Re: Lift Tabs, etc.

Dan & Jill Patch <dpatch1@...>

Joe Person wrote:

I can say this much, when overloaded (canard tip catching earth and ripping
from the airframe), the F22 bulkhead will fail catastrophically at the
canard attach points, while only bending the tabs, and not elongating the
bolthole in each tab. I ran this test in a forced landing 3 years ago...

and Ken Miller wrote:
I would wholheartedly agree except for one thing: real world
experience. I work on many canard aircraft in my shop. I can't remember
an aircraft coming in that I worked on that the canard lift tabs were not
damaged in some way.....

I for one appreciate the real engineering numbers (from Joe) backed up by
some real "experimental" observations (both Joe and Ken, on opposite sides
of the fence) rather than "gee it sure looks better thicker" (which, I'll
admit is true, but may not be very relevant).

FWIW, last weekend I looked at a LEZ (not mine!) that repeated Joe's "canard
tip catching in the earth" experiment - with the same results - bye, bye
F22, slightly bent lift tabs.

Perhaps what I'm getting from this is that you should go with the stronger
tabs and bolts if you think that you will be unable to build to plans and/or
maintain your tabs in an airworthy condition (i.e., undamaged). Although
it's a surprise to me, Ken's observations suggest that the poor quality in
this area not extremely rare. Super scary! Otherwise, the design and
operational strength of the "stock" 1/8" lift tabs should not keep you awake
at night.

One other point that may have been made, but if so I missed it, is that the
canard attach is a "system" that doesn't end at the tabs. If perfectly
designed, there should be a 50/50 chance that some other part of the load
path will fail under ultimate load rather than the lift tabs/bolts. Whether
or not this is true for Rutan's design I haven't bothered to figure out, but
knowing Burt, I suspect that the structural strength of each part of the
design is reasonably well matched to the anticipated load limit condition.
Logic then says that beefing up the lift tabs should have only a 50/50
chance of adding ANY useful strength at all. Adding strength to the lift
tabs should just move the failure to the next weakest link in the "system".

In particular, the critical member appears, empirically, to be F22. So
maybe it should be stronger? But then the super sturdy lift tab/F22
combination may fail the canard shear web and/or the backing plate, so they
ought to be beefed up too? But then the extra weight and stiffness will
require.... what???? Well, you can see why going down that path may not be
quite as simple (or useful) as it may appear, and the warm fuzzy feeling of
super strong lift tabs should be mostly an illusion.

Although not strictly relevant, I found the following quote from Rutan
circa, CP17 July 1978, to be interesting regarding the general process of
beefing up the design:

"Q. I am considering adding a few extra plies of glass in a few areas to
beef them up for extra strength is this OK?

A. No! More than likely the extra material will not be added where first
failure will occur anyway. The extra weight in most places will actually
weaken your airplane in that its maximum 'g' capability will be less and
failure on hard landings will be more likely. The best thing you can do for
optimum safety is to do perfect workmanship with the exact ply arrangement
in the plans. Also, stiffer structure can change flutter modes.

Enjoy the Holidays and get some air under your wings if you can!

Regards, Dan

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