Blowing an HPV fairing
How to make blown HPV fairings

11/1997 - An Article by Warren Beauchamp

At the WISIL skunkworks (Bill Murphy's basement) we have been creating blown canopies, nose-cones, and fairings for use on HPVs successfully for over a year. These plastic bubbles are aerodynamic, durable, and inexpensive to make. One may ask (OK, more than one asked), how exactly do you make those blown fairings and nosecones? Well, here goes...

Bubbles can be created out of almost any kind of plastic that is available in sheet form. The lower the melting temperature, the easier it is to make them. The types we have typically been using are:

  • ABS (cheap, low forming temp [200F], not too strong)
  • Vivak (low forming temp[250F], strong, cheap, slightly photo degradable)
  • Lexan (High forming temp[350F], expensive, extremely strong and stable)

The WISIL Missile, and Sean Costin's "boy in the bubble" fairings were made with .030" Lexan, and were a real pain. The main reason we used Lexan was because Sean found a source of a 200' x 4' roll, whereas we can only get Vivak in 4'x8' sheets. The main causes of the pain were the high forming temperatures (long oven heat up times), and the .03" thickness (too thin). We have since determined that between .06" and .09" is the optimal range of thickness to prevent buckling and warpage while riding at high speeds. Thinner than .06" and you need ribs for support. You can go thicker, but of course it is heavier. The bubbles seen on the "Practical HPV Fairing" page are made of .080" Vivak, which is much easier to work with.

The Oven
The hardest part is creating an oven that is big enough to hold a fairing, and to keep the temperature controlled properly. In the WISIL skunkworks facility, Bill, along with a host of other members of the WISIL builders contingent, built a 8' by 4' by 4' oven out of aluminum sheeting and aluminum "L" stock, and insulated it with high temperature fiberglass insulation. The oven is heated by placing 6 (surplus) electric base board heater elements on the bottom of the oven, along with two "cal-rods" (they look like big straight oven heating elements). The whole front of the oven opens (a 4'x8' door). The door had a glass windows in it, but it cracked, so now we just have a small peep hole to look in. The oven has a light bulb inside so we can view our progress. The fairing template hangs upside down from the top of the oven, so that when it is blown, gravity helps pull the bubble down.

HPV bubble oven

Don Barry, of Infinity Recumbents, made his oven a little differently, and, as his method is easier to build, I will illustrate it. He used Celotex tuff-R (an insulating board with aluminum backing on both sides) to construct his 4'x8'x4' oven. It has good high temperature characteristics, and is rigid enough to create a stand alone oven with. Aluminum tape and long drywall screws will hold it together. He used a large propane camping stove as the heat source (propane tank outside the oven), with a metal hose to feed the propane into the oven. A large sheet of metal over the burners disbursed the heat to warm the plastic sheet evenly. The sides and bottom of the oven are constructed of Celotex insulation, and the template rests on the top of the oven, with a gravity seal.

An oven thermometer is used to measure the temperature. More than one thermometer is recommended to detect hot spots in the oven. Hot spots are the most common cause of deformed bubbles.
The Template Sandwich
Bubbles are blown from a flat template which is made from plywood (3/8" or thicker). The flat sheet of plastic is sandwiched between the template, and a thicker (1/2" or thicker) sheet of plywood. The sandwich is all fastened together with drywall screws. A felt (or other high temperate material) gasket is placed under the screws to provide a good seal between the plastic and the backing plywood. The seal is very important. Things may expand when heated, so be careful here.

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Backing plate with neoprene gasket

Bubble Template Sandwich (top view)

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Template fastened to backing plate

Making a bubble
After the oven is constructed and the template is made, the fun begins.

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Template sandwich in oven

Heat the oven slowly to the proscribed temperature (some experimentation will be needed). Keep a close eye on your plastic sheet. When it starts to buckle you are getting close. When it starts to sag slightly, you are ready to blow. A standard 10 gallon compressor with a hand operated valve is all that is needed to provide pressure. A wire with gradations marked on it, or markings on the back of the oven may be used to measure how deep the bubble is. Blow the bubble carefully, watching for any potential blow outs.
If you make the bubble deeper than the template is wide, it will balloon sideways, over the sides of the template, and will be hard to remove from the template, as well as less aero. If hot spots are caught early, unhook the air hose. The bubble will deflate partially. The hot spot can be fixed, and the bubble can be re-blown (a few times).

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Newly formed bubble in oven

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Newly formed bubble

Be careful not to get the bubble too close to your heat source, as a runaway bubble can happen quickly. Once the bubble has reached a good shape (Don't try for perfect the first time), turn off the heat, and open the oven door, releasing the heat, while using small bursts of air to maintain the bubble's shape.

Once the bubble has cooled, you can remove it from the template, and trim the "tabs".

Needless to say, none of this would have been possible without Bill Murphy.

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Mounting the Bubbles
The bubbles need to be mounted to some type of light framework, as they are not rigid enough to be load bearing. Aluminum strap makes a good lightweight framework material. The bubbles may be mounted with self sticking Velcro, or pop rivets. (I like pop rivets). Check out the Bubble mounting page, and the Practical fairing pages for more information.

production.jpg (13369 bytes) Conclusions
While the initial oven and template construction may seem daunting, and the initial few blows will be frustrating, after a little experience you will be having bubbles popping out of your oven like gumballs from a candy machine.The costs and time of making fairings this way is significantly less than traditional methods, which consist of creating a "plug" (a "positive" mold of the fairing), creating the female mold from that, and the making left and right hand fairing halves from the female molds. It's not as stinky either. The shapes made this way are naturally aerodynamic, utterly smooth, and clear. The Vivak bubbles do scratch easily, and they can be painted. Painting the inside of the bubble makes the outside look very shiny. A completely clear fairing, or large clear canopy gets VERY hot in the summer sun. A veritable solar cooker.
Sean Costin blew a somewhat bigger bubble for his Monkeyhand laid back racer. It looked cool but there were difficulties with getting in and out, rigidity (it was a bit floppy), and the "solar cooker" effect.

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