Experiences Building the MW54 Turboprop


This article has been re-written and split into four parts
Pt 1   MW54 Gas Generator.       
Pt 2   MW54 Gearbox assy.         
Pt 3    MW54 Intermediate stage.
Pt 4   MW54 Autostart.                  


Preface

The MW54 Turbojet engine was designed and developed by Wren turbines LTD. It can be built in turbojet or turboprop format and can be supplied as just plans, raw materials, individual bits, kits for assembly and finished engines!
This article uses raw materials although I felt it wise to use a commercial NGV blank, Compressor and turbine wheel.
More infomation , plans etc can be found at their site
<http://www.wren-turbines.com>

I decided to build the Turboprop version which utilises the turbojet as a gas generator, the only differences being a different turbine wheel and no exhaust cone

Having previously built two Kj66’s the MW54 presented little difficulty apart from the generally smaller size making some of the tasks slightly more fiddly. The hole configurations i.e. 8 & 11 hole layouts may be found tedious to mark out accurately on lathes only fitted with a 60-hole detent system. Otherwise the drawings and general design are extremely good. Most notable features being the stepped shaft allowing the turbine wheel to be made a good fit on the shaft without the rear-bearing journal being affected. Also the "O" ring bearing support makes for a sensible modification enabling fits to conform to the ceramic bearing makers data sheets.


Construction notes

Wren turbines (who designed the MW54) will supply the drawings and a starter pack consisting of the raw materials for the front plate, Diffuser, Intake horn and Shaft tunnel.


Shaft

The shaft should be made out of EN24T. This steel also has a later identification number of 817M40T which may help anyone outside the Uk.
Although machining the shaft is straight forward it does need a great deal of care to ensure precision and concentricity. See page machining shafts after completion, the shaft and associated revolving components MUST be first statically and then preferably Dynamically balanced. If this is not done then the bearings are going to suffer!. 


Case

The case presented the first problem as I wished to use seamless S/S tubing however on the advice of Wren turbines the case should be very close to the diameter specified so I made a spotwelded case but with a separate joining strip outside of the butt joined case. This does mean more spotwelds but produces a clean, distortion free bore.
Do you recognise  the case? You should do! It's my thermos flask! Good thing is that there is enough left over for making the air filter.
The front and rear case ends were then machined to be a perfect fit in the case.


Compressor

Re-profiling the Garrett Compressor wheel seemed an horrendous task but turned out quite straightforward after making a vertical post to fit to the cross-slide and then making a cutter from an old piece of steel. Small cuts were made and the cross slide fed in until the profile was achieved ! The blade edges were cleaned up using a scalpel, The use of a magnifying glass is beneficial here.

mw reprofile.jpg (26882 bytes)

Wren later recommended that the compressor wheel should be re-profiled a second time Although I left mine at the first re-profile.
The second re-profile was the result of initial tests with the prototype turbine wheel. The production cast wheel performed even better so it is efficient enough to drive the compressor wheel with this profile, so the choice is yours!

Re-profiling the compressor wheel was easy but it will need rebalancing


Shaft tunnel & NGV machining

Right picture shows the initial drilling out of the shaft tunnel, A 3-point steady is needed stop any jittering of the work.
mach ngv.jpg (23956 bytes) mw_mach_tunnel.jpg (28235 bytes)

Left picture shows a KJ66 NGV being machined but it's the same process for the MW54


Combustion chamber

The combustion chamber is reasonably straight forward apart from the sticks. These are best bent to shape by first filling with lead or similar low melting point metal, bending and then melting the metal out again. The combustion chamber is now a "twelve stick" chamber as standard to enable better starting and improved low speed performance!

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  jet#06.jpg (31270 bytes)

The above sequence shows the parts of the 12 stick combustion chamber


Diffuser

Milling the blades on the diffuser can be best done with a 3-flute slot drill, They are available in short and long length versions. If you want to make the diffuser yourself but havn't got a lathe, Making a diffuser on a lathe shows how I made mine.

The Air holes in the diffuser are drilled at a quite shallow angle of 10deg. and must be accurately center punched at 1.2mm from the inside edge in order for the drill holes to appear on the other side in the right place. Center-punch these by grinding a point on a suitably sized diameter of piano wire, test punch to see if the correct distance is obtained.
You really must take care with these holes, a drilling table with a tilt base is recommended here.
The required 10deg drilling angle isn’t possible unless the front wedges have been milled first. It is then possible by using the drill bit with a brass tube extension through the gaps in the wedges.


NGV

The ngv employs a sensible flange mounting; this removes any chance of the NGV rotating or being crimped. It is possible to fabricate this item from sheet but if you do I would recommend that you drill 3 c/s holes down (almost through) each blade slot before assembly so that the weld will have full penetration of the outer band. Once machined, if the weld is inferior the NGV will distort with use and you will get hot spots or worse.

Two views of the combustion chamber with NGV and end plate in place 

Unless you have experienced building NGV's out of sheet then I would recommend that you buy a cast one to machine, They are cast in stainless steel  and are easily machined with tipped tools.

I felt that the NGV end of the combustor would be held more securely in place if a 0.5mm X 0.5mm flange was machined on the outside of the NGV!. This would also act as a combustion chamber end stop when assembling the engine


jet#01.jpg (29497 bytes)

Main components finished !.


Turbine wheel

On the MW54. the Turbine-wheel hole, is quite small, and almost impossible to machine with a conventional tool, so it's best to first drill the hole and then grind it to size, This is explained on the Machining turbine parts page.


Assembly

Assembly started with attaching all plumbing to the combustion chamber, followed by the NGV and back-plate. This was then mounted in the lathe and a dial gauge used to checked that the end-plate was square. Once the bolts and glowplugs were tightened, the case was removed from the lathe, the front end and shaft-tunnel bolted on.
The MW54 allows you to add the rotating components after completing the build! This is handy as the bearings can be checked without stripping the engine down.


 General

2.5mm s/s cap head bolts seem to be hard to obtain; they are not generally stocked due to low sales. Once I had located a supplier I purchased 200. These were 10 mm long so I machined them to length. However Wren will now supply these if required!


It would be sensible to use an ECU with this engine, as the spool up is quite rapid, it would therefore be easy to over rev. with nasty consequences, Auto start with the turboprop engine is almost a necessity! The FADEc is an excellent ECU.


The Plumbing is quite straightforward but take care with the injector needles. you need fine wire silver solder.
Drill the injector holes so that the syringe needle needs to be pushed though with a little force. Make sure that the 45 degree angle at the tip is slightly blunted and faced toward the primary flow in the ring main. Once soldered keep the needle hot while pulling the needle into a small loop by catching hold the plastic part of the needle with your hand. If this is not possible then reheat the needle and bend it to shape with a small screwdriver of rod.

 

  Picture above shows a gas check on the fuel line.All flames should be equal height. 

 

Note the even thickness of the Flow straightener vanes on the Diffuser. This is possible with quite basic equipment but might cause a headache on a small Lathe.

This picture shows the entry point of the pre-heat gas line into the combustion chamber.

 

 

Fuel manifold divider.
Note the Turbine rear plate is machined from s/s. This was a precautionary measure in case the temperature here would be much higher once the inter-stage had been added.


Detail shot of the Ring main and injector needles.

The ring main is a slightly loose fit over the combustion chamber end to allow for the Ni-Chrome wire retainers, Small holes were drilled in the combustion chamber to allow the wire to pass through.


The Temperature probe supplied with ECU's are slightly longer than the turbine case A zig-zag can be employed at the center to use up the extra length but if you do then take care to radius the bends. If you are actually building the Turboprop Then this probe can be kept straight due to the extra length of the engine.


The 4mm. Festo fittings have been replaced with 3mm. ones. I have also added one more for monitoring case pressure and pressure switch supply (for control of gearbox lubrication).


Two rails have been spot welded to the sides for easier mounting in the model.


Turbine oil supply is provided by using a "TEE" piece in the main fuel supply line a 0.6 needle is silver soldered in a short tube which is a press fit inside the 3mm nylon oil supply line. This restricts the flow to the bearings, the clear nylon pipe gives a visual indication of oil flow.


Observations

So far, the building has been a pleasant experience. 

You will need a decent drilling table and a good lathe, A milling machine would be a big advantage but it is possible to make a milling attachment for the larger lathes.


688 Full complement annular contact cageless ceramic bearings are obtainable from:-
IEC International Engineering Company  Ltd  <http://www.iecltd.co.uk> Min order 50 order
part no. 21D688/602 976   37.18 each.

Tel No UK 01202 676262  Contact person is Linda Wilson.


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