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Birdstrike calculation 6

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jetmaker

jetmaker

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Looking for some help here. FAR 25.631 refers to bird strike damage but does not provide guidance on how to calculate the impact load. The kinetic energy can be calculated from 0.5*m*v^2, but to calculate the force requires that a distance be known.

Any suggestions on how to solve this issue?

Thanks,

jetmaker
 
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As a quick starting point you could use the birds weight and factor by 9 (as in 9g fwd). This would give you a position to work from.

Mythbusters is pretty good, did you see the one about the pressurised fuselage which they shot with rifles, then progressed to explosives!!
 
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what are you applying birdstrike to ?

The leading edge of a wing, H/Stab ? ... don't use a force, use a non-linear impact model, i think LS-DYNA is one.

a piece of miscellaneous equipment ? ... you could use a force, but much more than 40818 suggests (9*8 isn't much), i'd think more like 100g (maybe 50g)
 
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Hi Jetmaker,

As a start you can download a thesis via Google by Koh Chee Chuan "Finite Element Analysis of Birdstrikes on Composite and Glass Panels". He uses non-linear analysis.

Andries
 
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here's an interesting relationship i hadn't noticed before ...

assume momentum = impulse
momentum = mass*velocity
impulse = force*time = (mass*Xg)*time

so airplane velocity = Xg*(impact time)

so if your airspeed is 440 ft/sec(= 300 mph)
and if your impact time is 1/100th of a sec, the X = 400/0.01*32.17) = 1200 ... hummm, i was pretty unconservative, but at least this gives you some rationale for picking a number (you pick other numbers, being the duration of the momentum transfer)

 
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interesting ... "the force of impact is equal to half of the mass of the bird, multiplied by the square of the speed of the aircraft (E=1?2 x Mass x Velocity2)." Apart from the semantics about saying a force has energy units, the inference i'd take is KE = work done = F*d, d = 1m ... equivalent to t = 1/airspeed.

looking at table 4, a very interesting table; i don't quite buy the numbers (but haven't recalc'd them), the change as airspeed changes is not proportional to v^2.

the FAR requirement is for a 4lb bird at Vc or an 8lb bird at approach speeds ... from table 4 you're looking at some BIG numbers, something like 4t. i would be a little uncomfortable applying a point force (or one distributed over a small area) for birdstrike ... your inlet shouldn't be able to pass it !
 
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jetmaker...

Go to
Search for following reports [downloadeable]:

Bird Impact Forces and Pressures on Rigid and Compliant Targets AD Number: ADA061313

Title: Determination of Body Density for Twelve Bird Species AD Number: ADA266452

Also try "bird strike" or "birdstrike" or "bird and hazard", etc...

Regards, Wil Taylor
 
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jetmaker,

You didn't say whether birdstrike is a hazard or not. If the inlet is for an engine, it's a problem. If it's for the air conditioning, that's another matter. Determine if the effect of a birdstrike has a detrimental effect on the pilot's ability to land the aircraft safely, in all applicable atmospheric conditions.

An anlysis technique I have seen models the bird as a "cylinder" with mass M and speed V. This "cylinder" is like the gelatin cylinder that *real* birdstrike tests apply, not the Mythbusters tests which used broiler hens.
The bird Force P then equals:
P*t = M*V*cos(alpha), where alpha is any sweep angle to the surface

When you arrive at a number for pressure, assume that the pressure is highest at the instant of impact, and zero when the bird has been fully "smashed". Hence, a triangular distribution of pressure w.r.t. time, and the average would be pressure you calculated. p[sub]0[/sub] would then be 2*p[sub]mean[/sub].

If you expect the bird to penetrate the surface, then you must consider the effects of the bird "material" upon what's behind the surface.

Density = 1, like water, is conservative - (most birds float, don't they?)

Isn't there any FAA advisory material on FAR 25.631?


Steven Fahey, CET
 
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SparWeb,

Thanks for the info, I'll check for an advisory. As far as the hazard goes, it is not a hazard so much as a design requirement that is mandated by the customer.
 
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I was involved in birdstrike analysis and testing for many years. I used non-linear FEA (DYNA-3D) which takes account of the bird's deformation coupled to that of the structure being impacted and the changes in material strength (strain rate effect). There is not a single force number you can use. Data exists for forces created by impacts on flat plates but these are not applicable to the case you have. Some empirical data exists for engine intakes but it is propatory The bottom line is that you need to either test or analyse, preferably both.
 
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There is an old RAE document which details a method of designing a wing leading edge to resist birdstrike. This can safely-ish be used for curved inlet LEs. It's good for metallic LEs. It works quite well for preliminary sizing and weights, and will probably pass a test. (Or an explicit FE can be used (one of the Dynas, Dytran or Abaqus Explicit), though last time I was involved (2000) the authorities still needed a test.) The document is Royal Aircraft Establishment Technical Report 72056, "The Design of Leading Edge and Intake Wall Structure to Resist Bird Impact". There is also a BAC report which is partly based on this, "BAe Bird Strike Design Manual", GEN/B44/30210.

The main reaon for keeping a bird out of an inlet is because you don't want it blasting back through the inlet aft bulkhead and potentially shutting down the engine by ripping off a FADEC box or whatever, or, worse, rupturing a fuel line and starting a nacelle fire.

I haven't got a copy of the full RAE report. If anyone knows of a digital copy please let us know.

However, I do have the GEN/B44/30210 report, and also a more concise Hawker Siddeley report which have most of the relevant data in them.

The GEN/B44/30210 report is 64 Mb; if you can receive this by e-mail then let me know a contact e-address suitably encoded (ROT 47 encoding worked well enough last time I did this). The Hawker report is just 1/2 Mb, but doesn't have the stuff in it about calculating the forces and pressures and doesn't account for material other than 2000 or 7000 series Al alloys, whereas the BAC report does all those things, plus it has a ton of references and background plus the workings for the various equations.

However, you may not need the whole document; the basic equation from the RAE report, as stated by the BAC report is:
Vp = 3127*t / cube_root(W*cos(alpha)^2) * antilog10(0.83 / (r^2 + 1.16*r + 1.56)) * 0.8*Ftu/63

where:
Vp is penetration speed in knots,
t is thickness in inches,
W is bird weight in lb,
alpha is LE sweepback angle,
r is LE nose radius,
Ftu is material ultimate strength in ksi.
NB: Antilog10(x) is aka Exp(x).

For the force, the BAC doument says that the RAE report has an empirical formula:

Fav = 1.18 * M^(2/3) * V^2 * cos(alpha)

where:
V is impact speed in knots,
M is bird mass in kg,
alpha is LE sweepback angle,
Fav is the average force in N.
(Apologies for mix of units here.)

I think this translates to
Fav = W^(2/3) * V^2 * cos(alpha) / 6.39

Fav in lbf,
W in lb,
V and alpha as before.

These average forecs are based on measurements during tests, and so aren't too bad. If you spread the force out from the LE back to the inlet attachment, take the distance around the attachments equal to the distance from the LE to the attachment station. NB: this assumes that the bird is comletely stopped by the LE.
 
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RPstress,
Sorry for going off on a tangent, but unless the RAE report is extremely long, I bet it could be reduced in size a fair bit with the right software. I can't pick up a 64MB e-mail, but I could point you to a few handy tools (read: free) for manipulating PDF files. (I'm assuming it was scanned at one point and saved as a PDF). If you don't mind storing large files, I undersand, but some of us do, so I thought I would point out the option.

Steven Fahey, CET
 
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Maybe if RPstress is ok with it and if somebody with a web-page that would be able to store the file for a bit if it were for general consumption? To enable it to be downloaded?
 
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PRstress

I'm very familiar with all the reports you reference but some of them will be/are propratory and not for circulation.

The problem with the reports is that they concern specific geometries and the failure modes reflect this. Accordingly if the design that Jetmaker is considering is significantly different from the report(s) then the simple equations on penetration velocity will no longer be valid. This is why analysis is more common these days.
 
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A correction to my post with the RAE document formula in it:

The correct formula (from a Hawker Siddeley conversion to Imperial units from the early metric of the RAE report) is

Vp = 3172*t / cube_root(W*cos(alpha)^2) * 10^(0.83 / (r^2 + 1.16*r + 1.56)) * 0.8*Ftu/63

where:
Vp is penetration speed in knots,
t is thickness in inches,
W is bird weight in lb,
alpha is LE sweepback angle,
r is LE nose radius in inches,
Ftu is material ultimate strength in ksi.

Apologies.
 
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