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- Rib Forming
An important consideration of the fuselage construction is the number of rivets to be installed at each joint of the aluminum angle. Many of the Texas Parasols have been assembled with two 1/8th inch rivets per joint - in the Builder's Manual, Richard Lamb suggests an alternative using a single 5/32nd inch rivet. The thinking here is that two rivets at the end of a piece of angle makes one rivet a pivot and the other a piece of aluminum that could be sheared off. Such is not the case with a single rivet.
The holes in my fuselage were drilled for a single rivet, so the decision was already made for me. This is the way I would have proceeded in any event as the larger single rivet ensures that there is adequate edge distance maintained at the join.
Here's an illustration using one of the fuselage joints in the Parasol - see Fig. 1. The material is 3/4 x 1/8th inch 6061-T6 aluminum angle and the hole size shown in the photo is #40 to fit silver clecoes. If we examine the overlap area of the two members we can see that we can center the rivet hole across the width of the angle face on the inside piece, but not so on the outside. If we did, the center of the rivet hole would end up being 1/4 inch from the end of the inside piece.
According to AC 43.13-1B the minimum acceptable edge distance is two times the diameter of the rivet. For the larger rivet this minimum distance would be 2 x 5/32 = 5/16 inch, which is 1/16 inch more than we have available.
Instead we need to visualize the overlapped area of the two members and center the rivet hole in that area. Fig. 2 shows this area as a rectangle measuring 5/8 x 3/4 inch. The center of this area would be 5/16 inch (.3125") from the end of the inside piece, meeting the requirement.
Now if we consider the same joint using two 1/8 inch rivets, we have an interesting situation. AC 43.13-1B states that rivet spacing should not be less than 3 times the diameter of the rivet for single rows. This is 3 x 1/8 = 3/8 inch between centers. The best way to meet this requirement while keeping maximum edge distance would be to position the rivets along one of the diagonals of the overlapped area. This is shown in Fig. 3.
If we satisfy the spacing requirement we find that the available edge distance is less than the required 1/4 inch for 1/8 inch rivets. If we meet the edge distance requirement the rivet spacing would be less than required. Either way, this joint does not meet the requirements of AC 43.13-1B.
There is also a practical concern if we consider how difficult it will be to buck the rivet closest to the perpendicular face of the angle - there's not much room, and likely not enough space to form an acceptable shop head.
All this being said, there are many examples of the aircraft flying with two rivets per joint, and they aren't exactly falling out of the sky. The fuselage, as detailed in the Texas Parasol drawings, is very strong - probably stronger than necessary. It's up to the builder to decide how they will proceed based upon the information that is available to them.
So here's more information. When the designer specifies the materials from which an aircraft is made they take into consideration many factors. Wherever the parts of the airframe are riveted together these connections may fail by tearing between the rivet holes (tension), by crushing of either the rivet or the material joined (compression), or by cutting through the rivet (shear). Tearing between rivet holes is a failure of the material joined, not the rivet. Shearing of a rivet is a failure of the rivet itself. Crushing can be a failure of either the material joined or the rivet itself. The strength of a structure is determined by the weakest component so the designer must find a solution that results in sufficient overall strength.
For our fuselage frame, the aluminum angle is definitely not the weakest component - the rivets are. If we were to try to pull apart two pieces of aluminum as shown in Fig. 4 the rivet would tend to be cut or "sheared" where the pieces join together.
The Texas Parasol design calls for two 1/8 inch rivets at the joins of the fuselage frame members, which gives the connections a particular shear capacity that is easy to calculate. The most widely used universal head rivets used in aircraft construction are MS20470AD type, which have a shear strength of 26,000 PSI.
It's important to note that we use the diameter of the rivet hole, not the rivet diameter, in our calculation. This is because the rivet will expand to fill the hole space when it is driven. We use a #30 drill to make a 0.1285 inch hole for a 1/8 inch rivet.
Using two 1/8 inch rivets the calculation is Ps = 2 (3.14159265 * 0.12852/4) 26,000) = 674 lb.
We can now calculate the shear strength of the single 5/32 inch rivet. We use a #21 drill to make a 0.159 inch hole for a 5/32 rivet: Ps = 1 (3.14159265 * 0.1592/4) 26,000) = 516 lb.
So, with the single rivet we respect the edge distance requirements but have 158 lb. less shear capacity. Fortunately we can easily fix this by adding gusset plates at the connections. These can be very small and the added weight is practically nothing. For example, at Station 303/4 along the top longeron there is a single perpendicular joint (shown in Fig. 1 above).
A very small gusset can be designed to fit between the angles in the same plane as the side of the fuselage. The rivets for the vertical member are 5/32 inch and the two outside rivets for the horizontal member are 1/8 inch. The gusset itself measures 25/8 inch wide by 15/8 inch high, and can be made from 0.032 inch or thicker aluminum sheet.
In order to determine the added strength we must first determine the strength of the
gusset itself. The tensile strength of 6061-T6 aluminum alloy is 47,000 PSI.
By examination we can determine the weakest part of the piece to be the cross section
taken at the top rivet. The net area of the cross section can be calculated by
multiplying the width by the material thickness and subtracting the diameter of the hole.
The gusset measures 7/8 inch across at the top rivet hole, so
For our example the strength would be Pt = 47,000 * 0.0229 = 1,076 lb. This is more than double the shear strength of the 5/32 inch rivet, so the gusset is quite strong enough.
What then is the net strength of the connection when we use the gusset? We already know the strength of the single 5/32 inch rivet at the join of the two angle pieces is 516 lb. The two 1/8 inch rivets either side of the vertical member together have a capacity of 674 lb (same as calculated above), which is more than the shear capacity of a 5/32 inch rivet - the top rivet would then have to be the limiting factor.
The shear strength of the top rivet determines the strength contribution of the gusset,
which we might assume to be another 516 lb. We could assume this - but we would be
wrong. There is an additional correction factor that needs to be applied for 5/32
inch rivets when used with sheet thicknesses below 0.063 inch. In our case that
factor is 0.925, so the answer would be
It's amazing what such a small piece of sheet metal can do.
Gusset plates are easy to design - just respect edge and spacing distances for the rivets. Also, don't use sheet aluminum thinner than 0.032 for 5/32 inch rivets - thinner stock acts like a knife edge and causes the joint to fail at a stress below the calculated capacity.
These supplemental gussets don't need to be larger because there's no need to add torsional strength to the truss structure. The diagonal and intercostal members are quite sufficient to keep the structure from twisting. According to Richard Lamb the gussets shown in the plans were a later addition that was done to keep the various members intact in the event of a crash. The structure itself is very strong for the application.
When you make up the gussets don't pre-drill the rivet hole that is common to the longeron and the vertical or diagonal member. This hole should be match drilled to the existing holes once the gusset is correctly clamped in position. The remaining gusset holes are then used to match drill the aluminum angle members.
This is the methodology I'm using for the construction of my fuselage. It allows me to respect the edge distance requirements while ensuring that the riveted connections are strong.