Steam speed records including City of Truro and Mallard

I understood (well, think I did) @Jamessquared; I'm afraid @Hermod went straight over my head.

 

1) No
2) See 1

Being kind ... The forces act instantaneously. Momentum is real, but in physical terms, change the forces = instantaneous response as described by F=ma. However - the real world experience of momentum when applied to a train travelling at more normal speeds is that (1) trains are very massive (i.e. have a high mass) and in comparative terms, the forces are quite small - so the accelerations are rather small. Set a 400 ton train rolling at moderate speeds along a perfectly flat piece of track, and it will appear to keep going indefinitely due to "momentum", but in reality it is gradually decelerating due to the friction and other retarding forces - just rather slowly - and it will eventually stop.

Tom

 

There is one mechanical feature about the A4 Pacific that seems to have been forgot about, but is well known and is even claimed by members within this forum as the reason for Mallard’s later failure.

This feature, a design flaw of sorts, also exhibits a well recorded phenomenon which is likely responsible for some of what we can see in the data the graph exhibits.

For myself, looking at the dyno roll on its own, it is apparent that the feature in question is not being taken into account by any of the posters here.

I await with interest to see if anyone else realises what is likely to have been providing sufficient extra power for Mallards run.

 

Is that anything to do with how the centre cylinder derives its valve events and therefore at speed how that cylinder “overdrives”?

 

The overshoot of the inner piston valve means that specific consumption gets even worse.
For a given amount of steam ihp goes down and we do not want that?
Take a look at picture 24 in the BR test bulletin for V2.
All speeds to be divided by 74 and multiplied by 81 and picture is valid for an A4 on a good day.
Six feet two and six feet nine.
The Diamond paper shows that the pressure difference can cut both ways and is not of big importance when going fast.
For tractive start effort and gauge clearance it can be life and death.

By the way:
Do someone know length and mass of 05002plus wagons

 

While on a macro scale we may do sums on the basis of a train going down say two miles of 1:200m followed by a mile of level track, in practice there will be a transition between the two over a distance, and also the length of the train comes into it, so there won't be an instant reduction in gravitational assistance, but instead it will gradually decrease until the whole of the train is on the level. And we may also wonder how level is level, and whether the gradient varies between the measured points. The closer you look at things the more complicated they get.

 

Interesting theory, but I'm not convinced. If, as I assume to be the case, you're suggesting whipping of the conjugated gear as creating an extension of the mid-gear cutoff beyond that normally induced by it taking up all the lost motion slack at speed, then no, I don't think it finds you significant extra power as you accelerate.

Let's say it's going to give you 60% with the outside cylinders at 45%. Extreme, but run with it a moment, a 33% increase in the admission phase in one cylinder. 11% more admission averaged across the 3. Or you could just wind the gear out to 50% on any other locomotive and get the same total admission with on average more expansive working and better use of the steam. But let's consider what else it's doing. Whip manifests itself as an increase in amplitude, but also as phase lag. So now the middle cylinder is not just running longer cut off (the portion of the stroke most likely to be heavily throttled through the ports by high piston speeds), but it's losing lead and also compression. As a result you're not getting the cylinder up to pressure as effectively as you could at the start of the stroke, reducing the cushioning on the middle big end, and generally giving yourself a world of pain. Porta was of the opinion that for really high speed running at long cut off you actually needed more lead than at short cut-off. This is contrary to the usual wisdom that the strongly increasing lead characteristics of Stephensons as you notch up was a good feature of 2-cylinder GWR classes, but if you assume that the shorter the cut off, the faster you're going (given constant boiler steam supply) the GWR setup still kind of holds together; it will certainly have helped their strong starting.

Really lead wants to be a function of both speed and cut-off (more lead with more of both); Wardale concluded for the Red Devil that where lead was not controlled independently of cutoff the constant lead characteristics of conventional Walschaerts was a fair compromise between something that would run and something that would start (and the Red Devil was not known for strong starting). Whatever the gear chosen, having it lose lead as it accelerates (independent of other settings changing) is more negative than positive.

 

The roll distinctly shows a mere 125mph.
propaganda gave the extra 1mph, a clear victory for British over the nazis.
But a legend all the same.

 

No, it does not.

What it shows is a series of five second intervals for which the average speeds over those five second intervals were taken, giving a clear 125mph sustained for nearly a mile.

The roll itself shows distance against time, drawbar horsepower and steam pressure.

The 126mph is shown by a point of instantaneous speed against distance on the roll.

Not propaganda. 125mph was undoubtedly achieved, and 126mph was later the correction after further examinations of the roll. However it is my contention that even further examinations of the roll haven’t given us the whole story. There is more to unravel.

 

Thank you for this. I need to point out that even on the LNER they never assumed the whip in the centre cylinder gave more than 10% than the outside ones when run at high speed. As with all aspects of Mallards run it is an additional factor to consider - did it hinder or help?

 

This is all very well, but if one is to talk about "authenticated" records, then really the record procedure needs to have similar characteristics to those other forms of transport at the time would regard as authenticating records. I think these would typically include actual authentication by an independent organisation, a a reasonable minimum distance for the speed to be measured over, assistance from gravity to be minimised and a minimum increment over the previous record. There's a valid argument to say that the special characteristics of rail transport make a number of those impractical, which is fair, but brings the obvious counter that in that case the records in question should not be counted as authenticated.

There's also the problem of rounding. Some say 125, some say 126, as if they were 1 mph apart, which is of course nonsense. Firstly of course the difference would not be 125 or 126 but (say) 125.9 and 126.0. And then if rounding to whole integers then conventionally 125.5 or anything over counts as 126, which leaves the rather counter intuitive concept that, say a speed of 125.5 sustained over a mile would be a more acceptable 126 mph than a speed of 126.0 recorded over a single second with lower numbers each side.

Folks may say that a lot of the preceding two paragraphs amounts to a discussion of angels dancing on a pin head, and I would not even attempt to dispute that. However what I will note is that all the bureaucratic administrations and conditions surrounding speed records in other spheres are there precisely to avoid such pin dancing.

 

Well, firstly it has not the slightest bearing [sorry] on what speed was achieved, but secondly if the differential travel and delay of the centre valve did generate more power then to me that suggests that the outside valve gear design was wrong, since its scarcely good to have two cylinders running incorrectly and one correctly.
But to me the whole business of overrun and valve gear whip is a complete red herring. Yes wear in the 2:1 gear introduces an inefficiency and loss in performance as the gear wears, but its just one inefficiency in the legion of inefficiencies that build up as a locomotive wears in service. I suggest it wouldn't even be discussed or considered, outside the relatively esoteric discussion of shopping intervals and overhaul costs, were it not for the elephant in the room that the centre bearing was simply inadequate for the job in hand. Look at it dispassionately: is it really acceptable engineering to have a bearing that fails at 110% of maximum design load?

 

“Two runs in opposite directions over the same course within one hour and at least 1% improvement on the previous record” to get the land speed record …

Tom

 

Hi Jim. I enjoyed your pun. That gave me a chuckle.

I agree it's a failure of the equipment. But we should be exploring all avenues and at least asking the question, no? Particularly when we're trying to work out the probable causes, what affected the locomotive, could it have done x, y and z, etc...

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth...

Regarding this:

I am in full agreement: but it is precisely because the railway sphere wasn't so organised as others that we have these debates (which sometimes throw up new ideas and evidence, and that is always interesting).

For myself, it is interesting to discuss and look at the original dyno roll, the closest we can get to the events on the day.

 

Tom's post (#177) is a helpful sanity check, particularly what we should expect to see where the gradient changed, subject to Jimc's point (#186) that the changes in gravitational assistance would be gradual. However the train was a lot less than a quarter mile long so the changes should show up fairly clearly. If the gradient profile is accurate then we need an explanation for the acceleration apparently reducing steadily (but remaining positive) over several miles rather than being much less on the level(ish) stretch and then more again on the next downhill.

 

As a complete aside, I came across the following - it is I think illustrative of the contemporary (mid 1930s) state of the art with regard wind tunnel testing of models. The comment relates to testing by the National Physical Laboratory that was passed to interested aeroplane designers, notably to RJ Mitchell and Sidney Camm of Spitfire and Hurricane fame:

In this connection, the later comments of Mitchell’s stress man, Clifton concerning Mitchell’s doubts about information derived from model testing deserve recording:​

"I think that Mitchell decided to make the wing as thin as he did, and I wouldn’t like to be positive about this, but my recollection was that it was against some advice from the National Physical Laboratory in that case where wind tunnel tests, I believe, showed that there was no advantage in going below a thickness chord ratio of 15%, whereas, the [Spitfire] wing was 13% at the root and 6% at the tip. . . subsequently it was found that when you made proper allowance for that, there was an advantage, as the testing could be shown to prove, in going thinner."
​

At that time, Hawkers had been advised by the National Physical Laboratory that their recent wind tunnel results had shown no drag penalty with the thicker Hurricane wing; however, the Laboratory scientists later found this advice to be incorrect. Fortunately Mitchell’s instincts were proved correct.​

(My emphasis - source: http://johnkshelton.blogspot.com/2012/06/r-j-mitchells-spitfire-close-run-thing.html)

Essentially the National Physical Laboratory ended up making a fairly major mistake in their analysis of aerofoils for aircraft. That is in the context of the time in which testing aircraft was pretty much "where it was at" with regard wind tunnel testing, and something that would have attracted a lot more experimental and theoretical effort than testing of rail vehicles. So perhaps not surprising if the figures that do exist for rail vehicle resistance at the time don't match very well real-world performance of full size vehicles.

Tom

 

The V2 tested in 1952 by BR was not very worn and the indicator diagrams taken at max steaming at 60 mph looks quer.

Mallard 126.5 is close to double and distubing forces go up squared with rounds per second.


The V2 diagrams can be seen on the last pages here:

http://users.fini.net/~bersano/english-anglais/BR-tests/BR_P&E_No8_LNER_V2.pdf

It seems thats one end of the mid cylinder alone is doing almost half the total work at 50% cut of.

This can not be beneficial for steam economy.

As written somewhere here economy is important because some of mr Andrews findings was an eleven second acceleration that needed 400 either horsepower or kW after miles of maximum steam rate.

If mr Martin cannot show us the digitized data from roll,maybe we can ask mr Andrews/Courier for his numerical data again.?
I cannot find them .
Especcially the analysis from NRM York

 

That's an interesting one. I wonder if the wing on the Typhoon also suffered from this erroneous advice...

 

To say the least!