For the same reason they don't go on the underside of the wing, there's nothing for them to do there.

Ridge Runner wrote:

senior wrote:Next question would be, why wouldn't you put them on top & bottom of stab??

I'm equally interested in this question, sent out a few email inquiries but no replies yet.

Because the horizontal stabilizer is an upside down wing, more or less.

Seems odd that a horizontal stabilizer that is equally flat on both sides (that doesn't create lift due to the shape of the airfoil), like on a Kitfox, Rans S6 or RR, would be affected by a VG at all.

Is it not that the VGs causes the air flow to stick to the elevator.
If you notice Vgs are on the rear of the stab about where the elevator hinges as opposed to the front (as in the wing).

Ridge Runner wrote:Seems odd that a horizontal stabilizer that is equally flat on both sides (that doesn't create lift due to the shape of the airfoil), like on a Kitfox, Rans S6 or RR, would be affected by a VG at all.

Correct me if my line of thinkin is wrong.
EG. when elevator is in the down position, it is actually lifting the tail due to the positive pressure from air flowing under the stab & pushing it up. It then makes sense that if you make the air stick to the top of the elevator as it leaves the stab it would create lift.

senior wrote:Correct me if my line of thinkin is wrong. EG. when stabilier is in the down position, it is actually lifting the tail due to the positive pressure from air flowing under the stab & pushing it up. It then makes sense that if you make the air stick to the top of the elevator as it leaves the stab it would create lift.

I think you may have meant "when the elevator is in the down position....." ?? then yes, it is lifting the tail because the air pressure from under the stabilizer being redirected downward is greater than the gravity acting on the weight of the tail section. As far as I know, the airfoil of the horizontal stabilizer of a small plane like the Kitfox, S6 or RR is just flat on both sides, like a piece of plywood. There's nothing much there to produce lift the same way a main wing produces lift. If I am wrong on this count, then I ask for forgiveness in advance and welcome correction.

On a curved surface, like the curve between the stabilizer and elevator when in the up position, I can see how VGs would work when going at a slow speed to keep the air attached to the upward angled elevator (like when coming in for a short 3 point)....

Image of a Savannah's horizontal stabilizer

image of the RR with flat horizontal stabilizer, it's kinda small and thin, hard to see I guess

But what about when trying to get the tail up at take-off, when the elevator is in the down position? Would VGs on the upper side of the stabilizer be beneficial for getting the tail up earlier?