Author Topic: P47 Thunderbolt Engine Cowl Tutorial  (Read 225 times)

Dave Winfield

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P47 Thunderbolt Engine Cowl Tutorial
« on: November 05, 2015, 10:48:00 AM »
I am posting this tutorial for two main reasons:
to help guide you through the assembly of the P47 Engine Cowl assembly (which can be a tricky assembly for some).

and because it involves a number of important techniques that can be of use in other areas of an aircraft model build.

In just these six parts, we use straight and curving cuts, inside and outside radiuses.
You have to roll hoops, use straight and tabbed joiner strips, build domed shapes,
deal with butt edge seams and install laminated formers.



This is a very straightforward assembly and can yield impressive results if you follow some basic practices.
Patience is a big factor here.
Allow glue and connections time to cure and strengthen.
Don't rush the build.
Use good sharp tools and cut with straightedges whenever possible.

Lets go...

The P47 kit houses a "fake" engine in a front "cowling" that attaches to the flat end of the fuselage.
It involves these six parts, two of which are formers and need to be laminated to thicker cardboard. (.75- 1mm thickness)

You can get away without scoring any parts for folding, but normally this is where you would do your pre-scoring.
Carefully cut out all the parts...I like to cut directly on top of the lines leaving as little of the edge lines as possible.

Its extremely important to cut the ends of the hoop parts with a straight edge.
These ends will butt join, and need to fit together tightly and evenly.
A scalpel or hobby knife and straightedge will give you the best seams




...
Once all my parts are cut out, I like to start preforming them into the rough shapes they need to be.
I use fingers, my palm, markers and pencils, and round dowels to shape the curving hoop parts.

Even the tabbed joiner strip must be curved to match, but remember to keep the printed side inside.

Make sure the hoop ends are properly curved and the seams must line up as evenly as possible.

You can also edge colour at this point, before you attach any joiner strips or tabs.




...
The large tabbed joiner strip fits inside the center section of the three hoop sections.
This will be your first assembly.

The tabbed inner strip is marked with a centerline at the center bottom location.
The ends of the strip are actually to the top.
It goes upside down, compared to the outer part of the cowl, which has its connecting seam at the bottom.

Use the center mark to locate the outer part, but you may need to transfer that centerline to the
blank side of the part. (Just hold the part up to a light, and trace the line on the blank side)


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Dave Winfield

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Re: P47 Thunderbolt Engine Cowl Tutorial
« Reply #1 on: November 05, 2015, 11:01:21 AM »
Once you are ready, start by attaching the outer part (center hoop section) to the tabbed strip.
Line up and glue one end of the hoop part at the (bottom) centerline on the tabbed part.

You only need to glue about 10mm (1/2") of the strip.

Wrap the other end around, butt up the ends, and glue it in place.
Your focus here is to join the two ends of the part, creatin a tight andeven connection of the ends.
Once again, you only need to glue a short section in place.

Once the ends are securely dried and glued in place, you can proceed to glue the rest of the tabbed strip
inside the outer hoop part....doing your best to keep the outer part centered on the joiner strip.
A craft paper glue will give you a string bond, while allowing you the time to reposisiton parts if necessary.
Take your time to line everything up nicely.

The tabbed joiner strip should fit perfectly within the hoop section, but if you have to trim a little
off the ends of the tabbed strip, thats okay. Cut as little as possible.




...
While the center section is curing some more, you can finish the two outer hoop sections of the Cowl.
Use the formers as reference for the overall shape of the parts.

Join the ends to form the hoops, making sure you have a tight and even connection.
Sometime edges don't line up perfectly, usually because of the way the parts were cut out.
After the sections are glued and joined...and cured...you can use a knife or fine scissors to trim the edges a little.
Don't cut too much!
Just even up those ragged joins.




...
Now we have all three sections of the Cowl ready to assemble.

One last time I will adjust the shape of the parts.
Smooth out the curves, make sure the curving parts match.
Use the formers as reference for the overall shapes.

You also need to test fit parts together before you start gluing.
But with this part you'll need to adjust the glue tabs first...


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Dave Winfield

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Re: P47 Thunderbolt Engine Cowl Tutorial
« Reply #2 on: November 05, 2015, 11:35:01 AM »
Because these three hoops connect together forming a domed shape, they meet at specific angles.
So you have to bend the joiner tabs inwards to allow the angled connections.

I use a wide, square ended Tweezer to fold each tab inwards the desired amount.
Go around the part slowly adjusting each tab.
I'll start with just one side of the assembly...I choose the larger (rearward) hoop connection first.
Its the easiest connection, and leaves you with a larger stronger part for the third (front) hoop.




...
Steps are the same when doing the second (front) hoop connection...

start by lining and gluing the bottom seam in place.
You only need to glue a small section at this point, no more than a third of the way around the hoop
making sure the center seams are properly lined up.

Once the connection is dry, cured and strong you can continue gluing the rest of the way around the part.
Use your fingers inside the part, and squeeze to form a tight bond.
All I use at this point is fingers and thumb.
Focus on the edge seam, keeping it tight and even.
Pressing it down to be as flat as possible.
Allow the part to fall into place as you glue...working up each side, and meeting at the top.
Don't work your way around the the part in one direction, because you will stretch the part as you go and it won't fit at the end.




...
Most important thing is keeping tight flat seams between the hoop sections forming a domelike curve to the surface.
Unless you use a lot of connecting sections, you will get a stepped effect, with slightly raised edges.

After the parts have completely dried and cured, there are a number of ways to burnish the seams flatter.
I like to use rounded end tools, applying pressure on the inside of seams, to roll, flatten and smooth out the connections.
You can also use smooth objects on the outside of parts to smooth out the connections.

Although the view is somewhat restricted, you can see some of the gluetabs inside the cowl...there has to be some tolerance for this.



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Dave Winfield

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Re: P47 Thunderbolt Engine Cowl Tutorial
« Reply #3 on: November 05, 2015, 11:35:45 AM »
...
Final steps are installing the formers.

The forward former is also the fake engine, and fits just inside the tabbed glue strip,
at the first seam in the cowling.
It will actually cover and hide the inner ring of tabs.

If you are installing some sort of spinning propellor, you may need to drill a center hole for that.
Its easier to do that before the former is installed.


I shape and sand (with files and sandpaper) the former until it just slips into place.
I don't want a tight fit which can cause visible "ribbing" outside.
But I also don't want it too loose...it needs to form the assembly into the correct shape.




...
Rear former goes in the same way...
sand and shape for the best fit.
It should fit at the base of the vent flaps.
(The cowl has all the hinged vent flaps marked...you have the option of slittling the flaps so they can fold outwards.)

Its important that this rear former is installed flat and even in the cowling,
so the cowing will position inline properly (not crooked) with the fuselage.






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