[vkou]

For the non-pilots here, roughly what is a forward-slip? I understand a little bit about flying, but not enough to grok the explanations/diagrams.

Is it a high-altitude, low-power approach to the runway, where full flaps are used to bleed off more altitude and speed? How is that different from a regular approach?

[pageandrew]

The ideal approach to landing is what's called a "stabilized approach", in which the aircraft descends to the runway at a constant 3 degree angle and constant descent rate. If the pilot finds themselves with too much altitude too close to the runway, in other words above that glide path, they need to more aggressively descend to meet the glidepath.

One way to aggressively descend would involve just pointing the nose down, but this has the effect of increasing speed (trading altitude for airspeed). You need a way to descend more quickly without increasing speed. Generally, this is done by adding more flaps (increasing drag), but in the case of the Gimli Glider, their hydraulic systems were down, so they could not add more flaps.

A forward slip is a maneuver in which you roll the aircraft such that the top of the wing and side of the fuselage are exposed to the oncoming air (relative wind), and you use the rudder in the opposite direction to keep the aircraft flying straight (with respect to the ground track). When properly executed, the rudder and aileron cancel each other out so you keep flying straight across the ground, but the nose of your airplane is not aligned with the direction of travel, and the top and side of your airplane is exposed to the oncoming wind, significantly increasing drag, and thus descent rate.

[reaperducer]

A forward slip is a maneuver in which you roll the aircraft such that the top of the wing and side of the fuselage are exposed to the oncoming air (relative wind), and you use the rudder in the opposite direction to keep the aircraft flying straight (with respect to the ground track). When properly executed, the rudder and aileron cancel each other out so you keep flying straight across the ground, but the nose of your airplane is not aligned with the direction of travel, and the top and side of your airplane is exposed to the oncoming wind, significantly increasing drag, and thus descent rate.

Sounds like drifting. If I get your drift.

[HeyLaughingBoy]

Interesting comparison. It is like drifting, but in a straight line (and in three dimensions).

[primis]

https://www.youtube.com/watch?v=AZ5ou3ObXAE yup!

[ilyt]

So basically like drifting the car but in 3D

[jvm___]

You Tokyo Drift the plane, exposing more of the side of the plane to the forward motion of the plane to increase the drag by using the side of the plane as a air-brake.

Instead of the most areodynamic way of flying straight, and the worst aerodynamic way of flying completely sideways, you power-slide the plane so that it's not dead-straight and the extra wind-resistance slows you down.

One of the pilots had sail-plane experience from riding a one-man ice-boat with a sail attached over an icy lake - which is where he learned the maneuver. You can slow your speed by the same trick, it's just not usually done with an airliner.

[pageandrew]

> One of the pilots had sail-plane experience from riding a one-man ice-boat with a sail attached over an icy lake - which is where he learned the maneuver

This is correct, but I want to note that every Private Pilot (in the US at least) learns this maneuver, and it is required to be demonstrated to an examiner on the Private Pilot practical checkride.

[CPLX]

While true, your comment is focusing on what I think is the second most important aerodynamic force in a slip, which is the wind resistance if the side of the plane.

By far the most interesting thing going on is that you’ve destroyed lift. The ailerons are acting as spoilers essentially so the plane starts to want to drop out of the sky.

I’m a pilot, and the sensation of a slip is definitely not that you’re rapidly losing forward groundspeed, it’s that you’re suddenly not gliding, and instead kind of dropping out of the sky.

[bombcar]

It feels different in different planes - I always felt the Cessnas were glued to the sky no matter how much I slipped, but the pipers would drop like a rock if I needed them to.

[subhro]

> Instead of the most areodynamic way of flying straight, and the worst aerodynamic way of flying completely sideways, you power-slide the plane so that it's not dead-straight and the extra wind-resistance slows you down.

That is actually not completely true. You are mainly increasing drag to lose altitude quickly WITHOUT gaining additional airspeed. Remember, backside of the power curve, pitch for speed, power for altitude. You could push the nose down to give up altitude in a power out, flaps out configuration, but you might overspeed your flaps by gaining airspeed. That is why you have the Tokyo drift.

[bonestamp2]

I used to fly a couple times/week for work and one I time I could see our runway out of my passenger window as we were landing, and we were heading directly for it. I just thought it was very windy, but I'm now learning that maybe Vin Diesel was at the helm that day. Very good explanation, thank you.

[bombcar]

That can also happen in a crosswind - the plane is flying in a liquid.

Just like to take a boat directly across a fast river you have to aim it “upstream” - same thing can happen with a plane on landing.

It’s hard to explain but one feels different from the other.

[bonestamp2]

I knew it could happen with wind, so that's what I assumed it was. I didn't realize the pilot could put the plane in that kind of slide. Very cool!

[quest88]

Spot on, but nit: Helps you lose altitude quickly, not slows you down.

[FartyMcFarter]

Whether you lose altitude or speed quicker depends on the plane's attitude. Both/either could be the case.

[quest88]

Uh..yes. But that's not the goal of a slip. You're describing the first flight lesson for every pilot. And it's true, but not relevant for the discussion of a slip. The goal of a slip is to quickly lose altitude without changing airspeed. If you need to quickly slow down, chop and drop and pitch up.

[upofadown]

It is where you fly the aircraft sideways through the air.

It is a way of dealing with the situation where the wind is not coming straight down the runway while landing a light power aircraft. You point the aircraft straight down the runway and add in enough bank to overcome the drift to the side. You use the rudders to prevent the aircraft from turning due to the bank.

Pearson was performing the forward-slip in the less common context of gliding. While flying gliders the problem is that they produce a glide angle that is much too shallow to ever be able to control exactly where you are going to land. Normally you use spoilers (AKA dive brakes) to control glide angle but the spoilers might fail or you might need a steeper approach angle than the spoilers can provide. So you fly sideways to force the side of the glider through the air and produce more drag. You still need to take into account any crosswind while simultaneously adjusting the glide angle while staying lined up with the runway. It's a bit tricky and it takes some practice to maintain the skill. That's why I get to demonstrate a forward-slip as part of my yearly glider check flights.

Pearson did this in an airliner with constantly changing control forces and completely nailed it on his first try. He touched down something like 100 feet past the threshold which is exactly what you want to do in a case where you don't have a lot of braking available. This was one of those times where you wanted the guy that lives aviation and not the nine to fiver.

[jacobsenscott]

There's an animation of the landing here https://admiralcloudberg.medium.com/a-mathematical-miracle-t...

[cf100clunk]

My only quibble with that animation was that it didn't show the state of the control surfaces, such as the rudder not appearing to have been moved when it was clearly at or near max during the real incident.

[caconym_]

I think it's fair to describe a forward slip as a maneuver that puts the airplane sideways to use the drag of the fuselage, which is now partially side-on to the airstream, to bleed off speed (energy, really). It's used in situations where you need to bleed altitude quickly without gaining airspeed, which would typically be the outcome of just pointing the nose down—you trade gravitational potential energy for kinetic energy.

[supergeek]

I'm also not a pilot but, based on a diagram below, it looks like you roll the plane and apply full rudder in the opposite direction. The opposing roll and rudder keep the plane tracking straight but the control surfaces fighting against each other add a bunch of drag to lose airspeed.

https://www.aopa.org/news-and-media/all-news/2018/july/fligh...

[rob74]

...plus the plane flying "sideways" also adds a lot of drag because it's much less aerodynamic than if the fuselage would be pointing forward.

[FartyMcFarter]

Essentially you fly slightly sideways towards the target landing spot on the ground, which loses speed and/or altitude quicker than you otherwise would.

This is done by applying rudder and ailerons in opposite directions (e.g. full right rudder and enough left aileron to still be heading towards the intended spot).

Typically this would be a bad thing as it feels weird and is inefficient, but in this case the whole point is to be inefficient in order to lose altitude without gaining too much speed.

[1659447091]

For those like myself who do better with visuals. Just under 2min direct explanation using a model plane

https://www.youtube.com/watch?v=BBqOyDviE-A

[johschmitz]

I like this one much better https://youtu.be/RKfG3lWCZ80 at 4:20 onwards. Also explains the difference between slip and skid.

[jholman]

Also not a pilot, but I did a little googling.

Here are a few key selections from https://pilotinstitute.com/forward-slip-vs-sideslip/

> Both forward and sideslips involve opposite aileron and rudder input. In a forward slip (typically used to increase drag and decrease altitude), the aircraft’s nose points away from the direction of flight, with the lowered wing facing the direction of flight. In a sideslip (typically associated with the final stage of a crosswind landing), the aircraft’s nose points in the direction of flight, regardless of which wing is lowered.

and

> To understand the difference between a forward and a sideslip, we need to understand the slip in general. A slip, at its core, is an uncoordinated turn. In other words, when the ball of the turn coordinator is not in the middle, you’re in a slip.

So in general, a slip is a situation where you're banking, but not turning, due to opposite inputs to ailerons and rudder (i.e. between stick and pedals).

As I read through all that, I thought I was coming to a decent understanding. However, finally,

> The jargon is unnecessarily confusing – you move forward during a forward and a sideslip, but you don’t move sidewards during a sideslip; you technically move sidewards during a forward slip!?

Hmn.

[cmurf]

It’s use case specific. And depends on perspective.

Forward slip is mainly used to lose altitude without increasing airspeed, such as emergency descent or when too high on approach for landing.

Side slip is mainly used to maintain lateral alignment with runway centerline during a crosswind landing.

The side slip is predicated on a crosswind, properly done the two forces equal. In effect like “leaning a shoulder into the cross wind” to stay laterally and longitudinally aligned with the runway.

Does a turn require a curve in the airplane’s direction? What exactly constitutes a turn? If a plane is in a bank, there is a horizontal component of lift. But if there’s no change in direction (either heading or ground track) is it a turn?

While there is such a thing as a slipping turn, it means an (insufficient) change in direction for the bank angle. If the rate of turn is zero, it’s just a slip.

Conversely I don’t see how you get a skid without a turn.

[barbazoo]

https://en.wikipedia.org/wiki/Slip_(aerodynamics)#Forward-sl...

https://youtu.be/lnXrXp_7tyE?t=103

[russdill]

Just imagine what snowboarder does to slow down. Turning the snowboard a bit sideways but leaning in such a way to keep going "straight".

[rzzzt]

Flying door handle first, with Déja Vu roaring in the background. (I might be exaggerating a little. Also not a pilot.) It looks like you have to use the rudder and bank slightly so that the plane is still flying "forward" but the nose points away from that direction.

[cf100clunk]

An extremely simplistic way to think of a forward slip is for the pilot to have the rudder pedals hard in one direction, stick/wheel hard in the opposite direction.

[tiahura]

Think drifting in a stalled Greyhound bus down a mountain (and the power steering pump is out).

[mhandley]

Generally to turn left you bank left using ailerons. This angles the lift vector to the left and the airplane will turn that way without using the rudder, but it's not fully coordinated - the plane won't be pointing properly into the turn - it will "skid" a little, creating more drag than necesssary. To coordinate the turn, a small amount of left rudder is used to yaw the plane left just enough.

In a forward slip the goal is to descend more quickly than you otherwise could. If you just point the nose down, the airplane will gain airspeed. If you're trying to descend quickly you'll overshoot where you're aiming. What you want is a lot more drag, which allows you to point the nose down (reducing lift) without gaining airspeed. Normally flaps, spoilers or airbrakes allow you to do this (depending on the aircraft). If you don't have these, as the Gimli glider didn't, what can you do to increase drag? Well, you can use the whole fuselage as an airbrake.

To do this, you can use the rudder to yaw the plane, so it's flying a little bit sideways. Say we yaw it to the left. If the wings have dihedral (they point slightly upwards) then the right wing now has a greater angle of attack than the left wing - the right wing produces more lift and so the plane will bank left by itself without touching the ailerons. If the wings are swept back, as on large jets, yawing left will mean the right wing is longer with respect to the airflow than the left wing, and it will bank left even more. To fly pointing left, but not turn left, you cross the controls: you use the rudder to yaw left, and the ailerons to bank right. The airplane is now flying partly sideways, pointing to the left of the flight path, with the right wing lower than the left wing, but if you balance rudder against ailerons correctly, you fly in a straight line. The fuselage is partly sideways to the wind, creating a lot of drag. The airplane is skidding downwards to its right, and you need to pitch the nose down to maintain airspeed and avoid stalling.

And then you've got to be fairly well coordinated as you come out of the slip so you keep flying in the direction of the runway.

Source: I used to fly gliders. Presumably swept wings increase the amount of aileron needed, but the principle is mostly the same.