Advantages of Turbo Props over Jets ??

[TenAndie]

Just wondering if anyone could please highlight the main advantages of a turbo prop aircraft over a jet.

The main ones im thinking about is more economical, better performance in descent after engine failure due feathering capabilities (less drag).


Any more guys ??


Cheers

[enicalyth]

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.

[TenAndie]

Thank You........thats just what i was thinking ;)

[enicalyth]

Tenny

I wish I could say all that really counts in one hundred words but it is a beaut of a subject. For all I have done I love prop aircraft dearly and surely there are fewer schools of pure joy than whamming a C-130 through the valleys as some do (not me) or batting 20min sectors in the Caribbean in a Dash 8 as still others do. Fair play to those who have done both. Gentlemen and women my hat is aloft, I was but a driver.

Talking about aircraft and engines and why they are the shape they are is for me still a passion. I suggest that if you can dig BEagle up and get him started on the A400M you will learn a massive amount about what can be done when you transform the conventional into the unconventional. I don't buy all the A400M puff because you can have what they say but not all at the same time. Brochurespeak and the art of its translation by the unwary is fraught.

You may care to know that during WW2 Rolls-Royce and Bristol collaborated to form "Rotol" propellers and they had a drawing office in a London suburb. Mrs "E" having dragged me to Europe to be cultured I had the chance to look up an old gent now in his nineties who is a by-word in all things propeller. When you consider also that the Polish gent who designed much of the Chipmunk was also an aerodynamicist or a stress engineer on Concorde (I forget which) these old guys stun me. I could tell you zillions about the A400M prop design, but then I'd have to kill you.

Anyway got to go and visit another cathedral I guess.

"E"

[Clandestino]

One economical advantage of turboprops over jets is that their drivers think of themselves as less worthy compared to jet jockeys and are quite happy with lesser pay.

[hydrop]

I remembered in the late 80s there was this Propfan which looks like a turbo fan with prop blades behind. It was supposed to be ultra fuel efficient and was planned to be used with the boeing 7J7.

I wonder if this hybrid design between turboprop and turbofan will resurface again given today's high fuel costs. :confused:

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

[White Bear]

I read a little about the Russian ‘Bear’ aircraft recently. IIRC the comment was that although the contra rotating props turned at a maximum rpm of only 750, each prop was 16ft in dia and the prop speed exceeded mach 1 by about .08.
Still it was amazingly fast with a max speed of 585 mph. No idea about efficient it was, or how much noise those prop tips made!
Regards,
White Bear.

[enicalyth]

G'Day again!

Please not the propfan again. The fluid dynamics of an unducted fan give it the dimensions it possesses. In the same way that a die manufactured to extrude a cold rectangular section of rubber possesses a weird and woderful shape when working with the molten subject so it often is with research models. You may visit many a hydrodynamic lab where a test model of dictinctly funny shape is rotating at distinctly funny speeds in a distinctly odd fluid flow to model something in scale more easily, cheaply and accurately than is worth doing on the real thing.

Many an engineer has therefore thought along the lines of "Hey, if I build a test model this funny shape to reproduce behaviour at entirely different speeds in a different fluid flow regime might it not do very well in its own right?" No because the necessary flight regime has to be flown at a scale of twelve inches to the foot and 6076.1155 ft/sec to the knot, knock seven bells out of the competition and not out of the airfame and its unfortunate passengers.

The propfan was a devout political endeavour rather than a carefully considered engineering project. It played to a political stage, had some pretty wacky actors as principals, some reluctant names including R Shevell himself and Billy Practice and some who thought "What the heck".

I admire the late Prof Silverman of Edinburgh who as much as any single man gave the project legs maybe as early as 1965-66. But he goofed on the propfan when he was past the peak of his powers and no shame to him. His was the field of marine propellers and steam turbines and brilliant ideas in the field of closed-cycle turbines. Who can doubt the genius of Siverman, GFC Rogers and many, may others?

Players in the propfan saga were NASA, some diligent and good names in the propeller world [one of whom I dined with recently and is in cracking form!], some whacking egos, GE engines and Douglas.

Computational Fluid Dynamics was primitive and the P&W engines then in the ascendancy were gas guzzlers and no mistake. But..

The fuel crisis that brought propfan its 15 minutes of fame had long since puttered out before someone at last put propfan out of its misery. The LRC propfan project had a mesmeric fascination that could glaze the eyes of a rattlesnake but thanks to the generosity of America in placing documents in the public forum read all about it with a thorough NASA web-search. Even without reading between the lines it is one heck of a lulu in the folly stakes.

In urban myth for many the propfan was so super-efficient that General Electric who were project partners secretly did a deal with P & W and R-R to hush the whole thing up. Yeah. And Elvis married Lady Di.

Propfan failed not through conspiracy, lack of funds or anything but it acquired an unstoppability beyond all reason as many an ill-suited love affair shows. It failed to meet its principal objectives unless it flew so slow that it consumed just as much juice anyway and more or less and did for retinal surgery and orthodontists what Attilla the Hun did for the mortuary business.

Surely not that bad?

In terms of Tractor v Pusher argument it is far easier to make propfan in its early development a pusher and leave tractor problems till later. Noise and vibration virtually define it as rear-mounted. Ahhh, we have to have a T-tail and unrelieved bending moment in the wings. No problem as such as long as we get deep stall and shaker solutions right. Bummer that we have to do it but okay, we will. Forget that we really need to understand what constitutes airframe-engine matching. Rear-mounted engines its gonna be despite having more strength in the tail than we'd really like. Now as for survivability of uncontained blade shedding, I wish you hadn't said that. But soemone will think of something.

Now as for contra-rotating blades or not an unducted fan is not contained in a diffuser like an axial compressor or fan stages and has all the drawbacks of a propeller in a free airstream Every single one except that people want to get from A to B at something like M0.72 cruise if they are going to buy into this aircraft.

The more you push turboprop or profan designs into direct competition with what are accepted as conventional aircraft the more rapidly they lose all their virtues and retain all their vices. They funnily enough morph themselves back into B737/A320 sorts of configs.

By all means read the Langley Research Center papers but it was a kooky idea from the '70s when computational fluid dynamics was in its infancy. The late Richard Shevell (Douglas)who had something to do with it said that the best place for a propfan was on a Boeing aircraft. He was not entirely joking.

I had also better come clean on something else. One of the screwballs on propfan was my own late father!

Guys back home! Still on the wagon and still under the cosh. Longing to get back home....


The "E"

PS speeling polees I cant be bothered to correct my mistakes. I just wanna come home!

[Flymani]

White Bear, how about the TU-114 airliner derived from the Bear:

220 pax
520 kts top speed
171.000 kg

And that was in the 50's. Nothing in the west compared until the jets. Noise/sound level; also very impressive!

[enicalyth]

flymani

I'm impressed.

Did it really do all this in one lash or is it a bit of gentle overstatement by our erstwhile adversaries and now sometime good friends?

I know little of but the centrefold in an Eagle comic I bought at an airshow stall in 1994 at Fairford. So...

London to New York? Despatch reliability? DOC? 220 pax each with 45kgs wanting to fly at highest available FL and 520kts? Airline wants minimum FUBO. Enginers want credible MTBO. Pilots want to live.

My father died advancing the cause of socialism in his old age when the Soviet era aircraft he was travelling in fell from the sky in tatters as many of them were, and still sadly are, wont to do.

On the other hand I have had a close call piloting inferior Western tat for nigh on thirty years, man and boy. Never an easy man to convince my old man and nor am I but assuredly your good friend in this matter.

If you say, flymani, all of the claims made on behalf of the aircraft were not only simultaneously achievable or even near-simultaneous and verified by the FAI and endorsed by an eminent chief accountant and his buddies the chief pilot and chief engineer as being comercially practical then I'm your man.

Otherwise I remain kindly sceptical. Gotta say a lovely looking aircraft. Have you flown it or in it? I'd be honoured to hear all about it if not, in the circumstances, wish to try it myself. Though come on, of course I would. What a legend. Any still flying? I'm up for it.

Best Rgds

the "E"

[nkrid]

enicalyth,

[edited. oops, posted a half-finished message]

Kind of curious whether you could suggest what the perfomance tradeoff in the Piaggio Avanti P-180 might be?

It seems rather fast for a tuboprop and rather efficient generally, especially compared to a lot of older bizjets.

I can't quite see why it hasn't been a roaring commercial success except that it's price range is similar to that of many smaller bizjets and might not do so so well as a turboprop given the ego factor i've always assumed (possibly quite wrongly) to be associated with a certain part of the bizjet market.

Given your rather detailed assessment of the perfomance limitations of tuboprops in general, you strike me as somebody who might be able to suggest the 'catch' in that design- if there is one.

[chrisstiles]

Talking about aircraft and engines and why they are the shape they are is for me still a passion. I suggest that if you can dig BEagle up and get him started on the A400M you will learn a massive amount about what can be done when you transform the conventional into the unconventional. I don't buy all the A400M puff because you can have what they say but not all at the same time. Brochurespeak and the art of its translation by the unwary is fraught

Is BEagle a pprune poster? Because I too was curious why the A400M had gone with props when all the other planes in that class seem to have jets (C17, C5, IL76, Antonov etc).

[cdb]

Well, from a non-expert perspective...

A400M is a C130 replacement. Having props means it can take off and land on dirt strips, and can lose speed much quicker than a jet - eg for tactical landings.

[Flymani]

enicalyth, if my post led you to believe that I have flown the TU-114, I am sorry.

Other than being increadibly loud there may be other shortcomings, so my post was incomplete...

[OzExpat]

I'm probably going to get myself into all sorts of trouble with this post but, what the heck! I'd venture to suggest that, over distances of around 200 miles or so, a Dash-8 or ATR will probably take about the same time and cost less to operate than a jet of similar payload capability.

About the only advantages that I can see with a turboprop over a jet is :-

it can operate to runways that are less well prepared and much shorter than a jet would like;
it's good for short hops, to act as feeders for larger aircraft; and
it's probably less wasteful to use a turboprop that seats up to say, 30 people, than a jet that might have capacity for 100 or so, but only 30 seats occupied on any given (short) sector.

[SKY's4ME]

It is true that in todays growing demand for more economical strives in aviation (Plain speak cutting costs!) the turbo-prop is under a renewed interest specificaly due to the facts mentioned above.

When you compare your typical Modern turbo-prop ATR42/72 over the new Emb145's etc there is very little difference in time sub 200nm but a huge diffrence in cost.

It is true that a new ATR72 is no cheap Aircraft to buy but it sure is to fly! ATR has orders for over 70 new TP's this year with Bombardier over 50 TP's.

Is this the end of the regional jets?
(Now I,ve done it)

[enicalyth]

If the public are happy to sit in a prop for 60 minutes when a fan would do it in 45....

Props deliberately target 66seats but most of the fans congregate around 120 seats hoping to carry more people over more sectors per day. The prop does not seriously start to compete until it can spend a lot of its cruise time above 13000ft pressure altitude because its burn gets leaner and its comfort gets better and it can speed up a bit.

A bash at the figures but I think this resume is a maybe a little thirsty.

Notwithstanding the differences in first cost. Suppose we have a sector of 200nm as Ozzy suggests and a battle of the twins.

A lightweight turboprop of 50000lb all up weight is competing with a bigger turbofan at 10000lb. The turboprop ascends to 13000ft pressure altitude but the turbofan 29000ft. I know I know but just wait. David and Goliath.

If the former has 650sq ft wing area (ATR-72/Dash 8 ish) the latter has 1000 (DC/MD ish). Both have about the same lift coefficient and a guesstimate of the parasite drag coefficients is turboprop 0.025 (ATR72) and the turbofan 0.020 (DC9).

The turboprop has an effective aspect ratio of 9 (from 12 absolute) and the turbofan may be 8 (from 11 absolute), the reductions in each case being a matter of aerodynamic efficiency.

So the turboprop chugs along at 480ft/sec and a lift:drag ratio of 13.5 to 1 whereas the turbofan flits along at 720ft/sec and L/D of 15.

The turboprop has little compressibility drag and its drag coefficient is about 0.03 compared to the turbofan which does suffer compressibility drag of say 6% raising its figure to almost 0.03 (remember these are referred to wing planform areas and not total wetted surface). I just need reasonable figures to estimate fuel burn.

If the prop efficiency is 85% the turboprop needs 3800ehp to provide the thrust to push it along. The turbofan needs 6600lbf of straight thrust.

Each ehp-hr consumes 0.45-0.50 lb of fuel and each lbf-hr consumes 0.60 lb of fuel. The trip is about an hour in the turboprop and 45 minutes in the turbofan burning 1800 and 3900 lbs fuel each. But the turbofan can carry twice the pax load and fly more sectors a day. But its more expensive to buy and operate.

But what if the load factors aren't high enough to justify the jet and if the prop can get at least half the trip done at a much higher pressure altitude, say 23,000ft?

The turbofan advantage starts to erode, that's what, as long as folks don't mind being thrummed along in a prop.

As hinted earlier big turboprops would really rather be jets and the A400M is the way it is because military and civil markets differ. And a ducted fan beats an unducted fan anyway and is easier to certificate. Remember blade shedding anyone?

PS my example FUBO figures turn out to be on the high side. If I'd guessed nearer 12kg/min for the prop and 1100kg/hr/engine for the fan it'd be better. But as the underlying assumptions are roughly the same for each it's still cox's pippins for garnny smiths and all good fun. Until the fans shrink and the weight plummets. All change!


Best Rgds from the "E" and KO Sally on tour. Caribbean next to see the relos.

And get sailing

[John Farley]

Is BEagle a pprune poster?

sure gets my vote for Question of the Month

[OzExpat]

Happy trails and happy sails to the E and KO Sally! Been to any good Jap resties lately mate? :D

[enicalyth]

Well a part of it anyway, in 1994, but I did some tucker. KO Sally is pretty hot on her japanese grub so we went to the miyama but most of the time she has been hauling me around art galleries and doghedrals. Strictly on the methodist moonshine. We are hauling off to sail for three weeks though. I was going to write about what makes a turboprop tick but KOS unplugged me and on my final warning. Hates aircraft she does because they take me away from home whereas we are both happy on the briny. We'll take Mr scroggs delightful 744 to ANU and the LIAT Crash-8 to EIS. Chill with the relos. I doubt I'll be allowed even a sniff of pussers rum and forget Callwoods. But some relos are DWI's, might get a seegar or three. Stand by for an exotic postcard or digishot from the air. Bashed out some ideas for Threadbaron when he hits Godzone Country. Am I hearing things, noises off about Reddo?

[redsnail]

G'day E,
Fascinating posts. I've learnt something :D

Been and returned from Godzone. Went out single, returned married.
Visited Sydney, Cairns and Noosa. The Big Fella is muttering about returning to Noosa Sept 06. :ok:

Flew the White Rat both ways. ;)