This post focuses the aerodynamics of level flight. There are plenty of references out there on this topic, however many of them are either way too complicated; requiring a doctorate in astrophysics to understand, or are so simple and vague that they do not allow you to put the whole picture together. This post aims for a happy median between the two. Believe it or not, the actual concepts which describe an aircraft in level flight are actually fairly simple and easy to understand. Hopefully you will agree as you read along.
How Flight Works
A good while ago, some smart guys (or maybe gals) discovered that if you throw something up in the air, it will come back down. Rather obvious to us, but an important discovery none the less. Then a little while later, a man named Isaac Newton found out that the force generated by a falling object could be quantified. Newton said that if you take the amount of stuff inside something and multiply that number by the gravitational constant, you could find out the amount of force something wants to fall to earth with. Basically, everything will fall down if you throw it up, and bigger things will take more force to throw up and will fall down harder.
So, if you want to make something fly, then you will need to generate more force than gravity does. The unfortunate part of this is that the downward force of gravity acts constantly on an object. So, if you throw a baseball up in the air, the baseball will eventually come down because you only were able to apply a force on the ball for a short period of time, where gravity was able to act on the ball constantly. In order to keep something suspended in the air (like an aircraft), you need to find a way to apply an upward force constantly.
A hot air balloon was the first aircraft which carried humans into the sky. Hot air balloons trap less dense, hot air inside their balloon. Because the hot air inside the balloon is less dense than the colder air outside the balloon, the balloon generates a force upwards. If the upward force generated by the buoyancy of the balloon equals the force of gravity pulling it down, then the two forces will cancel out and the balloon can fly. Fixed wing aircraft work the same way. The wings on an aircraft need to generate a force upward greater than or equal to the force of gravity pulling the aircraft down.
Lift production in a wing
So how does a wing work? Many pilots are taught the airfoil shape of the wing speeds up the air over the wing and slows down the air under the wing, creating a pressure differential which makes the wing rise. This is a very poor way of looking at lift production and is not fully correct. If this is so, flight with symmetrical airfoils (like what planes that fly upside down have), and flight for biplanes cannot be explained. So what’s really going on?
In reality, a wing produces lift rather simply. As an aircraft moves through the air, air impacts its wings. Because the wing is angled slightly upwards, some air is deflected downward. This causes the wing to be pushed upward, thus generating a force which opposes gravity. Yes, it is really that simple.
Factors which affect lift
The amount of lift a wing generates is based on the three main factors listed below (there are more but I’m trying to keep it simple).
Wing area: Bigger wings result in more lift because they are able to move more air downward. Most aircraft can adjust wing area by the use of flaps.
Airspeed: Faster airspeeds mean the aircraft’s wings are impacting more air and thus, are forcing more air downward.
Coefficient of lift (CL): The coefficient of lift can be thought of as the efficiency of a wing at producing lift. It is directly related to the angle of attack of the wing (the angle between the wing and the impacting air). Basically, as the angle of attack is changed, the ratio of the amount of lift it produces to the amount of drag it produces will change, meaning some angles of attack are more efficient than others. More on this a bit later.
Lift can be defined by the flowing equation:
Lift = Velocity^2 X Wing Area X CL
Increasing any one of these variables will increase the amount of lift produced. If any of these variables are zero, then no lift will be produced.
Drag and lift
As you already know, an aircraft’s wings must generate enough lift to equal or exceed the force of gravity pulling the aircraft downward. In order to accomplish this, the aircraft’s wings must impact air and change its direction. Unfortunately, this creates a force rearward called drag. Drag can be thought of as air’s resistance to movement. As a wing impacts air, the air applies a force which pushes back on the wing. This force must be counteracted by the aircraft’s thrust generating system. The more drag the wings produce, the more thrust the aircraft’s engines will have to produce in order to maintain level flight. If drag pulls back more than the aircraft’s engines push forward, then the aircraft will slow down.
Wing efficiency (Coefficient of Lift)
Contrary to what is commonly believed, a wing will produce lift at angles of attack of greater than zero degrees to less than 90 degrees. Meaning that a wing will generate some lift if it is more than parallel to less than perpendicular to the impacting air. However, the wing will only generate lift efficiently over a small range of angles. If the angle of attack is small, then the aircraft will have to move significantly faster through the air to generate enough lift to keep aloft. This additional airspeed will cause increased drag from the form of the wing, reducing efficiency. If the angle of attack increases much past 15 to 18 degrees, airflow over the top of the wing begins to separate and drag greatly increases. This makes the wing less efficient at producing lift. Usually a wing can efficiently produce lift between angles of attack ranging from 3 to 15 degrees. This concept is illustrated in the graph below. The graph represents the amount of thrust required to produce enough lift to keep an aircraft aloft at various angles of attack. The graph is not specific to any aircraft but is a generalization of all aircraft. The values of thrust on the Y axis are dimensionless and are used simply to illustrate the concept.
A few things become apparent from this graph. First, the graph has a minimum value. This value corresponds to the most efficient angle of attack or CL max. If you fly an aircraft at this angle of attack, it will achieve the best endurance and will glide the furthest distance. The speed of the aircraft which corresponds to this angle is called best glide speed, or best endurance speed.
In order for the angle of attack to decrease beyond CL max, airspeed will need to be increased to keep the aircraft aloft. This is accomplished by increasing power and allowing the angle of attack to lessen. As the aircraft’s speed increases, the aircraft will slowly become less efficient at producing lift, and a point will be reached where there will not be enough thrust available to counteract the drag produced by the wing and the body of the aircraft. This is the aircraft’s maximum speed.
In order for the angle of attack to increase beyond CL max, airspeed will need to decrease, and thrust will need to increase to compensate for the increased drag and decreased wing efficiency. In order to fly slower than the best glide speed of your aircraft, you will need to actually increase power. This is called flying on the back side of the power curve and is trained for during slow flight. If you further increase the angle of attack (past 16 degrees for most wings), CL will begin to drop quickly and the wing will quickly become less efficient. This occurs because the airflow over the top of the wing begins to separate and drag greatly increases. This phenomenon is best known as a stall. An aircraft can maintain level flight with fully stalled wings if it has enough power to compensate for the greatly increased drag which occurs as a result of the stall. Typically, most aircraft do not have enough thrust to power through a stall and will lose altitude when its wings are stalled.
To wrap things up:
1) In level flight, an aircraft’s wings must produce enough lift to counteract the force of gravity pulling the aircraft down
2) In level flight, an aircraft’s engines must produce enough thrust to counteract the amount of drag the aircraft’s wings and body produce.
3) Wings are only efficient at producing lift at certain angles of attack but can produce lift through a wide range of angles. Wings will produce lift when stalled.
4) The most efficient angle of attack of a wing is defined as the angle where the most lift is produced for the amount of drag produced. This angle is when CL is at its maximum and corresponds to the the best glide and endurance speed for that aircraft.
Please note: If you are an engineer, you will likely not like a lot of the things I mentioned here. I admit, this post does not nearly cover the entire picture and some assumptions are made. The purpose of this post is to help pilots understand some basic aerodynamic principles, and help clarify some of the things in pilot texts. It is impossible to describe these concepts fully in a few paragraphs.
Happy reading, Mike