Countersink HS/VS skins?

My first post. Hope to order the tail kit once I get back to the States.

I built a -4 tail kit back in '95 and a "pretty" building option was to countersink the HS/VS skins. Of course, this had to be done with much care. Does Van's support this and/or has anyone done this lately?

HS/VS skins

The standard method is to dimple the skins. Countersinking .032 skins is marginal, it can be done but dimpling is recommended. Dimpling is also much less stressful and quicker than countersinking. The finish is excellent in either case.

Another thing to consider is that some additional strength is provided by the nesting of the dimples over a countersunk skin sitting flush on the rib.
I'm no engineer so I don't know how much difference this would make...

Thomas

madsabre said:

My first post. Hope to order the tail kit once I get back to the States.

I built a -4 tail kit back in '95 and a "pretty" building option was to countersink the HS/VS skins. Of course, this had to be done with much care. Does Van's support this and/or has anyone done this lately?

Click to expand...

Mike,
I know of one local RV6 builder who did this. His HS and VS rivets do not lay down flush as nicely as other builders who dimpled the skins. Dimpling requires less time and skill to do, as well.
Another local RV6 owner (he didn't build his) has had constant problems with "smoking" (loose) rivets on both his wing rear spars. These areas were C/S rather than dimpled. I'd forget about countersinking those skins. I try to dimple everything I can. Skins and spars up to .040" thick can be dimpled.
Charlie Kuss

Some numbers

TShort said:

Another thing to consider is that some additional strength is provided by the nesting of the dimples over a countersunk skin sitting flush on the rib.
I'm no engineer so I don't know how much difference this would make...

Thomas

Click to expand...

Thomas ... the numbers are well known from the MIL-Handbooks for aircraft design.
A selected portion is here, copied from an old 1995 posting of mine on the Matronics List.

-------------- old 1995 post ---------------------

Following is the equivalent data for YIELD strength (in this case, defined
as a permanent set of 0.005 inch)

YIELD strength of 3/32 MS20426AD flush rivets in 2024-T3 (values in lbs.)

Sheet thickness ----- Dimpled ------- Countersunk

0.032 -------------- 209-------------- 132

0.040 -------------- 209 -------------- 153

0.063 -------------- 209 -------------- 213


This is actually a worse % loss than the ultimate loads, giving a 33% loss
in 0.032, and a 27% loss in 0.040. The previous post's figures were for
ultimate strength (failure), while the above are those loads that would
cause a small permanent set.


YIELD strength of 1/8 MS20426AD flush rivets in 2024-T3 (values in lbs.)

Sheet thickness --- Dimpled ------- Countersunk

0.040-------------- 367-------------- 231 (*)

0.063-------------- 506-------------- 321

(*) this is a "knife edge condition", and not approved. (Note: Not approved by Mil-Spec. - but approved by the FAA AC43-13 ... gil A)

----------------- end old 1995 post ---------------

The strength differences are substantial, but Vans has approved both configurations.

gil in Tucson ... who likes actual numbers....

Sorry about the formatting... I can't seem to insert blank spaces...