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..

SHOCKWATCH

One would often wonder why in flight aerodynamics we tend to differentiate the flight speed characteristics in terms of subsonic and supersonic flow and what are the hindrances in going from subsonic to sonic flow ....well the answer to that is the shock waves formation .....shock waves are the major hindrances in sonic and supersonic flight and in subsequent discussions the reason behind this will be uncovered.
In general disturbances in air travel at the speed of sound . If you consider a normal subsonic flight you will note that the the air ahead of the aircraft senses its presence and in 'layman' terms gets out of the way .However when the aircraft travels at the mach 1 the air ahead doesn't get the reaction time to get out of the way because before it can sense its presence the aircraft rams into the air .This causes massive increase in local properties of the air which includes increase in temperature ,pressure and density .One can imagine the plane to be ramming into wall of obstruction .This whole phenomenon summarizes the SHOCK WAVE formation.Shock waves not only give rise to huge drag to the aircraft but also cause the flow over the wings to dettach thus causing in lift condition .
Shock waves in general travel at speed greater than speed of sound.Speed of the shock waves generally depend upon the amplitude of the waves.These waves are highly energetic however they cover lesser distance as compared to sound waves because they dissipate energy very quickly in the form of heat .When a airplane crosses the sound barrier you may often note liquid condensation along with it ....this is mainly because of the rise in density of the air because of shock wave formation .

CONDENSATION EFFECTS


Once an aircraft has reached the drag divergence mach number there is tremendous increase in drag resistance offered to plane and thus a large amount of thrust is required to overcome it .However it is intresting to note that the wave drag at sonic flight is greater than that at supersonic flight .The reason for this is that in sonic flight the there is flow resistance (drag )plus flow being not attached to the aircraft ,however at higher speeds the flow again begins to attaches itself to the aircraft .hence we see that lesser total drag is offered in this case.
shock waves for flat thin bodies is normal to its area.These are called normal shock waves .The shock waves tend to turn around corner edges and angles giving rise to oblique shock waves which is most commonly seen in an aircraft.Now if the corner edges and angles are are large then shock waves which tend to follow the the curves dettaches .Such shock waves are called bow shock wave .Bow shock waves in general offer greater drag as compared to oblique shock wave .It is formed in case of blunt objects traveling at supersonic speeds .Bow shock waves have their application in space shuttle design wherein the shuttles are given a blunt shape so that they experience high drag and are slowed to a low enough speed for safe landing .

BOW SHOCK WAVE

Transonic flights are also to a great extent affected by shock waves .Although the aircarft is traveling at speed less than mach 1 there is local acceleration at points on the aircraft particularly the airfoil where the velocity crosses the sonic barrier .At these places very high wave drag is experienced and lift production reduces significantly . however the SUPER CRITICAL AIRFOIL has been hugely successful for high efficiencies of aircrafts in transonic range .This airfoil has characteristics such that the max.chord thickness is pushed aft so that the point at which shock waves are produced are pushed backwards and thus a major part of the airfoil is still available for lift .The other characteristics include thinner sections and less total drag .
Generally when an aircraft crosses the sonic barrier the huge opposition of drag is overcome by pitching nose down and going ahead.now as mach 1 is crossed the centre of pressure of wings moves ahead (due flow separation) and the aircraft pitches further nose down(this is known as mach tuck) and ultimately loses control and crashes.however with carnard configuration of the aircraft the plane can be stabilized again .
Now lets review the history of supersonic flight .The first aircraft to cross the sound barrier was the Bell X-1.The first SST(supersonic transport) aircraft were the TU-144 and CONCORDE.
The TU-144 has a very intresting story.It is named after TUPOLOV ,the great Russian aerodynamic expert and also its inventor .The development of TU-144 had begun well before its first successful flight in 1960's as an SST.Originally its development was aimed towards coming up with the worlds first fighter jet .it was encouraged by the RED ARMY during WW II and the project handed over to tupolov . However a number of factors caused a delay in its development mainly Tupolov being imprisoned several times during this period .Also the innumerable failures of the TU-144 during flight testing prompted the then impatient government to scrap the project .However post war government changed and tupolov along with his son continued on his endeavors and came up with the perfect plane years later.
In this modern era a number of hypersonic aircrafts are in development ,one noticeably being the NASA's X-43 has three stages of flight.Most of the supersonic and hyper crafts run on ram and scram jets ,which give high efficiency only at high speeds and very low efficiency at low speeds.
references:
www.grc.nasa.gov
www.centennialofflight.gov
www.en.wikipedia.com
introduction to aerodynamics of flight
introduction to flighthyperphysics.phy-astr.gsu.eduspaceflight.nasa.gov
http://selair.selkirk.bc.ca/aerodynamics1/High-Speed/Page2d.html
DREAM TILL U LIVE ,CHASE TILL U DIE !!!!!

SWEPT WONDER

Hello guys here I am with my own aeronautics blog and it will be known the AEROGENOUS zone .Here I will come up with discussions related to various aeronautics aspects, something on which I thrive these days. My writing skills I am sure won't match up to the other regular bloggers but my motive is to put my ideas through very clearly. So here we go .............

Okay so for my first topic I thought of picking swept wings .The reason for doing this is that the advantages that the swept wings have to offer as compared to any other wings are just tremendous and have always captivated me. This is the same very reason why most of the airplanes have been using the swept wings configuration .
the grunman X-29 with forward swept wings

swept wings may be forward swept or backward swept .The sweep which you will find on most of the commercial planes is backward swept.swept wings provide wonderful static and dynamic stability and also delays critical mach numbers being reached at various parts of the plane (will be discussed later ) and thus reduce drag by preventing shock waves formation .
Let us now view the various advantages of swept wing configuration one by one:

LESS DRAG :

Now if you consider simple aerodynamic you will know that any body kept inclined to free stream velocity will offer lesser drag than the one kept normal to it .So also is the case with swept wings and thus one can save on the extra thrust due to reduced drag and also increase fuel saving along with greater flight time .

YAW STABILITY:

In the case of a crosswind coming and affecting your flight path by turning it it in the longitudinal plane in a conventional non swept wing configuration aircraft you will alter the camber of the rudder to counteract this undesirable motion .However in swept wings due its tendency of returning to its original direction (static stability) the flight will do so without the pilot having to use the rudder .The reason for this is that when you yaw the inner wings will be further inclined to the free stream of air but the outer wings will now be normal to the free stream of air thus the drag on the outer side will be more compared to that of the inner side as a result the plane will now align itself back to the original position .

ROLL STABILITY :

Just like dihedrals swept wings also give you roll stability i.e. when the plane is rotated by external undesirable forces in the along the horizontal axis by small amounts, the wings produce disproportionate lift & become horizontal again .
This actually happens because the normal flow of air over the lower wings has greater velocity than that on the upper wings this is mainly because of greater sideslip velocity of air over the upper wings (try to work it out with a vector).Thus the lower wings produce more lift than the upper wings and plane rolls back to original position .

LESS INDUCED DRAG :
Now induced drag is basically vortex formation at the wing tips which disrupts air flow . Greater the lift at the tips greater will be the magnitude of the vortex and more the drag the airplane will offer .Now most of the swept wings are formed by giving a taper like cut to the normal wing . This increase the span efficiency factor (e) . Theories suggest that greater is the span efficency factor (it basically implies to elliptical pattern of lift production wherein max lift is at root and it dies off at the tip) less will be the induced drag and hence greater will be the lift .

ADVANTAGES AT TRANSONIC SPEED :
Even when the aircraft is flying below mach 1 (say arnd 0.8-0.9 mach) there will b areas in the plane where the flow will be accelerated and shock waves will b formed offering large drag .Most of the aeronautics guide suggest that one of these areas where the flow is accelerated is around the wing tips .Now the amount by which it is accelerated is directly dependent on the chord of wing tip .Lesser the chord at the tip lesser will be the wave drag and as seen above swept wings have a samller chord at the tip as compared to that at root thus they prevent wave drag formation and allow for a proper flow to take place .

When free stream air comes under the influence of the airfoil its velocity increases continuously from the leading edge to the maximum thickness if chord and then again decreases till the trailing edge in accordance with the continuity equation .So if the flow is transonic there are chances that mach 1 speeds will be reached at max.thickness of chord and thus causing wave drag .Therefore for transonic flows the thickness of the airfoil should be reduced so that velocity at the thickest part of the chord doesn't reach sonic ranges .actually the wavedrag associated with transonic flow is directly proportional to square of chord thickness/chord length(b/l).Now swept wings is nothing but conventional wing turned through some angle therefore the length of chord is effectively increased which implies nothing but the thickness of chord has been decreased and hence swept wings offer less drag at transonic ranges .






Swept wing configuration has its fair share of disadvantages as well:

• There is very large amount span wise flow of vortices in swept wings from the tip to the root which stalls the total wing together .this can however be prevented using wing fences which are vertical plates on wings which prevent the span wise flow of vortex.
• The efficiency of these wings at subsonic speeds are low compared to that at transonic speeds .
• Once the aircraft has crossed the sound barrier the efficiency's of swept wings further reduce offering large drag .
• Many planes are under research where in you can alter the sweep angles at various stages of flight conditions and thus increase flight efficiency .Such variable sweep aircrafts have been already made but each one encountered some flaw or the other .

Well I guess thats it for my first post ...if any flaws please mention them in the comments section .....adios .......

DREAM TILL U LIVE ,CHASE TILL U DIE !!!!!

references :

1. introduction to aerodynamics of flight
2. From the ground up-himalayan publications
3. www.nasa.gov
4. www.en.wikipedia.org
5. www.ae.metu.edu