EXCERPTS
FUNDAMENTALS OF DRAG AND LIFT (pp 6-7)
Imagine that you sit in the passenger’s seat of your friend’s Alfa Romeo, and you are being driven at some delightfully illegal speed on a newly paved straight road. You stretch out your hand, palm facing forward. You feel your hand being pushed back by the wind. This force becomes stronger as your friend accelerates to a yet higher speed, and becomes weaker as he decelerates when he notices a police patrol car parked at the side of the road. Being observant, you quickly conclude that the force on your hand is proportional to the square of the speed of the Relative Wind.
This force, which we will agree to call DRAG (a term which has no connection to unconventional attire), is due to your hand’s deflecting the oncoming air away from its original direction, equally upward and downward in this specific case. Whenever you want to change the direction of a flowing fluid, you must impose a force on the fluid. The fluid then pushes back with equal force. We owe this “tit for tat” insight to Sir Isaac Newton (1642-1727, best remembered for his encounter with an apple), whose third law of motion states: “for every action there is an equal and opposite reaction”. Substitute the word “force” for “action”, and you have it!
Having a penchant for scientific experimentation, you next turn your hand so that it is at an angle to the wind. The air is now deflected downward. In addition to the drag force on your hand, you now feel your hand also being lifted upward. This force component, you conclude, is proportional to speed squared just like drag. Since it wants to lift your hand, we will agree to call this force LIFT. As your friend pulls over and brings the car to a stop in response to the siren from the police cruiser behind, the lift force drops to zero, as does the drag. Clearly, movement is needed to generate these forces.
ANECDOTE ILLUSTRATING THE COANDA EFFECT (p 45)
Imagine that you have just been invited to your first meeting with the mother of your Intended. The antique Wedgwood tea set is beautifully laid out on the mahogany table, cups and saucers on little doilies, the teapot covered by a tea cozy. After the requisite four minutes of steeping, she invites you to pour.
You rise to the occasion, remove the tea cozy and pour. Alas! Instead of taking the planned path to the cup, the stream of tea chooses to follow the spout and come to rest on top of the table, forming a slowly spreading pool of fragrant Earl Gray.
This is where your knowledge of fluid dynamics will serve you admirably. You keep your composure, look reflectively at the mess and say “Aah! The Coanda effect!”.
Such sangfroid, combined with demonstrated scientific insight, so impresses the mother of your Intended that you instantly rise several notches in her esteem. Your education has not been wasted after all!
THE BASICS OF JET PROPULSION (p 60)
Imagine that you are skating at the local rink, and your long-time boyfriend starts ogling the just-arrived figure skating team. Your kneejerk reaction to set him straight has the result of bringing your skating to a halt, leading to an eyeball to eyeball confrontation with said boyfriend. You take off your glove and throw it at him.
This action has an unexpected outcome: you begin to move backwards. Next, you throw the other glove at him, and your backwards speed increases. Having a penchant for physics, you quickly come to the realization that Sir Isaac was right when he pronounced that force equals mass times acceleration and that for each action there is an equal and opposite reaction. The force which pushed you backwards was exactly equal to the mass of your glove multiplied by the acceleration you imparted to it when you threw it at the insensitive oaf.
This simple example illustrates the basic principle of rocket propulsion. Structure, payload and propellant, which make up the initial mass of the rocket (in the present example, you with the clothes you are wearing) is gradually depleted by ejecting the propellant (initially, your gloves), thereby generating a force which accelerates you in the opposite direction until only the structure (skates) and the payload (you, in your birthday suit) remain.