Aerodynamic smoothness of rivets

[Sparweb]

Working on a turboprop aircraft whose SRM defines aerodynamic smoothness in terms of maximum protrusion beyond the skin. It's tight enough that everyone says you can't have protruding head rivets. However, the purpose of the rivets is a doubler, and the doubler is a reinforcement to an antenna, and the antenna is a blade 16" long with a 10" chord. A big shark-fin blade that has plenty of drag. I am looking at the overall situation and considering whether the rivets really have to be flush for "aero smoothness" when the antenna will add many pounds of drag in the free air stream.
Other considerations:
-Fracture mechanics of countersunk rivets is poor compared to protruding heads
-The skin is very thin, chem-milled. Even low-profile rivets (NAS1097) are knife-edge in 0.032 thick skin.
-Blade antenna is stabilized by rigid internal structure but that means there are out of plane loads on this doubler. Small, but not zero.
-Boundary layer thickness in the area is at least 1/4" (this is not a guess, I have experience with this)

I think I'm gearing up for fight but is there really one to be had?
If the purpose of the smoothness requirement in the SRM has more to do with blending edges of repairs, and overall fuel economy of the aircraft in service, and "nice to have" then could an argument for structural integrity beat it?

 

[SWComposites]

Doubler is presumably on the inside?
Can you add an outside doubler, countersink into the doubler, and then add aero filler on the doubler leading edge?

 

[SWComposites]

And presumably you are getting an STC, so the SRM is not really applicable.
Is the antenna in an area where aero smoothness is not really important?
Do you have an aero expert on staff?

 

[Sparweb]

Doubler is inside, and preferred that way for ease of HFEC inspections. External doublers usually aren't as convenient, but now that you mention it, accessing this specific area from the interior every few thousand hours is trivial. Worth some thought.

You are correct about the STC, but my style is to refer frequently to SRM practices in the substantiation. It would be unusual to avoid the particular section about aero smoothness when so many other SRM practices are referenced. Maybe that's the issue in a nutshell, the inconsistency.

I do have an aero expert but more of a loads and dynamics person, not had to make a judgement like this before. The boundary later investigations I've done before tell me enough that a patch of several dozen rivets isn't going to affect the speed or fuel consumption of the aircraft one jot. The drag from the antenna is an order of magnitude more.

 

[MintJulep]

my style is to refer frequently to SRM practices in the substantiation. It would be unusual to avoid the particular section about aero smoothness when so many other SRM practices are referenced.

Who says that your references always must be affirmative?

Seems to me like you have valid reasons for not following the SRM. So....

Although the SRM call for flush rivets, in this case protruding head rivets are used because:
- The drag from the antenna is much larger than any contribution of the rivets.
- Fracture mechanics of countersunk rivets is poor compared to protruding heads
- The skin is very thin, chem-milled. Even low-profile rivets (NAS1097) are knife-edge in 0.032 thick skin.
- Blade antenna is stabilized by rigid internal structure but that means there are out of plane loads on this doubler. Small, but not zero.
- Boundary layer thickness in the area is at least 1/4"

 

[Ng2020]

What are the downstream consequences of installing such a prominent antenna in an aero critical zone?
Or does the aero smoothness requirement apply universally?
Can you give any indication of the type?

 

[WKTaylor]

Installs like this are rarely in the aerodynamic critical areas... so protruding head rivets around the antenna based would likely have no/0 drag effect... BUT... customers 'hate bumpy-head rivets' on their sleek flush riveted jets.

This is when I would recommend/specify... in rough order of preference...

Dimpled outer skin into dimpled or countersunk underside doubler... then buck-install solid aluminum flush rivets. WARNING: Cold dimpling May NOT be possible with 2xxx-T6 or 7xxx-T6 sheet metal.

Protruding shear/small-head titanium Hi-Loks with aluminum collars 'wet with sealant' in transition-fit holes.

NAS9306 FLANGED DOME HEAD blind rivets [= 'Unisink' blind rivets, CherryMax style = CR3245, CR 3255]
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1686843456/tips/CA-1011_CherryMax_manual_y27903.pdf[/url]

Or a protruding shear-head [low height] titanium blind bolts[ Huck or JO-Bolt style]
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1686843775/tips/CA-1010_Titanium_Blid_MaxiBolt_xrnhw6.pdf[/url]

Of course, MS20470 rivets can be installed and the protruding heads CAREFULLY milled down ~to 66% height.... to reduce height and look 'smoother'... but retain adequate head strength.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[rb1957]

yeah, but protruding head rivets on an OML look "bloody ugly" and very "agricultural" (more like a tractor than a plane).

If you don't have skin thickness for a CSK (even an LZ4) then your two choices are protruding head rivets (yuch) or an external dblr. Yes, this will require (if in the pressure cabin) a LFEC inspection, but this shouldn't be a problem. Of course you can extend the dblr beyond the stringer and so have an internal visual/HFEC inspection.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Sparweb]

Thank you for all the suggestions!

The definition of the critical zone is interesting in this case. Looking carefully, I found that the zone of the antenna doubler is only inches away from the non-critical zone. Basically, any area on the fuselage skin forward of the spar is critical for aero smoothness, while aft of the spar only the upper side is critical. This antenna is on the bottom just two bays forward of the spar. Any downstream effects of this installation just 24" farther back would be only slightly less than the effects at this position (and the existence of any effect is very dubious).

Another thing I noticed while checking the details: the aero smoothness standard accepts anything less than 0.030" above the skin surface. Since the head of a MS20470AD4 is 0.05" high, this is how I came to question using them. Are there any other types of protruding-head solid rivets that don't have such a large head? I've heard of a "Briles" rivet but never seen one in the wild. The Unisink fits under the threshold, but using blind rivets on the skin would probably cause more trouble than it solves. This is probably a blind alley (oh, pun intended) because even if I can find special rivets, there won't be strength data for design, and the guys with the rivet guns will need special tools to drive them!

Shaving the heads, as Wil suggested, sounds preferable. That could be a note on the drawing presented as an option to the installer.

This turboprop has been described as the pick-up truck of the sky, so I'm not daunted by the "ugly" argument.

 

[LiftDivergence]

Doubler is inside, and preferred that way for ease of HFEC inspections. External doublers usually aren't as convenient, but now that you mention it, accessing this specific area from the interior every few thousand hours is trivial. Worth some thought.

Hmmm... what fasteners are being installed and does the NTDM reference standard account for the fastener head overhang?

I see a lot of people get caught out by that... in analysis they assume detectable length is whatever the reference standard says, but usually for HFEC it is XX inches PAST the fastener head.

LFEC is is usually standardized from the SHANK.

Add on top of that, you need to be inspecting for cracks originating at the faying surface between the doubler and skin. So if your doubler is internal, you should be using external MFEC, which is not as sensitive as HFEC.

So if your options are:
1. Design with internal doubler and use external MFEC around fastener heads -or-
2. Design with external doubler and use external LFEC through the doubler

It is probably close to a wash in terms of inspectability tbh.

I see this all the time with DTA reports, people don't pay attention to the NDTM reference standards and set their detectable flaw size incorrectly.

Keep em' Flying
//Fight Corrosion!

 

[verymadmac]

What do the SRM skin repairs look like for this location rivet wise.

Edit: Noting that some times SRM repairs give themselves outs againest other restrictions in the SRM.

 

[Ng2020]

Liftdiv:
Instead did you mean: an inspection internally using mfec, through the doubler, for a lead crack in the skin?
(Typ assume cracks are growing in the worst case location, in this case - always at the faying surface regardless of doubler on inside or out. Assuming also antenna not removed and doubler gage> skin)

 

[LiftDivergence]

Ng,

This is what I mean (excuse the hastily drawn figures):

My assumption is that we are always inspecting the skin, not the doubler, since this should be the critical detail for load transfer in the reinforcement.


Keep em' Flying
//Fight Corrosion!

 

[WKTaylor]

FAA-AIR-90-01 Repairs to Damage Tolerant Aircraft, T Swift
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1686974105/tips/FAA-AIR-90-01_Tom_Swift_Repairs_to_Damage_Tolerant_Acft_qs4mzf.pdf[/url]

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[Sparweb]

Hi LiftDivergence!
You are quite right, the head must be accounted for, whether it is flush or protruding. And special-head rivets may be larger in diameter if lower in profile, thus delaying the detectability threshold. I believe the standard inspection tools on this aircraft for its external skin (per OEM insepction manual) is HFEC, which is unable to "see around corners".

I've taken the liberty to edit your diagram, since it's suitable to illustrate my external doubler considerations, too. My design concerns the lower skin, so in my application the diagram should also be upside-down. I didn't bother turning everything around!

Another consideration, and this may come out of left field for many of you, but I often see it on aircraft of this type: a self-adhesive polymer protective covering on the lower skin. It's most commonly seen on aircraft equipped to land on unpaved runways, to protect the belly skin from rocks thrown up by the nosewheel. It's one part of a "gravel kit" modification that many of these planes that I see. This polymer coating is about 0.04" (1mm) thick and it must be horrendous to remove for skin inspections. There might be more happy customers if I make this antenna design inspectable from the interior.

 

[rb1957]

I'd be surprised if we were looking for cracks under the head. I'd be looking for a 0.25" surface breaking crack (so the analytical crack length would be slightly longer) ... if I was using HFEC in the first place. Most of the time I try for visually detectable cracks, particularly if my plane is "a pick-up truck".

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[rb1957]

DHC8 has special inspection procedures low anti-gravel 3M tape (or paint).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[rb1957]

internal inspections ? so external doubler ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Sparweb]

RB,
Yes, I was considering an external doubler to try to make the inspections happen from the inside. Visual and HFEC.

Found some trouble with that idea today, where everything in "chapter 4", IE the Maintenance Planning Manual that defines airworthiness inspections, specifies all of the visual inspections be done from the exterior. I couldn't find any internal skin inspections. That's a bad start, even if I do ultimately plan to define scheduled inspections with HFEC.

In the same manual is a "note" about the gravel protection and, quite confusingly, it does not say to remove it. Instead it says to inspect the protective layer. Umm... but cracks grow whether it's covered or not... It is definitely not transparent. I'm still pondering the meaning of that statement, in case I haven't understood it or need information from somewhere else to make sense of it. I certainly have more reading to do, of course.

 

[rb1957]

"surely" there are visual inspections of the IML, like for corrosion ?

"surely" you can define your own inspections ? maybe as a "special detailed visual" ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.