Principles of Flight Theory Flashcards

1
Q

What is total drag?

A

Total drag is the sum of all of the aerodynamic forces which act parallel to, and opposite to, the direction of flight.

Total drag is the total resistance to the motion of the aircraft through the air.

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2
Q

What’s the aeronautical term for drag?

A

Air Resistance

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3
Q

What are the four types of drag?

A
Induced Drag - Forces associated with the production of lift 
1. Induced Drag:
        Induced drag is a by-product of the production of lift and is closely related to the angle of attack.
        To provide positive lift, the static pressure on the upper wing surface will be less than that on the lower wing surface. As the air flows rearwards, some airflow will 
        'leak' or spill around the wingtip from the high static pressure area under the wing to the low static pressure area above the wing. This causes a spanwise flow 
        component of air outwards away from the fuselage on the lower surface and an inwards component towards the fuselage on the upper surface. At the trailing edge of the wing where these upper and lower flows meet - both moving rearwards but with opposite spanwise (or lateral) components - a sheet of vortices is formed. At the wingtips, where the spanwise flow is greatest, by far the strongest vortices are formed. These are known as wingtip vortices. 

Parasite Drag - Forces not directly associated with the development of lift 
2. Skin Friction Drag: 
        Friction forces between an object and the air through which it is moving produce skin friction drag.   
        The magnitude of the skin friction drag depends on:
             - The Surface area of the aircraft. The whole surface area of the aircraft experiences surface or skin friction drag as it moves through the air.
             - Whether the boundary layer airflow near the surface is laminar or turbulent. A turbulent boundary layer disturbs laminar airflow and mixes more with 
                the air around it, causing more drag.
             - Roughness on the surface (including ice accretion) will increase skin drag. The transition from a laminar to a turbulent boundary layer may even occur
                immediately at the point of the roughness. Flush riveting and polishing help to smooth the surface and reduce skin friction drag.
             - Aerofoil thickness. An increase in aerofoil thickness increases skin friction drag from that aerofoil.
             - Angle of attack. An increase in angle of attack increases skin friction. 
3. Form Drag
         When airflow separates from the surface, eddies are formed and the streamline flow is disturbed. The turbulent wake formed increases drag. The is form drag.
4. Interference Drag
        If we consider the aircraft as a whole, the total drag is greater than just the sum of the drag on the individual parts of the aircraft. This is due to flow 'interference' at the junction of various surfaces, such as the wing/fuselage junction, the tail section/fuselage junctions and the wing/engine nacelle junctions. This flow interference creates an additional drag, which we call interference drag.
        
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4
Q

What effect does airspeed have on parasite drag?

A

At zero airspeed there is no relative motion between the aeroplane and the air. Therefore there is no parasite drag. As the airspeed increases, the skin friction, the form drag and the interference drag all increase.

An aircraft flying at a speed just above the stall may have only 25% of its total drag due to parasite drag. At a very high speed, the total dray may be due almost entirely to parasite drag (with practically no induced drag).

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5
Q

What is adverse yaw?

A

Adverse yaw is the natural and undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in lift and drag of each wing.

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6
Q

What’s a washout?

A

If the wing is built with an inbuilt twist (called washout), the angle of attack at the wingtip is less than the angle of attack at the wing-root. Therefore most of the lift force is generated on the inner part of the wing, while not so much lift will be generated near the wingtips. This can help prevent wing-tip stalls as well as reduces wing-tip vortices.

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7
Q

What are tapered wings?

A

A tapered wing has a shorter chord at the wingtips, therefore reducing the wingtip vortices and so the induced drag is less.

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8
Q

What effect does the aspect ratio of wings have on induced drag?

A

A long narrow wing of high aspect ratio (and therefore smaller wingtips) has weaker wingtip vortices, less induced downwash and therefore less induced drag.

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9
Q

What are the types of wingtip modifications and what’s their purpose?

A

Wingtip tanks and modified wingtips can reduce the leakage of the airflow around the wingtip and therefore reduce the formation of induced drag. Also, the installation of wing fences reduces spanwise flow and thus induced drag.

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10
Q

What is the formula for drag?

A

Drag = Cd x ½ x rho x V-squared x S

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11
Q

What is the formula for lift?

A

Lift = CL x ½ x rho x V-squared x S

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12
Q

What are the three axes of the aircraft?

A

Lateral (Lateral Stability) Y, Longitudinal (Longitudinal Stability) X, Normal (Directional Stability) Z

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13
Q

What is stability and controllability?

A

Stability is the natural ability of the aeroplane to return to its original condition after being disturbed without any action being taken by the pilot.

Controllability refers to the ease with which the pilot can manoeuvre the aircraft using the control surfaces.

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14
Q

Longitudinal stability is greatest when:

A

The centre of gravity is further forward.

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15
Q

What is wing dihedral?

A

Wing dihedral increases lateral stability. Dihedral is found on most low-wing aeroplanes, and is built into the aeroplane during design and construction. Each wing is inclined upwards from the fuselage to the wingtips, and adds to the lateral stability characteristics of the aeroplane.

As the aircraft sideslips, the lower wing, due to its dihedral, will meet the upcoming relative airflow at a greater angle of attack and will produce increased lift. The upper wing will meet the relative airflow at a lower angle of attack and will therefore produce less lift. It may also be shielded somewhat by the fuselage, causing an even lower lift to be generated. The rolling moment so provided will tend to return the aircraft to its original wings-level position.

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16
Q

What’s the purpose of the tailplane?

A

The function of the tailplane (or horizontal stabiliser) is to counteract residual pitching moments from the two main couples, i.e. it has a stabilising function.

17
Q

What’s a couple?

A

A couple is a pair of equal, parallel forces acting in opposite directions which tends to cause rotation because the forces are acting along different axes.

18
Q

At what degrees does the maximum aerodynamic efficiency of an aerofoil occur?

A

At the maximum L:D ratio, at around +4deg angle of attack for a conventional aerofoil.

19
Q

What’s the aspect ratio of a wing?

A

Span divided by chord