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Centuri Hummingbird

This web site contains a brief history of the Centuri Hummingbird, a model rocket boost glider, and directions and patterns to build a replica.

The Centuri Hummingbird boost glider was produced by Centuri Engineering Company of Phoenix, Arizona in 1973 and 1974.

During the launch phase, the weight of the motor causes the center of gravity of the rocket to be located ahead of the center of pressure, ensuring stable rocket flight per textbook. At apogee, after delay charge burnout, the ejection charge fires, pressurizing the front section of the rocket, which ejects the spent motor casing. The center of gravity of the rocket shifts to the mid-point of the length of the vehicle, just behind the wings, and the vehicle becomes a glider.

The Hummingbird doesn't have an airfoil, but instead relies on flat plate lift where the forward motion of the vehicle creates air pressure on the underside of the wings, generating lift.

Parts list:

Estes BT-20 body tube - 3 inch length

1/16" square basswood - two 3.5 inch lengths

balsa block - 1 inch by 2.5 inches for nosecone (or Estes NC-20), see construction text

Estes launch lugs - two 3/16" ID lugs, two inches in length

poster board - 1 sheet, white (1/64" thickness standard)

Krylon clear gloss paint #1301

Krylon gray primer #1318 *

Krylon forever blue #SCS-073 *

Sig Supercoat clear butyrate dope *

Top Flite Monokote white airplane covering

Heat-activated adhesive (Balsarite COVR2520 or Sig Stix-it)

Medium viscosity cyanoacrylate glue (CA) - Sig Manufacturing

CA accelerator (Sig Kwik Shot)

Canopy glue (Pacer 560 or equivalent)

* Used in making a balsa nosecone rather than the alternate Estes NC-20 nosecone

Equipment:

Covering iron (Tower Hobbies TOWR3250 or equivalent)

Suitable color inkjet printer with plain paper stock

Scissors

X-acto knife

Suppliers:

Construction:

The original Hummingbird was constructed of pre-printed 1/32" fiber board. Poster board has similar tensile properties and similar weight to the original parts, but poster board is half the original thickness at 1/64". It is important to have fiber board material as close as possible to the characteristics of the original parts to maintain strength and weight comparable to the original. The tail booms, wings, tail, wing support, and tail support are made by cutting out two pieces of each part from 1/64" poster board and gluing them together using heat-activated adhesive to form 1/32" fiber parts, very close to the originals. Two coats of Balsarite are applied to each 1/64" piece, allowed to dry, and the pieces are ironed to each other using a covering iron set to 225 - 250 degrees F. This forms the uncolored fiber parts. Use the parts outline to trace the parts on poster board and cut them out - two patterns make one part. I did try using plain 1/32" cardboard such as the type used in cardboard postal mailers, but found it to be too heavy, too flexible, and had too much gloss on one side to allow the color printed patterns to adhere.

The color patterns are printed on plain paper using an inkjet printer . The color patterns are printed, cut out, and the backside is coated with two coats of Balsarite adhesive, as is the front side of the corresponding fiber part to which the colored pattern will be ironed on. Allow the Balsarite to dry completely before positioning the parts over each other. Iron the printed paper on the underlying fiber parts with the covering iron using firm pressure, giving the adhesive time to activate. Heat-activated adhesive adds little weight or thickness, but does produce a good bond between the two layers of poster board and the printed color patterns.

Cut the body tube to 3 inches in length. Estes body tubes are brown. The Hummingbird had a white body tube. I used a piece of white Top Flite Monokote to cover the brown body tube to get a white body tube. One option would be to paint a brown tube with white paint, but paint adds a great deal of weight, and may alter the flight characteristics of the rocket as it is sensitive in terms of weight and balance. Filling the body tube spiral seam, priming, and painting would add too much weight in my opinion. Monokote is easy to apply with some practice. Peel off the backing, and iron it to the tube using the covering iron set on about 250 degrees. Monokote is a heat-shrinkable coating, and we don't want to shrink it, just activate the heat-activated adhesive on the back of the covering. If you choose to use this method and have no experience with the material, practice ironing some scraps on various round and flat shapes.

The original nose cone is unlike anything commercially available. The Estes NC-20 is a pointed cone. The original cone was a parabolic cone. I chose to hand carve a nose cone from a balsa block to closely approximate the original. After carving and sanding, the wood grain on the balsa cone was filled with multiple coats of clear dope, sanding with 400 grit paper in between coats. A coat of gray primer is applied, sanded with 400 grit paper , and the color coat is applied. Krylon "forever blue" is a close color match to the original Hummingbird nose cone. Some Hummingbirds had blue nose cones, others were white. Keep the paint coats light. The clear dope does a nice job of filling the grain and provides a very smooth surface, so little top coat should be needed to get great results.

If the replica is to be flown, glue the nose cone into the finished body tube using yellow (aliphatic) glue. If a plastic nose cone is used, cut a small piece of self adhesive printer label, attach it to the nose cone where it goes inside the body tube, apply glue, and insert into the body tube per the original instruction sheet.

Spray a coat of clear high-gloss Krylon on the colored fiber parts. This will prevent water damage to the water-soluble ink, and the high gloss paint gets absorbed into the paper, producing an even semi-gloss effect, very similar to the original parts.

Cut out the slots in the fiber parts where indicated, and assemble the rocket per the instructions. Use medium CA glue to attach the parts. It will stick to the painted parts and it dries clear. Attach the wings to the body tube using medium CA and use kwik shot to set the glue when the wings are positioned. Work quickly to align the parts after CA is applied as it sets in about 10 seconds. Glue the launch lugs to the wings using medium CA and to the body tube using canopy glue. Canopy glue is virtually the only glue that will bond strongly to the monokote used to cover the body tube. The CA used earlier will not adhere permanently to Monokote over a long term, and is only used here to get the wings aligned properly on the body tube.

My replica balanced at the same point as the original, just behind the main wing, and weighs the same as the original, which is 3/8 oz.

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