Radical Pratt and Whitney design

Apparently Pratt & Whitney thinks there's some value to the idea, or they wouldn't have filed the patent. They're not the kind of company that just files for a patent with any idea they come up with.

Speaking only for myself, I'd say that my first post in the thread says much the same thing. It is a "brilliant idea" with an "elegant solution." But will it be worthwhile? The OV-22 engine/crossdrive configuration is brilliant, but has been nothing but a headache for over a decade. I'd like to see more -- I like the idea. I don't think anyone is deluding themselves about potential headaches. I think we (or atleast I) have been guardedly optimistic. I've mentioned several potential problems in my posts already. The entire rear fuselage becomes part of the propulsion unit or a giant nacelle. Weight, complexity, reliability, cost. All potential negatives. But many of those things apply to turboshafts and turboprops, but we still build them to fulfill specific roles.
You're more skeptical about it (and we just listed several reasons together, so I'm not using "skeptical" as an insult like it is bandied about in this forum). There's nothing wrong with that either!


[edit on 4-5-2008 by _Del_]

Originally posted by mtmaraca
You can disagree with me, that's fine, but don't give me the condescending little smiley as if you couldn't possibly fit a 3m fan, but you can fit 2 2.1m fans.

Sorry if it came across as condescending, that was not my intention.

But as for your point, yes, in cases like this, that 90cm is a massive difference. Go check out the clearance under a 320 or 737 wing when the FAR/JARs are considered - its not that much with HBPR engines, never mind UHBPR engines.

I think there are better ways to deal with the space issue, like mounting the engine inside the fuselage and designing a conformal inlet to feed it. If I were designing the aircraft, I'd rather deal with that than the extra weight of the dual-fan system.

I disagree 100%.

Then you dive into a realm of structural problems in routing the flow, your load paths, more maintenance headaches and also suitable encasing for fan retention in the case of failure.

Not to mention the boundary layer resulting in pretty bad loading profiles across the fan disk - not good for both fan aerodynamic performance and structural integrity.



Work has been done on boundary layer ingestion - heck I've had to look at it previously, caused us a sh!tload of problems and is better left alone.


The best solution would probably be a twin pod nacelle, one with the core, the other just a fan driven off a driveshaft. There was an older aircraft with twin podded nacelles at the tail - damned if I can think of the name though.

Originally posted by Zaphod58
Apparently Pratt & Whitney thinks there's some value to the idea, or they wouldn't have filed the patent. They're not the kind of company that just files for a patent with any idea they come up with.

All companies file similar patents to cover themselves.


I wouldn't pay too much heed to that in itself.



P&W are piling ahead with the geared turbofan, and it is scheduled to be introduced in at least one aircraft design that I know of (assuming it gets the final go-ahead).

Originally posted by _Del_
The OV-22 engine/crossdrive configuration is brilliant, but has been nothing but a headache for over a decade.

To be fair, the Osprey's main problem is not linked to the engines or cross-drives.



Its the result of an aerodynamic characteristic of transition from horizontal to vertical flight. In a certain section of the flight envelope it seems a significant variation in thrust will exist between engines, despite the best of FBW and control laws in conjunction with aero testing.


The problem has been encountered on other programs in different countries looking at similar things.

Why not just place the fans in the wing roots like the Comet or how the Nimrod has her engines?

This is of course for a larger short-haul aircraft the size of an A320 that would require at least two engines.

Shattered OUT...

Originally posted by ShatteredSkies
Why not just place the fans in the wing roots like the Comet or how the Nimrod has her engines?

This is of course for a larger short-haul aircraft the size of an A320 that would require at least two engines.

Shattered OUT...

The Nimrod IS the Comet

But that's a good point. Personally I'm in favor of USB if you're going to have more than two engines (or fans in this case).

reply to post by _Del_


Nimrod:



Comet:



Are you absolutely sure? Ok, the first two Nimrod prototypes were Comet 4 airframes, but at this point it seems to me to be a different airframe, or at least a major overhaul.

Shattered OUT...


[edit on 4-5-2008 by ShatteredSkies]

I see a Comet airliner with a large fairing fitted to accomodate equipment and a weapons bay. Also a MAD stinger. What do you see?

It's a substantially different aircraft, but the first Nimrod was a converted Comet. The basic design remains much the same.
Much like a P-3 is a different aircraft than the old Electra (they've added fuselage sections,etc), though it is still essentially an Electra offshoot.

[edit on 4-5-2008 by _Del_]

A major overhaul/upgrade doesn't change the fact that it's still a Comet based airframe. Under all those changes is still the same old Comet.

That's like saying I have an HP personal computer, I quite literally take out all the inside components except maybe the harddrive and the ram, I swap out processors, motherboard, power supply, GPU, sound card, network adapter, everything that made the HP an HP is pretty much gone. Oh and I swapped out the case too for a slightly larger case that slightly resembles the older one.

So what it's still an HP?

Shattered OUT...

reply to post by ShatteredSkies


More like you took an old Dodge Charger and replaced the engine, suspension and radio. You still have a fancy old Charger.

reply to post by _Del_

But then you changed the Charger's body too, so much to the fact that it wouldn't immediately be recognizable as a charger, one would have to pay much closer attention.

But that's just me, I'll give up on the argument and say that small percentage that remained of the Comet in the Nimrod makes it a Comet cause it seems to me I'll be here a very long time arguing something I simply don't understand. I'm sure there will always be different views and opinions on it, and that's just what I have, an opinion.

Shattered OUT...

Originally posted by ShatteredSkies
But then you changed the Charger's body too, so much to the fact that it wouldn't immediately be recognizable as a charger, one would have to pay much closer attention.

So if you add an airdam, hood intake and a spoiler it's still not a Charger?

It's a semantic difference -- a good natured difference in opinion. I still consider it a Comet with extras bolted on. I can grant that is produced as it's own airframe, not as refurbished old airliners so one can call it a new aircraft, I suppose. We both win?

Originally posted by ShatteredSkies
Why not just place the fans in the wing roots like the Comet or how the Nimrod has her engines?

This is of course for a larger short-haul aircraft the size of an A320 that would require at least two engines.

Shattered OUT...

The engines in the Nimrod/Comet/whatever were/are turbojets and low bypass ratio engines. Thus, the fan diameter is small enough to fit inside the wing root - as an aside, the Nimrod's engines are too big for this, so the nacelle is incorporated into the wing rather than the engine.

Even so, this imposed such structural headaches and maintenance nightmares that it was discontinued ASAP (except for the dumb-as-f__k MOD who have them in the Nimrod - and then they wonder why they cost a fortune to keep running).



A semi embedded design has been explored (on the upper surface), however issues with the boundary layer affecting fan loading are still presenting problems IIRC and making the thing non-viable at the moment.

the re-wing and re-engine on the MR4A

www.rolls-royce.com...

they went as far as they can - quite alot of light-blue types want a totally new aircraft

and im sorry canada - its very obvious the original airframe is the comet , wether it is now sadly is moot - as when people who know anything about aircraft look at it they simply nod and think comet (whilst everyone else has never heard of the comet and say its a nimrod )

I thought about the thing for a while, and tried arguing the design to myself as if I were the design's creator. You know, putting myself in his shoes. Perhaps the assumption of dimished returns on adding another fan only applies if the gas core of the engine was making only enough power to spin one fan, which would mean that this engine was working at the very peak of its efficiency, and that any more components would merely reduce its overall power, versus complexity and weight. But a 100% efficient engine is pretty improbable.

So, I got to putting my self in the designer's shoes. Perhaps Pratt and Whitney figured out that the gas cores could take on a heavier fan load. At the same time two fans would 'supercharge' the gas-core better than one, making up for whatever power that was lost in adding a fan. Overall, seems initially like a more thermally efficient engine. There is also the possible issue of a very large turbofan's tips breaking the sound barrier alot easier than a low bypass turbofan, due to the tip of the blades having to travel a longer circular path at the same speed of any smaller circumfrance. Maybe smaller fans can spin at higher speeds without destroying themselves. And as a bonus perhaps, like the twin turbos on a Porsche 911, there might also be lesser enertia in two smaller fans than one, improving response.

I'd appreciate a response on my assumptions.

It looks like the primary benefit of this propulsion setup is being able to sell one large core to an airline rather than two small ones. Earlier posters have already discussed the point that fans are already as large as they can be if they are to hang under the wing of an A320/737. Both aircraft have (and will continue to have) only two propulsion pods. Assuming the thrust requirements of the aircraft remains the same, and since the number and size of the fans is fixed, it seems the only benefit of this concept is that supposedly you can produce the gas horsepower work required for the two fans more efficiently with one large core than you can with two smaller ones. There is some possibility to this as HBPR commercial turbofan cores are starting to get about as small as they can be without running into obscene mechanical difficulties. One larger core will have larger radius as well, which will buy you more LPT tip speed for a fixed wheel-speed, which will let you spin the LPT slower. That is going to be important since now that gearbox gets the thrill of transferring the shaft horsepower for TWO fans through a reduction. Have fun with that one little buddy.

I don't see any play on weight, as any potential weight savings in moving from two cores to one will most likely be offset by all the structural work needed to make the gearing and shaft layout work without this thing shaking itself to pieces.

This thing isn't going to fly on any regional transport or A320/737-type aircraft since the airlines and FAA won't allow them to fly on only one core. That's why I think this thing's best market is the small biz-jet class, since they would probably be able to get away with flying on one core; and since those engines are the smallest, they are already having the worst problems with minimum engine size.

You might want to look at BBC News - Farnborough story

Particular industry hopes rest on Pratt and Whitney, part of United Technologies, which is making a big splash again this year with its "geared turbofan" engines...

Innovative use of gearing allows the main front fan and the rear sections of the engine to operate at different, optimum speeds....

Meanwhile, Rolls-Royce, GE, and CFM are all also thought to be working on new ideas for " open rotor" engines.