Nowadays, I drink my coffee without any sugar in it. A little cream and I’m good to go. That wasn’t always the case, though. I’m not sure the age that most folks start drinking coffee, but I remember a coffee experience as far back as 7th grade. I was on a mission trip in Jamaica and had just poured a cup. In those days, I had a little coffee with my sugar. I mean I really poured it on. So when I found the bowl of sugar, I went to town. Sweet, delicious goodness. But then I took a nice big sip and realized I had a mouth full of what tasted like hot water from the ocean. That wasn’t sugar – it was salt! It’s amazing how two things that on the surface seem so similar can have two completely different effects. I found that aviation has two very similar maneuvers with very different effects: slips. There are forward slips and side slips, and you seem to be heading straight for both of them, and they both involve opposite rudder and aileron. So what’s the difference? Grab your coffee and let’s sip on some slips.

 

The Forward Slip
First, let’s look at the two types of slips. The first is a forward slip, which is using opposite rudder and aileron to bleed off altitude without gaining airspeed. If you are high on an approach, you might be tempted just to nose over to lose altitude. You’ll likely lose the altitude, but you’ll pick up a bunch of airspeed while you’re at it. Then to lose the airspeed you might have to level back off, or at least slow your descent, to bleed off the airspeed. Kind of defeats the point. By using a great deal of rudder in one direction (left, for instance) and opposite aileron (right, in this case), you effectively put the right side of the airplane into the relative wind. Much like a paddle placed into the water, when the edge of the paddle faces forward, there isn’t much resistance, but put the flat side forward and you increase drag, slowing your canoe. The same thing is happening to the airplane. Instead of moving aerodynamically through the air, you’re effectively putting the flat side of the airplane into the relative wind, increasing the drag and slowing you down. That increased drag is what allows you to lose your altitude without picking up the airspeed.

X-Plane 11 Cessna 172

Here we are high on the approach, so we’ve cranked in left rudder and then enough right aileron (bank) to keep us straight.  Notice the airspeed hasn’t picked up beyond our approach speed of 75 knots but we have a 1000ft per minute descent!

 

The Side Slip
The other kind of slip is a side slip. From a control input standpoint, you’ll find the side slip to be similar to the forward slip – opposite rudder and aileron. The main difference is how and when you use it. A side slip is used to counter the effects of a crosswind. On final approach, for instance, when you have a crosswind from the right. You bank the airplane to the right just enough to prevent the wind from blowing you off course. Then opposite rudder keeps the airplane heading in the forward direction towards the runway instead of the bank turning you to the right.

Cessna 172 X-Plane

Here the wind is blowing from the right.  Enough right aileron (bank) is used to counter the wind drift, and then opposite rudder is applied to keep the nose straight, preventing a true turn into the wind.

 

The Big Difference
Sounds a little confusing to keep the two concepts apart, right? Opposite aileron and rudder and you’ll slip. The big “aha moment” for me was when I pictured both maneuvers in a no wind situation. First, the forward slip.  Approaching an airport with no wind, you execute a forward slip by applying rudder in one direction and aileron in the other. You find a combination that keeps the airplane heading straight for the runway, even though it’s twisted funny. The more rudder deflection you use, the more aileron you will need to stay on a path to the runway. The bigger deflection means more drag and a greater altitude loss, too.

X-Plane 11 Cessna 172

There isn’t any wind on this approach, but we’re high, so we’re using a forward slip.  Notice how much of the right side of the airplane in turned into the relative wind, adding drag and keeping us slow for our greater descent rate.

 

Now in a side slip you aren’t trying to bleed off altitude. You are just using enough aileron to counter the wind drift, and enough opposite rudder to keep your nose straight in response to your aileron input. So why is it called a side slip? Consider your side slip in a crosswind, with a nice amount of right aileron banking you right and left rudder pulling your nose to keep you aligned with the runway. As you fly towards the runway, you maintain a straight path despite the wind blowing from your right. Now imagine you have those same control inputs in and the wind dies down to nothing. What is your plane doing? It’s maintaining a forward heading but drifting right in the absence of wind. A side slip is a maneuver where you drift to the side. It’s only because the wind is working to counter your side slip that you perceive it to be a straight ahead maneuver. You’re going sideways in the air, but because of the wind; straight ahead on the ground.

In a no wind situation, the forward slip maneuver would keep you tracking forward, while the side slip would have you sliding to one side or the other.

 

In summary, both of these maneuvers have similar opposite controls at their core. The main difference is that in a forward slip, you crank in rudder based on how aggressive you want to lose altitude and then match opposite aileron to stay straight. In a side slip, you crank in your aileron to counter the effect of the wind drift, and then match opposite rudder to keep your nose pointing at down the runway. I hope this helps you keep your slips straight and your coffee salted. Wait, or is it keep your slips to the side and your coffee sweet?

 

 

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12 Comments

  1. Stephen Kendall

    Really enjoyed this article.

    Reply
    • Clay

      Im glad you enjoyed it, Stephen, and thanks for reading!

      Reply
    • Randall Kelley

      Good article

      Reply
      • Clay

        Thank you Randall!

        Reply
  2. Babatunde bello

    This is a great flying instructions

    Reply
    • Clay

      Thanks for reading!

      Reply
  3. Doug Jackson

    Good explanation!
    Rudder leads forward slip
    Aileron leads side slip

    Reply
    • Clay

      Thanks Doug, and your statement is the perfect summary. We should placard that above the turn coordinator :).

      Reply
    • Josh Snow

      Nice summary…For the forward slip, I always thought it was helpful to first decide which wing I should drop (based on the crosswind direction). Upwind wing should be the one to drop. Once I decide that, I then lead with opposite rudder.

      Reply
      • Clay

        Excellent point, Josh.

        Reply
  4. James

    Great explanation as this is where I’m at with my training.

    Reply
    • Clay

      Thanks James! Keep up the training. Even when it’s tough, it’ll all come together and be worth it.

      Reply

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