The History of Stealth in Aviation II

The idea of creating stealthy aircraft capable of avoiding detection is as old as the idea of using aircraft in war. All the way back in World War I the Germans created the Linke-Hofmann R.1. This was an attempt to create an aircraft that the enemy would see straight through, and they did so by replacing much of the canvas on its body with cellophane. This ended up having the opposite effect, however, as it would catch the sun's rays and reflect them brightly into the enemies' eyes. Despite its failure though, it shows forward thinking on their part and represented a first step towards stealthy aircraft.

Linke-Hofmann R.1 image from the Ray Wagner Collection

These days stealth is generally referred to as L.O, or Low Observable. This description has five major elements to it:

1. Radar Cross Section (R.C.S.)

2. Acoustic Signature

3. Infrared (I.R.) Signature

4. Visibility

The topic of the day this time is Acoustic Signature, the second of these elements.

Acoustic signature is really just a fancy way of saying the amount of noise the plane makes. Before the development of Radar technology, Sonar (a similar technique that employs sound waves rather than radio waves) was sometimes used to track and identify planes. These days acoustic signature is most important in order to ensure that the enemy doesn't hear your presence.

In order to reduce this, manufacturers have employed a few different strategies.

One strategy that has been used for a very long time is to get rid of the plane's engines and other noise generating components. Essentially, this results in a glider that creates next to no noise and can fly completely silently above enemy territory. This was extremely popular in the 1940's and especially during the D-day invasions as gliders were used to parachute thousands of troops behind the Nazi front lines. It worked because of how primitive radar technology was at the time to where human senses were the first (and sometimes only) way to detect enemy aircraft. The trouble with gliders is that they require a massive wingspan in order to work, a feature that gives them absolutely massive Radar Cross Sections. Another issue is the lack of propulsion. Gliders work when travelling shorter distances relatively slowly, but modern aircraft are expected to fly halfway across the globe in hours - something that gliders simply cannot do with current technology.

British Airspeed Horsea Glider, Image in the Public Domain

That being said, another popular strategy is employing less noisy engines, after all they're the loudest part of a plane and decreasing the noise they produce can go a long way to decrease a plane's overall acoustic signature. This is especially popular with commercial aircraft - not because they need to avoid detection - but rather so that they don't disturb neighborhoods whenever they take-off or land from airports.The only issue with this is that many jets use combustion in their engines in order to generate thrust and no matter how noise efficient you can make those, they will always produce a detectable amount of noise. That is, unless you can design the plane to absorb and dampen that noise.

Here we once again have to mention the incredible design work that went into the B-2. The engines in the B-2 are mounted mostly inside the plane rather than underneath it as in commercial aircraft. This has the benefit of letting the entire body of the plane act as a dampener for the sound, and since the engines are slightly above the middle of the plane this also reduces the amount of noise that spills over from the rear end of the engine, while giving the bomber a unique advantage when it comes to its infrared and thermal signature.

Image of a B-2 Spirit in flight by tusrhode via Suwalls

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