Sideslipping

[uwochris]

Hey guys,

I have a question in regards to sideslipping (i.e. the slip used to counteract drift on a crosswind landing).

I realize that if the a/c were simply crabbing into the wind, the result would be an approach and landing where the wheels are not alligned with the runway, causing a lot of stress.

To avoid this, we bank the a/c into the wind. The purpose of this is to allow the horizontal component of lift to balance the effect of drift, correct? If this is true, why do we need to apply opposite rudder?

ie) if the HCL (or centripetal force) produced by the bank opposes the drifting tendency, what is the purpose of rudder? Is the role to simply keep the longitudinal axis aligned with the runway by overcoming adverse yaw, or must the rudder force balance the banking force.

In the forward slip, the HCL is opposed by the rudder. I just do not see the purpose of rudder in a sideslip, assuming my assumption that the HCL balances the drifting tendency is correct.

Perphaps my misunderstanding lays in the fact that the drifting tendency is NOT a force at all???

Thanks to all those who reply.

Chris.

 

[Illini Pilot]

i think of it more as using the rudder to align the rudder with the centerline, which causes the plane to lose its wind correction from the crab. this will cause a certain amount of drift, which is the counteracted by banking slightly into the wind, allowing some of the total lift to be horizontal. you said that we bank into the wind first during a sideslip, which we really dont. the first thing you should do would be to align the airplane, which is the whole point of the sideslip. now, the bank is induced in order to correct for wind. banking has nothing to do with the alignment of the longitudinal axis with the runway.

 

[Northern Lights]

If you only applied aileron, you would keep banking.
If you only put in rudder you would keep yawing.

You need to balance it out.

Rudder to keep the nose straight, and ailerons to keep from drifting.

The best way to see this is the next time you go flying. Try putting in only aileron or only rudder and see what happens.

 

[Mmmmmm Burritos]

Forward slips and side slips are aerodynamically the same thing. Forward slips are usually close to full control deflection whereas sideslips use just enough control deflection to meet an objective (line up with the centerline).

There is no "drift tendency" acting on the airplane. The airplane only sees air. You as the pilot see the ground and observe "drift". Obviously you have to correct for this before you touch down.

Rudder will cause its own horizontal lift which you must counteract with the bank. To break it down further, you need a bit more rudder horz lift than wing horz lift because of the stability characteristics of the plane. Now that you're in a slip, the vertical stability is such that the airplane wants to weathervane back into the relative wind. A bit more rudder will oppose that. But you shouldnt be thinking of this when you're flying. Just do it.

Rudder first to line up, aileron second to oppose perceived drift. Either one can "run out" and then that will be your maximum crosswind. Mooneys run out of rudder first and you'll end up landing in a crab. Seneca 1's run out of ailerons first and you end up rolling in the flare. Don't ask me how I know Anyway the slower you go while flaring, the less effective the controls will be, and the more you will need to input at the controls. So if you think you're fine at 200agl when the controls are close to full scale, you better watch out in the flare and be prepared to go around when it runs out.

 

[TonyC]

Hey guys,

I have a question in regards to sideslipping (i.e. the slip used to counteract drift on a crosswind landing)....


In the forward slip, the HCL is opposed by the rudder. I just do not see the purpose of rudder in a sideslip, assuming my assumption that the HCL balances the drifting tendency is correct.

Perphaps my misunderstanding lays in the fact that the drifting tendency is NOT a force at all???

Thanks to all those who reply.

Chris.

Forward slip, sideslip, what's the difference? Aerodynamically, they're both uncoordinated flight, where you intentionally use opposite aileron and rudder. The difference is only the reference to the ground and the purpose. In the sideslip you take advantage of the increased drag to increase descent rates without reference to the ground track or your alignment with that ground track. The airplane tracks along a straight line, but the nose might not (will likely not) be pointed along that straight line. The HCL is opposed by the rudder, and it may even be balanced by it - - not important.

IN the slip we use to land, we have an altogether different objective. Not only do we want to track along a straight line - - the centerline of the runway - - we also want the longitudinal axis of the airplane to be aligned with that centerline. In other words, we want the nose to be pointed down the runway, and we want to track down the runway.

We can accomplish the first goal by simply crabbing into the wind, and this is how we fly most of the approach to the runway. Crabbing aligns the track of the airplane, but not the longitudinal axis. What control do we have that will move the longitudinal axis? The rudder. So, we apply downwind rudder to align the longitudinal axis of the airplane with the centerline of the runway. The airplane twists, if you will, along its vertical axis. Great, now we start drifting with the wind.

So, how do we counteract the downwind drift? Bank. Banking into the wind produces a horizontal component that counteracts the drift and keeps our TRACK along the centerline of the runway.

Oh, by the way, the combination of uncoordinated controls also increases drag, so without the addition of power, the airspeed will decrease or the sink rate will increase, or both. So, if you plan on holding the attitude for long, you should consider adding power. Recall, the object of the slip in this case was NOT to increase the descent rate, as it was in the first example.

Some people teach establishing the bank first, then the rudder - - some the opposite order - - and some teach doing both at the same time. Theoretically, blending them both at the same time is ideal. If you're having a hard time understanding it, I recommend transitioning from a pure crab by first using the downwind rudder until your longitudinal axis is aligned with the centerline, and watch how you drift with the wind. Then, start using different bank angles to see how you can move downwind and upwind while the nose remains aligned with the runway - - not POINTED AT the runway, mind you, but ALIGNED WITH it. (Don’t forget about adding power - - if you let the airspeed decay, the rudder effectiveness will decrease, and this exercise will add to the confusion.) The object is to be aligned with, and tracking along - - the centerline.

 

[midlifeflyer]

There is no "drift tendency" acting on the airplane. The airplane only sees air. You as the pilot see the ground and observe "drift". Obviously you have to correct for this before you touch down.

I think you've hit on something here. The original question may reflect the common difficulty in separating air movement from movement relative to the ground.

(Chris, you are correct - there =is no= "drifting tendency" as an aerodynamic force)

In the crab, the airplane is merely moving forward in coordinated flight. The problem with landing in a crosswind is that the =air=, not the airplane, is moving sideways.

In any coordinated turn, when you bank, the nose turns in the direction that you bank. In the crosswind landing you are purposely making flight uncoordinated to prevent that from happening.

 

[avbug]

Don't get too wrapped up in the idea of "coordinated" flight. Coordinating the flight controls means using them in whatever manner one must to cause the airplane to do whatever it is that you want it to do. The position of the inclinometer (skid ball) has nothing to do with it. Reference to "coordinated flight" to mean that the "ball is in the center" is a common use term, but does not alter the fact that coordinated flight is use of the controls by coordinating them to make the airplane do what you want.

When I first started flying ag, I learned very quickly that a good way to get killed when spraying a field is to dip a wing to make a turn. Turns are made flat, when running down the field...especially when running low down a field. The wings are held level with the ailerons, and the rudders are used to move.

I like to do this with students, in a crosswind. I'll have them fly at five or ten feet down the runway, and keeping the wings level, move from one side of the runway, or one side of the centerline, to the other, using rudders. The inclinometer does not remain in the center of the instrument, but the flight is certainly a coordinated one. Coordinated use of the controls to accomplish the action desired.

When making a crabbed approach, one simply kicks out the crab at the bottom of the approach as one lands, and uses aileron to level the wings in the process. If one kicks out too high, one uses rudder to align the long axis of the airplane with the direction of travel (which hopefully parallels the runway centerline), and uses aileron to prevent drift by banking. Kicking out too high in a crosswind requires bank and the airplane will touch down on one wheel (the upwind main) first, whereas kicking out properly the mains should touch almost simultaneously and no more aileron control is required than to level the wings. No bank.

Horizontal component of lift is not centripital force.

Chris, simply put, the rudder's use in a side slip is to align the long axis of the airplane, as desired. In that condition, using aileron to bank into the wind to control drift, the use of the rudder is to point the nose of the airplane where you want to go. The longitudinal axis, much like Mastercard, should be everywhere you want to be.

 

[Pilot124]

Turns are made flat, when running down the field...especially when running low down a

Avbug I agree with you but when you just use rudders you introduce proverst roll that does allow one wing to travel faster than the other thus creating more lift and roll no matter how you look at it. I could fly a whole pattern using just rudder.

 

[TonyC]

Don't get too wrapped up in the idea of "coordinated" flight. Coordinating the flight controls means using them in whatever manner one must to cause the airplane to do whatever it is that you want it to do. The position of the inclinometer (skid ball) has nothing to do with it. Reference to "coordinated flight" to mean that the "ball is in the center" is a common use term, but does not alter the fact that coordinated flight is use of the controls by coordinating them to make the airplane do what you want.

In the Avbug flight school, terms can be redifined freely. In the aviation world, the term "coordinated flight" universally implies the use of aileron and rudder to keep the ball centered, that is, no slip or skid. Emphasis on coordinating (a different word with a different definition) the flight controls to make the airplane do what you want to do does not change the accepted definition of "coordinated flight." Yes, one must coordinate aileron and rudder to enter a slip. No, that doesn't make it coordinated flight.

... , whereas kicking out properly the mains should touch almost simultaneously and no more aileron control is required than to level the wings. No bank.

No bank, simultaneous touchdowns.... you're obviously NOT describing a landing with a crosswind.

 

[A Squared]

Re: Re: Sideslipping

Forward slip, sideslip, what's the difference? Aerodynamically, they're both uncoordinated flight, where you intentionally use opposite aileron and rudder. The difference is only the reference to the ground and the purpose.

Thank you Tony for adding a voice of reason to the discussion of slips. I dispair at the mindless insistence of so many that there is some magical difference between "sideslips" and "forward slips". When pressed, they invent fictional differences like "Forward slips are usually close to full control deflection whereas sideslips use just enough control deflection to meet an objective" Uhhh, no, compare the slip you might do in a power off landing in a flapless airplane where you just need the merest hint of additional drag to make everything work out perfectly, to an approach in a snarling crosswind which is right at the true limits of the airplane. ...so, which uses full deflection and which uses less than full deflection? Does the slip in strong crosswind suddenly become a forward slip because you're using full delfection. WHat if you're a little high on a calm day and you need just a little slip? does that become a "Side" slip just because you're using less than full deflection? what if you have a little crosswind *and* you're a little high and fast .... what kind of slip is that? side? forward? sideforward? Lastly, isn't a slip *always* done to meet an objective? If not why are you doing it?

Even more amazing are those that claim that "forward" slips and "side" slips use different proportions of rudder and aileron. (so far nont have popped up on this thread, but they are out there) Nope wrong again. Given airframe "A" at airspeed "B" , aileron deflection "X" requires rudder deflection "Y" Any more or any less and you're not in a slip, you're in an uncordinated turn. The airplane really does not care where it is pointing relative to the runway, nor whether you're slipping to lose altitude or to maintain a certian ground track, Aileron X requires rudder Y.

I recall clearly my initial flight training. I had one of those instructors who thought that there was some difference between "side" slips and "forward" slips he even had a third he called a "nose" slip. To this day I haven't a clue what a "nose" slip was. Anyway, I understood the physics aerodynamics of the slip perfectly adequately, but, because of this inane insistence on a non-existent difference, I was having difficulty. I was trying to understand how the slips were different, because I was told they were different. Once I figured out on my own that there is no difference, the troubles evaprorated. My experience is not unique, I have seen others who are obviously confused about the "different" slips, because thier instructors are preaching a non-existent difference, presumably because *their* instructors taught that slips were different and they themselves never grasped that *thier* instructor was wrong ... and the circle continues. Stop the madness, a slip is slip. period.

 

[floatflyer99]

A sideslip is a slip in a more-or-less level attitude.
A forward slip is a slip with more of a nose-down attitude.
A "nose slip"--I am speculating here--is a slip with a nose-up attitude.

In regards to the original question...
Chris--think of the dire consequences of landing a floatplane crooked or with too much drift, or a taildragger for that matter. With a tricycle geared a/c, landing with drift will cause a side force to act on the mains. Since every action has an equal and opposite reaction, and since the mains are aft of the c of g this force will cause the nose to pivot around the c of g (ie. straighten out). The only down side is the stress on the gear and the wear on the tires.

In a taildragger, this side force acting on the mains will cause the TAIL to pivot around the c of g (ie. swing forward and want to ground loop you).

I dunno the exact result in a floatplane but it could be very bad and very wet.

Keep the nose aligned with your track over the ground (or water) and everything will be cool. Do whatever it takes... just do it.

Further clarification/muddying of the waters...

Ask yourself why in the world you would add RIGHT rudder when the wind is coming from the LEFT.

To straighten the nose. It is not to counteract a force at all, but to align the a/c with its track.

Again, don't try to fly the airplane by virtue of aerodynamic reasoning. Aerodynamics is the STUDY of forces acting on an a/c in flight. Don't put the cart before the horse. You can bet no aerodynamics prof came up with the sideslip. Sideslippin' is good. It is a maneuver that makes perfect sense to any uneducated 1930's farmer/pilot landing on his grass strip.

Don't even try to arrive at why it is a good technique by virtue of cold aerodynamic analysis.

Cheers.
Hope that helps.

 

[Mmmmmm Burritos]

Re: Re: Re: Sideslipping

Forward slip, sideslip, what's the difference? Aerodynamically, they're both uncoordinated flight, where you intentionally use opposite aileron and rudder. The difference is only the reference to the ground and the purpose.

Thank you Tony for adding a voice of reason to the discussion of slips. I dispair at the mindless insistence of so many that there is some magical difference between "sideslips" and "forward slips". When pressed, they invent fictional differences like "Forward slips are usually close to full control deflection whereas sideslips use just enough control deflection to meet an objective"

Wow, someone needs to settle down. You tell me, if I say "Forward slips and side slips are aerodynamically the same thing" (like I did) and TonyC says "Forward slip, sideslip, what's the difference? " how does that make it so different? Did I royally offend you in stating the truth? The different slips are USED for different purposes, and USUALLY (like I said earlier) the forward slip is used to get down quickly by using close to full scale deflections while USUALLY a sideslip is used to a lesser degree in a mild crosswind. But apparently you live in a world with 35kt direct crosswinds every day and are always at a perfect height on final even when told to do a short approach. Of course there are always exceptions to something. Yipppeee that you figured that out. Very impressive.

And yes I have figured out that you can have varying degrees of either one.... even before you so told me so very eloquently.

I'm sorry that you are such a great asset to the aviation community that you were smarter than your instructor and therefore must be superior to all known mortals. All I stated was the truth and I got verbally annihilated. I guess I'll go hand in my instructor certificate now.

 

[Moonfly201]

It's all just a matter of degree .....

All I stated was the truth and I got verbally annihilated. I guess I'll go hand in my instructor certificate now.

"verbally annihilated" ? Did I miss something?

 

[uwochris]

Thanks again for the responses.

On a similar note... when you are in a twin and experience an engine failure, is the resulting sideslip any different from that of a single engine airplane (i.e. one that is performing a forward or sideslip)?

If I remember correctly, a ME a/c with its wings level and ball centered during an engine out operation WILL be sideslipping. This *seems* to be a different type of sideslip, but I figure in all cases, the a/c is flying somewhat sideways through the air.

Also, I believe that in the ME situation, you use opposite rudder to offset the adverse yawing tendencies, but you also need to bank the a/c to produce a Horizontal component of lift to balance this rudder force. When you are no longer sideslipping, the ball will NOT be centered... does this all make sense to you?

 

[Moonfly201]

Centered ball, no sideslip ...

If I remember correctly, a ME a/c with its wings level and ball centered during an engine out operation WILL be sideslipping.

No. When the ball is centered, there is no sideslip. During engine out operations on a twin, a certain amount of sideslip is necessary, and this will offset the ball.

 

[skiddriver]

Actually there can be a few degrees of sideslip in an aircraft flying with the ball centered. The ball positions itself in the slip race due to the orientaiton of the force vector resultant from the combined side force and gravity force. When these two forces "balance", the ball is centered (if properly calibrated). Sideslip is aerodynamic and is the difference between the longitudinal axis and relative wind in the aircraft's horizontal plane.

In experimental flight test, we instrument for both sideslip (using a weathercocking vane) and side force (using an accelerometer). The outputs are similar statically, but sometimes not exactly the same particularly if there is a force imbalance. They vary dynamically, as you probably would expect. For example, in a gusty crosswind landing, your sideslip is varying more than your ball would lead you to believe, as the effects of the crosswind lag the change in sideslip due to the effect of inertia as your aircraft tends to continue down it's original flightpath. Large planes (with more mass) have more side force lag, small planes (less inertia) have less side force lag.

 

[A Squared]

Re: Re: Re: Re: Sideslipping

Wow, someone needs to settle down. You tell me, if I say "Forward slips and side slips are aerodynamically the same thing" (like I did) and TonyC says "Forward slip, sideslip, what's the difference? " how does that make it so different?

Probably both of us need to settle down, but that's beside the point. Yeah, you got partial credit for saying "Forward slips and side slips are aerodynamically the same thing" but you lost all your partial credit points when in the very next sentence you repeat the same old meaningless and nonexistent "difference" between the two.

Slips to get rid of excess altitude and speed come in all degrees, from a touch of control deflection to full control deflection, dependign on the circumstances. Slips to land in a crosswind also come in all degrees, from very slight control deflection to full deflection. I don't know where you get this "usually" bit ... do you have some sort of statistical study?

As for the bit about my primary instructor, you've completely missed the point. It wasn't that I was smarter than him, Perhaps I was, perhaps I wasn't, it's not relevant. The point was to illustrate how insistence on a pointless and non-existent difference accomplishes nothing and actually creates confusion when students attempt to grasp a difference they have been told exists, but in fact does not.

 

[bigD]

Re: Centered ball, no sideslip ...

During engine out operations on a twin, a certain amount of sideslip is necessary, and this will offset the ball.

Are you sure about this? It's been awhile since my multi training and maybe I'm wrong here, but I thought the point of dropping a wing slightly was to eliminate any sideslip for maximum performance. The ball is slightly off center because the plane is slightly banked, but there's no actual sideslip through the air (have you ever flown a training twin with a string taped to the nose, like a glider?).

For the record, I'm using skiddriver's definition of sideslip, which is "Sideslip is aerodynamic and is the difference between the longitudinal axis and relative wind in the aircraft's horizontal plane."

Or pehaps we're just not making a distinction between applying sideslip-like deflections to the flight controls, and the plane actually being in a sideslip through the air?

 

[A Squared]

Re: Re: Centered ball, no sideslip ...

Are you sure about this? It's been awhile since my multi training and maybe I'm wrong here, but I thought the point of dropping a wing slightly was to eliminate any sideslip for maximum performance.

You're correct Big D, the bank should be just enough to keep the slip at zero. That being said a slip (aerodynamic slip, airflow not aligned with the axis of the airplane) into the operative engine will assist in maintaining directional control. You could lower VMC by banking 15 degrees into the operative engine, but performance would suffer.

 

[Moonfly201]

Re: Re: Re: Centered ball, no sideslip .

You're correct Big D, the bank should be just enough to keep the slip at zero. That being said a slip (aerodynamic slip, airflow not aligned with the axis of the airplane) into the operative engine will assist in maintaining directional control.

My head is really starting to ache. And I may be offbase.

The bank applied towards the operative engine assists in maintaining a constant heading. And the proper amount of rudder required for minimum drag results in the ball being offset ... which occurs because the aircraft is now in a sideslip.

Therefore, if a string was placed in front of the windscreen, like in a glider, it would be offset and not aligned with the fuselage, because the plane is in a sideslip to the relative wind.