View Full Version : Side slipping....problems?


Bear Cub
19th Jan 2001, 08:17
Question for the engineering and/or aerodynamicist ppruners of the world.

Often hear conversations about side slipping. Many say it is a good thing, many say it may be a bad thing.

I was reminded of this by a quote I just saw on another thread "and side slip but not in C152s".

I hasten to add, that I HAVE taken this out of context - this is just a question...what can you side slip nicely, what can't you..and why?

Reasoned arguments from both camps would be both welcome and educational.

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johnv
19th Jan 2001, 14:06
I always thought the only problem was sideslips with flaps extended in high wing aircraft because of the displaced airflow over the elevator.

Unusual Attitude
19th Jan 2001, 15:39
Spot on JohnV, I think its more on the Cessna models with the full 40 flaps as opposed to just the 30 flap versions.
In the manual for my C172N it states in big letters DO NOT SIDESLIP WITH 40 FLAP and I flew a 150 Aerobat once with 40 flap which had a placard saying the same.

I also looked this up on the net once (out of curiosity) and read a post by a bloke who tried it from a high altitude so that he had time to recover. He basically said that once it goes into the sideslip it wont come out again as the rudder effectiveness is lost. The only cure is to raise the flaps to re-establish the flow correctly around the rudder.

Not what you need at low level when trying to get into a tight field. Then again with 40 flap a 172 drops like a brick so if you need to sideslip aswell there's something badly wrong with your approach !!
If I'm trying to lose height quickly I tend to drop the flaps full and do tight weaving S manoeuvres instead whilst carefully watching the speed of course!

Hope that helps,

Regards

UA

Bear Cub
19th Jan 2001, 18:31
Interesting stuff....can anybody expand on this airflow stuff?

Also heard a comment about stalling the fin - same thing or a different problem?

Capt Crash
20th Jan 2001, 17:38
With the 152 the POH just states no slide slip. It could be a structual limit on the fin, there is an increaed side load in the slip.

I think it is a shame because slide slip is a very usefull technique to master.

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PULL UP!!!!!!

Cuban_8
21st Jan 2001, 01:40
BC,

Having studied an Aero Eng degree, I have had a bit of exposure to the business of sideslip, so for what it's worth, will add my thoughs!

As far as I was aware, it is an airworthiness requirement for an aircraft to be able to sideslip to a certain degree. This is not so critical on smaller aircraft, but consider approaches where a wing-down technique is used - the aircraft is in a constant state of sideslip. In this condition, the aircraft must remain easily controlable for obvious reasons.

Sideslip flight is sometimes a problem for aircraft with a high degree of wing sweep. Thus typically applies to large airliners, who's swept wings (there to reduce drag at high Mach no flight) mean that a lot of aileron control has to be deflected to maintain level bank attitudes on approach. I'm no real expert, but I suspect that this is the reason that crosswind limitations are defined in the a/c operation manuals etc.

With regards to the flap limitations on smaller aircraft, then this is most probably due to the flap disturbing the airflow over the tail control surfaces leaving them short of control power. This situation is akin to T-tailed aircraft going into deep stall and not being able to recover as the "dirty" air from the stalled wings passes over the tailplane.

I have never personally come across this problem with light a/c before, but then again, I have not flown a high wing or a Cessna! All light a/c that I have flown have not been sideslip limited.

Hope this helps,

Cuban_8

U.A. - Were there any implications with the C172 regarding landing with 40Deg and kicking off the drift??

foghorn
21st Jan 2001, 03:26
Sorry to hijack the thread, but does everyone else transitioning from a low-wing type into a cessna find that flap 40 landings are a real nightmare? The thing turns from an aircraft into a grand piano in the flare. I gave up trying to fly older Cessnas; I've had more luck on a later 172RG that did not have such big flaps.

Dave Incognito
21st Jan 2001, 07:17
Other a/c such as the PA28 limit slideslip duration to 30 sec. Upon asking, I was told by my instructor that it was a fuel flow consideration. Does anyone know how/where in the fuel system this is occurring? (float chamber perhaps?)

Have a good one, Dave.


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Flying is easy - just throw yourself at the ground and miss.

chipped prop
21st Jan 2001, 14:35
During my training days i was told not to side slip a c152 as this could possibly damage the flap mechanism if the flaps were extended.

AEROVISION
21st Jan 2001, 15:24
Let me add my two cents here.

Long ago when I was doing my type rating for the B-707, on one of the sim sessions the following took place.
According to the instructor, who had 15000 hrs on type, the sim flew like the real thing. And so were the engines. When you had to go to a certain power (N) setting, you had to move the throttle levers accordingly and at the right setting, all four of them were out of line with each other, always, and not a little bit. No autothrottle then. This was all good and well thil final approach came.
At flaps 15 at 2000 ft on the localiser, glide alive and gear down. power adjustment. On the glide, flaps 25, power adjustment because I always was above glide there. It took me some time to master the trade to be at the right settings and speed and be on the glide at flaps 25. Now picture this.
At one particular session I thought I had it all set up nicely and, as flaps 25 came, I was above glide again. But, instaed of adjusting the throttles again, in an instinct move, I side slipped the beast. It said whoooffee, and there I was, on glide on loc on speed, perfect. except for the instructor who behind me jumped from his seat and yelled where the hell did you pick up that trick.
After landing I explained to him that I picked this up when i was 14 when I flew gliders which at that time did not have flaps, nor spoilers nor speed brakes. So you always came in to land a tad high, (too low was no option), and then side slipped it in.
The instructor agreed that it would work in real life but advised me very strongly not to pull this off on my check ride. Which i did not cause by then I had mastered the trade.

Now, many years later I read an article in AOPA magazine, written by one of their regular columnists, Capt. Barry Schiff.
In that article he described the same trick, honestly, and he did it regularly. I forgot what he was flying at the time but is was either L-1011 or 747.
So, yes. You can side slip a swept wing airliner.

As for the C-172. No need to side slip. With full flaps, "hanging on the prop", you land on the threshold and stop on the threshold.

And then the Piper Cub. Aahh, those were the days. Came in to land on the grass strip, throttle idel, full flaps, flew through a thermal on final which gave you zero sink rate for 5 seconds, and then side slip it in, full flaps no problem.
But the coolest thing with the Cub was coming in on a high final, throttle back, switch off the engine, pull the nose up so the prop stops windmilling, full flaps and side slip it in. Which was nice.

AC-DC
21st Jan 2001, 16:33
Dave Incognito
Side-slip in the PA28 is limited due to the possibility that the centrifugal force on the fuel in the tanks will move it away from the feeding lines to the engine. This problem might happen when you fly with less than ½ tank. This is the explanation in my POH for a PA24. Anyway, if you need to side-slip for that long you have done something very wrong, go around.

Foghorn
I had the same problem. All you need to do is to seat and wait. You must be able to control you approach speed. As you reach the hedge pull the throttle and pull the yoke slowly towards you in order to keep the a/c floating above the r/w, Don’t worry about the high nose eventually the a/c will stall and you will find out that you have just perform a very good short landing.
Think about the way that tail draggers perform 3 points landing. Take you time and practice.

I believe that sid-eslip is a good manoeuvre to know. We were flying to Germany in a high winged twin. We spotted the airfield when we nearly were on top of it, maybe ¾ of a mile abeam the r/w. We were fast and high. I called final, cleared to land by ATC with a question if I can make it, my response was that I can only try.
Engines were closed, flaps went full down (30º) to increase drag, cross controls to sideslip and nose down to keep the airflow. We descended like a brick. We touched just behind the numbers and were about 5kt too fast.
But I must admit that I do not side-slip very often.

Now a question of my own.
I heard that in the States they have a Side slip and a Forward slip. The American Side-slip is what we call Crabbing and the American Forward slip is what we call Side-slip.
Any help/knowledge about this one?

If the above is correct, it means that the POH say ‘Do not Crab approach’ which does not make sense to me.
Therefore there can be two possibilities. The first one that I did not understand what I was told and the second, that the other block did not understand.
Thanks for your comments.

johnv
22nd Jan 2001, 03:07
Crabbing is still coordinated flight, you're just correcting for wind drift. A side slip is what you would use on a crosswind landing, the nose of the plane is lined up with the direction of flight, but you have one wing low and stop the turn with opposite rudder. A forward slip is what you would use to lose altitude on a turn from base to final, the nose of the plane is not in line with the direction of flight.
Another problem with slipping Cessna's is the fuel problem. You force the fuel in the tank away from the fuel line. I've experienced this first hand with an engine failure. I tried it again later at altitude and sure enough, hold it in a slip for long enough with one tank selected and look at your fuel gauge indicate zero and your engine will soon quit.

Code Blue
22nd Jan 2001, 04:53
Just to confuse matters further:

The Canadian Flight Training Manual describes:
a side slip as " a slip in which the aircrafts longitudinal axis remains parallel to the the original flightpath" This si the preferred technique for crosswind landings.

"a forward slip is one in which the longitudinal axis of the aircraft is at an angle to the desired descent path"

The C-172 POH recommends avoiding slips
with flaps extended
with less than 1/8 tank of gas because of unporting.

Why would anyone want to utilise the fuel selector on a C-172? I have always filled both tanks (and visually checked - the gauges are decorative not functional :) )and used the selector on both.


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invalid entry
22nd Jan 2001, 16:11
When practising double engine failures on 777 sim, it works beatifully when you get too high on finals. Enter a slip and you get back down sharpish. The instructor looks a bit confused because they aren't used to seeing it, but if it works then why not.

Squawk 8888
22nd Jan 2001, 18:33
The Gimli Glider (AC 767 that ran out of fuel at FL330) was side-slipped with no problems.

As for the terminology, in Canada we're taught 3 types of slips:

Sideslip- used for crosswind approaches, low wing into wind with opposite rudder to keep the nose straight.

Forward slip- used when you need to fly like a brick. Low wing into wind, opposite rudder to keep nose pointed to side with high wing, nose way down to maintain speed. Airflow hitting side of fuselage creates enormous drag.

Slipping turn- rudder opposite direction of turn. Great to have when too high turning base to final.

Never had a problem slipping 172, just don't do it with flaps greater than 20 degrees. I don't like how the 172 handles with full flaps anyway and I find that the buffeting spooks the pax.

johnv
22nd Jan 2001, 22:14
I've been thinking about the original question on the aerodynamics in the slip. In the 172 when you slip with full flaps you get an oscillation. I believe the reason for this would be the airflow would be displaced infront of the elevator making it ineffective causing the nose to pitch down. This would reduce the angle of attack, moving the airflow back over the elevator causing the nose to pitch up again, once again making the elevator ineffective.
The only reason Cessna would care about this since you can't stall and spin when your'e in a slip since you don't have enough elevator available to cause a stall would be in the event you needed immediate use of elevator for some reason (ie. traffic avoidance) it would not be effective. Any thoughts on this idea?

foxmoth
23rd Jan 2001, 01:05
When i learnt to fly (many moons ago now) it was on the c150 and i was taught an alternative to sideslipping for loosing height, if you need to come down VERY quickly, ie because of to high on a forced landing, kick on FULL L rudder then immediately FULL R rudder so the a/c fishtails, keep doing this until you are at the height you want, you don't need to worry to much about stalling/spinning as long as you keep to normal glide speed and you can do it with full flap, but the ROD is very impressive.
n.b. the B757 side slips very nicely.

[This message has been edited by foxmoth (edited 22 January 2001).]

Bear Cub
23rd Jan 2001, 07:36
Can I just check that I've understood this now.

In a side slip the aircraft is going forwards.

In a forwards slip it is going sideways.

Sideslipping is bad for swept wing aircraft but works well in a Boeing 707, 757, 767 & 777.

Cessna 152 must not be slipped at all.

Cessna 172 must not be slipped with 40° flap.

PA28's could run out of fuel if slipped - but no mention of tank selection (surely if one tank could "unport" - then the other side would be "pressurised".

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Hunting is bad!!
Support the right to arm Bears!!

AC-DC
23rd Jan 2001, 22:39
Well Done, You got it!!!

fly_boy
24th Jan 2001, 00:39
It depends upon the a/c type and the elevator design ("T" tail or otherwise). The blanking of lift on the elevator surfaces will cause nose up/down tendency dependent on design. Eg. when you have max. rudder input to one side (eg. right yaw), you expose the elevator surface to the opposite (left), hence, causing the elevator to lose relative airflow effect and therefore stall sometimes termed "elevator blanking". This may cause either a stall in conventional terms (nose drop) or nose up stall (deep stall?). Sideslip though, is commonly used to induce (induced drag) drag so as to lose excess height on finals. I quite comfortably (not the pax) did that 2 days ago to correct my miscalculated high and hot approach.

Wantok

Horsepower
24th Jan 2001, 03:06
Sorry guys, but:—

1) Until now, I’ve never heard of any limitation on sideslipping a 150/2 or 172 with full flap. Cessna seem to be equally puzzled by the various theories floating around on this and answered a query from a pilot on sideslipping—their response is given on the ‘Landing Long’ thread in the Private Flying Forum);

2) The only sideslip limitation I’m aware of on the PA28 (Arrow, anyway) is that “prolonged slips or skids which result in excess of 2000 feet of altitude loss...which could cause uncovering of the fuel outlet must be avoided as fuel flow interruption may occur when tank being used is not full”.

Hope this helps.

HP

Bear Cub
24th Jan 2001, 04:05
Well now look what you've done, HP!!!

Certainly buggered up that discussion - just as it appeared to be approaching the flare you seem to have declared a go around.

Anybody going to type in the relevant paragraphs from their POH?

(Don't you just love it when a plan comes together? [How should I know?])

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Hunting is bad!!
Support the right to arm Bears!!

Horsepower
24th Jan 2001, 14:21
:)) :) :) :)

Squawk 8888
24th Jan 2001, 19:19
I don't have it on me at the moment, but my Cessna POH for the model 172N says avoid slips with flaps greater than 20 degrees (italics mine). It doesn't say slips with flaps are prohibited.

Horsepower
24th Jan 2001, 22:08
Squawk 8888:— Correct. It’s a cautionary note, NOT a limitation.

Apparently it’s something to do with weakened airflow over the tailplane which causes mild oscillations, according to Cessna. Doesn’t apply to 150/2s.

Bear Cub:— I won’t fall into the same trap with your flap/crosswind thread! :)

Unusual Attitude
25th Jan 2001, 04:40
Bingo !!, found the answer hidden away in the depths of my C172 POH.

In the placards section it states 'Near flap indicator- "Avoid slips with flaps extended"'

Very nice but why ? most people would ask. If I've got an engine failure and need to get down into the only suitable field, you can bet your ass I'm going to sideslip with full flap if I have to. It would be nice to know what to expect though.

Fortunately if you then carry on through to the normal procedures section and read the bit on crosswind landings all is revealed.

It states the following -

"If flap settings greater than 20Deg are used in sideslips with full rudder deflection, some elevator oscillation may be felt at normal approach speeds. However, this does not affect control of the airplane"

Nice one, if that's the case then I'll be taking my chances with the oscillations in an emergency !!

I did wonder about the answer I gave in my previous post though. I definitely read somewhere that some bloke had tried it and that he said it was rudder blanking he had problems with ??
However I've been racking my brains and scouring my ATPL notes trying to find a way that would be possible and I cant think of one so he must have been talking balls.
Elevator Blanking is your answer then, I think I'll go and give that a try tomorrow if the weather is half decent, will be nice to try something new for a change !! :)

Regards

UA