side slip/forward slip
03-19-2009, 05:27 AM
#11
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Joined APC: Jun 2008
Position: USAF
Posts: 1,398
Originally Posted by UAL T38 Phlyer
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.
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.
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
Last edited by ryan1234; 03-19-2009 at 05:39 AM.
03-19-2009, 05:34 AM
#12
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Originally Posted by NoyGonnaDoIt
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? 
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.
03-19-2009, 06:07 AM
#13
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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.
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.
03-19-2009, 10:52 AM
#14
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Originally Posted by ryan1234
however landing on one gear and holding it there (or dropping it on the gear)
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.
03-19-2009, 11:01 AM
#15
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Originally Posted by NoyGonnaDoIt
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.
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.
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
03-19-2009, 03:46 PM
#16
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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.
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.
Last edited by Cubdriver; 03-19-2009 at 05:12 PM.
04-01-2009, 08:09 PM
#17
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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.
04-01-2009, 08:22 PM
#18
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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.
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
04-08-2009, 01:01 AM
#20
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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.
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