By Lauren Nagel

Introduction

When it comes to drone performance, there is a necessary trade-off between two important factors: payload and flight time.

Having a good payload capacity is increasingly important for delivery drones, cargo drones, search and rescue drones, and more.

Being able to predict how payload will affect your flight time can help you set realistic goals and expectations.

In this article we will cover:

1. Drone payload calculations
2. How to increase DC Motor RPM (and thrust)
3. Drone payload comparison with flight time calculator
4. Bonus: How to increase your flight time and payload capacity

Figure 1: Agricultural drone carrying a payload

Drone Payload Calculation

We can start with the assumption that adding payload decreases flight time. 
Most people assume this intuitively, but why is it true? For the purpose of this article, we are assuming that the payload is increased while the drone’s components stay the same. Note that decreasing the airframe’s weight is a good option too and it has the same effect as reducing the payload’s mass.

It ultimately comes down to this: to overcome a higher weight, more thrust is required from the propellers, which requires higher RPM, which draws more power from the battery, thus decreasing the available battery life and flight time.

Let’s look at it in a bit more detail:

When you add mass to a UAV, you increase the amount of thrust required to lift it off the ground.

When a drone is hovering, it is in equilibrium so the thrust is equal to the weight:

Figure 2: Drone hovering in equilibrium

For most drones, you want their max thrust to be about twice as much as is required to hover, which allows them to take-off, climb, and operate smoothly. Ultimately, greater mass = greater thrust requirements.

Further reading: How Much Payload Can a Drone Carry

There are a few ways to calculate thrust, but it is generally dependent on several factors, including propeller diameter and pitch, air density, velocity, and rotation speed (RPM), as demonstrated by this equation for dynamic propeller thrust:

Based on this equation, derived by Electric Aircraft Guy, there are a couple ways we can increase thrust: we can change the dimensions of the propeller, or we can increase the RPM. This equation is quite generic. For a more accurate thrust, consider testing with a thrust stand.

Since we are keeping our propellers constant, we will therefore have to increase the RPM.

How to Increase DC Motor RPM

To increase the RPM, we have to draw more electrical power from the battery, specifically more voltage.

Learn more: watch this video to see how voltage affects motor speed.

Battery capacity is often expressed in Watt-hours (Wh). If you have a capacity of 70 Wh, you can draw 70 watts of power for 1 hour or 1 watt of power for 70 hours.

The more power drawn, the shorter the life of the battery. And this is where we come full circle.

Since we will be drawing higher voltage (and therefore greater power) to reach a higher RPM, the battery life will deplete faster, and our flight time will be lower.

Drone Payload and Flight Time Comparison

Next let’s look at this in practice. We have developed a drone flight time calculator that allows you to input your drone’s information, such as drone and battery weight, battery capacity, and the # of propellers. It combines this information with propulsion data and estimates your flight time for you.

For this article, we will borrow data from this propulsion system with a T-Motor MN4014 motor and an APC 12x6e propeller in order to demonstrate the relationship between payload and flight time.

Figure 3: Drone Flight Time Calculator with sample data

With zero payload and the mass of the drone at 1.6 kg (battery included), we get a flight time of 39.6 minutes.

As we increase the payload by 0.2 kg increments, watch what happens to the flight time: 

Figure 4: Drone Flight Time vs. Payload graph

How to Increase Drone Flight Time and Payload Capacity

The equation below shows how battery capacity and power draw relate to the flight time of the drone. The more power you draw, the shorter the flight time will be:

There is one simple way that you can increase both the flight time and payload capacity of your drone, and it all comes down to one key variable: efficiency. Increasing the efficiency of your propulsion system reduces the power drawn from the battery, thus increasing your flight time.

Improving efficiency is not the subject of this article, but we have a full guide on How to Increase Drone Flight Time and Lift Capacity, which you can read if you are looking to improve your drone’s performance.

Conclusion

In this article we have demonstrated how drone payload affects flight time. The key variables are thrust, RPM, electrical power, and battery capacity.

If you have any outstanding questions or topics you’d like us to cover, leave us a comment below.