countersink skins? really?

I've just started bulding the tail of a RV7.
I read an article in Van's old newsletters about counter sinking skins that are .32 or .40
This is news to me. I thought everyone was dimpling all skins. Is this just old, out dated information? I havent' read what the thickness is of the skins I have for my RV7 tail, anyone know?

Thanks,
Lance

Some builders countersunk certain parts, but these days almost everybody dimples the skins. I personally dimple when given a choice. I feel I'm leaving more material for a stronger joint, and dimpling also benefits from the additional anti-shear of nested dimples. I'm not an engineer, so these may be naive assumptions...but hey, please correct me if I'm wrong!

Anyway, up to .040" thick I dimple when I can.

Dancer said:

I've just started bulding the tail of a RV7.
I read an article in Van's old newsletters about counter sinking skins that are .32 or .40
This is news to me. I thought everyone was dimpling all skins. Is this just old, out dated information? I havent' read what the thickness is of the skins I have for my RV7 tail, anyone know?

Thanks,
Lance

Click to expand...

Lance,
Certainly, a dimpled hole is stronger than one that is machine countersunk. But in many applications, that extra margin of strength will never be called upon and is not necessary. Several years ago, an Oshkosh Grand Champion winner was (the late) Tyler Feldman's outstanding RV-6. To produce the smoothest skins possible he chose to machine countersunk all of the wing skins. He knew that cosmetically a dimpled hole cannot approach the inherent smoothness of a machine countersunk hole.

Strictly speaking, the minimum acceptable material thickness to machine countersink for AD3 rivets is .032. There are documents out there that specify that but here is a link I am sure you can relate to. http://www.vansaircraft.com/pdf/section 5r9a.pdf I refer you to page 5 of the document. Note that even so, Van generally recommends dimpling anyway.

.032 sheet thickness leaves no margin for error because if you countersink the slightest bit too deep, you will surely produce a "knife edge" and over time may risk "smoking rivets" among other bad things. That said, I have machine countersunk many an .032 sheet and I sleep sound at night. My technique is to countersink the material just a tiny bit shy of maximum depth and then use a rivet shaver to flush the rivet to the skin.

You just have to look -- deeply....

Rick6a said:

Strictly speaking, the minimum acceptable material thickness to machine countersink for AD3 rivets is .032. There are documents out there that specify that but here is a link I am sure you can relate to. http://www.vansaircraft.com/pdf/section 5r9a.pdf I refer you to page 5 of the document. Note that even so, Van generally recommends dimpling anyway.
.

Click to expand...

Rick... there is a document specifying that.. it was in the old version of AC 43.13... it stated the minimum thickness for countersinking at 0.032 for 3/32 rivets...

The new (current) version of AC 43.13-1B refers you to the design guide that all structural aircraft designers use.... MIL-HDBK 5 "Metallic Materials and Elements for Fight Vehicle Structures"

Actual quote from this Handbook is...

b Values above line are for knife-edge condition and the use of fasteners in this condition is undesirable. The use of knifeedge condition in design of military aircraft requires specific approval of the procuring agency.

It is actually documented as sort of OK, but definitely not recommended.

The MIL-HDBK 5 is a free download from the govt. Assist site...

http://assist.daps.dla.mil/quicksearch/ search on full name given above - beware 79 Mb approx...

....but it is now obsolete, has been given to a private entity, and you need to buy it with $$$. I believe the first version is the same as the MIL version, but just re-labeled....

I say, don't do it unless you have to....

gil in Tucson

Lance,
Avoid countersinking where ever possible. You risk having smoking rivets in the future, less stength and added labor. Dimpling can be done on skins up to .040". Dimpled holes are stronger. Dimpled holes retain the pure aluminum cladding on the skin surface.
When countersinking, I've found that often the C/S depth varies from hole to hole. This requires you to drop a rivet in each hole to check whether or not the rivet head sits flush. Those that are not flush must be countersunk slightly deeper. All this adds a lot of time, which you won't have to spend, if you dimple.
Charlie Kuss

az_gila said:

.... MIL-HDBK 5 "Metallic Materials and Elements for Fight Vehicle Structures"

Actual quote from this Handbook is...

b Values above line are for knife-edge condition and the use of fasteners in this condition is undesirable. The use of knifeedge condition in design of military aircraft requires specific approval of the procuring agency.

It is actually documented as sort of OK, but definitely not recommended.

The MIL-HDBK 5 is a free download from the govt. Assist site...

http://assist.daps.dla.mil/quicksearch/ search on full name given above - beware 79 Mb approx...

....but it is now obsolete, has been given to a private entity, and you need to buy it with $$$. I believe the first version is the same as the MIL version, but just re-labeled....

I say, don't do it unless you have to....

gil in Tucson

Click to expand...

The company I work for has had the contract to maintain Mil-hdbk 5 for many years (since 1954). It was recently renamed MMPDS.

The web site is: http://www.mmpds.org/

-mike

mlw450802 said:

The company I work for has had the contract to maintain Mil-hdbk 5 for many years (since 1954). It was recently renamed MMPDS.

The web site is: http://www.mmpds.org/

-mike

Click to expand...

Hi Mike,

I'm assuming you no longer have the contract, is that true?

Do you know if the MMPDS latest version has any major edits from the last free version of MIL-HDBK 5 ("J" I believe)?

gil in Tucson

az_gila said:

Hi Mike,

I'm assuming you no longer have the contract, is that true?

Do you know if the MMPDS latest version has any major edits from the last free version of MIL-HDBK 5 ("J" I believe)?

gil in Tucson

Click to expand...

Actually, we still have the contract. The responsible party in charge is in my group and is just a few offices down the hall from me and I will ask about the edits.
A little history: The USAF had governmental responsibility of MIL-HDBK 5 from its inception until ~2003. In 2002 the FAA took over the governmental oversight and renamed the document MMPDS (Metallic Material Properties Development and Standardization). The first publication of MMPDS was technically identical to and overlapped publication of the last version of the HDBK 5.
We are now at MMPDS-02 and will shortly be releasing MMPDS-03. I believe the new releases contain new material data more than errata corrections so I suspect that the last version of the handbook ("J") is still substantially valid.

-mike

Countersink skins

I just recently rebuilt a crashed RV4 originally built in 1986 with 1400 hard hours on it, had good workmanship but was very well used and not had the best of maintenance in recent years. The entire aircraft was countersunk and had held up surprisingly well with only a few "smoking" rivets. The exception was the fuel tanks which had some real leaks around some of the countersunk rivets. Some older RV's used sloshing compound rather than Proseal but this one had Proseal although it was not very heavily applied. The fix was to drill out the offending rivet and replace it with a pulled rivet liberally dipped in Proseal. I do believe the eventual fix will be a new pair of dimpled tanks in the future. The aircraft suffered no wing damage and these tanks were obviously leaking before the incident with most leaks being nuisance weeping rather than severe leaks. Being a believer in the "dimples being stronger" point of view, this aircraft has a new exclusively dimpled front end but I have to admit the countersunk skins held up much better than I would have guessed.

Dick DeCramer
RV6 (Dimpled)
155 Hours
Northfield, MN

DickDe said:

I just recently rebuilt a crashed RV4 originally built in 1986 with 1400 hard hours on it.............. The entire aircraft was countersunk...........I have to admit the countersunk skins held up much better than I would have guessed. Dick DeCramer

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Dick,

Your first person observation is testament to real world conditions and worth far more than ten million words of mostly theory and personal opinion put forth by the well intentioned dilettante, many of whom populate these groups including me. As I previously stated...I have machine countersunk many an .032 sheet and sleep soundly at night. While I happily concede a dimpled hole IS stronger, your experience seems to validate a long held personal opinion that the extra measure of strength is really not necessary or ever called upon. Given that the original builder endeavored to machine countersunk the entire airframe, I would have to guess he was exceptionally skilled and/or patient to produce quality holes while avoiding knife edges to only end up with just a few smoking rivets. There is no margin for error. 1400 hours of operational service speaks for itself and is truly talking the talk and walking the walk. Thanks very much for sharing your first person observations.

When I built my -6 ('89-'93) I countersunk the skins on the horizontal and vertical stabilizer. Today there are no smoking rivets. They have held up fine. Would I do the same thing again? Absolutely not. Even though it works, it is definitely not as strong, and in my opinion, does not look as good as dimpled skins. My recommendation is that whenever given a choice...dimple!

Once in a while

In my "vast" experience - just finishing my empennage - I have countersunk a couple of skin rivets near trailing edges where the fit is tight. Now I have the "pop rivet" dimple set and that works pretty well.

I'm not planning to be a grand champion, but I am doing my work to my personal high standards. Dimpling satisfies those standards.