Helicopter airspeed calibration method for reducing test flight cost

Abstract

The invention relates to a helicopter airspeed calibration method for reducing test flight cost, which comprises the following steps of: S1, carrying out two times of stable level flight in different courses on all test flight speed points, and recording a flight path angle and a global positioning system (GPS) resultant speed of each test flight speed point in different courses; s2, calculating a corresponding wind speed vector according to track angles of two adjacent test flight speed points under different heading directions and a GPS resultant speed; the first test flight speed point is the preceding test flight speed point in the two adjacent test flight speed points; s3, performing wind speed correction on the speed of the first test flight speed point according to the corresponding wind speed vector, and calculating the vacuum speed of the first test flight speed point; s4, calculating the calibration air speed of the first test flight speed point according to the vacuum speed of the first test flight speed point; s5, repeating the steps S2-S4 to obtain the calibration airspeed of all the test flight speed points; and S6, performing linear fitting on the indicated airspeed of all the test flight speed points and the calibrated airspeed of the corresponding test flight speed point to obtain a helicopter airspeed calibration equation.

Description

Technical Field

[0001] This invention belongs to the field of aircraft scientific research and flight test technology, and specifically relates to a method for calibrating helicopter airspeed to reduce flight test costs. Background Technology

[0002] Airspeed refers to the speed of an aircraft relative to the surrounding air, and it plays a crucial role in helping pilots and designers understand the current flight status. Airspeed is typically measured by a pitot tube installed on the aircraft. Its working principle is to convert the measured dynamic pressure into indicated speed using Bernoulli's equation. However, due to the strong downwash generated by the helicopter rotor and the fuselage turbulence, the dynamic pressure measured by the pitot tube does not equal the dynamic pressure of the incoming airflow during flight, resulting in a certain degree of error. Therefore, before conducting research and performance test flights, helicopters need to undergo test flights to calibrate the indicated airspeed displayed on the instruments using standard airspeed to obtain accurate airspeed results.

[0003] Currently, standard airspeed can be obtained through various methods, including radar, photography, towed pitot tubes, standard aircraft escort, lead stick, and GPS. Among these, the GPS method has gradually become the mainstream method for scientific research flight testing due to its advantages of simplicity, high accuracy, low risk, and low weather requirements. The core of the GPS method is to obtain the helicopter's ground speed using high-precision GPS equipment, correct for wind speed, and then obtain the vacuum speed. This vacuum speed is then converted into a calibration airspeed based on the flight test environment and used as the standard airspeed to calibrate the indicated airspeed. The GPS method can be further subdivided into round-trip and trilateration methods. However, regardless of the method used, the prerequisite is stable and constant wind. If wind speed and direction change significantly during the flight test, the obtained airspeed calibration data is generally unusable. Therefore, the airspeed calibration method should have the function of determining the validity of the data.

[0004] The round-trip airspeed calibration method involves performing two level flights at the same indicated airspeed. Assuming the wind speed and direction remain constant during both flights, the ground speeds will be symmetrical, allowing for averaging to obtain the true airspeed and thus completing the calibration. The main problem with this method is its lack of mathematical rigor. While it assumes stable winds, it cannot provide approximate estimates of wind speed and direction, thus failing to demonstrate whether the assumptions required for the flight test results are met.

[0005] The trilateration method for airspeed calibration involves flying at the same indicated airspeed and following a given flight path, then using mathematical derivation to obtain approximate estimates of the actual airspeed, wind speed, and direction. Commonly used flight paths include equilateral triangles and squares, hence the alternative names "triangle method" and "quadrilateral method." Flight data from every three sides yields an estimated actual airspeed. The trilateration method can provide approximate estimates of wind magnitude and direction, thus demonstrating whether the wind remains stable and unchanging, and possesses mathematical rigor. However, in actual flight testing, the trilateration method is more complex and time-consuming, resulting in higher costs compared to the round-trip method. Summary of the Invention

[0006] Purpose of the invention: To address the shortcomings of the round-trip method in determining the validity of flight test results and the high cost of the three-sided method, a helicopter airspeed calibration method with reduced flight test costs has been invented, aiming to improve the economic efficiency of helicopter airspeed calibration flight tests.

[0007] A method for calibrating helicopter airspeed to reduce flight test costs, the method comprising: S1, perform two stable level flights with different headings for all test flight speed points, and record the track angle and GPS resultant speed of each test flight speed point under different headings; S2, calculate the corresponding wind speed vector based on the track angles and GPS resultant speeds of two adjacent test flight speed points under different headings; the first test flight speed point is the earlier of the two adjacent test flight speed points; S3, adjust the speed of the first test flight speed point according to the corresponding wind speed vector, and calculate the vacuum speed of the first test flight speed point; S4, calculate the calibration airspeed of the first test flight speed point based on the vacuum airspeed of the first test flight speed point; S5, repeat S2-S4 to obtain the calibrated airspeed for all test flight speed points; S6. Linearly fit the indicated airspeed and the corresponding calibrated airspeed at all test flight speed points to obtain the helicopter airspeed calibration equation.

[0008] Furthermore, S1 specifically refers to: Step 1: Under the specified configuration and at the given altitude, perform stable level flight and maintain the indicated airspeed required for the test flight based on the current test flight speed point A; Step 2: Use a GPS device to record the helicopter's flight path angle. f A,1 and GPS combined speed V A,1 ; Step 3: Change the flight course, with a course deviation angle between 60° and 120°. The flight altitude and indicated speed must remain consistent with those in Step 2, and the track angle must be recorded using a GPS device. fA,2 and GPS combined speed V A,2 ; Step 4: Maintain the heading and altitude unchanged, and adjust and maintain the indicated airspeed according to the next test flight speed point B. Use GPS equipment to record the track angle. f B,1 and GPS combined speed V B,1 ; Step 5: Change the flight heading, with a heading angle between 60° and 120°, maintaining the same rotation direction as in Step 3, and keeping the flight altitude and indicated airspeed as in Step 4. Use a GPS device to record the track angle. f B,2 and GPS combined speed V B,2 ; Step 6: Repeat the above steps until all speed points have been tested.

[0009] Furthermore, S2 specifically includes: Step I: Based on the obtained track angle and GPS resultant velocity, obtain the velocity vector coordinates of the first test flight speed point A. X A1 , Y A1 and X A2 , Y A2 :

[0010] Step II: Based on the obtained track angle and GPS resultant velocity, obtain the velocity vector coordinates of the next test flight speed point B. X B1 , Y B1 and X B2 , Y B2 :

[0011] Step III: Using the velocity vector coordinate points from Steps I and II, calculate the following intermediate quantities:

[0012] Step IV: Based on the results obtained in Step III, calculate the wind speed vector:

[0013] Where, mod( x , y )representx right y Modulo operation.

[0014] Furthermore, S3 specifically refers to: vacuum speed V T : .

[0015] Furthermore, S4 specifically refers to: Based on the density ratio δ between the current flight altitude and sea level altitude, the calibration airspeed corresponding to the first test flight speed point A was calculated. V c : .

[0016] Furthermore, in S6, before performing linear fitting, data points with abnormal wind speed and direction changes are removed.

[0017] This invention proposes a helicopter airspeed calibration method to reduce test flight costs, improve test flight data processing efficiency, effectively reduce the time and number of airspeed calibration test flights, reduce test flight costs, and can obtain wind speed and wind direction during test flights, and prove whether the wind satisfies the assumption of stability and invariance during test flights, thus possessing mathematical rigor. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the test flight maneuvers of the present invention; Figure 2 This is a schematic diagram illustrating the mathematical principles of the present invention. Detailed Implementation

[0019] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings.

[0020] Helicopter airspeed calibration methods typically include two parts: flight test procedures and data processing. Necessary flight test data is obtained through the flight test process. Based on the airspeed calibration principle, this data is processed and calculated to ultimately derive the helicopter airspeed calibration formula. For helicopter airspeed calibration flight tests, multiple speed points are usually selected for stable level flight. The speed range covers the helicopter's long-range speed (the speed corresponding to the lowest hourly fuel consumption under given configuration, weight, pressure altitude, and atmospheric temperature conditions), long-range speed (the speed corresponding to the lowest kilometer fuel consumption under given configuration, weight, pressure altitude, and atmospheric temperature conditions), and maximum cruise speed (the sustained maximum steady-state speed achieved under given configuration, weight, pressure altitude, atmospheric temperature, and power conditions, with a level flight overload factor of 1.0), to ensure the validity of the airspeed calibration results across each speed range. Therefore, considering the repetitiveness of the flight test and data processing steps at each speed point in the airspeed calibration method, and to avoid excessive redundancy, this invention will only describe a necessary number of speed points.

[0021] ①The specific flight test steps of this invention are as follows (e.g.) Figure 1 ): Step 1: Perform stable level flight at a given altitude under the specified configuration, while maintaining the indicated airspeed A required for the test flight; Step 2: Use a GPS device to record the helicopter's flight path angle. f A,1 and GPS combined speed V A,1 ; Step 3: Change the flight course, with a heading angle between 60° and 120° (clockwise or counterclockwise). The flight altitude and indicated speed must remain consistent with those in Step 2, and the track angle must be recorded using a GPS device. f A,2 and GPS combined speed V A,2 ; Step 4: Maintain the heading and altitude unchanged, and adjust and maintain the indicated airspeed according to the next test flight speed point B. Use GPS equipment to record the track angle. f B,1 and GPS combined speed V B,1 ; Step 5: Change the flight heading, with a heading angle between 60° and 120° (rotation direction remains the same as in Step 3), and maintain the flight altitude and indicated airspeed from Step 4. Use a GPS device to record the track angle. f B,2 and GPS combined speed V B,2 ; Step 6: Repeat the above steps until all speed points have been tested.

[0022] ②The specific data processing steps of this invention are as follows: Step I: Based on the track angle and GPS resultant velocity obtained in steps 1-3 of the test flight, the velocity vector coordinates of test flight speed point A can be obtained. X A1 , Y A1 and X A2 , Y A2 :

[0023] Step II: Based on the track angle and GPS resultant velocity obtained in steps 4-5 of the test flight, the velocity vector coordinates of the next test flight speed point B can also be obtained. X B1 , Y B1 and X B2 , Y B2 :

[0024] Step III: Using the velocity vector coordinate points from Steps I and II, calculate the following intermediate quantities:

[0025] Step IV: Based on the results obtained in Step III, the wind speed vector can be calculated:

[0026] Where, mod( x , y )represent x right y Modulo operation.

[0027] Step V: Calculate the vacuum velocity by correcting the velocity for wind speed. V T :

[0028] Step VI: Based on the density ratio δ between the current flight altitude and sea level altitude, the calibrated airspeed corresponding to velocity point A is calculated. V c :

[0029] Step VII: Repeat steps I to VI. By introducing the next velocity point C, the calibrated airspeed of velocity point B can be calculated. By analogy, the calibrated airspeed of all velocity points can be obtained. Step VIII: By selecting data, ensure that the magnitude of wind speed and direction changes at each speed point conforms to engineering experience, and use the least squares method to linearly fit the calibrated airspeed and the indicated airspeed to obtain the helicopter airspeed calibration equation.

[0030] Data processing examples I. Raw Flight Test Data

[0031] 2. Calculate the intermediate values ​​(X, Y, M, B) for each velocity point.

[0032] 3. Calculate the intermediate values ​​(Wx, Wy) between adjacent velocity points.

[0033] IV. Calculate wind speed, wind direction, and vacuum velocity.

[0034] V. Calculate and calibrate airspeed

[0035] VI. Determine the validity of the data and obtain the airspeed calibration equation through linear fitting.

[0036] This invention proposes a helicopter airspeed calibration method to reduce flight test costs. Compared to the traditional three-sided method (which requires three flights per test speed point), this method only requires two flights per test speed point, reducing flight test costs by one-third. Furthermore, this invention can calculate wind speed and direction during flight tests, possessing the ability to self-judge data validity. Compared to the three-sided method, due to the shorter flight test duration and shorter wind evolution time, it is easier to obtain valid flight test data, further reducing the risk of re-flight costs due to invalid data.

[0037] Regarding accuracy, the airspeed calibration flight test data of the same sortie were processed using both the present invention and the trilateration method. Using data correlation analysis, the two sets of results were defined as {X}. 1i =[x 1i ,y 1i ]|i=1…n},{X 2i =[x 2i ,y 2i The correlation coefficient between two sets of data (i=1…n) can be obtained using the following formula:

[0038] in

[0039] The correlation coefficient r ranges from -1 to 1. When r=1, the two sets of data are perfectly correlated; when r=0, they are completely uncorrelated; and when r=-1, they are also perfectly correlated, but with the opposite trend. The calculated correlation coefficient r between the two methods for airspeed calibration is 0.999. The helicopter airspeed calibration method proposed in this invention, which reduces flight test costs, has the same level of accuracy as the trilateration method.

[0040] In summary, the helicopter airspeed calibration method proposed in this invention can effectively reduce flight test costs by one-third while ensuring accuracy.

[0041] The mathematical principles of this invention are as follows: 1. Establish such Figure 2 The coordinate system shown has the Y-axis pointing due north and the X-axis pointing due west. oA1 and oA2 are the GPS velocity vectors in the two directions corresponding to velocity point A (see flight test steps 1-3 for details), FA1 and FA2 are the vacuum velocity vectors, and oF is the wind speed vector; while oB1 and oB2 are the GPS velocity vectors in the two directions corresponding to the next flight test velocity point B (see flight test steps 4-5 for details), and FB1 and FB2 are the vacuum velocity vectors. Since the magnitude and direction of the GPS velocity have been obtained through the GPS device, the coordinate positions of A1, A2 and B1, B2 can be calculated.

[0042]

[0043] 2. Draw perpendicular bisectors to A1A2 and B1B2 respectively, and then the corresponding D... A F and D B The expression for equation F is:

[0044]

[0045]

[0046] 3. Establish the perpendicular bisector D A F and D B Using equation F, the coordinates of the intersection point are calculated, thus yielding the wind speed vector:

[0047] Where, mod( x , y )represent x righty Modulo operation, and

[0048] 4. By correcting the velocity for wind speed, the vacuum velocity is calculated. V T :

[0049] 5. Based on the density ratio δ between the current flight altitude and sea level altitude, the calibrated airspeed corresponding to velocity point A was calculated. V c :

[0050] This invention proposes a helicopter airspeed calibration method to reduce test flight costs, improve test flight data processing efficiency, effectively reduce the time and number of airspeed calibration test flights, reduce test flight costs, and can obtain wind speed and wind direction during test flights, and prove whether the wind satisfies the assumption of stability and invariance during test flights, thus possessing mathematical rigor.

Claims

1. A method for calibrating helicopter airspeed to reduce flight test costs, characterized in that, The method includes: S1, perform two stable level flights with different headings for all test flight speed points, and record the track angle and GPS resultant speed of each test flight speed point under different headings; S2, calculate the corresponding wind speed vector based on the track angles and GPS resultant speeds of two adjacent test flight speed points under different headings; the first test flight speed point is the earlier of the two adjacent test flight speed points; S3, adjust the speed of the first test flight speed point according to the corresponding wind speed vector, and calculate the vacuum speed of the first test flight speed point; S4, calculate the calibration airspeed of the first test flight speed point based on the vacuum airspeed of the first test flight speed point; S5, repeat S2-S4 to obtain the calibrated airspeed for all test flight speed points; S6. Linearly fit the indicated airspeed and the corresponding calibrated airspeed at all test flight speed points to obtain the helicopter airspeed calibration equation.

2. The helicopter airspeed calibration method for reducing flight test costs according to claim 1, characterized in that, S1 specifically refers to: Step 1: Under the specified configuration and at the given altitude, perform stable level flight and maintain the indicated airspeed required for the test flight based on the current test flight speed point A; Step 2: Use a GPS device to record the helicopter's flight path angle. φ A,1 and GPS combined speed V A,1 ; Step 3: Change the flight course, with a course deviation angle between 60° and 120°. The flight altitude and indicated speed must remain consistent with those in Step 2, and the track angle must be recorded using a GPS device. φ A,2 and GPS combined speed V A,2 ; Step 4: Maintain the heading and altitude unchanged, and adjust and maintain the indicated airspeed according to the next test flight speed point B. Use GPS equipment to record the track angle. φ B,1 and GPS combined speed V B,1 ; Step 5: Change the flight heading, with a heading angle between 60° and 120°, maintaining the same rotation direction as in Step 3, and keeping the flight altitude and indicated airspeed as in Step 4. Use a GPS device to record the track angle. φ B,2 and GPS combined speed V B,2 ; Step 6: Repeat the above steps until all speed points have been tested.

3. The helicopter airspeed calibration method for reducing flight test costs according to claim 2, characterized in that, S2 specifically includes: Step I: Based on the obtained track angle and GPS resultant velocity, obtain the velocity vector coordinates of the first test flight speed point A. X A1 , Y A1 and X A2 , Y A2 : Step II: Based on the obtained track angle and GPS resultant velocity, obtain the velocity vector coordinates of the next test flight speed point B. X B1 , Y B1 and X B2 , Y B2 : Step III: Using the velocity vector coordinate points from Steps I and II, calculate the following intermediate quantities: Step IV: Based on the results obtained in Step III, calculate the wind speed vector: Where, mod( x , y )represent x right y Modulo operation.

4. The helicopter airspeed calibration method for reducing flight test costs according to claim 3, characterized in that, S3 specifically refers to: vacuum speed V T : .

5. The helicopter airspeed calibration method for reducing flight test costs according to claim 3, characterized in that, S4 specifically refers to: Based on the density ratio δ between the current flight altitude and sea level altitude, the calibration airspeed corresponding to the first test flight speed point A was calculated. V c : .

6. The helicopter airspeed calibration method for reducing flight test costs according to claim 3, characterized in that, In S6, before performing linear fitting, data points with abnormal wind speed and direction changes are removed.

7. A helicopter airspeed calibration system for reducing flight test costs, characterized in that, The system is used to perform the method as described in any one of claims 1-6.

8. A helicopter, characterized in that, The helicopter is equipped with the airspeed calibration system as described in claim 7.

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