My latest project is building a small, fast, 250-class quadcopter. Since it’s designed to be light and agile, keeping weight down is an essential part of the build. A lot of the total weight (~1/4) of the quad comes from the battery, so choosing the right size is a major decision.

I decided to model the flight time that different batteries would provide, based on their

Since the current drawn by the motors would be directly determined by the weight of the quad, I needed to find how much current my motors would pull for a given amount of thrust.

Of course, this is not a linear relationship, so I used a thrust table for the motors I am using to plot some points in GeoGebra, then created a quadratic function to fit them.

Next, I made a spreadsheet which takes a weight and capacity of a given battery, and calculated the flight time. It is available to view here.

The gist is:

- Find total weight
- Find thrust needed per motor (hovering)
- Find thrust needed per motor (if tilted at a certain angle)
- Divide this thrust by a constant, determined by how smooth/aggressive the flight is (~0.95 for smooth flying, ~0.5 for acrobatic flying)
- Use the Current-Thrust model to find the current supplied to the motor to sustain this thrust
- Multiply this by 4 to find the total current
- Using this current, and the battery’s capacity, calculate the flight time in minutes.

I will be interested to compare the predicted times with my actual flight times once I’ve finished the build.

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