side slip/forward slip
 

I still don't get it, about the definitions. Side slip is also known as the "down-wing method" while the forward slip is known as the "crab" method. On a crab you are "sideways" so shouldn't the crab be named as a side slip, because your longitudinal axis is parallel in the down-wing method, so name it as "forward slip."? :confused:

sideslip uses opposite aileron and rudder while forward is the difference of the wing correction angle as compared to track, no inputs just correcting for wind with a heading change to course.:D

Both are aerodynamically the same thing, which is a slip. The difference is in what they are used for and the order in which you make the control inputs...

In a forward slip, your puropse is to hang as much fuselage in the breeze as possible to create drag and bleed energy. You start by apply full rudder and then add enough opposite aileron to keep your desired track.

With a sideslip, your purpose is to fly the airplane on (and aligned with) centerline in a crosswind. You first apply enough aileron to offset the crosswind, then you apply the required rudder to keep the fuselage aligned with centerline.

Remeber that you will touch down one wheel at a time when landing using the sideslip in a crosswind landing. Try doing soft field landings in a crosswind to master the slideslip.

Better yet, only do 1-wheel touch and goes(assuming you fly a cessna). I've done it a few times in a piper, but definately much harder to do with the wider and longer wheelbase. Usually a good 10-15kt steady wind works well

Call me crazy, but I just think that doing one wheel touch and goes puts a lot of bad pressure on that one landing gear, especially if it's just the slightest bit gusty.

I could just never really understand all of the logic in not side loading the gear (when every take-off in a single does), yet landing on one gear. I've seen many students and ever other pilots drop it in on one gear trying to keep it corrected for x-wind (maybe a fixed gear might take it - but what about when you transition to a retract?)

Crab vs Wing-Low
 

Touching-down on a single-gear (and then lowering the other wheel) is not an undue-stress on the gear, whether it is fixed, retract, a 172, or an airliner. (Trying to do a single-wheel touch-and-go is probably beyond a beginner's capability)

Depends on the airframe, but in some airplanes (A-320, as I recall) you did wing-low during a max-crosswind landing. 747, more crab---the problem there was not dragging an outboard engine if the wing was too "down."

The issue is the direction of the touchdown loads. Gear, especially most retracts with oleo stuts, can take significant vertical impacts. And they can obviously take big aft-loads....that's what the brakes do. But sideways loads put the strain on the mechanism that makes it retract or stay in the locked position--generally, the weakest part of the gear. In fighters, the issue is often the tires...side-loads can peel the tire off the rim when heavy-weight.

As to your comment that sideloads exist on takeoff: not really, unless you are weaving. If you are tracking straight down the runway, don't confuse aerodynamic sideload on the fuselage (which is relatively small) with inertia sideload that you get at the moment of touchdown, if crabbing. That load can be huge.

Personally, I like wing-low and minimal side-loads where the airplane Manual allows it.

One thing (among others :D ) to clarify: A "crab" is not a slip and has nothing to do with a slip. A crab is =coordinated= straight and level flight.

Don't worry about the names of the two slips. I've heard people try to explain why they are called what they are and you can go nuts trying to figure it out.

rickair pretty much nailed the explanation: a slip is a slip - aerodynamically, they are all the same. What the slip is called is related to what the slip is being used for, with a "forward" slip used to lose altitude and a "side" slip used for landing in a crosswind.

You mean that you think sliding sideways across the runway on two wheels puts less pressure on the gear than moving straight ahead on one? :confused:

Excellent Point
 

Excellent point above, and agreed with the rest that followed. Don't worry about the semantics of what to call it. "A rose by any other name..."

I certainly have a lot to learn about even the basics... this observation just comes from seeing bent gear bolts on training aircraft - and wondering what caused it
I don't mean to pick the fly poop out of the pepper, but if the one wing is down that may not be vertical pressure on the gear only... perhaps that vertical pressure is directed slightly lateral the gear (at the retract's weakest point).

I agree with you with setting it down on one gear gently letting the other wheel down while using the rudder to maintain direction control... however landing on one gear and holding it there (or dropping it on the gear), while the weight of the aircraft rests on that one gear during deceleration and acceleration - seems that can't be good.

The side loading on the gear at take-off would be the torque, you're right not equal to the interia load at landing... no matter how much rudder you use at first the force is still acting on the wheel until the rudder becomes effective but with a single gear touch and go that force is large compared to the gear- at least my understanding

yes I know... call me crazy... but yes.... notice my words... touch and goes and slightly gusty.....
I have no problem and encourage drift control with BOTH wheels on the ground.... however you will only be reactive to drift with one wheel on the ground during a TOUCH and GOES.

I didn't say slide sideways on two gear, if you hold the aircraft on one gear the whole time it is not a direct vertical load on the gear... the weight of the aircraft puts a side load on the gear.

Not Torque
 

Ryan:

If you are applying the right amount of rudder for P-factor, there is no side-load from torque. You have a left-yawing moment from the prop, and a right-yawing moment from the rudder. I'll grant you from a statics & dynamics load/moment-diagram perspective that there is a slight side-load from the rudder, but it would be on the order of 10-20 lbs.

The aerodynamic side-load I am talking about would be taking-off in a stiff crosswind, and the side of your fuselage becomes a lifting-surface...."lift," in this case, being directed horizontally. The side of your fuselage is probably not a very good "wing," so the force generated there is not very significant--maybe 50-100 lbs (the same as trying to carry a sheet of plywood in the wind).

You're right; in a bank, some of the vertical component is directed laterally, but generally the bank agles and impact forces produce a smaller horizontal force than landing in a crab with a larger crab angle.

I may be misunderstanding you, but your posts seem to suggest that you are viewing the goal of a side slip landing to be touching down on one wheel.

It's not. The goal is definitely =not= to land on one gear and "hold it there."

The goal is to land the airplane longitudinally aligned with the runway, with no sideways drift (and, upon touchdown, to transition the control inputs to those required for a crosswind taxi).

From the point at which you decide to move into the slip, the control inputs you use will be those needed to maintain that alignment without drift, and may change all the way down to touchdown. Whether you land on one wheel (and to some degree how long you stay on one wheel during the rollout) is determined by wind direction and strength at the time those events occur. Not by the choice of a pilot who, oblivious to conditions, decides to land on one wheel.

Read the post I originally replied to. I agree (for the most part) with you here... just throwing this all out there for debate.

also I'm sure ewfflyer is a good, experienced pilot so I mean no disrespect to his post concerning touch and goes, just a respectful disagreement

Ok, now to move the discussion on a little bit. I find that students are easily taught to do forward slips. They like how it salvages a high approach. But they have great difficulty learning the hows and whys of sideslips. There is a great deal of difference between the two in this regard. A few will see the point of a sideslip at least in theory and a few will apply aileron into the wind when countering a crosswind, but none has any real feel for how much to input to apply or how to use the rudder artfully.

Lately I have been experimenting with a teaching technique just for this maneuver. It consists of an entire lesson flying low passes with a hard direct crosswind to an opposing runway. First I have them do about 6 passes skimming just above the runway while trying to hold the centerline. Then they reverse direction and do it the other way and try to hold the centerline going that way. It seems to work- they finally see that the only way to hold the centerline is either to crab which I will not let them do, or to apply a healthy sideslip in the exact proportion to the crosswinds.

I also think that sims can be used to teach the technique. The low pass idea started on my sim at home. I would crank the winds up until the airplane was no longer able to track the centerline. It dawned on me students could be shown the same thing in a real airplane on a windy day, but without landing. The whole point is you show them a maximum case scenario and how the only way it can be negotiated is with the proper technique. That's where most syllabuses fail to teach this- sideslip it is almost never taught under conditions where you must do it (and well) or you do not meet the criterion.

I was taught from the very beginning to depart the runway in a side slip when taking off with a strong cross wind and transition to a crab 50-100 agl. Now I transition much sooner but I never forgot what side slips were.

Ryan,

I am also confused as to why you would think there would be more stress on a wheel if you are only touching one down. If you are skilled enough to do this, the airplane is still flying...so you have less weight on the wheel than you normally would AND in order to do this, you must keep the aircraft straight, so the wheel is doing what it was invented to do... roll straight. in this situation, there is no sideloading of the gear because pilot's inputs cause the plane to go straight.

With that said, i disagree as well that this is a useful exercise. People make this WAY too complicated on their students in my opinion. Like i said in pearls other thread, a good approach will almost always lead to a good landing if you explain to a student that ailerons keep you over the centerline and rudder lines you up with the centerline. I think it does a student a great disservice to give them rote knowledge when they are trying to land. "ok, so if you have a right crosswind you are gonna use right aileron and left rudder, put the right wheel down first" Thats all BS...like a student needs to be thinking about all this when trying to flair. how about this is what rudder does (move the nose back and forth) and this is what ailerons do (keep you over the runway)

K.I.S.S.

ASA manual
 

Hi I am an old timer that wants to renew its CFI and found some confusing issues on the 2004 ASA manual:
Pivotal Altitude?? Are you not suppose to keep the same altitude on the 8 on pylons

I have found that a crab is easy on the long approach, but as you get closer to the ground side slip (the more drag the more stable speed) and touch down on one wheel will minimize side loads and also absorb sink rate better than 2 wheels.

No. Read the explanation and the formula carefully - pivotal altitude is groundspeed dependent and when doing a ground reference maneuver, groundspeed changes because...?

Better than the ASA manual, try the FAA's:

Airplane Flying Handbook

Thanks for the reply,
I have read the explanation on the manual, but to mention ALTITUDE instead of “reference point” or other word that would make less confusing to some one that has never used this expression before?
The same as REGION OF REVERSE COMMAND, maybe to use the expression behind the power curve would be more understandable?

That method (slip) is simple and easy to understand, I think it is important for students to understand the limits of it as well. I'm no test pilot, but I would bet that it is hard to get a 15kt lateral speed using the slip method under no wind conditions (as should simulate 15kt x-wind). After some tailwheel time, I'm more of an advocate of a crab up until the final moment, where the wind has little time to accelerate the aircraft...and you can basically roll the aircraft (if you have the wing down a little) over to the centerline.... just think the crab offers more options in various conditions and may offer a little more control during gusty conditions.....

The idea about the one gear being more stressed comes in a book from a guy named Leighton Collins (Richard's father) who was good friends with Langewiesche. He mentioned how landing gear is stressed during initial testing and generally loads are for both gear at the same time.

You're an instructor and you're the one who's there to explain it.
Or "stall"? Or "maneuverability", "stability" and "controlability" which all have meanings in aviation that are different than what the average guy on the street might think?

I don't think there's a profession, vocation or avocation that doesn't have it's "terms of art" - words and phrases that have special meanings. "Mint" to a collector doesn't refer to taste. Aviation is no different.

Just the way it is.

Myself I teach the crab on final and transitioning to the sideslip when approaching the runway (just before flaring).
Never tried a one-wheel touch and go myself, though it sounds interesting.
In regards to touchdown, I don't understand (like many others) why touching down with one wheel first would overstress anything at all.
Remember, if your airplane is "airborn" enough to touch down with just one wheel, and especially stay at one wheel, then it is still creating tons of lift, and there is basically no weight on the wheels anyway??

If the airplane has lost enough lift to touch down (one wheel) it has generally passed its critical AoA. As such there is not enough energy in the boundary layer to overcome reverse flow. There isn't much lift being generated at this point, especially since the aircraft has passed through the ground effect. Unless the airplane is flown into the ground and forward energy is placed into the ground (which usually causes a bounce), it must have weight on the wheels. On the other side... you would need to have enough dynamic pressure for the control surfaces to be able to bank the aircraft - this also puts a heavier load on the aircraft since less "lift" is being used vertically to overcome weight - but none the less produces some lift.

All in all... I can't imagine this being good for the tire itself.

touching down one wheel or two does not necessitate the aircraft wing passing the AoA at all. Ground effect would only stand to HELP. Ground effect equals more lift, not less.

The airplane should be flown onto the ground, which done right does not cause a bounce.

I know that the FAA is incorrect about a lot of things, but one wheel touching down before another has been taught for a long time. In a stiff crosswind there really isnt any way around it without drifting.

I don't even think it's an FAA issue. As you say, I think we can agree that landing with the airplane pointed in a diferent direction that it is moving across the ground is worse for the tires than landing on one wheel that is rolling straight ahead with no side loads.

If you are maintaining longitudinal alignment with no drift, it's not a matter of "making" one wheel touch down before the other. If there is a crosswind and you are maintaining longitudinal alignment with no drift, one wheel =will= touch down before the other.

Yep, that's what I said.

You're really overthinking this, with alot of technical explanations that are not really true.

If you were to touch down on one wheel, past the critical AOA, the airplane would most likely slam into the ground. However, when you flare in a crosswind and place one wheel on the ground first, I can assure you that you are still ways to go from the critical AOA and that you are still developing a significant amount of lift. Therefore your theory about this being hard on the tire and/or landing gear is imo false

Photon, I agree 100%. To ad further data to suggest that a forward slip landing and landing on one gear is ok, I have a TriJet Newsletter from McDonnell Douglas which discusses crosswind landigs. If has numerous diagrams and force polygons (for the physics geeks). Ultimately, they recommend the forward slip landing method for crosswind landings.

What I was trying to say, only worded much better.

thanks everyone for a great discussion! I learned a lot more than what I expected from a question that had to do with a technical term. I understand that mechanical inputs such as memorizing aileron into the wind, opposite rudder to keep nose aligned would never enable student pilots like me to grasp basic stick and rudder skills. Again, a great discussion.

So basically let me get this straight.... (since my explanations are not true):

You say that your are still a ways to go from stall speed (critical AoA) when landing on one wheel. Please explain "significant lift" with one wheel on the ground... what are we talking here 50kias? 45kias? (in a 172 let's say) or a touch and go (at whatever would be slowest speed to stay in contact with the ground, considering the ground effect). Basically you are saying you touch down a ways from your stall speed?

I may have rode the short bus to work here... but just curious why would you want to be all cross-controlled during a gusty day? That doesn't seem like positive aircraft control.


What I usually do is a little of both but mostly crab during approach and just a really slight bit of opposite rudder and roll back to centerline ... works pretty good.

The DC-10/trijet series was designed to be landed like that. Other companies tried different ways (i.e. Boeing) and preferred the crab til the end... some of which was because of where everything was located (wing tips, engines, etc)... I think the 73 has a max of 7 degrees wing down during landing (word on the street) before brushing either engine. This whole conversation we are having (along with some other factors) was what caused Boeing to look into castoring mains (B-52).

You all have valid points about the landing style I understand.... just playing the devil's advocate as well as some questions I've had about pragmatic instruction.

Are you thinking that somehow an intentional silp in which the pilot makes constant, moment-by-moment adjustments for changing wind conditions from a selected point before touchdown all the way to touchdown involves less positive control of the airplane than coordinated flight?

"Positive aircraft control" means getting the aircraft to do what you want. If there is, for example a 12G20 KT crosswind component and the goal is to touch down with the airplanes path and longitudinal axis both aligned with the runway, cross-controls (aka "doing what is necessary to produce the goal") at the point of touchdown is the way to produce "positive aircraft control" to meet that goal.

Or are you thinking that a slip to landing is something that is all or nothing? This might not apply to you but an awful lot of arguments about "crab and kick" vs "slip" are based on that misconception.

In order to perform a crosswind landing slip properly, you do not need to begin the slip 30 miles from the runway. That's obviously an exaggeration, but it's to make the point that you do not need to begin it 1/2 mile from touchdown or even 100' from touchdown. Or even 10. Plenty of pilots do just fine making the decision to begin the crosswind slip in the flare just a second or so (or less) from touchdown.

Now there's "positive aircraft control!"

What I really meant was that you made something very simple, very advanced by going in depth into what happens when an airplane stalls, which was not really relevant to the discussion.
Seemed more like a way to try and hide something that was wrong with a lot of technical explanations to try and throw people off? I might be wrong on that one though, just what it seemed like.

I would say, when I land in rather "severe" crosswinds (talking >10 crosswind component in a 172), I would imagine my speed at touchdown would be around 50 or so. If you want to stall the aircraft into a touchdown then the amount of correction you would have to put in would be rather significant, and holding centerline at this reduced speed would also be harder. You also land with less than full flaps, say 20 degree flaps, as to get your airplane more stable in the wind. If you try to stall a landing, at full flaps, with a at-the-limit crosswind, I can't imagine that would be easy to control at all.

It's not like I slip the airplane on a 600-feet final. The way I teach is the "crab transition to sideslip" method if you'd like. Crabbing down final, until at a position to flare, where I transition to a sideslip and land. We're talking approach speed of 65 (+50% of gust factor if applicable), and touchdown would not be much less than 50, so we're talking still ca 10kts IAS from Vso.
Remember also, that when stalled, the airplane still develops a "significant" amount of lift, just not enough to keep the airplane flying level.

I guess this goes as the crab and kick method. Even though all sorts of pilots will argue to the end of time which one is better between the three:
- Crab and kick
- Sideslip
- Crab and sideslip

Personally I still prefer the last one. You land with a minimum of sideloading on the tires and landing gear, and everything runs in the same direction they were supposed to go.
With the crab and kick I feel that, you have to be really good to pull it off, because if you wait to long, especially in a smaller airplane, that momentum you were carrying, will not hold long in a 15kts crosswind component type of wind, and sideloading will in that instance be imminent. Touchind down with the downwind wheel is also a problem? Making the airplane, especially a highwinged Cessna, susceptible to be blown over?
With the sideslip all the way down final, I don't feel comfortable beeing cross-controlled at 600 feet, so that's why I prefer to transition to it when I actually need it (right before touchdown)

Are we still talking about one wheel touch and goes or just general landing methods?If you all read my previous posts you'll see that it mostly pertains to that. I don't have any firm data to back up one gear loads... Just exploring all the critical dynamics of it

I don't see the major difference. In a one-wheel touch and go you just add some power to be able to take off again , in a sideslip touchdown you just let that power subside and let the other wheels settle onto the ground. I don't see where there is a difference in landing gear loads

Mr. Trome- we discussed this on another thread some months ago. You may wish to consult:

Eights On Pylons thread