Spitfire's drag - what was good, and what was not? (1 Viewer)

[tomo pauk]

Reading recently about the Fw-190As, one thing from the manual for the A-5/A-6 popped out as interesting: the A-5/A-6 were as fast, on about the same HP, on same altitude, as Spitfire MkV (Fw using the Max continous rating, 1180 PS) or Spitfire Mk.VIII/IX (Fw using the Emergency rating, 1440 PS). I wonder why the Spitfire was not faster, being powered by the V-12 (= inherently less drag)? Was it because of it's bigger wings, or the fixed tail wheel messed things in Mk. V/IX case (but still the Mk.VIII cannot come ahead, despite the retractable tail wheel), was it because the radiator(s) were draggy, or all parts played about the equal part? IIRC the Spitfire having thinner wing (someone with knowledge can chip in here, too), to offset the total wing drag? The Fw-190A-5 was also far heavier, especially vs. the Mk.V.
The chart can be found at Williams' site, the red dot represent's Mk.V's best value. The pink dot represents Mk.VIII/IX, 404 mph at 21000 ft (Merlin 66 aboard); the hi-alt versions of Merlin were able to propel the Spit at thinner air (less drag = more speed), but obviously they cannot be compared here. Maybe with the D-9.
The 'slower' lines of the Fw's speed are the one of interest (= with compressibility taken into account).

 

[Juha]

Hello Tomo
Spitfire's important drag-generators when compared other contemporary fighters, draggy radiator(s) and less inclined windscreen with sharp corners.

juha

 

[RCAFson]

The FTH of a Merlin 45 was about 13000ft, at 16lb boost, and at that Altitude the Spit V made about 370 mph. I think you need to compare engine output vs altitude and also use Allied estimates of the 801 engines output, since there may be differences between the Luftwaffe and RAF in determining engine output.

 

[drgondog]

I agree Juha but would add that the additional wing area was a parsite drag contributor in contrast to 109 and 190. The CD0 of the Spit was slightly lower than the FW 190, for example, but the wetted area was larger and the resultant flat plate drag comparisons favor the FW 190 by ~ 3%.

 

[tomo pauk]

Hello Tomo
Spitfire's important drag-generators when compared other contemporary fighters, draggy radiator(s) and less inclined windscreen with sharp corners.

juha

Thanks.

The FTH of a Merlin 45 was about 13000ft, at 16lb boost, and at that Altitude the Spit V made about 370 mph. I think you need to compare engine output vs altitude and also use Allied estimates of the 801 engines output, since there may be differences between the Luftwaffe and RAF in determining engine output.

I was comparing engine output with altitude, but don't mind to elaborate further on the matter.
The Merlin 45 running at 16 lb boost @ 13000 ft (1500+ HP) will be running at 9 lb boost @ cca 20000 ft, producing 1150-1200 HP there (all with ram effect). The Fw's 1180 PS means ~1164 HP (one HP is worth 1.014 PS), and are achieved at 19680 ft, with ram.
At those 13000 ft (~4030m), Spit's 1500 HP (370 mph) are to be compared with under 1400 HP of the Fw, and the Fw still comes ahead (almost 610 km/h, or almost 380 mp/h).

The data for the Spitfire VIII (from Williams' site):

Spitfire LF VIII	Merlin 66	
1,720 @ 5,750'   384 mph @ 10,500' MS
1,595 @ 16,000' 404 mph @ 21,000' FS

The value of 1595 HP at 16000 ft is for the static engine, ie. no ram. The altitude of the max speed at FS gear (21000 ft) points us at the altitude value with ram. So we compare 1595 HP and 404 mp/h for the Mk.VIII; the Fw-190A-6 making 410 mph with 1420 HP (=1440 PS) at ~20320 ft.

I agree Juha but would add that the additional wing area was a parsite drag contributor in contrast to 109 and 190. The CD0 of the Spit was slightly lower than the FW 190, for example, but the wetted area was larger and the resultant flat plate drag comparisons favor the FW 190 by ~ 3%.

Thanks - size indeed does matter.

 

[riacrato]

I am not so sure on the radiator, they seem to have a large (exposed) frontal area but iirc the German evaluation of the SpitV with DB concluded that the installation was rather efficient and with better Durchflusswiderstand than the contemporary Bf 109 radiator. Which aircraft had a better (less drag) radiator setup at the time (Mustang, Fw 190 D were certainly better srill some months/years in the future.

 

[RCAFson]

Thanks.



I was comparing engine output with altitude, but don't mind to elaborate further on the matter.
The Merlin 45 running at 16 lb boost @ 13000 ft (1500+ HP) will be running at 9 lb boost @ cca 20000 ft, producing 1150-1200 HP there (all with ram effect). The Fw's 1180 PS means ~1164 HP (one HP is worth 1.014 PS), and are achieved at 19680 ft, with ram.
At those 13000 ft (~4030m), Spit's 1500 HP (370 mph) are to be compared with under 1400 HP of the Fw, and the Fw still comes ahead (almost 610 km/h, or almost 380 mp/h).

The data for the Spitfire VIII (from Williams' site):

Spitfire LF VIII	Merlin 66	
1,720 @ 5,750'   384 mph @ 10,500' MS
1,595 @ 16,000' 404 mph @ 21,000' FS

The value of 1595 HP at 16000 ft is for the static engine, ie. no ram. The altitude of the max speed at FS gear (21000 ft) points us at the altitude value with ram. So we compare 1595 HP and 404 mp/h for the Mk.VIII; the Fw-190A-6 making 410 mph with 1420 HP (=1440 PS) at ~20320 ft.



Thanks - size indeed does matter.

The USAAF came up with these figures which differ widely from yours:

The same report gives the maximum output as 1750BHP:

II Summary

The German Focke-Wulf 190, EB-104 is a single place, low wing all metal monoplane, powered with a 1750 bhp BMW 801-D fourteen cylinder two row radial engine equipped with a two speed internal supercharger.

Fw 190 G-3 Performance Test

 

[tomo pauk]

The better (lower) Durchflusswiderstand (resistance to the flow) was pointing to the resistance to the coolant flow, not to the resistance to the air stream.
Looking at the photos of the Spitfire's wing under side, the radiator lacks boundary splitter (like P-51 had), or tunnel (like Bf-109F and later; Spiteful/Seafand seem to have the same layout, with the tunnel?). Further, the radiator is short, but tall, with wide opening at the front.

 

[tomo pauk]

The FTH of a Merlin 45 was about 13000ft, at 16lb boost, and at that Altitude the Spit V made about 370 mph. I think you need to compare engine output vs altitude and also use Allied estimates of the 801 engines output, since there may be differences between the Luftwaffe and RAF in determining engine output.

The USAAF came up with these figures which differ widely from yours:
View attachment 213143

The same report gives the maximum output as 1750BHP:

Fw 190 G-3 Performance Test

I thought we were comparing engine output vs. altitude
German data (from Fw-190A-5/A-6 manual) gives the max output of 1730 PS (1706 BHP), so it should be within factory tolerances vs. the 1750 BHP output from the report. But that is at ~1500 (4830 ft) of altitude (with ram; 600 m without ram), and there the 190A-6 does 600+ km/h (370+ mph). US chart gives 374 mph at 6000 ft, achieved with 1750 BHP.
For comparison, the Spitfire XII does 375 mph at 4,600 ft; another test gives 372 mph at 5,700 ft. Speeds achieved with 1720 BHP.

Sure enough, a (non turbo) engine doing maximum of 1700-1750 BHP at low altitude is never going to make that power at 20000 ft:

 

[riacrato]

The better (lower) Durchflusswiderstand (resistance to the flow) was pointing to the resistance to the coolant flow, not to the resistance to the air stream.
Looking at the photos of the Spitfire's wing under side, the radiator lacks boundary splitter (like P-51 had), or tunnel (like Bf-109F and later; Spiteful/Seafand seem to have the same layout, with the tunnel?). Further, the radiator is short, but tall, with wide opening at the front.

yes and it meant the spitfire radiator could effectively cool the db engine while having only about 50-60 per cent the size / frontal area of a 109 f or g radiator. of course things change once the intercooler is added.

 

[tomo pauk]

I wonder how much is the better cooling capacity to be attributed to the use of copper, instead of aluminum?

Here is what our drgondog (Bill Marshall) said in another thread, why P-51 was so fast:

Schmeud and Atwood stated 1st (Laminar Flow wing), 2nd (redesigned 51B/D radiator form and boundary layer control over inlet duct, 3rd (Meridith effect).

There is one other possible contribution, namely the windscreen/canopy design. Lednicer presented interesting potential flow model that showed the 51D design to be superior to Mk IX and FW190D canopy and slightly better than a P-51B.

The 51D design seemed to avoid flow stagnation at base of windscreen and therfore improved attached flow properties over top and rear surfaces of the canopy.

As known from the dive tests and limitations noted in manual, the dive speeds were rather high (higher than of P-47, for example). That should point us to the wings as being of relatively low drag coefficient (still the drag force was ~3% greater than Fw, because the wetted area was some 25% higher).
Spitfire was found lacking two things: boundary layer separator (or tunel, playing the same role?) for the radiator(s), while the inner shape of the radiator was ill suited to fully harvest the Meredith effect. Maybe the (under) wing placement was restricting the convenient radiator height, in order to sufficiently slow the air flow after it enters the radiator casing? Also, the radiator casing was restricted in possible length, since it was located between U/C leg space (when retracted) and flap - not allowing to the Meredith effect to happen at a desirable level.

The excerpts from this thread (here)do mention some other things: contrary to the radiator's exit flap of the P-51, that was progressively adjustable, the exit flap from the Spit (IX is mentioned in the article) have had two positions: fully open and partially closed, again messing with the M. effect. P-51s (Packard Merlin versions?), with propeller giving 1000 lbs while engine in full power, radiator's drag was 400 lbs, being couteracted with 350 lbs of thrust from the Meredith effect. So the cooling drag went down by an order of magnitude.

Finally, about the fixed tailwheel: Germans discovered that such a thing costs 12 km/h in their Bf-109Gs, or 7,45 mp/h.

 

[tomo pauk]

The Spitfire shared one feature with it's arch rival, the Bf-109, namely the wheels that were not fully closed up when retracted (maybe some 50% of the wheel was covered, for both planes). Germans concluded that wheel well covers can add cca 11-14 km/h for their Bf-109Gs (from Kurfurst's site).
Spitfire introduced the wheel well covers with Mk.21. From spitfiresite.com:

The undercarriage could be also fully enclosed in flight due to the new outer undercarriage covers.

 

[Tante Ju]

yes and it meant the spitfire radiator could effectively cool the db engine while having only about 50-60 per cent the size / frontal area of a 109 f or g radiator. of course things change once the intercooler is added.

It's a a bit tricky to compare radiator sizes by their numbers.. The Spitfire V used one rather large and heavy radiator of copper, the 109 used two smaller and lighter (though in total weighting about the same) coolant radiators.

 

[riacrato]

True, but the question was on drag. And the Spitfire seems to definetly have a smaller frontal area. So all else (total weight, cooling performance...) being equal, they seem to be better overall.

 

[tomo pauk]

Curiously enough, the early Fw-190s were featuring the wheel well covers, while the later ones did not. Instead, the covers attached at the wheel struts were of increased area, now covering maybe 75-80% of the area. With the every sub-version got both heavier draggier, no wonder the Fw-190A steadily got slower, from A-3 to A-8.

added: the wheel covers were removed due the installation of bomb/fuel fuselage rack, the presence of the rack making the covers operation impossible. Information via Dieter Hermann.
The wheel covers for the Bf-109 were installed in the K-4, but it was experimented much earlier.

 

[drgondog]

True, but the question was on drag. And the Spitfire seems to definetly have a smaller frontal area. So all else (total weight, cooling performance...) being equal, they seem to be better overall.

For the classic long lived fighters, all (except US) had a relatively small frontal area. The wing first, then wetted area second - are the prime contributors to Drag.

 

[tomo pauk]

Guess riacrato was thinkering of radiator's smaller area, rather than of whole plane.

Further on Spitfire: the Mk.III is stated as being equipped with 'wheel well flap', presumable a wheel well cover? However, the 'flap' is not easy to spot on the photos. Wonder if someone has some details about it?

 

[Edgar Brooks]

Sorry, finger trouble

 

[Edgar Brooks]

They're visible in some photos, both with blunt-tipped and elliptical wings; it only lasted as the Mk.III for less than a year, so photos are a bit rare. With the u/c down, the flaps were parallel to the ground, so only easily visible from front or rear.
The inflated dimension of the early Spitfire tyre was 24.51" (maximum,) with a hub diameter of 10"; this would give a maximum ground clearance, for the flaps, of 7.25" (18.4cm.,) possibly slightly less, which isn't much on a grass airfield, with oleos with nearly 5" of permissible travel.

 

[tomo pauk]

Many thanks