You ain’t wrong, Mooney. I am myself a perpetrator in this and just as guilty. I’ve always bought with the heart. Just happens to be my taste seems to favor oddballs. 
AdamFrisch wrote:
Very often you find piston think applied to TP’s and jets: “Oh, the second engine will burn twice as much fuel”. No, it’s simply not true. In piston twins yes, the efficiency goes down and they’ll burn about 30% more hp for hp compared to a single. That relationship is not true for gas turbines. A twin turbine with 1000hp total will burn as much as a single engine turbine with a 1000hp. You save zero cost in fuel. And in the case of engine overhauls, the gap is also less. A 1000hp PT6 will probably cost pretty close to overhaul what two 500hp PT6’s costs. Therefore, I do think the single engine TP market is inflated. Logic does not fully apply to it.
I know very little about turbines, but find myself wondering how that can be?
We’re always told that it’s very difficult to make a small efficient turbine engine, and 500hp is definitely on the smaller end of the scale, so you would expect that 2 × 500hp engines would be less efficient than one larger one.
For a piston engine, the engines are equally efficient whether you have 1 or two; what makes them inefficient is that you need larger engines (in case one fails, and because you need an excess of power to cope with the asymmetric condition) and typically they’re faster aircraft, and so will be less efficient.
For a twin turboprop, you will share the airframe penalty that a twin piston incurs, in addition to a drop in efficiency of the engines. So why would a twin be more efficient than a single with a larger engine? And why would a 500hp PT6 be much cheaper to overhaul than a 1000 hp version?
All I know is that the Eclipse that has two PW610’s put out 1800lbs in total, same as Cirrus Jet. Yet, the Eclipse will burn less fuel, go faster, and go higher whilst carrying more. Mustang has a little more power and will perform even better, and the fuel burn is on par with the Cirrus Jet. Granted, some of this might be due to the fact that the Cirrus Jet is restricted to FL280 (due to it lack of redundancy in system because it only has one engine), but still. Terrible numbers for the Cirrus with 80gph doing 270kts and almost no useful load.
Big turboprop engines are more complex and cost more than smaller fanjets to both buy and overhaul. You will not get any change from $350-500K+ on a PT6 overhaul for a TBM pr PC12. That’s probably very close to how much it would cost to overhaul two smaller PT6’s for the Jetprop, but this is just secondhand info. Hard numbers are difficult to come by as P&W don’t publish them. I would agree that really small turbines are harder to make efficient, so there is a slight advantage for the bigger ones, but not as much difference as pistons show. Maybe a 5-10% penalty.
Very often you find piston think applied to TP’s and jets: “Oh, the second engine will burn twice as much fuel”. No, it’s simply not true. In piston twins yes, the efficiency goes down and they’ll burn about 30% more hp for hp compared to a single. That relationship is not true for gas turbines
I don’t see how this can be true because the second engine must be dragged through the air, and its cowling is hardly smaller than that of a single engine with 2x the power. Also, the cowling doesn’t (?) deliver lift so the wingspan of a twin must be bigger by the width of the two engines, for the same lift.
If it is true it must be due to other factors.
There’s added drag, but there’s also more disc area. More disc area = more thrust/hp. Meaning, you can achieve better efficiency the more disc area you have. Helicopters that have big rotor discs need less power to hover, than an equally heavy one with a smaller disc area. Obviously, a second engine carries weight and drag penalties as pointed out, so it’s probably a wash in the end.
The propeller efficiency is also possibly higher due to being masked less behind the disc? ie. the cross-sectional area of a wing-mounted engine nacelle on a twin is lower than the cross sectional area of a cowling/fuselage on a single?
A lot of aeroplanes are “easy to fly” when you have a go in them for an hour or two, and nothing goes wrong.
A King Air is a delight to fly on a lovely day and satisfyingly straightforward in low IMC when you have mastered the avionics, However, an engine failure just after takeoff at high weight without auto feather is extremely difficult.
Similar comments would apply to most straight winged light jets, different issues, but superb until things go wrong.
I am sure SET will not be any different; clearly a lot of the difficult things on turboprops and light jets are to do with when one engine fails; SET’s are certainly difficult in OEI configuration.
High performance comes with strings attached, particularly in terms of system complexity.
Neil wrote:
I am sure SET will not be any different; clearly a lot of the difficult things on turboprops and light jets are to do with when one engine fails; SET’s are certainly difficult in OEI configuration.
Indeed this is why really the entire type rating training is focussed on losing an engine and dealing with system failures. There is no line training involved.
SET’s are certainly difficult in OEI configuration.
I am not sure if that is supposed to be self evident or if there is something else there. With your only engine gone, what else is there apart from setting up the best glide while steering away from terrain / towards the beach / the usual SEP stuff?
From vague memory of my 1.5hrs in the TBM850, the loss of power procedure is fairly simple and the main bit is bypassing the fuel controller and getting fuel to the engine directly. I read somewhere that in one Jetprop ditching that wasn’t done because the pilot was not aware of it.
