Turbine engine discussion

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by flyvulcan, Jun 9, 2016.

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  1. Jun 9, 2016 #1

    flyvulcan

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    In order to reduce the hijacking of Acrojets F16 thread which is about his aircraft, rather than the powerplant, I've started this thread to carry on the discussion about turbine engines as there appears to be keen interest in the subject.

    I'll start by adding to the discussion about turbofan engines on the F16 thread as follows:

    The PW610F of around 950lbf thrust has a fan diameter of less than 14" (ref PW600 | Pratt & Whitney Canada) so our 700lbf turbofan could possibly be a little smaller. The ability to fabricate a fan to suit this 700lbf turbofan engine is becoming both easier and cheaper as manufacturing technology improves.

    So maybe we can take up the discussion about small turbine engines here, rather than waylaying Acrojets F16 thread.
     
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  2. Jun 9, 2016 #2

    nerobro

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    How about smaller, and higher BSFC jets? Not that those P&W's are any slouch. I find it interesting that they're getting such a wide range of thrusts off the same core. I wonder how that relates to the design flight speed of an airframe...
     
  3. Jun 9, 2016 #3

    BBerson

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    Has any RC model jet engine builder made a turbofan?
     
  4. Jun 9, 2016 #4

    nerobro

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    Not that i'm aware of. They've made both turboprops and turboshafts though.
     
  5. Jun 9, 2016 #5

    Acrojet

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    Hope you don't mind if I chime....;-)

    Ironically the fans diameter is 14" (seems to be a common diameter. Consiquently, the Williams EJ22 had a 14" fan and the entire engine weighed 88lbs for 750lbf - don't know if that was ever officially attained, but it seems plausible)

    I've been told I can show you a cut-away drawing of the engines core with its now abandoned aft-fan module in favor of the front fan:

    image.jpg


    Peter
     
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  6. Jun 9, 2016 #6

    Acrojet

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  7. Jun 9, 2016 #7

    nerobro

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    I wonder how many times people will go for the aft fan concept. The short rotating assembly is really tempting, but you end up throwing out some pressure ratio in the bargain. And you end up isolating the shaft in a place that's hard to cool and lubricate.

    I still think they're neat. ;-)
     
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  8. Jun 10, 2016 #8

    TFF

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  9. Jun 10, 2016 #9

    Swampyankee

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    GE went the aft-fan route. It does have some advantages -- it eliminates the need for a coaxial shaft -- but the fan/turbine assembly is complex. They even did the same thing for their UDF. Since nobody else has tried, and GE does not use aft fans on any current engines, one could conclude that the aft fan is an idea whose time has come and gone, like acetylene headlamps.
     
  10. Jun 10, 2016 #10

    Doggzilla

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    Everybody keep in mind that most modern engines of this size arent really "modern" and are 25-45 years old by this point. They are hand made, or are priced at the original hand made price, even if they are CNC.

    Also keep in mind that there are loads of turboshafts that run in this power range, and have fairly decent specific fuel consumption.

    Replicating 25 year old tech with modern CNC is not an unreasonable goal in the least, especially as the price of the materials has dropped immensely since they originally came out.

    Dont forget that the first mach 2 aircraft were built using the same materials you can find in any decent machine shop.

    They already sell micro turbines for under $5k, there is not reason in hell we shouldnt have a decent General aviation turbine for the same or less than comparable piston engines.
     
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  11. Jun 10, 2016 #11

    D Hillberg

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    take a fan and mount it to a T62 T 32 150 hp @ 6000 rpm .......
     
  12. Jun 11, 2016 #12

    mcrae0104

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    Why do you think it hasn't happened, if there is no reason it couldn't?
     
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  13. Jun 11, 2016 #13

    DangerZone

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    Lousy BSFC of small turbines and high initial price..?
     
  14. Jun 11, 2016 #14

    mcrae0104

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    I think Doggz is arguing that there is no reason the price should be so high, and no reason the pressure ratios should be so low (leading to lousy BSFC). Is that fair, Doggzilla? I don't want to put words in your mouth.

     
  15. Jun 11, 2016 #15

    BBerson

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    Future sport engines cannot be introduced by large companies like Pratt or Williams because of liability.
    Future engine progress may come from the RC model industry as the demand for larger scale models get bigger and bigger.
    I searched for scale F-16 and found the largest now is 1 to 3.5, or slightly larger than quarter scale.

    I don't think they will ever get a single model engine large enough for a homebuilt.
    So two or four might work. Jetman is using four now.
     
  16. Jun 11, 2016 #16

    Glider

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    I have given up the idea of having a turboprop.

    The limited number of cycles available, before maintenance becomes a significant cost, troubles. $120k turboprop that has a 600 start limit is $200 per start (it hasn't lost all value subsequent to the magic 600, but there are maintenance events that ramp up after that limit has been reached).

    The fuel burn rate, the initial capital cost, those are somewhat tolerable I guess. But paying $200 into the kitty just because the engine started... I don't have that kind of money, it stings too much. Especially given that the intended use is to restart the engine in flight, perhaps multiple times, to self rescue.

    For someone just shy of L-39 ownership, I'm sure it is no factor.
     
  17. Jun 11, 2016 #17

    nerobro

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    There's a few good reasons. First, turbines are horrific at throttling. Reducing thrust also reduces BSFC, and can cause higher turbine and combustion chamber temperatures.

    Second, jets make a little air go really fast, which isn't the ideal for fairly slow GA aircraft. A turbofan setup helps. A turboprop is even closer...

    Finally, getting good pressure ratios as the engines get smaller gets more and more challenging. So lets say you're moving.... 1000cfm of air. (that's half the air of a cheap box fan..) If you're getting a 5:1 compression ratio, you're only moving 200cfm. That's the amount of air you get off a small desk fan. (these are points of reference, not math... having "something to think about" makes imagining what's going on, much easier for me)

    Lets start with that 20" diameter box fan. That's 314 square inches. Now, turbines typically keep the outer diameter of the steady, to take advantage of blade speed, and to not need to "also" push the air inward as it's starting to spin. At 5:1, the air passage area, is only 63 square inches. (assuming the air speed remains the same.. which may not be a good assumption.) Which works out to a 1.1" deep band around the edge.

    For a given airspeed, the boundary layer will have a pretty set thickness. Lets say that boundary layer is 1/4" At the intake, it's not a huge deal, you're losing 1/2" of your intake. By the time you get to the 5:1 compression ratio region, you're losing almost half your passage area to boundary layer.

    And now I bring up gaps. Lets say we maintain a .005 gap around the 20" front fan. That works out to .3 square inches of gap on that first fan. Or.. say... only 1/1000th of the fan area is "gap". That's a really good ratio. At the 5:1 CR area of our engine, that same .3 square inches is closer to 5/1000 of the fan area. But 5:1 is a pretty poor jet engine. only slightly better than model quality. At 10:1, your leakage area ends up being 1/100th of your fan area.

    Keeping the gaps small is important. Now core temperature matters a lot too. The as the engine warms up, and cools down, you can end up with some really wacky differential heating and cooling on the fans and turbine. This is what limits how close you can run the turbine blades to the housing, and defines how far the stators and rotors can be to each other.

    This is all "with a 1000cfm" engine. Which works out to 81 pounds per minute of air. Another perspective bit, is a J79 moves 169lbs/s or 10140lbs/minute.

    But 5:1 isn't a good compression ratio. We really need to look at something like 20 or 30:1. (the GE90 has a 40:1 overall pressure ratio) A point of comparison, the J-79 makes 13.5:1. And that's an old smoky engine design.

    And all of that assumes you're running at almost full throttle all the time. That's not realistic for GA.

    The only way I can think of to maintain good compression ratio, and variable RPM and Power settings, is with sealed combustion chambers. We "already do" pulsed combustion turbines... They're called turbocharged piston engines. This is where silly things like power recovery turbines, and such start making sense.

    And... that's a big mess.

    CNC isn't the magic you think it is.

    Which power range?

    Lets use the J79 as an example. The J79 first flew in 1955. So... 61 year old tech. Starting at the compressor, the engine has variable stator blades, to prevent compressor stall during startup, and ground idle. That setup has dozens of stacked tolerances.

    The engine uses combustor cans. They're simpler to make, and somewhat easier to design, as they can be individually built, and tested. Unlike annular combustors which need to be designed and operated more or less as a unit. They also need fewer injectors. The typical (scoff if you must) home made turbine uses a combustor can. So do some APUs.. which might be worth looking at.

    The J79 gets kinda crazy at the hot end. It's got stator and turbine blades that have cast in cooling passages. (this is beyond the typical machine shop..) The blades are used more or less as cast. But there is some very tricky things done on the blade mounts to allow them to survive the G forces they are under. Thankfully, on the GA turbine end of things, we could probally use a one peice turbine.. But blade cooling still is important.

    I've got titanium on my desk. And stainless steel. And aluminum of several grades. And various composites. (carbon, kevlar, etc..) A great many of the construction techniques are do-able at home. Forging is a bit.. pushing it. But with time, you could do even things like the carry through structures.

    That's not a great comparison. The number of precision surfaces in a model turbine are many fewer than in a GA piston engine. That's a reasonable way to figure out the cost of something. "how many precision surfaces are there". That's one reason why centrifugal compressors are so nice. They need many fewer precision surfaces to work well. The shape of the vanes are much less critical than on an axial fan, which needs each blade to be a good, and proper airfoil. Centrifugal compressors can also do their work without stators, which massively reduces the places you need to worry about precision.

    Given the cost of GA engines.. i think we "should" be able to get turbines for the cost of a new rotax... But would it be better than having a piston engine? Maybe, jets are very happy to run so long as there's fuel flow. A few "like" to have the ignitor running...

    But is that worth the tradeoff of being able to use less fuel? At least with a simple jet engine, you're going to be sucking down lots of fuel.
     
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  18. Jun 11, 2016 #18

    Swampyankee

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    Nerobro, you did a nice job.

    A large gas tubine, like a PW4088, is moving several thousand pounds of air per second -- in the neighborhood of 1.5 million cfm. Of course, most of that (about 90%) doesn't go through the core.

    Smaller engines -- the break is probably at about 50 lbm/sec -- cores tend to use centrifugal stages because an axial compressor's blades get too small, making them hard to make and more vulnerable to FOD.
     
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  19. Jun 11, 2016 #19

    DangerZone

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    I guess this could be one of those cases when you are both right, but might have slightly different perspectives. There are beautiful turbine engines made by PBS at almost every Aero Expo fair in Friedrichshafen (Germany) yet they cost a lot, from €30k upwards. The turboprop is even better, around 60kg with prop, €100k, burns something like from 20 gallons upwards an hour at best economy settings. Not really reasonable for hombuilt use, a similar power turbo ICE conversion might be burning half of that at same thrust settings, and cost much less initially.

    In other words, I think he is right that this is really possible. But the main problem is how to get the cost down and the BSFC up. Hopefully Acrojet will manage to achieve a better price/performance ratio.
     
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  20. Jun 11, 2016 #20

    nerobro

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    Off the shelf... maybe. The limited restarts on a turboprop come from what? IIRC, that's usually high turbine input temps, and potential burner liner damage. Since we're playing in a world of "lets dream up an engine." There's no reason one couldn't design for those problems....

    The drawback is engine size, and weight. We're looking at "replacing" a GA engine, so we've most likely got a lot of weight, and volume, to play with. Having an offset burner can (or two?), is a good way of isolating things off to the side, and allowing lots of space for flames to die down before they get to the precious turbine wheel.

    This is definitely a "if it's on the wishlist, it's an achievable thing". That is infinite restarts is not a pipe dream.

    Thanks, there's a fair bit of effort in that post. :)

    So how did you determine the 50lbs/second number? Solid guess? Math? Examples? I"m sitting here seriously considering buying some parts and doing some testing. ... but that's so far down my list of projects i'm not sure when I'd get to it.
     
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