Stalls and Slips (Sideslip and Forward-Slips)

[Eric McCandless]

Another thread discusses stalls and turning, which I found very informative.

 

But what about potential for stalls during intentional slips?

 

The importance of a balanced turn is emphasised, but in a slip the aircraft is intentionally unbalanced.

 

How does that affect the propensity for a wing stall? Will one wing tend to stall before the other, for example if more or less aileron is suddenly applied to make a change to the amount of slip and the airspeed is too low? Eric

 

 

[fly_tornado]

You mean a spin?

 

 

[Eric McCandless]

You mean a spin?

Depends how a spin is defined. I am talking about one wing stalling before the other causing the aircraft to pitch towards the ground. If one wing stalls before the other, then that may lead to a spin won't it? Eric

 

 

[rgmwa]

I think this was posted some time ago but it demonstrates the importance of making balanced turns, and what can happen if you don't:

 

https://www.youtube.com/watch?v=xwrfEsCiltc

 

rgmwa

 

 

[djpacro]

Generally, a stall in a skidded turn tends to an aggressive entry to a spin and a stall in a sideslip tends to be spin resistant. I will post a reference later.

 

(This for aeroplanes which conform to FAR 23, or similar, stall handling requirements)

 

 

[facthunter]

This has been done before fairly extensively. Does anyone have a link? Nev

 

 

[djpacro]

Nope, just my standard link to http://apstraining.com/ for excellent info on the subject - go to videos which are also on YouTube.

 

(edited - documents were available after free registration in the past but not now)

 

 

[peter]

Hi Eric,

 

I think you are asking what is the likelihood of and what happens if you reach past the stall stick position whilst deliberately sideslipping , for example, whilst trying to lose height during an approach.

 

When I did my tail wheel endorsement on the super cub at Aldinga this was discussed and attempted whilst practising sideslipping at considerable altitude. The cub virtually refused to stall so was not a problem however djpacro may be able to enlighten us as to the outcome in less docile aircraft

 

Cheers Peter

 

 

[Eric McCandless]

Hi Eric,I think you are asking what is the likelihood of and what happens if you reach past the stall stick position whilst deliberately sideslipping , for example, whilst trying to lose height during an approach.

When I did my tail wheel endorsement on the super cub at Aldinga this was discussed and attempted whilst practising sideslipping at considerable altitude. The cub virtually refused to stall so was not a problem however djpacro may be able to enlighten us as to the outcome in less docile aircraft

 

Cheers Peter

Sorry I may not have explained what I mean well enough. I don't have a lot of flying experience. There is discussion (in another thread) of not using too much aileron into a turn onto final and always maintaining a balanced turn using rudder.

A sideslip is a common technique for a crosswind landing. In a strong crosswind landing, I have used significant aileron and opposite rudder whilst flying at a relatively slow approach speed.I understand that when slipping the airspeed is actually higher than indicated due to the angle of incidence of the airspeed pitot tube to the airflow, so this establishes some safety margin.

 

Given that if I wasn't using opposite rudder to establish a slip, I would be turning as I am using opposite rudder to counteract the banking turn. How much more unbalanced can you be when using a high degree of into the wind aileron and opposite rudder? Isn't this the same as an unbalanced turn?

 

I suppose the airspeed over both wings would be the same in a slip, as opposed to when in a turn where the airflow is slightly lower for the inboard wing. But due to asymmetry of the airflow, would either wing be closer to stall than the other with potentially the same dangerous consequences as when turning?

 

Also when the crosswind is gusting, there is a need to sometimes use significant aileron (and rudder) movements, which if in a turn should be completely avoided. I am very conscious of steep banking and maintaining balance during slow speed banking turns, but less so when slipping. Should the same caution be exercised when in a slip?

 

 

[djpacro]

Page 4-10 of the FAA's Airplane Flying Handbook at http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf has a simple description of the stall from a skidded turn.

 

More detail in the APS videos at

 

From the APS video you will see several more factors not mentioned in the FAA book. In the skidded turn all the factors combine to cause rotation in the same direction. In the steady sideslip some of those factors act in one direction and the rest in the opposite direction. i.e. they tend to cancel out however the actual behaviour will vary from one type to another. I certainly haven't exercised enough types that I can make truly general statements in any more detail. I have even noticed some different behaviour amongst examples of the same type. I do most of this work in a Decathlon which is fairly docile at the stall so need to grossly mishandle it to get it to "bite" but today it really surprised the pilot in the front when I demonstrated it. I obviously knew what would happen and as soon as it stalled I took immediate recovery action but I was unable to stop it entering a spin and losing about 800 ft. He commented that he now understands why a balanced turn is important, especially at circuit height.

 

The stall in the sideslip was noticeable with the typical nose-down pitch and slight uncommanded roll. Preceded by significant buffeting - more like shaking and rattling. Some Decathlons reach the aft stick movement so no nose-down pitch.

 

The little Cessna 150/152 is generally much more aggressive in such exercises.

 

 

[Phil Perry]

Page 4-10 of the FAA's Airplane Flying Handbook at http://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/airplane_handbook/media/faa-h-8083-3a-3of7.pdf has a simple description of the stall from a skidded turn.More detail in the APS videos at

 

 

I agree most wholeheartedly with your comments regarding the C-150/152, these airframes, whilst being lovely docile aircraft to learn to fly in,. . . . can be quite a nasty little thing if you get a little too slow on the final turn, OR, mishandle a sideslip. . . . ESPECIALLY. . . with a high fuel load. I used to demonstrate this to students long ago, and a couple of them decided, (following this rather exhillarating experience ( ! ) that it was NOT the aircraft for them, which is sad really, because so long as you fly within the published envelope, you will NEVER have a problem, but with advanced students, ( You know,. . . the ones who think that . . . by now, they have a pretty good handle on things,. . . .) are sometimes aghast, when they see what a particular aeroplane WILL do, with frightening certainty, if abused in a certain manner. I really MUST add, that these excercises are best performed at a really Substantial vertical displacement above mean ground level. . . . . ! ! !

 

I had one particular student who already had 130 odd hours on type, ( C-150 Aerobat. 130 HP ) and had applied to take an aerobatic course, was really upset, when I allowed him to get into a Stall / spin at over 6,000 feet, and that it took him around 2,500 feet to recover the situation. . . . . Easy peasy at that height, but a bit dangerous in the circuit ??

 

The watchwords then must be. . . LISTEN carefully to your instructor, and if he hits you on the back of the head with a cricket bat and it hurts, then you'll know that if someone else does the same thing, it will probably hurt just as much.

 

Phil.

 

 

[Guest nunans]

Given that if I wasn't using opposite rudder to establish a slip, I would be turning as I am using opposite rudder to counteract the banking turn. How much more unbalanced can you be when using a high degree of into the wind aileron and opposite rudder? Isn't this the same as an unbalanced turn?

I suppose the airspeed over both wings would be the same in a slip, as opposed to when in a turn where the airflow is slightly lower for the inboard wing. But due to asymmetry of the airflow, would either wing be closer to stall than the other with potentially the same dangerous consequences as when turning?

 

Also when the crosswind is gusting, there is a need to sometimes use significant aileron (and rudder) movements, which if in a turn should be completely avoided. I am very conscious of steep banking and maintaining balance during slow speed banking turns, but less so when slipping. Should the same caution be exercised when in a slip?

If the plane is flying in a straight line, then most of the worries about steep turns don't apply. A slip in a straight line is a 1 G situation so the increased stall speed doesn't apply, actually when slipping, some of the lift is being generated by the side of the fuse and rudder so there is LESS lift required from the wing so the stall speed would actually be lower than in wing level flight.

 

The turns are a problem because often the pilot wants to maintain height in a turn so there is no choice but to come up with extra lift to offset the >1 g loading on the wing. This is done by adding power and pulling back closer to the stall stick position. If you aren't pulling back in your slipping approach, and you maintain attitude during entry and exit of the slip then you should be doing alright.

 

 

[djpacro]

... In a strong crosswind ... Given that if I wasn't using opposite rudder to establish a slip, I would be turning as I am using opposite rudder to counteract the banking turn. How much more unbalanced can you be when using a high degree of into the wind aileron and opposite rudder? Isn't this the same as an unbalanced turn?slip?

Such considerations should've been made in determining the max recommended crosswind for the type.

 

.... I understand that when slipping the airspeed is actually higher than indicated due to the angle of incidence of the airspeed pitot tube to the airflow, so this establishes some safety margin.Also when the crosswind is gusting, there is a need to sometimes use significant aileron (and rudder) movements, which if in a turn should be completely avoided. I am very conscious of steep banking and maintaining balance during slow speed banking turns, but less so when slipping. Should the same caution be exercised when in a slip?

Airflow around the static vents in a sideslip can also cause errors so there may not be a safety margin. Suggest you speak to your instructor about both issues.

 

 

If the plane is flying in a straight line, then most of the worries about steep turns don't apply. A slip in a straight line is a 1 G situation so the increased stall speed doesn't apply ...

Slip involves banking so increased wing lift required to give enough vertical component to support the weight. One effect is positive and the other is negative.Did a stall in a sideslip for my last instructor renewal test recently. It stalled at a higher indicated speed than a straight stall.

 

 

[motzartmerv]

Hi Eric. There is some good info in this thread as usual, and some that needs some more consideration. Your flying a J170 yea? Not a cessna 150/152?

 

You need to discuss this with your instructor, at kength. Ask for an extensive stalling briefing, and some 'extra' stall training.

 

The J170 is quite benign in a normal stall, without flap or power. But does have a tendency to drop wings in other config's. However a simple rule of thumb was touched on by DJ earlier, and that is that a sideslip is an 'antispin' manourvre, while a skid is a pro spin move.

 

So the standard sidelsip you use on your approach will generally delay any autorotation tendancy after a stall angle of attack is reached. You are actually in more danger when you 'kick it straight' after a sidelsip due to the yawing moment in the skidding direction.

 

Grab your instructor and go get it ugly mate, forums are great for info, but nothing beats being in the air while your instructor 'shows' you the effects. The J170 you fly has the ventral fin mod I take it?

 

 

[facthunter]

I add a few points. Sideslips done with a high airspeed are less effective and load the aircraft structure.

 

Sideslips without achieving much wing down are just horrible skids.

 

Obtain max comfortable bank and then try to stop it turning with rudder. You will eventually run out of rudder. If you don't want it to turn you will have to take off a bit of bank.

 

All the time you are doing this you will have to monitor the airspeed and have a margin above stall. Since you don't know what speed this will be exactly, start a bit faster and reduce it gradually till you find the pitch attitude that gives you the safe speed you want. Practice all this at HEIGHT till you are easy with the technique. Keep a look out for other traffic.

 

Don't' just go and do it on what I or anybody else has told you.. Get an instructor booked and make sure it is one who really knows what the right technique is. It is probably not used as much as it should be. Certainly you should be proficient in it IF you do it, especially near the ground.

 

Once you become really familiar you will do it to the flare IF YOU HAVE TO This would be to deadstick a plane into a small field where you would stay high and wash off the extra height when you are sure of getting in.

 

Coming out of the slip has to be very coordinated, or you will be dangerous or float more than you wanted and waste runway. Nev

 

 

[bilby54]

Sorry I may not have explained what I mean well enough. I don't have a lot of flying experience. There is discussion (in another thread) of not using too much aileron into a turn onto final and always maintaining a balanced turn using rudder.Given that if I wasn't using opposite rudder to establish a slip, I would be turning as I am using opposite rudder to counteract the banking turn. How much more unbalanced can you be when using a high degree of into the wind aileron and opposite rudder? Isn't this the same as an unbalanced turn? Yes but do not confuse this with a spin entry as the aircraft has not stalled

 

Also when the crosswind is gusting, there is a need to sometimes use significant aileron (and rudder) movements, which if in a turn should be completely avoided. I am very conscious of steep banking and maintaining balance during slow speed banking turns, but less so when slipping. Should the same caution be exercised when in a slip?

Hi Eric,

 

During a side slip the aircraft is flying unbalanced as you have stated which purely means that more of the fuselage, vertical fin and rudder are exposed to the airflow. This being the case, the aircraft will want to slow down so to counteract this the stick is pushed forward and airspeed (however it is presented to the airspeed indicator) will increase.

 

A sideslip is a manouvre that the aircraft is not really happy doing because if you understand stability and release the controls then the aircraft will fly directly into the relative airflow. A sideslip is a forced action on behalf of the pilot and the aircraft will not spin as the critical angle of attack has not been exceeded and the aircraft has not stalled. If you are on approach and need to sideslip, the aircraft will literally go wherever the stick is pointed and I doubt that will be into your lap.

 

Don't be worried about sideslips and if you want confirmation about this, then look up Tony Hayes advice on this forum