Turning an Airplane. What actually causes the circular motion in a banked (roll) turn? Basically I'm wondering if this is correct. Which essentially says that you need a torque to get the nose of the craft to turn and that this is provided by the rear tail surfaces. 
After trying to simulate airplane flight in a game engine, I'm thinking that link is correct. Although airplane flight is rarely described like that. 
If you just apply force in the direction of lift (and thrust), then during a roll, the horizontal component of lift causes the plane to get a velocity with a horizontal component. But the plane does not turn. The plane slips to the side while going forward. The horizontal component of lift does not stay perpendicular to the velocity vector so it doesn't move in a circle.
Imagine also a block sliding forward on ice (constant v) with a small thruster attached to its side (turned off). Now turn on the side thruster (pointed through cm). Does the block start turning in a circle or does it just get a horizontal component to it's velocity? Doesn't it need a torque, just like the plane to keep the thrust perpendicular to the velocity vector?
So am I out of my mind here or is this correct? 
 A: You're correct, the ice block will not turn automatically.  It will require a torque.  In aviation this is basically what is called coordinating a turn.  With an airplane, if the pilot does not provide the necessary coordinating torque via rudder/elevator inputs, the torque will be generated automatically via the weathervane effect, which tends to align the fuselage with the velocity.  So, you can effectively turn just by banking, if you're not anal about keeping your altitude and airspeed constant.  In a helicopter, it's a different story, as the helicopter doesn't have much of a weathervane effect at least at low speeds.  With a helicopter at low airspeed if you just bank without coordinating rudder then you just slide sideways.
A: As a student pilot with 100 hours and 300 landings, not to mention interest in engineering and physics, I love questions like this.
My first recommendation is get a copy of Stick and Rudder.
It's a delightful book, used by pilots for over half a century to understand the basics of flight.
My second recommendation is take an introductory flying lesson. It costs around $200 and is the most fun per buck you can have honestly.
Now to try to answer.
(Almost any question like this can be understood with a simple balsa wood glider or a paper airplane.)
First to understand is that in straight flight at constant speed, for a normal airplane with single main wing and a tail, the wing pushes up and the tail pushes down.
The wing supports not only the downward weight of the airplane but also the downward force on the tail.
The plane is nose heavy, and if the tail were suddenly chopped off, the plane would immediately go down.
Since the tail is some distance behind the wing, its downward lift creates a moment that holds the nose up.
The lifts, up and down, and thus the moment, are proportional to speed squared.
If for some reason the plane slows down, the moment decreases, the nose drops, and the plane speeds up.
If it speeds up, the moment increases, the nose rises, and the plane slows down.

This is the first thing you need to understand about airplanes - how the nose-heaviness and the downward lift on the tail cause its speed to be stable.
The exact speed it seeks depends on the angle of the tail surfaces.
The more they are canted upward, the slower the plane flies, and downward means faster.
(In case you're wondering how the wing gets enough lift when it is flying slower, it's because the whole plane tilts up, giving it more angle of attack.)
I know you are asking about turning, but this is the first thing you need to understand.
Second, how do you make it climb or descend?
That's what the throttle is for.
Just like in a car, you need more power to go up a hill, and less to go down.
In a plane, if you increase power, it speeds up, raises the nose, settles back to its natural speed, and keeps climbing.
If you decrease power, it slows down, drops the nose, and then settles to its natural speed, but on a descending slope.
OK, you were asking about turns.
The way you turn a plane is by putting it into a bank.
(With the ailerons, by monentarily turning the yoke left, and then re-centering it.)
Suppose you bank it 30 degrees to the left.
That tilts your lift vector, so that half of it is accelerating you to the left, and .866 of it is pushing up against gravity.
To keep gravity from winning, you apply back pressure on the yoke, creating more lift.
A turn has much in common with a climb, unless you want to descend at the same time.
You also need to give it some more power, to maintain speed through the turn.
Here's a 60-degree bank:  where you have to pull 2Gs of lift in order to maintain constant altitude, and add quite a bit of power. That's a pretty stressful maneuver, and you can't do it at slow speed because if you pull that hard it will stall. That shows you the similarity between a turn and a climb.
You were asking why doesn't the plane just accelerate sideways rather than turn?
If it did, then pretty soon it would be feeling this sideways wind on it.
Every airplane is a weather-vane. It turns into the wind it feels.
So, roundabout, that's the answer to your question.
A: To roll, the ailerons (moveable surfaces on the trailing edges of the wings, near the tips) move in opposite directions, increasing the effective camber of one wing tip & reducing that of the other. This creates a spanwise difference in the lift distribution.
If no other input is commanded by the pilot, the nose will drop; assuming you started in straight & level unaccelerated flight (SLUF), as the plane rolls the component of the lift vector opposing the weight of the plane drops (to zero when in a 90$^{\circ}$ banks).
In order to maintain level flight, the pitch angle must be increased; this also changes the thrust vector. In order to maintain unaccelerated flight (specifically to not lose speed), thrust must be increased (as drag increases with increasing pitch angle).
The differential lift distribution on the wing tips also creates a differential drag distribution; for most conventional airplane configurations, this comes in the form of adverse yaw (i.e., the nose tends to track away from the direction of bank). This is countered by application of balanced rudder force.
Summary: in an unaccelerated level turn, the side force on the vertical tail due to rudder deflection keeps the nose tangential to the circle, while the horizontal component of lift due to bank angle provides the centripetal force.
Emilio is correct that engine torque (gyroscopic coupling) can be an issue; it is typically negligible at low rotation rates.
EDIT (not enough rep yet to comment on other's answers): yes, the lift vector 'naturally' stays normal to the velocity vector by definition; it's the component of the aerodynamic force that is normal to velocity, while the component parallel to velocity is labeled 'drag'.
A: I believe that the banked turn could be done like this:


*

*Tilt one wing's surfaces up and the other's down to get the plane to bank. The lift force is now slightly off to the side. You are now sliding to the side, as mentioned in the link you posted.

*Get the plane to pitch "upwards" from its own point of view. The force at this point is off to the side and slightly backwards. I don't know if this would be done with the wings or the tail.

*Undo step 2. You are on a different heading, but still banked and still sliding.

*Undo step 1. You are on that new heading and no longer banked.


For the block-on-ice portion: you describe: any thruster that applies force along a line that passes through the center of mass will not cause the object to rotate. You can add a horizontal component of acceleration, as you suggested, but the block won't spin and you won't be able to get it to move in a circle with just that thruster.
A: I like this question, and I find the link to be of great utility, but I'll still go over several issues I have with its explanation.
First, we need to establish the completely plausible situation of a balanced plane operating normally, then being slanted to the left or right, and the fact that it will shimmy in that respective direction and downward.  This requires only a first-level force diagram.  The gravity vector is the same magnitude but rotated relative to the plane and the aerodynamic forces are unchanged relative to the plane.  There are several mentions of force moments in the link, but I...


*

*Don't find them useful or necessary for the explanation of aircraft turning and

*Don't see any reason to predict a moment imbalance in the aircraft's vertical plane


Consider that normally the wings exert a lift force upward (relative to the aircraft) and the horizontal tail exerts a lift force downward.  If we assume that we do not change the air velocity relative to the aircraft then changing the gravity vector (at CM) has no ability to create a net force moment in any direction.  Both lift and gravity act at the CM, and one has to move for us to obtain a moment (like they do in buoyancy problems).
In the simplistic view of a point force balance, steering the plane is not possible at all.  The link presents an attempt to resolve this conflict.

in order to avoid losing altitude, the pilot has to compensate with a bit of back-pressure on the yoke

I disagree with this.  The pilot has to put back pressure on the yoke in order to bank, but the idea that he/she does so to compensate for losing altitude shouldn't hold up to scrutiny.  Consider the fundamental perception of movement - velocity with no acceleration or reference frame is not perceptible.  The pilot would have to literally look at the altimeter and pull the yoke in order to compensate.  Provided he/she did so, the turn would actually happen, yes.
It is much more perceptible that you start pointing toward the ground as opposed to moving toward the ground.  Again, if you're losing altitude, you might notice eventually, but seeing the horizon move creates... a very instinctive correction by the pilot.
Here I will get into the superior resolution.

For instance, when an airplane is banked and moves sideways through the air (as in the parallel move discussed above), the relative wind will hit the airplane from the side it is banked towards.

If the plane is moving right/left and downward (as a result of the acceleration from the point force diagram) then the tail presents a large area directly to the wind.  This predicts a force moment exclusively in the aircraft's horizontal plane, which will point the aircraft down and right/left.  The pilot will then pull the yoke back (possibly imperceptibly) to compensate for the horizon's movement.  From a controls system analysis, the pilot's primary responsibility is likely to be maintaining the up/down orientation.  As any other control input changes this, the up/down orientation control will automatically respond very vigilantly.
This is my preferred explanation, and I should disclaim that I have very little experience directly with flying planes and my writing is strictly academic physics oriented.  With my current hypothesis, we have one major prediction to note:


*

*Turning left/right is a 2nd order effect from aircraft banking


The alternative to this would be that the orientation of the aircraft left/right directly imparts a moment (which you should remember I disagree with).  It is interesting to consider applying this to experience.  How fast does does the turn start once the aircraft is oriented in the banked position?  Does turning inherently result in a loss of altitude?  My explanation predicts "slow" and "yes".

One more point was brought up about the moment of the engines/propellers.  Unless you had counter-rotating dual (or more, even number) engines, this adds an asymmetrical component.  It becomes easier to turn left or right, depending on the direction of spin.  I know WWII era dogfighters were acutely aware of this fact in many situations.  I think it's obvious one could use this fact to their advantage/disadvantage.
A: When a plane banks the wing up-and-back (lift) force becomes partly sideways. When level average force of the 2 wings goes through the centre of gravity. When it banks the force is in front of the centre of gravity and induces a moment of force which turns the aircraft. The more the bank the more the force moves forward and more the torque. It's a 3D problem that is diff to imagine. 
To further complicate matters the outer wing has to move faster and experiences more drag which can prevent the turn and the plane will turn and dive the other way! I think there is another flap to correct for that if required.
A: If the aircraft  has a non-zero moment of inertia about the yaw and pitch axes, then a yaw torque and a pitch torque will be required to intiate the turn.
Even once a steady-state turn is established, the tail surfaces will typically need to be positioned differently, and will need to create different forces, than they did in wings-level flight at the same angle-of-attack.  An effect called "aerodynamic damping" exists which causes the rotation rates about all axes to tend to trend toward zero.  This effect is related to the fact in a turn, different molecules of the aircraft are moving in different directions through the airmass at any instant in time, since the yaw and pitch rotation rates are not zero.  As a result, in a stabilized turn (i.e. constant airspeed and constant rotation rates about all axes), the free-stream airflow can be viewed as being "curved" to follow the path of the turn.  This creates torques that would tend to slow the yaw and pitch rotation rates, if the tail surfaces were left in the same position as they were in during wings-level flight at the same angle-of-attack.  This effect is most pronounced in slow-flying aircraft, like sailplanes and ultralights, where the turn radius is not too large compared to the forward flight speed, or more precisely compared to the fuselage-lengths travelled per unit time.
Were it not for these effects, all you would need to do to estabilish a perfectly "coordinated" steady-state turn at the same angle-of-attack you were flying at with the wings level, would be to roll to the desired bank angle (and add some power if you wish to maintain altitude).  In reality, it's not quite that simple.  The effects described above cause the aircraft to tend to adopt a lower angle-of-attack in the turn than in wings-level flight, and also tend to cause the aircraft to fly in a "sideslipping" attitude, i.e. with the nose pointing slightly toward the outside of the turn in relation to the direction of the flight path at any given instant.  Learning to overcome these effects is a large part of what learning to properly "coordinate" a turn is all about.
A: Okay well I think a simple explanation is more intuitional here without going into the polemics of aerodynamic mechanics. An aerofoil is the design that creates lift by using Bernoulli principals of how fluids behave. One you know properly how an aerofoil works you can understand why the aircraft can turn in a bank.
In straight level flight the lift is equal to drag. if angle of attack is increased, or velocity is increased, then lift becomes grater than drag and altitude increases and vice versa. It is important to understand that the lift force exists in relation to the "relative airflow" which is the airflow being disturbed as a consequence of the aircraft's velocity. In straight and level flight lift is acting vertical to the the "chord line" of the aerofoil at the "centre of pressure". So it can only act in a singular dimension (up or down).
In a bank you have now induced another component of the lift vector. the horizontal.  This bank increases the load factor of the aircraft and disturbs the balance of lift and drag. therefore an increase in thrust will be needed to compensate for the increased g-force. If the increase in thrust puts lift and drag equal to each other it is possible for the aircraft to bank and fly the same heading as it were when it was in straight level flight (like swept wing high speed aircrafts are able to). However you want to change heading you need to increase lift in a bank (think of the direction of relative airflow on the aerofoil and the resulting effect of the horizontal component of lift). That resultant pitch up moment will cause an aircraft to turn. instead of slide. 
You should remember that an aircraft is always sliding  because in reality we are never flying in headwind. We are always side slipping and in smaller aircrafts like the trainer Cessnas I fly, we have to hold rudder inputs to ensure that we are not blown off course since the track (direction of movement in relation to the ground) is rarely the same as the heading (nose direction).
There are many other effects that cause the aircraft to drop the attitude or yaw adversely depending on aircraft wing, fuselage and powerplant design and  characteristics but that is a wholly separate discussion not a part of the question. 
hope that helps. 
