BALLOON BOAT

thingiverse

Here is the rewritten text: This water-running boat is a simple way of learning about using compressed air as a source of propulsion. It illustrates the concept of thrust and introduces a practical method to achieve a sealing solution with the intention to throttle air flow. Overview and Background THRUST In the same way that air pushes this boat through water, airplanes get propelled through air. This type of propulsive force is known as THRUST. Its computation is very important in the aeronautics world, and we think you would like to know how it's achieved. Let's take a look first at what's happening. The balloons apply pressure to the air, forcing it to exit. Even though the air exits, it resists movement and applies an equal force back to the balloons in the opposite direction. An action is followed by a reaction - this is Newton's third law! Since the balloons are attached to the strider, the air's reaction is effectively transferred onto it, and this causes it to move in the opposite direction. Action and reaction are equal in magnitude, so computing the force exerted on the air (action) will tell us how much thrust is applied to the boat (reaction). To quantify the force exerted on the air, you need to understand Newton's second law. For this, you first need to be introduced to the concept of "momentum." A body of mass moving at a certain velocity has a momentum equal to the product of these two (mv). We can increase an object's momentum by either increasing its speed or increasing its mass. Think of a snowball rolling down a mountain - as it rolls, it grows larger in mass and so its momentum increases (it is also speeding up...). In its purest form, Newton's second law states that a force is needed to make an object change its momentum over time. The value of such force is equal to the change of momentum desired over the time desired. So if we want a resting tennis ball to change its velocity to 5 m/s and achieve this in a process lasting 1 second, we will have to exert 10 times more force than if we wanted it to acquire the same velocity after 10 seconds. When the object moving is a solid, it's easy to think of the change in momentum as the product of the mass and the acceleration (hence F=ma). But when it's a fluid like air, we take the "snowball" approach. Sitting outside the strider, we see air mass exiting the strider at a certain velocity being increasingly delivered to the atmosphere. So the change in momentum of the air can be related to the change in mass moving at a given velocity. This change of mass over a given period of time is what's known as mass flow. There is a flow of mass of air exiting the balloons of the strider, and there is a flow of mass of air reincorporating into the atmosphere. If we think that the velocity of the air exiting the strider does not change with time during deflation (not an accurate approximation for our balloons but certainly for an aircraft engine!), we can estimate the force on the strider using two balloons to be 0.002N or 19.2 gf (grams-force, nearly the same as the weight of an AA battery). Perhaps not a lot, but definitely enough to move the strider. If no other forces would be acting on the strider, by the end of the 10 s period that it takes for the balloons to deflate, the velocity of the strider would be 312 mm/s. Unfortunately, Thrust is not the only force acting on the strider movement. There is Gravity, Buoyancy, and Drag. Drag is a force due to the friction between the strider body and the water. It depends on the area of the cross section of the floaters that is immersed in water. Although we won't try to compute Drag here, we can say that your strider will travel faster if less cross-sectional area is exposed to the water. More reason then, to balance it properly. What else could be subject to drag? Are the balloons offering a large cross-section area against wind? How would air drag compare to water drag on the body? Lesson Plan and Activity LEARN BY STEPS The recommended order to impart this lesson is: 1) THEORY: go over the concepts and general ideas 2) 3D PRINTING 3) POST-PROCESSING: work on the valve to remove the central wall. Make sure the screw attaches properly to the valve. Else remove any exceeding material. 4) ASSEMBLE: make a seal by cutting off a balloon nozzle (see pics). Follow the drawing (.pdf) to assemble and use screws M3x8. Balance boat to minimize Drag (see pics below) 5) TEST: ready to run!