AVRO Racing Baby
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by Joe Sullivan
Aug 20, 2005
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After
reading Don Ross's Rubber Powered Model
Airplanes, I decided to use the techniques
I learned on a Walt Mooney model design - the AVRO
534C Racing Baby model.
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is a "true biplane," meaning the
upper and lower wings are of equal dimension.
It is a single-bay, top-staggered, fabric
and wood covered plane. |
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Joe's completed model
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BACKGROUND:
The airplane first flew on April 30th, 1919. Built
by A.V.Roe and Co. with a Green
35 horsepower engine, it's top speed was said to
be 78-80 miles per hour.The 534’s designer
was Roy Chadwick, a well-known aircraft designer
for AVRO, whose designs include
Lancaster 4-engine bomber.
Seven Model 534s were built - three primary versions,
followed by one each of the “A”, “B”,
“C” and “D” variants. The
“C” version’s most famous pilot
was Burt Hinkler, who attempted to fly it from London,
England to Sydney, Australia in the 1920s. |
Avro Baby |
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the left is a photo of the Avro Baby.
This was the basis for the models’ color
scheme. I couldn't tell what color the fuse
was from this photo, so I went with purple! |
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Here’s another picture of the Baby,
with her designer Roy Chadwick. |
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Avro Baby and Roy Chadwick |
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I wasn't into this project for ten minutes before
I ran into a problem. The Mooney plans show the
model designated as the Avro 536 C Racing
Baby. After scanning the Internet I have
been able to come up with only one picture that
supposedly is of a “536”... |
Avro 536
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Avro 536 is described as
a 2-seat touring biplane. |
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Below is a copy of Walt’s plans that I purchased
as part of the Walt Mooney Plans Bag #3 from Peck-Polymers... |
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As you can see, the plans show the outlines of the
534 rather than the 536. So I’m going to call
it the Avro Racing Baby and let
sharper minds than mine work it out!
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WING CONSTRUCTION:
Ribs, ribs and more ribs! This was one of the
things that made it difficult to decide on this
model. I like the “lines,” but don't
enjoy cutting out all those ribs. Having to notch
them for spars seems a chore too.
I did not look forward to having to notch all
those ribs. On top of that, I was going to enlarge
the trailing edge slightly and notch it to accept
the ribs (for greater strength and support). Don
Ross makes his first appearance here.
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This is a photo of notching tools Don suggests
you build. I built two on the same board,
one side is 1/16 of an inch and the other
is 1/32. Both have some 120 grit sandpaper
glued to them. They really made the job of
notch cutting easier. |
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My first mistake was not using the correct thickness
balsa for the ribs. The plans call for using 1/16"
thick balsa for the center section ribs, then 1/32"
thick balsa for the remainder. I didn’t see
that last piece of info, so I happily cut out all
ribs from 1/16" thick balsa. So I needed away
to quickly sand down 36 ribs to half their thickness.
The answer came to me in the form of a sanding template
(see photo below). Using 1/32" thick Basswood,
I cut out a general shape for each of the rib sizes.
Then I placed each rib in the appropriate cutout
and sanded it down until it was as thick as the
template.
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Wings and sanding jig |
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TAIL CONSTRUCTION:
Walt’s plans call for a type of stabilizer
and rudder I haven’t built before. It’s
an airfoil shape on both sides as opposed the
flat stringer type I’m used to building.
Instead of laminating the outlines, I used a single
1/32" thick basswood strip soaked for 20
minutes in water and ammonia. It was bent around
balsawood forms cut to the rudder and stabilizer
shapes, taped, and baked in a 220 degree oven
for 20 minutes. If you want to see a step-by-step
method for laminating, check out Bill Carney’s
terrific tutorial
for making laminated parts in the SFA
Forum, and follow it to the letter.
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Finished rudder frame |
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After pinning the formed outlines to the plan, I
filled in the stingers as normal and waited until
the whole thing dried. The “flat” rudder
and stab were removed from the plans and 1/16"
square stock was glued over the stringers that represent
the ribs. This was done to both sides. Once the
assembly was dry, I sanded the airfoil shape.
In hindsight, I now realize that I could have avoided
using the center stringers. Just gluing the stringers
outside the frame and sanding them to shape would
have saved me some weight!
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TARGET WEIGHT:
Don is very insistent on watching the plane’s
weight, and rightly so. His approach to Peanut scale
models is to have a wing loading factor of about
.33 grams per square inch.
With the wings and tail feathers completed I decided
to calculate what my target weight should be. Each
wing is roughly 5.40 inches long by 2.6 inches wide.
Four of them (upper and lower pairs) plus the center
section equal a wing area of 62.53 square inches.
Multiplied by the wing load factor of .33 grams
that puts my target flying weight at 20.63 grams.
That's the “All Up”
weight , including model, prop and rubber motor
(by the way, the rubber is supposed to be 25-30%
of the all-up weight).
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