---
title: "Fuselage Structure: Longerons, Bulkheads and Skins"
description: "Fuselage Structure: Longerons, Bulkheads and Skins. Builder questions and answers from Van's Builder Technical Support."
---
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October 5, 2026
# Fuselage Structure: Longerons, Bulkheads and Skins
## Builder Tech Support answers on fuselage structure: longerons, bulkheads and skins
This article collects Builder Technical Support answers on fuselage structure: longerons, bulkheads and skins.
### Setting wing incidence
Wing incidence is set during fuselage-to-wing fit. The specific procedure varies by model (RV-4 through RV-14). If you are at this step and cannot find the incidence-setting procedure in your plans, call Builder Tech Support — this is one of the most common questions for builders working from older or partial plan sets.
### Overview
This article covers a recurring set of fuselage and skin build questions: when blind rivets are acceptable, what to do after a hole has been enlarged beyond spec, how to handle missed rivets after the assembly is closed, dimple sequencing, and resolving apparent drawing mismatches.
### Blind rivets as a substitute (F-711 longeron example)
If you missed pre-riveting the longeron before adding the aft deck and now cannot drive a solid rivet at an F-711 location, a blind rivet may be acceptable — but only after Builder Tech Support reviews the specific hole. Send a clear photo showing the affected hole and the part you would need to enlarge slightly to clear a blind rivet. Do not enlarge holes in primary structure without approval.
### Overview
This article addresses several recurring fuselage-build questions: bending the RV-12iS lower longeron angle, fit-up issues with year-to-year QB part variation, applicability of older Service Bulletins to newer kits, the laser-cut part replacement scope, and RV-7A to taildragger conversion.
### Wing incidence holes drilled oversize / non-round
If the wing incidence holes have come out slightly oversize or non-round so the AN5 bolts are loose, the priority is whether the hole position is still correct and the edge distance is preserved. If the holes are centered in the flanges and meet edge distance, contact Builder Tech Support with photos and measurements - the resolution may be to step up to the next bolt size, install a bushing, or fabricate doublers. Do not just leave loose bolts in place.
### Enlarged or oversized rivet holes
Holes get enlarged when a rivet gets drilled out, when a drill walks, or when a rivet gun slips. Standard remedies, in order of preference:
- **Step up one rivet size** (for example, AD3 to AD4, or AD4 to AD5) when edge distance still allows — match-drill the hole to the new rivet size.
- **Substitute an AN3 bolt** in a few locations where rivet bucking has failed repeatedly. Bolts are stronger than a bad rivet, but use them sparingly — not as a wholesale substitute.
- **Install a doubler** where multiple adjacent holes are enlarged or where structure demands it.
- **Replace the part** when edge distance is lost or the hole is in a critical structural location.
If you're unsure whether your situation qualifies for a step-up or needs a doubler/replacement, send photos and a sketch to Builder Tech Support — they will look at the location and call it.
### Hole enlarged after drilling out a rivet
When a drilled-out hole comes out oversized (for example, a #40 hole that's now closer to a #30, or a #30 hole that's now at .173" or .204"), the standard remedies are:
- **Step up to the next rivet size** if edge distance allows. AN470AD3 to AN470AD4, or AD4 to AD5, depending on hole size and location. Match-drill the hole to clean it up.
- **Substitute an AN3 bolt** in a few spots where rivet bucking has failed repeatedly. Bolts are stronger than a marginal rivet. Use sparingly — not a wholesale rivet substitute, and only when the location accepts it.
- **Replace the part** when edge distance is lost, the hole is in primary structure, or the step-up rivet won't fit without violating minimum edge distance.
Engineering will not accept a step-up if it violates minimum edge distance. If Tech Support tells you the part needs to be replaced, do not try to work around it — the call is based on structural margin in that location.
### Oversized hole from drilling out a rivet
When a hole comes out oversized (for example, a #30 hole drilled out and now visibly larger), the standard options:
- **Step up one rivet size** if edge distance allows. AN470AD3 to AN470AD4, or AD4 to AD5, with the next-up shank diameter.
- **Add a rivet on either side** of a bad hole only if engineering signs off — this is not a default fix.
- **Replace the part** when edge distance is gone or the location is structural.
If Tech Support tells you the part needs to be replaced — for example, an AN470AD4-6 step up that would violate the called-out 3/32 rivet edge distance — do not try to work around it. The decision is based on structural margin.
### How to get a faster answer
For any tank, wing, or skin question, include:
- Builder number and airframe (RV-7, RV-10, RV-12iS, etc.)
- Drawing or KAI page reference
- Clear, in-focus photos showing the part in context (not just a close-up)
- A description of what step you're on and what you've already done
### Adding a non-exhaust vent (e.g., air conditioning exhaust)
Aftermarket A/C units like the Zero Breeze need to vent condenser heat. A 2.5" hole aft of the wing trailing edge on the bottom fuselage is generally acceptable on most RVs, provided you maintain edge distances from structure and reinforce the opening as needed. Ask Builder Support for guidance on a specific airframe before cutting.
### What does KAI mean?
KAI stands for **Kit Assembly Instructions**. KAI is the modern step-by-step builder manual format used on newer kits (RV-12iS, RV-14, RV-10 revisions, etc.). Earlier kits (Legacy RV-12, older RV-7/8/9 batches) used standalone drawings (Dwg 36, Dwg 46, etc.) without the KAI step-through format.
If a service bulletin references "KAI page 06-04" but your kit shipped before KAI existed, contact Builder Support with your builder number and we'll point you to the equivalent drawing.
### I over-trimmed my lower cowl edge
An 1/8" over-trim on the aft lower cowl is cosmetic on most installations and does not affect airworthiness. The fix is fiberglass build-up and refinishing once paint is the priority. Send a photo if you want a specific assessment - over-trimming closer to the rear bulkhead or in a sealed area can matter more.
### Cowl louvers
If you have high CHTs and have decided louvers are the right next step (after baffle seal verification and timing checks), order the louver kit and install per the kit instructions. **Do not** drill temporary holes in the lower cowl without the louvers - this can *raise* lower-cowl pressure and make CHTs worse. Louvers work by creating low-pressure flow extraction, not just open area.
### Overview
If you're partway through a build and discover an extra notch, an extra hole, or a step that doesn't match what the plans show, you're not alone. Most of these are recoverable. Here's how to think about them and when to ask for guidance.
### Position-sensitive parts (e.g., WD-1043 forward cabin brace)
If a position adjustment causes screw holes to land slightly off, work the part through its adjustment range and find the position where the most holes line up acceptably. Small mismatches at non-structural screw holes can sometimes be enlarged slightly within edge distance limits, or filled and re-drilled. Always check edge distance to the bracket flange.
For any small build error, the safest path is: stop, take photos, note the page and step, and send to Builder Tech Support before re-drilling or modifying further.
### Overview
Riveting the forward and aft top fuselage skins on an RV is where bucking access gets tight. Here's guidance on pulled rivet substitutions, oversized holes, and damaged structure.
### When to contact Builder Tech Support
For any rivet substitution, missed flange, or out-of-spec slip fit, send photos with a scale (ruler) in the frame and your builder/serial number. Do not drive final rivets or proceed past the affected step until you have an answer.
### Van's Tech Q&A: A common RV-7/7A and RV-9/9A fuselage construction question
*Copied from Van's Aircraft Tech Q&A on 2026-10-01. Source: [A common RV-7/7A and RV-9/9A fuselage construction question](https://www.vansaircraft.com/faq/a-common-rv-7-7a-and-rv-9-9a-fuselage-construction-question/).*
**Official PDF:** [7\_9\_fuselage.pdf](https://www.vansaircraft.com/wp-content/uploads/2019/02/7_9_fuselage.pdf) (1 page). If the viewer below does not load, open the PDF with this link.
**Text of the PDF**
*Copied word for word from the PDF. Figures, drawings and table layouts are in the PDF above.*
**The most common RV-7/7A and RV-9/9A fuselage construction question?**
Reference drawing #28: In all of the following text the part references are for an RV-7/7A. The parts F-772, F-776, and F-704 should be F-972, F-976, and F-904 if you are building an RV-9/9A.
Regions of interest on drawing: Skins F-772, F-776, F-704 spar carry through (A-9 and A-10), and NOTE #1 (G-6 and G-7).
The drawing indicates three different rivet situations for the two forward rows of rivets where the spar flange of F-704 and the skin F-776 are "over lapped" by the skin F-772.
The first situation involves the tricycle gear “A” model kits only. The construction plans state that the “outboard” 5 rivets on both the left and right side of the plane (most forward row in F-776) need to be installed “double flush”. Having the rivet’s shop heads on the inside of the plane flush with the interior surface of the F-704 allows the tricycle gear leg socket to fully nest into position. To achieve this, the builder will have to machine countersink the F-704 on the inside (top surface of the flange of the F-704 if the plane is upright) so that the rivet, when driven, will end up flush with that surface. On the exterior, the F-772 will be dimpled and the F-776 and F-704 flange will be machine countersunk to accept the F-772 dimple. Obviously the F-776 is too thin for a machine countersink and the result will be an “over countersinking” of that skin and a slight countersinking of the F-704 flange underneath. This is standard practice.
The second situation involves the intersection points of the four forward floor stiffeners with the F-704/F-772/F-776 flange and skins. It is denoted on the plans with arrows and the comment “SEE NOTE #1”. In this case, the key point is that the builder should not for any reason dimple the flange of the F-704, as it would complicate the fit of the floor stiffeners. In this case the same exterior process should be used as in the first situation above. Dimple the F-772, machine countersink the combination of F-776 and the flange of F-704.
The third situation involves all of the remaining rivets (the non-special case rivets). You have two options. One is to dimple all three surfaces. The builder will find the F-704 a bit thick to take a good dimple so if the dimple is found insufficient, lightly machine countersinking the initial dimple will improve the fit (nesting) of the dimples of the F-776/F-772 skins. The second method is simply to continue the process used for the four floor stiffener locations. Thus machine countersink the combination of the F-776 skin and F-704 spar flange sufficiently to accept the dimple of the F-772. This will work fine.
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