Paper Airplane
Updated: Jul 16, 2020
In previous article, I've mentioned that angle of attack is 15 degree! some of you would have question that why is it between 15° to 18° only? If with the higher angle of attack, we can produce more lift, then why not 45, 60, 90 degree? why only about 15-18 degree is preferred?
Fig.2_As a result of Coanda effect
The above image shows that the air passing over the Aerofoil is deflected downwards, but why is that happening? does the shape is magical/enchanting? do they have gum over the wings? kidding 😅,but yes in a way the shape could be called magical. there is a theory which can make this statement more logical. Stick till the end, because I am going to share you this interesting science fundamental in much easier way.
you can skip the bracket below if you do not want to read in depth, you can directly read the topic of the article written after brackets. ⏩
Let's see mathematical formulae to create a lift to better understand what I am going to write down in the article.
Lift (L) : (0.5) x Density x {(velocity) (velocity)} x (Area of the wing S) x (Coefficient of lift, CL)
1) So according to the formulae, if we increase the density of the air then we can increase the lift, which we can not control. so we can not consider density to increase lift.
2) If we increase the velocity of the plane, it highers the lift, but it is not more practical and smooth. How? first of all it is a slow process and has very low control over the lift and hence, low control over the plane maneuver. it could cause discomfort to the customers as it makes plane fly with sudden altitude change. because we are increasing velocity which means drag will also increase. so this is not so considerable parameter either.
3) plane wing area, well this is much of design perspective, we know that the wider the wing span better the lift will be, but we also have to consider the stresses acting on wing while the plane is in the air. the plane wing could just cause a failure due to buckling force. so we better stay off changing the wing span.
4) at last, We have Co-efficient of lift. this is the factor we could consider increasing lift. the co-efficient of lift is the measure of how much lift a wing can produce. co-efficient of lift is linearly proportional to angle of attack for the normal flight condition. Angle of attack is defined as angle between the relative wind or Incoming wind and the Chord line of the wing. so normally we can change the lift performance by varying angle of attack or the design of the aerofoil.
Now what is Chord line right? It is the imaginary line connects the leading (front facing) and trailing (back side) edge of the wing's cross section.
Wing's cross section usually known as Aerofoil shown in above figure. The most reasonable AOA (angle of attack) is 15 degree, but it could be higher up to 18 degree. depends on aerofoil shape. What if it goes above that? plane enters in to stalling state. Stall is the state when, The amount of lift airplane produces decreases and due to which Drag increases. To understand how state of stall occurs, we need to understand the Coanda effect.
Answering our second question, Why does the Air or fluid changes the direction of it's flow path? It is due to the Coanda effect.
let us think of it at molecular level, take a look at below image.
DO IT YOUR SELF✅
you can do this experiment by your self, take a piece of paper, hold it with your hands as shown in figure. Now blow the air above the paper. you will notice that paper will try to move upwards. how did that happen? This effect is called Coanda effect. when you blow the air, it has high velocity compared to atmospheric air. higher the velocity lower the pressure. In addition to that, the high velocity air drags the air particles from nearer layer of the atmospheric air with it which creates vacuum (shown in figure with hatch lines). "V"states shows a region where vacuum is being produced. due to particles being dragged by Higher velocity air, blown by a person creates low pressure at region-2. The paper is light and free to float, so air molecule from region 1 (high pressure region) pushes the paper towards low pressure region which makes paper tend to move upward, when we blow the air. this effect is called Coanda effect. but what if paper is not free to move and fixed from both end?
Then the air from higher pressure region (region 2) will push the higher velocity air as shown in figure towards the curvature and deflect the direction of the air. Here region 1 also has higher pressure than the pressure near the curve of the paper, but air pressure from region 1 can not interact with low pressure region as paper is fixed and air can not move through the paper. This is what exactly happen with the Airplane wings and the Air flowing through it. It deflects the air and air exits at downward direction which is known as down wash. Another example of this effect is when you bring the plastic water bottle under a (laminar flow) tap water, the flow will deflect towards the bottle curvature as soon as it comes in a contact of the bottle. the image is shown below.
Now, let us move to the first question. why Angle of attack is ideal at 15 to 18 degree? at about 15 degree the air molecules follows the Coanda effect and move along the curvature of the wings which creates lift. but if angle of attack is more than 18 degree, air at the trailing edge started to get separated from the wing surface and become more turbulent. This state is shown below in very last condition. The separation point move further towards leading edge as the angle of attack increases (the situation of stall starts at critical angle of attack). In the last condition, air is not leaving in downward direction, which will not create down wash and no pressure difference will be created which definitely going to decrease the lift force.
It is similar to the hand which is kept out of the window of the car and if u turn your palm from facing road to facing towards incoming air. you will definitely feel the drag. same thing happens with wings. as angle increases the wind will resist and stop producing the lift.