MD-7 Large Scale Trainer

[Keyrigger]

A little over a year ago, I was given an MD-7 700 size helicopter kit (built by the first owner) to use for as long as I wanted. The fellow that gave it to me had two already that he wasn't flying too much and didn't need another one hanging on the wall not finished. I picked it up on my way back from Florida from a fellow in Daytona Beach and that, along with a few spares, was the start of an unorthodox project. It took the better part of a year to bring all the electronics and some noted missing or damaged parts, together to finish it. Almost a year later, I test flew it at Crosswinds and the results were nothing short of amazing. I drove to Florida for our end of year vacation and took it, along with my Heli-Babies, to fly at the Fall Classic F3C and Fun Fly in Boyette, Fla, just outside of the Greater Tampa Area (another GTA, lol). A mechanical breakdown causes the helicopter to slide sideways into the ground after attempting a medium altitude loop so a bunch of parts were ordered and I rebuilt it there. I test flew it and then proceeded to fly the pants off it for the remainder of the time I could spend at the club.

Fast forward to our meeting this month where I debuted it for those that were there and brought it to the field for an afternoon of flying. Near the end of the day, my fingers were getting a bit cold and I inadvertently flipped the throttle hold switch while low and in a high banked turn. Once again, into the weeds it went and this time did a whole lot more chassis damage. The rest seemed to have survived with the odd part showing its contact with the ground or other spots on the mechanics. I did find this time, parts that had been abused by the builder and seeing some other hidden flaws, came to the conclusion that it had been crashed before it was purchased. The main shaft of this beast is the largest I have ever seen on anything smaller than a 1/3rd scale scratch built machine and even those are for the most part, smaller. This one is 15mm diameter with a 10mm spindle, so it is fairly resistant to bends. In the two crashes, I replaced the spindle only once and the main shaft was untouched by both incidents. I test flew it today at Burlington and it was the best it has been so far. Naturally, if I had paid attention to the way the tail was setup, I would not have had to rework the servo direction and torque compensation. Oh well, I was able to sort it and now it will be "Normal" appearing to me in the future, lol. Take care.

Don

PS: The last two show the new quick release battery tray and the canopy I bought to replace the damaged green one.

[Keyrigger]

Well, it's been a while since I posted any new info on this helicopter, and today is as good as any for one. I flew this helicopter for the 2017 season and remainder of the 2016 season going through a few changes in gearing, boom length, and blade type. I ended up with it setup as an 800 size helicopter, running a 12t pinion to lower the head speed down to 825rpm at the bottom and 1070rpm at the top. The entire reason for this low head speed was for me to practice with a helicopter that I would need to be ahead of for every move it made. The flying of large scale helicopters requires that you be ahead of the helicopter so that you don't get yourself scrambling if you suddenly start flying through dirty air as you come in for a landing. The plan to join up with another pilot or owner to fly his (or her) machine in Team competition never materialized as the standard method is to buy a helicopter that someone else built and then fly it yourself, negating the need to find another pilot to fly your machine that you trust. Team is no longer a team event.

That didn't leave me with much that I could do with this helicopter the way it sat. The motor and gearing was specifically setup for low rpm and there is not much you can do other than putter around the sky doing the odd loop at full rpm and the occasional roll. I went down to the Fall Classis competition and fun fly this past October with the intention to try out F3C type flying. I knew that the gearing I had at the time, would only get me 1200rpm at full throttle but there was no chance to get anything else setup prior to heading south. When I arrived, the contest director offered me a much faster motor and I jumped on that as it would give me a good increase to 1850rpm governed at 92.5% throttle. I let him set the main adjustments to the helicopter so that it would rise straight up when collective was applied once the helicopter was up on one skid to compensate for the tail rotor thrust. I then headed up into the sky for some basic flight trimming and that is where I ran into trouble.

The tail rotor had a triple set of 120mm blades that had a lot of thrust at low rpm to counteract torque of the main rotor. Now, with the rpm nearly twice what it was normally, the amount of movement that the tail needed to counter torque was very little indeed. While in fast forward flight, the tail would violently wag from side to side. I started to turn down the amount of tail gain needed and this was successful but it took a lot of time. I had been used to the very long flights that the low rpm setup had given (over 20 minutes) and I failed to realise that the new motor was a very power hungry unit so as I finished the tail setup, I did a couple of passes at high altitude just to make sure, but the sound of a pulsing rotor told me that I had passed the safe flight time by a few minutes and now I had a few seconds to land. I started an autorotation about 200-150ft. up but a mistake, when I turned into the wind, of dropping the nose too much allowed the rotor to slow down to the point that the helicopter was now in a free-fall. At the last second, I managed to level out the helicopter and it pancaked into the ground, ripping the entire battery tray and chassis area around it out of the helicopter. The boom remained surprisingly straight and all of the shafts were still straight. Lost were the main blades, lower chassis, landing gear, and canopy. Time to rebuild.

While at that competition, I picked up another high voltage tail servo to match the ones that I had installed when I completed the first build. This allowed me to set the BEC to 8v from the current 7.4v setting. I could now remove the inline voltage regulator from the tail servo and clean up the wiring coming into the gyro. I still had one in the system for the cooling fan of the ESC as it was only rated for 6v. The inline regulator would not correct it all the way down but did enough to reduce the voltage so that the fan was not screaming. The tail blades have been set back to the length they were when I first put this helicopter together and although they didn't have the thrust to work with low rpms, they now may be more than needed for the new high rpm levels that this helicopter can now achieve. Now, if only the temperatures would co-operate or I could get some time in the studio with it for testing, sigh. Take care.

Don