Video #4: Angle of Attack

Video #4: Explaining what varying each of the items in the lift formula does, and how we utilise the angle of attack to control the aircraft.

Script (with weird spellings and lots of comma’s to make the helpful American gentleman narrator speak properly!)

Welcome to episode 4! You are still on the profpilot dot co dot ewe kay flight training video series. If you are a pilot, you may find going to the main site helpful to your pilot education, as it gives you information and reviews on courses and schools, as well as being filled with other fascinating nuances.

Going back to the joy of the lift formula, the best way to control an aircraft, would be the easiest one of these letters to have control of, in the real world. So which is easiest to alter? First, density. Can we change the density of air at will? No. So we can cross this letter out, as we cannot use it to control our aircraft. We do have quite good control over the velocity of the aircraft, so if we wanted to go up, we could increase the thrust to make us go faster. This would increase the letter v, and cause an increase in lift.
However, This is not very practical, and would not provide very acurate controls, and would take a while to react, rather than being instantaneous, which can sometimes be quite important. So this is also out. We cannot really change the area of the wing mid flight either. If that does happen to you, then something has gone very wrong indeed.

The coefficient of lift is the easiest thing for us to control, and more specifically for the cruise portion of flight, the angle of attack of the wing. This is the value we will be looking at in more detail, and more specifically, the angle of attack of the wing.

Before I explain the angle of attack, a couple of definitions. First, the wing chord. This is this green line drawn from the leading edge, to the trailing edge, of the aerofoil. Another definition you will need is the relative airflow. This is the direction of the air, relative to the wing, that has not been affected by it. You may remember this image of beauty from another episode. In it, the relative airflow is the undisturbed air at the top of the picture, here, as it has not been affected by the aerofoil. This air, here, immediately around the aerofoil, is not the relative air flow, as it has been curved by the wing. Relative Air Flow can also be abbreviated to R.A.F, not to be confused with the finest air force in the world.

The angle of attack is defined as the angle between the chord of the wing, and the relative airflow. So, you might have a low angle of attack, as in this picture, where the relative air flow and the wing chord lines are
nearly parallel, and this will produce a relatively small amount of lift. You may also have a higher angle of attack, where there is a larger angle between the relative air flow, and the wing chord. This will produce more lift,
than a smaller angle of attack.

Increasing lift, with increasing angles of attack, only occurs to a certain point. This point, where an increase in angle of attack, will now actually result in a REDUCTION of lift, is called the critical angle of attack, and it
varies depending on the design of the aircraft. As you can see, the aerofoil is increasing it’s angle of attack beyond this critical angle, so less lift is being generated. This can result in a stall, and these will be covered in
a later video.

So, The aerofoil we have been looking at is a cross section of the wing, assuming it has no moving parts. But what about the part of the wing with an aileron hanging off the back of it? Well, that changes the cross
section to something more like this. The leading edge remains the same, but the trailing edge now becomes the back of the aileron, and the wing chord is now made by joining these two points. This means that when
the aileron moves, the traling edge will also move , while the leading edge remains in the same place. The wing chord therefore changes to look like this, but the relative airflow stays the same. This has therefore
changed the angle of attack, from this. To this. So there is now more lift than before.

This is fundamental to how an aircraft controls itself, using its primary flight controls. Again, I am tantalising you with this picture as I have done for the last three episodes. Next time. I promise.

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