# Calculating Thrust from Propellers for Model Airplane

• reg
In summary, to calculate thrust for a model airplane, you can use the propeller size and pitch, as well as the engine power and RPM. The simplest method is to multiply pitch by RPM to find the air velocity and then use Bernoulli's equation and the prop's surface area to determine thrust. However, the actual calculation may be more complex as RPM can vary with the plane's speed.
reg
can anyone tell me how to calculate thrust for a model airplane. i know the propeller size and pitch and also the engine power and RPM.

Any help would be much appreciated

regards reg_ram

Well, the simplest way would be if you know the rpm, you can multiply pitch by rpm to find the velocity of the air, then use bernoulli's equation to find the pressure (and multiply by surface area traced out by the prop to find thrust).

The reality is probably a little tougher: rpm will vary with the speed of the plane - the faster the plane goes, the easier it is for the prop to spin. What ends up happening is that the horsepower generated remains roughly constant while rpm changes.

Calculating the thrust for a model airplane can be done by using the following formula:

Thrust (lbs) = (0.0000007 * propeller size (in)^3 * pitch (in) * engine power (hp) * RPM) / (1000 * 60)

First, you will need to convert the propeller size and pitch from inches to feet, as the formula uses cubic feet. Then, you will need to convert the engine power from horsepower to watts, as the formula uses watts.

Once you have all the necessary values in the correct units, you can plug them into the formula to calculate the thrust. Keep in mind that this formula is an approximation and the actual thrust may vary depending on factors such as air density and airfoil design.

Another way to calculate thrust is by using a thrust calculator, which can be found online. These calculators take into account additional factors such as airspeed and efficiency to provide a more accurate estimate of thrust.

It is important to note that while calculating thrust is helpful for determining the performance of your model airplane, it is not the only factor to consider. Other factors such as weight, balance, and aerodynamics also play a crucial role in the flight of a model airplane. It is recommended to consult with experienced model airplane builders or pilots for additional guidance and tips.

## 1. How do you calculate the thrust from propellers for a model airplane?

To calculate the thrust from propellers, you will need to know the diameter and pitch of the propeller, as well as the RPM of the motor. Then, you can use the following formula: thrust (in grams) = (diameter in inches x pitch in inches x RPM)/1050. This formula is an estimation and may vary depending on the design and efficiency of the propeller.

## 2. Is there a difference in calculating thrust for different types of propellers?

Yes, there can be differences in calculating thrust for different types of propellers. For example, a propeller with a higher pitch will produce more thrust, while a propeller with a larger diameter will produce more torque. It is important to consider the specific design and characteristics of the propeller when calculating thrust.

## 3. Can propeller speed affect the thrust calculation?

Yes, the speed of the propeller can affect the thrust calculation. As the RPM of the motor increases, the thrust will also increase. However, there is a limit to how much thrust can be produced, and going beyond this limit can cause the propeller to stall and decrease thrust.

## 4. How can I increase the thrust of my model airplane's propellers?

There are a few ways to increase the thrust of your model airplane's propellers. You can try using a propeller with a higher pitch, increasing the RPM of the motor, or using a larger diameter propeller. Additionally, ensuring that the propeller is properly balanced and the motor is running efficiently can also help increase thrust.

## 5. Are there any safety precautions I should take when calculating thrust for my model airplane's propellers?

Yes, it is important to take safety precautions when calculating thrust for your model airplane's propellers. Make sure to always double check your calculations and use appropriate units. Additionally, make sure to handle the propellers and motor carefully, as they can be sharp and may cause injury. Lastly, always follow the manufacturer's instructions and guidelines for your specific model airplane and its propellers.

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