Angle of attack,

Angle of attack,

For us, the seasoned airmen, the angle of attack may seem like a very useful and normal thing.  Even if you didn’t quite get it in the beginning.  By now I’m sure you have it within you.  Maybe for the usual cues, the position of the joke or the stick, the sounds in the cockpit, the mushy nose attitude.  For whatever the circumstances you are a master of the angle.

Even though life for us revolves around this topic in many ways we tend to forget all about it.  It is automatic, we don’t think in terms of angle, but everything we do while flying involves that angle.

Lets try and share with our rookie peers the benefits of mastering the angle.

This craft we call aviation is full of surprises.  Theory given to a new comer is just enough to get him of the ground and into so much happiness.  Similar to what Dr. Ian Malcom said in Jurassic Park, “Everything is fun and laughter until things start to go south” (I’m paraphrasing).

But it is true, when and airman does not understand that important bit of information from the book.  Things can go south awfully fast. 

Angle of attack like flying is not innate to us. Flying and  wings, behave very differently than what we like to think are the normal ways.  We put our feet on the ground and everything we do relates to that maxim.  To walk we apply a force to the ground which applies the exact but opposite force on us and we move. If we want to go somewhere we thrust ourselves by pushing the earth down and back and of we go. Even in a boat, we support the boat and its contents by pushing water, which sits, bellow the boat.

Well wings are not earth bound, and are not at all obedient to our maxim. But they do generate lift by applying force to the air, which in return applies an opposite force. Wings flow,  glide, slide, they even swirl. But above all, they don’t propel themselves with their lower portion, and that is a big difference.

They are bound by the same laws of physics that we all are.  A wing to fly will displace its weight in air, similar to a skier in water.  The moment the skier stops displacing its weight in water, is the moment he stops being on top of the water.  Skies also displace water with their lower part.

So the wing does it differently, it displaces the air mostly with its upper part.  And that’s really counterintuitive.  Not only are we not able to see the air we fly in, but the wing uses its cambered upper part to create lift.

Awesome!!

Wings glide through the air, developing lift mostly by displacing air with its cambered upper section, and that is a maxim of an air bound being.

So what about Bernoulli, and all of that?  Were they teasing me?

Well not really, all thought some misconception comes out of that paragraph in the books.

Couple of things you need to know about Bernoulli: one is that it’s real, other is that it is useful on the wing, but not in the way you think.

If from that information in the books you understood that you are sucked up in the air, you got it wrong.

Let me tell to you what does Bernoulli explain in lift on a wing and the efficiency of the upper section.  When you were studding meteorology, you remember that high pressure traversed to fill a low pressure, right?  Well, as the air moves over the wing it creates a low pressure like Bernoulli said, and what happens to a low pressure? It gets filed with air that surrounds it and that is at a higher pressure.

Air from bellow and in front of the wing tries to move upwards into its upper part. You see where we are going with this.  We now know a bit more of why the wing has such an efficient upper section.  It uses not only the air in front but also air that moves from in front and bellow to add to the total air managed by the upper section.

We have said to this moment that: the wing uses predominantly its cambered upper section to create lift, by displacing air equal to its weight.  We can check Newton’s third law here and Bernoulli’s principle also.

For the sake of argument, we will say that the upper section displaces in the vicinity of 70% of the total air managed by the wing. That means that the lower section contributes with around 30%.

So here we are knowing what a wing does and still not knowing what the angle is for?

Lets avoid getting to technical.  The angle of attack, which is the topic at hand, is the angle at which the air meets the wing.

First of all: who sets the angle of attack and    what does angle of attack have to do with lift?

Well, that is easy; the elevators set and correct the angle of attack.

For the second part of the question we will do an exercise.  With a pen, make marks on the tube that attaches to the joke where it meets the instrument panel and see that the angle you set in one position, will maintain a particular speed. (This is true only at 1 G. and stable power, and for the love of lift, take a pilot to see and avoid)

Draw marks in the tube; for example at 100 kt put a mark, and then pull the joke back inch by inch making marks until you see the stall speed approaching. At that point you are reaching the lowest speed you can fly under determined circumstances, but you are also approaching the angle at which the air will no longer be able to take the curve and be displaced by the upper section of the wing and there’s your last mark.  This is your stall speed and also your maximum angle of attack.

For the most part angle of attack relates to speed.  A lower angle of attack means a higher speed and a higher angle of attack relates to a lower speed.

Wings as we spoke, fly due to the fact that they displace air.  And they are able do this task by meeting the air in a range of angles.  The range is not immense but it is what it is.  A usual wing will meet the air and be able to displace it bellow an angle of about 17 degrees. 

On this subject let me tell you a short story, about 30 years ago I flew a Helio Courier, a beautiful aircraft with so many treats for the pilot.  One of the safety features of the Helio was that it had on the elevator a mechanism that prevented the pilot from pulling back on the joke so far as to stop the flow of air over the wing. It limited the travel of the elevators so that the wing could not reach the stalling angle of attack.

Amazing don’t you think?

Well even though most of the aircrafts we fly are capable of stalling, it is also true that they will only stall if you keep pulling on the joke back towards you, beyond the angle that prevents the flow of air over the wing.

Then why do we loose so much?

When we were talking about the 70/30 percent relationship between the upper and the lower sections of the wing.  We also realized that the weight of the aircraft was the important figure to be displaced in air to be able to fly.

Lets do some math.  Your average trainer weights about 2200 lbs. and 70% of that is around 1500 lbs.  Once your hand pulls the joke beyond the last joke mark and we go above 17 degrees of angle, we are bound to loose that amount of lift and the aircraft no longer flies but falls as an object towards the ground.

Sad stories relate to this particular happening.  It is truly important to understand and to practice flying at different angles of attack and recognize that the only way out of that fall is “letting the joke go forward”. 

If you were to be gliding down with no engine you sure will at some point reach the ground, but if you can manage your angle of attack, you will reach the ground in control of the aircraft, hopefully near the maximum angle of attack.

In regards to this matter let me assure you of something: If you understand angle of attack, practice and develop proficiency you will be a safer airman, for yourself and the ones who are onboard with you.

I wish you all, safe and pleasant flights on a safe and efficient angel of attack.

Luca Pineda

P.S.

For the airmen that fly rotary wings, lowering your “collective” is the way of reducing the angle of attack on those wings.

References,

AIH

PHAK

Stick and Rudder Wolfgang Langewiesche