Hobby Section ::: Learn to make a small aeroplane - No Cost just Time Required!

[luckyumer]

I am posting this article as it is, as i read!


AVRO Racing Baby

by Joe Sullivan
Aug 20, 2005


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.

This 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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[luckyumer]

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.


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.






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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[luckyumer]

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.




Finished rudder frame


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!



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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[luckyumer]

FUSELAGE CONSTRUCTION:

If I was going to get into trouble anywhere it was here, and I did. My first step was to try and construct a “square” fuse box using the two flat side panels (shown in photo below).




Well, just pinning the sides down didn’t work. Everything began shifting after I took it off the board to turn it over, and I ended up with the dreaded "banana fuse."



After this disaster I cut apart the two halves, removed the bracing, and sanded everything down to get the glue off it. Then I went back to Don’s book to see what he says about building a solid and square fuselage.

Don suggests that the section of the fuse where the wings mount can be seen as the main "box" of the fuselage, and should be reinforced. This fuse box helps support the joining of the two sides and provides torque and twist resistance during the remaining build. It also helps when flying.
In the photo at right I’ve marked off my fuse box area. I focused on joining the two fuse sides at this location. Basically, I used 90 degree bracing and gussets to fix the formers and cross members.

You can see that at the first, second and third vertical members, I’ve added gussets to make sure they do not flex front to back.






Don's advises that gussets should be cut so the grain of the wood parallels the long edge of the triangle. This takes advantage of the strength of the grain - something that never occurred to me. Also, I realize that the size of the gussets I used might be more appropriate on a P-30 than a peanut, but as I get more accustom to cutting them this way I'll make them smaller and lighter!



Notice also the gussets at top and bottom of the fuse box. This makes a very stiff structure. From here I can use triangles to make sure that the remaining fuse sections are square while gluing.



To the right you can see the squared fuse structure, ready to have the curved formers installed on top.

The plan calls for 1/32" sheeting over the formers behind the cockpit. Not having any 1/32" sheet, I sanded 1/16" sheet to half it's thickness, then soaked it in a water/ammonia bath for over an hour.







nose planking

With the formers installed, it was time to bend and glue the sheet wood over them. This turned out to be another major disaster. I spent an hour creating paper templates, cutting the balsa, and pinning it to the formers. There’s not a whole lot of wood to pin the top cover to, and as it dried the 1/32" sheeting stressed and twisted the tail section of the frame. So I abandoned this approach, and simply added stringers to the rear of the fuse.

I used 1/16" planking to cover the top front of the fuse, which would later be sanded down to 1/32" thick.


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[luckyumer]

THE PROPELLER:

Don’s recommends we determine the size of the propeller by taking the square root of the wing area and multiplying it by 0.8. This value for the Avro 534 is about 6.34 inches, but I went with a 6-inch prop. Don suggests a pitch to diameter ratio of 1.4 for scale craft. The remaining equations are based on these numbers.



The prop blank - designed and ready to be carved and sanded.



FINISHING THE FRAMEWORK

The image below shows progress so far. Stringers are now in place on the rear section of the fuse, and turtle decking sheet is installed around the cockpit opening. I also used small stringers to build up the front portion of the cockpit (a departure from the plan).


Progress so far


I tossed the propeller into my workbag so I could do a little sanding at lunchtime, when I proceeded to break it. It was either salvage these blades or carve a new one - I opted to save these blades and attach them to a new hub.

The nose block is 5 cross-grain laminated sheets, with a 1/16" diameter aluminum tube inserted as a prop wire guide. A raised rectangular form was built on the front of the nose block to frame a “grill,” made later from black screen door screening.

The cowl got several coats of balsa filler and sanding sealer. In this picture you can more clearly see the receiving boxes for the cabane struts.



The landing gear struts are 1/32" thick basswood, per the plan. The front struts each have a .020 music wire reinforcement imbedded in the middle of a basswood and soft balsa sandwich. The lateral connector is basswood also.

Wheels are four cross-grained circles of 1/16 balsa. The two center layers are solid circles, and the outer layers are rings glued to each side. The inside diameter of each ring was sanded at an angle before being glued on. Each wheel was then sanded round, drilled and fitted with an aluminum bearing. They were then finished with three coats of sanding sealer.

The completed framework



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[luckyumer]

COVERING THE WINGS:

This was the first undercambered wing I'd built. I applied glue stick to the entire underside of the wing and laid it upside down on the building board. After gently placing the tissue over the structure I placed my knife handle in the center of the wing, parallel with the leading edge, and rolled it back and forth like I was rolling out pizza dough. Then I just ran my fingers around the outer edges to smooth out any lumps.






COVERING THE FUSELAGE:

I cut out a template from copier paper with the words “AVRO” and “AVRO BABY” on it. White enamel paint used to spray-paint the lettering onto the tissue before covering.


Glue stick was used to attach all tissue, and the excess was trimmed from the edges with a small emery board.


Once everything was trimmed and dried overnight, it was sprayed with water and allowed to dry. The front section was then painted silver. The landing gear and cabane struts were painted gunship grey, and the inter-plane struts painted brown. Finally, the whole model was given a coat of clear gloss dope.This photo shows everything laid out and ready for assembly:


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[luckyumer]

DETAILING:

The rivets were done by sharpening the end of a .060 aluminum tube and pressing the end into the balsa. It produces a rounded area surrounded by a depressed ring which simulates rivets or screw heads.

Exhaust pies were made from a drinking straw. I selected one from a juice box because the size is closer to scale. One end of each exhaust pipe was cut square, and the other at an angle. They were painted “gunship grey” and glued in place.



Panel separations were done by dragging a pin across the wood surface.

The prop blades were salvaged from the broken carved prop. Don’s book has a chart showing what the tip angle should be for a given prop diameter and P/D ratio. I used that info to set the blades into the hub.



Rigging wires are monofilament fishing line. I poked small holes at the appropriate locations using a pin, pushed the filament into the holes, and secured it with a tiny drop of CA glue.



CONCLUSION:

Finished weight was a bit over 27 grams without the motor. With a wing area of 62.5 square inches, the wing loading is 0.43 grams per inch. That's just a tenth of a gram more than Don recommends for Peanut Scale planes.
If I add 25% more for the motor the weight goes up to 34 grams and the wing load jumps to .54 grams per square inch. I doubt I’ll use that much rubber.




If I were to build this model again I'd take the time to make better wood selections, sand the nose down a little more, lighten up the landing gear, and thin out the wheels more.





FINISHED - ENDED YOU CAN REPLY NOW

[Shaheen]

Nice sharing bro......welldone keep it up

[Abdul Moeed]

Nice WORK Sir G .. But kohi nahi banay wala ..

wasay acchi sharing hay .

[muneeb_af]

Ha ha ha very nice but i think k kisi k pas time nahi ho ga bnane k liye