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‫Welcome back.

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‫So let's analyze our airfoil now and let's see what forces it experiences.

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‫So here you had one velocity vector or mega times R that was horizontal and then you had another velocity

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‫vector, which was in red V.

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‫V.

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‫Omegle times are came from the fact that the model was actually rotating and then we saw we came from

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‫this dream tube effect and so our airfoil saw two types of air velocities.

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‫And since they are both vectors, then we could have a resultant velocity, which is this you?

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‫This is the resultant air velocity that the airfoil sees.

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‫And now we have to define a couple of angles.

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‫The first angle that we're going to define, we're going to define a Seeta angle.

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‫This is an Air Force pitch angle, and so you get it if you take your airfoil and this is your cord

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‫length in green and it ends here at its nose at the Air Force nose.

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‫So if you project this green line further than the air for a pitch angle is defined from this horizontal

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‫line up until this green line.

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‫So it gives you an idea of how much this airfoil is twisted upwards.

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‫Then you have another angle, which is from here to here, and let's call it Alpha, this angle is called

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‫the angle of attack is the angle that forms between this result and air velocity vector and then this

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‫green line that goes along the core length of the airfoil.

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‫In other words, you want to know how much the airfoil is twisted with respect to this resultant air

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‫velocity.

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‫And that's actually the most important angle in aerodynamics, because it's this angle.

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‫What determines what kind of aerodynamic forces your airfoil experiences because aerodynamic forces

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‫come from.

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‫Airflows or er velocity's, and then we can define another angle here, and that would just be the difference

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‫of Theta minus Alpha so we can say that it's PHI.

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‫So due to this resultant air velocity, this airfoil will experience and aerodynamic force that we're

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‫going to call the F sub A and now we have to decompose this vector and we're going to decompose it in

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‫two different ways.

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‫You can decompose it along horizontal and vertical axis.

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‫So this would be the horizontal component.

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‫And this would be the vertical component, so the horizontal component is along the same dimension,

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‫like the rotation of the airfoil around the center of the motorised rotation.

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‫And then the vertical component is perpendicular to that, we're going to call this component the DH

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‫and this H is not angular.

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‫Momentum is just a capital letter that I'm using to name this horizontal component of this D.F. sub

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‫a vector.

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‫And we're going to come back to this component later.

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‫But this component, the vertical one that's called the T., which is the differential thrust.

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‫So this is your aerodynamic force and this vertical component of it, this is what forms the thrust

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‫force.

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‫But that's not the only way you can decompose this force vector.

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‫You can also define to access that are parallel to this U vector and perpendicular to it.

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‫So this axis here would be parallel to the U vector.

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‫So this would be your fly angle then like here.

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‫And this axis here would be perpendicular to the U vector.

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‫So if you shift your you vector here, then you would have 90 degrees here.

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‫And now you can decompose this dates up a vector like this along these two axis.

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‫In orange.

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‫You have one vector here, which is this one perpendicular to the vector.

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‫And the other component would be parallel to the new vector, so it would be like this.

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‫So the component that is perpendicular to the vector, we call it the D.

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‫L, which is a differential lift force.

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‫So just for your knowledge, the lift force in general in aerodynamics is defined in such a way that

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‫it is perpendicular to the resultant air velocity vector that the airfoil sees and then the component

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‫parallel to the vector, that's what we call Didi's or differential drag.

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‫So again, in general, in aerodynamics at drag force is a force vector that is parallel to the vector.

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‫And so our objective now is to find the total thrust force that the entire blade generates, and once

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‫we know that, then we can simply multiply by two and then we'll know how much thrust the entire model

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‫will generate because each murder, remember, has two blades.

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‫You find thrust for one blade and then the other blade will have the same thrust.

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‫So that's why you can just multiply this by two and then you will know the total thrust force generated

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‫by one more.

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‫And then, of course, you find the thrusts of other three murders as well, and then you will sum them

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‫all up, thrust one plus thrust two, plus thrust three and plus four.

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‫And then you will know the total thrust that they are generating to lift the drone.

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‫But let's go back to our airfoil now.

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‫The thing is that in order to find the differential thrust force, you first have to express it in terms

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‫of differential lift and differential drag force.

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‫So you have to find DETI as a function of DL and D.

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‫D, so this will be your little exercise.

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‫So a little bit of work out in geometry.

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‫Based on this information here, you see the angles, right, and you see the vectors, so based on

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‫that, how would you write this DTA in terms of DL?

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‫And so try it out yourself and then you'll see the solution in the next video.

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‫Thank you very much.