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Sailplanes, among others, use a cotton string to show the sideslip angle. I've often heard that while circling in thermals the cotton string should show a slight slip angle to climb faster. This effect can be observed specially at high bank angles with heavy, high span gliders like the ASH-25. The german skid Wikipedia-article has even a subsection about it; however, without references. Flying like this has also the advantage of lessening the stall risk of the inner wing.

Is the indicated sideslip angle of the cotton string real or only apparent and due to the uneven incident airflow? Has this effect ever been scientifically measured and is it used in larger airplanes with FBW and sideslip angle sensors while turning?

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  • $\begingroup$ When the string is centered, it is an indication that you are NOT slipping (but rather, flying coordinated). By not slipping, you reduce drag. By reducing drag, you can climb faster, because less energy is wasted generating drag. $\endgroup$ – J. Hougaard Sep 11 '16 at 9:51
  • $\begingroup$ @J.Hougaard That is the point:it is said that large gliders climb better when the string is not centered. I would like to know if it really is like that and if someone has measured it. $\endgroup$ – Gypaets Sep 11 '16 at 9:56
  • $\begingroup$ Ah I see. I misread your question. I can confirm that I was taught the same thing when obtaining my glider license, but I did not question the scientific background :) $\endgroup$ – J. Hougaard Sep 11 '16 at 10:00
  • $\begingroup$ @J.Hougaard Feel free to edit if the question is not clear enough. $\endgroup$ – Gypaets Sep 11 '16 at 10:02
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    $\begingroup$ @J.Hougaard: Actually, you are not slipping if the string points slightly to the outside of the turn. Being ahead of the cg, it will show the local sideslip angle (and even exaggerate it quite a bit). This effect is very pronounced in two-seaters like the ASH-25. $\endgroup$ – Peter Kämpf Sep 11 '16 at 20:37
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Source: Wikipedia.

Origin of the theory

Klaus Holighaus, one of the famous glider pilots of the 70s, and himself a glider designer at Schempp-Hirth, is at the origin of a controversy when he recommended not to turn with the ball centered, and especially when climbing in thermals. Mr Holighaus did see two problems with the perfectly coordinated turn:

  • It doesn't reduce drag as in straight flight.
  • It reduces the aircraft margin for roll stability.

I'm not an expert in the field, so I'll just rely on the article by Richard H. Johnson published in Soaring Magazine and titled "Do you really want to keep the yaw string centered?"

Before going into Klaus Holighaus' theory, let have a look at lateral stability.

Roll stability: Dihedral angle

Unless you want a very agile aircraft, an aircraft must have positive stability on the three axis, i.e. if the aircraft straight and level attitude is unsettled by some event, the aircraft must have a built-in tendency to cancel the effect of the perturbation and to return to its initial attitude.

Roll stability is required to prevent the roll angle to increase automatically after a small unwanted roll input by the pilot or by a cross wind gust. The aircraft must return to a null roll angle automatically to prevent nasty consequences, including entering a spiraling descent.

Roll stability is mostly provided by the dihedral angle of the wing as explained in How does the dihedral angle work? In short: The dihedral angle uses the sideslip to create an opposite roll motion.

No sideslip: No dihedral effect

We know turning is not controlled by the rudder but by rolling the wing with the ailerons. But at some point the lowered wing aileron may also be deflected downward for turn rate control, and this deflection may happen to be excessive:

When the lowered wing's aileron is deflected downward, not only is its lift increased, but also its drag is increased, and a skidding turn will be induced.

At the same time the lowered wing, being closer to the turn center, is flying slower than the raised one. Slow speed, high angle of attack and more drag for the aileron can induce a stall: If the lowered wing or its aileron stall, drag on this side is increased and the aircraft is likely to enter a dangerous spiral.

However in most of the cases we can stop skidding:

The skid can easily be corrected for by adding some top rudder to keep the skid ball centered.

The solution implies to add drag to the rudder, which is not satisfactory from a performance perspective.

Get the best of both worlds

This is where the mild-slipping turn theory comes from: By turning with a slight slip instead of a perfectly coordinated turn:

  • We keep the dihedral benefit to stabilize the aircraft around the roll axis. This is safer.

  • The negative roll moment provided by the dihedral allows us to use less ailerons deflection, so less drag and more speed, and better climb in the thermal.

  • We use less rudder, so even less drag, and even better climb.

The article's author adds this explanation about improving lift with the sideslip:

Why slipping improves the climb in some gliders is not well understood. In generating lift, wings also generate spanwise flow, outward along the bottom and inward along the top of the wings. It is possible that there is flow separation near the tip of some circling sailplanes, and that slipping changes the spanwise flow enough to eliminate or reduce this. Separation can increase the drag significantly long before the tip actually stalls.

The article also enter in the details of why this is different when winglets are used on the wing. Slipping may induce a stall instead.

How much sideslip?

The optimum degree of sideslip depends to some degree on both the sailplane's wingspan and dihedral angle. After many hours of flying my 16.6 meter ventus a and similar sailplanes, I find that my best overall circling performance and handling characteristics occur while the canopy mounted yaw string is deflected about 10 degrees on the high side of the turn (a gentle sideslip actually).

Yaw string vs ball

The yaw string is also more accurately named the slip/skid string, as it doesn't indicates a yaw angle but a quantity of skidding/slipping, like the ball.

As @ymb1 commented, the string being on the canopy, ahead of the CG, its indication is exaggerated.

In a coordinated turn, the ball of the slip/skid ball indicator will be centered while the yaw string will be already indicating a slight amount of slip. This error will be magnified with actual slip.

The author estimates the actual degree of slip in his case is more accurately about half the indication of the string. See also Peter Kämpf's comment: "exaggeration is also caused by the fuselage shape [...]".

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    $\begingroup$ The yaw string exaggeration is also caused by the fuselage shape: Crossflow angles are higher than the actual sideslip angle. This and the location ahead of the cg will let it show at least twice the actual sideslip angle. Bonus point for using a Schempp-Hirth cockpit in the photo! $\endgroup$ – Peter Kämpf Sep 11 '16 at 20:34
  • $\begingroup$ That's a great answer! I won't, however, accept it because it doesn't answer if the airplane is really slipping. $\endgroup$ – Gypaets Sep 13 '16 at 15:13
  • $\begingroup$ @mns: "it doesn't answer if the airplane is really slipping": Of course it slips, that's the key point and clear in the answer, e.g. "By turning with a slight slip instead of a perfectly coordinated turn", "How much sideslip?" and "the actual degree of slip in his case is more accurately about half the indication of the string". $\endgroup$ – mins Sep 13 '16 at 17:53
  • $\begingroup$ @mins We can't rely on the string because it is ahead of the c.g. and will show an apparent slip even if we're coordinated... don't we have such an effect on the turn-coordinator too? Perhaps I'm being too nitpicky because I would like to see measured data instead of 'about half...'. If you ask other pilots who fly the same airplane I'm pretty sure there will be some saying 'about a fourth...' $\endgroup$ – Gypaets Sep 14 '16 at 5:43
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    $\begingroup$ @Gypaets -- just came across this old answer and associated comments-- one thing that seems to have been missed in the comments, was that the Holighaus article specifically recommends leaving the slip-skid ball about half a diameter off-center. The reading of the sllp-skid ball is not dependent on its fore-and-aft location in the fuselage. So yes, the aircraft really is slipping when flown as recommended in the article. Obvious the savings in sink rate is very hard to measure. It would be interesting to see data from controlled tests in still air, descending through thousands of feet. $\endgroup$ – quiet flyer Jun 25 at 11:59

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