Forward Slip with Crosswind

P

positiveR8

Well-Known Member
I've heard the argument that in a crosswind condition, a forward slip should be made into the wind (meaning the aircraft should be banking into the wind) because it is more effective. I can't picture why it would be more effective though, and I can't really find anything written about it.

In a normal coordinated approach to a runway in a crosswind, the aircraft is crabbing and effectively flying parallel to the relative wind. Wouldn't this mean that rudder deflection to the left or right would move the longitudinal axis the same amount left or right of the relative wind? I don't see how slipping either left or right would make that much of a difference.

Am I missing something?

*Edit* Just to clarify... I'm talking about a forward slip where you're trying to lose altitude. Not a side slip where you're trying to land the airplane with the nose pointed straight down the runway.
 
positiveR8 said:
*Edit* Just to clarify... I'm talking about a forward slip where you're trying to lose altitude. Not a side slip where you're trying to land the airplane with the nose pointed straight down the runway.
Click to expand...

They are actually aerodynamically identical. It has the same difference as a 30 degree turn to final compared to a 30 degree turn to depart the traffic patter. A turn is a turn a slip is a slip, as far as aerodynamics are concerned.

Slipping into the wind or away doesn't much matter, we will get to that in a minute. When you enter a sideslip the relative wind hits the aircraft at an angle. Here: http://www.pilotscafe.com/articles/...noperative-principles-in-a-twin-airplane.html This applies to twins, but you can move it to view the "side slip" to see how the wind passes over the aircraft.

Remember up top I said a slip is a slip? Well when you slip to maintain runway heading you do so into the wind, because of what was discussed with the RW. You can slip into the wind, which is the same as crabbing into it and fly straight w/o crabbing. Conversely, you can crab into the wind and slip away from it. Either one would give the same result, but number two might make other pilots question what drugs you are on.
 
shdw said:
Remember up top I said a slip is a slip? Well when you slip to maintain runway heading you do so into the wind, because of what was discussed with the RW. You can slip into the wind, which is the same as crabbing into it and fly straight w/o crabbing. Conversely, you can crab into the wind and slip away from it. Either one would give the same result, but number two might make other pilots question what drugs you are on.
Click to expand...

Like you said, it doesn't matter aerodynamically which way you do it. At Riddle there are some examiners who have failed kids on checkrides for "performing a forward slip in the incorrect direction."
 
positiveR8 said:
I've heard the argument that in a crosswind condition, a forward slip should be made into the wind (meaning the aircraft should be banking into the wind) because it is more effective.
Click to expand...

Kershner is the only one I've seen raise this issue. The idea is you can maintain a more pronounced slip by slipping into the wind, because the wind will help you on the runway final approach course. Too pronounced of a slip otherwise might cause you to drift.

However, this isn't quite true. For any given slip angle, even in calm wind, there is heading that a max performance slip will remain centered on the runway. It's just that the heading will be so far from that of the runway heading that it's a bit of a challenge to estimate. For instance, in a strong left crosswind, if you want to perform a forward slip to the right, you'd need to be pointed rather opposite the runway heading.

One additional reason to slip into the wind, though, is that it's easier to transition to the wing low crosswind landing method.
 
tgrayson said:
One additional reason to slip into the wind, though, is that it's easier to transition to the wing low crosswind landing method.
Click to expand...

I like that reason. Going from a right wing low to a left wing low would be rather dramatic and unstablizing on short final. Makes since to keep the low wing into the wind the whole time and only changing the degree of bank as the nose straightens out when transitioning from forward to side slip.
 
A side slip is nearly as effective as a forward slip in creating drag and reducing lift. In both cases you are forcing the relative wind to the side of the longitudinal axis, which of course produces parasite drag. You also lose some of your vertical component of lift because you are in a bank. It is for these reasons that you usually need to add power if you are in a side slip the entire way down final.

If you had the correct control inputs in to keep yourself lined up on final and you still weren't losing enough altitude or airspeed (the crosswind isn't very strong) you can always just add a bit more rudder and aileron and deepen the slip. Like tgrayson pointed out, if you wanted to enter a forward slip from a crab you would need to face the airplane's heading quite a bit away from the runway, which wouldn't make much sense.

A final consideration in forward slips in small SE a/c is the direction the relative wind is hitting the fuselage. It is always better to have the relative wind hitting the side of the airplane that the static port is on (usually left) so that you are getting ram air into the static port and your indicated airspeed is lower than true. If it is off the oposite side a slight vacuum will form over the static port and IAS will be greater than TAS which is dangerous in a slip.
 
chuckles1225 said:
A side slip is nearly as effective as a forward slip in creating drag and reducing lift....
Click to expand...

Note that a slip is a slip is a slip, there aren’t two different kinds aerodynamically, which means that a side slip is exactly, not nearly, as effective as a forward slip in creating drag.

Also, a loss of lift is not really a characteristic of any stabilized slip, since vertical forces are in equilibrium. While it’s true that the main wing won’t be supporting the entire weight of the airplane, this is true of any descent, because some of the weight is supported by a component of drag. The only unique characteristic of a slip in this regard is that the yawed fuselage also creates a side force which has a component that opposes weight. In knife edge flight, almost the entire weight of the airplane is supported by this fuselage side force, and a slip is just a less extreme version of this. Overall, I think it a large conceptual problem to characterize an equilibrium condition as the result of a force imbalance, so the altitude losing capability of a slip is best represented as purely a drag issue.
 
There's no right or wrong way to do it, but if you're slipping opposite the wind you'll have to make quite a change in rudder pressure to straighten the nose before touchdown. So the "into the wind method" tends to make things just a touch easier when rounding out/flaring.
 
chuckles1225 said:
IAS will be greater than TAS which is dangerous in a slip.
Click to expand...

Just a note, a slip is a stable maneuver, that is why we use it with little concern so low to the ground. Worrying about a couple knots discrepancy one side to another isn't necessary. Keep your speed up, as you should in any slip situation, and you won't have an issue.

An experiment to try, just to demonstrate a slips stability. Take the aircraft up to altitude, put it in a slip, and try to make it stall. Get it to a full slip, full rudder one direction and full aileron the other. Then yank back on that elevator and just hold. In a trainer you will find the aircraft will buffet like hell and just descend in a stalled slip.

Conversely, try this with a skid and watch how horribly violent the stall is. It is a great demo as to why we slip and why too much rudder in a turn is infinitely worse than no rudder.



tgrayson said:
is that it's easier to transition to the wing low crosswind landing method.
Click to expand...

I am just curious about the hype to wing low landings in tricycle geared aircraft. I personally have never once landed in a wing low position in any tricycle aircraft. It is essential in a tail dragger and I have used it with them.

So far though, up to high 30s sustained crosswinds I have never used this method. I crab it in, flare while crabbed, and just before the wheels touch kick the rudder to align. Just my opinion though, what does everyone else do/think?
 
If you wait to enter the slip until you're in the flare and fly crabbed the rest of the way down, and then only concentrate on putting the upwind tire down first, the crosswinds kind of solve themselves. Then you don't accidentally slip the wrong way and have to put in some kind of giant control correction.
 
shdw said:
I am just curious about the hype to wing low landings in tricycle geared aircraft. I personally have never once landed in a wing low position in any tricycle aircraft. It is essential in a tail dragger and I have used it with them.

So far though, up to high 30s sustained crosswinds I have never used this method. I crab it in, flare while crabbed, and just before the wheels touch kick the rudder to align. Just my opinion though, what does everyone else do/think?
Click to expand...
I'm assuming that you put in your crosswind taxi inputs as you slow down after landing, so the ultimate inputs are tyhe same (unless the wind was a quartering tailwind).

I transition out of the crab earlier than you so there is more of a chance of my ending up wing-low for landing. But the fact of the matter is that I never intentionally land wing-low. As I transition out of the crab, I do what's needed to keep aligned and counteract drift. Depending on the wind conditions and the airplane, that may result in a wing-low landing or it may not.

I agree with you that wing-low is somewhat hyped. It seems to be taught as if the goal was to land wing-low rather than to land longitudinally aligned with no drift.
 
shdw said:
Worrying about a couple knots discrepancy one side to another isn't necessary.

--

So far though, up to high 30s sustained crosswinds I have never used this method. I crab it in, flare while crabbed, and just before the wheels touch kick the rudder to align. Just my opinion though, what does everyone else do/think?
Click to expand...


I would be curious to know just how much airspeed error is present in a deep slip. I personally feel like it would be greater than just a few knots...but I can't say for sure.

As far as airliner style landing in a GA a/c I would be cautious. As soon as you line the longitudinal axis up with the runway centerline you will start drifting with the crosswind. This means that you need to time the rudder input just perfectly. Too early = side load, too late = side load. I have personally never landed this way and would definitely not consider teaching students this way. Too risky in my opinion.
 
chuckles1225 said:
I would be curious to know just how much airspeed error is present in a deep slip. I personally feel like it would be greater than just a few knots...but I can't say for sure.

As far as airliner style landing in a GA a/c I would be cautious. As soon as you line the longitudinal axis up with the runway centerline you will start drifting with the crosswind. This means that you need to time the rudder input just perfectly. Too early = side load, too late = side load. I have personally never landed this way and would definitely not consider teaching students this way. Too risky in my opinion.
Click to expand...


Nah, its easy, just takes practice, or if you like, you can transition to the crab about 100' AGL, then fly that the rest of the way down. As for the side loads, you can tell if you start to drift, so correct the drift with more crab, its a dynamic maneuver, its not simply timing, you don't have to time it perfectly, if it was that tight of tolerances a scuzz like me could never figure it out.
 
chuckles1225 said:
Too early = side load, too late = side load.
Click to expand...

This is precisely why tail draggers require this procedure, and I suspect it is where the procedure originated. In a tail dragger, CG location is behind the mains. The result is a directionally unstable aircraft, on the ground. In a tricycle gear it is in front of the mains, the result is a directionally stable aircraft on the ground. To sum that up:

Side load tail dragger = ground loop
Side load tricycle gear = nothing but a little stress on the wheels, likely not nearly as bad as a student flaring and falling in from 10 feet.

I land and teach to land full stall, every time in every situation unless I am demoing a wheel landing. So the side loads produced are negligible.


Side note:

The first time I heard this was from a WWII pilot at my local RC flying field, "A landing is a controlled stall so learn to fly any aircraft in stalled flight and landings will be easy." Perhaps the best advice I have received in my aviation career, second only to, "any landing you can walk away from is a good landing."



ppragmam said:
if it was that tight of tolerances a scuzz like me could never figure it out.
Click to expand...

:rotfl: Made my day with that one man.
 
Side load is not good for an RG aircraft and extreme side load is not good for any aircraft, but you already knew this.
 
shdw said:
Side load tail dragger = ground loop
Side load tricycle gear = nothing but a little stress on the wheels,
Click to expand...
...really? ..a little stress on the wheels, eh? Can you feel the side load at touchdown? If I can feel any side load, I'm stressed. Guess it shocks me back into the old Army Bird-Dog days..(eyes glaze and start rapid tics)
 
nosehair said:
...really?
Click to expand...

Well I should have added, depending on speed. I land and teach to land at full stall. Impact forces increase with the square of velocity, thus landing side loaded, at stall, has considerably less load than many folks landing 20 knots above stall. Finally, the real goal there was comparing the what if's between conventional and tricycle, to show that one = bent airplane and one = some stress.
 
You must log in or register to reply here.
Back
Top