Scaled Jets (F-16)

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DangerZone

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It seems to me a low cost turbofan would be better off using twin centrifugal compressors (staged) rather than multiple axial stages. Way cheaper to machine and assemble and would get you to the 9-10 pressure ratio range. A geared fan would massively increase cost and complexity of design/manufacturing.

No problem to find CNC machines capable of machining nickel alloys. Ceramic tools can wail on Inconel or hard 17-4 at amazing rates, but that machine time will be expensive for sure. Luckily we're talking pretty small parts here for this thrust range so material costs and machining time is lots less than 5000 lb. class engines.

IMO, getting a design that works well and reliably with decent SFC and surge margins will be a lot harder than machining the parts for it.

The key to this whole project is the engine and that's going to be the hardest part too I think.
Wasn't there some small turbofan/turbojet engine with dual centrifugal compressors (each in the 5-6 pressure ratio range adding one to the other to get a 10-11 pressure ratio range) available on sales a couple of years ago?

And what would be the losses compared to an axial compressor? Modern turbine engines seem to be losing 30% to 50% of power just to drive the compressor... For a scaled jet, such losses would be a major obstacle in achieving good performace.
 

Lucrum

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...And what would be the losses compared to an axial compressor? Modern turbine engines seem to be losing 30% to 50% of power just to drive the compressor... For a scaled jet, such losses would be a major obstacle in achieving good performace.
FWIW, 2/3 of the T-58's power produced is used just to turn it's 10 stage axial compressor
 

BoKu

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...Modern turbine engines seem to be losing 30% to 50% of power just to drive the compressor...
That's kind of like saying that a four-stroke reciprocating engine loses a lot of power compressing the fuel/air mixture during the compression stroke. That power isn't lost; it is an unavoidable investment in the combustion process. And the ideal gas law being what it is, a lot of it is recovered as the engine re-expands the exhaust gas through the turbine section.
 

BBerson

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To heck with the airplane; if you can supply engines that meet that spec and price point the aviation world will beat a path to your door.
Has any individual ever designed and built a Homebuilt size turbofan engine?
I asked John Monnett why he didn't use a turbofan instead of turbojet. He seemed insulted by the question, apparently suggesting it was impossible.
Then his test pilot said it just doesn't scale because of the blade clearance gap issue or something.
So, just wondering why they would assume a small turbojet works but not a turbofan?
 

DangerZone

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That's kind of like saying that a four-stroke reciprocating engine loses a lot of power compressing the fuel/air mixture during the compression stroke. That power isn't lost; it is an unavoidable investment in the combustion process. And the ideal gas law being what it is, a lot of it is recovered as the engine re-expands the exhaust gas through the turbine section.
That is just partially true, some engines lose more than others. That's why we have large differences in BSFC.

In the car industry this might be more obvious. A well designed turbo diesel like the latest 240HP VW 2.0TDi might consume a lot less than some other turbocharged engine in the same power range. Put a supercharger on an engine instead of a turbocharger and the losses rise even further.

Sure, some gasses will be reused to produce more compressed air, but this is far from saying energy is not lost in the process.
 

nerobro

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Williams has been doing this since the 1970s with the FJ-33, well before CNC or laser balancing equipment existed. It should be relatives straightforward to produce an engine with the same efficiency using off the shelf controls and CNC parts. The average machine shop actually exceeds most aerospace companies back when this stuff came out.
The problems come in with scale. The few thousandths of an inch blade clearance you have on a big jet engine, are really close to what you need on a small engine. Those little gaps become big leaky messes on smaller motors, as they're moving so much less air. And then you have thermal issues, the mass to area ratios are not favorable as you shrink down the engine. And then we need to talk about boundary layer effects. With a big engine, you have room to allow the boundary layers to be there. And then as things get smaller, keeping the fire alive gets a lot more difficult.

Scaling engines like this... isn't as easy as one would really like.

Wasn't there some small turbofan/turbojet engine with dual centrifugal compressors (each in the 5-6 pressure ratio range adding one to the other to get a 10-11 pressure ratio range) available on sales a couple of years ago?

And what would be the losses compared to an axial compressor? Modern turbine engines seem to be losing 30% to 50% of power just to drive the compressor... For a scaled jet, such losses would be a major obstacle in achieving good performace.
Pressure ratios, IIRC, multiply. I'd be really surprised if those compressors were running in the 5-6:1 range. I'd be really shocked if both compressors were operating at more than 4:1.

Turbine engines don't "lose" energy to driving themselves. They use it. Every engine needs some energy to run. You can do a little math, and come up with some shocking numbers. Horsepower is energy. Joules are energy. You can calculate the energy in a fuel, compare it to engine output and determine how much energy goes to cooling, and moving the engine.

Good performance (efficiency is how i'm reading that) comes from high pressure ratios, and high bypass ratios. Figure out how to get good pressure ratios on a small engine, and you'll make a fortune.
 

Lucrum

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The problems come in with scale.....And then as things get smaller, keeping the fire alive gets a lot more difficult.

Scaling engines like this... isn't as easy as one would really like....Good performance (efficiency is how i'm reading that) comes from high pressure ratios, and high bypass ratios. Figure out how to get good pressure ratios on a small engine, and you'll make a fortune.
I said as much earlier in the thread. Just not as detailed or as eloquently.
 

rv6ejguy

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Modern 3-5 inch aluminum forged/billet centrifugal compressors in turbochargers are around 5 to 1 PRs and over 80% efficient. Larger ones for this size engine should be better than that, especially if you went to higher strength materials than aluminum.
 

Doggzilla

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The problems come in with scale. The few thousandths of an inch blade clearance you have on a big jet engine, are really close to what you need on a small engine. Those little gaps become big leaky messes on smaller motors, as they're moving so much less air. And then you have thermal issues, the mass to area ratios are not favorable as you shrink down the engine. And then we need to talk about boundary layer effects. With a big engine, you have room to allow the boundary layers to be there. And then as things get smaller, keeping the fire alive gets a lot more difficult.

Scaling engines like this... isn't as easy as one would really like.



Pressure ratios, IIRC, multiply. I'd be really surprised if those compressors were running in the 5-6:1 range. I'd be really shocked if both compressors were operating at more than 4:1.

Turbine engines don't "lose" energy to driving themselves. They use it. Every engine needs some energy to run. You can do a little math, and come up with some shocking numbers. Horsepower is energy. Joules are energy. You can calculate the energy in a fuel, compare it to engine output and determine how much energy goes to cooling, and moving the engine.

Good performance (efficiency is how i'm reading that) comes from high pressure ratios, and high bypass ratios. Figure out how to get good pressure ratios on a small engine, and you'll make a fortune.
Im not really sure why you are talking down to people if you have both not read anything they said, and have little to no knowledge of any ACTUAL engines in that size range.

They have been making turboshafts for half a century that are somehow not limited by any of the thing you just listed. Williams has been making engines only slightly larger that somehow are not crippled by these effects either.

Even with a 180 degree turn of the ducts, turboshafts still nearly reach the efficiency we are looking for. Before the prop is counted. We are just counting the thrust the hot sections produce.

We arent talking some little single stage turbocharger based aluminum impeller piece of crap here.

The limitations you are talking about are on engines that cannot even match a turbocharger.
 

nerobro

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Modern 3-5 inch aluminum forged/billet centrifugal compressors in turbochargers are around 5 to 1 PRs and over 80% efficient. Larger ones for this size engine should be better than that, especially if you went to higher strength materials than aluminum.
I'd need to do some poking around. Sadly, most compressor maps I have access to, have max pressure ratios around 5, but that's well outside their efficiency islands. ... then again, I've not looked all that hard. :)

This is starting to feel like "new thread" time.

Everyone Else, you can ignore what follows.

Im not really sure why you are talking down to people if you have both not read anything they said, and have little to no knowledge of any ACTUAL engines in that size range.

They have been making turboshafts for half a century that are somehow not limited by any of the thing you just listed. Williams has been making engines only slightly larger that somehow are not crippled by these effects either.

Even with a 180 degree turn of the ducts, turboshafts still nearly reach the efficiency we are looking for. Before the prop is counted. We are just counting the thrust the hot sections produce.

We arent talking some little single stage turbocharger based aluminum impeller piece of crap here.

The limitations you are talking about are on engines that cannot even match a turbocharger.
Hmm.. white knighting on HBA. I would have never thought that would be a thing. I don't think Lucrum and I have any beef. So... you can put that to bed.

To quote yourself to you.

doggzilla said:
Williams has been doing this since the 1970s with the FJ-33, well before CNC or laser balancing equipment existed. It should be relatives straightforward to produce an engine with the same efficiency using off the shelf controls and CNC parts. The average machine shop actually exceeds most aerospace companies back when this stuff came out.

Most of that quote is poo. Why do we care about Williams making bigger engines in the 1970's. CNC actually rolled around in the 1960's. Why do we care about laser balancing, there are other methods for getting good balance. Those.. aren't constructive, or even accurate things to say.

The "average machine shop" is not better than 1970's machining. Definitely not better than "most aerospace companies" in the 1970s. Some shops have machining centers that are quite capable, but they still need good operators, and intelligent programmers to make them turn out even moderately good parts. (I listen to the laments of people in the aerospace industry daily, about that very problem.)

My response to you, was a constructive one, based on the context of what you posted. Your suggestion was that it would be relatively straightforward to turn out an equivalent engine out of an average machine shop. First, I don't think that's true. Second, scaling at 1500lb thrust engine down to 600lb thrust is going to take a whole shedload more engineering than "straightforward".

I don't think you're dumb. If I did, I wouldn't have bothered responding. It just seemed clear that you didn't understand what was involved in scaling an engine.

Turboshafts have been limited by everything I've listed, forever. Scaling down is a problem, and has been a problem, forever. For example, the WR24-7-2 has interference fits that wear into clearance upon starting. (Because you get all uppity about turbochargers, did you know that same technique was used by toyota on the gen 4 supra's turbochargers!)

Williams is just as limited as anyone else would be. They're just better at it than most. And more importantly, have found reasons and markets for the smaller motors.

Quoting you again..
doggzilla said:
We arent talking some little single stage turbocharger based aluminum impeller piece of crap here.

The limitations you are talking about are on engines that cannot even match a turbocharger.

And... this is again some word salad. Hard to digest, and even less tasty.

You're assuming I'm talking about converted turbochargers. And then you're calling them crap. Well... You're aware that the PT6 has a centrifugal compressor as it's final compressor stage right? And the FJ44.

The limitations i'm talking about, are the factors you must consider on every internal combustion engine.

So.... i'm going to assume the point you were trying to make was you thought I was talking down to you and others. Weaseling out the point of your other posts gets hard, as they don't point in one direction very well.

The problem with engines is that they are charging prices for hand made parts, yet everything is CNC now. There isnt anything required that cannot be bought off the shelf.
"Engines" or "Certified Engines". Either way, your statement isn't very solid.

The controls should probably come from turboshafts, as those are the most reliable systems from this power range, but otherwise the parts can be manufactured fairly easily to the standards we are looking for.
The controls are best made for the engine in specific. The size and weight of the control system is directly related to the fuel demands of the engine. Good metering comes from having pumps sized to match the demand.

The most expensive single tool somebody would need is the balancing machine, which is like $50,000 used, and which needs programming to be able to do whatever specific balancing task is required. These machines are what makes it possible to make small engines and turbochargers. The same machines balance full size turbines. It is the most important tool. They spin the part and then tell you which part needs a little shaved off of it, which allows you to make up for manufacturing imperfections. This dramatically improves the reliability of components, and is basically required.
If it's all CNC, and should be cheap, why are you now bringing in hand finishing up?

The parts can be made on CNC or cast, then they are balanced using the balancing machine to make up for any imperfections. This greatly improves reliability and simplicity.
In a turbine wheel, or other high stress or precision application, the typical part will be ~both~ cast, and cnc machined, or if it's really a stressed part, it will be forged, then machined. You're using CNC to refer to "cut from billet" which has it's own set of problems. If you're making say.. a dozen center shaft supports, and you have a large aluminum plate bolted to the tombstone that's going to go in your machining center.. you might discover that those parts all don't come out the same shape. Plates, billets, often have itnernal stresses that get exposed as you machine out of a plate. Whoops....

Controls are the most important component and so Im going to say its best to simply buy existing controls that are proven, most likely from turboshafts. Having familiar controls means that mechanics will be more confident working on them and be much more trusting of the engines. If new controls are developed, mechanics may refuse to work on the engines because they are unfamiliar and seem untrustworthy. Using existing proven control equipment will give people much more confidence in the engine, since it will seem much more similar to them. Thats really the only components I really think needs to be bought off the shelf.
You're going to need to convince someone to sell you those parts, unless you're talking about automotive fuel pumps, you're talking about jet engine fuel pumps. That's going to be a challenge on it's own. And they need to be about the right size to start out with...

Components like combustion chambers can be copied from large turboshafts if required.
Air does not scale like that. "Just copying" a combustion chamber will lead to crazy orifice sizes. Flow through a hole, not counting boundry layer effects, scales with the cube, not linearly. And then you need to consider the flow is going to be in a smaller space.

Bah. I'm not an expert. But I think you're in a weird spot on this. You want me to be wrong. Are you angry and maybe i'm pointing out things you didn't think about? I'm resisting doing the Canadian thing, because i'm not ~actually~ sorry.

I'll say it again, I don't think you're dumb. You've pointed out useful things in this thread. The link you posted to the F4 control systems was pretty neat. (I'll agree with others that it's not simple, just clever.. the two don't necessarily run in the same circles.)


 

DangerZone

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Pressure ratios, IIRC, multiply. I'd be really surprised if those compressors were running in the 5-6:1 range. I'd be really shocked if both compressors were operating at more than 4:1.
Good point, I don't trust manufacturer's specs also unless their numbers are verified. It's like the advertisement for my 250HP+ BMW, the manufacturer claims it burns only 12 liters of gasoline in the city and 8 liters on the highway for every 100km. The reality is 17 liters and 11 liters respectively, on a good warm day without the AC running. Turn the AC on and it rises to 18 liters and 12 liters for the same distance. Thus I guess these numbers are what the manufacturer claims is achieved in ideal conditions at very specific rpm and engine settings. On average, it is probably around 4:1 tops just as you said.

Turbine engines don't "lose" energy to driving themselves. They use it. Every engine needs some energy to run. You can do a little math, and come up with some shocking numbers. Horsepower is energy. Joules are energy. You can calculate the energy in a fuel, compare it to engine output and determine how much energy goes to cooling, and moving the engine.
Basically, you are right from a theoretical point of view. In reality, you are also right that these turbines use energy but the question is how efficiently they use it. It could be a language thing, for me all energy that is transformed into heat instead of work is 'lost' in efficiency calculation. I have no use of energy which rose my engine temperature instead of producing more output power or useful work. For example, at same power settings of 200HP and same speed, my engine will be burning 36 liters while my friends engine (different manufacturer) will be burning much less, around 28 liters. Or a better example with aircraft turbocharged engines, the Piper Malibu with a Continental engine had a range of 1500 nautical miles compared to the Lycoming's 1000 nautical miles range at same power and cruise speed settings. That's a big difference, a 50% difference in efficiency. So yeah, this energy was not lost into thin air, it simply turned to parasitic heat instead of work. Expecting a turbine engine to perform differently and not comply to these laws of phyisics might be a bit optimistic, but that's just my point of view. Maybe there is a way to turn all Joules from the fuel to useful energy, and I'd really like a turbine engine like that. :)

Good performance (efficiency is how i'm reading that) comes from high pressure ratios, and high bypass ratios. Figure out how to get good pressure ratios on a small engine, and you'll make a fortune.
These guys claimed to have used the compressor stages of a large automotive turbocharger with best efficiency on tests. To cut the costs down, they used these as the centrifugal compressor stage because small turbines have better use of such compressor than an axial flow one. So their turbojet/turbofan/whatever had a large hole/inlet in the front of the engine instead of the typical cone that turbofans/turbojets usually have. I haven't seen them since then at Aero Expos and have no idea how they are doing, it seemed like an interesting idea at the time. It could be they were led by the same idea to make a fortune cause small turbine jet engines were in demand. Now, with the recession going on in recreational aviation, it might be questionable if such a small engine would have a lot of buyers and be able to make a fortune.

Modern 3-5 inch aluminum forged/billet centrifugal compressors in turbochargers are around 5 to 1 PRs and over 80% efficient. Larger ones for this size engine should be better than that, especially if you went to higher strength materials than aluminum.
In terms of defining 'larger' turbochargers, what sizes would those be..?

... You're aware that the PT6 has a centrifugal compressor as it's final compressor stage right? And the FJ44.
Is there a see through of the Williams FJ44-4 engine to see what the compressor, fan and internals look like..?
 
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DangerZone

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Has any individual ever designed and built a Homebuilt size turbofan engine?
I asked John Monnett why he didn't use a turbofan instead of turbojet. He seemed insulted by the question, apparently suggesting it was impossible.
Then his test pilot said it just doesn't scale because of the blade clearance gap issue or something.
So, just wondering why they would assume a small turbojet works but not a turbofan?
Hovercraft ICE ducted fans have problems keeping the blades in place and preventing them from flying out of the hub at high power settings. And these are subsonic blades, less than 1m diamter and at lower power settings, around 100HP to 300HP. A small homebuilt size turbofan engine fan diameter would be what, something like the FJ-44-4 fan at around 60ish cm (2 feet)? Add power, add speed, add momentum (torque), and keeping these blades in the hub becomes a real challenge. And a costy one. But that's just my guess, there are probably other reasons too.
 

rv6ejguy

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Turbos come in pretty much any size you want from 1 inch diameter compressors to ones the size of your car. The purpose built ones for drag racing are still pretty efficient at 4.5 to 5 PRs.
 

nerobro

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I've started a new thread about small turbine engines to prevent Acrojets thread about his aircraft being waylaid any further. Shall we move the turbofan engine discussion over to that thread and leave this thread to be specific to Acrojets very interesting aircraft project?
Quite!

Acrojet, we're excited to see your engine, and your airframe. I wish you the best of luck.
 

Acrojet

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Quite!

Acrojet, we're excited to see your engine, and your airframe. I wish you the best of luck.

Thanks!

As we can see, the engine is a very big part of the equation of wether or not this endeavor works out.
The good news is this engine is essentially a "clean sheet" design. It was/is designed as such based on existing small turbojet technologies as its core. It will be a blend of engineering/fabrication coupled with efficiency and low cost.
It will also be a plug-and-play system. Everything will be included. Many times people/small companies will sell you a motor and then you need to go on a wild goose hunt to find all the components (accessories) to make the darn things run. I hate to use the analogy...R/C turbojet makers make petty good attempts to sell their engines as an "everything included" package. The same effort to do that same philosophy will be applied here.
FWIW, I was just told that the fan has been made. When I get pictures and schematics that I can share, I will..,,.

Peter
 
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