Hellenicopter

A few days ago I have published a pdf regarding the basic modifications I have made to the mechanics of the Trex 700E in order to install it in the Vario Jet Ranger fuselage. Here I elaborate a bit some steps that presented some difficulty.

The Tail Boom

In general, the Trex mechanics fit surprisingly well into the Vario fuselage. The main difficulty is the tail boom and the hub. Initially, I have tried to use the upgrade available by Align to convert a Trex 700E to an 800. Unfortunately, this doesn't work, the boom and the shaft are 3-4cm too short. I had to buy in addition a Trex-800 tail shaft. It is difficult to find aluminum tubes in the raw-material market with the right cross-section dimensions so you have to put together the tail boom from the upgrade with an extension that is cut off from a second tube (available from the original Trex-700E kit). I have tried to use a CF tube but it was not easy because the cross-section was not wide enough. I think that Align has chosen on purpose to use tubes with those non-standard cross-section dimensions in order to be forced to buy their tubes...

In my case, I have decided to merge the two boom parts towards the frame side and not the hub side. This is done because at the frame side it is much easier to put them together in a reliable way. A 20mm aluminum tube is used to merge the two boom parts from the inside without creating any problems to the shaft. The small part has been glued with epoxy glue to this tube (with Devcon 30min, still the best out there, much better than the 30min Zap epoxy) and the long full-length boom has been screwed on it with two short M4 screws (see picture in pdf). The short boom piece resides safely in the plastic gear box at the back of the Trex's CF frame. The long boom part is partly inside the same gear box.

The boom parts have to be assembled and secured before installing the mechanics at their final position.

Securing the Lynx rods to the boom before final installation of the mechanics in the fuselage.

Tightening the screws at the rear of the plastic gear box.

In brief, this solution works fine but I would prefer to have a full length tube instead. Unfortunately, although the Trex-800 boom has the right length, it is too large (25mm cross-section) to go through the fuselage's tail and doesn't do the job (I would have to change the plastic gear box and the tail hub too).  In addition, the Lynx Heli Innovations tail boom mounting rods (cut to the right length) have helped a lot to add stability. Those rods come already glued and screwed to their aluminum anchors and it is difficult to unglue them in order to adjust their length (you need a hot-air pistol at around 200 degrees Celsius and a lot of patience!).

Gluing back together the aluminum anchors to the tail boom mounting rods.

 

Connecting the boom to the fuselage

The connection of the GF (glass fiber) tail boom to the fuselage has been modified in order to be able to pass through it the Trex's tail tube. Two multi-layer 5mm-thick plywood plates have been glued on each side (fuselage sied and tail side). On the tail side two M5 drive-in nuts have been used for screwing the two parts tightly together (see pictures below).

The Tail Hub

The second "headache" has been the mounting of the tail hub. The tail of the fuselage already provides a quite big opening and it is almost perfect for the installation of the Trex-700E hub. Well, almost... I had to cut out a small opening for the hub's tightening screws as you can see in the picture below. This is a necessary "evil" as there is no other way to fit the hub. In the final assembly the metallic part of the hub barely overhangs out of the fuselage. In the real helicopter this part of the tail is covered by a canopy from above that is held with numerous screws on the body of the tail (see pictures below). So there is still place for hiding this with such a canopy made of glass-fiber or carbon fiber and held in place with screws as well (see photo below of a CF cover I made).

Picture showing the opening in the GF part of the tail to accommodate the tail hub.

Tail cover made of CF and epoxy. The lighter the better in order to avoid adding too much weight on the tail.

Finally, in order to better integrate the hub in the fuselage an aluminum plate has been made (1.5mm-thick) in order to screw the hub on the fuselage (see following pictures).

In order to avoid bad surprises and to reinforce the fuselage around the hub I laminated the internal surface of the fuselage with one 3K carbon-fiber sheet. In this way there is much less risk for the screws holding the plate and the hub to crack the fuselage and finally break it due to vibrations.