Question re: slipping

[uwochris]

Hey guys,

I got a Q regarding slips, and I hope some of you can shed some light on the issue.

Assume an a/c is coordinated and is flying S&L and there is no wind. If the a/c banks to the left, and uses opposite rudder, would the a/c technically still be slipping if the force produced by the rudder exactly balances the horizontal component of lift?

In a book I read, it says that the a/c will drift in this case. To me, though, it seems that a state of equilibrium will be established and thus, the longitudinal axis will be alligned with the relative wind, producing zero slip and no extra drag. I come to this conclusion based on the fact that all forces are balanced (i.e. no unbalanced side-ways force implies no drift).

Perhaps I am overlooking the fact that the moments/torques are not all balanced?? I understand that a state of equilibrium requires all forces and all moments/torques to be balanced. The issue with moments/torques is that if there is an imbalance, there will be a rotation, and not a sideways drift. So, if the a/c is still slipping in the case where the magnitude of the HCL balances the rudder force, what is causing the drift? Anyone care to explain?

Thanks a lot,

Chris.

 

[TIGV]

I'm guessing but I would say in the case of left wing low and right rudder then P-factor will play a roll in drift.
I remember on the old Aeronca Champ I used to start by teaching folks to give me a 30 to 40 degree bank with their feet flat on the floor, the adverse aileron yaw is so bad in that thing that you will have the bank established but for all practical purposes the aircraft will continue in a straight line while in a ' slipping ' condition, I used to do this to illustrate that in older aircraft the rudder is actually required to make the contraption do your bidding unlike the venerable Cezzy 150 and other modern flying machines with ' Frise ' type aileron control.
Just my .02.

 

[Donsa320]

Better find a better book, though I suspect that you have probably paraphrased it incorrectly. Anytime you stop a turn from occurring when the airplane is banked, you are slipping.

~DC

 

[PaulThomas]

Donza is correct. Drag would increase. The 4 knots direct crosswind that you may have doesn't have the aerodynamic has much as the air you're flying into.

If you were going on a straight line heading 360 but the nose was pointing to 270, the whole side of the airplane would be pushing against the mass of air rather than the nose and wings.

 

[uwochris]

So it is impossible to stop a turn with just enough opposite rudder to keep the nose aligned with the relative wind, and thus prevent any drift/ extra drag? ie) can you balance the horizontal lift force with the rudder force to achieve this goal?

 

[Donsa320]

So it is impossible to stop a turn with just enough opposite rudder to keep the nose aligned with the relative wind, and thus prevent any drift/ extra drag? ie) can you balance the horizontal lift force with the rudder force to achieve this goal?

Your statement is a contradiction. It makes no sense. If you are trying to correlate all this with ground track, though, for the purpose of aerial photography, ie; keeping the wings level and still maintaining track over the ground, then you are discussing SKID. But DRIFT, that is... departing a desired ground track, corrections can be made without bank by way of a skidding turn to a new heading which will keep you on a ground track. But fly along with some bank and opposite rudder to maintain TRACK, and you are SLIPPING.

Any application of rudder will cause the nose to no longer be "aligned" with the relative wind unless the rudder is required to offset some other opposite yaw force like adverse aileron yaw or offset thrust.

Remember in a coordinated turn the "relative wind" is constantly changing so as to to be coming from in front of the airplane

~DC

 

[nosehair]

To me, though, it seems that a state of equilibrium will be established and thus, the longitudinal axis will be alligned with the relative wind,

This is the flaw in your visualization of the relative wind. When you lower one wing, the horizontal component of lift "pulls" the airplane in that direction. The relative wind is now coming from an angle to that side, and it is this relative wind that tries to "weathervane" the aircraft nose around into the wind. That's why you can make the turn without use of rudder. But now you stop this weathervaning with opposite rudder, but the HCL still is "pulling" the airplane towards the low wing, and thusly, slipping sideways through the air. It is not "balanced" in the way that you are thinking.

 

[dmspilot00]

So it is impossible to stop a turn with just enough opposite rudder to keep the nose aligned with the relative wind, and thus prevent any drift/ extra drag? ie) can you balance the horizontal lift force with the rudder force to achieve this goal?

No, you will drift. For example, you can't do a slip to a landing in calm wind with the nose pointed down the runway.

 

[A Squared]

Chris,

Take a look at the airplane on the ground, wings level. Now think about which direction the forces act relative to the airframe.

The wings always produce a force *up* relative to the airframe.

The rudder always produces a force *sideways* relative to the airframe.

The rudder force is always pependicular to the wing force (unless you twist the fuselage, which isn't recommended)

You can't "balance" one force with another acting perpendicular to it.

Imagine you have two guys both pushing on a box which is on the ice. They are wearing crampons for traction. One is pushing North on the box, one is pushing East on the box. One cannot balance the other, the box moves northeast across the ice.

Same with the wing an the rudder. You can't balance lift with a rudder which is acting perpendicular to lift.

If you bank the airplane, there is a horizontal componnent of lift, and there is a horizontal component of rudder force, but when you use rudder to stop a turn, the horizontal components are acting in the same direction.

Lets say you bank right. The horizontal componnent of lift is acting to the right. then you feed in left rudder to stop the turn, the horizontal component of the rudder force is acting to the right also ... so you wind up with an airplane moving sideways through the air to the right, or in other words, slipping.

Does that help?


edit: Somthing else which might help. Like nosehair pointed out, you are not balancing the horizontal component of lift with the rudder, you are balancing the weathervaning tendency, or counteracting the tendancy to align into the relative wind. You are actually adding to the horizontal component of lift.

 

[uwochris]

Thanks for the responses.

A Squared- your post cleared up this issue quite nicely!