Fresh newbie in glider flying. I was told the joystick (aileron) and the pedal (rudder) should always be applied together when making a turn. My question is, what happens if I only pedals the rudder without applying the stick (aileron)? How is it different from applying only the aileron but not the rudder? And are there times when we should apply just one of the two?
To turn – any vehicle, not just an airplane – you need to generate corresponding centripetal force.
When you only apply rudder, the plane will turn a bit, but because unlike boat it has no keel, it will not generate much centripetal force, just fly somewhat sideways, which is called a skid. It will produce more drag this way.
Now any well designed plane has yaw-roll coupling, which will cause it to bank into the turn anyway. It is caused by the slight increase in speed of the outside wing, and the dihedral most wings have resulting in increase in angle of attack, thereby increasing lift on the outside wing. So you can make the plane turn with just the rudder, but it will be inefficient, and it will be slower as the yaw-roll coupling will take its time.
On the other hand if you apply just the aileron, the plane will bank into the turn as usual. However, there will be some slip, as the deflected ailerons cause an adverse yaw – the aileron deflected down, which is on the outside wing, causes more drag – so the plane will actually turn a little less than it should. Like skid, that will cause extra drag.
What you want to do is bank into the turn with ailerons, and use rudder to compensate the various drag asymmetries to keep the slip-skid ball centered. That will give you least drag. Different planes will need slightly different application of rudder.
There are cases where you will want to apply ailerons and rudder in different combinations (as mentioned in comments below):
- On the ground you steer with rudder, while using aileron to prevent wind from tipping the plane sideways – the landing gear provides the centripetal force in this case.
- During cross-wind landing, you need to align the plane with the runway, but as it flies relative to the air, it tends to drift sideways. You need to make it fly sideways relative to the air instead by banking into the wind while using opposite rudder to prevent turning.
- The same combination applies during cross-wind take off, aileron into the wind to prevent wind from tipping you over sideways and rudder to compensate for weathervaning.
- You can also use bank with opposite rudder to increase drag if you are too high and fast on the approach – this is called forward slip. Just don't let the airspeed decay too much, because stall in slip or skid is more likely to result in a spin.
If you apply (say) right rudder without any aileron input, the immediate effect is that the aircraft will yaw: the nose will swing to the right.
Note that this does not mean that the aircraft will turn! In the first moments after the aircraft yaws, the nose is now pointed off to the right while the aircraft continues to travel in its original direction. What the aircraft will "feel" is that the air is now blowing on it from the front left instead of head-on. We say that the aircraft is slipping to the left, because its track (the direction in which it is traveling) is to the left of its heading (the direction that the nose is pointing).
So, you have stepped on the right rudder pedal, causing the aircraft to yaw right and slip left. This has the following consequences:
- Your instruments will indicate a slip to the left. The yaw string will indicate this by moving to the right (meaning that the "head" of the yaw string will point to the left.) The inclinometer (the "ball in a tube") will indicate this by moving to the left. And your inner ear and body will indicate this by telling you that the centrifugal force from the turn is pushing you to the left.
- Drag will increase dramatically. In a glider, this means you will either slow down or descend more quickly, depending on what you do with the pitch. In an airplane, the same thing will happen unless you add engine power.
- Body lift. Since the oncoming air is blowing on the aircraft from the left, this will push on the aircraft and turn it to the right. (This pushing force is what causes the inclinometer to move, and your body and inner ear to feel the sensations I mentioned above.) This is, of course, not the "normal" way to turn an aircraft.
- Dihedral effect. The air blowing from the left will cause the aircraft to roll right. (Here's one way to think of this. The tips of the wings are "high up" compared to the rest of the aircraft. So when there's air coming from the left, it will push the wingtips to the right, causing the aircraft to roll right.)
Slipping is the same thing as being in uncoordinated flight. Being in uncoordinated flight presents two problems. The first problem is that it creates a lot of drag. (Sometimes you want drag, of course.) The second problem is that if the aircraft stalls while in uncoordinated flight, the downwind wing will stall first, which can produce a snap roll or a spin. Snap rolls and spins can both be recovered from, but if they happen close to the ground, the results can be fatal.
Using just the rudder and not the ailerons has some uses:
- When you recover from a turning stall, after regaining speed, you should level the wings by leaving the ailerons alone and applying rudder towards the sky. We use the rudder here because using the ailerons can exacerbate a stall.
- The procedure for recovering from a spin involves using rudder opposite the spin while leaving the ailerons neutral.
Using the rudder along with opposite aileron will produce a straight slip, which is useful for deliberately losing altitude or for landing in a crosswind.
As for applying ailerons without rudder: applying (say) left aileron without rudder will have a similar effect to applying right rudder without aileron. In fact, left aileron without rudder may even cause the aircraft to yaw to the right. (This is called adverse yaw.) As far as I know, the only difference is that the aircraft will bank to the left, which will ultimately result in a left turn instead of a right turn. The dihedral effect will still happen, just as if you had given right rudder; since you're pushing the stick to the left, this means that the dihedral effect is resisting your attempt to bank left.
Finally, on the ground, pretty much all of what I just said goes out the window. On the ground, the rudder is used to steer the aircraft and the ailerons are used to keep the wings level (if necessary). Unlike in the air, while you're on the ground, yawing the aircraft will cause it to turn immediately, thanks to the wheel or wheels.
With just rudder, your tail will kick out to one side, and you'll kind of skid around a turn in a really draggy manner.
With just aileron, the plane will turn but the tail will droop to the inside of the turn, also a kind of draggy maneuver.
When you coordinate both, the plane banks and the tail moves the fuselage around the turn in a nice smooth circle.
Aircraft can be designed (with relatively large dihedral) to fly without ailerons, using only rudder (and the roll coupling that the dihedral produces) for both directional and roll control. Without this designed-in coupling, rudder-only input generally results mainly in a skid, with only a small bank (and as noted in other answers, a very inefficient turn).
Many radio control trainers have been designed for rudder/elevator control; I owned and flew one many years ago that I could roll and fly in prolonged inverted attitude, even do outside snaps to a tumble that resembled a Lomczevak (and recover into level flight after two somersaults). Two-axis control is also common in R/C sailplanes -- Gentle Lady, Olympic 2 and Olympic 650, Two by Six, etc.
It's relatively rare for human-carrying aircraft to lack ailerons, though not unheard of (the Pou de Ciel homebuilt family mostly have no independent roll control), because in general, it's more useful for safety reasons to have direct roll control (perhaps with coupled rudder like the original Ercoupe) than to depend on high dihedral for roll -- crosswind landings, for instance, are far easier in an Ercoupe than a Pou. If you need a rapid roll correction, ailerons will give it; rudder and dihedral won't (there's some lag in roll because the dihedral requires the skid before it raises the forward/outside wing).
Aircraft work in three dimensions, not two, so the rudder is not used to 'steer' an aircraft they way you would a boat or a car. The three motions available to you are called pitch, roll and yaw and are controlled by the elevator, ailerons and rudder. All three are used together to change the direction an aircraft.
To change direction, you have to introduce a force that pushes you from where you are going to where you want to be. To go from straight and level due north to straight and level due west requires all three controls and a careful balancing act. Banking the wings left will change the lift vector from vertical into the corner pushing you in the direction you want to go, however with less lift, the nose will go down so you pitch up to keep the nose level, which also pulls you further into the turn. However, being banked means the plane will side slip can feel uncomfortable so you use the rudder to adjust the yaw to compensate. All of this pushing against the air to make the turn also robs you of energy that needs to be replaced, causing you to slow down or go down.
You can turn without the rudder and normally do, but you can not turn with the rudder only.
The controls all have primary effects of the control, and followon effects of the control. See Jeremy Pratt, section "Effects of Controls". https://stepbystepppl.wordpress.com/2013/07/02/ppl-exercise-4i-the-effects-of-control-surface/
If you are in level cruise (and you are in a reasonably "normal" airplane like a C-150) and give a rudder input, as many have pointed out in their answers, the primary effect is a yaw effect.
But, if you gently continue the yaw input, the yaw has a followon effect once the airplane settles into the new unbalanced attitude. The followon effect is a bank in the direction of yaw. The result therefore, if you are very gentle, is a coordinated turn.
My venerable mentor taught me to learn coordinated flight by flying exactly this way - in the cruise, fully trimmed, hands free and steering by gentle rudder inputs alone.
The major caveat is that every airframe is unique and you need to learn the particulars of yours. In my case, this of course was low level powered flight in a Cessna 150.