Most of the turbines are single stage centrifugal compressors with BSFCs of 1.0 or higher. The are bulletproof as far as reliability, but burn stupid amounts of kerosene. The axial flow multi-stage turbine requires multiple wheels that are built to much tighter specs. I think the key to getting the price down is to build the engine and build the plane around it. The engine placement on the Sub Sonex makes a lot of sense. You would need to design the plane to have the pilots sit up front, fuel in the center, and engine in the back in smaller designs.

I think it is time to have real turbines on new planes. Ski the turboprop all together. Either have piston prop planes or jets.

The smaller the turbine, the faster it spins. At those speeds, the ring is exerting TREMENDOUS forces on the blades.

For example, at speed, a single blade on the TPE331 (41,730 RPM) is exerting about 9,600 lbs of force, ~5 tons. If that blade has to support an outer ring, not only does that go up, but there are bending forces on the ring parts that span from blade tip to blade tip.

My guess is that such a design would result in potential fragmentation from stress cracking in the blade to ring transition.


You can get much higher compression ratios per stage with centrifugal compressors. So it takes fewer stages to get the pressure ratio up. Also, axial compressors require stators between stages. Thus centrifugal compressors are fewer parts is less cost.

Mike C.

Correct Mike. The compressor in my Helicycle was a single solid disc using centrifugal flow. It spun 64,281 RPM at 100% and idled at 42,525 RPM. It was reliable to the point that you could throw a screw through it and cause no damage, but the fuel burn was way high.

Mike, as I'm sure you know, converting thrust to bhp is a slippery issue.

But my BOTEC shows that, at 250 KTAS, 1000# of thrust is about 750 hp.

[An F-15 at 600 KTAS with a notional 50,000# of thrust is about 91,000 hp.!]

Jet engines are rated at static thrust, zero forward velocity. They don't make the same thrust at 250 knots. I don't know what that is for a PW610F, but PWC does, and it isn't 900 lbf.

A rough approximation is 2.5 lbf per HP. It is a bit like comparing apples and oranges, though.

Mike C.

Cmon Greg, I need someone to fly formation with me. Quick build kits and we'll be ready for Oshkosh Anything goes wrong, we just pull the chute..

Cmon Greg, I need someone to fly formation with me. Quick build kits and we'll be ready for Oshkosh Anything goes wrong, we just pull the chute..

Jet engines are rated at static thrust, zero forward velocity. They don't make the same thrust at 250 knots. I don't know what that is for a PW610F, but PWC does, and it isn't 900 lbf.

900lbs of thrust ala the Eclipse feels more like 600HP per side when comparing a similar weighted 325 a side P Baron.

Perhaps its just too apples to oranges to compare thrust and HP but the Eclipse is making a whole lot more than 700HP on takeoff!

Mike


350 HP is roughly 900 pounds thrust (lbf). Basically, a Pratt PW610F like that on the Eclipse.

They sell new for around $300K depending on volume and supply agreements.

If you can sell them for $50K, that would be a game changer.

I suggest you start a company to do that.

Mike C.

It's harder to make an axial flow turbine because the air tends to swirl with the blades, needing stators to correct the airflow. A centrifugal compressor is very easy to make, but it's harder to get high compression ratios out of it. Many engines use a combo of both, especially gas turbines.

I also agree - we have the technology today to make turbines cheaply, we just don't for various reasons.

Concerning thrust vs velocity...

Even though you get some "ram" effect of giving the inlet air momentum, not all of that will be converted to increase the total pressure of the turbine inlet, correct? There will be pressure loss as the air slows down to enter the turbine. The exit velocity of the exhaust is constant so having an inlet velocity means there is less total velocity change between inlet and outlet, therefore less momentum change of the air meaning less thrust?

Thrust is force, change in momentum is force so is this a valid way to look at it?

I'm just waiting for someone to take this further:

http://www.flyingmag.com/news/video-ge-builds-tiny-3-d-printed-jet-engine

This would reduce manufacturing expenses, in regard to tolerances, allowing more complex ideas, like axial compressors, and higher bypass engines for fuel efficiency...all assembled at the same time. I love 3D printing.

I went out and watched every video I could find on this little jet. It's seriously cool, and so are the videos. Too bad it's not much more than a toy, as Todd mentions.

I thought the same thing: a little more power, a second seat even if tandem, and a range of 500nm... The airframe would need to grow to carry the increased weight of pax/fuel - these turbines suck (fuel)!

All the figures I could find assume flight at 10,000 feet - I wonder how it would do at 20,000. But the Sonex people don't seem to think of this as a real traveling machine - rather more of a toy or exhibition plane.

That would make one awesome RC F-16. I wonder what the actual cost of this thing would be, and what type of printer you need to get this level of precision. And, how durable the alloy parts will actually be. Wwill take many more years of R&D, and testing before this thing pushes people.

It's clear that 3D printing is a game-changer, just wish SSYS stock would reflect that. <sigh>

Did the subject of a 2-seater come up? Just wondering....