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?
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.
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.
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.
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.
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.
https://www.aopa.org/news-and-media/all-news/2018/july/fligh...
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.
Sounds like drifting. If I get your drift.
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.
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.
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.
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.
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.
If you have the chance (and didn't already do so), make a few flights in a glider with an FI.
I thinks it's a different kind of flying and allows you to be more confident in certain situations such as slips, slow flights and engine out landings.
Here are some suggestions. As one of the child comment stated, go fly a glider for a few hours. It will immensely help with flying "not straight". I started it for gaining better understanding of flight but ended up finishing a commercial ticket on glider as well, because I enjoyed it so much and also just for the bragging rights.
Secondly, cross control is perfectly safe till the time you pull back on the stick. That is how you get into cross control stalls, the starting configuration for a spin. When you are slipping, you should actively trim nose down, (that you should have done already, remember you are approaching to land), and maintain just a little forward pressure as you fly sideways. I would recommend, grab a competent CFI and get cross control stalls nailed. I know it is not a part of PPL, something I am very pissed about. Hell, PPL in US does not even need spin training. Bollocks, if you ask me....
Private Pilot ACS Section IV Task M requires both demonstration and knowledge of "Forward Slip to a Landing"
https://www.faa.gov/sites/faa.gov/files/training_testing/tes...
It's actually a really nice way to land, I wish powered aircraft had this.
Anything a small plane can do a big plane can (very few exceptions), so they can slip just fine, like they can do a 60° bank just fine - but it becomes VERY uncomfortable for the passengers - who now will feel they’re “outside the plane”.
Only if you try to maintain altitude. If you let it descend it can be a 1G maneuver and no one will be able to tell unless they look out the window (at which point they will probably freak).
Have you actually flown a forward-slip?
It might feel unnerving on the first attempt but once mastered, it becomes a normal procedure one can rely on as a pilot.
Some planes require it for landing, e.g. aerobatic airplanes with a high angle of attack - otherwise the pilot would not be able to see the runway.
Yes, I'm a Private Pilot. I've done them dozens of time. Still feels weird to be flying sideways and cross controlled.