ThrustXL Ver 1.02

An accurate RPM to thrust converter

by Rod Badcock

Spurred on by Martyn McKinney's comments ThrustXL ver 1.02 is here! This development could not have been made without the data contributors at RCGroups and This is now unlike the previous version and the following has been incorporated: Please download the MS Excel file from here and the prop measurement datafiles, (csv format) from the link; these should both be unzipped and placed into the same directory as each other.

Why do I need ThrustXL?

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

The Spreadsheet has two main worksheet user interfaces: 'thrust' and 'Predictor'


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.

Please note: record the calculated prop coefficient .
This coefficient can be used to predict the thrust curve at any future time.

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

Well I hope that is of use to anybody - it has certainly become a useful tool for myself.


DOWNLOAD Program File here:

DOWNLOAD Data Files here: