Relates a propeller’s airspeed with the rotational frequency and diameter. It is closely related to the angle of attack of the propeller blades.

The sudden reduction in lift of an airfoil as the angle of attack is increased. For most airfoils this occurs at about 15 degrees. Note that airspeed in only indirectly responsible for stalls.

The ratio of the current air density to that of standard conditions at sea level.

The cross-sectional shape of a wing or propeller.

The speed of movement relative to the air.

The angle between the chord line of an airfoil and the relative wind. Note that, for most non-symmetrical airfoils, even if the angle of attack is zero there will still be lift generated.

The difference in shape between the upper and lower surfaces of an airfoil. Symmetrical airfoils have no camber.

The length of the chord line of an airfoil.

A straight line connecting the leading edge and the trailing edges of an airfoil.

The different entities that the calculator is designed to edit. Motors, propellers, airfoils, etc.

The equivalent altitude above sea level when temperature effects are taken into account. It can have a profound effect on the efficiency of wings and propellers.

A dimensionless quantity characterizing the drag produced by an airfoil.

The ability to produce useful work from a given unit of energy. Wasted energy is usually turned into heat, sound, or light. It is a dimensionless quantity.

The amount of electric charge moving through a medium. Usually measured in amperes. Think of it as the amount of electrical flow. The higher the current, the higher the electrical resistance and corresponding efficiency losses.

The degree that a material opposes the movement of an electric current. Measured in ohms. The wasted energy is normally converted into heat.

The ability to do work. It is a scalar quantity (i.e., it does not specify a direction). It is closely related to torque, which is rotational force. Common units are the joule, kilowatt-hour and foot-pound.

A gearing mechanism for reducing the RPMs while increasing the torque of a motor. They permit the use of much more efficient larger diameter propellers at some small cost in overall efficiency and weight.

Cycles per second. A unit of frequency.

The power losses in an electrical component due to heat.

A dimensionless quantity characterizing the lifting force produced by an airfoil.

A dimensionless quantity measuring a speed relative to the speed of sound. It is about 750 MPH (350 m/s). As an airfoil approaches Mach 1, both its lift and drag increase dramatically.

The maximum thickness of an airfoil as a percentage of the chord.

A line drawn from the leading edge of an airfoil to its trailing edge equidistant from the upper and lower surfaces. The line can be curved.

Motors are electric; engines are internal combustion. Cheap motors in toys are inrunners that use ferrite magnets. The input voltage determines the RPMs and the input electric current the torque. The motors that we use have a linear relationship between torque and RPM for a given voltage. Maximum torque occurs at zero RPM and results in maximum electric current being drawn. Maximum RPM occurs when there is no load on the shaft. The power produced by a motor is the product of the RPMs and the torque.

RPMRevolutions per minute. of the propeller when it is absorbing 100 watts of power. Defined to be measured at zero airspeed and standard atmospheric conditions. Note that this is not the power going into the motor, but the power available to the propeller.

RPMRevolutions per minute. of the propeller when it is generating 10 newtons of static thrust. This is about 2.25 pounds of force. Defined to be measured at zero airspeed and standard atmospheric conditions.

The electrical current drawn by a motor when the output shaft is allowed to rotate freely. Below this current level, the motor will not turn. Normally measured at 10 volts - sometimes at 8. In theory, this quantity is not dependent on the input voltage. In reality, inexpensive motors require higher current for higher voltages.

EnergyThe ability to do work. It is a scalar quantity (i.e., it does not specify a direction). It is closely related to torque, which is rotational force. Common units are the joule, kilowatt-hour and foot-pound. per unit of time. Common units include the watt and horsepower.

Relates the forward velocity with the power required to turn a propeller. It is closely related to a propeller’s efficiency. It is sometimes used to help determine the correct propeller diameter to use.

An airscrew. Ultimately, it is the thrust produced by the propeller that counts. Every other power system component is there in a supporting role. The bigger the diameter, the better.

Twice the distance from a propeller tip to the center of the hub. This must be as big as possible in order to maximize the propeller efficiency.

The distance that a propeller would move forward in one complete revolution if moving through a solid. If the pitch is too high, then the propeller will be stalled if the forward motion is too low. Some propellers are stalled at the beginning of the take-off run. On the other hand, high pitch is needed for fast moving models such as pylon racers.

A dimensionless quantity characterizing the dynamic behavior of a fluid. It is determined by the viscosity of the fluid, the velocity of the flow, and the size of the object.

How quickly something rotates. Think HertzCycles per second. A unit of frequency. (cycles per second), not MPH.

Revolutions per minute.

Speed controls modulate their output voltage in order to control the rotational frequency of a motor.

A ToolTipA ToolTip is a small text box that appears when the mouse pointer is hovered over a screen object. The box disappears when the mouse pointer is moved away from the object or after a specified period of time, usually a few seconds. is a small text box that appears when the mouse pointer is hovered over a screen object. The box disappears when the mouse pointer is moved away from the object or after a specified period of time, usually a few seconds.

Rotational or angular force. Also called a moment. It is a pseudo-vector quantity. The units are the same as for energy - newton-meter and foot-pound. Joules are not used to measure torque.

Relates a motor’s input current with the available torque.

The difference in electrical potential energy between two points. Think of it as the velocity that electricity moves through a circuit. Measured in volts.

Relates the rotational frequency of the motor to the input voltage. Net input voltage * voltage constant = RPMRevolutions per minute. .

Unit of electrical power. 750 watts is about one horsepower.

Non-symmetrical airfoils still generate lift even if their angle of attack to the airflow is zero. The negative angle at which they generate no lift is called the zero lift angle. For most airfoils this is about -3 degrees, but it is very dependent on the Reynolds number and the exact shape of the airfoil.