As steam is much less dense than water, a correspondingly larger surface area is needed to provide enough airflow to cool the steam back down. The thrust was significant enough to offset the drag of the duct the radiator was enclosed in and allowed the aircraft to achieve zero cooling drag. Less commonly, power steering fluid, brake fluid, and other hydraulic fluids may be cooled by an auxiliary radiator on a vehicle. Car manufacturers have inundated consumers with marketing proclaiming AWD as a safety feature, although the advantage of AWD over FWD occurs in accelerating, not braking or steering. Some safety experts advocated a performance-based occupant-protection standard rather than one mandating a particular technical solution (which could rapidly become outdated and prove to not be a cost-effective approach). Just ten vehicles are believed to have been made; one drawback was that due to the extra weight of the electric drive, the vehicles were less efficient when running on their engines alone than standard Audi 100s with the same engine. Engines for stationary plant are normally cooled by radiators in the same way as automobile engines. As they are so dependent on airspeed, surface radiators are even more prone to overheating when ground-running.
However, pressurized systems were also more complex, and far more susceptible to damage - as the cooling fluid was under pressure, even minor damage in the cooling system like a single rifle-calibre bullet hole, would cause the liquid to rapidly spray out of the hole. Several aircraft were built using their design and set numerous performance records, notably the Heinkel He 119 and Heinkel He 100. However, these systems required numerous pumps to return the liquid from the spread-out radiators and proved to be extremely difficult to keep running properly, and were much more susceptible to battle damage. This is called the Meredith effect, and high-performance piston aircraft with well-designed low-drag radiators (notably the P-51 Mustang) derive thrust from it. In effect, the evaporative version is operating between 80 °C and 560 °C, a 480 °C effective temperature difference. At first glance this appears to be much less temperature difference, but this analysis overlooks the enormous amount of heat energy soaked up during the generation of steam, equivalent to 500 °C.
Such a system can be effective even with much smaller amounts of water. Efforts to develop this system had generally been abandoned by 1940. The need for evaporative cooling was soon to be negated by the widespread availability of ethylene glycol based coolants, which had a lower specific heat, but a much higher boiling point than water. It is generally a limitation of most cooling systems that the cooling fluid not be allowed to boil, as the need to handle gas in the flow greatly complicates design. In Germany, the Günter brothers developed an alternative design combining evaporative cooling and surface radiators spread all over the aircraft wings, fuselage and even the rudder. Racing aircraft such as the Supermarine S.6B, a racing seaplane with radiators built into the upper surfaces of its floats, have been described as "being flown on the temperature gauge" as the main limit on their performance. Aircraft with liquid-cooled piston engines (usually inline engines rather than radial) also require radiators. For most uses, this solved the problem of cooling high-performance piston engines, and almost all liquid-cooled aircraft engines of the World War II period used this solution.
Such designs were seen mostly on World War I aircraft. In 2005 a simple method of reducing noise was identified: Keep aircraft at a higher altitude during landing. Another effect that is especially important in aircraft cooling is that the specific heat capacity changes and boiling point reduces with pressure, and this pressure changes more rapidly with altitude than the drop in temperature. Although it is more difficult to build an aircraft radiator that is able to handle steam, it is by no means impossible. There are some unique differences, depending on the stationary plant - careful planning must be taken to ensure proper air flow across the radiator to ensure proper cooling. Huntington had begun long negotiations with Harriman about consolidating the Los Angeles electric railways beginning in 1907. There had always been a difference between the two men as to the purpose of the railway, with Huntington seeing the PE as a means to facilitate his real estate efforts, and Harriman seeing it as part of the Southern Pacific's overall transportation system in Southern California. There is no way to predict which random number the transmitter and receiver have chosen to use as the next code, so re-transmitting the captured code has no effect.
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