ThomasP
Chief Master Sergeant
About the use (first use?) of the Meredith effect,
I cannot say authoritatively that the Hurricane was the first production aircraft to incorporate the Meredith effect, but Hawker engineers knowingly introduced aspects of the effect in the production cooling system, with the intent of reducing the cooling drag. Although they were aware of the potential in the Meredith effect (it had originated in the UK after all) and understood most of its theoretical finer points as to what would work, the Hawker design team did not pursue the maximum possible effect for practical reasons.
One reason was the cost of manufacturing what for the time would have been a fairly sophisticated assembly.
Another reason was available space and weight for the assembly. On first look this might not seem correct, after all the Hurricane was a fairly large airframe and the radiator was already in a similar location to that of the P-51, and it might be possible to rearrange the bits and pieces to maintain an acceptable CG. Keep in mind, however, that the Hurricane was intended to be an evolution of an older biplane design into a monoplane (similar to the evolution of the Wildcat), not a largely new design.
Still another reason was the expected top speed and sustained speeds of the new aircraft. The radiator system of an aircraft must be able to meet the sustained speed cooling requirement indefinitely, and the maximum power cooling requirements for various lesser periods (e.g. WEP for 5 min, Military for 15 min, Climb for 30 min, etc). Everything else being equal, the greater the speed, the greater the Meredith effect. In fact the Meredith effect becomes more efficient with square of the inrease in speed. The Hurricane II was effectively 100 mph slower than the P-51B, so if the exact same assembly as used on the P-51B ( at 430 mph) was used on the Hurricane II (at 330 mph) it would only be abut 56% as effective as on the P-51B.
Instead the Hurricane design team relied on a shorter but larger orifice inlet, with a boundary layer lip/notch, and a somewhat shorter exit chamber with a variable outlet area via a movable flap. The boundary layer lip/notch prevented air flow stagnation at the radiator inlet, the short inlet chamber combined with the larger orifice allowed enough air to slow down (via the pressure head effect) to efficiently absorb heat while passing through the radiator, the radiator heated the air up, and the short exit chamber with variable area outlet acted like an ejector nozzle depending on the aircraft speed, power setting, engine temperature, and flap position. In order to keep the controls simple, the pilot adjusted the flap depending on the engine temperature, not for maximum Meredith effect. The normal flap setting was used for maximum sustained speed which would correlate to a sustainable engine temperature, the more closed settings were used for higher speeds where higher air flow helped keep the engine cool temporarily, and the more open settings were for lower speeds at high power settings (while climbing for example) or while idling/taxying and during take-off .
If my information is correct the P-51B cooling system reduced the cooling HP loss by about 80%, or from about 400 HP to about 50 HP.
Using the P-51B system on the Hurricane II would have reduced the cooling HP loss by 45% (i.e. .56 x .80), or from about 260 to 143. This would have resulted in a speed increase of about 9 mph at 12,000 ft and about 13 mph at 19,000 ft. (i.e. if there was no reduction in cooling HP loss the top speeds would have been 9-13 mph less than they were in actual service.)
The actual system used on the Hurricane II only reduced the cooling HP loss by about 90 HP, or from 260 to 170, for a reduction of ~35%. This resulted in a speed increase of about 7 mph at 12,000 ft and about 10 mph at 19,000 ft.
So to sum up, for the loss of 2-3 mph top speed, the Hurricane II cooling system using a reduced effectiveness Meredith effect, saved an undetermined amount of time in development (check out the amount of time the NA team spent in wind tunnel and flight tests before they settled on a final design) and an undetermined amount in development cost, reduced the cooling system structure weight (probably) by about 50 lbs, and reduced the cost (probably) of the cooling system structure by about 50%.
I cannot say authoritatively that the Hurricane was the first production aircraft to incorporate the Meredith effect, but Hawker engineers knowingly introduced aspects of the effect in the production cooling system, with the intent of reducing the cooling drag. Although they were aware of the potential in the Meredith effect (it had originated in the UK after all) and understood most of its theoretical finer points as to what would work, the Hawker design team did not pursue the maximum possible effect for practical reasons.
One reason was the cost of manufacturing what for the time would have been a fairly sophisticated assembly.
Another reason was available space and weight for the assembly. On first look this might not seem correct, after all the Hurricane was a fairly large airframe and the radiator was already in a similar location to that of the P-51, and it might be possible to rearrange the bits and pieces to maintain an acceptable CG. Keep in mind, however, that the Hurricane was intended to be an evolution of an older biplane design into a monoplane (similar to the evolution of the Wildcat), not a largely new design.
Still another reason was the expected top speed and sustained speeds of the new aircraft. The radiator system of an aircraft must be able to meet the sustained speed cooling requirement indefinitely, and the maximum power cooling requirements for various lesser periods (e.g. WEP for 5 min, Military for 15 min, Climb for 30 min, etc). Everything else being equal, the greater the speed, the greater the Meredith effect. In fact the Meredith effect becomes more efficient with square of the inrease in speed. The Hurricane II was effectively 100 mph slower than the P-51B, so if the exact same assembly as used on the P-51B ( at 430 mph) was used on the Hurricane II (at 330 mph) it would only be abut 56% as effective as on the P-51B.
Instead the Hurricane design team relied on a shorter but larger orifice inlet, with a boundary layer lip/notch, and a somewhat shorter exit chamber with a variable outlet area via a movable flap. The boundary layer lip/notch prevented air flow stagnation at the radiator inlet, the short inlet chamber combined with the larger orifice allowed enough air to slow down (via the pressure head effect) to efficiently absorb heat while passing through the radiator, the radiator heated the air up, and the short exit chamber with variable area outlet acted like an ejector nozzle depending on the aircraft speed, power setting, engine temperature, and flap position. In order to keep the controls simple, the pilot adjusted the flap depending on the engine temperature, not for maximum Meredith effect. The normal flap setting was used for maximum sustained speed which would correlate to a sustainable engine temperature, the more closed settings were used for higher speeds where higher air flow helped keep the engine cool temporarily, and the more open settings were for lower speeds at high power settings (while climbing for example) or while idling/taxying and during take-off .
If my information is correct the P-51B cooling system reduced the cooling HP loss by about 80%, or from about 400 HP to about 50 HP.
Using the P-51B system on the Hurricane II would have reduced the cooling HP loss by 45% (i.e. .56 x .80), or from about 260 to 143. This would have resulted in a speed increase of about 9 mph at 12,000 ft and about 13 mph at 19,000 ft. (i.e. if there was no reduction in cooling HP loss the top speeds would have been 9-13 mph less than they were in actual service.)
The actual system used on the Hurricane II only reduced the cooling HP loss by about 90 HP, or from 260 to 170, for a reduction of ~35%. This resulted in a speed increase of about 7 mph at 12,000 ft and about 10 mph at 19,000 ft.
So to sum up, for the loss of 2-3 mph top speed, the Hurricane II cooling system using a reduced effectiveness Meredith effect, saved an undetermined amount of time in development (check out the amount of time the NA team spent in wind tunnel and flight tests before they settled on a final design) and an undetermined amount in development cost, reduced the cooling system structure weight (probably) by about 50 lbs, and reduced the cost (probably) of the cooling system structure by about 50%.