Aileron control in flow separation...and elevator

[pstaney]

I’m trying to get an appreciation of the relationship between max Cl and the amount of flow separation one can expect. For example, consider a wing with a constant chord, camber, and no wash out. Omit any external influences on the airflow (no fuselage, tip effects etc) and consider low speed flight to minimize compressibility effects.

Question 1; at maximum Cl, is there a general range of flow separation over the aft portion of the chord? Not looking for exact numbers, of course, but a number that can be considered reasonable. For example, aft 10-20% could be in separated flow.

Question 2: if there is some separated flow, then is it reasonable to expect for the wing conditions mentioned above (and at max Cl), that an aileron would experience a “snatch” to the up position?

Question 3; if the wing was at or above the stall angle of attack and the aileron is indeed “snatched” to the up position, is there any reason to believe that applying the aileron to the DOWN position would itself help RE ATTACH the flow (un stall the wing)?

These questions may sound strange, but the answers WILL lead somewhere, and I look forward to explaining further once I get some understanding of the above. Thanks for the replies, looking forward to all your thoughts. Stan

[SNS3Guppy]

Pstaney,

Typically an airfoil experiences airflow separation by 25-30% chord, to some degree. So-called laminar airfoils experience it around 50%. A properly designed airfoil (which opens a very wide range of possibility in interpretation) shouldn't be experiencing any "aileron snatch" at CLmax. None.

Lowering the aileron will increase the angle of attack by increasing the angle of the chord line. Remember the chordline runs from the leading edge to the tailing edge, and lowering the trailing edge also lowers the chord line, or increases the angle of incidence...airflow remaining relatively unchanged in the free airstrea, angle of attack is therefore increased.

At the same time, you're also changing camber by lowering the aileron, which alters the coeficient of lift, too. CLmax will occur differently for that wing station.

Raising the aileron, generally speaking, decreases lift and also alters the coefficient of lift, generally decreasing it, and also decreasing drag.

How these affect an airplane really depends on the specific airplane. Applying aileron close to a stall may be helpful or hurtful in roll control, depending on the specific aerodynamics of the aircraft in question.

[pstaney]

My generalized wing was not meant to have any specifically designed aerodynamics to prevent aileron snatch, nor enhance laminar flow. For example, a simple NASA 2412 airfoil. For airflow separation, I wasn’t thinking necessarily about non laminar flow, but more along the lines of when the angle of attack is at the stall, so that neither the turbulent boundary layer nor any of the remaining laminar flow can actually stay attached to the complete upper surface.

For example, past pictures of a “stall” sometimes show tufts of wool or cotton streamlined back on the upper surface, but for those near the trailing edge I think they were no longer “streamed”, and were often even pointing forward. I don’t have a reliable source for this, so I’ll try to make this question a bit simpler:

Would it be correct to say that GENERALLY for the simple situation I posted originally, if this wing is at an aoa that it is just stalled, can we expect some flow separation on the aft 20% OR SO of the chord, and hence cause an up force on the aft section of the chord due to pressure differential between surfaces?

Thanks for being the first responder Guppy. I’m not sure if you were answering YES, or NO, to my questions.

Stan

[SNS3Guppy]

Stan,

As I said before, you've already got airflow separation long before you start moving toward CLmax. In normal flight, airflow separation is occuring closer to 30% chord. You never have laminar flow, and always have some degree of separation, aft of about 25-30% chord, or as far back as 50% chord on laminar flow wings. never mind the last 20%...that's always in some state of separation in varying degrees. As you approah to higher and higher angles of attack, the evidence of this separation increases, but it's always there.

Would it be correct to say that GENERALLY for the simple situation I posted originally, if this wing is at an aoa that it is just stalled, can we expect some flow separation on the aft 20% OR SO of the chord, and hence cause an up force on the aft section of the chord due to pressure differential between surfaces?


I'm not really sure what you're asking. What up force...are you talking about airflow causing suction causing an upforce? Are you talking about the disparity between the center of pressure on the upper surface of the wing vs. the lower surface? Airflow separation, which isn't exactly the best description, isn't sucking up the aileron or applying an upward force (Lift) on the aft portion of the wing. It isn't picking up the aileron. It's turbulent airflow. A better way to think of it is loss of lift, rather than lifting anything up.

When the airflow separates, it stagnates, and the local flow velocity drops. When it drops, pressure rises, and what you have is an increase in pressure, as well as burbling air, aft of the flow separation. It's not a vacum back there, it's just not benifiting you.

If you've seen vortex generators, these serve the function of not separating airflow, but injecting high energy air into areas where separation and loss of lift is occuring. This can increase lift by keeping energized air (and lower pressures due to increased velocities) closer to the wing/boundary layer, as well as put unstagnated airflow around the controls surfaces.

Not trying to be difficult with your question...I just don't understand exactly where you're going.

[pstaney]

When the airflow separates, it stagnates, and the local flow velocity drops. When it drops, pressure rises, and what you have is an increase in pressure, as well as burbling air, aft of the flow separation. It's not a vacum back there, it's just not benifiting you.

So…how can an aileron do it’s job in just such a situation you describe? With flow over the aileron separated long before CL max, as you say, and then if the pilot provides an input to deflect the aileron to the up position, how does this separated airflow provide a downforce on the wing?

Again, not talking of any specific wing, just an airfoil with these characteristics. And I appreciate your replies, I’m just trying to better understand the forces that could be present on flight controls at, or just past the stall. Maybe I should have just asked that instead.

[hawk37]

I didn’t want to post because I’ve no real knowledge in this, but here I go.
I would say that yes, an aerfoil can expect to have flow seperation over the rear portion of the chord when it is close to the max CL point. Even though there is no meaningful contribution of lift from this part of the upper surface, the lower surface is still providing lift along the whole chord.
The snatch that you refer to would not happen because the ailerons are linked. But there would be a force on both ailerons trying to force them up, but since they are linked, one will not go up and the other down. This force is from the positive pressure on the underside of them. I seem to recall that both aelerons will float up in flight for the reason that the resultant force on them from upper and lower aerfoil pressures is up.
The response from SNS3Guppy deals mostly with laminar flow, when he says flow seperation often about 30% chord. I’d have to say that perhaps it’s laminar flow changing to turbulent flow within the boundary layer at about 30%, and what you say I agree with. The laminar flow usually degrades to turbulent flow, which is not actual seperation. In fact the extra energy in the turbulent flow helps keep the flow attached to the upper surface. And as said, this is why boundary layer energizers are often added to wings, and in fact the slots in flaps do the same thing by injection of hi energy air. I do not think the flow over these flaps are laminar, by someone can correct me. Once this turbulent flow can no longer stay attached, the flow seperates, and you get the tufts on an upper side of a wing start to steam backwards in places, as you described.
Now you ask how an up aeleron can provide a down force in separated flow, I will really go out on a limb here, and say that it can’t. It’s the aeleron on the other side that goes down and raises it’s wing.
And now…..this is from my general knowledge….so take with a grain of pepper somewhat!!
Hawk