<< Page 48 Page 49: Page 50 >>   Acoustic Stealth Shhh! Be vewwy vewwy quiet...   Sound is possibly the least important of the four aspects of stealth, but it has been given significant thought. The sound of an aircraft usually alerts you to its presence, rather than revealing its precise location. This allows you to start looking more thoroughly, or to point your eyes and your sensors in roughly the right direction. Also, weapons cannot be guided by the sound an aircraft makes – while anti-aircraft artillery and missiles can be guided visually or by radar or infra-red. Still, it is best that an aircraft flying over enemy territory not be given away by its sound, if its sound would be the first thing to alert the enemy to the aircraft’s presence. Not being heard at least buys the pilot more time and gives him the element of surprise. For military cargo aircraft and spyplanes, sound may mean the difference between being shot down or not. There are basically three approaches to making an aircraft not be heard. The first is to fly very high. An airplane ten miles above the earth will not come any less than ten miles away from someone on the ground. Even the loudest of aircraft can get away with flying that high and not being heard (unless they generate a sonic boom). Most military aircraft fly comfortably at 50,000 feet and not-so-comfortably at 60,000 feet. That is well outside of hearing range. The problem with this approach is that bombs delivered from lower altitudes tend to be more accurate, so most combat aircraft do eventually have to fly low enough to be heard. If nothing else, they must take off and land, although they ideally will enter and exit enemy territory at altitude. The second way to avoid being heard is to fly near or past the speed of sound. This guarantees that you will not possibly be heard until you are flying past your enemy. Unfortunately, it also means you WILL be heard once you are starting to fly away from your target, even if you are flying very high. You do get the element of surprise, though, at least from humans (if not from radars). Another problem with this approach is that almost all aircraft need to use afterburners in order to fly this fast, and that the air being slowed down from these speeds as it enters a jet engine gets quite hot. This means almost no jet can fly at the speed of sound for more than 10 or 20 minutes before their engine is too hot and/or fuel runs out. This is enough for a high-speed low-altitude dash once the target is near, but not for the whole mission – the rest of the mission is flown higher and slower. (At the speed of sound, you’re doing 12 miles a minute, so even 10 or 20 minutes gets you pretty far). One exception to this rule is the F-22, which can sustain supersonic speeds without the use of the afterburner, a capability called “supercruise”. The F-22 is the only combat aircraft that can fly its entire mission above the speed of sound. (It has been claimed, but not demonstrated, that the Eurofighter Typhoon can also do this. And, of course, so could a Concorde, but the Concorde is not a combat aircraft). The last approach to not being heard is to, well, actually make a quieter airplane. Two things cause an airplane to make noise: the slipstream (the air being made turbulent as it flows past the airplane) and the engine. To lower slipstream noise, you have to make the airplane smoother and reduce the places where air moves in turbulent, erratic, or otherwise non-constant or non-smooth ways. Luckily, these same measures also lower drag, so in general an airplane that experiences little air resistance (which is one of the primary goals of any airplane’s design) also produces little noise as it moves through the air. Similarly, engines that are more efficient tend to produce less noise. An efficient engine is one that moves large amounts of air by a little bit, while an inefficient engine takes a small amount of air and moves it a lot. Because of this, high-bypass-ratio turbofans make very little noise. In fact, a 777 about to land makes about as much noise from the flaps down, gear out, wings and fuselage at a high angle of attack, etc, as it does from its two GE90 engines. For the first time, airliner engines are so efficient that they make as little noise as the rest of the airplane makes when it moves through the air. Unfortunately, though, combat aircraft tend to have inefficient engines. Taking a small amount of air and moving it a lot means you can go faster (because of the “moving a lot” – equal and opposite reaction, right?) and means your narrower engine accelerates quickly when you need a lot of thrust immediately. So combat aircraft are pretty loud, except for the cargo aircraft and mid-air refuelers that are basically adapted airliner designs. The one big exception to this is the tank-killing A-10 Warthog. It was built to loiter for extended periods of time over the battlefield. Since, for this role, endurance is more important than speed, it was given big high(ish)-aspect-ratio wings, and two nice big slow high-bypass-ratio turbofan engines. These make the A-10 one of the quietest airplanes in operation today. Even though it can fly nowhere near the speed of sound, you don’t hear it until it’s right on top of you. Nothing illustrates better how quiet the A-10 is than an airshow Heritage Flight. Often at airshows, the US Air Force flies modern aircraft alongside older combat aircraft from World War 2. Usually, this involves an A-10 finishing its demo by doing a few passes flying in formation with a P-51, a small single-engine propeller-driven fighter from World War 2. The amazing thing is, when they go by, you can’t really hear the A-10 over the noise of the little P-51’s engine and propeller. So, as you can see, acoustic stealth is really a low priority, and usually comes as the by-product of other design goals (such as fuel efficiency, endurance, speed, or high-altitude flight). Mission planning might take into account the need to not be heard, but even that mission profile – fly subsonically and at high altitudes until approaching the target, then fly lower and faster – is a profile primarily designed to avoid the enemy’s weapons, not so much acoustic detection. The one time when acoustic stealth was top priority was when troop movements in Vietnam were observed from high-flying, glider-like aircraft. These were extremely efficient (less drag, less noise, and long endurance) and quite slow aircraft. They could fly high, but then they would have a harder time carrying out their recon mission (especially at night), so they had fly low and quietly. Lockheed designed a few airplanes around this philosophy, with huge thin wings and slim fuselages (less turbulence in the slipstream), special engine cowlings to muffle engine noise, and very slow-spinning propellers. These designs originated with the X-26 experiments (basically a Schweitzer glider), followed by the powered X-26B which became the operational QT-2, which evolved into the Q-Star. The Q-Star really epitomized this unusual design philosophy, possibly the most acoustically stealthy powered aircraft ever. Experience from this program led Lockheed to design the YO-3, a more refined, less unconventional design. These aircraft could fly quite low at night over enemy camps without being heard, which is really remarkable. And that's it for acoustic stealth. Probably not what you have in mind when you hear about stealth or low observability, but I bet you learned something... :)