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I am perplexed on the dynamics of the forward slip. I am illustrating here: enter image description here

The intended ground flight path is axis +y, and the plane is forward slipping by banking right + left rudder.

In order to keep being on the intended path, forces on x should add up to zero. However, the lift has a component to the right side (+x). I don't understand how the rudder is able to counteract this. The lift can be modeled as a linear force, thus acting on the plane's center of mass; instead, the rudder provides a torque that can change future geometry, but certainly not provide a linear force. What am I missing?

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  • $\begingroup$ Do the answers here not explain it well enough? aviation.stackexchange.com/questions/60774/… $\endgroup$ Nov 4 '21 at 14:11
  • $\begingroup$ P.S. lift would be on the Z axis of your diagram. $\endgroup$ Nov 4 '21 at 14:14
  • $\begingroup$ @MIchael Hall yeah, I've visited this before! But, while it clarifies between the two slips, it doesn't make me understand the dynamics. I've ignored the z axis, I have no problem there. $\endgroup$ Nov 4 '21 at 14:21
  • $\begingroup$ The Z component partly counteracts the weight, sure, I understand this. I am referring to the horizontal contribution of the lift due to banking. $\endgroup$ Nov 4 '21 at 14:25
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    $\begingroup$ 1. Answers are editable. 2. It isn’t your responsibility to edit your question to match the answers that are given. 3. If the light came on and you identified the problem, you can answer your own question explaining the confusion or error in your premise. 4. I’d answer, but heading to work…. ;) $\endgroup$ Nov 4 '21 at 14:38
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You are confusing some of the directions. For a forward slip in the +y direction, you have to bank left. The lift and rudder vector should both point in the opposite direction.

That would still leave a sideways component. However, the airframe will also generate some sideways lift! In fact, that is exactly the reason why you have to make some bank angle at all; if the airframe only resulted in drag, you could perform a forward slip wings level.

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  • $\begingroup$ Yes, a right bank with full left rudder sounds like the plane may be upside down in the immediate future, and not happily slipping. $\endgroup$ Nov 4 '21 at 12:10
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    $\begingroup$ @tedioustortoise I'm confused by your comment. Sounds like you're just describing a forward slip, which doesn't result in being upside down? $\endgroup$
    – Sanchises
    Nov 4 '21 at 12:40
  • $\begingroup$ " if the airframe only resulted in drag, you could perform a forward slip wings level.'-- almost but not quite. You'd need a slight bank in the same direction as the deflected rudder, to neutralize the sideforce from the rudder itself. I once tried to replicate this with a radio-controlled swept-wing flying-wing aircraft with no fuselage but it did have a very skinny tail boom with a big rudder at the end of it. Wasn't able to consistently demonstrate the need to bank in same direction as rudder, need to repeat in calm air with stabilization, autopilot, etc, would be a fun project. $\endgroup$ Nov 5 '21 at 16:45
  • $\begingroup$ Crosswind landings using the "wing down" method in such an aircraft would be extremely "interesting". You'd actually have to hold the downwind wing down. A little hard to wrap your mind around that concept... Or if you try the "kick out the crab, keeping wings level" method of crosswind landing-- if you make the "kick" too early and keep holding the rudder deflection till you touch down, you start accelerating (accumulating "drift") not in the downwind direction, but in the upwind direction! There's a mind-bender for you-- $\endgroup$ Nov 5 '21 at 16:49
  • $\begingroup$ @quietflyer Nice. Indeed hard to wrap your head around. Does that mean that with a large enough wing sweep and a small enough fuselage you could forward slip wings level after all :) $\endgroup$
    – Sanchises
    Nov 5 '21 at 17:42
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Think it through in steps.

When, say, right rudder is applied just wings level, what has happened is that the camber of the fin/rudder has increased and its chord line offset to the left, imparting a yaw torque that yaws the body until a new equilibrium is reached.

The angle of the fuselage to the airflow, that its static stability force wants to seek, its weathervaning trim angle you could say, is offset to the right. The airplane yaws right and stabilizes at a new yaw trim angle of attack as long as the rudder is held. It's as if you bent the trailing edge of a barn weathervane and it now points into wind but with an offset.

At that new trim angle, the fuselage is operating at a lateral angle of attack, and the thrust line is also offset in the same direction. The plane starts a flat turn from the lateral fuselage lift and offset thrust line (if there is power on). So I'm in a wings level skidding turn in the direction of the rudder application.

When I lower the wing the opposite direction, I introduce side slip forces that oppose the lateral forces induced by the yaw's lateral lift. At the right bank angle, the magnitude of the side slip forces precisely cancel out the lateral yaw and thrust (if there is any power on) forces, the net result on the body's mass is neutral, no turning occurs, and you are in a forward slip.

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  • $\begingroup$ Give me a few months to process this :) $\endgroup$ Nov 5 '21 at 10:22
  • $\begingroup$ A highly imperfect analogy, but may help with the metal picture: you are driving on a giant table top of ice with studded tires where you drift or skid through turns with the ass end out. The car skids around in an arc to the right say. Then the driver moves a magic lever and somehow makes the tabletop tilt to the left. The car now wants to slide downhill to the left, as its limited traction is slewing it around to the right. If the driver gets the tilt angle just so, the left and right forces cancel each other out and the car goes straight, even though it's pointed to the right. $\endgroup$
    – John K
    Nov 5 '21 at 12:58
  • $\begingroup$ "A side slip is similar, but less rudder is applied relative to aileron, so that the side slip forces are stronger; there is a net surplus of side force in the direction of the slip,"-- I think this is incorrect. A net surplus of side force would cause a net acceleration-- also known as a turn. Complete with all the dynamics that go with turning-- heading change, etc. Not sliding-- surplus force causing sliding is an Aristotelian concept, not a Newtonian one. All forces must be balanced in both cases $\endgroup$ Nov 5 '21 at 16:55
  • $\begingroup$ (ctd), and the only difference between the two, is that "sideslip" is the term we prefer to use when the function of the slip is to exactly counteract the crosswind component and allow the aircraft heading to be parallel to the ground track so the wheels are aligned with the ground track for a nice landing, and "forward slip" is the term we prefer to use when there is a little or no crosswind and the nose is cocked to the side relative to the ground track (better not land that way!) and the main purpose of the slip is simply to increase drag (or to increase forward visibility.) $\endgroup$ Nov 5 '21 at 16:59
  • $\begingroup$ And what term to use when practicing 3000' in the air and we can't see the direction of the ground track anyway? It's completely arbitrary, take your pick, or just call it a "slip". $\endgroup$ Nov 5 '21 at 17:00
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First, we must realize the purpose of a forward slip is to increase drag, which allows the aircraft a greater angle of descent for a given airspeed.

This manuever serves the same purpose as dropping flaps to increase drag.

Importantly, ailerons are used to control lateral track, using the "slip" to counteract the tendency of the aircraft to fly in the direction it is pointed.

It is the horizontal component of lift, created by controlling bank angle with ailerons, that offsets forces in the opposite direction.

Because your aircraft has right rudder in, you must slip to the left. The wing lift vector in the diagram, therefor, needs to be pointing in the opposite direction.

So, the change in relative wind when right rudder is applied stops the right yaw and tries to push the entire plane to the right. The left force provided by the horizontal component of lift compensates for this.

Notice the wing is also pulling the plane in the y direction, but in a very draggy way. Thus, the aircraft is (deliberately) uncoordinated.

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