birdstrike analysis question - wing leading edge (FAR25) 1

[Haplo787]

Dear forum members,

I am looking at a birdstrike analysis of a leading edge of a commercial aircraft.
The penetration velocity is calculated per RAE TR72056 report.
Ref post written by RPstress in thread

http://www.eng-tips.com/viewthread.cfm?qid=192021.

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


Could someone clarify a few items about that approach please:
1) does the above formula account for rows of fastener holes (attachment to ribs)?
2) in the later post by RPstress in the above thread the "0.8*Ftu/63" is mentioned as a coefficient to account for difference in materials. Why the 0.8 reduction?

Thank you
Alexey.

PS I do not have a copy of the TR72056 report, maybe some of answers to my questions are there...

 

[IRstuff]

Yes, some of the answers are in the report. Bear in mind that the equation from the report is empirical.

They mention that the leading edge penetration involves fracturing along the line of rivet holes, so presumably, the equation includes the effect of those holes.

The 0.8 appears to be a fitting factor to their measured data, but they only used that in the context of the titanium and nickel alloys they tested.

TTFN
faq731-376

Need help writing a question or understanding a reply? forum1529

 

[RPstress]

Bad quality and only 15 ot of 29 pages copy of RAE report attached.

As IRstuff says, it's emprical. This and the reports listed below were mostly (all?) based on curve fitting impact test results. The 0.8 reduciton for stronger metals is feature of that approach; it gave a passable and conservative fit to the data but would be pointless nonsense for a material less than 1.25 times as strong.

This is an incomplete list of crap I've collected over the years on simple birdstrike stuff:
RAE TR 72056 "The Design of Leading-Edge and Intake Wall Structure to Resist Bird Impact",
Hawker Siddeley Armstrong Aviation Design Handbook vol. 2 sections 13 & 14,
BAC GEN/B44/30210 v3 International Birdstrike Manual,
AFFDL-TR-77-60 "Bird Impact Forces and Pressures on Rigid and Compliant Targets",
AFFDL-TR-77-80,
AFFDL-TR-77-134,
AFFDL-TR-77-150,
Bombardier B39 Chapter 10 Section 001 Birdstrike was written in the mid to late '90s based on these old equations.

There are minor differences between these but nothing really significant (from memory; I haven't looked at these at all since about 2011 and I can't really remember what's in the AFFDL reports). The modern way is to spend about 9 man-years developing explcit FE analysis models (which don't work very well for composite LEs). The above reports allow some rapid preliminary sizing, but are of course only valid for metal structures similar to those tested. The RAE report (or maybe it was the Hawker report) mentions the presence of riblets at about 8" pitch. Not usual in modern LE designs... Airbus have used glass/honeycomb LEs on fins in recent years. I read that Glare ones have been used somewhre on Airbuses but have no info to back that up. Such designs would be well outside the scope of these reports' methods.

I can post the non-copyright stuff (basically not the Bombardier) that might be hard to find if you don't have it and can't find it, but I'm not sure of the file size limits at Engineering.com. These old scans are not modern searchable PDFs and get quite big. The AFFDLs are of course online somewhere. Like a fool I didn't keep a record of where I found them.

 

[GrandpaDave]

FYI: I spent a week, in the early 1980s, in San Antonio, TX. at SW Research taking a course in Penetration Mechanics.
I was there on a "boondoggle" with an associate who was a penetration mechanic guy. It wasn’t my-cup-of-tea but I enjoyed
being there. In those days the East Germans along with the Russians were the bad-guys, so we were working on runway penetrators.

I’ve been retired since 2000… but SW Research is probably still involved in penetration mechanics and such. --- Grandpa Dave

 

[Haplo787]

Dear all,

thank you for taking time and effort to answer my post.

RPstress, thank you very much for the report, it helped us a lot.

With kind regards,
Alexey.