What happens when you put vortex generators (VGs) on both sides of a symmetrical vertical tail?

I understand a horizontal tail with a classic foil like a NACA 0012 has a max cl of about 1.5.

I understand VGs add about 0.6cl.

What happens when you put VGs on both sides of the vertical tail?

For sure it will allow lower speed or better control at lower speeds, but will it actually bump the max cl from 1.5 to 2.1 at positive and negative angles of attack?

That's a 40 % increase in performance!!!

So.... can I decrease my vertical tail by roughly 40% and still get a similar EFFECTIVE vertical tail volume ( vertical tail moment)?


Just because VGs can delay the flow separation (with no guarantee of success), it doesn't mean you have increased your tail volume. In fact, you have the exact same tail volume unless your vertical tail is separating at small sideslip, which probably indicates bad design or bad manufacturing.

If the VGs do increase the max Cl, it means you may be able get to a higher slip angle, perhaps more crosswind capability if your rudder is up to the task.

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  • $\begingroup$ I'm not sure adding Vg's to both sides of a vertical tail will increase the cL at all since you're effecting both sides at the same time, and not increasing differential pressure or increasing the velocity of only one side, no? $\endgroup$ – Fred Jul 22 '19 at 19:04
  • $\begingroup$ @Fred VGs do not modify the camber, so I don't see how it can increase the lift slope. VGs serve to energize the boundary layer and delay flow separation. $\endgroup$ – JZYL Jul 22 '19 at 19:20
  • $\begingroup$ I understand Vg's can act the same as a slat on a wing with a NACA2514 foil or similar. The Zenith 701 uses a slat. A copy of the Zenith 701, the Savanah, uses vg's instead of slats for less drag and higher cruising speeds, or do vg's just allow for a higher AoA ( therefore higher cL)? $\endgroup$ – Fred Jul 22 '19 at 19:26
  • $\begingroup$ @Fred I don't have the wind tunnel data or the flight test reports, so I can't surmise on the particular design. However, if there's an area of separation on the wing, especially on flaps down at nominal AOAs, VGs have the potential to reattach the flow and decrease the overall drag. $\endgroup$ – JZYL Jul 22 '19 at 19:43
  • $\begingroup$ @ jimmy Vg's do allow for a higher AOA, and therefore a higher climax, as per the Zenith design school: zenithair.com/stolch801/design/design.html. Vg's don't change the slope of the lift slope, but rather extends it. $\endgroup$ – Fred Jul 29 '19 at 15:52

It doesn't work that way on vertical tail surfaces. There aren't really issues with LE stall on rudders that you need to delay.

My old '68 Cardinal had a STOL kit that added VGs to the vertical fin. They aren't mounted at the LE like a wing, to delay separation at the LE, they are installed just forward of the hinge line to keep flow attached to the rudder itself over a wider range of movement than without. What you got from it was a bit more rudder effectiveness at the lower speed the plane was capable of because of the wing mods. It doesn't allow you to make the surface substantially smaller, which will degrade your yaw stability.

You see that on most airplanes where VGs are added to vertical surfaces. They are almost always just forward of the hinge line.

If you really want a much smaller vertical surface, you can do an all-flying surface like Zenith, but you still need substantial fixed side area for passive weathervaning. The Zenith does this with large slab sided fuselages.

However you could get good natural passive weathervaning with an all flying surface by using an anti-servo tab on its trailing edge.

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  • $\begingroup$ How does adding a anti-servo tab on a all flying horizontal elevator add stability? I assume it acts as a slatted flap and adds somewhere between 0.7 and 1.3 cL. $\endgroup$ – Fred Jul 22 '19 at 14:52
  • $\begingroup$ An anti servo tab works to aerodynamically fix a stabilator surface at a given incidence by creating a counteracting force to pitching moments in the surface and results in stick free stability like having a fixed stabilizer. Trailing edge goes up, anti-servo tab goes up more, providing a counteracting moment. At a certain spot it's all balanceed.The sizing and gearing of the anti-servo tab to the movement of the stabilator determines the overall pitch feel force characteristics,and the gearing can be mechanically biased to change the speed at which equilibrium is found,giving you pitch trim. $\endgroup$ – John K Jul 22 '19 at 17:06
  • $\begingroup$ If you put an anti-servo tab on an all flying rudder like a Zenith's, it would just be centered with the rudder centered and if the airplane yaws the rudder will want trail, displacing it from neutral, and the displacement makes the anti-servo tab move in the same direction, tending to drive the rudder back to neutral, and this makes the all-flying rudder tend to act as a fixed fin. Not sure how well it works in practice, although in pitch they are quite effective. $\endgroup$ – John K Jul 22 '19 at 17:11

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