Takeoff dolly

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With the help of the club guru I managed to fly this thing a few times on Saturday 3 October 2009. However, a big problem is the takeoff. There is nowhere to hold this thing for a launch throw unless you're willing to try that weird Zagi method. We ended up with me holding it on my finger tips like a waiters tray and running. After a few steps to get some speed, power is applied and it goes right away, narrowly missing my fingers. If you don't get it right it plows into the ground and breaks a prop! (despite the propsaver which surprised me, but maybe I had too many elastic bands on it)

A brief discussion brought mention of bungee and wheeled dolly launch mechanisms. Bungee is kinda hard to find, so I considered the dolly idea more seriously and ended up with this contraption. Using the correx signs and packing polystyrene to hand I have made a 3 wheeled dolly. If this works well I'll keep it, otherwise I have some ideas for changes and alternate materials. And there is always the option of hi-starting it with a fishing rod.

First, spend Sunday afternoon making a foam cutter. I did this as quickly as possible so it is a bit rough. The wire is heating coil from a dead hairdryer that has been in the garage for about 8 years for just this purpose. The transformer is one that arrived recently to supply 110 volts for a soldering iron I bought labelled 220v but is actually 110v and glows red on 220. It so happens that the secondary winding is just right to overpower the cutting wire. I then borrowed a variac (adjustabel output voltage transformer) from Dad so I could adjust the voltage going into the transformer, thus adjusting the heat. This works very well indeed.

The bit of plywood with a skewer in it at bottom left is the wheel cutting jig. Place the polystyrene on the spike, slide the plywood along the ruler clamped to the bed (photo left) until cut depth is reached while rotating the foam smoothly, retract smoothly at end of cut. If you hesitate you get a lumpy cut so practise the motions to get it slick.

So, a bit of experimentation produced a jig with which to cut wheels and here they are. I then drilled the center holes so I could fit drinking straws that fit on some sosati sticks I bought. These straws form 'bearings' that turn very easily on the sticks, and protect the foam from deformation.

And the final result is this dolly that the wing can rest on. The vertical strips provide stiffness without too much weight. It is fairly high off the ground to provide plenty of clearance for the prop. Hopefully the prop won't chop it to pieces on takeoff. The large wheels roll very easily. It started rolling on the kitchen floor with minimum throttle, so it should manage the grass strip with a bit more throttle.

Design issues revealed by attempted takeoffs

Sadly, 3 takeoff attempts resulted in prop strikes every time, with the wing making it into the air but without the prop, which was saved nicely by the elastic bands breaking. I tried cutting some clearance at the field, but that was not nearly enough. It is being modified to:

  • Hold the wing much higher above the platform, so the prop has more than enough clearance.
  • The front supports will be moved outward so the prop can go straight through the middle.

Hopefully this will solve takeoff issues. If it doesn't I'll have to fit tricycle undercarriage to the wing itself.

Modified dolly

The wing supports were detached and reattached both wide apart (the front ones), and further back so that as the wing moves forward the prop cannot hit the decking at the rear end. I have just thought that I must attach a piece between the front and back wing supports so there is no way for the rear edge to dip down between them.

Now there is some latitude for the plane to move sideways on takeoff, and thus hit the side supports. If it proves to do that (it did) on every takeoff attempt, I will fit tricycle undercarriage to the wing itself. The other alternative it to make an even wider dolly, and I don't like that as it won't fit in the car!

RESULT

Ultimately the dolly thing did not work out. After the mods above the wing stayed secure until it lifted off. At that point the dolly slowed down quickly and the prop shredded the rear wing supports on the way past. Therefore, I have fitted undercarriage which is working very nicely.



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Last modified: February 18 2013 16:01:50.