August 23, 2013

Scorpion Mk2 assembly day 7: engine and fuel system.

The engine install described in this section is with our Merlin VT-80.

This engine is simply fantastic. It is extremely reliable and features the best thrust to weight to price ratio on the market. Additionally this is one of the most integrated engine. The only accessory that is left outside of the engine is the fuel pump. ECU and valves are located under the inlet cowl. This makes a very clean install and clears up most of the radio tray surface as you will see later. Also the engine setup is extremely simple and is a matter of seconds.

The first step is to install the Grumania pipe provided with the kit. the long aluminium tab is designed to secure the bellhousing. However I recommend to add two miniature screws placed at 120 degrees at the bottom of the latter for additional securing. The pipe itself will be secured from the side of the bellhousing thanks to two sheet metal screws going into the plywood engine rail. I actually added two hardwood blocks to the rails at this location to make the drilling easier.

On this picture one can see the hard wood block as well as the sheet metal screw.

Note that it is important to install the engine first before securing the pipe to adjust its position properly. However I found out that the Grumania pipe is very tolerant to the engine placing. In my case the VT-80 nozzle is flush with the bellhousing entry. I have tried positions up to 35 mm inside the bellousing with no notable difference in thrust or engine/ pipe cooling.

The VT-80 being quite narrow, it requires extension tabs to mount on the rails. I used pieces of 3 mm carbon fiber laminates cut to the appropriate shape. 6 socket head sheet metal screws are used to fasten the tabs to the rails.

The engine is placed as far to the rear as possible to facilitate CG placement. With this setup there is actually no balancing lead in the nose and the LiPo batteries are placed as far aft as possible in the nose bay. The FOD screen used is the one provided by Jets-Munt.

The VT-80 pump is place on the right engine rail, close to the engine. Both fuel pump feeder line and exit lines are short.

The engine loom is protected by our MIL spec wrap-around cable sleeve and will not suffer from an overheat condition. The loom consists of the power leads, a radio input cable and a terminal output cable. The terminal plug is located on the radio tray. The VT-80 terminal is very small and could be left inside the plane. However the engine is so dependable that I don't use it anymore and permanently removed it from the installation.

The fuel tank shown below is the 3.2 liters version. This gives a fuel endurance of 20 minutes which is a bit overkill. I have installed a 2.2 liters tank since then and saved a substantial amount of weight at takeoff weight, making the Scorpion Mk2 performance with this engine very convincing.

The tank is secured a the front thanks to a couple of rubber bands. The rear is locked with a foam block velcroed on the main bulkhead.

I used the excellent GRB Jet medium size CAT on this setup. The air trap is located at the rear of the tank above it. It is secured with some velcro and a custom cut foam block.

The servo wire that you see passing above it is the speedbrake cable. It is long enough so that I put the engine hatch on the wing without having to disconnect the servo plug.

Here is a view of the rear block.