Mosquito aerodynamics?

[spicmart]

In the case of the wide, low radiator ducts, as used on the Spiteful, there was actually boundary layer thickening on the roof of the duct. The boundary layer air is not moving very quickly relative to the aircraft, so this reduced the effectiveness of the radiator. I can't recall exactly how much of the radiator the boundary layer covered, except that it was around 1/8 the depth of the radiator. Which is significant. This was from tests done of the production style Spiteful radiators.

Another solution was to provide a duct the entire length of the radiator duct, gong above the radiator.

Is this what you mean?

Bf 109 F
This shallow bypass air duct for boundary layer extraction is only available on the Bf 109 F.
Due to the increased flow velocity of the air flowing through, the channel only reduces the air resistance which the cooler housing generates.

 

[nuuumannn]

Following on the discussion about radiators, here are some images of the P-51's radiator for comparison with the others posted here already. The Mustang's opening and its boundary layer diffuser are plainly evident.

Intake

The oil cooler is also inside the orifice, with the radiator further back.

Intake inside

This is the oil cooler outlet under the aircraft's belly.

Oil cooler outlet

Further aft of that is the radiator outlet. Ignore the bump to its rear, it is a concession to modern avionics.

Radiator exhaust i

The size of the radiator and orifice from where the Meredith Effect thrust element exits can be seen here.

Radiator exhaust ii

 

[wuzak]

Is this what you mean?

Bf 109 F
This shallow bypass air duct for boundary layer extraction is only available on the Bf 109 F.
Due to the increased flow velocity of the air flowing through, the channel only reduces the air resistance which the cooler housing generates.

Yes

 

[spicmart]

So did the Spiteful also have this feature?
Messerschmitt gave up on this after the 109 F-version for whatever reason.
Maybe the cost-benefit ratio was not worth it. It doesn't seem too complex though.

 

[pbehn]

So did the Spiteful also have this feature?
Messerschmitt gave up on this after the 109 for whatever reason.
Maybe the cost-benefit ratio was not worth it. It doesn't seem too complex though.

As I understand it, it preserves the boundary layer but by reducing the flow through the radiator. So you need a much bigger radiator to do the same amount of cooling which defeats the purpose of it.

 

[wuzak]

So did the Spiteful also have this feature?
Messerschmitt gave up on this after the 109 for whatever reason.
Maybe the cost-benefit ratio was not worth it. It doesn't seem too complex though.

No. The production radiator duct was a plain duct, without boundary layer duct.

It was investigated, though.

 

[wuzak]

As I understand it, it preserves the boundary layer but by reducing the flow through the radiator. So you need a much bigger radiator to do the same amount of cooling which defeats the purpose of it.

The problem identified for the Spiteful was that the boundary layer thickened in the duct and reduced the effective area of the radiator.

 

[pbehn]

The problem identified for the Spiteful was that the boundary layer thickened in the duct and reduced the effective area of the radiator.

I was talking about the 109 set up, preserving the boundary layer worked but reduced the efficiency of the radiator. (that's how I understood it anyway).

 

[Gemhorse]

Anyway, the DH Mosquito's design lead on also to the DH Hornet, which had a level speed of around 487mph and was a study (at the time) of streamlining, particuarly of the Merlins 130/131's cowlings... all superb considering there was largely wooden structure involved, leading onto the first wood & metal being glued, DH using this new 'Redux' adhesive on the Hornet, all very new development that DH were pushing ahead with, and they then followed-on with the jet Vampire...

 

[pbehn]

Anyway, the DH Mosquito's design lead on also to the DH Hornet, which had a level speed of around 487mph and was a study (at the time) of streamlining, particuarly of the Merlins 130/131's cowlings... all superb considering there was largely wooden structure involved, leading onto the first wood & metal being glued, DH using this new 'Redux' adhesive on the Hornet, all very new development that DH were pushing ahead with, and they then followed-on with the jet Vampire...

Part of the Mosquito was its cooling and general streamlining, but it was a bomber initially. Compare to a Lancaster, the "boundary layer was passing over a cheese grater by comparison.

 

[fliger747]

Yes the surface smoothness and ability to carefully design and control shape gave advantages to the wooden construction. I am not sure about the weight for strength as opposed to more conventional alloy construction. Not that WWII aircraft were designed for a long life span given a typical combat aircraft lifespan, but the wooden wonders of various sorts did not hold up well to adverse climates such as in the humid tropics. This probably limited the applicability of such airframes to a post war environment where longevity became a consideration.

 

[pbehn]

Yes the surface smoothness and ability to carefully design and control shape gave advantages to the wooden construction. I am not sure about the weight for strength as opposed to more conventional alloy construction. Not that WWII aircraft were designed for a long life span given a typical combat aircraft lifespan, but the wooden wonders of various sorts did not hold up well to adverse climates such as in the humid tropics. This probably limited the applicability of such airframes to a post war environment where longevity became a consideration.

Comparing wood to metal, wood has less torsional strength, this just requires different methods of design and construction. There were some problems in the far east but more concerning adhesives than the actual wood.

 

[chuter]

--->According to Lee Atwood<---, only the P-51 and Mosquito got Meredith Effect to pay off. Everyone else (WW2) tried to adapt it to existing designs and failed so those engineers decided the Mustang's and Mosquito's speed must be due to the wing in the Mustang's case and clean airframe in the Mosquito's case. He said the Mustang's wing was actually a failure in the field, as far as laminar flow speed gains are concerned.

Apparently the 109F's boundary layer bypass didn't pay off as duct stall wasn't eliminated and a larger cooler was seen as a an easier and slightly better performing option.

 

[fliger747]

The wood construction was in effect a very early composite. And with such a material the adhesive or binder is an essential part of the material. "Wood" is perhaps a misnomer for this material. The Hornets posted Post War to SE Asia did not hold up well, indeed to issues with the adhesive. Various models of the 737 used adhesive rather than complete riveting, which resulted in the famous "Zip Top" that successfully landed in Maui after a terrifying failure of the forward fuselage shell.

Wood, depending on the species and grain alignment, and layering can be engineered to quite sophisticated properties. Most Alpine Skis still have wooden cores. My powder skis have a bamboo core! That the prototype Hughes "Spruce Goose" used wood composites was quite an accomplishment for the era. Of course had the plane ever been produced the idea was to use alloys.

 

[Shortround6]

Apparently the 109F's boundary layer bypass didn't pay off as duct stall wasn't eliminated and a larger cooler was seen as a an easier and slightly better performing option.

The radiators have to be sized to the engine and expected airflow.

Which 109 was designed to which engine?
The DB605 in the 109G made 15% more power 800 meters higher (thinner air) than the DB601N engine that the early 109Fs used.
You are going to need bigger radiators, it is just a question of how you arrange them.

 

[fliger747]

I would expect (but do not know) the effect leading edge radiator ducting has on stall characteristics. Certainly they would have very different and probably not better flow over the top of the wing than a regular leading edge section. I would expect an increase in tail plane buffet near the stall at a minimum.

 

[pbehn]

I would expect (but do not know) the effect leading edge radiator ducting has on stall characteristics. Certainly they would have very different and probably not better flow over the top of the wing than a regular leading edge section. I would expect an increase in tail plane buffet near the stall at a minimum.

In late model piston engine aircraft air intakes on the leading edge for water or oil cooling were very common.

 

[wuzak]

The radiators have to be sized to the engine and expected airflow.

Which 109 was designed to which engine?
The DB605 in the 109G made 15% more power 800 meters higher (thinner air) than the DB601N engine that the early 109Fs used.
You are going to need bigger radiators, it is just a question of how you arrange them.

The radiators are also sized for normal continuous power, not 30 minute climb power or 5 minute WEP.

It's part of the reason the time limits exist - the temperature can't be controlled at the higher power settings for too long.

 

[Shortround6]

Thank you for that but it doesn't change the basics,

The 605A was rated for 19% more power 950 meters higher up for the 30 minute rating and 13.6% more power 400 meters higher continuous power.

The 605 made 30 more PS continuous 650 meters higher than the 601N did for it's 30 minute rating.

The 605 was also putting a bit more power into the supercharger.

Basically you needed 14-15% bigger radiators (or 14-15% more cooling capacity) at a minimum for the 109G than you needed for the 109F.

 

[chuter]

The radiators have to be sized to the engine and expected airflow.

Which 109 was designed to which engine?
The DB605 in the 109G made 15% more power 800 meters higher (thinner air) than the DB601N engine that the early 109Fs used.
You are going to need bigger radiators, it is just a question of how you arrange them.

OK - I'll walk you through my reasoning. The Meredith Effect requires non-turbulent airflow to work. The 109F used a bypass duct to aid in achieving this. This duct took up space in the wing and required some effort to produce but would be worth it due to the improved cooling efficiency (non-turbulent airflow) and much reduced (if not eliminated) cooling drag. While the concept looked great on paper and obviously passed some level of testing prior to production, when the G came out with its more powerful engine requiring additional cooling due to its additional power (as you noted) rather than adapt the bypass cooling duct system with its apparent efficiency and drag characteristics by expanding the duct downward they took the cheaper and easier way by removing the bypass duct and expanded upward into the void left by the removed bypass duct. They wouldn't have done this if the duct clearly worked. Look at the Mustang, with the switch to the Merlin the cooling duct expanded downward into the airflow to handle the additional cooling requirement without a drag penalty.