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Old Feb 07, 2012, 10:12 AM
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United States, WI, Fond du Lac
Joined Sep 2008
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Got to work on the nacelle shells. The top and bottom are different profiles with the bottom longer to accommodate the landing gear. I made the lower form on the lathe. I glued a plywood firewall to a plywood piece cut to the plan view and filled it with foam. Then I turned it to shape.

I covered the form with the same heat shrink window film I used to make the canopy mold. Then I added a layer of 6 oz. cloth and a layer 2 oz. cloth. I used MAS low viscosity resin and slow hardener. I covered the wet epoxy with another layer of the heat shrink film and shrunk it tight. The slow cure hardener allowed me to heat shrink the film without prematurely curing the epoxy.

The result was better than I'd hoped. I've done some vacuum bagging of epoxy lay ups and it gives an almost perfectly smooth surface to the finished part. The trouble is that it only works on developable surfaces; compound curves are limited by the ability of the bag to stretch. The shrinkable film gets around that problem.

The result isn't perfect, there are some wrinkles at the tail end of the nacelle where the film couldn't quite conform to the mold but this will be MUCH easier to finish than without the film.
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Old Feb 08, 2012, 08:30 PM
newb
United States, LA, Walker
Joined Jul 2008
20 Posts
A little late, but what is the advantage of skinning the wing with DT foam versus FFF?
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Old Feb 08, 2012, 09:31 PM
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United States, WI, Fond du Lac
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Quote:
Originally Posted by will in the 98 View Post
A little late, but what is the advantage of skinning the wing with DT foam versus FFF?
There are a few reasons. DT foam weighs less (12-14g/sqft vs 17g/sqft for FFF) so I saved about an oz. It's flatter than FFF (though you do have to pick through the sheets at the $Tree store to find the good ones). DT foam is thinner than FFF which helps some too.

I didn't use DT foam for the fuselage because the sheets are too small. Even if they weren't, it's tougher to heat form than FFF. It's softer and more sensitive to the temperature used to form it. On my Ki-46, I used it for the nacelles and pulled seven or eight half shells to get 4 usable ones.

If I had depron available locally I probably would have gone with that. 3mm or possibly even 2mm but it's a relatively expensive material when you have to pay for shipping.
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Last edited by pmullen503; Feb 08, 2012 at 09:59 PM.
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Old Feb 12, 2012, 11:34 AM
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United States, WI, Fond du Lac
Joined Sep 2008
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Fitting the nacelle halves is pain because you have to compensate for wing's airfoil, taper and dihedral. Making an accurate mirror image for the other side only complicates the process.

Here's how I fit the nacelle halves. First, I cut and fit the right shell. Then replaced it onto the mold and marked the outline. To make the mirror image I transferred the distances from the center line of the mold to the marked line to the other half of the mold. Next I replaced the untrimmed, left shell onto the mold and used the new marks to draw the cutting line for other half.

This was very convenient with a round, turned form but it works with other shapes. You just half transfer the shape of the right side of the trimmed shell around to the left side of the form and vice versa using some convenient reference line.
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Old Feb 16, 2012, 12:18 AM
Use your imagination....
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Turkey, Izmir, Seferihisar
Joined Oct 2007
1,165 Posts
Nice project...

Cem
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Old Feb 18, 2012, 12:41 PM
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United States, WI, Fond du Lac
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I finished fitting the nacelle shells. Their combined weight was only 30g. The top was glued into place and the underside will be secured with screws so I can service the retracts and gear doors.

I made the cowl mold and shrunk 3 liter bottles around it to make the cowls. The mold is made of 5 pieces held together with screws. Because the middle of the cowl is a larger diameter than the ends I couldn't use a simple one part mold. The center section of the mold features a wedge that can be removed after the bottle is shrunk and the screws removed allowing the rest of the mold pieces to be removed. A detailed decription of how I made the mold is here.

I'll add plywood rings with locating pins and magnets to secure them to the nacelles.
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Old Feb 19, 2012, 07:26 PM
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I worked on the gear leg today. The leg is made from 1/8" piano wire. I bent the lower part and mig welded it to the leg. I didn't know if it would be strong enough but I clamped the first one in the vise and tried to break it and the wire bent first.

I took some DuBro collars and ground a flat spot on then where they touch the wishbone and cut some brass disks (simulates disk brakes) to reinforce the joint. All the parts were clamped into the jig shown below and silver soldered together. Note the allen wrench used as for the axle. A round axle would have been soldered in and impossible to remove. The wrench only touches at the corners and so wasn't permanently soldered in. Somehow I kept the solder out of the allen screws threads so both collars still work. If the didn't I figured I could still secure the axle with a drop of CA glue.

These took me all day to make so if someone knows an easier way to make these wishbone type gear legs, I'm all ears!
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Old Feb 20, 2012, 04:32 PM
Watt Waster
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Joined Oct 2010
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MIG Weld vs Brazing

Quote:
Originally Posted by pmullen503 View Post
... The leg is made from 1/8" piano wire. I bent the lower part and mig welded it to the leg. ... These took me all day to make so if someone knows an easier way to make these wishbone type gear legs, I'm all ears!
You might want to investigate Brazing methods. It is more like soldering a joint in electronics. A brass rod, coated or not, is normally used to bond the metal parts, which don't have to be the same metal type. In most cases a flux is used to help remove oxygen that might contaminate the strength of the bond. It is a very old way of joining various types of metal together and was used for race car frames for a very long time with excellent results. The key point is the bond is many times stronger than needed for this application and is faster to complete the work. You do need a flame that gets hot enough, but for small jobs you can use small cylinders and torches. No need to spend more then $100 for tools that will out last for a few dozen tasks like this. Expect the tools to last for many jobs of this nature and take a few years or more before replacement cylinders of gas are needed, unless you like to leave the flame on while you do other stuff.
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Old Feb 20, 2012, 04:57 PM
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United States, WI, Fond du Lac
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Quote:
Originally Posted by Tsavah View Post
You might want to investigate Brazing methods. It is more like soldering a joint in electronics. A brass rod, coated or not, is normally used to bond the metal parts, which don't have to be the same metal type. In most cases a flux is used to help remove oxygen that might contaminate the strength of the bond. It is a very old way of joining various types of metal together and was used for race car frames for a very long time with excellent results. The key point is the bond is many times stronger than needed for this application and is faster to complete the work. You do need a flame that gets hot enough, but for small jobs you can use small cylinders and torches. No need to spend more then $100 for tools that will out last for a few dozen tasks like this. Expect the tools to last for many jobs of this nature and take a few years or more before replacement cylinders of gas are needed, unless you like to leave the flame on while you do other stuff.
Brazing would have worked too and it's easier to get a neater looking joint. I was out of the right kind of rods and oxygen so I used the mig welder which was ready to go because of recent snow blower repairs.

One advantage to the welder is that the holding fixture could be made of wood. The heat is so concentrated and brief that the rest of the piece doesn't heat up too much.
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Old Feb 25, 2012, 02:41 PM
MULTI ENGINE FANACTIC
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SAO PAULO,S.P.,BRAZIL
Joined Dec 2000
547 Posts
Xcellent job!

I had the opportunity to pilot one Beech 18 in the Boituva´s Skydiving Centre at Sao Paulo state, in my cowntry.

Unforgettable!
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Old Feb 25, 2012, 06:33 PM
ARFs make me BARF
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United States, MI, Roseville
Joined Dec 2000
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Neat. I like this ship. Twins are so much fun!
Mark
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Old Feb 25, 2012, 08:00 PM
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Joined Mar 2011
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too many brake disks

Don't mean to be picky but I think that the Beech only had a disk on the inside of the wheel

Tim

Quote:
Originally Posted by pmullen503 View Post
I worked on the gear leg today. The leg is made from 1/8" piano wire. I bent the lower part and mig welded it to the leg. I didn't know if it would be strong enough but I clamped the first one in the vise and tried to break it and the wire bent first.

I took some DuBro collars and ground a flat spot on then where they touch the wishbone and cut some brass disks (simulates disk brakes) to reinforce the joint. All the parts were clamped into the jig shown below and silver soldered together. Note the allen wrench used as for the axle. A round axle would have been soldered in and impossible to remove. The wrench only touches at the corners and so wasn't permanently soldered in. Somehow I kept the solder out of the allen screws threads so both collars still work. If the didn't I figured I could still secure the axle with a drop of CA glue.

These took me all day to make so if someone knows an easier way to make these wishbone type gear legs, I'm all ears!
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Old Feb 25, 2012, 08:59 PM
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Quote:
Originally Posted by tim48v View Post
Don't mean to be picky but I think that the Beech only had a disk on the inside of the wheel

Tim

True, but they are there to strengthen the solder joint between the gear leg and the collar. I suppose I could grind down the inboard side.

Pat
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Old Feb 26, 2012, 01:47 PM
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United States, WI, Fond du Lac
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Got to work on the gear doors. After carving out a recess in the wing for the retracted wheel I marked out the door locations on the nacelle shells.

Then I cut slots to accommodate the hinges. I removed the hinge pins and put the pair of hinges on a single long wire to keep them aligned. After epoxying them in I cut the doors free.

I had hoped to use a simple torsion spring through both hinges to open the doors and a thread to pull them closed but the thinnest wire the LHS had is too strong to use for the spring. I'll have to come up with something else.
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Old Feb 26, 2012, 04:19 PM
pilot
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United States, OR, Bend
Joined Mar 2007
124 Posts
Those gear doors look really nice.

The DLG guys will often use .020" guitar string ( and other sizes) for pull/spring control surfaces, perhaps you know a guitar player?

Jeff
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