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T-Rex 700 Scale Hughes MD500 Build 28Jul12

Posted by tanker in Scale Helis, TRex.
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MD500

MD500 fuselage with T-Rex700E mechanics

With the SAB Goblin 700 now becoming our performance heli combined with the fun we’ve had with the Align Cobra we decided to convert our Align T-Rex 700 into a scale machine as well. Unlike the Cobra we wanted to get a bit of performance out of it and not fly it strictly scale. We chose the Hughes MD500 fuselage from RC Aerodyne. The three things we like most about this fuselage is its looks — it is certainly easy to see! — its size and its ease of installation.

Build/Installation

The fuselage shipped quickly and without incident. Installing our existing T-Rex 700 into the fuselage turned out to be a bit more difficult than we anticipated but in general it was quite easy especially compared to how other scale builds could be. Unlike the Cobra it required absolutely no modifications to the stock T-Rex 700 other than removing the tail boom supports, fins, and landing gear. Before starting the install we updated the stock Align 3G flybarless system to the latest V3.0 firmware and test flew it. We also prepped the Hughes fuselage by painting the inside gray just because we thought it looked better than the white fiberglass. We also beefed up the fuselage with extra epoxy on the plywood and fiberglass joins and also reinforced the main plywood former with carbon fiber stock. We then attached the gear to the fuselage per the instructions.

MD500 nose off

The nose comes off for easy access to the mechanics. Test fit, test fit, test fit before gluing the tail. Stock T-Rex 700E mechanics with only the gear and tail boom supports and fins removed.

Installing the mechanics into the fuselage is fairly straightforward. Basically you remove the tail case from the 700 and then simply insert it into the fuselage. The tricky part is that the fuselage is actually designed for the T-Rex700N. The E is shorter so we had to add about a 1/2 inch of additional wood risers on top of the full set of included risers to get the proper height. We went through multiple test fittings and had to cut the top cross former to clear the ESC. We epoxied in a new, stronger, former. More test fitting showed the rear ball link on the swash hit the fuselage so we had to grind out a small gap for clearance. We also realized that the fuselage was slightly tweaked and it was not possible to get the fuselage perfectly square with the mechanics. We concentrated on getting the head centered and the tail centered and then did the best we could with the rest of the alignment. We were also careful to ensure the tail case had enough clearance. It turns out the mechanics are not straight in the fuselage (thankfully it turns out it doesn’t matter) and are off centerline in the front by about 1/4 of an inch. After all of this prep work we finally screwed the mechanics down to the plywood and added some extra screws for good measure.

All that was remaining was to glue on the tail boom cone. Further test fitting showed that the included slot in the tail tube was not in the correct spot so we expanded the slot (lengthening it about two inches) and taped off the unneeded gap. We then glued the tail cone being careful to keep it square. Finally we reattached the tail case and screwed on the vertical stab keeping it vertical.

MD500 compared to 700

The new MD500 compared to the original T-Rex 700E canopy. As you can see it is a bit larger!

The fuselage ships with plastic windows that you need to cut out and glue inside the doors. We decided to keep them off for the test flights. The doors are hinged and the doors latch closed. Both seem a bit fragile so we taped them shut with Blendurm tape. The windowless doors are plenty big enough to allow access for battery arming without the need to open the doors at all. Finally we also added two additional metal clips to reinforce the two screws used to attach the nose.

Flying

For our first test flight we were careful to check center of gravity. The batteries needed to be a couple inches further forward than the stock T-Rex 700 but this was not an issue. We then screwed the nose on and clipped it. Spool up was uneventful and the heli lifted off without issue. It was immediately apparent that the heli was going to fly without any problems. It of course feels a bit heavier in the air but surprisingly not dramatically so. After one flight of just hovering around we landed and checked everything. All seemed well so  we prepared for another flight. The instructions warn of doing aerobatics with the fuselage and especially of boom strikes. Without the tail boom supports there could be a bit more tail flex but the fiberglass tail tube is fairly rigid. Well there was no better way to find out than to just try it! We did some basic forward flight and noticed that the size of the heli gets it blown around a tad in high winds but the 3G seemed to handle this just fine. Forward flight characteristics are normal with no odd pitching or other issues. Tracking seemed fine so the slight alignment issue with the mechanics and the fuselage do not seem to make any difference in flight. After getting comfortable we did a wide loop without a problem then did a series of rolls and forward flips (we figured back flips might be pushing our luck). Inverted flight was no problem but it sure does look weird. We then landed.

Careful post flight inspection showed no signs of cracking or stress on the fuselage. We now have six aerobatic flights on the fuselage without any issues or signs of stress. We have decided to keep the windows off to improve cooling and allow easy access to the batteries. In the air you can’t tell the difference. We are getting about 6:30 of aerobatic flight time. You certainly cannot 3D the heli but basic aerobatics and aggressive flight seem to be no problem. The key is probably to simply keep all maneuvers smooth. The heli has a great presence on the ground and in the air and the fuselage is very easy to see in the sky. All in all we could not be happier with it.

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