23 Mar 2015
There are 4 basic forces that act on an aircraft in flight; thrust, lift, weight and drag. Of these forces the most complex is drag. A thorough understanding of how drag affects the flight of an airplane is necessary if one is to exercise complete control over their aircraft. Drag can be broken down into 2 different types, induced and parasitic. It affects both the lift and thrust of the aircraft, and determines best glide speed as well as power required for an airplane. Drag in itself is not so difficult to understand, but as is so often the case, it is how drag interacts with the other forces that make it unique and complex.
There are four basic forces that act on an airplane in flight; thrust, lift, weight, and drag. Of these forces, it is important to take a deeper look into what drag is and how it really affects the aircraft. The total drag acting on an airplane in flight can be divided into two specific types; Induced drag and parasitic drag. Induced drag is produced by the wings and is simply a byproduct of lift. Parasitic drag is caused by moving an object through a fluid (such as air). In order to fully understand what is happening during flight and through different angles of attack, we must fully understand how drag is formed and the relationship between drag, power and airspeed.
Parasitic drag is primarily made up of form drag and skin friction, and is the easier of the two to understand. As the aircraft moves through the air, air particles nearby absorb some of the energy from the aircraft through friction. So the once relatively still air particles now are being accelerated as the aircraft passes. This transfers some of the aircrafts energy to the surrounding area. This is simply friction and specifically, skin friction. Form drag is disruption of air particles as a particular shape passes through them. Obviously an aerodynamic shape will require less energy to move through a fluid (air) than something not so aerodynamic. An example could be a sleek new Lamborghini compared to an old school Volkswagen hippy van. The Lamborghini will be more efficient and require less energy to move through the air. In the real world, parasitic drag not only comes from less aerodynamic aircraft designs, but mainly from landing gear, wing struts, antennas, or basically anything attached to the aircraft that disrupts the airflow over it. The formula for calculating parasitic drag is:
Parasitic Drag = CD * ½ p V2 * S
Since the coefficient of drag, air density, and frontal surface area cannot be changed (for a particular aircraft already manufactured), they are considered constants and are not important to us right now. The important thing to notice is V2. It can therefore be stated that Parasitic Drag is directly exponentially proportional to the aircraft's speed. In other words, as the speed of the aircraft increases, the parasitic drag of the aircraft increases. This is primarily due to the sheer number of air particles that come in contact with the aircraft. The higher the airspeed, the more molecules the aircraft comes in contact with, thus the more drag is imposed on the aircraft.
Lift can be defined as the force that acts perpendicular to the relative wind, and in the plane of symmetry. Bernoulli's principle basically states that the pressure and velocity of a fluid are inversely related. So as the velocity of a fluid increases, the pressure decreases. This is true of an airfoil. The velocity of the fluid (air) flowing over the longer, curved shape of the top of the airfoil must increase. Therefore the pressure over the top of the airfoil must decrease. The result is a relative low pressure area over the airfoil or wing of an aircraft. This in turn produces a relative high pressure area under the airfoil. The high pressure area found beneath the wing is what we refer to as lift. As the aircraft speed or velocity is increased, the lift on the wing is also increased (See figure 1).
Induced drag can be defined as the force that acts parallel to the relative wind, but in the opposite direction of thrust. It is a byproduct of lift, as stated earlier, and is somewhat related to the angle of attack of an airfoil. In its simplest form, one can think of it as this; at slower airspeeds, it is necessary to increase angle of attack to maintain enough lift to hold a constant altitude. As the aircraft increases in speed the angle of attack is decreased due to the airfoil producing more lift, thus reducing induced drag.
What is actually happening is the airflow ahead of the wing undergoes what is called upwash. As a consequence, the air that flows over the wings arrives there at a deflected angle, or is forced up. The air behind it is then forced downward. Since lift it created perpendicular to the relative wind, the resultant lift now is slightly deflected backward (see figure 2). This adds to the drag vector of the aircraft and is the induced drag. Factors that affect this induced drag are weight of the aircraft, design of the airfoil, aspect ratio of the wing (chord compared to the length) and angle of attack. Ultimately the determining factor is lift. The more lift the wings have to generate, the greater the pressure difference between the top and bottom of the wing. The greater this pressure difference, the more induced drag will be present.
Now that we have a firm grasp of what drag is we can discuss how it will affect the aircraft in flight, and what the pilot can expect (see figure 3). As you see in the Total Drag chart, parasitic drag increases with speed of the aircraft and induced drag decreases. The result is a total drag curve that decreases as airspeed increases to a point, then increases as airspeed continues to increase. It is important to understand this because the total drag curve and the required power are the same. If you think back to the definition of thrust, you will recall that it is the force required to move the aircraft forward through the air, or simply, the opposite of drag. If drag then is increased, thrust must also increase to remain moving in a forward direction. Then again looking at the Total Drag chart, you will notice the power required to sustain flight at slower airspeeds is greater than the power required at faster airspeeds, to a certain point. This region of the graph is referred to as the back side of the power curve. It is necessary for pilots to be proficient in the phase of flight because these airspeeds are typically the encountered during takeoff and landing. In the case of instrument conditions, or a fouled runway, the pilot may be force to execute a missed approach or go around. The pilot then has to get the aircraft to climb at low airspeeds, while avoiding an aerodynamic stall. This is not so hard to accomplish in itself, but add the stress of a long flight in instrument conditions, lots of chatter on the radio, at a large airport, among a myriad of other possible distractions, and you begin to see the danger.
Suppose for a second that you are flying around in a Cessna 172. You are just taking in the sights, putting along at 3000 feet. All of the sudden the engine quits... What do you do? Again referring back to the Total Drag chart you can see that somewhere in the middle of the curve is the minimum drag speed. This is the speed at which the total drag exerted on the airplane is the least. There are two important things to take away from this. First, this is the speed that will yield the greatest fuel economy. Second, this is the Best Glide speed of the airplane. A Cessna 172 has a best glide speed of 65 knots indicated airspeed (KIAS). This airspeed will give you a glide ratio of about nine to one. That is for every 1000 feet of altitude; you have about one and one half miles of forward distance. Notice, I did say forward. Turning the aircraft causes more load on the wing which requires more lift. And we know more lift equals more drag. So in the previous example, you would have approximately four and one half miles to either get the engine restarted, or get the aircraft safely on the ground.
It goes without saying that it is important for a pilot to know every aspect of the aircraft he is flying. I would assume just about every student at Embry Riddle could state the four forces that act upon an aircraft in flight. However, as I have discussed, something as simple as the force of drag could have disastrous consequences if not fully understood and respected. Drag is both good and bad, but understanding drag is all good.
Skinner, Craig, The Wonderful World of Drag. Retrieved from http://www.guelphgremlins.com/file/public/Drag.pdf
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