Birdstrike calculation 6

[jetmaker]

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

 

[saksmito]

If you need a force you could try using the size of the bird/2 (length/2) as a mean value (from head to the center of mass).
Anyway I agree with the previous answers. If you have an impact you are studying a dynamic load. You have a very high energy in a short time (dirac delta). Materials don´t always behave in a linear way when they are facing this kind of load, and even though you get an accurate force the structure should fail.

 

[IRstuff]

Sure, a classic example is the explosively formed projectile (EFP) that's been in the news from Iraq recently. The same shaft of copper under a steadystate compression against armor would buckle and crumple. But in the dynamic case, the same shaft punches a hole through the armor.

TTFN

FAQ731-376

 

[RPstress]

Another birdstrike formula clarification:

The 0.8*Ftu/63.4 is for non-aluminum LEs. For decent 2000 and 7000 series Al just miss it off. (They found that strength correlation for Ti-6-4 and Inco 718.)

The Als tested were L73 (2014A-T6), L72 (2014A-T3) and L88 (7075-T6). Anything very different from that had probably better have the strength factor in.

Here is the metric version:

Vp = 98t * exp(1234/(r^2 + 30r + 1000)) / (W*cos(alpha)^2)^(1/3) * 0.8*Ftu/437

 

[DavidDDH]

RPstress
I would be interested in the GEN/B44/30210 report if you are still interested in providing it.

My email will definitely not take 64 Mb, but I do have an FTP site that it could be loaded to. If the report is still available let me know here and we can figure a way to share the address.

Thanks, David
Aircraft Stress / DER

 

[RPstress]

It's ancient stuff (data from the seventies and early eighties) but still kind of interesting.

Like a lot of things, more sophisticated analysis (mainly explicit FE) has taken its place for a lot of work, but there's nothing new and up to date (that I know of) for quick estimates/sizing.

I also have the original RAE report (TR 72056, 1972) now, too. It's only about a megabyte.

 

[plasgears]

I worked on the early composite fan blade development at GE, and they learned to make sure the birds were defrosted before the test. It throws off the data.