Crush box rule interpretation

[emotepix]

Hi All

I've just started a frame up on my RF86 FC and of course this falls under the GCS's D.6.b) and c) in the FCS (page 53, crushable nose structure and side panel intrusion protection). FF rules apply to FC in this instance.

I have a nose box from an RF88 FF and I'm trying to decide whether or not it's legal.

What's the interpretation of "minimum cross section of ... 31 sq. inches" mean? Cross section which way? From the front?

Is that a total frontal cross section at the widest part? Or at the front panel of the box?

Thanks!

[R. Pare]

The rule states that that cross-sectional area is the minimum requirement at a point 15.75 inches forward of the pedals. Just measure up the nose from the pedals to that point, and measure what the section area is at that point.

If you are not sure what is meant by "cross section", just imagine that you have sliced off everything forward of that point, and then measure the width and height of that cut face. That will give you the sectional area at that point.

[emotepix]

Thanks for your input.

While I do know what the words mean, I'm used to seeing them a little more qualified, as in "the frontal cross section of the structure, taken at the minimum point, should be no less than xxx".

There's a leap in logic in the wording as it now stands (a comma in the place of the words "taken" at" or "measured at" that leaves room for ambiguity, thus my query to this wonderful band of people.

Thanks again for the clarification!

[Purple Frog]

Yes, grasshopper, many times the GCR contains sections that truly are a "leap of logic". Some folks find the chasms nice places to jump in with unique interpretations.

OBTW, having 'flight tested' a nosebox in the past year... a word to the wise, don't scrimp.

[emotepix]

Thanks Frog!

The box I have is a works one, I believe. It's made from the requisite gauge and type of alu and it looks plenty sturdy enough.

However, it's an FF1600 box and the FC nose won't fit in top of it fully because of the front wing spar.

Having seen some legal crush structures made from open tubing, though, and since the wing spar is made of thicker stuff than the crush plate material, I wouldn't think that I'd be bending the rules if I cut a channel into the crush box to take the wing spar.

(Or of couse I could go with a later model lift nose and a moustache... and a new crush box due to the lift nose... (sigh) there goes the budget...)

Thanks again, all.

[RobLav]

I too went through an aluminum nosebox. The bottom rivets sheared off one by one during the crash - not good. I now build these with countersunk screws at the bottom.

[emotepix]

Time for that aluminium attachment for the wire welder?

[John Robinson II]

WElding might make the material around the weld more brittle. If Rob's crush structure used structural rivets, I bet they would not have sheared. Since I am not proficent at buking solid rivets I went the more expensive route and bought the cherry apex rivets.

John

Note, I might be wrong, but I dont think a crush "box" made of open tubing fits the rules.

[emotepix]

If it's in the shape of a box and it's made of the specified material, then the rest, I guess, is open to interpretation.
I'll look into the avex option, thanks!
Cheers

[R. Pare]

Note, I might be wrong, but I dont think a crush "box" made of open tubing fits the rules.

The structure rules call for it to be made up of a min. .050 6061-T4 alu sheet. If a steel or alu tube frame were made and then clad with the alu sheet, it would probable get thru tech OK. A simple tube frame structure with no sheet won't ( or shouldn't).

[Bob Coury]

John is correct. The heat affected zone in welded aluminum joints decreases its strength-if memory serves me correct-somewhere around 50%.

[John LaRue]

Shouldn't get through tech, but on the DB1 they always did.

[R. Pare]

The DB1 goes under the pre-'86 rules, so they aren't required. Somehow the 3 cars that were built in '88 or '89 have been ignored in tech!

[John LaRue]

You have that right. haha

[Wren]

John is correct. The heat affected zone in welded aluminum joints decreases its strength-if memory serves me correct-somewhere around 50%.

I've done pull tests on 6061-T6 that was welded with a MIG gun and we saw a decrease in strength from 21ksi uts to 17ksi uts. The fracture was right at the edge of the heat affected zone.