Drag

Question 1

Drag definition

Answer 1

Drag: force which opposes the forward motion of an airplane
●opposes thrust
●equal to thrust in unaccelerated, straight-and-level flight

Two main forms of drag:
●Induced – Product of Lift
●Parasitic – Everything else

Question 2

Induced drag

Answer 2

• Inversely proportional to the square of the speed. If speed is decreased by half, induced drag increases fourfold

• Increases total drag as airspeed decreases

• Can be directly controlled by the pilot
•Speed
•Flaps
•AOA

At low speeds a wing must fly at a higher AOA; result, more high pressure air from the lower surfaces comes around (or spills over) the wingtips, forming powerful vortices called — WING-TIP VORTICIES

At high speeds there are correspondingly less powerful wingtip vortices and less induced drag

Question 3

Wingtip vortices

Answer 3

●a by-product of lift
●caused by the joining of the low and high pressure areas from above and below the wing
●spiraling vortex is generated

Question 4

Infinite wings

Answer 4

•No Wing Tip Vortices
•No Down Wash
•No Induced Drag

Question 5

Finite wings

Answer 5

•Wing Tip Vortices
•Downwash
•Induced Drag
•Must Operate at Higher AOA to Produce Same Lift as Infinite Wing
•Less Lift At the Same AOA as Infinite Wing

Question 6

What determines induced drag

Answer 6

•Velocity
•Air Density
•Lift Required
•Characteristic of Wing

Question 7

Rectangular wing

Answer 7

•Area Does Not Match Lift Distribution
•Root Works Hardest
•Higher Local AOA
•Tips Work Least
•Lowest Local AOA

Question 8

Elliptical wing

Answer 8

•Area Matches Lift Distribution
•Airfoil Sections Work Equally

Question 9

Droop tips

Answer 9

•Downward Deflection of Spanwise Flow
•Vortex Forms Farther Away

Question 10

Endplates

Answer 10

•Block Airflow Around Tip
•More Parasite Drag

Question 11

Winglets

Answer 11

Most effective at Low Speed and/or High Angle of Attack

Question 12

Weight

Answer 12

•Induced Drag Increases with the Square of Weight
•Big Factor

Question 13

Span efficiency factor

Answer 13

•Induced Drag Decreases with More Efficient Wings
•Elliptical = 1
•Others = .85 to .95

Question 14

Wing span

Answer 14

•Induced Drag Decreases with the Square of Wing Span
•Big Factor

Question 15

Air density

Answer 15

•Induced Drag Decreases with Greater Density
•Increasing Pressure
•Lower Altitude
•Colder Temperature

Question 16

Velocity

Answer 16

•Induced Drag Decreases with the Square of Velocity
•Big Factor

Question 17

Wake turbulence

Answer 17

•Wing Tip Vortices
•Weight
•Span
•Velocity

Question 18

Ground effect

Answer 18

The Tendency For An Airplane to Float Above Just Before Touchdown.

PROXIMITY TO THE GROUND REDUCES UPWASH, DOWNWASH, AND WINGTIP VORTICES, DECREASING INDUCED DRAG

OCCUR’S WHEN AN AIRPLANE FLIES WITHIN A DISTANCE FROM THE SURFACE EQUAL TO ITS OWN WINGSPAN OR LESS

INDUCED DRAG IS ONLY ABOUT HALF IT’S VALUE WHEN THE WING IS AT 10% ITS SPAN ABOVE THE GROUND. RESULTING IN LIFT-OFF AT A LOWER THAN NORMAL SPEED

Question 19

Ground effect aspect ratio

Answer 19

•High AR, Low Di
•Less Ground Effect
•More Pronounced with Low AR

Question 20

Ground effect continued

Answer 20

•Reduction of induced flow causes significant reduction in induced drag
•No direct effect on parasite drag
•Thrust required at low speeds will be decreased
•Reduced induced angle of attack and change in lift distribution
•Smaller wing angle of attack will be required to produce the same lift coefficient
•If a constant pitch attitude is maintained as ground effect is encountered, an increase in lift coefficient will be incurred
•Generally, the reduction in downwash at the horizontal tail increases static longitudinal stability
•Requires additional up elevator at a specific lift coefficient
•Flying down into ground effect will produce a nose-down change in pitching moment

Question 21

Nose down pitch

Answer 21

•A slightly nose-heavy aircraft will be more stable and less susceptible to stall at a low speed
•Nose will tend to drop when the throttle is reduced
•A tail-heavy aircraft will be more unstable and susceptible to stall at low speed

Question 22

Ground effect airspeed system error

Answer 22

•Due to the change in upwash, downwash, and tip vortices, there will be a change in position error of the airspeed system
•Static source pressure increase
•Produces a lower indicated airspeed and altitude

Question 23

Ground effect landing

Answer 23

•Ground effect must be understood and appreciated
•If flying an airplane into ground effect with a constant angle of attack
•Lift coefficient increases
•Thrust required decreases

Question 24

Ground effect takeoff

Answer 24

•Upon leaving ground effect
•Requires an increase in angle of attack to maintain the same lift coefficient
•Increased induced drag
•Increased thrust required
•Decreased stability and a nose-up change in moment
•Usually a reduction in static source pressure and increase in indicated airspeed

Question 25

Ground effect drag reduction

Answer 25

•1 wingspan = 1.4% decrease in Di •¼ wingspan = 23.5% decrease in Di •1/10 wingspan = 47.6% decrease in Di

Question 26

Downwash

Answer 26

Created by wingtip vortices Due to the spiraling in of the vortex Upwash is also a side effect Highest impact near the wing tips Reduced as it moves toward the wing root

Question 27

Boundary layer

Answer 27

Two types of airflow over an airfoil: ●Laminar ●Turbulent One of them is the skin friction between the molecules of the air and the surface of the aircraft. The skin friction causes the air near the wing's surface to slow down. This slowed down layer of air is called the boundary layer. The boundary layer builds up thicker when moving from the front of the airfoil toward the wing trailing edge.

Question 28

The Reynolds effect

Answer 28

Another factor is called the Reynolds effect, which means that the slower we fly, the thicker the boundary layer becomes. The effects of the boundary layer on lift are contained in the lift coefficient and the effects on drag are contained in the drag coefficient. Which flow type occurs within the boundary layer at a given point of the wing's surface depends on the wing's form, the surface's roughness, the chord length, the airspeed and the ratio of density to viscosity of the air. Reynolds combined all those factors (except the surface condition) into a non-dimensional number known as Reynolds Number Re. Re = (air density/air viscosity) x air speed x wing chord The Reynolds number is dependent on the weather conditions, the wing chord and the airspeed. Re increases as the airspeed, the air density and the wing chord increases. •At low airspeed and small wing chord (as with a model aircraft) the air viscosity is a dominant factor, whereas with the full-sized aircraft the viscosity effects of the air are insignificant while the aircraft's mass inertia becomes more dominant. •That's why you should not expect a scaled model aircraft to have the same flight characteristics as its larger counterpart.

Question 29

Parasite drag

Answer 29

•CAUSED BY ANY AIRCRAFT SURFACE WHICH DEFLECTS OR INTERFERES WITH THE SMOOTH AIRFLOW AROUND THE AIRCRAFT • PARASITE DRAG INCREASES BY THE SQUARE OF THE AIRSPEED • THREE MAIN TYPES OF PARASITE DRAG = •1. FORM DRAG •2. INTERFERENCE DRAG •3. SKIN FRICTION DRAG

Question 30

Skin friction drag

Answer 30

•Laminar Boundary Layer •Low Drag •Turbulent Boundary Layer •High Drag

Question 31

Skin friction and form drag

Answer 31

•Streamlined Objects •Skin Friction Drag Greatest •Blunt Objects •Form Drag Greatest

Question 32

Profile drag

Answer 32

•Parasite Drag of Wing •Skin Friction •Form •Laminar Flow •Less Skin Friction Drag •Turbulent Flow •More Skin Friction Drag •Laminar Flow Wing •High Speed Flight •Extend Laminar Boundary Layer •Reduce Viscous Drag •Typical Airfoil •Maximum Thickness Forward •Approximately 25% chord •Laminar Flow Airfoil •Maximum Thickness Aft •Approximately 40% chord (or greater) •Laminar Flow Wing Problems •Surface Irregularities •Limited Angle of Attacks •Stall

Question 33

Interference drag

Answer 33

•Drag At Juncture of Two Bodies •Greater Than Sum of Individual Parts •Intersection of Boundary Layers •Vortices

Question 34

Form drag

Answer 34

Caused by the separation of airflow from the surface of a structure. Related to both the size and shape of the structure protruding into the relative wind Based on the shape of the airplane, how well it is streamlined, and how much frontal area it has

Question 35

Interference drag

Answer 35

Created when the airflow around one part of the aircraft interacts with the airflow around an adjacent part, such as the wing root and the side of the fuselage. Corrected by adding aerodynamic cowlings, nacelles, pylons and fillets

Question 36

Skin friction drag

Answer 36

Due to air molecules giving up some of their kinetic energy as they contact the skin surfaces of the aircraft structure The structure is covered with seams, rivet heads and variable rough surfaces

Question 37

Cooling drag

Answer 37

Result of Cooling of Reciprocating Engine •In addition it is very difficult to get the airflow over the engine to pass evenly by each of the cylinders. •The result is uneven cooling of the cylinders with the front cylinders generally running much hotter than the rear cylinders. •Using an air-cooled engine means that there are few options on how to pickup cooling air and were the heated waste air can be dumped.

Question 38

Leakage drag

Answer 38

Air Leakage From Pressurized Aircraft

Question 39

Wave drag

Answer 39

Wave drag is caused by forming shock waves around the aircraft as the speed of sound is obtained Delta and swept back wings change shape as smoothly as possible in order to overcome wave drag Supersonic flight would be very difficult to achieve and require very powerful engines coupled with extremely strong wing material if delta and swept back wings were not used