Not a jet.


Not when limited to FL250.

Jet efficiency requires flying higher. Flying higher requires two engines. It is that simple.

Mike C.

It isn't making the systems work, it is making the systems work RELIABLY in the face of PROBABLE engine failure. When you had two engines, the power source reliability was effectively assured because you had TWO engines, now it is not. This flows down through all of the reliability analysis of every system. Most of the work of certifying is showing how the systems work in the face of FAILURES.


The size difference between PW610F and FJ33 is not enough to greatly affect SFC. Further, the installation compromises for an SEJ eat up all the SFC advantage and probably more. So the SFC argument only works slightly when considered in isolation, not in an actual installation.


Mostly correct. There are installation compromises (angled engine axis, oversized V tail, trim drag) that do compromise the SEJ somewhat, but the primary issue is that the SEJ isn't allowed to fly high because it isn't reliable enough to be a space ship in super thin air. If it was, it would achieve most of the efficiency of the twin.


There are many worse answers. For example, adding an APU.

Every jet maker knows what it takes to certify a jet. None of them are even talking about an SEJ.

The maker who wanted the CHEAPEST and MOST efficient jet, Eclipse, used TWO engines. What does that tell you?


For 50+ years.

If you think there is a compelling advantage to being a single, wouldn't that have happened by now? Well over a hundred twin jets have been certified, not one single.

Jets need to go high to be efficient. To go high you need two engines. Cirrus has accepted they will not be allowed to go high with their single. Thus the SF50 will be inefficient. There's nothing to argue here, that's the way it is.

Mike C.

What's different about a single turbo prop?

It can be efficient in the 20s.

Even there, though, being a single isn't a huge advantage. The TBMs and PC12s are burning similar fuel as I am with two engines, and oddly enough, their engine reserve is quite a bit higher for their one engine than my two.

Both the extra fuel burn and the high engine hourly costs are a direct result of being a big block PT6, the -6x series.

Mike C.

They don't build Mits anymore. Stop comparing the 2.

I think we have a long long way to go

Besides it imperative we reiterate everything at least another six or seven times. We've got to make 50 pages at least

I suspect the SF50 will (like many other turbines with hydraulic gear) have a gas cylinder for emergency gear extension. Or, if it's electric, there would still be plenty of battery left after an engine failure to lower the gear.

As for whether the chute lowers the gear automatically or not, I assume that will depend on the gear extension speed and what happens if you bust it.

In the Citation, Vle is 250 and I have no idea what happens if you drop it faster than that. In my Bonanza, Vle is 145 and the gear doors will be bent higher than that, but the gear will still come down.

Is a single jet is more efficient than a twin?

We do have some data that is almost directly head to head under similar conditions:

Cirrus **unverified** published data: 62.8 GPH, 300 KTAS, FL280.

EA500 **observed** data: 62.9 GPH, 305 KTAS, FL270.

Note that if the EA500 flew at FL280, the fuel flow would go down a bit and speed up a bit, so the twin would improve a bit over these numbers.

Now the EA500 goes up to FL410 and really improves its efficiency. The poor SF50 is stuck down low.

A twin jet is MORE efficient than a single because it can fly high, and it is about same efficiency at the same altitude and airspeed.

NO TWIN PENALTY.

Yes, this is different than piston airplanes.

Mike C.

I'd say there is a twin penalty: Two engines to repair (I've read your arguments about the economies of scale on the PW vs the FJ, but your underlying assumption is that the FJ won't reach critical mass. I consider that a very big assumption.)

Those published numbers from Cirrus change the economics of the SF50 a bit if they are point in time numbers. Since the chart is range vs speed, it's entirely possible that they used block to block averages instead of point in time.

Your Eclipse numbers are point in time. The block to block on those things is probably closer to 70? (Please, one of the Eclipse owners chime in here).

By definition this is an emergency situation. You may not have hydraulics, electrics, etc. This is why a free fall or blow down system (gas cylinder as you say) would be preferred.

The emergency could be, oddly enough, unable to extend the gear. This isn't a question with the SR series, but would the AFM say belly it in or pull the chute?

Hmm, is Cirrus going to get an ELOS (equivalent level of safety) approved that says they don't need a manual gear extension method because they have a chute? They did that for the spin recovery requirements on the SR series. A 6000 pound plastic airplane with a ton of fuel on board skidding on a concrete runway might be a hull loss anyway, but I would want an emergency method to at least try before I do that. I've read of far too many people with gear problems who were successful with the emergency extension.


If you pull the chute, the landing gear (and the rest of the airplane) will never be used again, and you are extending it just for energy absorption, so I don't think Vle matters much. Whatever ends up sticking out of the bottom is what you will get.

The fact the SF50 is a retract adds an interesting wrinkle to the whole chute story.

Another question will be how the gear legs fair when impacting under chute. Unlike the SR series, the gear legs are mostly straight and so they may punch up through the wings. I worry about compromising the fuel compartment and then having a fire. In the SR series, the gear legs fold out and don't punch up through the wings.

Mike C.

That's not all of the 1 versus 2 argument.

Liability: the single engine airframe maker AND engine maker are taking much higher liability on the single engine jet and they have to fully load that cost into just ONE engine. Every engine failure in the SEJ is FAR more likely to be a lawsuit. It doesn't take many of them to radically change the cost structure of the product. Long term, Williams has to extract money from the engine reserves (HSI, OH parts) to cover this, too, and that is from half the units.

Systems: the airframe maker will have to make more complex systems to compensate for the redundancy limitations of the single engine.

Design and certification: more costly and harder with a single engine for both airframe and engine maker. That has to be amortized over the units sold, now half for the engine maker.


So are the SF50 numbers.

The PW610F engine is in service so we can get an idea what the HSI and OH costs are per hour.

The FJ33 is not in service in any meaningful way. All of its hourly costs are TBD.

It would not surprise me in the least if the FJ33 costs about twice per hour as the PW610F.

Also, don't forget the speed ratio. The EA500 is about 20-25% faster, which means engine hourly costs are 20-25% less per mile.

Mike C.

Good question. I'd wager they've designed them to fail in a safe and useful way.

Or a CAPS save.

Its interesting to hear from an engineer. I suspect Cirrus has one under employ though. It seems you assume you are the only one.

Or a CAPS save. :D