Look at the ratios on the new GE or RR turbofans. Even the Williams Engine in the SF50. A super majority of the thrust is now from the bypass fans.
Fundamentally, a turbofan is a gas generator core turing a bypass fan in the front of the engine within a common shroud. The bypass fan provides additional compression to the gas generator and significant volume of air for thrust which bypasses the the gas generator.

A turboprop is a gas generator turning a fan which is not enclosed with in the engine shroud. The prop, pay or pay not provide additional compression to the gas generator sections and the residual thrust of the gas generator may or may not provide additional thrust to the prop.

At the end of the day, the difference between the two forms of the turbine engine deal with how the primary thrust generating fan is placed, the direction of the gas generator thrust and if the fan is enclosed within the same shroud as the gas generator.

Therefore, if a turboprop can be optimized for 25K feet, so can a turbofan. It is a question of the desire, requirements and money to make it happen. Nothing there deals with physics.

Tim

MU2s? Too expensive to build, too labor intensive. Still a massively capable machine, there's nothing that can replace it from any maker today.


Transferring piston experience to jets. An example is assuming a single jet is less cost than a twin jet.

Mike C.

Everything will happen much faster. An SR22T pilot has to learn to think as those speeds.

If you depart a major metro area and you get at 10,000 ft, you are going 200 knots, climbing 2500 FPM, 20 nm from the airport, had 3 altitude assignments, had 4 frequencies, had 3 or 4 vectors, maybe had a reroute thrown in for good measure.

All in the first 5 minutes after liftoff. That's the busiest time for me in my MU2, by contrast an ILS to mins is serene.

Now add night, weather, schedule, etc. Whether that pilot has 1 or 2 engines is insignificant to his workload. Fail a generator and the SEJ pilot workload just shot up while the TEJ pilot has a minor nuisance.

Also, most SR22T pilots don't fly in the flight levels all that often. With the SF50, it will be almost every flight. It isn't that hard, but it far less tolerant than a piston fixed gear single for mistakes.

If SR22 pilots are the target audience, then the SF50 will likely be their first turbine, first jet, first retract, first pressurized, first air conditioned, first known ice, first flight level airplane. That's a lot to take in at once so I expect some tough love during their initial course and a hefty mentoring period, probably 50 hours or more. Some of those jet egos are going to get bruised.

Cessna doesn't think 182 pilots are ready for a Mustang.

Jets demand a type rating for a reason.

Mike C.

Another way to meet the pressurization requirement is to have check valves in the system and outflow valves that are protected against going above 15000' incase of a normal mode failure. Control the outflow instead of redundant inflow. All it needs to do is give enough time to make a reasonable descent down to 15000' then.

I think you just said what I said but you used a lot more words. But their are some hurdles you are ignoring.

Turbofan blades do not adjust pitch and pure jet thrust like high altitude for efficiency.

A unducted fan has been tried and did not work what else do you feel,can be done to find a replacement for the turboprop engine?

That's not true:

http://flightaware.com/live/findflight/kteb/kpdk/

I asked Flightaware tonight for ALL recent flights TEB to PDK. 9 came up, all business jets. The altitudes they got were:

C56X FL400
C750 FL430
C550 FL380
C680 FL400
CL60 FL340
GLF5 FL430
CJ3 FL430
C56X FL400
C560 FL400

The lowest one, FL340, was a CL60, filed for FL360, flew at FL340, perhaps a turbulence thing. It was the ONLY one where they didn't fly at or HIGHER than the filed altitude.

It isn't difficult to check these things out.

Mike C.

Tune the jet for the correct altitude. I am getting really far outside my depth here, but if I follow it correctly, you need to decrease the compression ratio in the gas generator, add ducting from the bypass through the hot section to reduce temp in the hot section, increase the bypass ratio.

Tim

That's not true:

http://flightaware.com/live/findflight/kteb/kpdk/

I asked Flightaware tonight for ALL recent flights TEB to PDK. 9 came up, all business jets. The altitudes they got were:

C56X FL400
C750 FL430
C550 FL380
C680 FL400
CL60 FL340
GLF5 FL430
CJ3 FL430
C56X FL400
C560 FL400

The lowest one, FL340, was a CL60, filed for FL360, flew at FL340, perhaps a turbulence thing. It was the ONLY one where they didn't fly at or HIGHER than the filed altitude.

It isn't difficult to check these things out.

Mike C.

Some cautions here. The CJ2+ is going to climb faster with two and twice weight than the single with one and half weight. It just has less frontal area per pound of thrust and a smaller wing per pound of thrust than the SF50, so more engine goes to climb and less to drag.

Fuel flow does not scale linearly with thrust. You assumed 1900/2500 fuel ratio. Doesn't work that way.

A 600 nm trip is not two hours with a 300 knots cruise. It is more like 2.2 hours. Climb and descent is not 300 knots over the ground.

The single engine on the SF50 is aerodynamically not as efficient due to canting, pitch effects, and pitch trim drag of the V tail.

The CJ2+ at 311 knots is pulled back. The SF50 at 300 knots is top speed. Fuel flows don't compare.

Basically, you can't really scale the SF50 as half a CJ2+. Too many variables not in alignment.


Yes, I was only talking about the concept of a light twin jet and using the Eclipse as an example of the efficiency possible, not saying it represents what one should get.

Cirrus could be building a Garminized, solidly supported, less weird, composite version of the Eclipse with two PW610Fs. Now that would be SOMETHING!

Mike C.

If you lower compression ratio from compressor you lower thrust. You can't take bypass air and reroute into compressor without losing thrust from fan.

Early jets used a centrifugal compressor that had lower compression but was quickly replaced a axial compressor because of efficiencies.

Physics does play a part here, there simply is not a metal strong enough and light enough and cheap enough to use for pitch adjustable blades. Someday maybe one will be created.

Pathetic yes, but still would have been a longer flight and burned a butt load (technical term) more fuel if they had not climbed all the way up.

They are taking it now. It is pass/fail. So far, every other student has failed and there are many (Gulfstream, Stratus, Piper, Diamond, Eclipse, and a few more...). They do not grade on a curve.

The graduate students are not building SEJs. Hint, hint.

Mike C.

Cheap Jet = homerun sales.

I don't think the chute makes that much difference here, nor the 1 versus 2 engines. It is all about being a cheap jet.


Ah, easy then, no deliveries necessary.


Don't believe you. Those folks won't sit still for 3 weeks in a type initial course just to fly a slow jet.

Why aren't those folks buying Eclipse, Mustang, etc? Because the Hamptons folks aren't owner flown.

Mike C.

Conceptually, none.

My airplane could be described as having ultra high bypass unducted variable pitch turbo fans.


When you optimize the turbofan for 25K, the fan will turn out to look like a prop. The rotor diameter will grow to the point the shroud is no longer feasible, so it becomes unducted, and it gets much fewer blades.

Unducted turbofans have been tried:



Looks remarkably like a prop, no?

More info:

http://en.wikipedia.org/wiki/Propfan

Don't think this is going to satisfy the jet ego requirements, and and where can you stick this on an SF50?

Mike C.

Hey Mike. I think that after 39 posts on the subject, we understand that you think building a SEJ is a bad idea, destined to fail.

The SF50 is a SEJ. I believe it's too late for you to save Cirrus.

Can we move on with life now?