At first this
may seem a slightly strange blog topic. I am a member of the local U3A Science
and technology group. Each member gives a presentation usually 1 hour long but twice a year short centred
around an object members present. For my next presentation I have chosen this
topic. It combines my interest in modern history with aviation technology.
The allied
forces in WW11 had diametrically opposed philosophies on bombing. The RAF found
that daylight bombing produced huge losses. From early on they switched to
night bombing. Because navigation was imprecise this had to be area bombing.
Even area bombing demanded development. As the war progressed a number of
navigation and operational aids were developed. These aids were very necessary.
A detailed study in 1941 revealed that half of all bombs fell more than five
miles from their target.
The American
philosophy was quite different. They used heavily gunned aircraft and what they
thought was a superior bombsight called the Norden sight. Using this approach
they thought it would be possible for bombers ( called Flying Fortresses ) to
fight their way to targets and then bomb with precision in daylight. Bitter
experience showed this was not possible. Firstly German aircraft could inflict
big losses on the Fortresses and secondly precision bombing was not possible in
the murky north European weather.
What the
Americans needed was a long range fighter to escort the bombers to the target.
Such an aircraft evolved over several years as the North American P-51 Mustang.
( the US called fighter aircraft “pursuit” planes hence the P designation). The
Mustang was originally designed to a British specification at the start of the
war. It used a turbocharged Allison engine but only performed well at low
altitudes.
The Mustang
was designed with a then new laminar flow wing. Essentially this wing was
designed for lower turbulence by a flatter aerofoil which gave lower drag. Even
so this couldn’t explain why the Mustang was so much more slippery than others.
It was found to be a good 30mph faster than other aircraft with the same power.
The reason, not realised for years, was that the Mustang had been designed with
flat sides rather than the usual rounded ones. This meant it had reduced wave
drag in accordance with the area rule. This rule states that minimum transonic drag requires approximately constant cross
sectional area. Where the wings increase the area the fuselage needs to have a
smaller cross sectional area to compensate.
Another
source of improvement was in the belly radiator of the water cooled engine.
Work by NACA established that the lip of its cowling needed to be set away from
the airplane structure. Close to the airplane the airflow is static at the
surface rising rapidly with distance from the surface. This region is known as
the boundary layer. If the radiator takes in boundary layer air it creates
turbulence and drag Also the radiator
could be designed so that the waste engine heat expanded air within it giving a
small jet thrust.
However the
big change came with the use of the licence built Merlin engine with its two
stage, two speed supercharger. This boosted the engine power particularly at
altitude by about 30%. The revamped Mustang was designated the P-51B or P-51C
according to the factory in which it was built. The revised model also had
extra fuel tankage in the fuselage.
The final
factor in improving the range was the use of drop tanks. These temporary
underwing tanks were designed to be dropped when empty enabling the normal
fighting capacity to be restored.
The use of
the revised Mustang to escort US bombers was extremely successful. The Mustang
could fly with the bomber stream as far as Berlin.
The impact of
the bombing campaign is still debated but there is no doubt it contributed to
ultimate victory.
No comments:
Post a Comment