How to Calculate BLDC Motor Thrust?

What is a BLDC Motor?

A BLDC Motor that is brushless has no brushes, as the name suggests. The rotor and the stator are the two distinct parts that make up a brushless motor. BLDC Motor has revolutionized the world of electric motors, offering numerous advantages over their brushed motor. In this comprehensive guide, we will explore the principle of operation of brushless dc motors, highlighting their key features, benefits, and applications. Whether you are an enthusiast looking to build your own drone or someone interested in understanding the inner workings of these motors.

meps 2207

What is BLDC Motor Thrust?

Thrust is a reaction force described quantitatively by Newton’s third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that system. The force applied on a surface in a direction perpendicular or normal to the surface is also called thrust. A fixed-wing aircraft propulsion system generates forward thrust when air is pushed in the direction opposite to flight. This can be done by different means such as the spinning blades of a propeller, the propelling jet of a jet engine, or by ejecting hot gases from a rocket engine. Reverse thrust can be generated to aid braking after landing by reversing the pitch of variable-pitch propeller blades. Rotary wing aircraft use rotors and thrust vectoring V/STOL aircraft use propellers or engine thrust to support the weight of the aircraft and to provide forward propulsion.

BLDC motor thrust, to put it simply, is the maximum upward force your drone can create when it is fully charged.

Refer to Wikipedia: https://en.wikipedia.org/wiki/Thrust

Why does Thrust Matter for FPV BLDC Motor?

Oftentimes, thrust is the primary consideration when selecting a bldc motor. It is, after all, the force that gives your drone its lift and enables it to carry out those amazing aerial stunts. Higher thrust leads to faster acceleration, although efficiency and current draw are also important considerations. Selecting a bldc motor and propeller combination that requires high current will overtax your batteries and reduce their longevity. Ascertain that your battery’s maximum discharge rate is sufficient for the work if your drone uses a considerable quantity of electricity when it is operating at a high throttle.

Therefore thrust is certainly an important aspect to consider when choosing a bldc motor. If you want to learn more about the knowledge of choosing motor, read this guide on  How to choose fpv drone motor a detailed guide 2023 meps-quadcopter-motors

Calculating Thrust-to-Weight Ratio For FPV Drone

By adding up the entire weight of your quad components (all-up weight, or AUW) and comparing it to the estimated thrust for your motor, prop, and lipo battery combination, you can calculate the thrust-to-weight ratio of your possible quad construction. You can determine how light or heavy your quad feels in the air by figuring out its thrust-to-weight ratio.

A quad’s total weight can be easily determined by summing the weight of all of its components and payload (such as a GoPro, if you want to fly one) in grams. The wiring and other accessories, such as battery straps, will require an additional 20–25 grams.

It seems sense that you can estimate this and change it later if you haven’t chosen any parts yet besides your frame. Use the product information to determine the grams that each of the other parts you’ve picked for your quad, such as the flight controller or video transmitter, weighs.

It’s trickier to calculate thrust without first purchasing the necessary parts and conducting your own experiments. Due to the limited product information available, we can only estimate thrust. Because it’s simple to use and provides a reliable estimate of the amount of thrust to anticipate, this estimation approach is excellent. This technique works with any bldc motor that has thrust table information. As an alternative, you might Google bldc motor thrust testing that have previously been carried out.

You must view the motor thrust tables and navigate to the motor’s product details in order to estimate thrust without having to purchase your parts beforehand. You can see how the quad motors work with a particular prop and battery combination in these tables. Find out how much thrust a certain prop and battery combination can produce in grams at maximum throttle.

Example Thrust-to-Weight Ratio Calculation:With MEPS 2207 for Example

Assume for the moment that the total weight of our 5-inch quad was 1000g.

MEPS 2207 thrust chart:

meps-2207-motor-thrust-chart

We can determine that MEPS motors 2207 are generally suitable for 5-inch quads by using our motor criteria. In the chart (which is boxed in red), we discovered the thrust is 1641.2g throuh the match of meps motors 2207, propellers and battery (shown by the voltage) . We’ll multiply that by four because we’re making a quadcopter, and the result is 6564.8g of thrust.

So for this match, the thrust-to-weight ratio is 6564.8 : 1000 ≈ 6.7 : 1

If this is the kind of quad you want, you may decide with your thrust-to-weight data. Generally speaking, you want to aim for a ratio of at least 2:1 so that your quad can stay at half-throttle. This exercise will help you to alter your fpv motors need based on the type of flying you choose to accomplish. To change the feel, you can also change the sections that make up your total weight.

The more thrust to weight ratio, your quad will have greater responsiveness. Quads with high ratios are typically built by freestylers and racers, whereas cinematic pilots seek out lower ratios for more controlled flying. Cinematic flyers might like a ratio of almost 2:1. Freestyle pilots typically utilize quads in the 3:1–7:1 range. Additionally, quads with a ratio of at least 15:1 are frequently observed.

Determine the thrust-to-weight ratio of your hypothetical quad so that you have a benchmark for future designs. The key to your first build is to make a well-informed guess and work your way down from there. You will be able to feel the thrust-to-weight ratio of your quad once you build and fly it, and you can use that sensation to figure out what ratio you want in a rebuild or your future build.

Other Powertrain Components Are Selected Based on BLDC Motor Thrust

Propeller

You can use this as an opportunity to adjust and acquire a higher or smaller thrust-to-weight ratio, or you can utilize the same props (or props with the same length, pitch, and blades) to get the thrust-to-weight ratio you computed before. The predicted thrust of your quad may now be calculated after you’ve selected your fpv motors and determined the type of propellers utilized. Pitch and prop length increases usually yield faster speeds at the expense of higher power draw. Props that are shorter also spin faster and are more responsive. It is typical to fly with a propeller that has two or three blades. The quad is more efficient and requires less power when there are fewer blades on the props. The quad becomes more inefficient while using more power as it has additional prop blades.

ESC

The amp rating is the most crucial consideration when choosing an ESC. An ESC’s specifications will indicate if it is 15A, 30A, 40A, etc. The continuous amps that your ESC can manage are shown by the amp rating. The continuous rating and the burst rating are the two amp ratings for ESCs. The amount of amps your ESC may pull continually is indicated by the continuous rating. This is equivalent to the current you use for the most of your flight. The burst rating on your ESC indicates how many amps it can pull for a brief period of time, such as when you throttle to 90–100%. These two amp ratings are crucial since, in some circumstances, your ESC may fry or catch fire if its amp value is too low, particularly while operating at high throttle.

Let’s check the thrust chart of the fpv motor of our choice and examine the Throttle and Current column.

meps-2207-Throttle-and-Current-column

Since your burst rating can be interpreted as 100% throttle, you must select an ESC with a burst rating of at least 36.5A when using this motor/prop combination. You can use the amp draw at 80% throttle to calculate the continuous amps required for an ESC. Therefore, we would require an ESC that is rated for at least 21.4A continuous for our example motor/prop pair. Generally speaking, it’s ideal to use an ESC that is rated for a little bit more amperage than what your bldc motor and propeller combination will require both consistently and sometimes. Returning to our example, we selected an ESC with a 45A continuous and 55A burst rating for our motor/prop combination.

Battery

Since we have already selected our quad motors, we can use the thrust table of our bldc motor to ascertain the cell count lipo that was utilized.

meps-2207-motor-Throttle-and-Current-column

Cell Count 1S 2S 3S 4S 5S 6S
Nominal Voltage 3.7V 7.4V 11.1V 14.8V 18.5V 22.2V
Fully-charged Voltage 4.2V 8.4V 12.6V 16.8V 21V 25.2V

 

Quad / Prop Size Lipo Capacity Range
2.5inch or smaller 200 – 500 mAh
3inch 650 – 1000 mAh
4inch 850 – 1300 mAh
5inch 1000 – 1800 mAh
6inch+ 1500 mAh+

Our 1750 Kv fpv motor’s voltage column shows us that the voltage ranges from 24.7 to 25.2 V. It is reasonable to believe that 6S lipos are required for this design as a fully charged 6S lipo has a voltage of 25.2V.

The larger the capacity, the heavier the battery. That’s why the capacity of the battery is not as large as it should be. Since we are building a 5 inch FPV, we chose a 1500mAh lipo.

As for the discharge rate of lipo batteries, we can calculate it according to the following formula:

  • Continuous C Rating = 50% of Burst C Rating = Max Current Draw / Capacity
  • Continuous 60C = Burst 120C ≥(37.9V * 4 motors) / 1500mAh (1.5Ah)

We have chosen a 6S, 1500mAh, 120C lipo battery for this example fpv.

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