Thread: Vortex Generators

Originally Posted by rick130

I think it was more for drag reduction (reduced fuel consumption) at cruising speeds, wasn't it ?
Hence very attractive for passenger aircraft operators.

Reduced Low Speed turbulence at the wing tip, For take off and landing speeds was what the Boeing Eng told us back in 1990...

Increased stability at lower speeds.....

Originally Posted by JDNSW

Spitfire wings were elliptical not parabolic, otherwise your description of the advantage is correct. The reason elliptical wings are rarely used is nothing to do with stall characteristics, but because the wing shape is very expensive to build. The Spitfire actually had excellent stalling characteristics for the type and era.

Most wings will stall all at once unless the wing has washout or twist, regardless of the shape in plan. The reason for arranging washout is so that the ailerons remain effective near the stall rather than the suddenness of the stall, and pilots in the Spitfire era were taught to control roll near the stall, as when landing, using rudder rather than ailerons.

John
(who does have a tailwheel endorsement)

Thanks for correcting me John.

My interest in aerodynamics mostly comes from when I was involved with sailing, and later a small part in some research on blades for medium large wind turbine. But I have not looked at the books for many years.

Anyway, tonight I pulled out a paper I have about wing tip tailoring - not to check on what John said, but to respond to Rick's later post.

It confirmed that the optimum spanwise lift distribution and wing planform is elliptical, as John corrected me. My memory let down.

It said that, quote:
"...elliptical wings are not only difficult to make," (as John said) "but more importantly, they also have undesirable stalling characteristics."

Originally Posted by Bush65

Thanks for correcting me John.

My interest in aerodynamics mostly comes from when I was involved with sailing, and later a small part in some research on blades for medium large wind turbine. But I have not looked at the books for many years.

Anyway, tonight I pulled out a paper I have about wing tip tailoring - not to check on what John said, but to respond to Rick's later post.

It confirmed that the optimum spanwise lift distribution and wing planform is elliptical, as John corrected me. My memory let down.

It said that, quote:
"...elliptical wings are not only difficult to make," (as John said) "but more importantly, they also have undesirable stalling characteristics."

I am not sure what the undesirable stalling characteristics are - the Spitfire is the only important aircraft ever made in any numbers with elliptical planform wings, as far as I can think, and every reference refers to them as a delight to fly. Not necessarily to land, but that was because of the narrow track and poor forward visibility in the landing attitude, rather than stalling characteristics.

The problem with building the elliptical wing is that everything is curved - the easiest wing to build is a constant chord one (not all that common, but they do exist - e.g my first plane, an Auster, and there are a lot that are constant for part of the span, e.g. my second plane, a Cessna 180), and after that one with leading and trailing edges straight, which is most modern aircraft, possibly most aircraft ever built.

I had to learn a bit of aerodynamics to get my licence, and have accumulated a large aeronautical library over the years - plus an even larger sailing library.

John

Originally Posted by rick130

I think it was more for drag reduction (reduced fuel consumption) at cruising speeds, wasn't it ?
Hence very attractive for passenger aircraft operators.

My brief comment about winglets in the 1st post was badly stated.

An important advantage achieved with winglets is reduced drag and resulting fuel consumption. But what is happening to get these reduction?

And Mike may be correct as well.

Aerodynamic lift and drag are associated, you can't have lift without drag. And there are other forms of drag/resistance besides that associated with lift.

The vortices at wing tips are bad. They reduce lift and increase drag.

Air flow over wings is 3 dimensional and complex at the wing tip.

The planform of a Spitfire wing is the optimum shape as the lift reduces toward the tip because its chord and thickness reduce to such small values. This minimises the tip vortex and the induced drag.

Tapered wings and swept back wings are used to reduce the tip vortex. A delta wing can be thought of as not having a wing tip.

Because lift increases with velocity, wings are a compromise between low drag at high velocity and high lift at low speed (for take off and landing). This explains why high performance military aircraft land and take off at such high velocity - they can't produce enough lift at low velocity. Devices such as flaps are used to increase lift at low velocities.

A low speed, high lift airfoil, with substantial camber, will have larger vortices than a high speed low camber, laminar flow airfoil.

Lift also increases within a range of increasing angle of attack.

The downward motion of air ahead of the wing, caused by pressure disturbances, resulting in a change to the angle of attack. With tapered wings, the pressure disturbance and downward air flow is greater at the wing root. The result is a higher angle of attack at the tip, especially near stall when the wing is producing a high lift coefficient.

It is undesirable for the wing tip to stall first, so designers will use wing twist to reduce the angle of attack of the wing tip. But at cruise the tip may produce negative lift.

Downward deflection of ailerons, especially if they go all of the way to the wing tips, will increase the vortex.

Instead of air flowing from the wing leading edge to the trailing edge and down to produce lift, in the tip vortex, flow occurs around the tip from the underside to the top.

Winglets in the vortex, tailor the pressure distribution and modify the vortex so that more of the wing is producing lift and induced drag is reduced.