Slips

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Slips

Postby jimclarke » Tue Jan 25, 2005 1:50 pm

What is the difference between a forward slip and a side slip, and how do you perform each one?


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Re: Slips

Postby beefhole » Tue Jan 25, 2005 7:01 pm

Hmm, I'll stick to the sideslip since it's the only one I've actually done.

A sideslip is done to counter a crosswind (as you probably know). One wing is dipped into the crosswind, and then opposite rudder is applied to keep the nose straight. For example-

Landing on runway 6, winds 095/15

You would dip the right wing into the wind, then apply left rudder to keep the nose on the centerline.

I would go consult me books for the forward slip but I'm being yelled at to get off he computer.  >:( ::)
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Re: Slips

Postby Hagar » Tue Jan 25, 2005 7:08 pm

A sideslip is often used to lose height rapidly & safely when landing in a restricted space. Mainly by light aircraft & gliders operating from small fields. Point the wing at the runway & use opposite rudder to compensate. I'm told that it's impossible to stall from a correctly performed sideslip.

I've never heard of a forward slip.
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Re: Slips

Postby Boss_BlueAngels » Tue Jan 25, 2005 8:25 pm

I don't know if maybe it has to do with geographic location... but for private pilot training here in the US the Forward slip is what Hagar just discribed.  Used for landing.  

Also, it is possible to stall from a forward(side) slip if you don't know what you're doing.  Since you are in a crossed control situation (left aileron, right rudder) it can be a potentially hazardous situation.  But... if you dip the wings and the nose at the same time you'll be just fine.  Just don't do it when trying to shallow up your descent. lol  Or you'll impact the ground rather quickly with a flash of alluminum.  But, as Hagar said... you're perfectly safe when you do it, perfectly. :D

And BTW, man, I missed doing forward sips, they're so fun!  Specially for the people on the ground... "That guys landing sideways!!" lol  
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Re: Slips

Postby beefhole » Tue Jan 25, 2005 9:14 pm

I was taught the terminology was sideslip. perhaps they're one in the same.
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Re: Slips

Postby beaky » Tue Jan 25, 2005 10:23 pm

 Everyone so far is more or less right, but let me try to sum it up:

A forward slip is done to lose altitude over less distance without increasing airspeed (generally used to get past an obstacle close to the approach end of a short runway) . Use rudder to point the nose to the side somewhat (into the wind if there is a crosswind; otherwise slip so that the side you're sitting on is facing forward, so you can see better), and at the same time lower the wing that is now forward.  The excess drag of "flying sideways", and the reduced efficiency of the wings at such an angle  will help keep the airspeed down while consequently reducing lift to give you that steeper descent. Your rate of descent  will probably be higher than normal too, which can be very helpful in an emergency (fire, deteriorating weather, etc.). But not too fast...!!
 A sideslip is used to maintain alignment with the centerline when there's a crosswind. Rudder is used to keep the nose aligned with the runway centerline, and the upwind wing is lowered just enough to keep the airplane from drifting to one side or the other.
 Bear in mind that in both cases, you will be slightly "cross-controlled" (right  rudder with left aileron, for example), so you need to be careful. Stalling on final in a cross-controlled state  can easily lead to a spin, with insufficient altitude for recovery. Definitely more likely w/ a forward slip than a sideslip, as there's usually a lot more rudder involved.

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From the USDOT Flight Training Handbook, ca. 1980
Last edited by beaky on Tue Jan 25, 2005 10:49 pm, edited 1 time in total.
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Re: Slips

Postby beefhole » Tue Jan 25, 2005 10:56 pm

OOHHH great, that clears it up. I've done a sideslip, looks like the forwardslip is for REALLY bad crosswinds.
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Re: Slips

Postby Hagar » Wed Jan 26, 2005 4:18 am

Thanks Rotty. I suspected as much but, being old-fashioned, a forward slip is still a sideslip to me. The aircraft is still moving sideways through the air so it's sideslipping. Far less confusing & easier to visualise. The term "forward slip" seems an oxymoron to me. How can an aircraft slip forwards? ??? Explain that to a student.

I've been interested in aviation since I was 6 years old & read many books on the subject over the years. I got my gliding licence when I was 16 & my first job (in 1960) was with a flying club. I've been in the aviation industry all my working life. This is the first time I've ever heard the expression.

OOHHH great, that clears it up. I've done a sideslip, looks like the forwardslip is for REALLY bad crosswinds.

No Beefy. You have the wrong idea. Nothing to do with crosswinds. This "forward slip" is simply a different term for a sideslip used to lose excess altitude quickly on the approach, typically to get into small fields over high obstacles (like tall trees) on the boundary. Some pilots might change the direction of the slip several times during the approach. When done properly by an experienced pilot it's beautiful to watch. There are other times it can come in handy but you can do it when there is no wind at all. I think this proves my point about the confusion.
Last edited by Hagar on Wed Jan 26, 2005 5:11 am, edited 1 time in total.
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Re: Slips

Postby beaky » Wed Jan 26, 2005 6:18 am

Thanks Rotty. I suspected as much but, being old-fashioned, a forward slip is still a sideslip to me.
Think of it this way: the term "slip"indicates the plane is not going entirely nose-first into the relative wind ( I know it's not that simple, but you get my meaning?). So now to describe slipping along one's ground track (the runway centerline), with the low wing leading the way, the term "forward slip" makes some sense. Likewise, to describe slipping across one's track, just quickly enough to compensate for wind blowing across that track, the term "sideslip" is more appropriate. Anyway, that's how I clarified it for myself; I needed to do that, 'cause these maneuvers are two very different animals! I guess you could call them  the "sideslip" and the "forward sideslip"...
I've been interested in aviation since I was 6 years old & read many books on the subject over the years. I got my gliding licence when I was 16 & my first job (in 1960) was with a flying club. I've been in the aviation industry all my working life. This is the first time I've ever heard the expression.
Doesn't surprise me... reminds me of the US/UK discrepancy between "angle of attack" and "angle of incidence". Let's face it, you people talk funny.  ;)  Doesn't matter, as your introduction to slips was probably more practical ("sideslip this way, or this way"...). Probably for the best, 'cuz a demonstration is worth a thousand words... but it's odd that you've never seen the term "forward slip" in a book; I own a few that mention it by that name, i.e. the USDOT manual  shown above. Maybe it's a relatively new distinction in books...

No Beefy. You have the wrong idea. Nothing to do with crosswinds. This "forward slip" is simply a different term for a sideslip used to lose excess altitude quickly on the approach, typically to get into small fields over high obstacles (like tall trees) on the boundary. When done properly by an experienced pilot it's beautiful to watch.
He's right, beefy. And not only is it beautiful to watch, it's a joy to execute! Very handy sometimes- I've often done it to salvage a high approach, as well as for clearing obstacles.
There are other times it can come in handy but you can do it when there is no wind at all.  
I've found the forward slip is much easier to execute without much crosswind... if your nose is pointed to the right into the wind and your downwind (left) wing is low, you may not be able to check that drift very well. As for yawing away from the crosswind: I've never tried it. With a good crosswind, I'll stick with a sideslip or a good ol' crab (coordinated turn into the wind) .

BTW beefhole: Some aircraft are placarded with a prohibition against slipping (side or forward) with full flaps. C172s are one type; supposedly, with more than 10 degrees of flap, you could "blank out" the tail during a slip, which could cause the nose to rise or drop when you don't want it to, possibly encouraging a stall. Some pilots poo-poo that; I have no interest in proving them wrong, but the theory seems sound to me.  Always check first before you slip. And if you're landing on a short runway with obstacles nearby, if you have to go around, you're better off with no more than one notch of flaps anyway- there might be obstacles at the departure end, too...
Last edited by beaky on Wed Jan 26, 2005 6:22 am, edited 1 time in total.
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Re: Slips

Postby Hagar » Wed Jan 26, 2005 6:31 am

Here's an example of a sideslip approach. Call it a forward slip if you wish. I spend a lot of time just hanging around the local airport watching aircraft take off & land. Sorry for the quality of the shots. They were taken some time ago before I got my new camera.

This is aerobatic display pilot Denny Dobson doing a typical landing. It's impossible to see over the nose of the 2-seat Extra 300 from the rear seat so he appproaches at 45 degrees to the runway. The Extra has windows below the wing so from this angle he can see the runway clearly throughout the approach. It might be a tad exaggerated as Denny is a bit of a show-off. ;)

Lining up & starting the slip.
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Believe or not the aircraft is heading straight for me.
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Still at 45 degrees to the runway & right on the spot. He will now kick it straight for a perfect touchdown.
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PS. The wind direction was almost straight up the runway.
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Re: Slips

Postby beefhole » Wed Jan 26, 2005 4:43 pm

D'OH! For the record, I knew that. I swear  :-/
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Re: Slips

Postby beaky » Wed Jan 26, 2005 7:20 pm

Cool... pictures explain a lot. That's a great example of what I call a forward slip. No need to apologize for the quality; it's a great aviation moment, esp. the 3rd shot. Reminds me of this one I took at the Sussex, NJ show a few years back: Roger Lenhert doing his "Flying "Farmer" bit in a stock J3. He usually starts with a nail-biting low-speed loop right after takeoff, then proceeds to do all this wonderful low and slow stuff that's just amazing. Here he is , floating (I swear) from well outside the frame of this shot to a point well to the left. Very masterful control of throttle is needed to do this, not to mention the aileron and rudder work. So here's a question: is this a slip, or what?
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He's pretty well crossed up here, at a very low airspeed, very close to the ground. I haven't been able to pull this off yet in the FS9 Cub, but I'm working on it...
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Re: Slips

Postby Hagar » Wed Jan 26, 2005 7:33 pm

LOL. I forget what the aerobatic guys call that but it looks more like a vertical knife-edge manoeuvre. You would need a lot of faith in the engine to do it that close to the ground. :o

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Strange that. I was born & still live in the original Sussex - 3,000 miles away, give or take a few. ;)
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Re: Slips

Postby beaky » Wed Jan 26, 2005 8:05 pm

Lenhert's good. I imagine he's lost power at least once doing this stuff, but the beauty of the J3 is that even that low, you'll walk away from your "arrival" if you act quickly, I bet. There's another guy I've seen around here named Stan Segalla; does the same thing: crazy stunts in a stock J-3.  Are you familiar with the "Flying Farmer" routine?
 Sussex Airport (FWN) is home to "The Biggest Little Airshow in the World"... it never disappoints. I've seen some of the very best there... have a few more pix i should really get around to posting sometime (not tonight- I'm gonna fly the sim for a change!). Hmmm... I wonder if Beefhole is totally confused now, or what?? ;D
Last edited by beaky on Wed Jan 26, 2005 8:07 pm, edited 1 time in total.
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Re: Slips

Postby Hagar » Wed Jan 26, 2005 8:22 pm

[quote]There's another guy I've seen around here named Stan Segalla; does the same thing: crazy stunts in a stock J-3.
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