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mstram
26th Jan 2004, 06:07
In a *slipping*, *descending* turn, what is the effect on the angle of attack, if any?

In my ppl training, no mention was ever made on the effect on a.o.a. from a slipping turn.

A discussion came up on another board, re: the safety / wisdom of slipping turns to final, from a too-high approach, and I'm looking for some aeronautical info on the subject from any of the experts here :)

Mike

Miserlou
26th Jan 2004, 06:55
That's a tough one to get your head around if you don't choose an exact moment and set of parameters.

To look at a straight descending side slip such as one might normally do at the end of the final turn. One wing is low and it's full area is presented to the airflow. The other wing is partly blanked by the fuselage. To maintain a constant bank angle another force must be present to balance the different amounts of lift each wing produces. This is, of course, the reason for the aileron in the opposite direction to the rudder input.

This is, perhaps, where the discussion really gets interesting. The aileron then, increases the angle of attack of the one wing and reduces that of the other.
You may prefer to say that the aileron increases the lift co-efficient on one side and decreases that of the other but then one must consider the three different lift co-efficients (that of the plain sections of both wings and those of the two parts of the wings where the ailerons are fitted) and their respective maximum co-efficients of lift.

This complicates the discussion unnecessarily for the purpose of the stated argument as when the pitch angle is increased (up elevator), the wing stalls from the outboard aileron on the side of the applied rudder.

Regarding your discussion of the wisdom/safety it should be noted that the most common error in performing a slip is the failure to raise the nose enough which results in a higher speed and consequently lower angle of attack.
Quite alarmingly high rates of descent can be achieved but are also quickly recovered from by centralizing the aileron and rudder.

A thoroughly safe and extremely useful technique and an essential one for many taildragger types with limited forward vizibility in the landing attitude.

ft
26th Jan 2004, 19:04
As you slip, the air streams around the fuselage above and below it.

In a high-wing aircraft, this will mean that the air that went around the bottom of the fuselage will stream up against the leeward wing, increasing the AoA of that wing and increasing the lift, thus rolling the aircraft away from the slip (which usually means towards level flight).

This is the reason for high-wing aircraft being inherently laterally stable to a larger degree than low-wing aircraft, where the air will be impacting the wing after having gone over the top of the fuselage, decreasing the AoA of the leeward wing.

If you have dihedral on the wings, a slipping turn will increase the angle of attack of the windward wing, as the air coming in from the side has a component perpendicular to the wing going up. That is why dihedral provides increased lateral stability. The opposite is of course the case for a wing with anhedral.

Cheers,
Fred

John Farley
26th Jan 2004, 20:19
mstram

If you want to see why the AOA increases on the leading wing during a sideslip then take a sheet of A4 paper and fold it in half (short side to short side) crease the fold and then open the sheet to represent a wing with a modest dihedral angle. Hold the paper up in front of your face looking along the crease line as if the dihedral wing is flying towards you. Tip the crease line slightly leading edge up so that you see the underside of the wing. Note you see the same amount of the bottom of both surfaces. Now apply a slip angle and notice how you (the oncoming airflow) now sees the bottom of the leading wing but the top of the trailing wing. This asymmetry in AOA has come from the geometry of the dihedral angle.


As has been said there is a bit of work needed to feel at home in the air and be quite confident that you know what you are letting yourself in for when it comes to using slipping turns on finals..

The following may help (sorry if it is a bit basic)

1 At a safe height start with a straight descent in the landing configuration at your typical approach speed. Trim it and let go of the wheel or stick.

2 Smoothly apply some rudder. If the aircraft changes its bank angle then the aircraft has dihedral effect (whether or not the wing has dihedral angle).

3 If the roll is in the direction of the applied rudder then the dihedral effect is said to be positive (which is normal on certificated aeroplanes) and the roll arises from an imbalance of lift resulting from an increase in the AOA of the leading wing and a decrease in the AOA of the trailing wing. (as you saw with the paper)

4 Counter the roll with aileron and adjust the bank until your heading stops changing. You are then said to be doing a ‘steady heading sideslip’ beloved of certification and flight test communities as a means of gaining data about a variety of subjects. You will now need a lower nose position to maintain your original IAS due to the drag of going sideways. Thus a sideslip is useful as a means of increasing your descent rate without increasing speed as well as improving your view in the direction you are travelling (not the direction you are pointing).

5 The significance of doing all this in a turn (sticking on rudder and countering the roll with aileron) as opposed to straight flight is that for a given airspeed the AOA values of both wings will be higher because you are turning. This means you are closer to the stalling AOA than in steady heading flight. Just as you are in a balanced turn when compared to balanced straight flight if both are at the same IAS.

6 If you fly slow enough or turn tight enough or a combination of both you will stall any aeroplane. Doing this at a time you have rudder applied turns this stall approach into a spin entry manoeuvre.

FlyingForFun
26th Jan 2004, 21:02
John F,take a sheet of A4 paper and.....One thing which I have always had a huge amount of trouble with is visualising three-dimensional angle-of-attack problems. Thanks for such a simple solution! :ok: Hoping this technique will work for other problems.... but I can't think of another suitable problem to try it on right now!

FFF
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Field In Sight
26th Jan 2004, 23:02
FFF,

I had exactly the same problem visualising why the AoA increases with sideslip. Even on my FIC the explaination didn't give me that warm fuzzy feeling inside of understanding.

Then I read the book "Stick and Rudder" which uses a view from infront of an aircraft at a slip angle to demonstrate the same thing as a the A4 paper method.

It's what I will use from now on for explaining it to students.

I work on the principle, If I can explain it withought me feeling like I am bullshi%^ing :} I understand it.

FIS.

Intruder
27th Jan 2004, 00:28
A simple way to look at is that in a slip you are flying less efficiently. Therefore you will need a higher AOA to maintain the same speed.

sycamore
27th Jan 2004, 02:44
A nd to carry on from John`s visual aid, if you turn the paper upside down you can see the effect of Anhedral, Di`s Welsh sister...!

Just a couple of point`s to add; be very careful if you do any prolonged, or extreme sideslips with flap down, as you may find the nose suddenly drops, stick comes back, and the a/c may flick, as the effect of flaps may blank off the fin/rudder;;

Also, if its a twin with fuel in the wings, the fuel outlet on the inside wing may become uncovered, and it may stop the inboard engine.....This may well be somewhat eye-watering, and require a change of underwear upon recovery....!!!! :ooh: :ooh:

mstram
27th Jan 2004, 12:24
Thanks for all the replies.

When I have used the slip I have always maintained at least 1.3 (more like 1.5 probably) Vso, (judging by control response / wind noise more than ASI), and not more than a 25-30 degree bank. I've been taught that those are "safe"parameters. (172's primarily, but also in gliders lately (1-26's and 2-33's so far).

I've done deliberate spins in a 172, that required somewhat "artificial" inputs (though they could very well be produced by the unwary), i.e. full back-elevator / full rudder all with no-flaps extended. Have also done a "skidding" turn/spin in a glider and the subsequent recovery.

I *haven't* done much practicing on slipping / excesive-bank/ low airspeed leading right up to the stall. I *will* do that the next time I go flying (Next summer ? .. -12 -> -20C here right now !)

Mike