Measuring fuel flow rate is an important part of evaluating gas-powered unmanned aerial vehicle (UAV) engines. It allows engineers to monitor fuel consumption at different throttle settings and understand how fuel delivery changes with engine operating conditions.
In this article, we’ll explain what fuel flow rate is, explore different sensor technologies, and demonstrate how to measure fuel flow rate using the Tyto Robotics Flight Stand 60 and Fuel Flow Sensor.
Table of Contents
1. What Is Fuel Flow Rate?
1.1 Volumetric Fuel Flow Rate
1.2 Mass Fuel Flow Rate
2. Types of Fuel Flow Sensors
2.1 Turbine Flow Sensors
2.2 Positive Displacement Flow Sensors
2.3 Transit-time Ultrasonic Flow Sensors
2.4 Thermal Flow Sensors
3. How to Measure Fuel Flow Rate with the Flight Stand 60
3.1 Hardware Installation
3.2 Software Setup
4. Conclusion
1. What Is Fuel Flow Rate?
Fuel flow rate is the volume or mass of fuel passing through a system per unit of time. When measured appropriately within an engine's fuel system, it can be used to determine fuel consumption.
1.1 Volumetric fuel flow rate:
Volumetric fuel flow rate, represented by the symbol Q, measures the volume of fluid that passes through a system per unit of time. The SI unit is cubic meters per second (m³/s), although flow rates are commonly expressed in milliliters per minute (mL/min).
A simple method to calculate volumetric flow is to fly your drone, then record the time flown and the volume of fuel consumed. Dividing consumed fuel volume by elapsed time gives the average volumetric flow rate for that interval of time. This approach is useful for obtaining a crude estimate, but does not reveal short-term changes in fuel flow as throttle, flight conditions, or engine loads vary.
For flow through a fuel line, volumetric flow rate can be expressed as:
Q = A × v
Where:
Q = Volumetric flow rate
A = Cross-sectional area
v = Average fuel velocity across the area
1.2 Mass fuel flow rate
Mass fuel flow rate, represented by the symbol ṁ, measures the mass of fuel that passes through a given point per unit of time. Essentially, instead of measuring how much space a fluid occupies, it measures how much “material” is actually moving through a system. The SI unit is kilograms per second (kg/s), although it is sometimes expressed in grams per second (g/s).
Mass flow rate is particularly useful for comparing fuel consumption under different conditions because it accounts for fuel density, allowing engineers to compare the actual mass of fuel delivered rather than its volume alone.
It can be calculated from volumetric flow rate using the density of the fuel:
ṁ = Q × ρ
Where:
ṁ =Mass flow rate
Q =Volumetric flow rate
ρ = Fuel density
For more information on evaluating fuel efficiency and reproducing flight conditions, read our article on How to Improve UAV Fuel Consumption.
2. Types of Fuel Flow Sensors
There are several ways to measure fuel flow, and each sensor type works a bit differently. Knowing how they work can help you choose the right one for your drone testing needs. Below, we’ll look at some of the most common fuel flow sensors used in UAV applications.
2.1 Turbine flow sensors
A turbine fuel flow sensor contains a small rotor that turns as fuel passes through it. The turbine rotates at a speed proportional to the flow rate. Magnetic or optical pickups detect this rotation and convert it into a flow rate reading.
Figure 1: Schematic of a turbine flow sensor
Turbine sensors are particularly suitable for low-viscosity fuels such as gasoline, making them a good choice for UAVs powered by internal combustion engines (ICEs). These sensors offer good repeatability and high measurement accuracy across a wide range of flow rates.
However, turbine sensors contain moving parts. Make sure the fuel is free of debris and air bubbles, and regularly inspect the moving components for wear.2.2 Positive displacement flow sensors
Like turbine flow sensors, positive displacement (PD) flow sensors measure volumetric flow rate. However, instead of measuring the rotation speed of an impeller, they use moving components, such as gears or pistons, to separate the flowing fuel into small, fixed volumes. Each movement transfers a known amount of fuel through the meter. The flow rate is calculated by counting how many times this process repeats over time.
Figure 2: Schematic of a positive displacement flow sensor
Given their mechanical design, PD sensors are well suited for measuring low flow rates and can provide high measurement accuracy.
However, low-viscosity fluids can leak through small clearances, affecting measurement accuracy. Fluids with higher viscosity, like diesel, reduce internal slip and improve accuracy.
2.3 Transit-time Ultrasonic flow sensors
These ultrasonic sensors send sound pulses with and against the direction of fuel flow. The difference in travel time is used to determine flow velocity and calculate volumetric flow rate.
Figure 3: Schematic of an ultrasonic flow sensor
Since ultrasonic flow sensors have no moving parts obstructing the fuel flow, they generally introduce little additional pressure loss.
However, transit-time measurement requires sufficient acoustic transmission through the fuel; excessive bubbles or suspended material can scatter the signal.
2.4 Thermal flow sensors
Thermal sensors infer flow by using a heated sensing element to measure how much heat is carried away by the passing fluid, allowing the sensor to determine mass flow rate.
Figure 4: Schematic of a thermal flow sensor
Like ultrasonic sensors, thermal sensors have no moving parts, reducing mechanical wear. Since their measurements depend on the thermal properties of the fuel, these sensors must also be appropriately calibrated for the fluid being measured. They also offer good sensitivity at low flow rates.
However, contaminants can accumulate on the sensing element and affect measurement accuracy. Fuel compatibility is another important consideration, as many thermal sensors are designed for gases rather than liquid fuels.
3. How to Measure Fuel Flow Rate with the Flight Stand 60
The Tyto Robotics Flight Stand 60 uses an inline mechanical type fuel flow sensor with a measurement range of 20 to 800 mL/min. For more information, refer to the datasheet here.
Fuel flow measurements can be recorded during internal combustion engine testing alongside thrust, torque, RPM, temperature, and airspeed.
These synchronized measurements allow engineers to examine fuel delivery under different engine operating conditions.
To measure fuel flow rate, first follow the user manual to set up the Flight Stand 60 and mount your ICE engine and propeller. Then, follow the steps below to install and configure the fuel flow sensor.
3.1 Hardware installation:
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Retrieve the following items from the Flight Stand 60 Auxiliary Sensor Box (SKU: FP37):
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Fuel flow sensor kit:
- Fuel flow converter unit
- Jeti fuel flow sensor and amplifier
- Wires and screw terminals
- Fuel flow sensor and fuel tank support plate
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Open the fuel flow sensor kit, take out the support plate and place it flat on a work station as shown in the figure below:
Figure 5: Support plate with 6-holes on the left and 4-holes on the right
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Install the fuel flow sensor on the support plate with inlet on the left, and outlet on the right. Use the M3 screws and lock nuts, make sure to install the nuts on the same side as the flow sensor, as shown below:
Figure 6: Fuel flow sensor on the plate, inlet on the left, and outlet on the right
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Install the fuel flow converter unit on the support plate with two M5 - 10 mm button head screws and two M5 nuts.
Figure 7: Support plate with fuel flow sensor and fuel flow converter unit
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Prepare a 5 V DC power output to be connected to the converter unit with the 2-POS screw terminal; this can be any 5 V power adapter on the market, or a USB extension cord with the VCC and GND connected to the correct position on the screw terminal; make sure the screw terminal is fully tightened.
Figure 8: USB cable with 5V and GND to the fuel flow converter unit
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Connect the Jeti amplifier unit with one side to the JST white header, and the other side’s 2.54 mm header to the Brown - Red - Orange (BRO) input on the converter unit.
Figure 9: Jeti amplifier interconnects on the fuel flow converter unit [B: Brown, R: Red, O: Orange]
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Connect the Jeti flow sensor with the fuel flow converter unit through Green-terminal-JST-cable (SKU: 47CG).
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Connect the 5 V power supply (USB or power adapter).
Figure 10: Fuel flow sensor and converter unit fully installed and connected on the support plate
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Do not insert the fuel line until you are ready to install the fuel tank.
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Install the support plate assembled with the flow sensor and the converter unit on the Flight Stand 60 upper L-brackets, behind the FMU; make sure that the fuel flow converter and the fuel flow sensors are facing downward and you have an empty space on top of the plate behind the FMU; you may install the fuel tank on the plate afterwards.
Figure 11: Install the support plate with the sensors facing downward
Fastening torque:
- M6 - 30 mm flange socket head screw and nuts: 12 Nm
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Connect the fuel flow converter unit to the FMU (RPM port) on the side.
Figure 12: Plug the 3-POS JST wire from the fuel flow converter to the FMU’s RPM port
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(optional): You may design a custom support plate to enable you to mount your fuel tank on top of the fuel sensor support plate
Figure 13: Fuel tank sitting behind the FMU on custom support plate (not included)
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You may then prepare the fuel line for the engine. NOTE: set the throttle to OFF, close the CHOKE, and make sure the ignition unit is OFF when preparing the fuel line.
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You may place the fuel tank directly on the fuel tank and flow sensor support plate, right behind the FMU; you will need to make your own fixture to secure the fuel tank as seen in figure 13.
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Prepare three sections of fuel lines to be connected:
- Fuel tank to fuel filter
- Fuel filter to fuel flow sensor
- Fuel flow sensor to carburetor on the engine
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Cut the fuel lines to the right length, and then drop in two clamps for each line, except the one that will go into the fuel tank.
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Prepare a fuel filter and carefully check the inlet and outlet side.
Figure 14: Fuel filter direction
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Plug a section of fuel line to the inlet of the fuel filter, and then drop the other end to the fuel tank; clamp the line to maintain the watertight on the filter.
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Plug another section of fuel line to the outlet of the fuel filter, and then connect the other end to the inlet of the fuel flow sensor; clamp the line on both ends to maintain the watertight.
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Plug the last section of fuel line to the outlet of the fuel flow sensor, and then connect the other end to the fuel inlet of the carburetor; clamp the line on both ends to maintain the watertight.
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Use a hand pump to supply fuel to the engine so that the engine can start; the engine will not start dry.
3.2 Software setup:
The sensor reads the flow rate as pulses and it has a lower limit of 20 ml/min. Please always plug the flow sensor converter in the FMU’s RPM port. Even though it is also possible to plug it to the Flight Stand DAQ unit, we do not recommend doing so due to the limited cable length.
To set up the fuel flow sensor:
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Plug in the sensor and power it up with 5 V, then open the Flight Stand software.
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Go to the Hardware tab -> ETS60-FMU -> find Rotation speed, click on the smaller gear button on the right and set the divider to 1.
Figure 15: Divider setting for flow rate sensor
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Then go to input transformation -> New, with the following information.
- Name: Fuel Flow Sensor
- Unit type: Other: ml/min
Formula input:
- a: ETS60-FMU - Rotation speed
- Formula: 1.16*a
- System limits: 0 to 0
Figure 16: Input transformation for flow rate reading
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Save the input transformation.
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Go to Powertrain mappings -> Powertrain 1 -> Extra mapping -> Add extra input/output -> Choose “Transformation - Fuel flow sensor” in the drop down menu.
Figure 17: Extra mapping of Fuel flow sensor in the software
Conclusion
Measuring drone fuel flow rate is an essential part of evaluating fuel-powered propulsion systems. Accurate fuel consumption data helps engineers understand engine performance and estimate a UAV’s operational endurance.
When combined with thrust, torque, and RPM measurements, fuel flow data provides a more complete picture of propulsion performance, helping engineers evaluate engine performance and compare propulsion configurations.
Looking to measure fuel consumption during UAV engine testing? Explore the Tyto Robotics Flight Stand 60 to learn how synchronized measurements can support your propulsion testing process.