Appendix 12.1 — Birdimpact Forces — The Physics
Aerodromes and Air Navigation
 Standards Branch
 Aerodromes and Air Navigation
 Standards
 Wildlife Control

TP 13549
 Acknowledgements
 Dedication
 Preface to the second edition
 Foreword
 How To Use this Book
 Introduction
 Chapter 1
 Chapter 2
 Chapter 3
 Chapter 4
 Chapter 5
 Chapter 6
 Chapter 7
 Chapter 8
 Chapter 9
 Chapter 10
 Chapter 11
 Chapter 12
 Chapter 13
 Chapter 14
 Chapter 15
 Appendices
 Glossary
 Acronyms
 Production Team
 Colour Plates
Introduction
This book features a series of tables that assist readers in understanding the impact forces that are generated by birds of various weights at a variety of speeds:
 Table 12.1 — Bird Impact Forces vs. Speed
 Table 12.2 — FAR 33 Engine Certification Standard Bird Weights
 Table 5.5 — FAR 23, 25 & 29 Airframe Certification Standard Bird Impact Forces
 Table 5.6 — FAR 33 Engine Certification Standards (Old) Bird Impact Forces
Knowledge of 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 birdimpact forces.
Impactforce 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,
 impactsurface shape, and
 impactsurface rigidity.
To simplify the calculation, the following assumptions were made:
 impact speed is equal to the speed of the aircraft;
 impact angle is 90 degrees;
 bird shape is spherical;
 bird is deformed by one half of its size on impact;
 aircraft impact surface does not deform; and
 aircraft impact surface is flat.
Birdimpact force mathematical equation
The birdstrike impactforce equation was developed with the assistance of Mr. A.C. Tribble of the Advanced Technology Center at Rockwell Collins. The equation was derived as follows:

The energy transfer — or pressure — that results from a bird strike to an aircraft hull can be estimated through relatively simple calculations. Taking the simplest approximation — where the bird is at rest and 'sticks' to the aircraft after the collision — the change in a bird's kinetic energy is
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.

The force that the bird felt — the same force that the airplane felt — is given by
We can estimate the bird's mass, m, and the aircraft speed, v, with ease. The key parameter then is the distance d over which the impact is delivered.

As a first approximation, let's assume it is half the distance traveled by the aircraft in moving through the birdimpact event. If we further assume that the bird can be represented as a sphere, we end up with

If we assume the bird is spherical, then the bird's size depends on its mass according to the relation
where is the bird's density.

Combining the two previous expressions gives
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