During horizontal short field takeoffs, the engines are angled forward and upward and the flaps are fully deflected to generate maximum lift. Pitch and roll control are provided by thrust vectoring and differential thrust (the V-22 has conventional swashplates). Below is an image of a V-22 during takeoff. (source)

The advantages: Short/Vertical takeoff. More control of the aircraft.

You mention increased efficiency, but I don't see how that would be possible. The most efficient aircraft are those which generate lift over a large span.

This is because the air you move doesn't need to be accelerated as fast when you are moving large volumes of air at a time. This why gliders for example have such long wing spans.

The problem with gliders however, is they need very long runways to take off and land.

To get the best of both worlds many aircraft use wings which change airfoils depending on what the aircraft is doing. For example deploying flaps during takeoff.

In flight ailerons might cause a bit of parasitic drag, but it's certainly less than 4 rotating propellers pushing in different directions would be.

If you wanted to get even more lift out a standard wing to have shorter takeoff you could use flaperons. Which are basically ailerons that extended the entire wing and are used for control and generating lift.

One aircraft I can think of which merges both worlds together is the V-22. It has 2 rotating propellers which can provide differential thrust, and an airfoil with a solid flap built in to make takeoff as short/efficient as possible.

However outside of the specific military roles this aircraft provides, I don't see much use for it for the general public. I you really need that kind of control helicopters are always available.