fpv.cat
EN

Chapter 21 · Extras

The numbers, instantly

The sums that keep coming up when you pick parts, fly in a group or tune. Change the values and the result updates itself.

Reviewed: · Found a mistake? Let us know

Motors

Theoretical motor rpm

KV × voltage. Useful for comparing motors of different KV on different batteries. In flight, with a prop on, the real rpm is lower.

At nominal voltage—
With a full battery—

Charging

Charging time

How long an empty battery will take to charge at the current you choose. To make it last longer, charge at 1C or less.

Approximate time—
Current at 1C—
Charge rate—

The last stretch of the charge is slower, so the real time is usually a bit longer.

Battery

Maximum battery current

Capacity × C. Many brands inflate the C figure: take the result as a theoretical maximum.

Theoretical maximum current—

At the field

Battery state per cell

Enter the total voltage shown by the OSD or the multimeter, at rest, and you’ll know what state each cell is in.

Per cell—
State—

Reference: full 4.20 V · storage 3.80–3.85 V · land 3.5–3.6 V · minimum 3.0 V.

Tuning

P/D balance: raise the gains without losing it

Once you’ve found the P and D that work well on your drone, the balance (P ÷ D) shouldn’t change. Enter your current values and the new D you want to try: we’ll tell you which P goes with it.

P/D balance—
New P—
Change in gain—

Raise it little by little and fly a battery between each change. Stop when propwash no longer improves or the motors get hot: that’s the drone’s mechanical limit.

Filters

PT1 low-pass filter: what it removes and what it costs

Choose the filter’s cutoff frequency and any frequency you like: you’ll see how much signal gets through and how much delay it adds. Try it with motor noise (300–500 Hz) and with the drone’s movement (below 50 Hz).

Signal that gets through—
Attenuation—
Delay at this frequency—
Delay of slow movement—

A PT2 attenuates twice as much per octave and a PT3 three times as much, but they also add more delay. The delays of each filter add up.

Sticks

Rates comparer and converter

Betaflight has five ways of writing rates and they all draw the same thing: how many degrees per second the drone rotates depending on where your stick is. Enter yours (or paste the diff), look at the curve and convert them to another type without the drone feeling any different. Handy for copying the rates of another pilot or a simulator.

Paste a Betaflight diff

It calculates with the same formulas as Betaflight. If you use a simulator, put the same rates in it: that way what you learn there will serve you at the field.

Design

Thrust, current draw and flight time

Before buying motors, props and a battery, see how the combination will turn out: the thrust-to-weight ratio, at what throttle point it hovers, how much current it draws and how many minutes you’ll fly.

Thrust-to-weight ratio—
Maximum thrust per motor—
Hover throttle—
Current at full throttle—
Current in hover—
Maximum rpm under load—

Flight minutes (using 80 % of the battery)

Steady hover—
Cruising or filming—
Freestyle—
Racing—

It’s a simplified model with typical values for FPV props and motors: allow for ±20 %. The real data for each motor (the manufacturer’s thrust tables) always take precedence.

Flying in a group

Video channel planner

Tell me how many pilots will fly and with which system, and I’ll suggest the channels that interfere least with each other. It takes into account channel spacing and intermodulation: two transmitters close together generate “ghost” signals (2·f1 − f2) that can stomp on a third pilot.

Digital systems (DJI, Walksnail) show the frequency in MHz in the goggles menu: set it as a fixed frequency. HDZero uses the Raceband channels. In the EU, without a licence, the limit is 25 mW at 5725–5875 MHz; power levels or channels outside this band require an amateur radio licence.

Radio

ExpressLRS range: the link budget

Add up the power and the antennas, subtract the path losses and compare it with the receiver’s sensitivity at each packet rate. The result is the theoretical range and, above all, the margin you have left at a given distance.

Link budget—
Theoretical range (free space)—
Range with environment and margin—
RSSI at the distance—
Margin at the distance—
Fresnel zone (radius)—

Approximate receiver sensitivities for each mode. Free-space theory is very optimistic: what really limits you are obstacles, antenna orientation and height (keep the Fresnel zone clear). Use the margin: below 10 dB, turn back.