Imaginary Locomotives

[DenysW]

29 minutes ago, Flying Pig said:

Not actually technobabble insofar as it means what it says and refers to a real activity that yielded meaningful data.

Less than you'd expect, I fear. I don't believe that it would be possible to scale-down a steam locomotive, given the temperature differences between the steam and the smoke versus the wind, especially given the vertical movement of the exhaust versus the horizontal movement of the air, and the mixing of the one into the other. Full-scale of a 3m (wide) by 4m (high) locomotive puts a rolling road into, say, 10 m diameter half-pipes, with a 300 m approach (30 diameters) and 100 m exit (10 diameters) to straighten the airflows.

 

If you baulk at the cost of this, your data is only as good as the validity of your scaling-down assumptions. I've seen 10:1 physical models of water-flow, which are conventionally scaled on Froude Number, and then seen the subsequently-built real thing, and they're fairly close but not exact.

 

If you don't care about the exhaust you'll get closer scaled-down than if you do, but you may end up with the LMS design that didn't do the exhaust well. And, probably, politically had to be different from the LNER design.

 

Computational Fluid Dynamics is the modern answer, but has even more dependence on assumptions and simplifications (and mesh size and choice) than physical models.

[Compound2632]

5 hours ago, DenysW said:

Justified by the techno-babble of wind tunnels testing.

 

5 hours ago, Flying Pig said:

Not actually technobabble insofar as it means what it says and refers to a real activity that yielded meaningful data.

 

The question of streamlining was the subject of research sponsored by the four railway companies collectively, starting in the late 20s, using, among other resources, the wind tunnel at the National Physical Laboratory. The LMS management were sufficiently impressed to recruit Johannson from NPL to the staff of the Scientific Research Laboratory at Derby, where a wind tunnel was set up, for the final design of the Coronation streamlining. [All IIRC - one of my erstwhile NPL colleagues wrote an article on this, of which I have a copy - somewhere.]

 

I dare say Chapelon's work on internal streamlining of steam passages could also be derided as technobabble...

[Flying Pig]

4 hours ago, DenysW said:

Less than you'd expect, I fear. I don't believe that it would be possible to scale-down a steam locomotive, given the temperature differences between the steam and the smoke versus the wind, especially given the vertical movement of the exhaust versus the horizontal movement of the air, and the mixing of the one into the other.

 

 

I think what you'd be looking for is an understanding of the airflow over the body, which is what wind tunnels were developed for in the aeronautical field.  That will change with the scale of the model due to a change in the Reynolds number which relates the density and viscosity of the fluid to the dimensions of the body around which it is flowing.  Specifically, laminar (smooth) flow at one scale may become turbulent at another, which would make results meaningless when applied to the real thing.

 

However, as this was understood and wind tunnels were well established by the time the streamlined locos were being developed, I would be surprised if there were not techniques to compensate for it, in order to make the data useful.

[tythatguy1312]

scratch that maybe we should de-streamline an A4 just to see how effective that'd be. Even then both the Peppercorn and Thompson A1's, which are both quite close to the A4's mechanically, showed less capability for speed, implying the A4's streamlining would've helped in that regard.

[Compound2632]

10 minutes ago, tythatguy1312 said:

Even then both the Peppercorn and Thompson A1's, which are both quite close to the A4's mechanically, showed less capability for speed, implying the A4's streamlining would've helped in that regard.

 

I think it's a doubtful assumption that the lack of streamlining made a significant difference, compared to the other aspects of design that were different, notably cylinder and valve dimensions and valve gear. A point to bear in mind is that the later locomotives were designed for a time when absolute maximum speed was not a critical measure of success.

[rockershovel]

This all sounds to me, and taken in conjunction with the NYC Hudsons (which had two radically different streamlining styles, but mostly ran without either) as though the 90-120mph speed range in which steam locomotives operated in express passenger service, simply isn't fast enough for the benefits of streamlining to be evident - whereas the benefits of steam passage and draughting development, being largely invisible but directly related to observed performance can be accurately assessed. 

Edited January 4, 2023 by rockershovel

[Compound2632]

7 minutes ago, rockershovel said:

This all sounds to me, and taken in conjunction with the NYC Hudsons (which had two radically different streamlining styles, but mostly ran without either) as though the 90-120mph speed range in which steam locomotives operated in express passenger service, simply isn't fast enough for the benefits of streamlining to be evident - whereas the benefits of steam passage and draughting development, being largely invisible but directly related to observed performance can be accurately assessed. 

 

Really, for virtually all of the time, the 70 mph - 90 mph range. 90 mph+ running was always the exception, and certainly not planned for on a routine basis.

[MikeOxon]

On 03/01/2023 at 17:53, rodent279 said:

I think something like this would sit rather well on a Duchess or on a Bulleid Pacific.

 

 

 

 

 

3801 looks a bit like something I tried on a GWR 'Castle' - in this thread  18th Jan 2014

 

[The Johnster]

4 hours ago, tythatguy1312 said:

scratch that maybe we should de-streamline an A4 just to see how effective that'd be. Even then both the Peppercorn and Thompson A1's, which are both quite close to the A4's mechanically, showed less capability for speed, implying the A4's streamlining would've helped in that regard.

 

Did they?  In normal running, I very much doubt it!

 

There is a major mechanical difference between some of the A4s and both the Thompson A1/1s and the Peppercorn A1s; the 'bird' batch of A4s and some others had double Kylechap exhaust, which ups the power significantly and allows the A4 to gain an edge for speed even before any other consideration such as steam passage design are taken into account.  But the difference was not enough to prevent the Thompson and Peppercorn engines doing A4 work to A4 timings.  If you wanted to prove that the streamlining had any significant effect, you would need to provide evidence of noticeably better performances than those from the Thompson and Peppercorn pacifics from single chimneyed A4s in order to compare like for like, and I doubt that this can be done.  I'm sure you can provide examples of very good runs from single chimneyed A4s, but not noticeably better performances or higher speeds on average in day-to-day running over the later pacifics, except the A2s which had 6'2" driving wheels and could not be expected to achieve the very high speeds of the 6'8" engines; they could get up into the 90s all the same, though!

 

Streamlining doesn't do much if anything up to about 120mph, and doesn't really make much difference until you get to Shinkansen/TGV speeds, which renders it absolutely pointless on steam locomotives except as styling, and while styling may attract passengers and sell tickets and is worthwhile on that basis, it makes no difference whatsoever to the running of the trains at speeds below 120mph.  Witness the NYC Niagaras, which were not streamlined and could pull heavier trains at higher speeds in daily booked timetable running (1,000tons at 100mph, the only steam timetable that has  ever required booked 100mph running with any load, anywhere) than the streamlined Hudsons or Hiawathas.  External streamlining doesn't make steam engines go faster, never did and never will.  In fact the several tons deadweight of the casing might even slow the loco down...

Edited January 5, 2023 by The Johnster

[toby_tl10]

38 minutes ago, The Johnster said:

If you wanted to prove that the streamlining had any significant effect, you would need to provide evidence of noticeably better performances than those from the Thompson and Peppercorn pacifics from single chimneyed A4s in order to compare like for like, and I doubt that this can be done.

Why? The Thompson A1/1 and Peppercorn A1s were all fitted with double chimneys.

 

In fact, all Thompson pacifics had double chimneys. Only 10 of 15 Peppercorn A2s had single chimneys throughout.

[62613]

10 hours ago, Compound2632 said:

 

Really, for virtually all of the time, the 70 mph - 90 mph range. 90 mph+ running was always the exception, and certainly not planned for on a routine basis.

I read somewhere (O.S. Nock, so might not be entirely correct) that pre - WW2, speeds on the ECML were restricted to 90 mile/hour maximum for normal service, and if you exceeded that, you needed a good reason why. Something to do with the signalling.

 

[DenysW]

The case for the prosecution, m'lud:

6 hours ago, The Johnster said:

Streamlining doesn't do much if anything up to about 120mph, and doesn't really make much difference until you get to Shinkansen/TGV speeds, which renders it absolutely pointless on steam locomotives except as styling

The case for the defense (from Johansen in 1936, as mentioned above by @Compound2632 and reached by googling Air resistance Steam Locomotive Data OR Measurement)

 

From the summary: "The results show that, with fast trains of conventional design, air resistance may absorb large powers, of which up to 75 per cent or more can be saved by streamlining. Valuable reductions of power at high speeds can be achieved by modifications involving no radical departures from conventional forms of locomotive and coaches."

From the introduction, referring to Victorian research: "and that, at speeds exceeding 60 m.p.h., the atmosphere is the most important individual source of resistance to passenger trains of conventional design."

The models were 1/40th scales (LMS Royal Scot + 6 LMS coaches, or 3 LMS + 3 LNER coaches) and 1/12th scale (Flying Scotsman or 10,000 both + 1 coach), set by the size of the wind tunnel and the desire to have the wind, if possible, at an angle to the train. The 1/40th scale 'Ideal' locomotive was of polished wood.

 

So, irrespective of the actual outcomes at full-size under the constraints of full-scale construction, the work was rationally based in pursuit of a rational target. Shame it doesn't seem to have translated well to full-scale.

[Compound2632]

1 hour ago, 62613 said:

I read somewhere (O.S. Nock, so might not be entirely correct) that pre - WW2, speeds on the ECML were restricted to 90 mile/hour maximum for normal service, and if you exceeded that, you needed a good reason why. Something to do with the signalling.

 

Braking distance - after all Mallard's famous run was during a series of brake trials.

 

Nock, as a Westinghouse employee, is as far as I can tell pretty reliable on signalling and brakes.

[rogerzilla]

According to Nock, they used double blocking for the Silver Jubilee, except on the sections of line with newer automatic signalling where it could go no faster than other trains as the automation couldn't do double blocking.

[rodent279]

7 hours ago, The Johnster said:

Streamlining doesn't do much if anything up to about 120mp

 

I have always understood that in railway applications, streamlining does not really benefit until speeds are in the 70-80mph region. As the forces on the train due to wind resistance increase non-linearly, with the square of speed, the force required to overcome wind resistance at 100mph is about half as much again as at 80mph. That is a significant amount, and applies just as well at 60mph into a 40mph headwind.

Of course, since sustained speeds above 80mph were the exception rather than the rule with steam, and the mileage run at or above those speeds was a small fraction of the total, you are right to say that it didn't make much difference overall. 

Edited January 5, 2023 by rodent279

[Compound2632]

But it's altogether more complicated than the simple surface are argument suggests, as the effect of turbulence has to be taken into account. So what happens between and under carriages matters a great deal, irrespective of the streamlining of the locomotive.

[MikeOxon]

Every cyclist knows that air resistance is very significant at quite low speeds!  I suspect, however, that the drag is mainly due to turbulence in the slipstream.  A railway engine has the advantage of having a long train behind it, which plays a very significant role.  The same is true with ships, where Destroyers used long slim hulls

 

EDIT pointed out by JimC below that the case of surface ships is fundamentally different

Edited January 5, 2023 by MikeOxon

[Compound2632]

12 minutes ago, MikeOxon said:

Every cyclist knows that air resistance is very significant at quite low speeds!  I suspect, however, that the drag is mainly due to turbulence in the slipstream.  A railway engine has the advantage of having a long train behind it, which plays a very significant role. 

 

Ah, but, as I posted, the train itself generates very significant turbulence - it is a long way from the ideal smooth surface of a ship's hull. Any streamlining advantage of the A4s or Coronations was probably only really coming into play when hauling the Silver Jubilee, Coronation, or Coronation Scot set trains.

[rodent279]

I guess that's because the area of the front of the locomotive, and in fact the surface area of the locomotive,  is small compared to the surface area of a 12 coach train?

[Compound2632]

5 minutes ago, rodent279 said:

I guess that's because the area of the front of the locomotive, and in fact the surface area of the locomotive,  is small compared to the surface area of a 12 coach train?

 

Rather more because the train has an irregular surface - notably the gaps between carriage bodies and the space between the underframe and track - that gives rise to turbulent air flow. The GWR's bow-ended stock was a first step at ameliorating this for general-service stock; the BR Mk 2 carriage introduced rounded corners and the Mk 3 skirts between the bogies, which undoubtedly made a difference for 100 mph running on the electrified WCML and 125 mph running of the HST. The LNER Silver Jubilee and Coronation sets had skirts and the articulation helped reduce the size of the gap between carriages.

[JimC]

2 hours ago, MikeOxon said:

.  The same is true with ships, where Destroyers used long slim hulls

Ships are not comparable at all I'm afraid. The desirability of long thin ships is all to do with wave making in water. Its a very big subject and not relevant here, PM me if you want to discuss. 

 

With trains I believe a significant issue is cross winds. Frontal area isn't as big a factor as one might expect. 

[rockershovel]

The 20th Century Ltd was an exemplar of streamlining applied to carriages, as seen here

 

[RedGemAlchemist]

On 03/01/2023 at 17:53, rodent279 said:

I think something like this would sit rather well on a Duchess or on a Bulleid Pacific.

 

 

 

These are quite smart. Might have to look for one...

 

On 03/01/2023 at 08:36, billbedford said:

To keep any oil thrown up from the journals saturating the boiler insulation. Think Bulleid's and fires. 

This is exactly the reason the splashers were there.

Also for context on the photo I shared a few days ago, it's an image of Mallard on I think the Yorkshire Moors Railway in 1985, running for a brief period without streamlining due to the casing undergoing major repairs.

[62613]

1 hour ago, rockershovel said:

The 20th Century Ltd was an exemplar of streamlining applied to carriages, as seen here

 

Looking at those drivers, do solid wheels have any advantage over spoked ones. I think Porta mentioned somewhere that they did.

 

[tythatguy1312]

2 hours ago, 62613 said:

Looking at those drivers, do solid wheels have any advantage over spoked ones. I think Porta mentioned somewhere that they did.

They do. They typically have better structural integrity and smoother running compared to spoked wheels.