ThrustXL Ver 1.02

An accurate RPM to thrust converter

by Rod Badcock

• Expanded thrust measurement database (72 props) for wider prop selection
• Overlay of measurement data on the predicted curve for comparison
• Separate 'predictor' that produces a thrust/RPM curve from a single measurement
• Graphing of all results

Knowing the static thrust values (with current and voltage values also) from a basic Tacho measurement helps reduce risks with the batteries and speed controllers.

This conversion allows a close to optimum prop selection for a given motor and power system without the need for a dedicated thrust test stand. Static performance is no indicator of how it will perform in flight, but by obtaining a reasonable balance of static thrust and pitch speed, one can at least have a conservative flight-testable system [which won't result in instant melt-down, and can then be tweaked for optimum performance].

The program

This is the main section where thrust data is predicted based on a database of measurements carried out on 73 different props. The user interface is quite simple and requires input in two areas; selection of the prop type from the drop-down box and entry of the measured RPM for that prop. The program then predicts the thrust for the RPM, and displays a graph of the selected data point, predicted thrust curve and the measurement data points.

Three additional worksheets are used for this window: 'Coef.' contains the prop and coefficients database, 'Curve' contains the predicted curve data and 'Measured' contains the looked-up thrust-stand measurements.

This is the section intended to remove much of the tediousness of characterising a prop / thrust measurement. Ideal propellor behaviour will have a quadratic Thrust to RPM behaviour. Previously most prediction software has considered idealised propellors and has not accurately predicted thrust behaviour.

In this implementation a single thrust-stand measurement can be used to generate a close fitting thrust/RPM curve for a given prop. The algorithm used evaluates the measurement data for the quadratic coefficient and produces the thrust graph.

The user interface requests three parameters: description of the prop, measured thrust and measured RPM. The program calculates the prop coefficient and produces the thrust graph.

The relationship between thrust, RPM and the coefficient (k) is:
Thrust = k . (RPM)^2

Rod