Question on Anhedral and airflow

[kamrav]

I need some help if possible, please

With regard to trailing edge vortices and wake turbelence, textbooks all indicate airflow on the lower surface of the wing moving towards the wingtip, and airflow on the upper wing surface moving towards the root, thus forming a vortice. There is also the pressure differential to consider when looking at the whole effect. However, almost all the sketches show wings with dihedral.

There is always the possibility that it is just pure coincedence, but another pilot almost convinced me that when standing in front of a airplane with anhedral, the airflow on the upper and lower surfaces might be the opposite as to the abovementioned.


Any help will be appreciated. Thanks

[Field In Sight]

C17
They look like winglets to me.

FIS

[bookworm]

You're too easily convinced, kamrav!

The anhedral/dihedral doesn't make a difference to the direction of circulation if the wing is producing lift.

[avoman]

Air escapes around the wingtip from the high pressure area below to the low pressure area above. This distorts the airflow in the directions noted relative to the chord. This is a continuous process as the airfoil moves forward through the air thereby producing the spiral vortex. Anhedral etc is not relevant.
I suppose the vortices must be reversed relative to the airframe in inverted flight!

[kamrav]

FIS, thanks for the photo

Bookworm and avoman, thats exactly why I asked the question, I was almost convinced, not completely, and this forum seemed like the best place to get the best answer, even if I had to ask a stupid question.

Thanks again

[Dick Whittingham]

Just to add a bit of confusion: It is generally true that the boundary layer flow is outboard on the bottom surface of the wing and inboard on the top. Swept wings, however, bring in another factor. In the area behind the point of max curvature, where the pressure is rising as you move aft - the adverse pressure gradient - the pressure changes push the boundary layer outboard.

This is interesting in that because sweep brings high lateral stability swept wings are often combined with anhedral for the opposite, balancing, effect. You could say, without too much rigour, that in practice anhedral means sweep and sweep means an outward flow of the BL on the top surface.

This outward flow of the BL is one of the factors leading to early tip stall on swept wings - until or unless corrected by an aerodynamic fix of some kind.

Dick W

[jerrytug]

Makes you wonder,does a Harrier for example need a lot of stick to roll from inverted to erect with the diheral and the sweepback both trying to keep it upside down?

[Right Stuff]

jerrytug - please tell me how the aircraft knows which way up it is! The answer is, of course, no

[Field In Sight]

Woooooaaahhh there Right Stuff, easy fella.

I think jerrytug has a valid point.

Consider a high and STRAIGHT wing aircraft. When flying right way up, the lower CofG acts as a pendulous stabilising force.

Roll it over to inverted flight and the aircraft now has a destabilising pendulous force.

Now consider the reverse happening for a low STRAIGHT wing aircraft.

The aircraft might not know which way is up, but the CofG certainly does (and the pilot's breakfast also does)

FIS.

[bookworm]

The aircraft might not know which way is up, but the CofG certainly does

Yes but unfortunately the C of G doesn't know how to communicate this to the aircraft in a useful way.

Lateral stability is about the reaction of an aircraft to sideslip. If the aircraft slips right, the mechanism of lateral stability produces a roll to the left and vice versa. The roll angle of the aircraft with respect to earth vertical is not relevant.

Dihedral is one mechanism for lateral stability, as is a high wing. The latter has nothing to do with pendulums, but is caused by the interference between the fuselage and the wings when a sideslip is present.

[Dick Whittingham]

An aircraft with sweep and anhedral has a high degree of lateral stability when flying inverted. Fact.

I did a BofI on a Gnat, abandoned during an aeros display, which recovered to inverted flight and droned around quite happily until it carried out a textbook forced landing upside down.

But we are talking sideslip stability here, not roll rate, so the Gnat did and the Harrier should (no practical experience here) roll just as rapidly from inverted to upright as it did when rolling inverted in response to aileron input.

"Pendulum effect" is just an analogy to help the visualisation of the effect of high wings, high tails, T tails etc. The mean sideways force is applied above the aircraft CG, so tends to roll the aircraft upright.

Dick W