V0.19: Gas Turbine Engine for RC Aircraft

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ppemawm
Posts: 518
Joined: Fri May 17, 2013 3:54 pm
Location: Manhattan New York

V0.19: Gas Turbine Engine for RC Aircraft

Postby ppemawm » Mon Sep 09, 2019 4:41 pm

This is a small gas turbine engine for use in radio controlled (RC) model aircraft. The engine was designed and developed by Kurt Schreckling in 1992. It is remarkably simple design that can apparently be created using basic machine shop tools by experienced RC hobbyists using readily available materials.

This is a good PartDesign project for intermediate level users. Although the assembly is quite simple it requires the use of most of the basic workbench tools. My FreeCAD model is based on Schreckling's plans found in his book at https://rclibrary.co.uk/title_details.asp?ID=2515. His book is a good primer for turbojet design theory and RC application with plenty of photos of how to manufacture the engine.

All the bodies were created using only the PartDesign workbench "in context" in one file as is my standard practice for creating these type of assemblies. No assembly workbenches were used, since for my purposes, there is no need for constraints among the bodies. Fasteners were added from the Fastener add-on workbench.

Several screenshots are included with some brief comments for new users that may be interested in the process:

Picture1.jpg
The turbojet engine is rated at 30N maximum thrust with a specific thrust of 27 N/kg on diesel cut with 15% petrol or kerosene. Maximum rotation speed is 85,000 rpm. Outside diameter is 110 mm and length, 220 mm. Total weight with accessories is only about 1 kg!

The model tree for this assembly is organized as a collection of subassemblies and individual bodies. The Part container is used for subassemblies. Each subassembly is located in the top assembly using the Part placement properties. Bodies are then located in their respective positions relative to the subassembly using their Placement properties. Sketch attachment is used only for locating the body features.
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Picture2.jpg
This is an exploded view of the assembly which can be easily done using the X and Y placement properties of the subassembly Part containers or the individual Bodies. Major components include a 2D backswept radial compressor impeller, vaned diffuser, combustor with vaporizer coil, an axial flow turbine with inlet guide vanes, and an annular jet exhaust.

The schematic shows the basic components and the gas and bearing oil flow in the engine. The bearings are high speed precision ball bearings lubricated with oil mist.
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Picture3.jpg
A cross-section of the engine assembly can be inspected using View>Clipping plane as shown in this screenshot. Note the few parts and relative simplicity of the engine design.

The most complex bodies to model are the vaporizer coil, twisted blades for the turbine wheel, and the inlet guide vanes. The compressor wheel and diffuser are relatively simple to model since the blades are two dimensional.
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Picture4.jpg
The twisted blades of the turbine wheels are manufactured in practice by slotting the rotor rim and then twisting the blades with a claw tool as shown in the inset photo. The blades are then shaped by hand grinding the channel between blades until they approximate the blade design shown in the second inset.

This can be simulated using sketches and additive sweeps. After some trial, I found it was necessary to use two sweeps: one, using the hub, root, and midplane cross-section sketches and the other using the midplane and tip cross-sections sketches. The blade can then be duplicated around the circumference using the MultiTransformation and Polar pattern tools.
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Picture5.jpg
This is the process used to create the turbine inlet guide vane housing which should be self-explanatory for the intermediate user skill level.

It starts with a master sketch in the XZ plane of the revolve cross-sections and the blade location. This master sketch is then Sketcher>CarbonCopied for each of the features. The sketch geometry is toggled to construction geometry as appropriate for each feature. This approach results in an exceptionally robust parametric model as long as the master sketch geometry is not changed. The linked dimensions can be changed but not the actual geometry.

I chose to start with the outer diameter revolve feature, but you could also start with the inner hub revolve. The first sketch of the revolve feature is used as the master sketch. Subsequent feature sketches are CarbonCopies. Note that when you carbon copy the sketch it kicks it out of the Part Container which then must be dragged and dropped into the body.

The Groove tool is used to trim the blades to the proper shape using a slightly modified CarbonCopy of the master sketch.

The compressor impeller was created with the same master sketch approach.
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I will place the model on DropBox via PM for anyone that may be interested as long as it is only used for instructional or educational purposes. I have found that trying to recreate someone's model by recreating the model tree step-by-step can be a good alternative to tutorial videos as long as you are familiar with the basic GUI tools.

The version I used is shown below which is behind several revisions currently available, but I found that the latest versions are not yet stable enough for this type of project.

OS: Windows 10
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.17352 (Git)
Build type: Release
Branch: master
Hash: ec0049921b813ee78c1fd4ea304e70904a3208f7
Python version: 2.7.14
Qt version: 4.8.7
Coin version: 4.0.0a
OCC version: 7.2.0
Locale: English/UnitedStates (en_US)
"It is a poor workman who blames his tools..." ;)
kisolre
Posts: 1243
Joined: Wed Nov 21, 2018 1:13 pm

Re: V0.19: Gas Turbine Engine for RC Aircraft

Postby kisolre » Mon Sep 09, 2019 5:17 pm

Impressive as allways :-)
Could you elaborate on CarbonCopy kicking sketch out of Part container? Since it happens while editing the sketch how is that possible?
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ppemawm
Posts: 518
Joined: Fri May 17, 2013 3:54 pm
Location: Manhattan New York

Re: V0.19: Gas Turbine Engine for RC Aircraft

Postby ppemawm » Mon Sep 09, 2019 6:42 pm

kisolre wrote:
Mon Sep 09, 2019 5:17 pm
Could you elaborate on CarbonCopy kicking sketch out of Part container?
Sorry for glossing over that.
What I should have wrote more precisely is that a copy (Edit>Duplicate selected object) of the CarbonCopy is kicked out of the Part.
When I have multiple features that depend on a single master sketch, I make a copy of the CarbonCopy rather than a new CarbonCopy for each feature. Perhaps that is not the best way or a bad habit?
"It is a poor workman who blames his tools..." ;)
kisolre
Posts: 1243
Joined: Wed Nov 21, 2018 1:13 pm

Re: V0.19: Gas Turbine Engine for RC Aircraft

Postby kisolre » Mon Sep 09, 2019 7:03 pm

ppemawm wrote:
Mon Sep 09, 2019 6:42 pm
When I have multiple features that depend on a single master sketch, I make a copy of the CarbonCopy rather than a new CarbonCopy for each feature. Perhaps that is not the best way or a bad habit?
That is actually quite clever. If you have one full CarbonCopy it is easy to find it in the tree and duplicate it instead of making Master visible, selecting if from sketch, wandering of Ctrl/Alt/... combinations for non paralel/out of body sketches...
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miniellipse
Posts: 71
Joined: Tue Apr 24, 2018 9:09 pm

Re: V0.19: Gas Turbine Engine for RC Aircraft

Postby miniellipse » Fri Sep 13, 2019 10:43 pm

Wow...thanks for sharing.
ppemawm wrote:
Mon Sep 09, 2019 4:41 pm
This is a small gas turbine engine for use in radio controlled (RC) model aircraft. The engine was designed and developed by Kurt Schreckling in 1992. It is remarkably simple design that can apparently be created using basic machine shop tools by experienced RC hobbyists using readily available materials.

This is a good PartDesign project for intermediate level users. Although the assembly is quite simple it requires the use of most of the basic workbench tools. My FreeCAD model is based on Schreckling's plans found in his book at https://rclibrary.co.uk/title_details.asp?ID=2515. His book is a good primer for turbojet design theory and RC application with plenty of photos of how to manufacture the engine. ....