Advantages of Turbo Props over Jets ??

[enicalyth]

I'll try

Tenny!

Try it this way. What are the disadvantages?

Public perception and slowness maybe? Why slow? Because the blade tips travel at a speed that is the vector sum of aircraft and propeller velocities. For a conventional prop when its tip speed exceeds M0.85 it gets noisy and uncomfortable for pax as well as starting to decline in efficiency. So for a conventional prop this restricts out-and-out speed.

The other side of the coin is that for a good match of engine and airframe the band of speeds for which good efficiency can be attained is quite broad. For what the prop designer calls "power coefficient" if you keep that constant then the speed range for which efficiency is good and relatively constant is 2:1

Hey I'm gonna lift off at maybe 130kts true, climb at 160 indicated and be flat-out-charlie at 280kts true! She's gonna be an efficient baby in most parts of the flight regime.

There are two thermodynamic rules to remember. The colder the air going in and the hotter the air coming out the more efficient the engine is likely to be. The faster the engine can turn the better too. Unfortunately the conventional prop can't efficiently get to the really cool air. A nagging problem is that even a fast turning engine may be no use because it has to be geared down so that the prop tips don't go too fast. So make a virtue out of a vice and keep the conventional prop where she's best, low and slow.

Props do something else pretty neat. In the simplest analogy they are an actuator disk into which one heck of a lot of energy may be imparted to the air so the conversion of power into relatively constant thrust is very good. And that actuator disk is spinning right from the word go and therefore at take-off, sea level it is surprisingly efficient. Great take off perf. A jet loses thrust by quite a margin as it picks up speed down the runway.

It is by no accident at all that Lower and Upper Airspace are as they are. Prop aircraft are staggering aloft above FL250. Jets are getting into their stride and the faster they go, because they can being propless, the higher they can go into cooler and cooler air with less resistance and density. All good things come to an end and at FL390 the jets are beginning to have a dicey grip of the situation. A good ten thousand feet or so above the prop. They have Mach number troubles too. But at least that is down to the forward speed alone and not a convolution of aircraft and propeller speed.

Now if I jump back a stage or two. We are going to accept for the moment the actuator disk aircraft as a concept. You have hit that nail on the head, she can descend all right if the power is slammed off and if you feather she is clean. Props can be tractor or pusher. Funnily enough having a plate close behind the prop (the engine) increases thrust but I am not going to prove it. Depends on the plateness of the plate for one thing. But the flow and drag over nacelle and wing take most of it away again. So the pusher may have it over the tractor except for getting air in to breathe and cool the hot parts. Grey area and public perception has more to do with it than it should.

Given that prop aircraft are operating low and slow their drag has to be exceptionally well managed. That pretty much means single isle for a start and though the ratio of fuselage diameter to wing span is pretty sleek it is rather spoiled by the fact that wings, producing drag as well as lift, tend to be of relatively lower area with respect to the whole than in something like a 744. To counteract this they have a wider span but the chord length reduces. Do that and the thickness reduces because lifting shape cannot be instantly created. Meaning the ability to withstand bending reduces and the top and bottom skins are thicker to carry the stresses involved and are proportionately heavier than you'd really like. Less room for fuel too so try as you might to juggle the paramaters the efficiency of the wing on say an ATR72 is more of a compromise than on a B777 or a B744. I'm thinking of a factor called Oswald's "e".

In plainspeak this is something of a double whammy in that the more you tailor the aerodynamics to better the turboprop aircraft performance the quicker the law of diminishing returns takes effect. The Boeing B744 may have an aspect ratio of only 7.67 compared to 12 for an ATR72-500 but the effective value of the latter is really only 75% of that, not 86%. Effective Aspect Ratio is a critical factor in determining how much drag the wings induce and though of course the 744 is massively more draggy than the ATR the drag contributions are more equably spread. No way am I saying that designing a 744 is a dawdle only that the faster you want a turboprop to go and deliver your pax within an acceptable timescale the harder it gets to meet all the criteria and still have the overweening fuel efficiency you crave. There comes a time when you have to say, sod it, she'll have to be a jet.

It's a helluva question to answr properly in one hit and I am guilty of telling the truth in small measure. But hopefully the gurus will put me right and steer you to the good oil.