How to assemble a truss structure rear fuselage on an Ultralight Aircraft

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Assembly of a rear truss structure aluminum fuselage for a

Belite Ultralight Aircraft

It’s easy to assembly a rear fuselage for a Belite ultralight airplane!

1.  PARTS INVENTORY CHECK

Your kit should have the following items in it:

A)    Pre-riveted frames, constructed from 2024T3 aluminum, 7/8” x ½” x .063”.

B)     Longerons, with tabs already attached.  Also constructed from 2024T3 aluminum.

C)     Rear post, with pre-welded rudder hinge point. 

D)    Gussets.  Gussets are made from either .032 or .040 aluminum, 2024T3 or 6061T6.

E)     Truss pieces, constructed from 2024T3 aluminum.

F)      Rivets – commercial grade.  If you are interested in using aircraft grade rivets, we encourage you to purchase them directly from Aircraft Spruce, or the supplier of your choice.  We do not supply them.

G)    Top skin aluminum, pre-routed. 

Here is the pictures of the parts:

Section Frames, Six of them, A through F.

The frames are ‘A’ through ‘F’ as shown on the blueprings.  Each is riveted together.  All are easy to identify by comparison to the blueprints, with one exception:  One of these frames has a dimension of 19 by 19 1/8”.  Don’t confuse the vertical dimension with the horizontal dimension on this one frame; look carefully at your blueprints.

Longerons with gussets.

 The longerons come pre-riveted with gussets.

Truss sections.

 The truss sections are all labeled as shown.  You may also notice color coding, which we do to help you sort out where the truss and frames rivet together.

Rear post.

The rear post is pre-welded for rudder attachment.  DO NOT RIVET to the top of this post as it has to receive the vertical stabilizer, rivets would obstruct inserting the vertical stabilizer post.

Gussets.

 The gussets are obvious in use, especially after reviewing all photos.

*PLEASE NOTE that we are now using small triangular gussets INSTEAD of the small wrap-around gusset shown on the bottom right of this photo.*

Top skin, predrilled.

The top skin is also drilled and ready to rivet in place.

2.  WORKBENCH AND PROJECT PREPARATION.

We recommend that you prepare your absolutely flat workbench with a centerline (denoting the center of the fuselage and positions for each of the frames.  You can see these lines throughout our photos.

The fuselage is built upside down.  The top longerons are placed on the workbench; the frames are riveted in place; everything is placed absolutely dead square to the workbench (90 degrees to the table top.)  Use squares, clamps, and whatever else you need to keep it all completely square.  Measure twice and drill once.

All holes should be de-burred.  It is our experience that the 2024 aluminum alloy makes very clean holes, with very little chipping or burrs on the edges of the holes. 

We supply commercial grade rivets with the fuselage.  There is no place on the fuselage that requires aircraft grade rivets.  However, if you encounter low quality commercial grade rivets, don’t hesitate to drill them out and replace as appropriate.  (As an example, you can tell a bad rivet by a non-uniform pull force.)

The entire fuselage may be reassembled, prior to riveting, using Clecos, which we do not supply.  Having the entire fuselage in its final form may provide you with a great deal of certainty that you are doing it correctly, but this choice is up to you.   If you choose to use Clecos, your fuselage project may look like this:

Clecos in use

And here is another pictures of clecos:

More clecos in use

 Let’s get to work on putting this fuselage together.

 

3.  FUSELAGE ASSEMBLY.

Lay the top longerons and small rear gusset plate on the work bench.  (Remember, this is upside down.)

Top longerons.

Now rivet the rear plate in place.  (We recommend riveting this plate in place, even if you are using Clecos.)

Rear plate and top longerons

 Place the “A” frame in place as shown in the photo, below.  Note that a cross brace has also been placed on this A frame; make sure that the A frame is absolutely square.   The cross brace supports load from the fuel tank, which eventually rests on the cross member of the A frame.  Use any aluminum for the cross brace.

'A' frame in place, along with cross brace

Rivets at base of "A" frame

 Rivet the A frame in place.  In the photo above, you can see a typical rivet pattern in the A frame.  Our gussets are now cut on our CNC shopbot and are much ‘prettier’ than shown above.

"A" frame centered on workbench and riveted in place

Note how the center of the A frame has been centered onto the workbench centerline.  Both sides are riveted in place.

Weight placed on top of "A" frame

A weight has been placed on the A frame to keep it from moving. 

Now, place all frames in place and rivet.  Note that all frames are exactly where they need to be, and we use weights to keep them in place.  The longerons are matched exactly with the beginning of our table (barely visible in lower left corner):

A through F frames in place

Rivet holds frame to longeron

Each frame is held by a single rivet at this time.  A second rivet will be drilled and placed, later.  The gap in the frame is expected, as shown in the above photo.  And although you can’t see it in the above photo, the rear of each frame has been beveled so that it will clear the interior of the longerons.

Additional longerons are then placed along the opposite side of each frame. 

Longerons on other side of fuselage

Rivets in gussets

The rivets are placed into the new longerons and frames as well, as shown above.  Everything needs to be square and line up, as shown in the photo below.

Fuselage view down center top

Fuselage view down inside

Rear gusset

The rear gusset is used to hold the longerons in place.  Note that rivets have been used to keep the longerons together, but the rivet pattern has not yet been filled out.

NOTE:  Taildraggers use FOUR of these plates (quadrupled thickness); tricycle gear aircraft use TWO of these plates (doubled thickness).

Rear post

 The rear post is also held in place temporarily with rivets.

Another view of rear post

Small Wrap gusset

NOTE:  the gusset in the above photo has now been replaced with two triangular gussets, one on each side.  It’s much easier to fabricate. 

Large wrap gusset

The other end of the rear post will accept the vertical stabilizer.  It is notched into the longerons, then held in place with a doubled wrap around gusset.

Figure 27  Rivets on wrap around gusset

Now it’s time to start adding truss sections, starting with the rear gusset, then moving forward to other frames:

Rear post with truss added

Another truss added

All of the truss sections are added and riveted in place:

All trusses in place

And one reinforcement is added, parallel to the rear post.  It is in the photo below, immediately flush to the rear post.  Because the rear post may not be riveted, this parallel structure provides compressive strength to the rear assembly.  

Rear post with parallel reinforcement.

Typical gusset; two rivets per frame

Each frame and gusset is riveted out.  Note that each frame now has two rivets connecting it to the longerons.

Big gusset on rear post

You will also need to flip the fuselage over and add the top skin. It is possible to place without clecos, but go very slowly, line it up very carefully, and make sure every frame and centerline is square.

Top skin in place.

After riveting, it’s Done!  And ready to attach to the fuselage cabin.

The rear fuselage must be covered with fabric (after assembly to cabin) in order to have adequate strength.  Make sure you review final assembly requirements prior to covering.