Model Rocket Aerodynamics
Aerodynamics is the study of the way air flows around various shapes and the forces that are created in this process. Moving air can exert considerable force on an object. Properly used, it can stabilize a model rocket in order to predict its path. Uncontrolled, aerodynamic forces can rip a model rocket to pieces.Drag is created by a rocket during flight as it pushes air out of the way, slides through the air, and allows the air to close in behind it again. The shape of the rocket and the way air flows around it have a major effect on the amount of drag produced.
Several forms of drag are of interest to model rocketeers. They are as follows: Friction drag, Pressure drag, Interference drag, Parasite drag and Induced drag.
Friction Drag
When air moves over the surface of a model rocket, friction results between the air and the surface. The rougher the surface the greater the friction drag. To overcome the effects of friction drag, it's important that the entire surface of the model rocket be as smooth and glossy as possible.Pressure Drag
The impact of air against the surface of a model rocket is called pressure drag. A model rocket moving through the air at 250 feet per second doesn't give the air enough time to get out of the way. The air must be shoved aside by the nose which creates pressure drag.Base drag, a form of pressure drag, is created as air pushed aside by the nose cone moves past the rear of the rocket. The air begins to swirl and become turbulent, which retards the motion of a rocket. This is known as a wake. To reduce base drag a boat tail, tapering the rear end of a rocket, can be used.
Interference Drag
This drag is caused by the interference in the flow of air between the surfaces of the body tube and the fins of a model rocket. A rocket with three fins will have 25% less interference drag than a rocket with four fins due to the decrease in fin surface area.Interference drag can also be reduced by the use of fillets. A fillet is a reinforcement of the joint between the fin and the body tube of the rocket to improve the rocket's aerodynamics and to strengthen the fin mount. A satisfactory fillet for most small model rockets can be made with a bead of glue.
Another method of reducing interference drag is to move the fins forward (toward the nose) by a distance of about one body diameter. This slightly forward location of the fins permits the airflow to smooth out behind the fins before it reaches the base.
Parasite Drag
This drag is caused by anything that sticks out from the body tube and interrupts the smooth flow of air. A major source of parasite drag on a model rocket is the launch lug. The launch lug can amount to as much as 35% of the total drag of a model rocket. However by locating the launch lug in the fin-body joint of one of the fins reduces the parasite drag to about 20% of the total drag, a 15% savings.Induced Drag
This drag is created by the lifting force caused by airflow over the surface of the fins. The majority of induced drag is generated by the tip of the fins. As air flows past the tip of a fin it swirls, known as a vortex. The larger the vortex the greater the induced drag. The simplest way to reduce tip vortex is to shape the fin tips properly. An absolutely square fin tip has the lowest induced drag.The shape of a fin also has an influence on the amount of induced drag. Swept fins create the most induced drag. They also tend to flutter or vibrate, at high speeds, which creates additional drag.