I've been asked a variation on that question many times since I purchased a kit to build one and I've put together some of the questions with my answers.

The Wren MW54 Gas Turbine

Before we go any further I'd just like to state that I regard myself as just an average model builder (on a good day anyway), and I'm certainly not an expert in an engineering workshop, or indeed on jet engines. Neither have I any connection with Wren Turbines, other than being a happy customer. Please be warned now that these things are very addictive!

The MW54 is a very small working gas turbine (or microturbine as they are called) designed by Mike Murphy and John Wright of Wren turbines.

MW54 shown here with ruler for scale

The finished engine is 6 inches long and 3.5 inches in diameter. It weighs 800 gms (engine only) and produces 12 lbs of thrust at an almost unimaginable 160,000 rpm.

Front Inlet	Rear Outlet

To put that in perspective, it weighs less than an OS120 4-stroke, an absolutely top notch (and very expensive) 91 sized ducted fan unit produces about 8.5 lbs of thrust and an OS91 4-stroke produces about 8lbs of thrust. An engine will fly planes up to about 3 times the available thrust. In real terms that means the MW54 will fly a model up to about 20 lbs all up weight (including fuel) quite comfortably.

MW54 shown with a Laser 70 for comparison

An MW54 can be used in a plane that will fly on a 91 sized ducted fan, but be aware that the airframe may well need reinforcing to cope with the additional stress.

There are a range of options. You can buy the plans from Wren, machine/make all the parts yourself and I believe you can build the engine for about £200. It goes without saying that you need a well-equipped workshop (lathe, spot welding gear etc.), and you need to be able to use it with considerable skill. Well, I guess that counts most of us out including me. At the other end of the scale you can buy one ready made and tested. The MW54 itself can be purchased as a bare engine for £1095, or in full auto-start configuration with everything you need to put it in a plane for £1375. Alternatively you can purchase one of the MW54 derivatives such as the RAM 500 (£1750) or Simjet 1000 (£1650), again in full auto-start configuration. Sorry, while prices have come down that's still too much for me.

There is another way....

Wren supply a kit of parts for the engine for £714 and you can put it together yourself with a relatively small amount of work. The rest of my Q&A concern the kit engine.

What Wren have done is produce a set of parts that have all the difficult machining and fabrication completed. What remains to be done is time consuming, but quite straightforward and you save on the labour cost as well as getting to know your engine very well. You will also need further items to actually run it, but absolutely no more than another £100.

Well, yes it is but many of the parts are expensive (two very high specification ceramic bearings at £44 each for example), require exotic materials (the turbine wheel is made in something called Inconel 713 and produced and tested for flaws to aerospace standards) or simply need very high precision machining (the rotor shaft). The kit is very complete and contains all the parts you will need to make a running engine (with a very minor and inexpensive exception - bit of brass sheet for a retaining tab).

There is a Garrett compressor at the front of the engine. When this spins it forces air under pressure into the main body of the engine, the combustion chamber. Here, the air mixes with vaporised paraffin and burns. The heat makes the air expand and forces it out of the rear at very high speed and the difference between the momentum of the air going in and the air coming out is what produces the thrust. On its way out of the engine the air turns a turbine wheel which absorbs some power and in turn keeps the compressor turning by the connecting rotor shaft. When there is a surplus of power in the hot air leaving the engine, the engine will self-sustain and produce thrust.

Exploded component diagram	Assembled cross section view

Not in the near future. At the moment these things are relatively low volume items utilising expensive or hard to make bits. It is possible that our friends in the Far East may have a go but I wouldn't hold your breath. On the other hand, if you look at the progress that has been made in, say, the last five years (and not just with respect to jets, e.g. electric Lancasters with retracts etc.) who knows? What is quite clear is that the ducted fan is dead and that turbines are reaching the stage where they are becoming accessible to the average chap. I haven't got a clue what the score will be in another 5 years time (if you have a working crystal ball please have a butchers and let me know).

The kit comes with a full set of drawings for the engine and machining instructions for everything. These are a master class in workshop practice. Fortunately, the hard bits are done for you!. The kit also comes with separate instructions for making the kit itself. These are excellent and if you follow them to the letter your engine will run. It is as simple as that. Wren tell me they have never heard of a kit engine failing to run (even mine ran beautifully at the very first attempt, and I really, really didn't dare believe it would!). Wren also do the occasional open day, these are first class and well worth going to, you can learn an amazing amount very quickly.

There are four main chunks of work. You have to drill and/or tap some 50 holes. The drills and taps to make the threads are included in the kit. Drilling and tapping holes is not difficult, even if you have never done it before. You have to drill 3 awkward air cooling holes and one hole for a lubrication pipe. These are tricky because of the angle they have to be drilled at, but if you follow the instructions they do come out correctly. There are some minor parts to make such as gluing fine stainless mesh to a ring to make an air filter, and assemble the jet tail cones. Finally, everyone's perceived major problem: balancing the compressor, shaft and turbine. I put this off for several weeks but it was dead simple in the end - just took some time. The instructions cover this in great detail some I won't go into except to say that the general idea is to assemble the components on the shaft, with bearing, and put the bearings inside a loose fitting tube - piece of copper pipe is fine. You the rock the tube back and forth on a flat surface and the heaviest point of the assembly will shake down to the bottom. You then grind off a little bit and try again until it won't settle anywhere. It is then balanced. It may sound crude but it is extremely sensitive. If you are really worried about this, and you don't need to be, Wren will do it for you for a charge of £40 (takes a 2-4 hours to get right) and they can also check it on their electronic balancer.

Finally, you screw it all together!

There are two items you absolutely must have. The first is a drilling machine for the drilling and tapping. You will need one with a gap between chuck and drilling table of about 8 inches (you could get away with 6 inches I think, but 8 will give you more room to work). You should be able to get one for somewhere in the range of £50-£100. The second is a mini-drill to do the grinding while balancing the turbine wheel. Everyone seems to use a Dremel although there are other makes that are probably just as good. I purchased a Dremel set in a case with forty accessories for about £80 and it contained everything I needed. Other than that, basic hand tools really, most will already have them. You will also need a few drills that are not supplied and a countersink (all available from Chronos Ltd and others). Other than that you will need some tubes of Loctite "Lock & Seal", Holt's FireGum, silicon sealer & silicon grease (try a plumber's merchant).

Wren reckon that it is about 20 hours work. That probably isn't too far out, I spent about that in the workshop plus a lot thinking/silly worry time. All it takes to build this engine is time,care, patience and a drop of the good old common sense.

Wren are very helpful. They had several calls from me along the lines of "Hello, is that Wren? This is Simon Bisson and I've had another disaster …". Needless to say, there were no real disasters and everything came out fine. Even if the worst came to the very worst they will simply sell you another bit, not the end of the world really! I would also recommend that you join the Gas Turbine Builders Association (GTBA).

That depends. If an engine is really out of balance then it will scream, if indeed it will run at all. An out of balance MW54 will refuse to run up over 70,000 rpm. I've never heard it but it is apparently truly painfully ear-splitting. As the balance improves the scream will reduce to a whistle, and a well balanced engine will not whistle at all. It is still loud and the sound is that of a giant blow torch which I suppose is what it is. The sound dissipates very rapidly with distance and in the air they are virtually silent. You will hear the occasional roar but that is it, indeed, pilots I have spoken to say that it is a real problem because the only sign you get that it has stopped is a puff of smoke - you can't hear it. I understand the normal BMFA noise test does not apply to turbines.

I don't know yet! I think that it is generally accepted that you should have at least your 'B' test before even attempting to fly a jet powered plan (and I'm still working on my 'A'). A jet has different characteristics to a propeller driven plane. The thrust available to a propeller driven plane decreases the faster it goes whereas a jet produces pretty much the same thrust at any speed, so they can fly a lot faster. Jets also suffer from "turbine lag", it takes time for the engine to accelerate in response to more throttle. In the case of the MW54 this shouldn't be too bad because it is so small and hence the moving parts have a relatively low inertia. Wren have told me that the MW54 will spool up from idle (35-40,000 rpm) to full power in under a second. The final effect is residual thrust. At 50,000 rpm the MW54 still produces about 1 lb of thrust so even at idle it is still powering the aircraft along. Depending on the airframe it may be necessary to land dead-stick to avoid it coming in too fast. I'll let you know when I get there!

Yes - If you are stupid enough to put your fingers within 6 inches of the inlet at full power they will be sucked in and will be turned into mince. This is a very bad idea because it may damage the bearings and the blood may cause the rotor shaft to go rusty. These engines have a very powerful "suck". If you are stupid enough to wave your hand over the rear of the engine while it is running you will get very, very badly burnt by air at 600 degrees centigrade and if you loose the hand it will be your fault for being a complete moron.

Treated properly they are no more dangerous than an I.C. engine. Let's face it, virtually any I.C. engine is potentially capable of doing serious damage if you stick your finger/hand/whatever in the prop. You would think this was self evident but I'm sure you all know someone who is always suffering from "prop rash". They probably think they are unlucky. I think they shouldn't be allowed to go near an engine (and yes I have been bitten, once, and I don't intend to let it happen again).

A jet needs to be treated with appropriate respect. The GTBA have published safety rules and if you stick to them you will be fine. Specifically, you must have a fire extinguisher handy and nobody must stand either in-line with the turbine wheel or at any point behind the engine. Fire is always a potential hazard particularly in the event of a crash. I have not heard of a Wren turbine wheel bursting - they are very carefully tested and you get the test certificate - but it could happen. Big dose of common sense is called for; don't stand in a dangerous place.

I hope these notes have been of some help. I'm still starting on this journey myself and having great fun. These engines really are awesome and now coming within reach of all of us. I hope that I may have encouraged you to have a go if you are interested, it is now possible and you do not have to be an engineering genius to do it. If you have put the engine together yourself as I have then you will really understand it. More to the point, if it should need attention you will be able to do it whereas someone who has bought a box off the shelf will have to send it away and simply pay the bills.

The first time your engine fires into life is incredibly satisfying. All I've got to do now is improve my flying so that I can stick it a plane!

Installation in a Kangaroo sport trainer	Or how about 2 in this scale ME262

MW54 configured as a turbo-prop	Turbo-prop installation in a Tucano

If anyone has any more questions, please post them on a Bumpy Green forum (start a Jet one!). I check in quite frequently and will try and answer them. If I can't help I'll try and point you towards someone that can. Got to go, there is a MW54 test stand that needs building!

Pictures reproduced with the kind permission of Wren Turbines
http://www.wren-turbines.com


Article by Simon Bisson for Bumpy Green
7th December 2001