Left behind because it seemed overwhelmed by the reactors or jets ,with the search for energy saving, the propeller of the boat and aircraft becomes again a technical solution leading. The reactor was a symbol of power and modernity but its low efficiency (20 to 30 % max)is a fact and the propeller comes to the front of the stage with 80 to 95 % efficiency for the contra rotating propeller
Before discussing the characteristics of marine or air screw propellers, introduce some concepts that will help us better understand the functioning of propulsive screw propellers (aircraft, ships, ventilation)
A ship screw propeller, an aircraft screw propeller or fan ventilation, have different forms , because the implantation constraints, and the ambient fluid are different, but the principles generate the propulsive force, explained here, are identical..
As Newton explained to us, the force is equal to the product of the mass per acceleration F= M x AThe thrust force will be proportional to the mass of the accelerated fluid. To generate the same force we can::
|Accelerate slightly a large amount of fluid. Propeller large size generating little lift on the blade surface) this case corresponds to a propeller lightly loaded.||Greatly accelerate a small amount of fluid. (Propeller small size generating a lot of lift on the surface of the blade) this case corresponds to a heavily loaded propeller.|
- A little story for imaging this relation between mass and acceleration; Let us make a canoe race. As oar, we have the choice between: shovel Pizza or a coffee spoon. Knowing that pushing hard on the pizza shovel,or rowing quickly with the coffee spoon, we will spend the same amount of energy.
In your opinion, who will waste the most energy in leakage around the surface, transmitting force to the fluid? one who rowing at high speed with a teaspoon, or one who chose the pizza shovel? You choose what, instinctively without thinking?
There is a cheat, he rowed at high speed with a pizza shovel!All that to say that the greater the propeller, the less there will be leaks, and the better you will. This applies to ship screw propeller, aircraft screw propellers or fan ventilation. A further more theoretical point of view, but very important, allows us to evaluate the profitability of our effort relative to the speed of movement: the propulsive efficiency