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200,000rpm 20Hp gearbox 2

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smi3th

smi3th

Aerospace
Nov 11, 2011
6
US
I hope this is a good place to ask this question...I'm a physicist that got into something where I don't have any practical experience; A friend of mine, an automotive transmission-designer, recommended this website.

I have an engineering problem that can be solved by a small turbine engine developing about 20hp. These things exist, but the shaft rpm is in six figures and I need an output rpm in the low four figures.

Does gear-technology even exist to deal with these speeds and power levels?

The simple answer I need is size and weight, and someone who already makes these things.

If all I can get is sympathy and some discussion of the technological issues, even that would help <grin>.
 
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You may find your answer on an aircraft modeling forum.

A quick search found a turboshaft engine of 7.5HP @ 9000 RPM:


This one is a bit bigger:


Here is a list of other manufacturers (not all have turboprop/turboshaft):


Are you sure that a turbine is really your optimal solution? Is specific fuel consumption a factor or does your engine only need to output power for a relatively short period of time?

Von Braun might point out that a hydrogen peroxide powered pump works pretty good. If you want to consider going that route, contact my old friend Juan:


I don't think Juan has dealt with H2O2 turbopumps per say but his catalyst packs are excellent. He certainly is up to the challenge. This guys VW gives you the idea (turn down your headphones):


If you provide more details on your project, you can get more help.
 
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I'm not aware of gears that can survive what you propose.

There are other ways to get power out of a high speed shaft. One such:

... which is basically an AC generator that runs at turbine speed, in their case 25k..75k rpm, maybe a bit more. Turbine and generator are coaxial, coupled by a skinny solid shaft. The AC is rectified, stored in a battery to deal with transients, and inverted to drive an AC bus at a regular powerline frequency.



Mike Halloran
Pembroke Pines, FL, USA
 
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that's very helpful. Thank you, all. I had no idea that "model" airplane hobbyists had gone to such extremes.

I see that these are all single-stage turbines, and their fuel efficiency is about three pounds per horsepower-hour. These also seem to be a nominal five horsepower, and so I would need two such engines to develop a decent ten hp, about half of what I would have liked. Their fuel consumption is about six times what a good multistage turbine uses, and for one to four hours running time the fuel consumption is more than I want to carry, but barely tolerable.

I see one of those turboprop units actually has a gearbox, and evidently survives the 25 hours between recommended overhauls, although it's not clear if that is at full-speed. 25 hrs is about six times the 4 hours of running time I need, so that would work. That suggests that someone has done something with a gearbox design that survives a useful amount of time.

In terms of technology, this does not seem that far off. I spend some time as an engineer in the aerospace industry, and it's axiomatic that an engineer's estimates are off by a factor of pi. A physicist, on the other hand, is happy to get within an order of magnitude or so <grin>, and we seem to be somewhere in that box.

I have looked at the turbine driving an alternator; the electrical interface is a different kind of transmission. The electrical power would feed an inverter, be converted to high-frequency A. C. and used to drive a motor that would turn a propeller at some thousands of rpm. I ultimately need to turn a prop. The motor to be relatively small and light would need to run at some high speed (a few tens of thousands of rpm?), and have a modest gearbox of its own. All these things add to cost and system complexity. The ridiculously-high-speed gearbox is preferable for those reasons, but may not be practical to build and have last any length of time. On the third hand, if it was inexpensive enough and highly-reliable for four hours, that would be acceptable.

Do any of you forumites know more about gearbox feasibility, or is there another forum where I could pose this question? I looked over the forum lists here for quite a while, and finally settled on this one as the best place to start.
 
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Not too sure of the exact speeds but air tools often have very high speed turbines, which presumably need gearing down (efficiency is not an issue).

Frankly the electrical approach offers so many advantages that it would be my primary option.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376
 
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A friend of mine spent much of WW2 rebuilding B29 turbosupercharger speed increaser gearboxes. Actually, what he was doing was building entirely new ones from spare parts stock, because when they failed, they basically fragged everything within and around. ... and their output shafts were spinning at what today is a pretty modest speed.

I think if your gearbox self-destructs after four hours, you're going to get really good at building new ones. ... and you'll get really tired of doing so.



Mike Halloran
Pembroke Pines, FL, USA
 
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I need the gearbox to work *reliably* for four hours. If I know it became unreliable after, e. g., 10 hours, I will just replace the gearbox after each four-hour operational cycle. I could live with that.
 
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I know that helicopters commonly use turbine engines coupled to gearboxes to power the blades... somewhere to look for answers?
 
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Hi Smi3th, do you have nothing else to power but the propeller? If you have a significant sensor load, there is a definite advantage to the proposed AC generator.

All the turbines I provided links are of the turboprop/turboshaft type and therefore have a gearbox. I would at least talk to them about possibly providing a scaled up solution.

Are you sure you need a turbine at all? Why not use an internal combustion engine? It will be tough to match the simplicity, cost, and weight when you consider fuel consumption over 4 hours.





You have of course stumbled on the basic problem with turbines for propeller or rotor drive- they seem like a great option but their rated HP is at high RPM and low torque but your output is low RPM and high torque. At low HP, usually internal combustion engines are used. Burt Rutan built a small one-place aircraft with a 20HP industrial engine that did 110+ MPH cruise:

 
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I understand the advantages of an A. C. generator, and also the disadvantages of a power-processor driving a not-that-small high-speed 10-20 hp motor via a high-frequency polyphase inverter. I have designed power processors, from tens of watts to hundreds of kilowatts. It's not a trivial matter, particularly as regards, size, weight and cost, both manufacturing and engineering. In this case my electrical load is negligible by comparison with the other issues.

Thank you for that two-stroke engine suggestion. I had not considered two-stroke engines, and did not know of those you referenced. A two-stroke engine will surely be quieter than a four-stroke with all the valve-noise, etc.

It is true that the two-stroke engine delivers its torque directly. I see that the one you referenced has about half the fuel consumption of the single-stage turbojets but about double the fuel consumption of more efficient multi-stage turbines [which weigh about a quarter of what the 15hp, 2-stroke engine does. I note the 28 Hp model is somewhat lighter and more efficient on a per-Hp basis.]. For a short mission duration, the reduced fuel payload appears to be a tradeoff against the increased engine weight, and I will add this into the decision-matrix.

I still find it interesting that the small "model airplane" turbojet manufacturers can make a gearbox that apparently survives 25 hours at six-figures rpm. I will query them further about that.
 
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Smi3th, I'm sure there are many more power-plant options available than those provided here. I would make some posts on aircraft "home-builder" or modeling forums. Your application is right in their alley and they have a lot of practical experience.

Here is a larger turbine to consider:

 
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This presentation from an Israeli source claims your application likely falls into the range where 2stroke and Wankel types dominate:


I have also read some things about a strong inverse relationship between HP and fuel consumption per hp for turbines/turboprops. Seems intuitive.

Here is another 2 stroke engine that delivers power in your range:

 
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The Israeli presentation from that engine manufacturer naturally favors *their* products. They don't evidently make turbines, such as do the hobbyists you mentioned or the high-tech (multi-stage) turbine manufacturers such as Azmark Aero Systems (formerly M-DOT), who can make turbines with weight of 2 Hp/lb and fuel consumption well under a pound per Hp/hour of fuel. That's the order of magnitude of range of performance available in the marketplace today.

The gap I found was in the over-100K rpm, under-50 Hp range, which appears to be a technology-limit for gearboxes.

I have asked the Wren and JetCat folks for gearbox life information at max rpm; their info-pages say 25 hours maintenance interval, but it's not clear if the gears or lubricant holds up that long running at-the-limit.
 
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Sounds like you are on top of it. I agree with your general sentiment that any of the published numbers are to be taken with a grain of salt. To really compare apples to apples you will need to do some comparison testing/weighing.

I wouldn't throw out the UEL engines. The presentation does list a number of engines besides their own. It is important to note that the Isrealis purchased UEL to meet a need for their UAV designs and they use a number of engine types from different sources.

The nod of approval from the Israelis still looks pretty good to me at least for initial consideration. I've worked on another UAV project with another Israeli firm and found them to be exceptional engineers with a lot of UAV expertise. Their homegrown industry has an impressive UAV development record.

You may find your optimal solution is still a turboshaft given your intermediate 4hr flight time...but I wonder if you will find it worth the extra cost.

 
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Cost is an interesting variable. I never would have thought of hobbyist sources, but look at the Horsepower-per-Dollalr one can get!

A turbine has very few moving parts, and the potential for very decent fuel economy. The question of headwind or tailwind and planning for the whole activity can affect the fuel needed by a factor of easily three, and in some circumstances it appears the 5-7 Hp designs are barely-viable, but viable enough, and the cost is quite low enough. An improvement in that technology by merely a factor of two would get me in the ballpark for this application. I don't think there has been a push to advance that technology up to fill the gap from below.

Perhaps that will happen soon.
 
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>>>
A turbine has very few moving parts, and the potential for very decent fuel economy.
<<<
The potential has so far been under- realized.

The Capstone units do surprisingly well.
They include a recuperator that brings fuel consumption down to only 15-ish pct worse than an equivalent Diesel.



Mike Halloran
Pembroke Pines, FL, USA
 
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