u/on_aircraft

After my first post here a few days ago, I played around with eCalc some more and adjusted the design parameters.

Drivetrain

I’ve now laid out the drivetrain so that, with the maximum design payload of 3 kg (including a safety margin — the actual maximum payload will be 2 kg), the aircraft has a thrust-to-weight ratio of roughly 2:1 for heavy-lift operation.

For the maximum-flight-time configuration (doubling the battery capacity + around 400 g payload, e.g. a thermal camera), the thrust-to-weight ratio is also now closer to 2:1.

Battery

I’ve also decided to build the battery using 21700 cells, specifically the Molicel INR21700-P50B. By chance, anyone has any experience with this cell?

The cells are rated for 50 A continuous discharge (60A burst), so with a 5P configuration I should theoretically be able to draw around 250 A continuously.

If I apply a conservative 20% derating due to the maximum design flight temperature of 35 °C, that gives me roughly 200 A continuous.

eCalc is estimating a maximum current of 202 A. That seems acceptable to me, but I’d really appreciate some opinions on this. Would you consider that too close to the battery’s continuous limit?

ESCs

The ESCs are also fairly close to their rated continuous current limits.

They should never actually see the maximum rated current in normal operation, but is designing this close to the continuous rating considered common practice, or would you recommend more headroom?

Motor temperature / limiting thrust

My biggest concern at the moment is the motor case temperature.

The >80 °C readings in eCalc — am I correct in assuming that these occur when the motors are operating at or close to maximum throttle?

If so, is there a way to configure ArduPilot + Pixhawk 6C so that I can limit the maximum available thrust to, say, 80% during normal operation, while still allowing 100% thrust in an emergency situation such as a motor-out scenario?

For reference, in the 3 kg payload configuration, eCalc estimates a climb rate of around 4.9 m/s. I definitely don’t need that kind of climb performance, so limiting the available thrust seems like it could be a good way to reduce motor/ESC/battery stress and temperature.

My main question is: would limiting maximum thrust primarily reduce climb rate, or could it also significantly compromise maneuverability/stability to the point of becoming unsafe or uncontrollable?

I’d be interested to hear how you guys would approach the design.

After my first post here a few days ago, I played around with eCalc some more and adjusted the design parameters.

Drivetrain

I’ve now laid out the drivetrain so that, with the maximum design payload of 3 kg (including a safety margin — the actual maximum payload will be 2 kg), the aircraft has a thrust-to-weight ratio of roughly 2:1 for heavy-lift operation.

For the maximum-flight-time configuration (doubling the battery capacity + around 400 g payload, e.g. a thermal camera), the thrust-to-weight ratio is also now closer to 2:1.

Battery

I’ve also decided to build the battery using 21700 cells, specifically the Molicel INR21700-P50B. By chance, anyone has any experience with this cell?

The cells are rated for 50 A continuous discharge (60A burst), so with a 5P configuration I should theoretically be able to draw around 250 A continuously.

If I apply a conservative 20% derating due to the maximum design flight temperature of 35 °C, that gives me roughly 200 A continuous.

eCalc is estimating a maximum current of 202 A. That seems acceptable to me, but I’d really appreciate some opinions on this. Would you consider that too close to the battery’s continuous limit?

ESCs

The ESCs are also fairly close to their rated continuous current limits.

They should never actually see the maximum rated current in normal operation, but is designing this close to the continuous rating considered common practice, or would you recommend more headroom?

Motor temperature / limiting thrust

My biggest concern at the moment is the motor case temperature.

The >80 °C readings in eCalc — am I correct in assuming that these occur when the motors are operating at or close to maximum throttle?

If so, is there a way to configure ArduPilot + Pixhawk 6C so that I can limit the maximum available thrust to, say, 80% during normal operation, while still allowing 100% thrust in an emergency situation such as a motor-out scenario?

For reference, in the 3 kg payload configuration, eCalc estimates a climb rate of around 4.9 m/s. I definitely don’t need that kind of climb performance, so limiting the available thrust seems like it could be a good way to reduce motor/ESC/battery stress and temperature.

My main question is: would limiting maximum thrust primarily reduce climb rate, or could it also significantly compromise maneuverability/stability to the point of becoming unsafe or uncontrollable?

I’d be interested to hear how you guys would approach the design.