Aerodynamic Parameters

Chord Length: This is the length from the leading edge of the blade to the trailer edge.

Angle of Attack: This is the angle between the relative airspeed and the chord of the wing. In general, a greater angle of attack is associated with an increase in lift.

Cl: Cl is coefficient of lift, which is usually determined experimentally, but can be calculated as well. A lot of data is available for different airfoils that exist today. Cl is affected by the shape of the airfoil, the angle of attack, and the Reynolds Number.

Reynolds Number Reynolds number is a dimensionless number that is the ratio of inertial force divided by viscous force. It is typically used to account for dynamic similarity. One of main differences between full scale aircraft/helicopters and model aircraft/helicopters is this difference in Reynolds number. When testing an engineering scale model, it is usually important the Reynolds number and the Mach number are the same for the model and the full size design. Reynolds number=D*V*r/mu where D is a charactersitic length (chord), V is velocity, r is density, and mu is viscosity.

Mach Number Mach number is a dimensionless number that is the ratio of an objects speed divided by the speed of sound in that medium. It is typically used to account for dynamic similarity. When testing an engineering scale model, it is sometimes important the Reynolds number and the Mach number are the same for the model and the full size design. Mach number=V/Vsound where V is velocity and Vsound is the speed of sound in that medium. The closer the model/full size design is to 1 or higher, the more important it is to match the Mach number. This is because as the speed of an object apporachs the speed of sound, the density of the fluid increases greater where it contacts the object.

Cd: Cd is coefficient of drag, which is usually determined experimentally, but can be calculated as well. For an airfoil, Cd=Cd0+Cdi where Cd0 is drag coefficient at zero lift and Cdi is the induced drag coefficient. Cdi is the unfortunate byproduct of lift described above, the equation for Cdi is Cdi=Cl^2/(pi*AR*e), where Cl is the coefficient of lift, AR is aspect ratio, and e is an efficiency factor.

Lift: Lift is just what it sounds like; lift is created by a pressure differential between the top and bottom of the blade. For typical airfoils, lift is directly related to the air speed and angle of attack. The equation to calculate lift is L=.5*Cl*r*v^2*A, where L is lift, Cl is coefficient of lift, r is density, v is relative air velocity, and A is wing area.

Drag: Drag is also like it sounds; drag is the force opposing motion of you helicopters blades and is also an unfortunate byproduct of lift. The drag from the blades spinning is why a tail rotor is required. A lot of people mistakenly believe that the tail rotor is there just to oppose the main motor’s torque; however, in a vacuum not only would a helicopter not fly, but it always would not have a constant rotation. It would rotate slightly as the blades accelerated/decelerated, but after they reached a constant speed, the helicopter would not try to turn. In air, the constant drag of the air against the blades drive the requirement of a tail rotor. You may have noticed that if you damage your main blades your helicopter now turns more to the left (counter clockwise), this is because there is an increase in air drag force. Therefore, any reduction in the drag will result in less force needed to keep the helicopter straight. The equation to calculate drag for airfoil is D=.5*Cd*r*v^2*AR, where L is lift, Cd is coefficient of drag, r is density, v is relative air velocity, and AR is aspect ratio.

Lift to Drag Ratio: This is a very important number, it measures the ratio of lift (yeah, cheers!!) to drag (boo, hiss!!). There is a constant compromise in aerodynamics between lift and drag, it is difficult or impossible to increase lift without some kind of increase in drag. The question becomes how much more lift can you get versus how much more drag.

Aspect Ratio: This is a ratio of the length of the blade divided by its chord length. High aspect ratio wings have long thin wings, while low aspect ratio wings have shorter thick wings. An example of an aircraft with a higher aspect ratio is a glider, while most fighter jets have low aspect ratios. A higher aspect ratio has a greater lift to drag ratio, which makes it well suited for gliders. Since fighter jets travel at high speeds, reducing drag is an important part of their design.

Camber: Camber is another measurement that defines the shape of the airfoil. If you created a line with all the points halfway between the top and bottom of the airfoil, you could create the mean camber line. Camber is the difference between this line and the chord line. In a symmetrical airfoil, the camber is 0 and the mean camber line is the same as the chord line. An increase of camber will typically create more lift at a lower angle of attack/lower speed, along with more drag.

Airfoil: A section of the blade, this section clearly shows the chord length, the thickness of the section, as well as the camber. The shape of the airfoil has a large effect on the Cl and Cd.