Salto
May 21, 2004, 01:26 AM
Hello all,
Even though I wasn't sure we wanted to split this forum off from the general building one, I can now see it's good to have all those links posted in the stickys here. Thanks everyone who contributed there.
As my contribution to help kick off the new forum I thought people might be interested in the following method I've used for making carbon tail booms. I described this a year ago on the Alegro forum.
I've yet to get a vacuum system up and running so I developed this method for consolidating uni carbon and carbon sock around a balsa tail boom for a 1.5m electric glider. The method could also be used for making a hollow boom over an aluminium mandrel.
Here is what I did; Tapered a piece of 16mm dia low density balsa down to
12mm dia at one end. Cut a groove along the top for two pushrod plastic outers. Installed the pushrods then filled the groove over with epoxy/microballoons, sanded smooth.
Laminated one layer of unidirectional carbon/epoxy lengthwise over the balsa using as little epoxy as I could. Slid on a piece of 25mm heatshrink tubing, shrunk this down using as little heat as possible, let cure overnight.
Removed heatshrink after lightly scoring longtitudinally with a knife (easy). Laminated one layer of carbon sock/epoxy, soaked up as much epoxy as I could by running the boom through paper towels held tightly in my fingers, this also pulled the carbon sock out nice and tight. Slid on and shrunk another piece of 25mm heatshrink, let cure.
Removed heatshrink, and admired result - 440mm long, 16mm tapered to
12mm, 20g, looks perfectly round and straight, with a nice 45 deg. cross
woven flat topped surface, no sign of excess epoxy or air voids. In fact, looks just like it was vac bagged.
For a first attempt I was happy with the 20g weight which included the two
control outers. Mark Drela indicates 15g for the Allegro E lite boom, which I
assume is without control tubes.
Next time I will do the uni and sock at the same time, which will allow me to
soak up the excess epoxy from the inner layer as well (I did not attempt to
remove any excess from the uni this time as I was afraid of disturbing the lay of the uni fibres). Also, I would seal the balsa first to prevent it soaking up too much epoxy. These details should save 2 to 3g. A hollow boom could be done over a waxed aluminium mandrel which would save the weight of the balsa (another 5g).
During heat shrinking, the whole assembly gets just too hot to touch (say, 50
to 60 deg C.). This thins out the epoxy nicely, which guarantees no dry spots, and helps all the excess run out the ends. It also speeds up the cure.
Here's some pics of the finished results, plus another smaller one I built for a micro slope soarer.
Graham.
Even though I wasn't sure we wanted to split this forum off from the general building one, I can now see it's good to have all those links posted in the stickys here. Thanks everyone who contributed there.
As my contribution to help kick off the new forum I thought people might be interested in the following method I've used for making carbon tail booms. I described this a year ago on the Alegro forum.
I've yet to get a vacuum system up and running so I developed this method for consolidating uni carbon and carbon sock around a balsa tail boom for a 1.5m electric glider. The method could also be used for making a hollow boom over an aluminium mandrel.
Here is what I did; Tapered a piece of 16mm dia low density balsa down to
12mm dia at one end. Cut a groove along the top for two pushrod plastic outers. Installed the pushrods then filled the groove over with epoxy/microballoons, sanded smooth.
Laminated one layer of unidirectional carbon/epoxy lengthwise over the balsa using as little epoxy as I could. Slid on a piece of 25mm heatshrink tubing, shrunk this down using as little heat as possible, let cure overnight.
Removed heatshrink after lightly scoring longtitudinally with a knife (easy). Laminated one layer of carbon sock/epoxy, soaked up as much epoxy as I could by running the boom through paper towels held tightly in my fingers, this also pulled the carbon sock out nice and tight. Slid on and shrunk another piece of 25mm heatshrink, let cure.
Removed heatshrink, and admired result - 440mm long, 16mm tapered to
12mm, 20g, looks perfectly round and straight, with a nice 45 deg. cross
woven flat topped surface, no sign of excess epoxy or air voids. In fact, looks just like it was vac bagged.
For a first attempt I was happy with the 20g weight which included the two
control outers. Mark Drela indicates 15g for the Allegro E lite boom, which I
assume is without control tubes.
Next time I will do the uni and sock at the same time, which will allow me to
soak up the excess epoxy from the inner layer as well (I did not attempt to
remove any excess from the uni this time as I was afraid of disturbing the lay of the uni fibres). Also, I would seal the balsa first to prevent it soaking up too much epoxy. These details should save 2 to 3g. A hollow boom could be done over a waxed aluminium mandrel which would save the weight of the balsa (another 5g).
During heat shrinking, the whole assembly gets just too hot to touch (say, 50
to 60 deg C.). This thins out the epoxy nicely, which guarantees no dry spots, and helps all the excess run out the ends. It also speeds up the cure.
Here's some pics of the finished results, plus another smaller one I built for a micro slope soarer.
Graham.