Jimmy HuntPitot TubeP09233

Airframe Measurements

1)  DEVICE Name: Pitot Tube

The Pitot - static tube is used to measure the velocity of the object the Pitot tube is attached to, through a fluid. For most applications that fluid is air and for this the project that is also the case.

A pitot tube works by measuring the Total or Stagnation Pressure, Po, which is measure at a hole pointing into the flow that creates a stagnation point; and the Static Pressure, P, measure through a port that is perpendicular to a given flow. The operational range of a pitot tube and especially for this project, the Bernoulli Equation (Eq 1.) can be used to calculate the velocity of the craft from the measured Total and Static pressures from the Pitot tube.

Po= Pstatic+ 12 ρ V2

Eq 1. – Bernoulli equation to calculate velocity, V

In most pitot tubes today, there is temperature compensation. This means that the Pitot tube is measuring the Static Temperature of the flow through the static pressure port. With most applications and this project the working fluid is air and that can be considered an ideal gas, therefore the ideal gas equation (Eq. 2) can be used to find the density. This density can then be used in the Bernoulli equation the find velocity.

ρ= PstaticRT

Eq 2. – Ideal Gas Equation to find density, ρ

The Pitot tube is important because it provides the pilot or the control system with the important measurement of velocity. The velocity measurement is important because the pilot or the control system needs velocity to help them navigate, find aero coefficients that are dependent on velocity, for example coefficient of Drag and Lift, for the control system, flight times (e.g. ETA or time to a waypoint), and fuel consumption. This makes the Pitot tube an important part of this project because it will be one of the important sensors that will help make this UAV autonomous.

2)  Design Specification:

The main requirement that is expected out of the Pitot tube is velocity, whether it is velocity or a voltage that can be converted to a velocity. The next set of design requirements involves physical attributes, power requirements, and inputs. The design requirements for a suitable pitot tube are as follows:

1.  Weight 1 oz. or less

2.  Smaller than 1” x 1” x 1” for a chip and no longer than 6” for pitot-tube

3.  Capable of measuring both static and dynamic pressure

4.  Measure Speeds from 0 to 100 mph

5.  Resolution of ± 1 mph

6.  Input voltage of 3.3V

7.  Sample rate of no less than 1 sample/second

These design specs (1, 2, 4) came from the customer (Airframe B, P09232) in which the sensors had to be light and small, so as to leave more room and weight for the payload that the UAV is to carry. Some of them also can from the MCU (Microcontroller Unit), which uses 3.3V (6). Other design specs (3, 5, 7) were set by the team thinking from the point of view of the control systems team; in this case the MAV Controls team was consulted, P09122.

3)  Pugh Analysis:

After the different types of sensors were laid out as to what was needed to fly a UAV, each member picked and then researched that sensor. In regards to the pitot-tube, there were three pitot-tubes that fit the bill of being capable of meeting the customer needs. These three pitot-tubes are, in no particular order: 1) Eagle Tree Systems Airspeed Microsensor; 2) Eagle Tree Systems Airspeed Microsensor w/ eLogger; and 3) Space Age Control Pitot-Tube 300933.

The first step in the Pugh Analysis is the Concept Screening Matrix. The selection criteria in the Concept Screening Matrix was based on what the team as a whole felt was most important to accomplish the customer’s needs, along with one or two extra selection criteria’s that were important to that individual sensor. For the pitot-tube, the individual sensor selection criteria’s are Airspeed, Live Data, and Calibration. Using the standard Eagle Tree Systems Airspeed Microsensor as the reference, each sensor was given a “+”, “0”, or “-“, depending on how it measured up to the reference sensor. After this was completed, the Space Age control pitot-tube was dropped and the two Eagle Tree Systems Airspeed Microsensors moved on to the Concept Selection Matrix. The main reason that the Space Age Control Pitot-tube did not move on was the fact that this was just a shell of a Pitot-tube. To make this a functioning Pitot-tube, pressure transducers, temperature transducers, tubing and wiring would be needed and bought separately. This greatly increased the cost, as a single pressure transducer was more expensive than the Eagle Tree Systems Airspeed Microsensor.

In the Concept Selection Matrix, the same selection criteria were used along with the individual selection criteria for each individual sensor. The difference is that in the Concept Selection Matrix, a percentage out of 100% is given to each selection criteria. These percentages were discussed and decided upon as a team. Then each sensor was rated, 1 through 5 on each selection criteria and then weighted according to the percentage assigned to that selection criteria. Once each selection criteria was weighted, they were added up and the sensor with the highest score was chosen. The sensor that was on top was the Eagle Tree Systems Airspeed Microsensor w/ eLogger. The reason that the other Eagle Tree System was not picked was mainly that the two Eagle Tree Systems are the same Airspeed Microsensor which only measures max velocity. With the addition of the eLogger, the capability of the Airspeed Microsensor is greatly increased because the eLogger with the software packaged with eLogger, allows the storage and analysis of data, along with real time telemetry.

The Eagle Tree Systems Airspeed Microsensor w/ eLogger is the best system because it is ready to go out of the box, directly outputs velocity, is able to store and analyze data, real time telemetry capability, very small and very light. It is also a complete kit which helps with troubling shooting and interfacing issues.

4)  Components Specification:

In the appendix there are more detailed specs as to what the eLogger and Airspeed Indicator can handle, how they work, and what they output. To use the Airspeed Indicator and the eLogger, all that needs to be done is to simply connect them together as shown in the manuals. From the Pitot-Tube, there is a Static port and a Pressure port. Using the tubing that came with the kit, the Static Port is connected to the minus sign on the Airspeed chip and the Pressure port is connected to the plus sign on the Airspeed chip. Then the chip can just be directly connected to the eLogger following the pattern displayed on the eLogger.

Once this is done the eLogger can be connected to the planes battery for standalone power or connected to a computer with the eLogger software by USB. When the eLogger is connected directly to a computer, live telemetry can be viewed while the eLogger still records data. The Airspeed chip itself directly output velocity to the eLogger and the eLogger records data anywhere from 1-10 samples a second depending on what the user desires.

Mounting is simply done with a clamp to hold the Pitot-tube and the Airspeed chip and eLogger can be placed on small plates that can be placed where they are needed.

To use this device, simply power the eLogger through the plane’s battery or a computer. This device is not sensitive to anything, vibration, EMF, or temperature. Plus this device never has to be calibrated before each test. With the temperature measurement that the Airspeed chip takes, the airspeed is calibrated for the current local atmospheric conditions. In terms of maintenance and cost, there is no maintenance that needs to be done for this device and if it breaks, the best course of action would be to by a new system since it is already fairly inexpensive.

5)  Test Plan:

There are four tests planned to test and verify that the Eagle Tree Systems Pitot-tube works as claimed and is accurate.

Test #1

Wind Tunnel Test of ETS Pitot-Tube in small and large wind tunnel at RIT. This will be done against a hot wire probe, pressure transducers, and/or and anemometer. This test will help to verify that ETS Pitot-tube is accurate; can hold a steady velocity reading without outside noise, like wind; and find the lower limit of the Pitot-tube, ETS claims 2 mph. This test will be done against observations not time.

Test #2

“Car test” of ETS Pitot-tube against anemometer. This test will help to see the Pitot-tube’s delay through live telemetry, also how the Pitot-tube is across changing speeds and higher speeds, along with the addition of possible winds, angle of attacks and side slip angles to check that the Pitot-tube will work with these angles. This will be done on a car with the Pitot-tube pointing forward directly into the flow, along with the anemometer. Both will be fixed to the outside of the vehicle. One person will drive and call out when to record, while one person records the live data off of the computer with the ETS system attached while another person records the data off of the anemometer. This test will also be against observations not time.

Test #3

This test is a time delay test. With a certain length of tubing there is going to be a certain time delay as the column of air in the tubing changes and that change is picked up by the Airspeed chip. It is important to determine this time delay to know how much tubing can be used to achieve a reasonable time delay. This is also important for the future knowledge of the Controls Team so they can account for or if necessary, change the length of the tubing to achieve a desired and acceptable time delay.

Test #4

This will be the final test and will take place in Airframe A. This is an integration test of all the sensors that will be placed in the “Sensor Box.”

6)  Test Results:

Test #1 – Small Wind Tunnel at RIT

This wind tunnel can only go up to 5 mph. This was perfect to test the lower limits of the Pitot-tube. The benchmark for this test was the anemometer. Table 1 shows the results of this test.

Anemometer / ETS Pitot-Tube
m/s / mph / mph
1 / 2.2 / 0 ~ 2
1.3 / 2.86 / 1 ~ 3
1.5 / 3.3 / 3 ~ 4
2. / 4.4 / 4

Table 1 – Small wind tunnel results.

When the speed was below about 5mph, the Pitot-tube was having troubling making a consistent reading. This is most likely due to the fact that to Pitot-tube is reaching a stall point and cannot accurately read the velocity. This is not a problem due to the fact that if the plane is flying this slowly, it has already stalled and will nosedive and pick up speed above this level. However this test does confirm that the lower limit is 2 mph ± 1mph as claimed by ETS.

7)  Final Design:

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