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How to build a large RC jet model

An RC jet plane model of the Lockheed U-2 spy plane.

DeskProto user Mike in Australia makes RC (Radio Controlled) model planes.
These are no simple hand-held devices made from a DIY airplane kit: this jet plane model has a wing span of 2.5 meters ! Watch it in the impressive video at the end of this page.

The resulting model U2 plane
The resulting model plane, in Mike's workshop.

The Lockheed U-2 is an American reconnaissance aircraft ("spy plane"), flying at ultra-high altitudes. Commissioned as top-secret in 1955, the plane became famous when in 1960 pilot Gary Powers was shot down in a U-2 over the Soviet Union.

Screenshot: 3D CAD data of the fuselage
3D Scan data of the plane's fuselage.

The CAD-model was created as a combination of 3D scan-data and new modelled surfaces: the fuselage was scanned and the wings were designed as they need to be all flying surfaces and therefore required to be accurate. The wing profile design was essential to make the model aircraft fly fast.

DeskProto Screenshot
STL file loaded in DeskProto, with toolpaths for the left side of the fuselage.

Mike states: "The two-sided milling in DeskProto is fantastic!! For all flying surfaces as I'm able to get absolute accuracy. The trailing edges are as thin as 0.5mm and if the top side and bottom side were to be off this would show up as wings are over 1200mm. The mirror feature allowed me to make a left wing and a right wing from a single design in CAD therefore there was no need to design a left and right wing separately."

The fuselage sides are created as two symmetric halves. In DeskProto the toolpaths have been calculated with setting "Use upper half of geometry". Mike continues: "This is also a very important tool for me in DeskProto as I import the complete fuse. Again I mirrored the left side to become the right side, as 3D scans and/or the original model can be inaccurate, resulting in two different sides not being able to match."

Machining in tooling board
Wing, machined from two sides
Machining the master-models, in tooling board.

Milling has been done on a AutoGrav 2010 CNC machine from Germany (manufacturer no longer in business). Mike uses OBO tooling board for the master-models, 500 grade for the wings and 400 grade for the fuselage. Various cutters were used, for example the tool to cut the fuselage was a 150mm long, 12mm diameter large flat tip as this works best for large surfaces. Wings were milled with a shorter 10mm diameter flat tip.

Inside part of the mold (the core)
Outside part of the mold
Thin-walled part in the mold
The two mold halves for the left side of the fuselage, and the actual part being produced.

As low weight is needed to achieve a great flying plane, it is built from thin-walled panels. Mike starts the production process by machining a master plug, shown in the left image.

The next step is to make a negative mold, created by hand-layup of gelcoat, polyester and fiberglass reinforcement over the master plug. These negative molds, shown in the center image, need to be quite strong and stiff as they may not deform in the final production step. They can be reused multiple times.

The final flying part is produced in a negative mold: first wax and a PVA (Polyvinyl Alcohol) based Parting Film are applied, next the epoxy resin and the light weight glass reinforcement, and finally some layers of strong plastic foil that is securely taped along all edges. The space between negative mold and plastic foil then is vacuumed (25 inches of mercury, which applies a pressure of 2.6 kg/cm2). The image on the right shows excess resin being absorbed into a layer of paper, through tiny holes in a first layer of plastic foil. A second layer without holes seals the vacuum around the mold. The vacuum will compress all materials to achieve maximum strength for the flying part.

The images above show one of the molds for the fuselage: you can see the hole for the transparent cockpit window, and the attachment point for the jet's air inlet.

zoomed in photo whowing the rivets
The level of detail is amazing: the separate panels and all rivets are clearly visible
(some transparent windows still are missing here).

For the fuselage's 3D scan a 1:48 scale U-2 model was used, as this was the best way to achieve a scale and detailed result without having to draw the fuselage in CAD from scratch. The 1:48 model was then scaled up in DeskProto using 5 times magnification in order to achieve the final required dimensions - this was very easy to do.

The resulting model aircraft
The result is a complete U-2 plane, ready for take-off.

Specifications of the flying model:
  • Wing span 2500 mm
  • Weight 2520 g AUW (All-Up Weight)
  • Powered with a single EDF 70mm diameter with a brushless motor
  • Battery is a 6S, 25V, 2200 mAh (milliAmpere hour. 6S stands for 6 cells in series)
  • Flying time 20-30 minutes
The engine works with a propeller fan built in: 10 blades connected directly to a brushless motor. EDF stands for Electric Ducted Fan, which is a propeller fan mounted inside a duct (cylinder). For "small" models Mike uses 25V, for larger models 52V.

Flying video still
This YouTube video shows the U-2 aircraft model flying.

Mike concludes: "For me the toolpath for machining is the most important part as it determines success or failure: I haven't failed yet due to the ease of using DeskProto".
As we see it many more critical issues needed to be dealt with to create a flying airplane model like this. Mike has surely has proved capable to deal with all these issues !