# How does a plane turn?

1. Aug 3, 2007

### ank_gl

how does a plane turn???

damnit i am not able to figure out what does a rudder do in an aircraft.
an aircraft banks, lift has a component perpendicular to the fuselage in horizontal plane, it plays for the centrifugal force, aircraft makes a turn.

if rudder is moved keeping the bank zero, nose rotates in a horizontal plane but craft keeps on going on in the same direction. rudder IS producing yaw but is not able to turn the plane.there is no force pushing it to turn

if rudder is not there, wont the craft turn??? what is rudder trying to balance??
what the hell is rudder trying to do??:grumpy::grumpy::grumpy:
i am sure i am missing something(geez i have wasted 2 hours thinking about it)
i know i am wrong somewhere:uhh::uhh: but dont know where. plz help

2. Aug 3, 2007

### K.J.Healey

Well can't the plane turn multiple ways?
Lets call our three axis, CABIN, WING and HEIGHT
First its no question that the plane can rotate about the CABIN. It turns the left wing(aileron?) one way and the right the opposite. Like a corkscrew. This doesn't turn the plane, but rotates it while it travels in the forward direction.
Then just readjusting the ailerons on both wings the pilot can "pull up" and this induces a change of direction with only two rotational motions.

Heres some detailed info:
http://en.wikipedia.org/wiki/Flight_dynamics

The rudder itself is an aileron that can change the planes yaw, or the rotation about the vertical axis. Basically what you call a turn.

Real airplanes use both methods together, thus keeping the plane from pitching too much, or having too much yaw.

3. Aug 3, 2007

### ank_gl

geez this is what you tell kids, i know about all that stuff.. i know how one controls a plane.

""The rudder itself is an aileron that can change the planes yaw, or the rotation about the vertical axis. Basically what you call a turn.""
i dont think so, plane has to be banked first for a turn. and this is what i am asking, banking is enough for the plane to turn, what is rudder doing???
plz try to understand. i am in a situation like "you know everything and you are missing one point" this is killing me. duh.. why i am soo dumb

4. Aug 3, 2007

### K.J.Healey

Ah.
I believe the rudder turns the plane back. By TURN I mean NOT changing the flight path, but keeping the fuselage of the plane pointing toward the direction of motion.
The plane doesn't always move in the direction of the vector from tail to nose. It can be off slightly and not change the flightpath by use of the rudder.

So imagine no bank, the rudder would change the rotation/yaw of the plane. The flightpath, for sake of our argument, wouldn't change the path much, but it off-centers the tail-nose vector compared to direction of motion.

You can use this effect in a banked turn to keep the nose of the plane pointed in the direction the plane is actually moving. Where in an un-ruddered banked turn the nose would be off due to the changing in velocities of each side of the craft.

5. Aug 3, 2007

### LURCH

I think the best way to address this question is by looking at two alternative scenarios; executing a turn without using the rudder, and executing a turn using only the rudder. These are the two scenarios to which you refer in your original post. First, I should briefly state that there are two kinds of drag acting on the aircraft, induced drag and inherent drag, and that induced drag is caused by the wings generating lift. So we look at our first scenario:
Now, as stated above, there is a certain type of drag that is the direct result of generating lift. When the aircraft banks, it does so because the ailerons have been deflected, one downward and the other up. This deflecting of the ailerons results in the movement of the wings, one upward and the other down. If you think about it, you'll probably see that for one wing to go up, and the other to go down, one wing must be generating more lift, and the other less (of course, some of the "downward" motion of that wing may be the result of generating downward lift, but because the wing is moving in the direction gravity wants it to go, much of its motion can be seen as a decrease in upward lift). So, if generating lift causes drag, you can see that the wing that is now generating more lift is suddenly experiencing more drag, and the other wing which is generating less lift, experiences less drag. This causes the nose to want to yaw toward the upper wing. if you're turning left, the nose will try to yaw right. This is aerodynamically inefficient, can be uncomfortable for the passengers, and also reduces the pilot's control of the aircraft. The rudder is used to keep the aircraft "pointing into" the turn.

However, the rudder can also be used on its own, to execute a turn in an aircraft where, for example, the ailerons are stuck and won't move.

Ah, but there is! You see, if the rudder is used to make the aircraft yaw to the left, thrust from the engine is now pushing off to the right, making the airplane go left. BTW; this will also result in the aircraft banking to the left, for reasons I could go into, if you want.

I hope you can now see that without the rudder, the aircraft willing indeed turn, but that turn will be "uncoordinated", making it sloppy and uncontrolled. Likewise, with only the rudder and no ailerons, an aircraft can still be turned, but that turn will be ugly.

Last edited: Aug 3, 2007
6. Aug 3, 2007

### ank_gl

""So imagine no bank, the rudder would change the rotation/yaw of the plane. The flightpath, for sake of our argument, wouldn't change the path much, but it off-centers the tail-nose vector compared to direction of motion.

You can use this effect in a banked turn to keep the nose of the plane pointed in the direction the plane is actually moving. Where in an un-ruddered banked turn the nose would be off due to the changing in velocities of each side of the craft.""

oh yes, this is what actually i thought. a rudder WILL affect the flightpath but the rate of turn will be too slow that way, maybe negligible.
but if i have already banked, why do i need a rudder???
i guess to change the tail-nose vector direction to the circular path. but if so, how do i calculate the force required to maintain the tail-nose vector in the direction of flightpath??

7. Aug 3, 2007

### Danger

An aeroplane can change direction under the influence of either the ailerons or the rudder alone, but it's not pretty. Using them in concert makes it happen right away and with a minimum of slippage.

8. Aug 3, 2007

### ank_gl

oh i get it now LURCH. but if i want to design a rudder, as you say, it is used to counter the yaw induced because of banking, i need the rudder to generate this much of force, right???
just need to confirm it

9. Aug 3, 2007

### ank_gl

i know that
so the rudder is used for better control of the system?? i guess so

10. Aug 3, 2007

### K.J.Healey

Yes, it removes some of the adverse yaw effects induced by the banked turn.

11. Aug 3, 2007

### LURCH

Right. So the amount of force you need to apply to the rudder depends on a number of factors; bigger planes have bigger ailerons, producing more drag, but they also have bigger rudders, multiplying their effectiveness. Also, in many aircraft, the rudder pedal is connected to the rudder by hydraulic and electric motors, and the amount of pressure with which the pedal resists your foot is controlled by a computer. So unfortunately, we can't just say, "a 1.5g left turn at 500 knots requires 6.25 lbs of pressure on the left rudder pedal" or something like that. That's why there's a "ball" in the instrumant panel, to help the pilot coordinate his turns.

12. Aug 3, 2007

### Danger

Sorry guys; somehow I missed seeing posts #4,5 & 6 before my previous response.
I just wanted to add that in a real flight situation, the throttle and elevators are usually manipulated as well to compensate for the overall loss of vertical lift during a turn. (The actual lift remains the same, but it is no longer directed straight downward.) Otherwise, the plane will lose altitude.

13. Aug 3, 2007

### ank_gl

hey isnt that upward
and lift does change, only its vertical component remains unchanged. horizontal component increases

Last edited: Aug 3, 2007
14. Aug 3, 2007

### ank_gl

oh no, i didnt mean that, i meant the calculation of force which i ll need for the rudder to balance the induced yaw resulting from banking

15. Aug 3, 2007

### caslav.ilic

If I recall my classes correctly, the rudder action needed to sustain coordinated turn for conventional configurations is not a critical condition for sizing. For single-engine airplane the sizing criterion would be flight in crosswind, and for twin-engine airplane flight with one engine out (which is tougher than crosswind, hence the huge fins on twin-engine transports).

--
Chusslove Illich (Часлав Илић)

16. Aug 3, 2007

### ank_gl

hey chuss, i didnt get much of what you said above, can you please explain
and what are the criterion for sizing???

Last edited: Aug 3, 2007
17. Aug 3, 2007

### caslav.ilic

So, when you want to size a control surface (or anything at all for that matter), you need to look at all the conditions in which it must operate -- the envelope -- and consider the extremes.

One thing that rudder is used for, is for coordinate turn. Check.

Another time for rudder action is when the airplane is landing, trying to keep in line with the runway, but there is a crosswind (wind perpendicular to runway direction) which would tend to slide the airplane from the approach path. So, for the airplane to keep on course, it has to fly with a non-zero yaw angle to the total relative wind (usual + crosswind) -- hence, the rudder must be deflected.

When a twin-engine airplane experiences loss of one engine, the other engine will introduce yaw moment around CG, which again has to be countered by rudder deflection to keep the airplane from yawing.

Of these three, as far as I understood, the needed rudder authority for coordinated turn is never an issue compared to what is needed for crosswind landing and especially asymmetric engine loss. Hence, when dealing with single-engine airplane, the designer has to decide upon the maximum allowed crosswind and size the rudder to keep airplane at certain yaw angle accordingly, and for twin-engine aircraft, size the rudder so that it can keep the airplane at zero yaw when flying with only one engine.

I can't help you with particular procedures and sizing formulas, but some are certain to be found in aircraft design books -- a usual mix of statistics, empirics and some theory. I vaguely remember one author mentioning that sizing the fin in general is tricky bussines, and that it is not that unusual to make corrections even after the first prototype has flown.

--
Chusslove Illich (Часлав Илић)

18. Aug 4, 2007

### rcgldr

At "cruise" speed in a powered aircraft, which is well above stall speed, the rudder is almost useless. Radio control gliders meant for high speed at slope sites rarely have rudders. There are powered equivalents of these gliders that are also rudderless.

Regarding adverse yaw, this can be reduced by setting up the ailerons so a roll control input results in more upwards movment than downwards movement of the ailerons, but this only works for a limited range of air speed and corresponding angle of attack. Using rudder inputs to control adverse yaw isn't airspeed limited. Note that the yaw only occurs while an aircraft is rolling, once it stablizes into a fixed bank angle, the adverse yaw goes away.

Rudder is more important for slow speed flight, such as thermal gliders, both full size and radio control, since coordinated turns reduce drag. For gliders, slow speed turns (as in thermal turns) require a bit of opposite aileron to keep the relatively slower moving inner wing from sinking, and a bit of positive rudder and moderate up elevator to maintain a coordinated turn (keeping the fuselage lined up with the direction of travel).

Also it's not just the size of the rudder, but the length of the tail boom; the distance from the center of mass to the rudder (torque = force x radius, so increase the size for more force, or increase the radius). Most powered aircraft use a relatively short tail boom and a larger rudder, while most gliders use a long tail boom and a smaller rudder (less drag).

Rudder control is also important for take offs and landings.

Regarding rudder's affect on an aircrafts roll response, this is called yaw to roll coupling, and the amount of this effect depends on the effective dihedral. If there's no effective dihedral (a straight wing), then rudder inputs just yaw the aircraft and the fuselage effectively becomes a wing. This is demonstrated in knife edge flight (flying straight with the wings banked to vertical), where rudder effectively becomes an elevator, and fuselage becomes the wing. High powered rc aircraft can do knife edge loops, I'm not sure if full scale aerobatic aircraft can do this.

updated - Some rc aircraft have a lot of dihedral, which creates a lot of yaw to roll coupling. This allows a model to get by with just rudder and elevator, using the rudder to induce roll.

Last edited: Aug 4, 2007
19. Aug 4, 2007

### ank_gl

thanks guyz, i was getting so confused with this.
apperently, i did thought the same way as jeff and chuss say, but i was a lil confused regarding its use.
anyways thanks

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