any thing on the acceleration of the Spitfire

thedab said:

I got this for the G-2 A-4


that A-4 don't look right it should be about 630km/h on 1.42 ata

5000m is about 16400ft and 618km/h is 384mph it look like could be the mkIX with the 61Merlin Spitfire F. Mk. IX BS.428

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That is normal, while Soviet 109 tests gave appr. same, sometimes even better results than the German ones, their tests with 190s systematically gave worse results than the German or Allied 190 tests. Maybe because the fuel used. But also the Soviet combat pilots tended to think that the "Messer" was more dangerous opponent than the "Fokker". It tended to be other way round in the West

pbehn said:

Surely by the time the merlin Spitfires acceleration was considered poor it shouldnt have been in service. The Typhoon should have replaced it as should the Griffon Spitfires both planes and their engines took time to sort out. The later model Merlin Spitfires were making the best of a bad job.

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There wasn't a big difference in acceleration at 5000m between Fw 190A-4, Bf 109G-2, Spit F.Mk.IX and Bf 109G-2/R6 (in order from best to worst) in the Soviet test results, but one must remember that while Soviet figures for 109G-2 and G-2/R6 were even slightly better than official German figures the 190A-4 clearly underperformed in Soviet tests. The accelerations of P-39 and P-40E were also fairly close together but clearly worse than the first group.

And according to Wade's figures, Spit LF IX and XVI clearly accelerated better than Tempests

GregP said:

Thanks Aozora, good info ...

Wonder why they think the acceleration of the SPitfire is rather poor?

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I know I have seen a similar comment somewhere else but can't think where (Now it's going to nag me...) The paragraph goes on to warn against cruising at +2 boost @ 1,900 revs which means accelerating from (say) 240 mph TAS to 370 mph @ +16 lbs and 3,000 rpm...at a guess that was slow when confronted with an Fw 190. cruising at a much higher speed.

I think the acceleration wasn't poor at all. The time from when you realize you are under attack until you are at combat speed from cruise speed was probably the best or ONE of the best, but still didn't help you from being shot down when surprised.

The P-38 had the problem twice over since there were two mixtures, prop governors and throtles along with the gunsight lamp switch.

The solution was to SEE the opponent before he shot you down and to get into a combat state before you were killed. Beware the "Hun in the Sun" was not a casual warning to neophyte pilots.

There is more to calculating acceleration than Power to weight ratio.

You have to solve for Force - Drag= Mass x Acceleration.

In General, Acceleration will be highest when the start of the acceleration is initiated at low speeds where the Drag Polar shows total drag at minimum.

For example a Bf 109G-6 and a P-51B-1 with 1650-3 (high altitude) engine starting at 380 mph at 29,000 feet will demonstrate very nice acceleration against the 109 because for Him the D=T at max velocity and there remains no 'excess thrust over drag'. On the other hand a 109 and Mustang at 250 mph at 22000ft (DB 603 FTH) should initially pull away from the Mustang until somewhere around 320 mph, when the Mustang steadily increases and pulls away from the 109.

For Thrust, for reciprocal engine, you need to know the prop efficiency, the Horsepower and the Velocity to derive prop thrust at That Velocity, plus calculate the exhaust thrust. The Exhaust Thrust is typically between .11 and .14 of Prop Thrust. Prop Efficiency is a function of diameter, number of blades, RPM of blades (as function of engine RPM and gear ratios) then table look up. Most WWII fighters were in the range of .85 for medium to high speeds.

Drag calculations require CDo for that airframe, then Induced Drag may be calculated for each incremental velocity, altitude and Weight condition, until max thrust and Velocity are attained.

For a Spitfire with various two stage supercharged Merlins (or a Mustang) the Horsepower is a function of Boost and altitude.

In other words "It Depends"

Anyone know anything on propeller developments?

I was reading about Operation 'Insect' (supplying Spitfires to Malta 21 July 1942) and came across this quote from Pilot Officer Noel John Ogilvie, RCAF:

'These new Spitfires had laminated wood hydrodynamic propellers, giving us much greater acceleration ... '

You don't need any of the calculations above if you have comparative tests of the planes involved. ... it is all laid out for you in the test reports. If you don't have comparative tests of the planes of interest ( and we appear not to have them), you need some information for Bill's calculations that usually isn't available to the average forum member. While there probably IS a very detailed study of the P-51 (at least some variant) around, finding the same data for the various aircraft of interest will not be a simple task, though it well might be out there somewhere.

I've seen Bill's posts on the P-51 before so I know he has the data on that airframe, probably at least the P-51B-15, P-51D, and P-51H at any rate. But the detail data on late model Bf 109's, Fw 109's, Yaks, Lavochkins, and Spitfires, while it may be out there, isn't readily located other than general performance numbers ... sometimes only at the critical altitude.

So it isn't the strong suit of a non-aeronautical engineer to go dig these data out and make the calculations.

In the absence of the required data, I'll lean on power to weight ratio and assume the propeller effeciences for all combatants were within a stone's throw of one another, since when I look them up, most are. Down low, the Yaks, Lavochkins, P-51's, Spitfires, Fw 190's and Bf 109's were probably very close in general performance with the nod in armament going to the cannon + MG equipped aircraft. As you go higher, the Soviet planes fall off pretty fast, having simple, single-stage superchargers for the most part and lower-HP engines.

I'd expect the Soviet planes to be non-conterders above 15,000 feet or so realtive to the 2-stage aircraft, and their acdceleration would also fall off at height. But at 3,000 - 8,000 feet, the Soviet fighters, while certainly no ground attack wizards, were consumate dogfighters and pretty fast. So if you're talking acceleration above 15,000 feet, then the Spitfire, P-51, Bf 109, and Fw 190 were probably in there while the Yaks and Lavovhkins weren't. Above 25,000 feet the Spitfires, P-51's and Bf 109's were good but the Fw 190's had fallen off at 22,000 feet or lower.

Late model Spitfires were good way up high, as was the very scantily-produced Ta-152 series that never managed any decent combat results.

Down at sea level to 10,000 feet you had the Sovet fighters as well as the OTHER lower-altitude fighters in the fray, including the P-38. At sea level, even a P-39 was a very dangerous opponents and was probably faster than you thought though not quite as good in acceleration s higher horsepower opponents.

What we need is a quantitative study with acceleration numbers and we don't seem to have one. The graphs avialable don't seem to cover all the intersting possibilities.

I predict a never-ending discussion (or argument) on this one, but it probably won't be a US fighter in the lead unless the F8F Bearcat and/or F7F Tigercat are thrown in as players. The F7F held the US time-to-climb record well into the jet age and that argures well for it's power-to-weight ratio.

Where calculated acceleration moves into the art vs theory blend is in two areas. The first is assigning a propeller efficiency in order to perform the Thrust = Prop Efficiency x Horsepower x 550/V(in feet per second). The second is, as Greg says, obtaining reliable CDo data.

The rest of the factors all involve deriving Total Drag.

The Total Drag varies by altitude and velocity as both CDo and CDi (induced drag). For those that have Dean's America's One Hundred Thousand look to page 604 for a list and rank for the HP specified, starting velocity at 250 mph at SL.

The P-38L at 16880 GW, two Allison V-1710-110/111 at 1600HP each and Dean's reference total Drag= 1676 pounds yields an initial acceleration of 4.13 ft/sec per sec.. The P-47M = 4.02, the P-51D= 3.85, the P-63A = 3.57, F4U-4 = 3.33, the F6F-5 = 2.6 and the P-40N = 2.24.

The P-51D noted above at 10208 pound GW, has less acceleration than the P-51B/C with 1650-7 engines but equivalent loaded GW of 9820 pounds has an initial acceleration of 4.0.

Good information, Bill.

I noted in Dean's 100K that total Thrust was calculated on an efficiency of .8o (probably low for variable pitch prop and that his calculations Only accounted for the Propeller Thrust - not including exhaust thrust.. Exhaust Thrust is an additional (.11 to .13) x Hp according to Hoerner in section 14-3.

In section 14-9 he gives CDo as follows:
P-51 = .017
Me 262 = .021
P-80 = .020
Bf 109 = .033
B-17 = .024
B-29 = .033
Me 163 = .012

Bill's post above makes me dredge up a partial Cd0 table I did awhile back. Please see below.



The jets are partially blank because I just added them from Bill's post above. The formulas for a jet will be different from a propeller plane. It is difficult to get accurate horsepower from jet thrust (or agreement) because you have to know the speed the plane is flying, and jet thrust varies with altitude, too, just like the power from a piston engine. So while we may know the static thrust of, say, an ME 262, I do not know the thrust at the critical altitude, even though we do know the speed.

Cd0 is the zero lift drag coefficient.
You multiply it by the wing area to get the equivalent frontal area in square feet, add the horsepower, and you can get the horsepower per square foot of frontal area, which is a very good indication of an aircraft's speed and ability to accelerate. Notice the Fw 190's HP/F and you can understand why it was able to accelerate away from many attacks.

This table is not too good yet because I have listed the installed horsepower … not the horsepower at the aircraft's critical altitude … it was a place to start and I got sidetracked before I went further. But if you were to change the HP column to HP at critical altitude, you'd have a VERY good indication of speed and climb ability relative to one another.

Bill, if you see any mistakes, please point them out.

If anyone is interested, I have the references for the Cd0 numbers.

GregP said:

Bill's post above makes me dredge up a partial Cd0 table I did awhile back. Please see below.

View attachment 267275

The jets are partially blank because I just added them from Bill's post above. The formulas for a jet will be different from a propeller plane. It is difficult to get accurate horsepower from jet thrust (or agreement) because you have to know the speed the plane is flying, and jet thrust varies with altitude, too, just like the power from a piston engine. So while we may know the static thrust of, say, an ME 262, I do not know the thrust at the critical altitude, even though we do know the speed.

Greg - you don't need HP when dealing with a jet.. just the Thrust as a function of altitude. The only reason to jack with HP is to derive the equivalent Thrust. Most jet engines have both SL static Bench test data as well as plots of T as a function of air density and Velocity.

Cd0 is the zero lift drag coefficient.
You multiply it by the wing area to get the equivalent frontal area in square feet, add the horsepower, and you can get the horsepower per square foot of frontal area, which is a very good indication of an aircraft's speed and ability to accelerate. Notice the Fw 190's HP/F and you can understand why it was able to accelerate away from many attacks.

The Hp/flat plate area is at best a Kentucky windage 'guess'. The total Drag for any specific velocity is the Parasite Drag plus Induced Drag. For example - at 250mph, SL, 1720 Hp MP and 80% efficiency - the flat plate equivalent total drag = 4.1 + 1.1 = 5.2 sq ft. At 10,000 feet the P-51D at 10,208# GW, 1720Hp, 250mph the Parasite drag plus Induced Drag equivalent flat plate Drag = 4.1 + 2.05 = 6.15 sq ft. You can't reliably get acceleration with 'equivalent flat plate'. You need the Total Thrust and the total drag with respect to CDo and CDi in Force... so to get there from flat plate equivalent you then multiply the total by Q... but why bother when you have to calculate the CDi and Cdo anyway?

From there it is straightforward to do T-D= [W/g] x A; A = g x(T-D)/W so looking at GW is hugely important.

This table is not too good yet because I have listed the installed horsepower … not the horsepower at the aircraft's critical altitude … it was a place to start and I got sidetracked before I went further. But if you were to change the HP column to HP at critical altitude, you'd have a VERY good indication of speed and climb ability relative to one another.

Bill, if you see any mistakes, please point them out.

If anyone is interested, I have the references for the Cd0 numbers.

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You may recall my 'epic' dialogues with Soren when he casually threw out "HP to W" as the acceleration final answer?

As a refresher I pointed out that Acceleration = zero at FTH and max V? And that the Hp to altitude at MP plot looks so very different between a 109, 190 and 51 and that the difference of Thrust = f(altitude) for constant boost and RPM, vary a great deal when comparing the three a/c.

Last but not least, there are three variables that do not have universal equivalence between those three ships -
Eta = propeller efficiency which varies a little bit depending on the velocity of the airframe and Power settings.
e = Oswald Efficiency of the wing and wing geometry
Delta Thrust = between (.11 to .13) times the Propeller Thrust for given HP and Velocity of the airframe.

When Soren pushed for turn calcs, I pushed back citing a real analysis went beyond standard performance calcs because they, if carefully considered, are only for 'point' calcs and do not take into account;
high Angle of Attack contributions to drag (both additional form drag and trim drag in a turn, for example),
the variance in Drag as a function of an aircraft losing velocity in the sustained maneuver until equilibrium is attained,
the 'bleed' of powerplant efficiency as velocity reduces - and last but not least
the rating of the engine as a function of altitude and supercharger stages.

Dean does perform the calcs, and by scrutinizing the data he gives you, one can derive the e (.80) and the Eta (.80). Personally I would have used .85 for the Propeller Efficiency in the 250mph to 400 mph range and more inclined to .85 for Oswald simply because those values enable one to derive the flight test drag data for SL max velocity results Slightly better.

Hi Bill,

I missed your epic post dual with Soren, and I wonder how to find it. It probably won't be listed under "Drgondog versus Soren, the Real Story." The reason the jet entries are missing some cells is because I have no idea of the thrust at whatever critical altitude is for a given aircraft. When they cite thrust, it is usaully max static thrust, not thrust at best altitude. There seem to be very few "thrust versuis altitude" charts for the early jets, at least that I have stumbled across.

If anyone knows of a good way to find these discussions, I'd love to read them. Sometimes Drgondog and I have clashed but, in the end, he always had / has a good point. It's tough to find him definitely in the wrong. On the other hand, I sometimes look at a problem backwards. The curse of poor toilet training rears it's ugly ... head occasionally. Hopefully less ocasionally these days.

Cheers.

Gentlemen,


In David Birch's book on "Rolls-Royce and the Mustang", page 15, it gives the following information about the Spitfire IX, Spitfire Vb, and Mustang (Allison powered):
At sea level and a speed of 250 mph in level flight the Mustang (V-1710-39) at a weight of 8,625 pounds required 480 horsepower, the Spitfire Vb (Merlin 45) at a weight of 6,525 pounds required 580 horsepower, and a Spitfire IX (Merlin 61) at a weight of 7,170 required 600 horsepower.

From this one can see how clean the Mustang airframe was compared to the Spitfire. Perhaps one could derive acceleration from the above data?

Eagledad

thedab said:

Hi I'm trying to find anything on the acceleration of the Spitfire, as the forum which I'm on a lot,

everybody tends think that the Spitfire has poor acceleration, but I was looking at the mk5 it has a very

good power to weight (yes I know it should be thrust to weight but this is all I know) and looks

to be better than the Yak-3,which everybody say has a excellent acceleration. so now I'm looking

for anything on the acceleration of the Spitfire ( and yes I have looked on Aircraft performance, but came up dry )

so if anyone know anything on the subject, can you help, as I want to know more on the subject

many thanks Ian

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Over and above airframe factors you have engine characteristics. If you have a Merlin on low revs/high boost (say under 2,000rpm) for max economy the pick up is going to be quite slow.
You start to accelerate then the revs come up and as they do the boost comes up, but you have a long way to go before you are getting into those max power levels.

Contrast that to a Sabre, where its MWM setting wasn't that far off its max power...because it was a low boost high revving. engine.

Take a Merlin 66 in a Mk IXLF Spit: Most econ cruise is 54% of max speed. Max cruise on weak mixture is 81%.
Sabre IIb in a Tempest: Most econ is 66% of max speed. Max cruise on weak mixture is 90%.

So you'd expect the engine pickup to be far faster on the Sabre, since it is already revving quite high and delivering closer to max boost.


So you can't ignore the engines when you are calculating this.

eagledad said:

Gentlemen,


In David Birch's book on "Rolls-Royce and the Mustang", page 15, it gives the following information about the Spitfire IX, Spitfire Vb, and Mustang (Allison powered):
At sea level and a speed of 250 mph in level flight the Mustang (V-1710-39) at a weight of 8,625 pounds required 480 horsepower, the Spitfire Vb (Merlin 45) at a weight of 6,525 pounds required 580 horsepower, and a Spitfire IX (Merlin 61) at a weight of 7,170 required 600 horsepower.

From this one can see how clean the Mustang airframe was compared to the Spitfire. Perhaps one could derive acceleration from the above data?

Eagledad

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Yes - given the proposed Hp at a specific boost. For example the Allison 1710-39 had a given 1150 Hp maximum at SL.

Assuming that the Propeller/engine efficiency = .8 and that exhaust thrust = 11% of Prop thrust then the steady state Thrust + HP*(.8)*550/V ; at 250mph (assume SL for the comparison above for P-51/Mark I).

480*.8*550/(1.467*250) = 576 pounds at 250mph at SL for prop; add 11% = 1.11*576= 639 pounds@480HP

Assuming the applied thrust at WEP is near instantaneous (and engine doesn't blow) then the Thrust Force@1150 HP at 250 mph
1.11*(1150*.8*550/(1.467*250)) = 1518 pounds at 1150HP

Thrust (new) - Thrust (equilibrium @250mph) = Mass * Acceleration = W/g * Acceleration = 8625/32.12 * Acceleration

(1518-639)*32.17/8625 = Acceleration = 3.27 feet/sec^2

So, find the HP at full Throttle for each Spit and arrive at the respective accelerations.

Note: in real world model, you would have to know what the CDo is for your a/c and calculate a.) reduction of Thrust as Velocity increases, and b.) the Drag components of Induced and Parasite Drag resist the 'new thrust' at t=0, 1, 2, 3, etc.

The flat plate Drag CDo will not change with V until M>.55 (small) but Induced Drag varies with CL^^2:

Q changes as the system accelerates and velocity increases and varies with square of velocity, no change to density at constant altitude

I just found a bit more on that G-2 and it does look that it was running on 1.42ata
on the frist page of this [In Development] Yak-9 - Dev Blog discussion - War Thunder - Official Forum someone has put up some charts with the G-2 on it with it engine power and weights

GregP said:

The F7F held the US time-to-climb record well into the jet age and that argures well for it's power-to-weight ratio.

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Are you sure you didn't mean the F8F? I'm pretty sure the Bearcat would outclimb the Tigercat...

My Figures for Cd0 at low Mach numbers are Spitfire IX 0.018 (Ackroyd and Lamont) and P-51 0.016 (Loftin). Ackroyd argues that the difference has much to do with the better use of the Meredith effect in the Mustang.

At high Mach numbers, above about 0.70 the Cd0 of the P-51 rapidly rises above that of the Spitfire IX. This Ackroyd attributes mainly to the thinness of the Spitfire wing. It was not an intentional aspect of the design. When asked about the figures Joe Smith replied.
"We had in the late twenties, evolved a guess-work correction for the drag increase, due to compressibility of the tip sections of the Schneider Trophy planes' metal propellers, to
enable us to analyse their performance. However we certainly saw no reason to apply this data to the Spitfire wing."

There are so many variables that the figures are endlessly debateable.

Cheers

Steve

thedab said:

I just found a bit more on that G-2 and it does look that it was running on 1.42ata
on the frist page of this [In Development] Yak-9 - Dev Blog discussion - War Thunder - Official Forum someone has put up some charts with the G-2 on it with it engine power and weights

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Is that "our" stona posting in that thread?