A Review on Bird Strike Capability on Service-Age Aircraft Canopy

*M.T.Ahmad, M.R. Ajir, R. Varatharajoo, F. Mustapha, R. Zahari, A.R. Abu Talib,

M.F.Abdul Hamid

Department of Aerospace Engineering,

University Putra Malaysia,

43400 Selangor, Malaysia

*Email: tarmizi1@eng.upm.edu.my

Identifiers: High Speed Impact, Bird Strike, Aircraft Transparencies, Pressure Vessel

Abstract:           Bird strikes to aircraft have resulted in major aircraft damages coupled with severe/fatal pilot injuries. Analysis of operational bird impact statistical data indicates that the trend of damaging bird impacts on civil and military aircraft is continuing to rise. Impacts to the aircraft transparency system also continue to rise resulting in a continued flight safety risk to the aircraft and the aircrew. University Putra Malaysia sponsored by the Air Force had initiated a program to develop Bird Impact Test Facility to evaluate aircraft transparency system for bird strike capability. This paper describes the design, fabrication, testing and calibration of the Bird Impact Test Facility. The baseline test results are presented through the use of post test photographs and calibration of pressure and impact speed diagram using high speed camera.

INTRODUCTION

Bird-strike-resistant acrylic/polycarbonate laminate transparencies have become the preferred materials for many existing aircraft and have been a part of the transparency system design for new aircraft. Aircraft windshields, canopies, and windows must endure a harsh environment that includes moisture, extreme temperatures, chemicals, and ultraviolet light, and as a result are subject to the same age-related issues as other aircraft components [1]. One particular issue that has affected many polycarbonate and acrylic transparency systems is bird strike capability. In some cases, particularly fighter aircraft, age-related degradation had caused a reduction in bird-strike capability. A new F-5 windshield is rated for an impact velocity of 400 knots for a four-pound bird [2], but the manufacturer of F5 aircraft does not provide information on the bird strike capability of the F-5 aircraft canopy.

Though there are not many instances on record of crashes that have resulted from large birds smashing through the canopies of jet aircraft however because the tactics of modern warfare are increasingly calling for high-speed, low-level attack missions, military aviator is taking a serious look at the chicken cannon tests in order to evaluate their canopies to withstand such impacts [3].The bird strike capability of the F-5 canopy is not yet evaluated and the bird impact resistance system had yet to be tested.

The data is used to evaluate the F-5 canopy, hence providing substantial safety to the aircraft programs.

OBJECTIVES OF DEVELOPING BIRD-STRIKE TESTING

To assess the bird-strike capability for the F-5 canopy, UPM with support of the Air Force, performed test and evaluation on service-aged F-5 canopy. The objectives of this effort were to:

a. Conduct bird-strike testing to evaluate the F-5 canopy bird-strike capability.b. Assess the additional risk for the F-5 fleet due to a decreased level of bird- strike

protection.c. Develop capability of conducting similar high speed impact test for other purposes.

The test facility was designed in accordance to test facility’s requirement stated in ASTM F330-89 (2004) Standard Test Method [4] for bird impact testing of aerospace transparent enclosure. This test method may be used for bird impact testing of aircraft crew compartment transparencies and supporting structure to verify the design; compilation of test data for use in verification of future transparency and supporting structure design and analytical methods; and comparative evaluation of materials. This test method covers conducting bird impact tests under a standard set of conditions by firing a packaged bird at a stationary transparency mounted in a support structure.

BIRD IMPACT TEST FACILITY

Air Gun

The design, testing and calibration of the Bird Impact Facility were done at UPM whilst the manufacturing was out sourced to a local steel fabrication plant. The Bird Impact Test facility consists of an air gun capable of propelling a 4 lb bird in excess of 500 knots, supporting

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structures and an enclosure for the test article. The air gun consists of pressure vessel connected to compressed air supply and a barrel (Figure 1).

The air gun is of 6 in bore and has a 19 ft long barrel. The compressed air reservoir of 30 ft2

capacity can be pressurized to 250 lb/in2 and is separated from the barrel by a butterfly valve. On operating the butterfly valve release handle high pressure air will be dispensed rapidly to the barrel and the projectile is accelerated down the barrel towards the target. The gun is fixed and the target mounted in position to give impact on the desired point. Maximum impact speeds up to 1000 ft/sec (600 knots) can be achieved with birds of 4 lb in weight by extending the barrel to 60 ft as compared to the present capability of 500 knots.

Figure 1 – Bird Impact Test Facility

Timing apparatus

Impact velocity is measured by the bird projectile passing through well drawn marker lines positioned between the gun muzzle and the target. The time interval between traveling the marker lines is measured by a high speed camera. The accuracy of the speed measurement is estimated to be +/-2 %.

Bird projectile

The 4 lb thawed bird projectile is wrapped in a clear plastic material (cling foil). The total weight of the projectile is adjusted to 4 lb 0 oz immediately before firing by the addition of pieces of chicken parts. The foam plastic barrel plug is used to ensure an accurate fit of the projectile in the barrel of the gun and thereby obtain repeatable velocity/gun pressure characteristic which ensures that the required impact velocity is achieved in the test. The purpose of the cling foil is to prevent breakup of the bird before it impacts on the target.

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Air Gun Test and Calibration

The pressure vessel was first tested at 1.5 x working pressure (250 psi) as required by the local industry authority. The muzzle velocity of the air gun was calibrated using a high speed camera at frame rate between 10000 to 20000 frame/sec as shown in table 1 and graph 1.

Start frame

End frame

Frame taken

dist travel

m

Total frame

per second

Time travel

Speed, m/s

DateTest No.

Result Press

2852 2606 246 1.5 20000 0.0123 121.9 14/0/09 Test 1 Ok 50psi

2256 1870 386 1.5 20000 0.0193 77.7214/06/09 Test 2

Not Satis 100psi

3001 2790 211 1.5 200000.0105

5142.18

21/06/09 Test 3Not Satis 100psi

733 621 112 1.5 10000 0.0112 133.9325/06/09 Test 4

Not Satis 100psi

1383 1284 99 1.5 10000 0.0099 151.52 12/07/09 Test 5 Ok 100psi2158 2064 94 1.5 10000 0.0094 159.58 12/07/09 Test 6 Ok 150psi1473 1448 25 0.5 10000 0.0025 200 19/07/09 Test 7 Ok 200psi

Table 1-Calibration Data

y = 0.484x + 98

R2 = 0.9449

0

50

100

150

200

250

0 50 100 150 200 250

Pressure, PSI

Vel

ocit

y, m

/s

Graph 1 – Calibration Curve of ‘Chicken Cannon’

The first test was carried out using sabot made of high density polystyrene foam reinforce by two layers of fiber glass. At higher pressure (>100 psi) the tests were not satisfactory due to failure of the sabot. Subsequent tests were successfully carried out by using a foam barrel plug (cylindrical shaped polystyrene foam) and chicken wrapped in clear foil and coated with grease. This method was adopted for the actual trails. The calibration curve was used to determine the required pressure for the specified test velocity.

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The Pressure-Velocity Calibration Curve was compared to calculated values and the result is shown in Graph 2. The actual calibrated velocity values and the calculated values deviate at higher pressure. This is due to frictional loss that was not considered in the derived equation.

Muzzle Velocity

0

50

100

150

200

250

300

350

0 50 100 150 200 250 300

Pressure psi

Vel

oci

ty m

/s

Series1

Series2

Graph 2 – Velocity Versus Pressure Curve

The equations that were used to calculate muzzle velocity as a function of pressure is shown in Appendix A.

BIRD IMPACT TEST TRIAL

Description of Test Specimens

Figure 2-F5 Canopy

The canopy (Figure 2) consists of a single, biaxially stretched acrylic plastic panel, Military specification MIL-P-25690, edged with acrylic resin impregnated fabric laminate and mounted in a rigid frame structure. The acrylic panel is sealed to the frame at the aft end by a combination sheet silicone rubber and tetrafluoroethylene film expansion joint and is secured with flushed-head screws. The panel is attached to the frame side beams by retaining pins inserted thru matching integral nodes laminated to the panel and side beams. Nylon fabric with chloroprene coating across the hinge lines is cemented to the side beams and the acrylic plastic panel to form pressure-tight joints for cabin pressurization requirement. The entirely acrylic panel is stressed.

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CalcActual

Details of the test specimens are as follows:

Aircraft Type

Pt/No S/No Date Manufactured

TSN Age

F5E 14-13300-1 DUM/AF/05/038 Dec 1983 3569.07 15 yrs 7 monthF5F 1560009961399 298 Dec 1983 3816.32 15 yrs 7 month

Both canopies were visually inspected for defects prior to test and followings observation were made:

a. Optical quality of the transparencies appears good with no sign of internal craze that may indicate structural failure or material degradation.

b. The holes for screws attachment at the forward and aft end of both canopies appears to be free of cracks or elongations.

c. The hinge nodes of the canopy hinges appear to be in good condition with no visible cracks or defects.

The canopies were mounted with the rigid frame sandwiched between two rigid steel channel structures to simulate the mountings of the canopy to the aircraft frame joints.The mounting structure was arranged so that the line of fire of the compressed air gun was parallel to the canopy contour and that the impact was on the highest curvature of the panel under test. With this arrangement the impact angle of the test canopy corresponded to that with the aircraft in level flight at low altitude cruise speed. Figure 3 shows the canopies mounted on the test structures before the test.

Figure 3 Canopy Mounted on Test Structure

Methodology of Bird Impact Test

The ‘bird’ (chicken) had been stored in deep freeze and thawed out for 24 hours at room temperature (about 25oC) prior to firing. The mounting structure was arranged so that the line of fire of the compressed air gun was parallel to the canopy contour and that the impact was on the

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highest curvature of the panel under test. Adequate support is provided by clamping the canopy frame with steel bars on each side of the canopy. This is essential if the full potential resistance of the canopy transparency is to be realized. The forward edge is fitted with wooden board to maintain canopy rigidity similar to its fitment to windshield. Two high speed cameras were used to record the impact points during the test. Another high speed camera of 10,000 frames/sec was used to measure the speed of the ‘bird’ projectiles.

Using high-pressure air, technicians fire 4-lb chicken carcasses from a 19-ft long launch tube at speeds exceeding 350/400 knots to simulate a direct bird-strike. The impact force is estimated at 374 KN [Appendix A]. Videotapes and high-speed motion picture cameras operating at speed of 1000 frames/sec provides visual data on what actually occurs during a simulated bird-strike. This information is used to determine the ability of the test article to withstand the impact and the damage caused during the strike.

TEST RESULTS

Figure 4 shows the result of the bird impact test. The results of both bird impact test carried out on the service-aged canopies showed that both canopies failed the impact test. The failure of the in-service canopies to withstand the prescribed test condition was probably due to either one of the following reasons:

a. Age-related degradation that reduces the bird-strike capability of the canopies.

b. Both the F-5 canopies are not design for bird strike capability of 350/400 knots.

The Bird Impact Test Facility was able to conduct bird impact test to a certain degree of success. The high-speed motion picture cameras operating at speed of 1000 frames/sec provides only the final picture of the test but visual data on what actually occurs during a simulated bird-strike could not be satisfactorily replayed. Hence meaningful information on the actual impact damage mode was not realized.

Figure 4 Canopy Acrylic failed Bird Strike Test

DISCUSSION

The tests were done at vessel of pressure 170 psi impact speed of 350 knots (186 m/s) for the F5-F and at 220 psi impact speed of 400 knots (206 m/s) for F-5 E canopies respectively. It was

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observed that both the service-aged canopies failed the impact test. The failure of the in-service canopies to withstand the prescribed test condition could be due to age-related degradation that reduces the bird-strike capability of the canopies or alternatively the failure could be due to both the F-5 canopies not design for bird strike capability of 350/400 knots. The acrylic materials of the canopies did not exhibit defects due to age-related degradation and previous fatigue tests [5] show that the material properties of similar service-aged canopy are still within its structural integrity. Hence age-related degradation can be safely ruled out.

To substantiate the reasoning that the canopy are not designed for bird strike capability of 350/400 knots a literature research was conducted and it was revealed that the windshield (stretched acrylic) of F-15 aircraft (similar configuration to F5) has a bird-strike capability of 405 knots (600 knots for polycarbonate laminate with cast acrylic) whilst its canopy (stretched acrylic) bird-strike capability is only 170 knots [6]. Similar reasoning can be applied to the F5 canopies but however this finding needs to be validated by the canopy manufacturer.

With regards to the assessment of the additional risk for the F-5 fleet due to a decreased level of bird-strike protection the test does not provide any indication of decreased in bird-strike protection since the test was conducted at higher speed presumably higher than the designed bird strike capability of the canopy. Hence to determine the risk factor, bird strike information on the canopy need to be provided by the manufacturer and more test need to be carried on serviced-aged canopy.

With regards to visual data the high-speed motion picture cameras operating at speed of 1000 frames/sec provides only the final picture of the test but visual data on what actually occurs during a simulated bird-strike could not be satisfactorily replayed. This shows that the frame rate of 1000 frames/sec is not suitable to record visual data of the impact point.

CONCLUSIONS

The final test trials exhibit the capability of UPM Bird Impact Test Facility to conduct actual bird impact test on aircraft canopy. The visual data on what actually occurs during a simulated bird-strike could not be satisfactorily replayed due to frame rate of 1000 frames/sec is to slow to exhibit meaningful records of the impact test.

The bird-impact tests carried out on both the F5-E and F5-F in-service canopies show that both canopies could not withstand the bird strike capability of 4lb bird at 350/400 knots. The possible cause could be that the canopy is not designed to the bird strike capability similar to that of the windshield which is either made of stretched acrylic or acrylic sandwiched between polycarbonate laminate.

In term of aged-related degradation of the canopy, the bird impact test does not provide any data to substantiate any indication of decreased in bird-strike protection since the test was conducted at higher speed than the designed bird strike capability of the canopy. Fatigue test result does not show that the acrylic material of the canopy is adversely affected by environmental condition.

For continuous improvement in flight safety and continuous airworthiness of F5 aircraft fleet, in order to reduce the potential for damage from a bird-strike, the following principle should be addressed:

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a. The Air Force should continue to evaluate data to ensure that certification standards reflect real-life bird-strike risks.

b. The OEM should be consulted to continually evaluate and modify testing methods to ensure that they reflect actual bird-strike scenarios.

c. Details of bird strike capability of F5 canopy need to be obtained from the OEM and further test may be conducted to validate the bird strike capability of service-aged canopy.

References

[1] Heath, J B R, Gould, R W. Degradation of the Bird Impact Resistance of Polycarbonate: National Research Council Of Canada Ottawa (Ontario);1983

[2]Technical Manual TO 1F-5E-36 Chapter 6 Cockpit Enclosure Group Inspection

[3] Richardson, W.J. Serious Bird Strike-Related Accidents to Military Aircraft of Europe and Israel: List and Analysis of Circumstances. Proceedings and Papers. International Bird Strike Committee (IBSC) meeting no. 23, May 1996. London, U.K.: IBSC, 1996.

[4]ASTM F330 - 89(2004) Standard Test Method for Bird Impact Testing of Aerospace Transparent Enclosures

[5] R. Zahari, F. Mustapha. Fatigue Life Prediction of an F5 Canopy. Technical Report No.02/09

[6] Hugh Darsey. ASTM F7.08 Subcommittee on Aerospace Transparent Materials and Enclosure Technical Seminar, Washington DC, November 2004.

[7] Transport Canada. Annual. Bird Strikes to Canadian Aircraft: 1999 (and previous years) Summary Report. Transport Canada, Aerodrome Safety Branch. Ottawa: Transport Canada, 1999.

Appendix A:A.1 Bird Velocity CalculationA.2 Bird Impact Force CalculationA.3 F 15 Windshield and Canopies

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Appendix A

A.1 Bird Velocity at end of Gun Muzzle

To facilitate bird velocity calculation the following assumptions were made:

a. Full expansion of air occurs at the cannon exit.

b. The average pressure within the barrel is used to propel the bird.

c. The weight of the sabot is negligible small as compared to the weight of the bird.

d. Air and frictional resistance are not taken into account in order to simplify the equation.

e. Outside the barrel disregarding drag and wind the bird is subjected to a projectile motion trajectory

A.1.1 Bird Velocity Mathematical Equations

The equation was derived as follows:

a. The forces acting on the sabot is equation to mass of bird multiply by bird’s acceleration

b. The muzzle velocity is given by

c. The average pressure is given by

d. The distance traveled by the bird is given by

e. Along the horizontal direction the velocity is

f. The vertical velocity follow the equation of motion for constant negative acceleration g

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g. The impact velocity is the

A.2 Bird-impact Forces

A.2.1 Impact-force calculation assumptions

There are a number of factors that affect the impact of a bird strike. These include impact speed, bird weight, bird density, bird rigidity, angle of impact, impact-surface shape, and impact-surface rigidity. To simplify the calculation, the following assumptions were made:

a. impact speed is equal to the speed of the aircraft;

b. impact angle is 90 degrees;

c. bird shape is spherical;

d. bird is deformed by one half of its size on impact;

e. aircraft impact surface does not deform; and

f. aircraft impact surface is flat.

A.2.2 Bird-impact force mathematical equations

The bird-strike impact-force equation was obtained from the Advanced Technology Center at Rockwell Collins [7]. The equation was derived as follows:

a. The energy transfer that results from a bird strike to an aircraft hull can be estimated by change in bird’s kinetic energy. Assuming the bird is at rest and ‘sticks’ to the aircraft after the collision the change in a bird’s kinetic energy is

(1)

Where, W is the work, F is the force, d is the distance over which the force is delivered, m is the mass of the bird and v is the velocity of the aircraft.

b. The force that the bird felt (the same force that the canopy felt) is given by

(2)

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The bird’s mass, m, and the aircraft speed v can be estimated. The key parameter then is the distance d over which the impact is delivered.

a. As a first approximation, assume distance d is half the distance traveled by the aircraft in moving through the bird-impact event. Another assumption is that the bird can be represented as a sphere, the impact force is

(3)

b. If we assume the bird is spherical, then the bird’s size depends on its mass according to the relation

(4)

where ρ is the bird’s density.

c. Combining Equation 3 and 4 gives

(5)

For 4 lb bird r = 7.727 cm (3.042 inch) ρ = 938.5 kg/m3 (0.03391 lb/inch3) the Force F = 11.7349V2 where velocity V is in m/s.

d. The impact point will be at an angle Ө i.e. angle between horizon and line tangent to the highest point at the leading edge of canopy, hence the impact force will be

T = F Cos Ө = 11.7349 V2 Cos Ө (6)

The angle Ө (Figure 6) varies from 10o (straight and level) to 40o (deep dive and tactical turn).

Knowledge of the impact force and the potential for aircraft damage are critical in the design and certification of aircraft components. This section summarizes the methodology applied in the calculation of bird-impact forces.

 Aircraft Speed - Knots (m/s)

Bird Species & Weight (lbs.)

100(57.44)

150(77.17)

200(102.89)

250(128.61)

300(154.33)

350(180.06)

400(205.78)

450(231.5)

Chicken 4.0 F (KN) 38.7 69.76 124.23 194.10 279.50 380.46 496.92 628.90

Impact Force T(KN) Ө = 10o 38.11 68.70 122.34 191.15 275.25 374.68 489.37 619.35

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Impact Force T(KN) Ө = 40o 29.20 52.63 93.72 146.43 210.86 287.02 374.88 474.45

Table 1-Approximate Bird-impact Forces (KN.)

Table 1 shows approximate impact forces by bird weight and impact speed. Accordingly, we see that a four-lb bird striking a windshield at 400 kts generates a direct impact force of up to 497 KN. While the data provide compelling evidence, suggesting more stringent airframe bird impact certification standards may be required, the potential costs involved in upgrading the bird-worthiness of the current fleet of legacy aircraft would be enormous.

A.3 F15 Windshield and Canopies

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