Drag of radial-engined fighters

[tomo pauk]

Reading the 'America's hundred thousand', one can find two different Cd0 values for the P-47. One being 0.0213 (pg. 113), another one being 0.251 (pg. 598 ). Ie. the 1st value is in the ballpark with Spitfire (single stage engine) or P-39, another one seem much more believable. IIRC The Fw-190A (A-3?) was credited with CdO of ~0.0245.
I'll ask people to share their data here, about all radial-engined fighters

 

[davebender]

5.4. DB601E
5.5. DB605A.
5.8. Merlin X
6.1. Merlin 61.
6.1 to 6.2. Allison V1710 series.
6.3. Ju213A.
6.9. DB605B
7.0. DB603A
7.5. Merlin 66
12.6. P&W R1830
13.4. R2000
14.7. BMW801D.
15.0. R2800
16.0. R2600.

These will vary a bit depending on specific engine installation. However they should be in the ballpark.

 

[tomo pauk]

Maybe we could get this ball rolling with the attached table? The XF8F was to be at Cd=0.019, despite its whacking thick wing (18% TtC at root) - better than any Spitfire??
(please open the pic separately)

 

[fastmongrel]

5.4. DB601E
5.5. DB605A.
5.8. Merlin X
6.1. Merlin 61.
6.1 to 6.2. Allison V1710 series.
6.3. Ju213A.
6.9. DB605B
7.0. DB603A
7.5. Merlin 66
12.6. P&W R1830
13.4. R2000
14.7. BMW801D.
15.0. R2800
16.0. R2600.

These will vary a bit depending on specific engine installation. However they should be in the ballpark.

Why would the Merlin 61 and the 66 have such different frontal areas. Surely the 66 is just a 61 with a different supercharger ratio.

 

[Elmas]

The frontal area of an inline engine it is not very significant if you do not add the frontal area of the radiator (s)......

 

[wuzak]

Why would the Merlin 61 and the 66 have such different frontal areas. Surely the 66 is just a 61 with a different supercharger ratio.

There were some changes, but the frontal area should definitely be the same.

 

[wuzak]

The frontal area of an inline engine it is not very significant if you do not add the frontal area of the radiator (s)......

Not necessarily. Leading edge radiators, such as those used in the Mosquito, present little or no more frontal area than that of the section of wing.

And in many cases the frontal area of the radiators is significantly smaller than the surface area of the radiator - see the Mustang.

Annular radiators are only slightly bigger than the engine behind them, and the P-39 had a radiator completely enclosed within the fuselage, presenting no frontal area whatsoever.

And if you are going to include cooling mechanisms in frontal area, you should perhaps also consider the are of cowl flaps - which could be significant on radials (and annular radiators).

 

[swampyankee]

There have been studies about this, including wind tunnel tests. Except for the P-51, which probably had the best designed cooling system of any mass-produced piston-engined aircraft, there is very little difference in the zero-lift drag coefficient of aircraft with radial engines vs those with liquid-cooled inlines (there were air-cooled inlines). So, believe the 0.0213. And believe the 0.028 that I've seen for some marks of the Bf109. And take them all with a grain of salt.

As to frontal area? For subsonic aircraft -- a group which includes every piston-engined aircraft -- nose shape is unimportant as long as there's no separation. The total surface area (wetted area) is. As for air cooling? All aircraft engines are air-cooled; it's just that liquid-cooled engines do it with an indirect heat exchanger. I've got (or had; I've moved twice since I printed them) printed copies of several AIAA papers on the subject, and all agreed that cooling drag is not significantly different between radials and liquid-cooled inlines.

 

[swampyankee]

Not necessarily. Leading edge radiators, such as those used in the Mosquito, present little or no more frontal area than that of the section of wing.

And in many cases the frontal area of the radiators is significantly smaller than the surface area of the radiator - see the Mustang.

Annular radiators are only slightly bigger than the engine behind them, and the P-39 had a radiator completely enclosed within the fuselage, presenting no frontal area whatsoever.

And if you are going to include cooling mechanisms in frontal area, you should perhaps also consider the are of cowl flaps - which could be significant on radials (and annular radiators).

For all the radiators, there is a diffuser in front, to slow the air, a radiator, which will transfer heat, and further slow the air, and a duct return the air to the free stream. Shockingly, this is exactly the same as what happens with an air-cooled engine. The only potential advantage for a radiator is that you're a bit more free about where you can put it relative to the engine.

 

[Shortround6]

No matter what engine type, there is cooling drag. It can be major like in a 9 cylinder radial engine of 55in diameter and no cowl or it can be minor or even converted to positive thrust like the Mustang was supposed to do. It is a bit different than aerodynamic drag of the installation. There is a certain amount of work that needs to be done in cooling the the engine and that requires a certain amount of airflow.
The P-39 installation (or the Mosquito) had two elements of drag. The airflow through the radiator matrix and the drag in the ducts (or wing sections) leading to and from the radiators. The cross section of the aircraft may not have changed but there was drag caused by the cooling system. Engineers/designers just got a lot better at designing the installations as time went on.

And lets please remember that 1940 was only the 37 year of flight and streamlining had come a looooong way in just a few years.

late 1920s liquid cooled engine installation.

Nice cowling on the engine but the radiator was a maximum drag set up. ejector exhausts?

Some of the 'larger' aircraft weren't much better.

Gained in "frontal area" but the overall drag?

For the Lockheed Vega

The cowling was worth 20mph in top speed even with the "same" frontal area.

And "frontal area" of an engine was only of importance IF it was a dominating feature of the aircraft.

Sticking a 3.0 sq ft inline six on a plane that sat the passengers side by side didn't gain much over the same plane fitted with a 7.5 sq ft radial

For single seat fighters the frontal area of the engine becomes more important but since a seated pilot with an adequate view over the nose has a larger frontal area than most V-12 engines extra small engines don't really have much of an advantage.

There are a lot of other factors that come into the drag differences between the engine types.

 

[drgondog]

The CDo, or total parasite drag at zero lift, for virtually all WWII fighter aircraft is dominated by the wing - not the frontal area. Wetted area becomes significant in the discussion when comparing a big fighter like the Mustang to a Bf 109 but not near as much as the wing profile drag

The profile drag of the Mustang NAA 45-100 wing as well as the friction drag of the filled and primed and painted wing was simply much less than the wings of the Bf 109, P-47, Spitfire, etc.

Frontal area is important when free stream air flow is brought to a 'halt' as a stagnation point. Whatever that area of stagnation may be, it becomes flat plate drag for that relative area. That is one reason to be careful about canopy windshield design or uncowled engines.

 

[Elmas]

Very simplistic ( Bernoulli and Thermodynamics inside the ducts etc .....)

but, between this situation

and this situation

drag is different, isn't it?

 

[Shortround6]

Maybe

Frontal area is different for sure. Drag may not be inline with the change in frontal area though.

For a given fighter design ( same payload= armament, equipment, crew, fuel) using roughly the same power engines the wings are going to very close (P-36 vs P-40, Ki 61 vs Ki 100 etc) which means that while the wings form the largest part of the drag they don't affect the difference we are discussing.

Things were changing very rapidly in the 1930s and there were few good wind tunnels available ( most were small and/or limited in speed of the air stream) so a lot of 'streamlining" was based off hunches/guesses/intuition which turned out to be wrong given later actual knowledge.

for example early GeeBee Racers went from this

to this

using the same engine in a quest for streamlining.

The early 30s saw the change from pure water to Prestone cooling which allowed higher temperatures and smaller radiators for the same power.

Please member that it was only about 10 years from radiators like this

to the P-51 radiator set up.

 

[Timppa]

Reading the 'America's hundred thousand', one can find two different Cd0 values for the P-47. One being 0.0213 (pg. 113), another one being 0.251 (pg. 598 ).

The first value is incorrect, or at least not representative to the actual wartime P-47. Note the time of the test, 9/11/41. Only one protype, XP-47B was flying at the time (without guns etc. as seen from the photographs).This Cd0 value is much better than that of the F4U-1, with the same engine, but the latter was still faster.

Experience gives engineers some kind of baloney radar, they can look figure with a nasty suspicious eye and think if it looks and feels right.
I think Dean was a good engineer, but he missed this one (taking it at face value).

 

[Balljoint]

This was supposedly the great almost equalizer.

NACA cowling - Wikipedia, the free encyclopedia

 

[Shortround6]

It was in 1932-33 but then liquid cooled engine installations got better. The previously mentioned Prestone (glycol) cooling fluid made it's debut. As aircraft speeds increased the fixed exit slot/area of the original NACA cowling no longer worked at all speeds and cowl flaps were developed. Making the inlet of the cowling smaller also helped drag but sometimes cuffs were needed on the prop blades or prop spinners were used to keep air flow from reversing on the ground and air flowing forward around the prop shaft and out of the cowling with out cooling the engine.

It was a continual see-saw between the two types of engine with one or the other rarely enjoying a dominance over the other for very long. Post war commercial use has as much to do with operating economics as performance.

 

[tomo pauk]

FWIW: the cooling drag of the Fw-190A-8/A-9 (radial engine) was 'worth' 0.073 m^2, vs. 0.039 for the Fw-190D-9 (V-12, liquid cooled), as the 'equivalent flat plate'. In square ft, that would be 0.786 vs. 0.42.
Fuselage (including leaks, cabin, interference, imperfections): 0.156 m^2 vs. 0.1393. Two categories combined give 0.229 (A-8/A-9) and 0.1783 (D-9). Total equivalent flat plate was 0.485 m^2 vs. 0.444 (external intakes add a bit drag for the D-9, 0.026 vs 0.009 m^2 for the internal intakes of the A-8/A-9).
All values are for 'schnellflug', high speed condition (ie. not for climbing ('steigflug')). For climbing, it was 0.623 vs. 0.596.

Looks like just switching to the liquid cooled engine gave the reduction in Cd0 of 10% for the Fw-190?

 

[gumbyk]

Maybe

Things were changing very rapidly in the 1930s and there were few good wind tunnels available ( most were small and/or limited in speed of the air stream) so a lot of 'streamlining" was based off hunches/guesses/intuition which turned out to be wrong given later actual knowledge.

for example early GeeBee Racers went from this

View attachment 263192

to this

View attachment 263193

using the same engine in a quest for streamlining.

That also has a lot to do with propeller efficiency. Only that portion of the propeller outside the engine cowling provides significant thrust. Smaller cowling, more efficient propeller. This is greatly simplified, but you get the idea...

 

[Shortround6]

It could be. The difference between the P-40 and the experimental P&W test hack with the R-1830 was judged at 8% more for the radial. Not sure how they counted exhaust thrust though

A problem with air cooled engines is that only air that passed within 3/16 in (4.76mm) of the engine did any cooling and while a somewhat turbulent air flow could bring fresh air into contact with different engine parts, turbulence equals drag. There was a lot more to cooling and air cooled engine than the size of the inlet and outlet of the cowl.

Most high powered engines ( above 500hp ?) used sheet metal baffles to force the air through the fins as any air that passed along the engine outsidethe fins did little or nothing to cool the engine but only contributed to drag.

two row engines had more complicated baffling to ensure equal of front and back rows.

Trusting to luck like they did in the 1920s

wasn't going to work with cylinders making 85-120hp each.

Many museum engines have the baffles removed to provide a better view as do many company pictures of engines.

 

[Shortround6]

That also has a lot to do with propeller efficiency. Only that portion of the propeller outside the engine cowling provides significant thrust. Smaller cowling, more efficient propeller. This is greatly simplified, but you get the idea...

Actually the red airplane (type R) has larger cowling than the black and yellow airplane (type Z) and is the later airplane. and faster.