### Winning Camp: Aeronautical Engineering Quiz-1

Q1.    Why did the early aeroplanes have small holes in the wing covering near the trailing edge?
Q2.    What are the effects of large camber on the upper surface of the (slow speed) wing?
Q3.    What are the advantages & disadvantages of the convex under surface on the (slow speed) wing?
Q4.      Explain the significance of high lift/drag ratio.
Q5.      Is high lift/drag ratio of the wing always preferable?
Q6.       Describe a common method used in aeroplanes to get variable camber of the wing.

Q7.       Draw a simple flap & a split flap. What is the advantage of split flap over the simple flap?

Q8.      What is meant by aspect ratio? What are its values for sailplanes, gliders, transport aircraft,
fighter planes & hypersonic planes?

Q10.   What are the wing-tip vortices and induced drag?

Q11.  Why ‘induced drag’ is called so?

Q12.   Explain what happens when an aircraft wing stalls?

Q13.    What is the stalling angle for most of the slow speed aerofoils?

1. those are not holes actually those are vortex genrators used to stop sepration of the air in the edge
and using skin fricton drag to reduce form/pressure drag

cl increases as well as cd..there is more sepration and hence induced more drag and more circulation leading to more downwash leading to more lift

3- lesser sepration,better cl/cd ratio,ability to achive higher A.O.A, you do have a space for control surfaces also..but high speed required,lower cl,

4- the lift/drag ratio tells how efficiciant the aerofoil is under a given "genrally all" angle of attacks the aircraft is always designed to do more lift and lessre drag {unless it`s a fighter/stunt aircraft} so usually more lift/drag ratio airfoils are prefferd

6- deployment of split flaps/ control surfaces

7-simple flap only used to make drag to stop the aircraft during landing
but split changes the shape of the airfoil making more lift at lower A.O.A
stalling the aircraft at landing

8-aspect ratio is the ratio of wing is ratio of total wingspan to camberline length
my apologies for the exact values but i can arrange em in ascending order

hypersonic<fighter<transpot<gliders and sailplanes is the same thing

9- pros - lesser velocity required,better gliding ratio,efficiant
cons- more structural load at the wing root spar {bending moment},more drag

10- at the end of wing the high pressure air at the bottem goes upwards to top and make a
vortex sheet no lift is produced

11-cause it is induced by genration of lift no lift no induced drag

12. lift goes max and so drag ,the upper end of the foil experiances sudden drop in pressure and
tempraure so vapour condeses over aircraft and aircraft is in least speed

13- 12 to 13 degrees

1. 1. The holes in the early wings had a different purpose. I give you a hint. The early wings had fabric as the wing skin/covering.

2. Lift increases but also the drag (form).

3. The wing has more depth and hence higher strength to weight ratio. There is a little loss in the Lift.

5. High L/D is not always preferable e.g. when you want to glide down steeply or establish a speed record or even while taking off/landing, the AOA does not correspond to the highest L/D.

7. Split flap gives a smooth upper surface of the wing.

8. Sailplanes may have aspect ratios in excess of 20.

11. Because this type of drag is the result of the downwash (downward velocity) ‘induced’ by the wing-tip vortices. Hence the name.

12. (a) Flow separates from the surface of the wing
(b) Flow becomes turbulent.
(c) Lots of eddies/vortices are formed. Hence drag increases.
(d) Downwash reduces & hence Lift reduces.

2. How does the camber of wing helps in creating additional lift and decreasing g head resistance?

1. Camber increases the circulation around the wing and hence we get more lift. However the size of the low pressure wake behind the wing also increases. Hence the drag also increases.

3. Why helicopters have lower ceiling than fixed wing aircraft ?

1. Helicopters generate lift by rotary motion of the wings/blades. As altitude increases, the max lift that can be generated by the rotor blades reduces since the density of the air reduces. We cannot increase the lift by increasing the speed of the rotor blades. We do not want shock wave to sit on these rotating blades. So we try to keep the max speed on any section of the rotor blade below the speed of the sound. Thus helicopter cannot fly faster just to get required lift especially at high altitudes. Thus the height/altitude/ceiling of helicopter is limited.

Aeroplanes have fixed wings. Even if the speed of the air exceeds the speed of the sound, there is not much problem. Thus aeroplanes can get required lift (Lift = Weight for straight and level flight) even at high altitudes where density is low by just going faster.

The above explanation is given based on Aerodynamics. In general, there is another limitation of engine power also imposed due to decreasing density with increase in altitude. This applies both to the helicopter engine (turboshaft) and the aeroplane engine ( turboprop, turbojet, turbofan).

4. What do we mean by term moment and how does itivary from torque?

5. Moment expresses the turning effect of the force.

Torque is generally linked with rotary motion. The moment associated with rotary motion is often called as torque.

6. Why do birds dont have tails for stability? ? ?

7. What is the difference between AERODYNAMIC CENTER and NEUTRAL POINT ?

1. Aerodynamic Centre is for the wing, whereas neutral point is for the wing-tail combination.

8. How unstable aircraft become stable?

1. Made artificially stable by giving automatic commands (with help of computers)for movement of control surfaces.

2. Fly by system

3. Fly by Wire called FBW or EFCS (Electronic Flight Control System) as is there in Mirage 2000 fighter plane.

9. What is the difference between windmill and wind turbine?

1. Priyanka Singh, you may like to answer this.

2. Wind mill directly converts wind energy to mechanical energy is used to grind grain and pump water.
Wind turbine converts wind energy into electricity, which can then be used to power electrical equipment, stored in batteries or transmitted over power lines.

3. Thank you.

10. At neutral point dCm/dalpha =0 . . . . . So can we say that it is an aerodynamic center of an aircraft? ? ?

1. Yes, you may call it. Good idea.

11. What is saw-tooth leading edge ?

1. the dog tooth stops span-wise flow hence leading to less drag and it makes more average lift per unit span practically

2. A dogtooth is a small, sharp zig-zag break in the leading edge of a wing. It is usually used on a swept wing, to generate a vortex flow field to prevent separated flow from progressing outboard at high angle of attack. Thus it improves airflow and reduce drag at higher speeds.

12. What is auxiliary airfoil ?

1. It is 'a secondary airfoil such as a slat, flap, or tab, that supplements or aids flight in some manner, as by creating an additional force or by providing a smooth airflow'.

13. somebody please explain strees-tensor the simplest way possible. acc to my idea it`s per unit length change in tensile stress on body

1. This comment has been removed by the author.

2. Stress tensor has 9 components of stress on different planes and different directions. (three mutually perpendicular planes and three directions for each plane).

You can view the diagram at the following link:-http://www.efunda.com/formulae/solid_mechanics/mat_mechanics/stress.cfm

14. As the aspect ratio increases. . . The lift slope increases. . . But why did the stalling angle decreased as stalling is related to separation and will aspect ratio change the amount ofseparation

1. As aspect ratio increases, downwash because of wing tip vortices reduces. Therefore induced angle of attack (ratio of downwash to free stream velocities) reduces. It means effective angle of attack increases since Effective AOA = Geometric AOA minus Induced AOA. Now stalling depends on effective angle of attack and not on geometric angle of attack. Thus Geometric stalling AOA reduces (Effective stalling AOA remains the same).

2. can we say that as AR increases the amount of air coming from below the wing reduces. resulting in lesser pressure gain above wing surface that pospones the stalling of wing..
(alpha stall (wing) > alpha stall (airfoil))

3. As aspect ratio increases, the contribution of adverse tip effect reduces. Hence overall lift coefficient increases and the induced drag coefficient decreases.

15. https://youtu.be/p5l3fD2WIQc

What is the cause of these vortices formed between the jet engine and wet ground?

1. It is the result of suction/low pressure in/around the air intake. Moisture gets condensed and can be seen in the vortices. Vortices are created because of viscosity of the fluid.

16. This comment has been removed by the author.

17. WHAT IS THE DIFFERENCE BETWEEN UNIFORM FLOW AND LAMINAR FLOW?

1. Laminar flow occurs in case of viscous fluids when the Reynolds Number is low. In such a case, one layer of the fluid slides over the other smoothly without any intermixing e.g. flow of oil in a pipe. At any cross-section of the pipe, the velocity of the oil is changing in magnitude i.e. it is maximum at the centre and zero at the pipe surface.

Uniform flow is that in which the velocity vector does not change with space. Flow of ideal fluid in a pipe of constant diameter or freestream flow of air (undisturbed flow far way from the moving body) past a moving body like aeroplane etc.

A uniform laminar flow is that in which the velocity vector may vary across the flow cross-section (due to viscous effects) but not along the flow direction e.g. flow of oil in pipe of constant internal diameter.