Chaff was quite effective as a means of creating false targets on the screens of enemy radars. Small clouds of the material lingered in the air long enough to clutter radar screens and confuse real targets from false ones.
Strip chaff was bundled in light cardboard packets about the size of a paperback novel. Hundreds of small individual pieces of foil, barely bigger than lengths of wire, would scatter as soon as the slipstream tore open the surrounding cardboard. From altitudes of 20,000 feet it took several minutes for the chaff to reach the ground.
Ribbon chaff, also packed in light cardboard, was one long piece of aluminized material. It looked like a length of shiny magnetic tape from today's cassette music players. One end of the ribbon was attached to a rectangular piece of cardboard which acted like a parachute, with the ribbon dangling down below as it fell to ground.
The strips of chaff were cut to particular lengths, according to the radio frequency band of the radars meant to be countered. Lengths of four to ten inches were used, though individual packets typically contained but one length. Shorter lengths were intended to counter high frequency radars and longer strips were used for lower frequencies. Each individual strip of chaff acted as a reflecting dipole (antenna) to send echo signals back to the radar receiver.
Ribbon chaff acted similarly, but one twisting and turning ribbon could fool radars of many frequencies. As the reflective ribbon twisted and untwisted in descent, the length between twisted segments varied, causing the reflective lengths to effect higher and then lower frequency radars. Ribbon chaff did not create the large false images on radar screens that strip chaff clouds did, but the versatility of varied frequency reflectivity made it useful in some applications.
One problem with aluminized mylar chaff, both strip and ribbon, was its effect on hitting the ground. If the material landed in pastures of grazing cattle, the stuff would be eaten by the animals just as if it were grass. Accumulations of the metalized strips eventually caused stomach problems for the livestock. Farmers were not happy when their cows ate the "window".
When ribbon chaff got down near the ground, it often caused quite different problems. Long twisting ribbons of the metalized foil, draped over electric power lines, occasionally played havoc with public utilities. Electrical power blackouts, sometimes effecting wide areas, were caused by the ribbon chaff. Military training flights were soon barred from dispensing ribbon chaff over the continental U.S..
Another difficulty with chaff bundles arose from moisture and the cold of high altitudes. Military aircraft, with bundles of chaff loaded into their dispenser hoppers, often sat out in the rain. Cardboard wrappers around the chaff bundles absorbed water, sometimes enough to soak the chaff within. When the airplanes climbed to altitude, where temperatures are well below freezing, the chaff packets froze solid. Chaff bundles dispensed in a frozen state tend to remain as solid bricks and the slipstream does not scatter the material as intended. Those chaff "bricks" posed a hazard to people and property on the ground too.
The effectiveness of chaff depends upon a variety of factors. It is much less useful today, with the advent of very high speed airplanes, than it was in the 1940's and 50's.
Once a bundle of chaff is tossed or dispensed from an airplane, its relative velocity stops almost instantly. The real radar target, the airplane, quickly moves away from the slowly descending chaff. The cloud of chaff continues to appear on the radar scope, unless special echo processing techniques are employed, and the airplane's echo can be seen moving away from its protective chaff. Discerning which echo is the airplane and which is the chaff is not difficult. The obvious motion differences, between the high speed airplane and the nearly motionless chaff become a problem.
The advent of radars which discern targets of different velocities ( high, medium and low ), by echo signal processing, has greatly diminished the utility of chaff. These radars feature signal processing circuits, known as moving-target-indicators (MTI), which quickly ignore stationary targets and portray only moving ones to the radar operator.
In WWII days, however, chaff was widely used and proven very effective, when properly employed. For example, One airplane dropping a stream of chaff bundles could form a long cloud echo pattern on a radar scope, masking airplanes flying through it. Several airplanes dropping multiple trails of chaff could obscure many airplanes.
An airplane dropping random clusters of chaff creates multiple echo patterns on the radar screen. Several airplanes, flying along in parallel, or better yet crossing tracks, could create a screen-full of false targets. Sorting out the real from the false echoes took time and a lot of work for radar operators, in the early days.
Some techniques were developed to extend the usefulness of chaff in protecting high speed airplanes. Chaff rockets, fired ahead of the airplane, dispensed reflective material in the path of the craft. The radar echo of the chaff would then include the same airspace as the plane flying through the chaff. This technique was not very effective, and certainly much more costly than dropped chaff bundles. It also failed to solve the immediate velocity difference between the fast moving airplane and the low velocity chaff.
Today's modern equivalent of chaff is the flare. Flares are pyrotechnic devices which are used to decoy infrared (IR) homing missiles. The guidance systems of some ground-to-air anti-aircraft missiles employ electronic sensors which track the heat signals of airplanes. The heat of jet engines is easy for a missile to separate from the cold IR characteristics of blue sky. The missile tracker systems simply home in on the heat source.
The purpose of the flare, for the target aircraft, is to confuse the incoming missile and induce tracking errors causing it to miss. Where chaff used to be effective in the low frequency (radio wave) frequency spectrum of radars, the flare is still moderately effective in the very high frequencies of IR. A great deal of sophistication has evolved in the design of IR trackers and IR decoys meant to counter them.
Effective use of chaff, in earlier times, and IR flares today, depends upon the timing of their use. Knowing when to dispense such decoys is critical. The problem for the flight crew is determining when, how and if to dispense the life-saving decoys. In fact, that's the crux of the survival problem, and the subject of an entire treatise outside the context of this presentation.