Another Q for the experts.

[PLAGUEz]

Well, since my 'non-lethal AA missile' thread went so well. I thought I'd run this by you.

 

How about an A-10c with propellors instead of jet engines, in the exact place. How would it perform? Lets say its load is reduced. Could it ever be useful?

[Weta43]

You mean something like this ?

or this :

The propfan concept was developed to deliver 35% better fuel efficiency than contemporary turbofans. In static and air tests on a modified Douglas DC-9, propfans reached a 30% improvement over the OEM turbofans

 

Might be a plan if they weren't actively trying to retire it....

Edited April 4, 2014 by Weta43

[RIPTIDE]

Well, since my 'non-lethal AA missile' thread went so well. I thought I'd run this by you.

 

How about an A-10c with propellors instead of jet engines, in the exact place. How would it perform? Lets say its load is reduced. Could it ever be useful?

Intuitively I would guess that propellers represent a larger and more vulnerable point of failure for the threats that A-10s would have to face.

[Bushmanni]

At first the project proposals that concluded in A-10 were turboprops but when a suitable turbofan engine became available it was chosen as the powerplant. I don't remember why the change was made.

[sobek]

I would also say that external fan blades are probably more fragile. Also, propfans are *incredibly* loud compared to turbofans, which has it's own drawbacks on the battlefield as well as in everyday service.

[Yurgon]

Also, propfans are *incredibly* loud compared to turbofans, which has it's own drawbacks on the battlefield as well as in everyday service.

 

In this regard, there are some highly fascinating chapters in "Warthog - Flying the A-10 in the Gulf War" by William L. Smallwood.

 

Some of the A-10 pilots were told to prepare for night time combat missions during the build-up for Desert Storm. Although they had never trained for that (it was the A-10A back then), they soon figured out that they would be really stealthy at night. Barring radar detection, they would be both invisible and inaudible if they flew as low as 5000 ft at night IIRC. That offered them the cover of darkness over terrain with hardly any cover at all and the pilots soon began to appreciate this way of flying.

[dumgrunt]

http://en.wikipedia.org/wiki/FMA_IA_58_Pucar%C3%A1

 

has worked for a number of "budget" airforces. im not sure how well they fared in the falklands, but it would be hard to tell owing the argies tactics.

[Scrim]

Well, as it says in the article, it's a COIN aircraft, something the Argentinian military didn't seem to grasp. Using them probably cost more Argentinian lives for the pilots shot down than British servicemen killed by them.

[dumgrunt]

i wouldnt be so dismissive. the article also states that the pucar was the only aircraft to to score a confirmed argie air to air victory during the entire war. (remarkably similar to the A-10s only a2a victory except the pucar had to do it with 7.62 machine guns, not a 30mm cannon) it was also the only aircraft that could operate from the islands themselves especially after that famous bombing run by the poms. it was also noted as being able to sustain a substantial amount of punishment.

 

if it was in the right hands it would probably be better at CAS than the skyraider which the seppos were using only a decade before in Vietnam.

 

also if i recal correctly, the pommy boots on the ground had a particular fear associated with the pucar. so i doubt it could be deemed useless even in a medium intensity conflict like the falklands

Edited April 5, 2014 by dumgrunt
..

[Scrim]

That air to air kill was against an un armoured, unarmed scout helicopter on a MEDEVAC mission, not exactly impressive.

 

If using a plane costs your own side more KIAs than the enemy, it's probably not used properly. The Sky Raider caused more hostile casualties than friendly ones ;)

[EtherealN]

Well, since my 'non-lethal AA missile' thread went so well. I thought I'd run this by you.

 

How about an A-10c with propellors instead of jet engines, in the exact place. How would it perform? Lets say its load is reduced. Could it ever be useful?

 

Something to remember: in a turbofan (like on the A-10), most of the thrust is actually not from the "jet" itself. A turbofan has two parts:

 

1) "The jet", this is where air enters the engine, gets compressed, meets jet fuel, is ignited, and the added energy both drives the fan and of course gives thrust as it is expelled.

 

2) "The fan", this also doubles as intake for the compressor of "the jet", but most of the air pulled in by the fan never enters the actual "jet engine", it just gets pushed back same way a propeller does. On the A-10 it's a "high bypass" turbofan engine - that is, most of the air pulled in never enters the engine proper.

 

In the case of a "turboprop", you have the "jet engine" part, but instead of it being integrated with a fan and a shroud, you are using it to drive a conventional propeller. The Pucara actually has the same type of actual powerplant as does, say, the A-10C or the F-15 - and for that matter, the Ka-50. The difference is what exactly you do with it.

[aaron886]

The Pucara actually has the same type of actual powerplant as does, say, the A-10C or the F-15 - and for that matter, the Ka-50. The difference is what exactly you do with it.

 

That's a bit like saying a wankel car engine is the same as a piston boat engine. They share some theory but that's most of it.

[sobek]

That's a bit like saying a wankel car engine is the same as a piston boat engine. They share some theory but that's most of it.

 

Actually, the core of jet engines is remarkably more similar than that because *most* of them use axial flow jet engines. Some use radial flow or combined axial radial flow, but even axial and radial are remarkably more similar in design than a Wankel and a Otto motor.

[AlphaOneSix]

Centrifugal. :P

 

But yeah, the process is identical when comparing axial to centrifugal flow designs. The only difference is the path the flow takes.

[Bucic]

The question is pretty easy to answer actually.

 

First and foremost switching to turbopropeller engines without rearrangement of the engine mounts/nacelles would render the turbopropeller engines close to useless due to imposed limitation on propeller blade spans.

 

Even with the propfan concept the fuselage-prop clearance would be somewhat low. And regarding propfan's low noise characteristics. I'll believe it when I see some hard facts. Physics says at the same effort open-props are louder.

 

Let's say it was possible to fit some open-prop engine onto A-10. "How it would perform then?"

Well, first you need to acknowledge that at the centre of aircraft design is its future role. As such its projected mission profile is drafted and performance-critical areas of it are identified. Other consideration get on the board as well (e.g. noise levels, resistance to gunfire, maintainability etc.).

And now to answer your question. You take the given propulsion performance charts and put them against your mission profile. The profile that 'matches' best indicates the propulsion you should select. Long story short :)

 

Basic engine characteristics:

1. Engine thrust vs speed (higher speeds require jet engine)

(an example of a simplified one)

http://www.exl.at/helicopter/turbinen_bilder/vortriebswirkungsgrad.gif

2. Engine efficiency vs altitude (turboprops are more efficient for low-to-med altitude mission profiles)

Edited April 5, 2014 by Bucic

[VincentLaw]

The only difference is the path the flow takes.

Axial flow compressors can be stacked for extra compression, while you can only have one layer of centrifugal compressors.

[Weta43]

Not saying it's that answer for the A-10 (were an upgrade on the cards), but I think PropFans will probably become something like mainstream in the end (if a bit niche), and end up in this sort of role...

 

Basic engine characteristics:

1. Engine thrust vs speed (higher speeds require jet engine)

(an example of a simplified one)

http://www.exl.at/helicopter/turbine...rkungsgrad.gif

2. Engine efficiency vs altitude (turboprops are more efficient for low-to-med altitude mission profiles)

 

Those would point you at Propfans for the A-10 though..

 

 

 

regarding propfan's low noise characteristics. I'll believe it when I see some hard facts. Physics says at the same effort open-props are louder

 

The noise could be an issue (not so sure it matters at 5000m with a sniper pod), but still

 

McDonnell Douglas .. modif(ied) its company-owned MD-80. (replaced 1 engine with PropFan) .. test flights were conducted .. exhibiting a 30% reduction in fuel consumption over turbo-fan powered MD-80, full Stage III noise compliance

 

Even though the external blades are more vulnerable, those disc areas are still more space than blade

[dumgrunt]

That air to air kill was against an un armoured, unarmed scout helicopter on a MEDEVAC mission, not exactly impressive.

 

If using a plane costs your own side more KIAs than the enemy, it's probably not used properly. The Sky Raider caused more hostile casualties than friendly ones ;)

 

 

exactly, tactics. the tactics that are used to employ them are separate from the aircraft's potential effectiveness. if you are talking pure attrition which you seem to be, its a moot point because the argies were employing the pucar in a maritime strike role, against what were some of the best air warfare maritime vessels of the day....

 

and well yes it is impressive when you think that the harriers were up against, A-4s, Mirages and F-4s, all of which are better fighter aircraft, yet the only A2A victory was by a pucar?

the a-10 victories were both against utility helos, no different at all.

 

it has payload, loiter and survivability, all important in a CAS aircraft.

Edited April 6, 2014 by dumgrunt
.

[AlphaOneSix]

Axial flow compressors can be stacked for extra compression, while you can only have one layer of centrifugal compressors.

 

Doesn't make what I said less true. I suppose then it should be noted that the centrifugal stage is always the last stage of compression before the combustion section, but you can have several axial stages prior to the centrifugal impeller.

[Bucic]

Those would point you at Propfans for the A-10 though..

As far as I'm concerned 'not proven' ~ 'currently inavailable'

Also, even slight increase in engine mounts length could lead to higher weight increase than just mere 'by how much the lengthen mounts are heavier' in overal A-10 airframe weight.

 

The noise could be an issue (not so sure it matters at 5000m with a sniper pod), but still

I was rather commenting on their 'not proven' state in general.

 

Even though the external blades are more vulnerable, those disc areas are still more space than blade

I'd even go as far as saying that more exposed doesn't necessarily mean more vulnerable, unless the the A-10's engine nacelles provide some significant protection.

[Scrim]

exactly, tactics. the tactics that are used to employ them are separate from the aircraft's potential effectiveness. if you are talking pure attrition which you seem to be, its a moot point because the argies were employing the pucar in a maritime strike role, against what were some of the best air warfare maritime vessels of the day....

 

and well yes it is impressive when you think that the harriers were up against, A-4s, Mirages and F-4s, all of which are better fighter aircraft, yet the only A2A victory was by a pucar?

the a-10 victories were both against utility helos, no different at all.

 

it has payload, loiter and survivability, all important in a CAS aircraft.

 

Disagree. If the Pucara air to air kill had been against a Harrier, that'd have been impressive. But it was against a helo on a MEDEVAC mission. Not impressive, but rather of very questionable legality. Neither can I say that an A-10 taking down an Mi-8 is terribly impressive. It's well within the capabilities that you'd expect of both the AC and the pilot.

 

Very much doubt it was ever employed for maritime warfare. Proper AC could get there from the mainland, and the armaments of the Pucara would be hard pressed to cause casualties on the deck crews, not to mention the non existing chances of actually penetrating the armour of any ship present. Or penetrating past the CAP and AA to get to the ships for that matter. I mean, this is the plane that would attack enemies on the ground, and be far more likely to get shot down by them than to actually kill or wound any of them (there is not one recorded British casualty due to a Pucara air to ground strike).

[sobek]

Axial flow compressors can be stacked for extra compression, while you can only have one layer of centrifugal compressors.

 

Says who?

 

Besides, the reason for using centrifugal compressors is their significantly larger compression ratio per stage. A few axial stages with one centrifugal stage can offer the same compression as a greater number of axial stages. The typical reason for not using them is that they usually increase the diameter of the core, hence they are usually employed when engine core length is more of an issue than diameter.

[EtherealN]

That's a bit like saying a wankel car engine is the same as a piston boat engine. They share some theory but that's most of it.

 

Aside from the fact that that's a bit wrong, I think you are trying to read my post at engineer level, while I am explaining things at newbie level. ;) You know, the spirit of what the OP was asking. :)

 

The point is that you can have the same basic theory going on in an engine even though one "looks" like a "jet engine" and the other looks like a "prop", and the third looks like a "helicopter". That is, we need to consider the difference between the powerplant itself and how the power that is generated is translated into thrust. And in the case of a turbofan, actual thrust comes from the same principle as does a "prop": the engine turns a blade assembly that pushes air aft. In the case of the A-10, the majority of thrust comes from that principle - just as it does from a P-51 - rather than having heated/expanding gases push you. (They contribute a non-trivial portion, but in the case of the A-10, the majority follows the same principle of mechanical work on air as does a prop.)

[VincentLaw]

Says who?

Okay, you are right. engines with multiple centrifugal compression stages do exist e.g. the Rolls Royce Dart. I was just stating something that I think a professor said years ago, but having a degree doesn't make you right.