At high angle of attacks, the prominent problem is the separation of airflow. And if this airflow separation is near ailerons, we lose lateral controllability.

And the solution is making the flow turbulent.

By adding slots in a wing at correct positions, we are capable of re-energising the flow. It is the same thing a vortex generator does (I presume).

But what device is best suited for the current case?

From an unknown source, I found:

At high angles of attack, the airflow over the ailerons can be separated from the surface. Vortex generators, positioned just forward of the ailerons, are designed to re-energise the boundary layer and help to prevent this separation. This makes the ailerons more effective at high angles of attack

(I do not how effective this reason is.)

And therefore, please consider the case of Leading Edge Slats (which creates a slot) too. At a high angle of attack, a LE slat helps by re-energising too. And If we have a turbulent flow (by employing LE slats) in the early stage, (I think) it is better than the case where we put vortex generators near ailerons, which may encounter an already separated flow.

Edit: The aspect of lateral controllability dictates the conditions where one wing can be at higher AOA than the other. Will Slats not fail to provide the desired effect due to its symmetrical effect (increasing CLmax simultaneously for both and stalling one of the wings and making into a spin)?

Does the "symmetrical effect" apply to VG and slots near the aileron too?


2 Answers 2


Vortex generators can be used at the leading edge to energize flow and delay separation. They are widely used as an add-on device to improve low speed behaviour of a range of light aircraft, and in that usage they have a similar effect to a slot or slat in that they both increase stalling AOA and improve aileron response in the stall. I call them "poor man's slats".

However, slats are able to energize the boundary layer to significantly higher angles than leading edge vortex generators. As an add-on, VGs are great, but if your objective was to maximize AOA at stall, you would go with a slot or slat. A VG might raise stalling AOA from 15 to 19 or 20 degrees, whereas a slat/slot is good to the mid 20s. Similarly, a VG can increase a wing's Clmax by maybe .3 to .5, whereas a slat/slot adds about 1.

To answer the question specifically, for maximum benefit for lateral controllabilty at the highest AOA possible, you would use a slot/slat. VGs would be a second choice, unless your objectives were more modest, in which case if they do the job, they would be a much cheaper and simpler solution.

There was a period back in the 40s when leading edge slots in front of the ailerons were popular as a way to maintain aileron authority through the stall (Globe Swift, Stinson, etc). Later, designers just included sufficient wing washout to achieve the same goal without the manufacturing hassle and drag, and fixed slots disappeared.

Modern transport airplanes and specialty STOL airplanes will use slats across the full span to increase stalling AOA and maximum lift the wing can make, as opposed to just improving aileron control. Same with leading edge VGs, although the benefit is only about half. On light twin engine airplanes, VGs have miraculous benefits for single engine performance.

  • $\begingroup$ To me, the biggest difference between VGs and slats is that the former are always added later as quick fixes for some deficiency whereas the latter are intentional parts of the design from before the first metal is cut. $\endgroup$ May 8, 2021 at 5:17
  • $\begingroup$ I think the aspect of lateral controllability is missing in this, where one wing can be at higher AOA than the other. Will Slats not fail a little due to its symmetric effect? $\endgroup$ May 8, 2021 at 10:41
  • $\begingroup$ @PeterKämpf that's pretty much what I meant by add-on device but yes in general. A few homebuilts like the Savanah do go with VGs as a slat-alternative to avoid the complexity/weight/cost/drag of a fixed slat. Once I was building (never finished) a Pegazair homebuilt, making wing parts, and I didn't want to fabricate its fussy automatic slat system, so I decided to leave it out and use VGs with a reprofiled LE. I'd estimated that a Clmax increase of .4/5 instead of 1 would only raise the minimum speed from 28 to about 32 mph, a small enough penalty to make it worth leaving them out. $\endgroup$
    – John K
    May 8, 2021 at 12:27
  • $\begingroup$ With the Swift ultralight foot-launchable sailplane, you literally "add on" the vortex generators ("vortilons"), mounted on the leading edge, in front of the elevons, one on each wing) every time you assemble the aircraft-- they velcro on-- $\endgroup$ May 8, 2021 at 14:28

Nature provides the perfect answer with soaring bird's slow flight wingtips, as seen with condors and eagles, the slot.

Leading edge devices do increase Coefficient of Lift and do increase stall Angle of Attack, but placing the slot directly in front of the aileron effectively makes it a separate 2nd wing with its own airflow.

This Junkers design was first seen in the 1920s and worked well to improve low speed roll control in passenger aircraft such as the Ju 52. The slotted aileron/flap can extend the entire length of the wing.

As with other high-lift devices, they do have a drag penalty at higher speeds, which is why the albatross keeps a single tapered wing tip. Modern airliners are able to retract their leading and trailing edge devices for the same reason.


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