Technical Interview 2

Question 1

What are the disadvantages of a swept wing?

Answer 1

• Poor lift qualities
• Higher stall speeds as a consequence of the poor lift qualities
• Speed instability at low speeds
• A tendency to stall at the wing-tips

Question 2

If you climb to FL300 at a constant IAS of 250 kts, what would happen to the TAS?

Answer 2

TAS will increase as density decreases with increasing altitude. A constant IAS will give constant dynamic pressure.

Question 3

What is dihedral?

Answer 3

Upward inclination of a wing from the root to the tip.

Question 4

Why do we measure speed as a Mach number?

Answer 4

As the speed of an aircraft increases, there is a decrease in the distance between the aircraft and the influence of the advancing pressure waves. The aircraft begins to catch up the pressure waves. At higher speeds there is also a change in the flow and pressure patterns around the aircraft, which results in undesirable characteristics, shock waves start to form and drag increases.

These effects are due to the compressibility of air. So its important to know the speed of the aircraft in relation to the potential
effects of compressibility. Mach number is a measure of compressibility.

If the aircraft speed through the air (TAS) and the speed of sound in the air through which it is flying (the local speed of sound) is known, this will give an indication of the degree off compressibility.

So its much safer and easier to use a Mach number to express a limit at which compressibility effects start to appear.

Question 5

How do high lift devices do?

Answer 5

High lift devices are surfaces designed to increase lift during some phases or conditions of flight. They produce an increase of camber and this increases the lift coefficient at a given angle of attack and increases CLMAX.

Question 6

What do you prefer, a high or low wingload?

Answer 6

I prefer a low wing loading.

Wing loading is a useful measure of the general manoeuvring performance of an aircraft. Wings generate lift owing to the motion of air over the wing surface. Larger wings move more air, so an aircraft with a large wing area relative to its mass (i.e., low wing loading) will have more lift at any given speed. Therefore, an aircraft with lower wing loading will be able to take-off and land at a lower speed (or be able to take-off with a greater load). It will also be able to turn faster. Increase in wing loading means higher load factor, and hence higher stall speed.

Question 7

Why do some airplanes have dihedral wings?

Answer 7

Because dihedral provides positive static lateral stability (rolling).

Question 8

What are the lift qualities of a swept wing?

Answer 8

Poor. When you reduce the amount of air flowing parallel to the chord line, you reduce the amount of lift the wing creates.

Question 9

What is anhedral?

Answer 9

Downward inclination of a wing from the root to the tip (pensar en la letra A, tiene la misma forma).

Anhedral is used to reduce dihedral effect in aeroplanes with excessive “dihedral effect” (an aeroplane with high mounted and sweptback wings).

Question 10

What is a wing tip?

Answer 10

A wing tip is the part of the wing that is most distant from the fuselage of a fixed-wing aircraft.

Question 11

What are winglets and what is the advantage of winglets?

Answer 11

Winglets are vertical aerofoils which form part of the wing tip. Winglets partly block the air flowing from the bottom to the top surface of the wing, reducing the strength of the tip vortex, thus reducing induced drag and fuel burn

Winglets on the 737-800 have demonstrated drag reduction in the 5-7% range, which increases range and fuel efficiency

Question 12

What are the advantages of swept wings?

Answer 12

1 - High Mach cruise speeds, they delay the airflow over the wing from going supersonic

2 - More stable in turbulence, as they produce less lift they are less responsive to updraughts

Question 13

Why is anhedral used instead on some aircraft?

Answer 13

Anhedral is used to reduce dihedral effect in aeroplanes with excessive dihedral effect. For example, a high mounted and sweptback wing would give excessive “dihedral effect”, so anhedral is used to reduce “dihedral effect” to the required level.

Improves roll maneuverability

Question 14

Where does a swept wing stall first?

Answer 14

At the tip. The amount of spanwise flow (air flowing perpendicular to the chord of the aerofoil) compounds (se acumula) as you approach the wingtip, decreasing the wingtip’s effective airspeed and thickening the boundary layer.

Question 15

What is wing loading?

Answer 15

Wing loading is the ratio of aircraft weight to wing area.

1. If the wing area is small and the aircraft is heavy > High Wing Loading
2. If the wing area is big and the aircraft is light > Low Wing Loading

Wing loading is a useful measure of the general manoeuvring performance of an aircraft. Wings generate lift owing to the motion of air over the wing surface. Larger wings move more air, so an aircraft with a large wing area relative to its mass (i.e., low wing loading) will have more lift at any given speed. Therefore, an aircraft with lower wing loading will be able to take-off and land at a lower speed (or be able to take-off with a greater load). It will also be able to turn faster. Increase in wing loading means higher load factor, and hence higher stall speed.

Question 16

On a high wing anhedral airplane, like a BAE146, and a high wing straight winged aircraft like a Fokker 50, why they don’t have dihedral wings if it has so many advantages?

Answer 16

Because high wings already increase static lateral stability, using dihedral wings would further increase this stability, this can lead to Dutch roll

Anhedral is used to reduce dihedral effect in aeroplanes with excessive dihedral effect

Question 17

What are wing tip vortices?

Answer 17

Wing tip vortices are a byproduct of lift. Air will flow from areas of higher pressure towards areas of lower pressure. When a wing is generating lift, the air pressure on the top surface is lower than outside the wing tip and, generally, air pressure on the bottom surface is slightly higher than that outside the wing tip.

This causes air to flow inwards from the tip towards the root on the top surface and outwards from the root towards the tip on the bottom surface. The pressure difference at the wing tip will cause air to flow from the bottom surface to the top surface around the wing tip and this rotating airflow generates the tip vortices.

Question 18

How does an aerofoil work?

Answer 18

An aerofoil, also known as an airfoil, works by taking advantage of the properties of airflow to generate lift. When an aerofoil is placed at a positive angle of attack to a moving airstream, it causes the airflow to be deflected and accelerated over the curved upper surface of the aerofoil. This results in a lower air pressure on the upper surface of the aerofoil than on the lower surface, creating a net upward force perpendicular to the airstream - lift.

The lift generated by an aerofoil can be explained by Bernoulli’s principle and Newton’s third law of motion. Bernoulli’s principle states that as the velocity of a fluid (such as air) increases, the pressure exerted by the fluid decreases. This principle explains why the air pressure is lower on the upper surface of the aerofoil, where the airflow is faster.

Newton’s third law of motion states that for every action, there is an equal and opposite reaction. When the airflow is deflected and accelerated over the curved upper surface of the aerofoil, it creates a downward force on the airflow. By Newton’s third law, the airflow exerts an equal and opposite force upward on the aerofoil - lift.

The shape and design of the aerofoil also play an important role in generating lift. The shape of the aerofoil is designed to create a favorable pressure gradient that results in lift. The camber or curvature of the aerofoil is also important, as it determines the amount of lift that can be generated at a given angle of attack.

In summary, an aerofoil works by deflecting and accelerating the airflow over its curved upper surface, creating a lower air pressure on the upper surface than on the lower surface, and generating lift by the reaction of the airflow to the deflection and acceleration. The shape and design of the aerofoil are also important in determining the amount of lift that can be generated.

Question 19

What is the critical angle of attack?

Answer 19

The angle of attack which produces maximum lift coefficient, also called the stalling angle of attack. Any further increase above this angle will make it impossible for the airflow to smoothly follow the upper wing surface, the flow will separate from the surface, causing CL to decrease and drag to increase rapidly

Question 20

On a tapered wing, where would the stall occur first?

Answer 20

At the tip. The amount of spanwise flow compounds as you approach the wingtip, decreasing the wingtip’s effective airspeed and thickening the boundary layer.