For some time now I am wondering how a tilt rotor like the Agusta/Bell AB-609, the V-22, and the XV-15 are controlled? I know how helicopters work and I also know how airplanes work. So far I haven't been able to find a decent explanation online.

In a helicopter you have to pull the collective towards you to increase thrust (lift generation) of the mainrotor whereas you have to push the throttle away from you in a jet to increase thrust. How about when both control devices are merged into one? Silly question perhaps but entirely against my own logic hence worth mentioning.

With the nacelles entirely horizontal it's obvious the control surfaces direct the aircraft like with a normal jet/airplane. However, what happens when the nacelles are turned vertical, and even more confusing to me, what happens when they're somewhere in between helicopter and airplane stance?

How does it pitch up and down, since the surface of the rotordisc does not seem large enough relatively to the body of the aircraft to have it pitch up and down by simply altering the rotor pitch in the front and rear half of the disc, like is the case with a helicopter.

How does it roll in helicopter mode, simply by increasing lift on the one side and decreasing it on the other?

How does it yaw, again the same story with the small disc size. It seems too small to tilt the discs in opposite direction like the yaw function of a Chinook does?

How do the rotors generate lift in helicopter mode and thrust in airplane mode? Does it have a variable blade pitch like a helicopter with a constant output RPM, or does it have a fixed blade pitch like an airplane and a variable output RPM? I know that props and turboprops can alter blade pitch as well, but normally it is set to 100% and the RPM is altered to gain or loose speed. So far I haven't been able to notice a helicopter-like rotor hub that grants the ability to alter per-blade pitch.

Please take 5 minutes to provide me with a backed up and usefull answer if you have one, I am very curious!

Jp

Here is everything you need - and there has to be a similar standard volume on fixed wingers.
Beyond that everything depends on design choices and how you define the UI (user interface aka. stick, throttle, collective and pedals) and which - more or less nifty - control laws you put into the small black boxes, that bear so meaningful acronyms as AFCSD or whatever.

What strikes me a bit about your question, is that one has to clearly differentiate between the collective (which changes the blade root feathering angle, which, in turn, together with blade torsion and inflow determines the sectional angle of attack, which finally is at the basis of all the aerodynamic stuff, which happens around the blade) and the actual throttle (which controls the power produced by the engine by means of altering the fuel flow). For a helicopter this differentiation is clear. For a fixed winger these two controls also exist and are named prop-pitch and again throttle respectively.

So, before answering your question, I'd like to know, if the e.g. V-22's rotors work as fixed pitch props in plane mode or not?

Well, the bottom line remains: I, as a theoretician, am unable to answer your question. It's more for the operational people - some AF techs even Boeing engineers maybe. But I am interested in their answer, too!

So basically your question is the same as mine: is increasing engine power or increasing blade angle of attack the key producer of the propelling airflow.

I am aware that in both helicopters and fixed wing two entities are present: (1) engine power, controlled by a throttle handle or lever and (2) blade angle, controlled by either a collective (helicopter) or prop pitch lever (fixed wing).

What I am also sure of is a helicopter requires (near) full engine power setting to operate so the engine power can be considered a static variable (lets leave the small dynamic end range on the power curve, controlled by the governor, aside). If the engine would be running at anything less than full power, applying blade pitch, the dynamic variable, would generate a resistance higher than the engine could overcome resulting in an engine kill. More or less the same theoretic principle of disengaging your car's clutch when having the engine run at 1000 RPM versus when having the engine run at 3500 RPM. The 'work' or torque an engine can cope with is much higher at high (as in: maxed out) power settings than at low(er) power settings.

As far as I'm informed it's exactly the other way around with fixed wing aircraft. Blade pitch is constantly set to max, making it the static variable, whereas engine power is altered to generate less or more thrust, making it the dynamic variable.

So the issue that leaves us in the dark is: which of both methods applies to e.g. the V-22 family?

First answer has a chance of winning a week at the Seychelles!

Fixed wing these days should be considered analogue to a car with an automatic transmission; pitch = gear, throttle = gas pedal. You decide the amount of power you want, the gearing sets the best gear/pitch to transfer the power to the road/air.

As for tilt rotor, I would think they might not have used blade angle, but instead used computer controls/FBW to achieve desired slip/yaw near the hover speeds.

First thing to understand about tiltrotors is they're neither the best airplanes, nor are they the best helicopters, but rather an acceptable compromise between the two. Their wings generally don't have the lift area required for efficient glide in the event of dual engine failure, and their rotors don't have the best autorotative characteristics either. Their blades, or "proprotors", are airfoils designed to be a compromise between lifting surfaces like helicopter rotor blades, and thrusting surfaces like an airplane's propellers. Their engines are specially designed to be able to operate like turboprop and turboshaft engines, AND to be able to do it throughout 90 or even 100 degrees of rotation.

Most helicopters have an engine governor that automatically adjusts engine power to maintain the rotor RPMs at a specific speed. The more the pilot increases blade pitch, the more drag on the rotor system, the more engine power is needed to maintain rotor RPMs. Some smaller helicopters require manual throttle control in the form of a motorcycle-style throttle on the collective lever; the pilot must make a throttle adjustment for every collective adjustment to keep from overspeeding the rotors or letting the RPM's bleed off.

In prop planes, depending on their complexity, the pilot may have a simple throttle control with the propellor mounted directly on the engine shaft, or he mave have control over the prop RPM's and/or blade pitch settings. If you ever hear a larger turboprop aircraft like a C-130 landing and the engines revving up and the prop sound get deeper, they're feathering the props to produce reverse thrust.

Anyway, in a nutshell, a tiltrotor combines all these technologies into one. And this requires a robust flight computer(s) system to manage it. While in helicopter mode, the engine fuel flow is automatically adjusted to maintain a constant prop-rotor RPM, while the control stick manipulates the prop-rotors much like a helo.

Roll is accomplished through differential lift between the rotor systems. Pitch is accomplished with either the front or rear of the rotor disks producing more lift. Yaw is done mechanically by adjusting the engine rotation. This is one of the reasons why tiltrotors have more than a 90 degree rotation capability. If you want to yaw the nose to the left, the right engine is tilted slightly forward, and the left engine is tilted slightly backward. Chinook helicopters yaw in the same manner by differential tilting of the rotor systems.

During airplane mode, when the proprotors are required only for forward thrust, the RPM's are reduced and the blade pitch angle adjusted to a more efficient angle. Control is obviously provided by the control surfaces on the wings and tail.

Where it's tricky is during the transition modes. As the pilot pushes the knob or switch to tilt the rotors forward from the vertical, the flight computers take into account the nacelle angle and forward airspeed to electronically "fade" the control inputs from the proprotors to the control surfaces. Think of it this way, with the nacelles tilted forward at 60 degrees, and the airspeed at 50-ish knots (I'm just throwing numbers out there, I'm not a tilt-rotor pilot, and actual numbers would vary between specific airframe models anyway), let's say that 2/3 of a stick adjustment in pitch would go to the proprotors, and the remaining 1/3 would go to the elevator on the tail. At some nacelle angle and/or airspeed combination, the control inputs would be phased completely to the control surfaces. Then during the transition back to helicopter mode, the reverse process would take effect.

From what I read on an article about the original V-22, there wasn't a throttle or a collective in the cockpit, but rather a thrust controller or power controller. There was a large handle on the left side of the seat that the pilot would push forward to get more thrust from the proprotors/engines. I read the biggest change the pilot's had to learn was that in helicopter mode, they had to push the power controller forward to increase vertical lift, instead of pull back or up like a collective. They MAY have changed the entire principle of operation in the current V-22B configuration; and from what I understand about the AW609, the control system is completely different, with a collective-style control. (The BA609 was renamed the AW609 since Agusta Westland took full control over the program)

Now for the caveats: these tiltrotor principles of operation are based on what I've read about the XV-15/V-22/AW609 development programs. It may not reflect the current operational configurations of the V-22B's in service today, or any developmental refinements made to the AW609 since it's still in final stages of development. Everything else is based on my first-hand knowledge of helicopter and fixed-wing aircraft operation. If there are any Marine Osprey pilots out there, please feel free to jump in and correct anything that requires correcting.