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Is it possible for pilots to make a right or left turn, using just the ailerons and without rudder input in all airliners? If not why so, for what reason?

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    $\begingroup$ I think people flying propeller-driven Cessna's and such use this way of turning. I don't know how frequent this is. It's probably not considered "good airmanship" but it can be done. $\endgroup$ – user7241 Dec 17 '17 at 10:10
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    $\begingroup$ Your wording of this question "rudder input" instead of "rudder movement" makes it tricky or interesting to answer because this is actually a feature of advanced fly-by-wire system. $\endgroup$ – user3528438 Dec 17 '17 at 19:13
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    $\begingroup$ @user3528438 It is not a feature of an "advanced" system. Turns on any yaw-damper equipped airliner (707, DC-8 to today) should be flown without any rudder input. I think this is more along the lines of the answer that is being sought, rather than the complex aerodynamics and glider answers so far. $\endgroup$ – user71659 Dec 17 '17 at 19:26
  • $\begingroup$ @user71659 You know about yaw dampers but not about the flight dynamics behind it? You have to share your knowledge on this. $\endgroup$ – user7241 Dec 17 '17 at 19:41
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    $\begingroup$ @jjack "roll-yaw decoupling" $\endgroup$ – user3528438 Dec 17 '17 at 19:50
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It depends on the aspect ratio of the wing and the lift coefficient. Short, stubby wings at low angle of attack will not create much adverse yaw with aileron input, especially when the ailerons have differential linkage (more deflection on the trailing-edge-up side and less on the opposite side). Differential gearing is also beneficial in reducing stick forces, so many small aircraft with manual controls employ it. Therefore, turning with ailerons alone is possible with low aspect ratio wings.

Airliner wings have aspect ratios between 7 and 11, so here it is helpful to reduce the lift coefficient by flying fast and lower than at maximum cruise altitude if you want to turn with the ailerons alone. If you allow rudder movements but just want the pilot to rest his/her feet on the floor: Almost all jets have yaw dampers, that is a device which will deflect the rudder automatically such that yaw is minimized. Therefore, airliners can be flown with the feet off the pedals and still ailerons and rudder will keep the plane in a coordinated turn.

With gliders and their high aspect ratio wings, however, using only ailerons will mostly yaw the aircraft, especially at low speed when the lift coefficient is high. The wing also has high roll damping from its high aspect ratio and low flight altitude, so the roll angle achievable before yaw becomes dominant is small. The yawing condition will now create its own rolling moment due to the dihedral effect of the wing. Unfortunately, this effect counters the intended rolling motion and prevents the aircraft from turning with ailerons alone. If adverse yaw is large enough, the aircraft will not even begin to roll but will only yaw with aileron input. Some glider pilots even apply cross controls when entering a thermal in order to maximize their rolling speed (rudder into the turn and aileron against to provoke more yaw and let the dihedral effect roll the glider).

If you find yourself in a high-performance glider with a jammed rudder, speed up in order to fly turns with the ailerons (and elevator) only. The lower lift coefficient will reduce adverse yaw so direction can be controlled with ailerons and elevator only.

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    $\begingroup$ I have just noticed that the question text (but not title) mentions airliners. From riding in the back of many airliners, I have noticed that inboard ailerons are used in cruise, and spoilers may be used in conjunction with ailerons, at least when the flaps are extended. I would guess that these would both reduce the adverse yaw? And perhaps also so would the yaw damper? $\endgroup$ – sdenham Dec 17 '17 at 18:20
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    $\begingroup$ @sdenham: Inboard ailerons are a specialty of Boeing and their main purpose is to reduce wing torsion. A high true airspeed reduces roll damping, so very little rolling moment is needed and adverse yaw is low at high speed. Spoilers are used mostly in the approach phase and open the gap between wing and flaps. In that case roll control with spoilers does indeed lower adverse yaw. A yaw damper works on the rudder so the pilot doesn't have to, so this would be against the spirit of the question. $\endgroup$ – Peter Kämpf Dec 17 '17 at 18:27
  • $\begingroup$ @PeterKämpf Are inboard ailerons and flaperons the same? $\endgroup$ – user7241 Dec 17 '17 at 18:32
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    $\begingroup$ @jjack: In a way yes, inboard ailerons cover the kink in the trailing edge where inboard and outboard flaps would collide. They move trailing edge down when flaps are extended and as ailerons at high speed. True flaperons, however, mix aileron and flap inputs. The inboard aileron is only one of each at a given time. $\endgroup$ – Peter Kämpf Dec 17 '17 at 18:54
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    $\begingroup$ @copper.hat: Once the aircraft has rolled, turning is inevitable. Adverse yaw can indeed prevent the aircraft from rolling such that no turn results, not even a very slow one. Maybe I should clarify that part. $\endgroup$ – Peter Kämpf Dec 17 '17 at 19:04
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For most aircraft, it is. Deflecting ailerons only results in adverse yaw: the aileron deflecting downwards has higher drag than the one deflecting upwards, and the nose turns away from the direction the pilot wishes to bank into.

enter image description hereImage source

The relative magnitude of this effect depends on wing span and airspeed: hardly noticeable in jet fighters, highly disturbing in gliders.

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  • $\begingroup$ Just to clarify: When the pilot banks the aircraft using the ailerons, he wants to point the lift vector in the direction towards the center of the turn. Adverse yaw works against it however, so it's counterproductive if there's too much of it. $\endgroup$ – user7241 Dec 17 '17 at 12:19
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    $\begingroup$ Adverse Yaw does not cause aircraft turn. It only causes yaw. Yaw is not turn. Yaw is a deflection of the aircraft flight path from the aircraft longitudinal axis about the vertical axis of the aircraft. Turn is a continuous change in the aircraft light path direction of motion through the air mass. The aircraft can be in a continuous established sideslip angle as a result of a yaw inducing force, and still be maintaining a constant heading... i.e., zero rate of turn. Lift, pointed in a direction other than perpendicular to the horizon, is what creates turn. Nothing else. $\endgroup$ – Charles Bretana Dec 18 '17 at 2:58
  • $\begingroup$ @CharlesBretana you'll need to tilt your lift vector to compensate for rudder input if you want to fly in a straight sideslip path: bank to left, right rudder pedal. If you don't bank to left and give only right rudder pedal, this causes an angle of sideslip which will turn the plane as long as you press on the pedal. $\endgroup$ – Koyovis Dec 18 '17 at 12:18
  • $\begingroup$ @Koyovis. This is only true if the force from the rudder is different from the sideways force from the sideslip angle. With no other yaw force on the aircraft, if you have right rudder, that does create a leftwards force, but it also creates a right yaw (sideslip) which itself creates a rightwards force that opposes the leftwards force from the rudder. In a multi-engine aircraft with one engine inop, you need bank IN the direction of the rudder to compensate because the Yaw from the rudder just eliminates yaw from operative engine, and sideslip is zero. $\endgroup$ – Charles Bretana Dec 18 '17 at 14:17
  • $\begingroup$ @Koyovis, It works in exactly the same way as it does in pitch. The elevator pulls the tail down and, because it's behind the CG, creates a nose up pitching moment (torque). Lift on the wings lifts the aircraft up, and because it's also (generally) behind the CG it produces a Nose down pitching moment. Because the moment arm of the elevator is so much longer the torques balance, but the Up force from wings is much larger than the down force from the elevator and the aircraft is able to fly. The force from the rudder, and the force from the sideslip angle of the fuselage act in the same way. $\endgroup$ – Charles Bretana Dec 18 '17 at 14:22
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In fighter simulators I almost never used the rudder: I just rolled to almost 90° then applied the elevators...

IRL I tried this with gliders (we even had to learn how to fly if one of the 3 controls were lost - elevators are the worst, one of the other two you can get by without much easier). You can turn with gliders really well by just using the ailerons, and compensating with the elevators to not drop your nose too much (and if you are rolling, then your elevators are partially working as rudders anyway). Not good enough to do thermals with great efficiency, but good enough to get back home and land.

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    $\begingroup$ In fighter simulators I almost never used the rudder: I just *yawed* to almost 90° then applied the elevators - rolled, surely...? $\endgroup$ – Grimm The Opiner Dec 18 '17 at 10:53
  • $\begingroup$ @GrimmTheOpiner : yes, I overlooked it, English is not my native language $\endgroup$ – vsz Dec 18 '17 at 11:53
  • $\begingroup$ In the F-4 Simulator, (before leading edge slats were added), the adverse yaw was so extreme at high Angle of Attack that use of aileron would put the aircraft out of control. To roll the aircraft at high Angle of Attack, we used the rudder exclusively. As an instructor, in fact, when a student was flying the aircraft at high AOA, (like for stall series maneuvers), we would mash our knees together on both sides of the stick to ensure that the student did not introduce any aileron. $\endgroup$ – Charles Bretana Dec 18 '17 at 15:22
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I just came across your question by accident regarding turning an aircraft with the use of rudder.

The simple answer is YES.

You NEVER lead a turn using Rudder. Its a combination of of Aileron to lead into a bank (left or Right). Back pressure is then applied on the yoke/ stick to maintain altitude. Reason, additional drag encountered resulting in a loss of lift. Back pressure provides this additional lift required to maintain altitude.

The rudder's part is all this is to balance the turn. This means that aircraft "slip" or "skid" during a turn. Basically slip into towards the centre of the turn or skid outwards not maintaining a fixed radius turn in otherwords.

The "Ball" on the Turn Coordinator should be centered during the turn. So if the ball go to the left "then stand on the Ball" = left rudder to balance and coordinate your turn.

All this is a very smooth coordinated movement in practice.

If I may, just go one stage further regarding Adverse Yaw.

I also see some comments on the subject. Adverse yaw is one of these "Strange but True" issues.

Firstly, if you were to gently deflect the rudder (the Action) either side of center in normal controlled flight, two things will happen:

  1. It will produce a movement about the vertical axis.This is the Yaw. The Primary effect of rudder.

  2. Lots of groaning and moaning from the aircraft as if to say, Hey what are you doing to me ?.. reason the aircraft is being knocked out of balance.

The Secondary effect of rudder is to Roll. If you prolong the defection of the rudder the aircraft initially yaws followed swiftly by a rolling movement about the longitudinal axis.. dangerous if maintained..spiral dive follows..

Now, the main part. The adverse bit.

When a turn is initiated using aileron to lead into the turn the up-going aileron enters a reduced pressure area. Remember relatively low pressure on top surface of wing. While the down-going aileron enters relatively higher pressure area.

For example: Left turn.

The left aileron is deflected upward and the right aileron moves downward. As the chord line on the left wing is altered in an upward direction (The Chord line runs from the leading edge directly to the trailing edge of the wing or a control surface.), this effectively reduces lift on that wing. The opposite happens on right wing.. Lift increases. Result a left-hand bank should occur.

But, oops. a Surprise awaits us, We started a Left turn using Aileron but we initially get a RIGHT YAW effect..just as if we applied Right Rudder. So now after scratching our heads for a while, lets see what happened there....

In brief, as the lift changes on the outer parts of the wing, so also does the associated drag. Known as Induced Drag. A By-product of lift.

So the down going left wing, has reduced lift and reduced drag by association Right wing has increased lift and increased Induced drag.

Net result drag on left side - Reduced. Right side increased, so its acts like a "brake effect" on the right side, and relieved on the left side. Produces a Yaw to the RIGHT initially.

Solution: In aircraft nowadays the angular movement of the ailerons is arranged so that the up-going aileron travels further up than the down-going one. For example full up deflection say 50 degrees, Full Down movement 25 degrees and proportional in between.

This limits the effect of Adverse yaw due to up-going aileron traveling further into lower pressure area and likewise, down going moves down less in higher pressure area as the Chord line is altered differentially. This system is known as Differential Ailerons.

One further development on this was the Frise Differential Aileron, which allows the top side leading edge, of the aileron, to protrude into the airflow on the down-going aileron, creating a Parasite Drag on that side to counter the Induced Induced drag on the other wing. Combined its known as the Frise Differential Aileron.

You might need to read the last paragraph again, but the penny will soon drop for you.

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  • $\begingroup$ Welcome Gerry: That's a good description of adverse yaw, but not the topic of the question (turn without rudder). You may need to re-focus your post. $\endgroup$ – mins Dec 18 '17 at 10:44
  • $\begingroup$ Thank you. I put in an brief article a few hours earlier on the main topic regarding turning using rudder. This was just an add on as i noticed alot of comment on adverse yaw. I did prefix this by saying i was going one stage further prior to adding this bit. Gerry $\endgroup$ – Gerry Dec 18 '17 at 10:58
  • $\begingroup$ Correct indeed, but that's not how SE works, we prefer only answers that are directly answering the question, hence "comments", even good ones, may be deleted. $\endgroup$ – mins Dec 18 '17 at 11:19
  • $\begingroup$ Gerry, the Frise aileron is about lowering control forces. The parasitic drag you describe actually mostly occurs outside of the normal operation range of a Frise aileron. $\endgroup$ – Peter Kämpf Dec 18 '17 at 11:38
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Of course! Neither the rudder, not the ailerons, actually turn the aircraft. It is LIFT that turns the aircraft. It "turns" the aircraft in the direction that the Lift vector (perpendicular to the wings) is pointed in. So to turn, you need to point the Lift vector in the direction you wish to turn, (roll into a bank) and apply some Lift (Pull back on the stick/yoke).

To turn up (increase pitch attitude) leave the aircraft in zero bank angle, and pull on the yoke. To turn left , place the aircraft in a left bank, and pull on the yoke, etc.

To roll into a bank in a coordinated manner, you generally need both ailerons and rudder, but if you're not picky about being uncoordinated, you can roll into a bank with either one.

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    $\begingroup$ The airstream can exert forces and moments in 6 degrees of freedom on the airframe, lift is only one of them. $\endgroup$ – Koyovis Dec 18 '17 at 2:55
  • $\begingroup$ Any forces exerted by the Airstream on the aircraft are Lift. And they can be exerted in any direction. The rudder produces both Lift and Yaw, but these are not the same. None of these Lift forces are yaw. Yaw is a measure of the deflection of the fuselage from the flight path about the vertical axis of the aircraft. Yaw is a torque - a force causing a rotation, Lift is an ordinary force, causing an acceleration. Defining Lift as only that component of all aerodynamic forces perpendicular to the wings is an artificial engineering simplification to aid in visualization and computation. $\endgroup$ – Charles Bretana Dec 18 '17 at 3:04
  • $\begingroup$ Aircraft can be turned by the rudder alone: it creates a torque about the z-axis of the aeroplane. Not sure where you got your references from when you say Any forces exerted by the Airstream on the aircraft are Lift. $\endgroup$ – Koyovis Dec 18 '17 at 4:50
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    $\begingroup$ @CharlesBretana You wouldn't call aerodynamic drag force "lift" $\endgroup$ – user7241 Dec 18 '17 at 9:51
  • $\begingroup$ @jjack, Actually, I would. The "force" we call drag is a fiction, it's just another artificial construct invented by engineers and Mathematicians to make it easier to think about lift and perform calculations. All Aerodynamic forces are applied locally, to each point on the aircraft. Lift itself is just a artificial summation of all the tiny aero forces applied all over the surface of the aircraft, and "drag" is just the mathematical component of all those tiny forces that lies parallel to the flight path. ("Lift" is the component perpendicular to the flight path). $\endgroup$ – Charles Bretana Dec 18 '17 at 14:00

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