Trade Study
The Trade Study at https://rotorlab.app/sweep sweeps one variable of a build across a range and re-analyzes the whole aircraft at each point. Instead of asking "what does 500 g of payload do to endurance", you see the entire curve from 0 to 1500 g at once, with the best point marked. Use it to size a battery, set a payload budget, or pick a propeller against a target figure.
Each point is a full re-analysis of the build with just that one variable changed, so every interaction is captured: more payload raises hover current, which raises sag, which lowers usable thrust-to-weight. It is a first-order model; use it to understand the shape of the trade-off, not as a specification.
Setting up a sweep#
- Pick the Build to sweep: Current build (from the app) (the working build), the Example profile, or any Library: build. A note confirms which build the sweep is running on.
- Pick the Variable to sweep. The available variables are:
| Variable | Unit |
|---|---|
| Payload mass | g |
| Battery capacity | mAh |
| Dry mass | g |
| Prop diameter | in |
| Prop pitch | in |
| Motor Kv | KV |
| Altitude | m |
- Set the From and To ends of the range (each variable prefills a sensible range) and the number of Steps (default 21, allowed 3 to 40).
- Pick what to Plot. The sweep recomputes as you change any of these.
Note
A battery capacity sweep carries a realistic mass penalty: the pack's mass scales with its capacity, so a bigger pack weighs more in every figure downstream. That is what makes the endurance-versus-capacity curve flatten out instead of climbing forever.
The metrics#
Any of these can be plotted; the rest are still computed at every point, so switching the plot is instant:
| Metric | Unit | Better |
|---|---|---|
| Hover endurance | min | higher |
| Cruise endurance | min | higher |
| Range | km | higher |
| Thrust-to-weight | higher | |
| TWR under sag | higher | |
| Hover efficiency | g/W | higher |
| Top speed | km/h | higher |
| All-up weight | g | lower |
TWR under sag is the thrust-to-weight ratio after the battery's internal resistance drops the voltage at hover current: the usable figure on a real pack rather than the full-charge number. Sweeping payload or capacity against it shows where a build stops having real thrust headroom, not just paper headroom.
Metrics that do not apply to the build (cruise endurance on a pure multirotor, for example) simply show no value.
Reading the result#
- The headline shows the best value of the plotted metric and the variable setting that produces it, for example "best hover endurance at Battery capacity = 9,000 mAh".
- The chart draws the metric across the range, with a green marker on the best point.
- The table below lists a subsample of about eight rows across the range. The best point is always included and highlighted, even if the subsampling would have skipped it.
Sizing with a sweep#
Two worked patterns:
- Sizing a battery: sweep Battery capacity and plot Hover endurance. The curve rises, flattens, and can fall as pack mass overtakes the added energy. The marked best point is the capacity worth paying for; beyond it you are lifting battery to carry battery. Then switch the plot to TWR under sag and confirm the chosen capacity still leaves acceptable thrust headroom.
- Setting a payload budget: sweep Payload mass and plot the metric your mission actually needs (Hover endurance for station-keeping, Range for transit). Read off the payload at which the metric crosses your minimum requirement; that crossing, not the zero-payload figure, is the number to put in the spec. Check All-up weight at the same point against any regulatory weight limit you operate under.