SpitfireV speed at +12lbs.

[VooDoo]

what the Merlin 45 gave the Spit V over the Spit I was about 20 additional mph speed above 16k when both are running at +12 boost.
So the main purpose of the SpitV+Merlin45 was to enchance performance at heights above 10.000 feets.

In the real world that came in handy
Against what ?

I think you'd find those specialized Merlin low alt engines like the 45m and 50M to be quite fun arena planes. See here
Was there but dont know production numbers of these engines. Were them widely used ? And where ? In England ? FW190 interceptors ? Or in Med ? Or just both of it ?

[VO101_Isegrim]

Originally posted by gripen
Well, then I quess the Spitfire had really excellent inlet design because in every case in the flight tested critical altitude seems to be at least 2k higher than given in the power charts. Another possibility is that flying speed used for power curves is very low.

gripen

Keep in mind that there were many extarnal modifications, ie. the Spit I you mentioned was tested w/o the additional armor plates, windscreen, which came with a 9mph speed decrease (IIRC), and as such a reduction in rammed FTH. I don`t know if the Spit had any special inlet design, to me it looks like a simply hole on the fusalage, but I would hardly think it would have anything to do with it. Anyway, if you have any info on the details of the "really excellent inlet design", please share.

Looking on the Bf 109 G-2 and G-6, for example, the additional drag of the G-6 slowed it down by -25 km/h at FTH compared to the G-2, and interacted with the decrease of FTH of 400m (1300 ft). You may notice the exactly similiar decrease in case of MkV W.3134, with and w/o snowguards fitted (370.5mph/20.1k and 365/18.8k). The loss of FTH decreases power available at altitude, which decreases speed, which again decreases the power and so on until it balances out.

So, IMHO it`s simple a case that the Merlin III (and possibly the others) curves were done using later Spits with more extra drag from new equipment, such as filters, armored windscreen etc, and comparing their FTH with the earlier, cleaner ones makes little sense. Looking at MW`s rammed power chart for Merlin III, the FTH is at around 17.5k-18k ft or so. This agrees very well with the measured FTH of Spit I (Merlin III) of N.3171, at 18.9k ft. I dont know what equipment the plane exactly had, the difference is not great, and easily explainable by extra drag from newer equipment or difference with indidvidual planes. As an example for the latter, the well known JL 165, the same plane in different test was found the have a FTH of 14 400 and 13 800. Same plane, just different tests!

In any case, the point is that MW`s chart shows dynamic powers in max level speed flight, and this should be taken into account when compared to, say, a static chart. Some might come to false conclusions about high altitude performance while being busy comparing apples and oranges.

Voodoo : The acceleation figures were calculated by greg shaw - he posted in this thread, ask him about the details of the calcs - , based on available max SL speeds at given powers and weight. Thus they are based only indirectly on German testings, but should reflect the relative merits very well. After all, it`s not hard to see that 2000 HP will accelerate a 3362 kg plane better than 1840 HP at 3859 kg heavy one, especially if the latter needs more power to obtain the same speed.

[Flyboy]

a kinda newbish question, why does the performence curve become the same for the different boost settings in a certien altitude? (merlin III about 16k, merlin XII 14k, merlin45 17k)

[VooDoo]

The acceleation figures were calculated by greg shaw - he posted in this thread, ask him about the details of the calcs
OK. Gwshaw = Greg Show ? If so, Greg, could you repost calculation here ? Or just input data ?

After all, it`s not hard to see that 2000 HP will accelerate a 3362 kg plane better than 1840 HP at 3859 kg heavy one, especially if the latter needs more power to obtain the same speed.
Who is 1840 hp ? Spit14 ? Oh no... Not again... As for calculations - 5-10% in the propeller efficiency makes big difference. And 2000/1840 - its nonrammed figures and wo exhaust  thrust - so it make almost no sense for calculating real in flight acceleration. That is why Im looking for the real test data - calculations are not so good .

[gwshaw]

I'll see if I can dig through stuff and find the original posting from the Air War board. Unfortunately I don't keep very good archives of my own posts.

I still have the spreadsheet to do all the performance comparisons and could do them over again, if I find the time. Probably more accurately this time, I have better power data for some aircraft. Russian aircraft will probably be slightly worse, the Spitfires slightly better, most US and Germans stay about the same. The Fw 190A-8 will be lower, and the F4U and F6F will be significantly better.

I did the calculations at 250 mph @ SL to eliminate ram as a major concern. None of the planes should need ram to achieve full MAP, and all will lose power proportionally due to ram heating of the intake air. The Spitfires were figured using ram power figures, take ram out and it should gain 50+ hp and accelerate slightly better.

Exhaust thrust isn't much help at 250 mph, another reason why I did the calculations at that speed, although I may start at 200 mph next time. HP is 50% more effective than exhaust thrust at that speed, the same old thrust (lbs) = 375/250 * hp. Turn that around and 100 lbs exhaust thrust is only equal to 66 hp.

So a 1800 hp engine with 150 lbs of exhaust thrust is getting the following:

2700 lbs thrust from engine * .8 (propellor efficiency) = 2160 lbs thrust

150 lbs thrust from exhaust, about 7% propellor thrust.

That will certainly help, but since I'm not taking it into account on any of them the effect should be fairly neutral.

I'm using 80% prop efficiency, most should be right in that range at 250 mph.

The figures aren't absolutes, but should be pretty close relative to each other.

Greg Shaw

[VO101_Isegrim]

Originally posted by gwshaw
None of the planes should need ram to achieve full MAP, and all will lose power proportionally due to ram heating of the intake air.

Greg Shaw

Just to take the discussion on a sidetrack, I wonder if anybody has a solid answer for why planes with fixed gear ratio loose power below FTH as speed/ram increases ? My guess is the air is already well compressed by the time it reaches the air intake, and since the supercharger just tries to keep it`s speed constant, it find it harder to further compress already dense air. In the process it does extra work, which goes into waste (I would guess they get rid of the extra pressure with a wastegate or something to avoid overboosting). Any thoughts?

[gwshaw]

Power decrease below FTH is due to the temperature increase at lower altitudes.

sgrt(abs temp fth / abs temp new altitude) * hp @ FTH.

Example:

1500 hp @ 5000 ft
abs temp @ 5000 ft - 278.46

SL abst temp - 288.36

sqrt (278.46/288.36)
sqrt (.9656) = .9826

1500 x .9826 = 1475 hp @ SL at same manifold pressure.

Ram does the same thing, unfortunately I haven't figured out how to get the temp increase from ram to make the adjustment.

Greg Shaw

[Flyboy]

thanks for your answer greg.
theres only 1 problem, i have no idea what does all the initials mean (FTH, sgrt, abs, fth, abst, SL, sqrt, Ram )

[VooDoo]

I still have the spreadsheet to do all the performance comparisons and could do them over again, if I find the time. Probably more accurately this time, I have better power data for some aircraft. Russian aircraft will probably be slightly worse, the Spitfires slightly better, most US and Germans stay about the same. The Fw 190A-8 will be lower, and the F4U and F6F will be significantly better.
Im looking forward... Changes looks same as I thought. But Ive found some data on P-51: 3.85 ft/s2 WEP S/L from 250 mph, and 3.23 ft/s2 MIL 10-15K feet from 250 mph.

although I may start at 200 mph next time
No need. 250mph is close to 0,7Vmax and so its comparable to russian tests.

And one question about meaning of the calculated acceleration. Is it "instant acceleration" at 250mph speed or average for 250mph-Vmax ? Average for 0-250mph ?

[Pyro]

Originally posted by gwshaw
Power decrease below FTH is due to the temperature increase at lower altitudes.


Ram does the same thing, unfortunately I haven't figured out how to get the temp increase from ram to make the adjustment.

In addition to the temperature difference, there is also the greater pressure difference between intake and exhaust.

The formula to calculate the temp increase from ram is simply (V/100)^2.  The input velocity is in MPH and the output is in degrees C or K.

[gwshaw]

Thanks Pyro,

There is increased backpressure at lower altitudes that will need more power to push the exhaust out. Didn't think about that.

I'll put that formula in my list and start trying to compute ram power values for some engines.

Just to make sure I'm following your right.

For 250 mph TAS

(250/100) ^ 2

2.5 ^2 = 6.25 C

So @ SL that would raise temp to 288.36 + 6.25 = 294.61

sqrt(288.36/294.61) = 98.93 * 1475 = 1460 hp rammed.

Greg Shaw

[HoHun]

Hi Flyboy,

>a kinda newbish question, why does the performence curve become the same for the different boost settings in a certien altitude?

Good question, not at all newbish :-)

The reason is that above the so-called "full throttle height" ("FTH", also called "critical altitude"), the supercharger can't generate the desired boost.

At sea level, the Merlin 45's supercharger can generate about +26 lbs/sqin boost pressure. That's much more than the engine can handle, so it has to be throttled back.

At 12000 ft, the Merlin 45 generates +16 lbs/sqin, exactly what the engine can handle. We can open the throttle fully - thus we call it "full throttle height".

Above 12000 ft, the boost provided by the supercharger drops off because the ambient air gets thinner and thinner. At 14000 ft, it's down to +12 lbs/sqin. At 18000 ft, it's down to +9 lbs/sqin. The higher you get the further it drops.

As low-altitude power depends on the strength of the engine, you can get more power from the same engine by making its components stronger. (There's a limit of course - internal combustion requires certain pressures and temperatures to work.)

However, above full throttle height, it doesn't matter whether your engine has a robust crankshaft, strong rods and pistons that don't melt at high temperature. Its power is inevitably limited by the amount of oxygen in the air provided by its supercharger.

To get more power there, you need to do something about your supercharger. You can either spin it faster by increasing engine rpm or by installing a two-speed gearbox, or you can mount a bigger supercharger to feed more air to the engine at high altitude.

If you leave the supercharger as it is, the high-altitude part of the power curve won't change at all even if you improve your engine so that it can run at higher boost pressures. That's why the performance curves coincede for the different boost settings coincede in the graph above.

Regards,

Henning (HoHun)

[Pyro]

Originally posted by gwshaw
There is increased backpressure at lower altitudes that will need more power to push the exhaust out. Didn't think about that.

I imagine that accounts for some power loss too, but what I was really getting at is that your charge weight goes up as the pressure differential between your intake and exhaust increases.  The lower the pressure of the residual exhaust gas, the more charge is drawn into the cylinder provided that the intake pressure remains constant.

[gwshaw]

Originally posted by VooDoo

Im looking forward... Changes looks same as I thought. But Ive found some data on P-51: 3.85 ft/s2 WEP S/L from 250 mph, and 3.23 ft/s2 MIL 10-15K feet from 250 mph.

Sounds about right, I don't know weight that is at, and don't remember what weight I was using for my figures.

although I may start at 200 mph next time
No need. 250mph is close to 0,7Vmax and so its comparable to russian tests.

And one question about meaning of the calculated acceleration. Is it "instant acceleration" at 250mph speed or average for 250mph-Vmax ? Average for 0-250mph ?

It is instant acceleration, ie fps. So if you are going 250 mph (250 * 1.47 = 367.5 fps), and accelerate at say 5 fps. After 1 second you would be up to 372.5 fps (253.4 mph). Then you can recalc again using the new speed, which will change the hp - thrust relationship, and both drag values. So the next second will have a slightly different acceleration value. You can continue that up until you reach vMax, the finer the time granulation the more accurate it will be.

Greg Shaw

[gripen]

gwshaw,
After looking through quite many sources I still can't find any support for your claim that the RR rated engines with some sort of fixed RAM effect or something. Actually my impression is that ratings by RR are without RAM.

As an example the Merlin 45 (XLV) in the mw's chart does about 1200hp at 16000ft with +9lbs boost (3000rpm I assume). And in the real world speed tests rammed FTH were around 18800-20800ft depending on conditions (the lowest value is with snowguard), in the climb tests the AA.878 did still about 17000ft at about 200mph TAS with 3000rpm +9lbs.

As for  comparison the V-1710-81 (according to "Vee's for Victory) did 1200hp with 47"  MAP and 3000rpm  at about 14000ft without RAM (chart p. 271) and in the real world speed test in the P-51A (p. 157) could keep this MAP up to about a bit over 15000ft at around 390mph TAS.

gripen