Airfoil: Curves that revolutionized Aviation!
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Roughly, 100,000 flights take off and land every day all over the globe. Every hour someone experiences their first flight.
The grandeur of the big bird fascinates them.?
But how many times does this question pops up?
Why are the wings on the aircraft not like rectangular blocks???
The above illustrations kind of answer's the question here.
This shape that we observe is an airfoil. An airfoil has a characteristic shape with a curved top and bottom surface in order to have the desired flow required to generate maximum lift.
Another question that arises is why we need an airfoil shape when a flat plate could fly nicely just like we see folded sheets and wings on butterflies and bumblebees.
How did we land up with this design?
So, let’s turn some pages back in the book of aviation and dive into the chapter on Airfoil origin.?
The origin started with German aeronautical pioneer, Otto Lilienthal.
From childhood, Lilienthal took great interest in the study of birds and flight. Such was his devotion, that he would strap onto himself a pair of 6 ft long wings and flap them while running downhill in the hope of taking off.
His initial experiments were unsuccessful, but they sowed the seed for what would be a prime contribution to the field of Aeronautics.
Through his observations of bird flight and experiments with differently shaped wings, Lilienthal discovered that the curvature in a bird’s wing is crucial for flight. This curvature of the wing is termed a camber .? This was the beginning of the transition from using flat wings in gliders to curved wings.
Using these gliders, Lilienthal became the first human to launch himself into the air, fly, and land safely. Unfortunately, during one of his test flights, his glider went into a stall and nose-dived to the ground, and he died the next day in the hospital. Lilienthal's last words were: “Sacrifices must be made”.
His research became the basis for the next major chapters of the book.?
Turning to 1901 when the two hobbyists, the Wright brothers (Wilbur and Oliver Wright) came up with different ideas to improve flight characteristics. They carried out a number of flight tests.?
During their test flights, they observed that there was adverse yaw acting on the aircraft which was the reason for their unsuccessful flight tests.
?(The issue with flat plates was that the Coefficient of Drag observed on it was way higher and was undesirable).
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This made them question the original theories (which might be due to their wrong interpretation of the Lilienthal theory) which made them go back to research and prototype testing before the original full-scale model testing.?
For a comparative study, they tied an airfoil shape and a flat plate shape on their bicycle and started cycling to generate airflow.?
This ride brought the transition needed from flat plate design to airfoil design.
We started with symmetrical airfoils which have the same curvature on the top and the bottom.?Due to this shape of the airfoil, it could generate lift only when provided with an angle i.e. the lift generated at zero angle of attack was nil.?
So, this provided room for more research, the Wright brothers made a? rudimentary wind tunnel of their own design for prototype testing. But the early results through the wind tunnel testing were misleading and made them conclude that thinner airfoils provided better results. So, the Wright brothers worked more on thin airfoils.
The research didn’t just stall there.?
During the 1930s, the U.S National Advisory Committee for Aeronautics (NACA, present-day NASA) developed tests on ‘families’ of airfoils’. With the advent of better wind tunnels, the misconception of leaning towards thinner airfoils got rectified and the later tests proved that incorporating airfoils with camber provided better results. Here, the German aerospace engineer Max Michael Munk who formerly worked in NACA made a remarkable contribution to the thin airfoil theory.?
A lot of history in the last 80 seconds so let’s revert back to the present.?
We broadly classify airfoils into 3 main categories:
The division is based on the design of the airfoil shape which ultimately leads to the difference in the flow and their lift generating characteristics.
The non-symmetrical airfoils were designed to counter the problem faced by the symmetrical airfoils of not being able to generate lift at 0 angle of attack. The non-symmetrical airfoils have camber which enables the airfoil to generate lift even at negative angles of attack.
The development of supercritical airfoils was like reaching another milestone in the aviation industry. These airfoils have the characteristic of delaying the formation of oblique shock wave which leads to separation when the aircraft travels at transonic range.?
Airfoils are not only an essential factor while we are considering aircraft wing design but play an important role when it comes to the design of propeller blades, turbine blades, etc.
Symmetrical airfoils are used in the tail of aircraft and aerobatic aircrafts. Non-symmetrical airfoils are used in the wing of the aircraft, propeller blades, and also in turbine and compressor blades.?Supercritical airfoils are used in business jets, airliners and transport, and various military aircraft.?
Since curious minds will never stop wobbling and thereby the research will never be stagnant.
So, the present research is being done on the design of the airfoil required for the Mars Helicopter rotor.?