Vortex generators

[Rynoth]

In level flight in designed CG range - when the aircraft flies slowly and stalls - what SHOULD happen is that the horizontal stab stops providing enough down force to keep the nose up because it stalls or at least provides not enough "down lift". The tail rises and the nose drops and the plane accelerates out of a stall.

So anything that keeps the horizontal stab flying "better" and providing more efficient down force - compromises your ability to have the nose drop and fly out of a stall.

I think you have this backwards. A typical stall is loss of lift at the main wing, which causes the nose drop. Coincidentally, a cross controlled-stall (wing drop) is one main wing stalling before the other main wing. In neither case should the horozontal stab be stalled.

A horoz stab stalling prior to the main wing is a tailplane stall, and is generally considered bad, particularly when caused by icing. Also things like putting a turboprop engine into beta or reverse in-flight can "blank the tail" causing a tailplane stall which can also be bad. The tail is essentially "flying" the main wing, and its better to lose lift on the main wing and retain some pitch control versus losing (downward) lift on the tail and losing all pitch control.

[sonex1374]

The main wing is going to stall at whatever its critical angle of attack (AoA) is (based on its airfoil characteristics), and that happens irrespective of the horizontal tail (consider how a flying wing can stall even without a tail). The horizontal tail's job is twofold: counteract the airfoil's pitching moment (again, a characteristic of each airfoil) and command a desired angle of attack (within the capabilities of the horizontal tail).

If the tail is "effective" (meaning it has the ability to create sufficient lift in either direction to do it's two jobs), then it can adequately command an AoA sufficient to stall the main wing. If the tail is ineffective or marginally effective, it might not (e.g. the Ercoupe "stall-proof" tail).

Equally important is the ability to command AoA while the main wing is stalled. If the tail is ineffective during this flight condition (such as flying in disturbed or "dead air" coming off the main wing like with a T-tail), you might not be able to command a lower AoA necessary to un-stall the main wing. This deep stall condition might be unrecoverable.

In both of these cases, having a tail that is sufficiently effective is important to safety. Even though more tail is better from a control stand point, as with everything there is a trade off. Carrying around more tail than you typically need is draggy, heavy and wasteful, so designers minimize the tail effectiveness to only what they believe is required.

Vortex Generators (VGs) are one of several ways to make the horizontal tail more effective, and VGs have advantages: they're simple to apply, lightweight and low drag. Compared to some other methods of increasing tail effectiveness, they're a pretty good deal. Whether you need them or not is up to you and your demands on the plane.

On final thought is that increasing tail effectiveness might pose some unexpected risk. By increasing the effectiveness you might notice a sharper stall break because the tail can drive the main wing into the critical AoA quicker; you might be able to command a deep stall condition that you couldn't previously; and you might be able to generate greater loads on the tail as a result of improved effectiveness. These are the things that should be explored in flight testing, but I wouldn't tend to think they're likely a problem in a Sonex.

Jeff

[Rynoth]

Just to beat a dead horse, the fact that releasing backpressure (bringing the stick forward) during a stall shows that the tail is still effective (unstalled) in this condition. Stall recovery is (typically) not a matter of un-stalling the tail, it's lowering the angle of attack on the main wing.

[Sonex1517]

Yep, next time I’m posting about religion, politics and sex....yep.....

[WesRagle]

Hey Robbie,

Yep, next time I’m posting about religion, politics and sex....yep.....

The discussion is fun :-)

I have wondered if adding VGs to just the outer wing panels of a Onex would have much the same effect as the "wash out" on some certified aircraft. Might be able to get a pronounced pre-stall buffet while still having full aileron authority. Don't know.

Wes

[vernd]

Along this line of thinking just wondering if anyone has tried small wing fences on the wing tips.

[Area 51%]

The tail stalling prematurely on the Cessna Cardinal is why they introduced a slotted stabilizer in the later models.

One disadvantage of a flying tail, like the Cardinal or Cherokee - it's a straight airfoil. When you have an elevator you are essentially getting a highly cambered airfoil when you have elevator deflection.

Don't tell the "Raptor" pilots about this. They think their airplanes are pretty hot stuff.

[Bryan Cotton]

Don't tell the "Raptor" pilots about this. They think their airplanes are pretty hot stuff.

They have vectored thrust. Essentially they are cheating.

[509sx]

With an AR=5 or so Hershey bar wing, with no twist, Otto Shrenk says the spanwise lift distribution will be midway between an elliptical distribution and the chord distribution. The lift falls off as the tip is approached. I would not expect VGs to be needed out near the tips, since CL never gets near stall there.
On the airfoil section 64-415 (if I'm not mistaken). Although this is a NACA laminar flow section shape, the roughness and waviness requirements to actually support laminar flow are not easy to achieve. Furthermore, the pop-rivet rows act as roughness elements, the first of which will cause turbulent wedges with about 15-deg half-angle to form, chewing into any laminar flow achieved. Of course, bug splats on takeoff and landing also create turbulent wedges, so unless the skins were formed very carefully, and all rivets countersunk and filled, and the paint very smooth, and no bug splats, it's unlikely that we are actually achieving laminar flow on our wings. The good news is we don't have to be nutzo about keeping them clean, since there's likely nothing lost if they're a little dirty.
I was part of a small Boeing team of aerodynamicists that put laminar flow technology on the tailfeathers of the 787-9 and -10 airplanes, and worked laminar flow technology for Boeing and MDC for a couple of decades.
Art
Sonex 509 TD Corvair

[GraemeSmith]

OK - back to the dead horse......

Before I post pictures, video and some numbers can someone please tell me:

Legacy Sonex
Well built to plans and true wings

Is there any? Or should there be any? " Washout" or twist so the wingtips and aileron's stall after the wing roots?

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Save me going to the firesafe at the office and digging through the drawings I got with the plane

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I'm thinking the wings are true flat Hershey bars from eyeballing and some measuring - but something as small as a half degree would be hard to measure out in the tie downs with the tape blowing in the breeze.

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TIA - then I will share some test flight thoughts.