71. If a tuning fork is struck and held close to the ear and slowly rotated about the vertical axis in one rotation how many times is there no sound?
In mediums which are fluid (e.g., gases and liquids), sound waves can only be longitudinal. So when the tuning fork is held near ear, sound can be heard, but when it moved up or down, the sound cannot be heard. In one rotation tuning fork moves up and back to ear and then down and back to ear. The sound cannot be heard twice ( one, when the fork is above the ear and second time when the fork is bellow the ear). Sound can heard only when the fork is near the ear.
72. Vortex generators
a. move the transition point forward.
Why is this right? Vortex generators don't delay the appearance of the transition point? Please advice
Laminar boundary layers are uninterrupted smooth flow that reduces drag. At the same time they are very fragile and can easily separate from the wing surface when they encounter an adverse pressure gradient.
Whereas turbulent flow speeds up the airflow and gives it more forward momentum. As a result the turbulent boundary layer stays with the surface much longer before it separates from the surface, which in turn delays the stall.
Vortex generators energize the airflow and generate a turbulent boundary layer at early stage whereby transition point move towards the leading edge.
73. Why cabin window shades have to be pulled up before takeoff and landing?
Answer by: Suganthan Nadarajah
This is to provide better visibility in case of accident. The first aiders can see if there is any fire in the cabin, and if any seen, can direct the passengers to use the safe exit passage.
74. Why there are no parachutes on passenger aircraft?
Answer by: Jeshan Mishel
Cost Factor - parachutes on board equate to increase weight and hence resulting in increase fuel consumption and maintenance.
Storage - Large number of parachute storage in a limited spaced aircraft may pose a problem.
Temperature - Evacuation at altitudes above 40,000 feet with temperatures between -45 0C and -60 0C is not compatible for human survival.
Knowledge - Not every passenger is comfortable with jumping off planes and handling parachutes.
Practical issues - Evacuating 300 people of varying ages off a plane may be challenging.
Air crashes are rarely seen in recent times due to the advanced technology and effective safety measures in place. It is apparently 25 times safer than any other form of transport.
There are many reasons: