Saturday, March 21, 2009

[flight dynamics] twist, dihedral and wingtips

Great article on 2 December 2001, at aerospacewebdotorg, by Doug Jackson.

In answer to the question, ‘Why does an aeroplane’s wing twist?’ he addresses two issues: twist and the dihedral. Perhaps he should also address ‘angle of attack’.

Flight theory

If you look at a wing, as in the diagram below, you see a flat base and a curved top. As the air flows faster over the curved section, this creates low pressure and the difference from the high pressure base creates lift.

After that comes the Lift/Drag ratio and that depends on a number of things. Suffice to say that the higher the ratio, the more effective is the power.

If the base of an aeroplane’s wing is horizontal to the ground, the plane’s not going to take off; in a not dissimilar way, a sailboat will not go directly upwind. So the whole wing needs angling upwards towards the leading edge and this is angle of attack.



Twist

Doug Jackson writes:

In most aircraft, the airfoil twists down as we move along the wing further from the fuselage. This is referred to as "washout." Twist is applied to wings so that the outboard section of the wing does not stall first.

When an aircraft is pitching nose up and increasing its angle of attack, the airflow over the wing eventually reaches a point where it becomes turbulent, causing a loss in lift. By twisting the outboard portion of the wing down, the stall is delayed in that area, simply because the angle of attack is lower in that region.

Why is the outboard portion of the wing so important? It is because that is where the ailerons are located. By maintaining lift on the outboard portion of the wing, the pilot is still able to maintain roll control of the aircraft in the event of a stall.

This phenomenon of twist is well known in the sailing world too and is one reason why the solid wing boats, even Cogito:




… will not replace the soft wing:




… that reason being the capability of a soft sail to twist at the top. Cogito, in the top pic, was able, at great expense, to create twist and thus was untouchable on the race course. It’s the world champion.

Back to aeroplanes.

Dihedral

Dihedral is the upward angle of the wing from the vertical when seen from the front, or nose of the aircraft. If each wing is angled 5° up from the horizontal, then the wing is said to have 5° of dihedral.

Here is an example of dihedral:




The opposite of dihedral is called anhedral, and, of course, refers to a wing that is angled down. A good example of an aircraft with geometric anhedral is the Sea Harrier:




Effective dihedral is a little different:

Many aspects of an aircraft's configuration can effect its effective dihedral, but two major components are wing sweep and the wing location with respect to the fuselage (such as a low wing or high wing). As a rough estimation, 10° of sweepback on a wing provides about 1° of effective dihedral, while a high wing configuration can provide about 5° of effective dihedral over a low wing configuration.

All this talk of anhedral and dihedral leads to the question of why one would want use either of these on an aircraft. The simple answer is they provide lateral (roll) stability. Let's consider an aircraft rolling to the right. As it does so, the right wing produces more lift than left wing, causing the rolling motion. At the same time, however, this increased lift creates an increased drag, which causes the aircraft to yaw to the left, an effect known as adverse yaw. This is why pilots need to apply rudder in the direction of the turn.

Now let's consider the advantages of dihedral. When an aircraft with dihedral is yawing to the left, the dihedral causes the left wing to experience a greater angle of attack, which increases lift. This increased lift tends to cause the aircraft to then return to level flight. I know this is very confusing in words, but if you stick your arms out in the air and recreate all of these motions, it should make sense.

The end result of all of this is that dihedral tends to make an aircraft more stable.

For some aircraft, like fighters, stability really isn't a good thing. A slight instability in an aircraft leads to increased maneuverability, which is highly desirable in fighter and attack aircraft. This is why most aerobatic planes and military fighters utilize some amount of anhedral.

I know the Spitfire used dihedral:




… and was a fighter but almost all modern fighters use anhedral, e.g. the F18:




Large transports have both a high wing and a considerable amount of wing sweep, both of which create a large amount of effective dihedral. To counteract this large amount of dihedral, some geometric anhedral is required. Otherwise the aircraft would be overly stable, making turns extremely difficult, and an aircraft that can only fly in one direction isn't much use to anyone.

Anhedral applies to bombers as well.

Wingtips

It’s been shown in both the aeronautical world and in the aquanautical, that if you put perpendicular or angled tips on the ends of the wings [foils], you increase their efficiency.

This is because of the theory of flight. Those unbalanced pressures above and below the wing [or to windward and leeward in a yacht], cause air to try to make its way around the tip to the other side, creating vortices and drag.

To avoid this, rivet a vertical panel on the end of the wing and you’ve instantly increased the wing’s efficiency. This especially applies to yacht keels and was the centre of the controversy over Australia II’s victory in the America’s Cup in 1983.

So, you either know all this already or now you’ll look more closely at wings next time you’re near them.

3 comments:

dearieme said...

"As the air flows faster over the curved section": why does it do that, James?

Lord T said...

Basically it is because the curved section is longer so it has to travel faster to make the trip. Physics. The volume of air remains the same of course and being spread out over a larger area makes it less dense and creating an area of lower pressure which helps pull the wing up.

You can improve this by using flaps (which extent the wing length and thus the distance) and/or changing the angle of attack (The way the wing is orientated) You use both methods when taking off. Flaps and pointing the aircraft up slightly.

James Higham said...

Thanks, Lord T - the physicist to the chemist.