FLEXIWINGS

This article of the Aerogenous zone is by Rishikesh Karande

Ever thought of actually flying like a bird? Well, aero engineers across the globe are also trying to achieve the same goal.

But what is so special about any birds’s flight? Flapping..feathers..wings which change shape..? Actually all of it! All the research that has been done on bird flight has proved that their method of flying is much more efficient than any other method we humans use at the same speed. Here's a small article on how and why we are trying to emulate the high performance flight pattern of a bird by changing the wing shape.

Ever observed how an eagle soars in the skies? The pics here shall give an idea:

Fully spread wings to soar with separate feathers at the tip to reduce drag.Also when the wings flap upwards there is separation of feathers at the tip to rdeuce the drag .

Smaller planform area reducing drag and maintaining lift at high speeds

Gull wing type shape at extremely rough and high speeds. The eagle’s actually hoverin with its wings!

Check up the curve during the soar..

Check out the wingtips..

We need to remember that drag is proportional to surface area and square of velocity. This is also true with lift. Now, as the weight of an eagle is fixed(in flight), it needs a fixed amount of lift to stay airborne; and extra lift to climb. So if the wind speed is more than required, the eagle reduces its wing area such that lift reamains same and drag does not increase unnecessarily. Also, when the wind speed is low, it increases its surface area to get the minimum lift required.

We can also notice the curvature of the eagles's wings when its soaring..It forms a sort of winglet at the tip which GREATLY reduces induced drag by curbing the motion of wingtip vortices. The winglets on most passenger aircrafts have been an outcome of this flight feature itself. The eagle also applies something similar to the gull-wing in very high winds and it actually tends to hover (stay at that particular place) without flapping a wing !

When I say all this, it actually sounds very simple and natural. But what we need to remember is that if we are successful in applying the same methodology for any man-made fixed wing aircraft, we can actually ensure that the plane flies at its max efficiency at all speeds. Consider a glider as a simple example. Its long, narrow wings help it to fly at low speeds with very little drag. But if we try to fly that same glider at approximately 100-200kmph, it'll definitely give more lift. But the drag encountered at that speed will be humongous! That reduces the lift to drag ratio of the plane as a whole which reduces its capability to fly at those speeds.

Another perfect example is a passenger aircraft. Say..the 737. It may be observed that while taking off, the flaps and slats are fully or partially extended. This is done to increase the lift coefficient at the take-off speed which is much lesser as compared to the cruise speed of the aircraft. When the cruise speed is reached, the flaps and slats are taken in (hence reducing area and drag). The area can be reduced as long as the lift is maintained at the required value by increasing speed. This exercise of increasing and decreasing the area is done to maintain the efficiency at take-off as well as cruise speed. We cannot fly in cruise with the flaps extended as it hampers the range and reduces efficiency by increasing drag.

The above examples highlight the area and speed relationship. Accordingly, other factors governing lift and drag can also be varied with speed so as to maintain efficient flight. Such as the camber can be reduced as speed reduces with an increase in area or angle of attack so as to maintain lift. This is as cambered airfoils tend to give more drag at less speed. Another parameter that can be varied is the dihedral or anhedral as seen in the front view. An increased dihedral reduces the upward component of lift and improves roll stability. Also, more sweep angle improves roll stability and decreases the lift as the air doesnt flow exactly perpendicular to the span.

The clark y gives way less drag as compared to the highly cambered chen(at low speeds that is..)

So all this basically implies that different speeds and conditions call for different wing configurations. A bird accommodates for these changes easily (using its muscles) so as to maintain efficiency..so how can we? Well..NASA has began conceptualization of a flexiwing concept whose wing changes shape according to speed. However, the major challenge in realising this concept is the large number of actuators required in different directions to realise the large number of degrees of freedom required. The video here shoes the concept rather clearly..

http://www.youtube.com/watch?v=jHZ6JCnCPHk&feature=PlayList&p=4B3B6F49B8F8BEDB&playnext=1&playnext_from=PL&index=15

One available actuator is called the electroactive polymers. These are polymers which change shape in a particular direction when an electric current is passed in a specific direction. Research is still on over these..

This concept definitely is made for the future to optimise the range and fuel consumption of passenger aircrafts atleast..