Principles of Flight --2

Different types of drag: Form drag Skin friction Profile drag Wing drag Induced drag Parasite drag Interference drag This section deals with drag and forces in balance. In no particular order, here is a list of different types of drag. Total drag is the sum of all of the drag forces acting on the glider Principles of Flight --2 D Puttock based on the work of W G Scull

Wing drag and parasitedrag. Wing drag is self explanatory; parasite drag will be defined later on, but for now you can consider it as the drag attributable to the non lifting parts. Parasite drag can be further subdivided into two Principles of Flight --2 Wing drag TOTAL DRAG Parasite drag D Puttock based on the work of W G Scull

Form drag and skin friction. Form drag is the drag due to the shape of the body. Skin friction is self explanatory. Principles of Flight --2 Skin friction Parasite drag Form drag D Puttock based on the work of W G Scull

We have divided total drag into wing and parasite drag, and the parasite into form drag and skin friction. Principles of Flight --2 Wing drag Total Drag Skin friction Parasite Drag Form Drag D Puttock based on the work of W G Scull

The wing drag components are, skin friction, induced and form drag Principles of Flight --2 Skin friction Wing drag Induced drag Form drag D Puttock based on the work of W G Scull

Drag breakdown Form drag and skin friction appear twice, and so are common to all parts of the airframe. Induced drag is peculiar to the wing. Principles of Flight --2 Skin friction Wing Drag Induced drag Form drag Total drag Skin friction Parasite Drag Form drag D Puttock based on the work of W G Scull

Different types of drag: Form drag √ Skin friction √ Profile drag Wing drag √ Induced drag √ Parasite drag √ Interference drag What have we accounted for so far? Interference drag is an affect when two parts join, like a wing and fuselage, airflow conflicts and we have interference drag. Principles of Flight-2 D Puttock based on the work of W G Scull

Profile Drag —the sum of skin friction and form drag for the whole airframe. Form Drag ---is due to shape. Certain shapes will disturb the airflow less than others. Principles of Flight-2 D Puttock based on the work of W G Scull

Form Drag Each of these shapes has the same frontal area, but offers a different resistance. Streamlining reduces resistance up to a point. When the length is four times the width, skin friction becomes a problem. Skin friction depends on both surface smoothness and surface area. Principles of Flight-2 Resistence 100% 50% 10% D Puttock based on the work of W G Scull

Skin Friction The upper surface of the plate is smooth, and the lower surface is rough. The rough surface not only slows the airflow more, but also influences the airflow to a much greater distance. The depth of the region through which the airflow is slowed is called the boundary layer. Principles of Flight-2 The boundary layer D Puttock based on the work of W G Scull

When airflow is slowed in an orderly manner, it is called laminar, and if disorderly, turbulent. Most wing sections have turbulent airflow beyond the thickest part. This point is called the transition point. Skin friction and form drag both increase as the speed increases. Together they are called profile drag. Principles of Flight-2 laminar turbulent Transition point laminar turbulent D Puttock based on the work of W G Scull

Profile drag/ airspeed relationship Drag increases as the square of the airspeed. Double the airspeed from 30 to 60, and the drag increases by a factor of 4. Treble the airspeed from 30 to 90 and the drag increases by a factor of 9. 9 4 1 60 30 90 Airspeed Principles of Flight-2 Drag units D Puttock based on the work of W G Scull

Drag = CD.½ρV².S Drag is commonly expressed as a coefficient. (CD) Principles of Flight-2 D Puttock based on the work of W G Scull

Induced drag These picturtes show the pressure distribution. This distribution is almost eliptical for most wings. The pressure difference reduces towards the tips. Air tends to equalise causing a flow from high to low pressure. - + Principles of Flight-2 Chordwise distribution Spanwise distribution - + D Puttock based on the work of W G Scull

Airflow over the top Airflow over the bottom Airflow over the top surface will tend to be inwards, and that over the under surface will tend to be outwards. As the two airflows meet at the trailing edge, a spiral motion is imparted to the flow, the greatest effect being at the wingtips. These are called trailing edge and wingtip vortices, the energy dissipated is induced drag. As the pressure differences are greatest at low speeds, induced drag will be considerable at low airspeed and reduce progressively as the speed is increased. Vortices can be lessened by having less wing tip around which the airflow can take place, by altering the ratio of the span to the chord. Principles of Flight-2 Vortices D Puttock based on the work of W G Scull

Aspect Ratio= Aspect ratio is the ratio of the span divided by the chord. These three wings have the same area, but the aspect ratios are different. The first is 36/5 =7.25 45/4 = 11.25 60/3 = 20 Aspect ratio is the designers choice, modern gliders are around 20. Principles of Flight-2 Span Chord 36’ 5’ 45’ 4’ 60’ 3’ D Puttock based on the work of W G Scull

Aspect ratio of non- rectangular wings. Aspect ratio = Mean chord = A.R = = Aspect ratio = If the wing does not have a parallel chord, then the aspect ratio is determined like this. Take a little time to ponder the derivation of this formula. Principles of Flight-2 Span chord Area span Span Mean chord Span X span area span² area D Puttock based on the work of W G Scull

Induced drag/airspeed relationship If we have 10 units of drag at 30 knots, doubling the speed to 60 knots gives a quarter of the drag, and trebling it to 90 knots gives one ninth of the original 10 units. This follows the inverse square law. Principles of Flight-2 10 9 4 1 60 30 90 Airspeed D Puttock based on the work of W G Scull

Profile drag/ airspeed relationship Remember this graph? 9 4 1 60 30 90 Airspeed Principles of Flight-2 Drag units D Puttock based on the work of W G Scull

Summation of drag forces Principles of Flight-2 Here the two graphs have been superimposed. Minimum drag speed has been shown The stall line introduced Total drag Drag units stall Adding the two graphs together is not complicated, it is just a Matter of adding two heights Together at various speeds Induced Drag Profile drag speed Minimum drag speed D Puttock based on the work of W G Scull

Test Wing drag comprises of 3 types of drag, what are they? Streamlining is a measure to reduce what type of drag? What is the difference between a laminar and turbulent boundary layer? What does transition point mean? What is the relationship between profile drag and airspeed? Does induced drag have the same relationship with airspeed as profile drag.? Explain why the minimum drag speed is not the minimum speed the glider can fly. Principles of Flight-2 D Puttock based on the work of W G Scull

Principles of Flight --2