# Small turbofan for VERTOL concept (I'm aloud to dream)

Discussion in 'New Member Introductions' started by Jay Marshall, Dec 19, 2005.

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1. Dec 19, 2005

### Jay Marshall

#### Guest

Hi all.

I've been working on a VERTOL concept in my warped little mind and I've been trying to sorce some two small turbofan engines with about 800-1000 lbf of thrust, Williams International do the FJ33 which is a bit big dimension wise, aparently they do smaller engines but I can't get any specs on them.

Has anybody got any ideas for a small turbo fan.

Cheers Jay.

I'm a forum virgin so I hope I push the right button.

2. Dec 19, 2005

### orion

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Physical size and thrust are somewhat interdependent so if the FJ33s are too big, it's unlikely that you'll find anything in the thrust range of the fan design that is too different. Yes, Williams makes other engines but many of those are delegated to military missions so even if they'd be willing to provide you with the basic data, it is unlikely that you'd be able to purchase them if your project ever got that far. Also, keep in mind that any engine of this size designed for a military mission is most likely a short life product (5 hrs or less).

Pratt and Whitney also make a line of smaller engines (as choosen by Eclipse) but they too have envelopes similar to that of the Williams products.

But your problem may be a bit more basic. First off, turbofans of this size are quite rare (as in none at all) and the only ones that could come even close to what you're after would be the ones mentioned above. But those have thrust ratings a bit higher than what you seem to be after. As far as I know, there is simply nothing in the range you describe.

The other issue is price. Even one of the Williams engines could buy you a very nice house or small mansion. And when it comes to jets, fans especially, there is no such thing as "entry level".

In short, dreaming is good. Unfortunately, there is at times a point when you have to wake up and deal with the real world. And in the real world there is no such thing as an "affordable" small jet engine, turbofan or otherwise.

I know of one or two projects where private individuals are working towards designing and building their own fan engines but at this point I don't think any one of them is even close to any form of actual hardware.

3. Dec 20, 2005

### CNCRouterman

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Why???

Bill,
Accepting your post as accurate, I still wonder WHY there are no affordable fan jets.

Please forgive my ignorance, but can you walk us through the main elements of turbine development?

Prior to buying my CNC Router, I had entertained buying CNC metal working machinery instead, after all, I could have bought 2 or more medium/small metal working CNC's or one modest sized one with a fair amount of tooling for the same money I spent on my router. Fortunately, I recognized that the metal machining industry in my region was in a nasty slump at that time, and I steered clear.

I do still have a hankering to pick up a rotory table 5+ axis machining center capable of cutting the exotic metals. In fact one of the demonstrations being cut when I was shopping was a turbine component, for a turbocharger I think.

The relevence being that I would expect that the capital equipment entry cost to making jet turbines would be reletively low. That flies (pun intended) in the face of observed entries (or lack there-of) in the "low"-cost turbine market.

As I know there is much more to running a CNC Router job shop than just buying a CNC Router, what are the most significant cost related aspects to producing a jet turbine for aviation use (even experimental)???

4. Dec 20, 2005

### wally

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here is my .02 worth
These are some of the dificult issues to be overcome designing a jet engine.

The materials used to make the hot section blades is tricky and expensive stuff. I think remembering blades are sometimes grown or pulled from a molten vat under vacuum and somehow formed as a single crystaline structure.

Bearings are working at very high speed and temps in small turbines and require special oil spray system for them to survive.

The laws of physics being what they are, if a workable small turbine is designed, that is the way it must be for it to work - and is probably already patented by P&W, GE or Williams.

Combustion design is cut and try for the most part. I remember hearing the early BMW-RR 710 start before they changed the burners. It made an awfull howl/racket. They knew it would need redesign from the test cell but mounted it and flew it (2 of them) on the prototype GV for a while. And they had to put bigger oil tanks on it because the carbon/carbon oil seals leaked so much. The oil tank size/ range was less than the airplane fuel range.

The fuel control will have to respond to a lot of parameters. Startup, idle, accel/deccel, top temp limits, top rpm limit, burner pressure limits and a host of unknown issues that are only guessed at until the prototype is spun up for the first time.

I just chatted with a jet engine guy I work with, besides the above, he mentioned burner design must be capable of keeping the fire going with all this accel/deccel too.

The engine will need to have a usefull range of thrust. The early Lear and Sabreliners had huge brakes just because the JT-12 idle thrust was not much different than full thrust!

Oh, it needs to be light as possible and it would be nice if it had a self starter.

I wish you great luck!
Wally, also a dreamer

Last edited: Dec 20, 2005
5. Dec 20, 2005

### orion

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Wally is actually on the right track.

There are several issues that keep turbines out of the "affordable" category. The first is of course the issues of development. Creating an efficient engine is of course much more than just bolting a bunch of parts together. The engine has to be designed as a matched system in order to achieve the efficiency most of us are after from a bypass fan. This requires specialized knowlege as well as the abitliy to accurately model and analyze the hardware as well as the flow of air, fuel, and the combustion gases. In short, there is a rather substantial amount of development that has to be done in order to come up with the optimal configuration and parts.

The next part is structural. For manned airplane use the parts need to be efficient and have a reasonably long life span. This requires specialized knowlege of the service environment the parts are subject to, as well as a suitable background in metallurgy and fabrication processes. As above, this falls under engineering and development.

Once you reach fabrication, there is more to making turbine parts than just hogging them out of metal. That in itself may be OK for a short life engine or for a technology demonstrator, but not for a production unit. For the latter, a number of the parts (high temperature applications)need to be cast out of relatively exotic alloys and under very specific environmental conditions (vacuum) in order to achieve the highest purity and the least likelyhood of voids or surface defects.

The other parts too need special attention since under load the materials will tend to creep and thus eventually start to wear. The flow of air and hot gasses will also wear on the various surfaces, thus eventually degrading the critical parts, substantially reducing the engine's efficiency and performance capabilities.

Coupled to all this are unique (expensive) components such as the rather specialized bearings, fuel delivery systems, lubrication, etc.

And finally you have to examine the issue of manufacturing. How many parts (or engines) can you make and how many can you realistically sell. Despite the seeming interest, the likelyhood is that the actual market may be relatively small and if by chance it does pick up, even then the production numbes will be low enough taht effects of mass manufacturing will most likely not be there. After all, our industry is not (nor ever will be) on the scale of Detroit and so most of the prices we have to pay will be as for a very specialized market or product line.

In short, all this translates into . For manned application it's not a good idea to look at things like the small engines that are becoming so popular in scale jet RC airplanes. Yes, those are impressive little developments but they tend to be short lived. As such, the processes that are used in their manufacture should in no way be considered viable candidates for reliable manned flight.