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Drive Parameters

DP02.gifThis is the propeller diameter expressed in inches. Stored in Planes database. The power required to turn a prop goes up as the 4th power of the diameter. There are pros and cons to 3-bladed props. They look cool (when the motor's not running) and save a little ground clearance. However, they're significantly less efficient and that's a BIG con.

DP03.gifThis is the propeller pitch expressed in inches. Stored in Planes database . The number represents the number of inches the prop would advance if it were a screw in an inelastic medium. Pitch is normally measured at 75% of the distance from the center to the tip. The best pitch for most electric applications is between half the diameter (P/D=0.5) and the diameter (P/D=1.0). This assumption is used in some of the tools. TIP: try modeling a 6-10 pitch as an 8.5 or 9.0.

DP04.gifThis is a propeller constant used in calculating the power absorbed by a propeller. Stored in Props database. It indicates the manufacturer’s generic prop shape and its’ effect on power. A prop with a higher Kprop absorbs more power for a given diameter and pitch. This number is difficult to measure, and much of the data is empirical. However, there is substantial variation in prop factors (usually pitch) in what you buy which often overshadows the accuracy of other numbers.

DP05.gifThis coefficient accounts for efficiency differences between props, e.g. 2- vs. 3-blade. Stored in Props database . This coefficient is new to Version 2. The coefficient Kpitch from Version 1 is still used but can only be changed on the database screen .

DP06.gifThis is the number of revolutions the motor must make to rotate the prop one revolution. Stored in Motors database. There is a gearbox efficiency number in the ecalc.ini file. You can change this by editing this file. ElectriCalc will not let you put unreasonable numbers here.

DP07.gifThis is simply a brief description of the prop and manufacturer. Stored in the Props database .

DP08.gifThis is the Props database control. If you click the big button, you jump to the Props database. The smaller buttons step you back and forth through the database. Use CTRL_P as a shortcut to access the database. This selects different prop types. This includes Kprop, Keff, Kpitch, and a description of the prop.

DP09.gifThis is the rotational speed of the prop in thousands (K) of revolutions per minute. Dividing the motor RPM by the Prop RPM yields the gear ratio. When using a gearbox, you need to be aware that the motor is turning faster than the prop. There is a limit to how fast a motor should be allowed to turn. These limits are due to such factors as type of bearings, brushes, how the windings and magnets are secured, etc. These limits range from about 20K to 50K+. Check your motor data sheet.

DP10.gifThis is the power supplied to the prop. It is an empirical equation which generally provides a reasonable approximation. This is the power into the prop from the motor/gearbox. How much of this actually moves the plane through the air is a function of the efficiency of the prop. This is a complex function. Best to experiment or share the experiences of others.

DP11.gifThis is the ratio of the power into the motor(s) to the total weight of the plane. This is a “rule of thumb” measure of a plane’s flying ability. In general, 50 watts/lb. is indicative of a plane with “adequate” power, with higher numbers implying greater aerobatics capability. This is based on brushed “cobalt” motors. Ferrite motors generally require somewhat higher numbers for the same flight performance. Although this is a handy rule-of-thumb, ElectriCalc provides more important data such as climb rate and cruise time. Watts per pound doesn't guarantee your plane will climb.

DP12.gifThis is the power loss in the gearbox. The benefits of using larger, slower-turning props generally outweigh these small losses. The gearbox efficiency factor is found in ecalc.ini and can be changed using a text editor.

DP13.gifThis is the theoretical speed of a plane with zero drag, based solely on the pitch and RPM of the prop. Planes with low drag can fly faster than this number because of the 'lift' generated by the prop blades and the 'unloading' of the prop at higher speeds.

DP14.gifThis is the force in ounces or grams provided by the motor(s) to pull or push the plane through the air. The difference between thrust and drag is what can provide acceleration and climb. There is controversy about the value of static thrust measurements as a figure of merit. If you study a typical thrust graph , you will see why these measurements can mislead. Lower-pitched props yield the highest static thrust, but this drops off more rapidly than a higher-pitched prop.

DP15.gifThis is the force in ounces or grams due to the drag of the plane. This includes drag due to creating lift as well as drag due to the shape of the plane. At higher speeds, this second component is dominant, and is proportional to the square of aircraft speed.