Wind-Triangle Computers

Whiz-Wheel Wizardry

April 1, 2000

Philip Dalton’s E-6B computer continues to set the standard

In the beginning, pilots had their hands full just getting off the ground and keeping their machines in the air. But as engines and airframes became more reliable than a politician’s promise, pilots began to venture farther afield.

Pilots flew from one place to another using the most basic form of navigation. It was called air piloting and involved flying from one landmark to the next, a method now known as pilotage. They eventually discovered that simply aiming at a distant landmark resulted in flying a curved track (called a loxodromic curve) when under the influence of a crosswind. Such a flight path is similar to the curved track experienced by later pilots who were to use an automatic direction finder (ADF) to home in on a radio beacon without crabbing into a crosswind. As a result, a lengthy cross-country flight often consisted of a scalloped track, a series of curved segments connecting en-route checkpoints. This obviously increased flight time and distance. Those who flew above the clouds without benefit of landmarks and without knowing how much crab to apply frequently ended up substantially off course and occasionally lost.

The frustration of incessantly drifting downwind of the intended route eventually led to the development of the first practical navigation instrument, the drift indicator (later known as the driftmeter). This device was mounted on the cockpit floor and included a scope through which a pilot could look down and observe objects on the ground passing directly beneath the aircraft. The pilot then rotated a glass plate (in later models) etched with parallel lines until ground objects appeared to move parallel to these lines. The drift angle was then read directly from a drift-angle plate, and it was added or subtracted (depending on the direction of drift) from the true course to obtain the required true heading.

The disadvantage of the driftmeter was that it could be used only when the ground was visible; it was useless above clouds.

A pilot could determine a wind-correction angle by plotting a wind triangle (also called a triangle of velocities) on the aeronautical charts before departure, but most preferred to use the fine art of "guesstimation." (The first aeronautical charts in the United States, by the way, were Rand-McNally Air-Trail Maps.)

For those who may have forgotten, a wind triangle consists of three lines, each of which is a vector. The direction and length of the ground vector represents true course and groundspeed; the air vector represents true heading and airspeed; and the wind vector represents wind speed and direction.

A number of mechanical computers that resembled drafting tools were developed in the late 1920s. These allowed pilots to make navigational computations but generally were too unwieldy to use in the confines of a cramped or open cockpit. Smaller models were either inaccurate or too difficult to read during flight. The most elaborate of these devices was the Le Prieur Navigraph, a large, strange-looking, spider-like contraption that could be used only aboard aircraft equipped with a navigator’s station.

Some pilots of the 1920s planned their cross-country flights using published deduced or dead-reckoning tables. For example, a pilot flying a 100-mph aircraft would consult the 100-mph table and enter it, using the forecast wind speed and wind angle (the angle at which the wind was predicted to blow across the true course) to determine wind-correction angle and groundspeed. Similar tables were used to extract other in-flight data. Considering all the fuss and bother, however, most pilots preferred to simply point their aircraft and go.

What might be the most significant event in the development of navigational computers occurred on June 11, 1930. This was when Philip Dalton enlisted in the U.S. Naval Reserve. He earned his wings of gold a year later on June 25, 1931, as Naval Aviator #4654. It was during his flight training at Pensacola, Florida, that Dalton confirmed the need for a small, user-friendly computer. As a research physicist and consulting engineer who had been educated at Princeton, Cornell, and Harvard universities, he was well-qualified to undertake the development of this much-needed device.

His first such invention was introduced in 1933. It was a compact pocket-size, time-speed-distance computer based on the principle of a circular slide rule. This was the first "whiz wheel" and was called the Model A. It was immediately replaced by the improved Dalton Aerial Dead-Reckoning Slide Rule, Model B. Pilots heralded the Model B and could not understand why such an essentially simple device had not been developed sooner.

Dalton’s next challenge was to develop a computer capable of solving wind triangles so that pilots could determine in advance the predicted groundspeed and the heading required to maintain a given true course under any given wind condition. He apparently did not have too much difficulty achieving his goal, because he introduced the Mark VII only a year later.

One problem with the Mark VII was the steadily increasing cruise speeds of new-generation aircraft. Dalton’s computer was suitable for speeds of up to only 160 mph. The computer could have been lengthened to accommodate higher speeds, but it would have been too long and, being made of 1930s plastic, prone to breakage.

Dalton solved the speed problem by developing the Model G computer. This clever design incorporated an "endless tape" that could be scrolled through the computer. The Model G, however, contained mechanical gearing and was considered by many to be too heavy and bulky to be used as a handheld computer during flight. It nevertheless proved popular with both the U.S. Navy and Britain’s Royal Air Force as a computer that could be strapped to a pilot’s leg. The U.S. Army Air Corps, however, continued to express the need for a light, handheld computer that could be used throughout a wide range of airspeeds. Dalton eventually solved this problem by designing a computer with a rectangular slide that could be moved vertically within the instrument to the desired airspeed. The Army was jubilated with the results and dubbed it the Model E-6, which was the sixth and final iteration of Dalton’s handheld wind-triangle computer. When the Model B speed-time-distance computer was added to the reverse side of the E-6, it became known as the venerable Dalton Dead-Reckoning E-6B Computer, which has been in worldwide use ever since its introduction at the beginning of World War II.

The clouds of aeronautical history have obscured any reason there might have been for using the letter E in E-6B. Perhaps the implication made by pioneer navigator Maj. Gen. Norris Harbold is an appropriate explanation. In his book, The Log of Air Navigation, he says, "Phil Dalton was killed in an airplane [accident at the age of 38] in 1941, but his mark, E for excellent, had been made in air navigation."

Although early E-6Bs were constructed of aluminum, the scarcity of this metal during World War II led to the widespread use of plastic. Another reason that E-6Bs were made out of plastic instead of aluminum was to allow for the use of fluorescent markings on black plastic. This made the scales more legible under "black light," the Army Air Corps’ method of cockpit lighting at the time. Of the more than one million E-6Bs that have been manufactured in the United States to date, a small number of wartime units were made of brass, and these are highly prized by collectors.

Although various other types of wind-triangle computers were developed in other countries for use by their military pilots during World War II, not one continued to be manufactured much beyond war’s end. This is a significant commentary on the success and popularity of Dalton’s E-6B. I have studied and used the military computers developed by Germany, Great Britain, Japan, and the USSR (Union of Soviet Socialist Republics) during World War II. Not one is as accurate or easy to use as Philip Dalton’s E-6B.

With the exception of adding a density-altitude scale to the speed-time-distance side of the computer, the E-6B has remained largely unchanged and continues to be the computer of choice, despite the variety of electronic E-6Bs that have been introduced in recent years.

The U.S. Air Force continues to use them, too, although the jet-speed version is called an E-10. These incorporate scales for calculating Mach number, static temperature (ram-air temperature corrected for temperature rise due to compression), and so forth.

There are a number of reasons that might explain why pilots overwhelmingly prefer a manual E-6B to its battery-powered, microchip counterpart.

First, the E-6B is a status symbol, and many pilots are proud to be seen with one. This is similar to the manner in which math and engineering students in college used to enjoy strutting around campus with slide rules dangling conspicuously from their belts. (I recall buying a used E-6B after my first flying lesson. No, I had no use for it so early in my career, but the computer looked so cool and identified me as a pilot.) Second, the E-6B is absolutely, positively Y2K compliant and will not fail when a battery dies. (Plastic E-6Bs, however, have been known to warp and become junk when left on the glareshield of a parked airplane on a hot, sunny day.) Third, they are relatively inexpensive.

Perhaps the most significant reason for the E-6B’s popularity and longevity is the elegant simplicity with which a pilot can construct a wind triangle. The required plotting consists only of making a small pencil mark on the computer to represent the wind velocity. Once that is done, the pilot needs only to adjust the sliding scale to the applicable true airspeed and rotate the compass rose to the desired true course. Voilà! The computer has been set up in a way that allows the pilot to visualize the entire wind triangle. The two unknown quantities (usually groundspeed and true heading) are then read directly from the computer and can be visually verified as being reasonably correct solutions.

For these reasons, the majority of flight and ground instructors continue to recommend the "old-fashioned" E-6B instead of an electronic model. Although slightly more accurate than an E-6B, electronic computers are prone to input errors that result in erroneous data that may not be as readily detected as when using an E-6B, a classic example of "garbage in, garbage out."

It is worth noting that although some E-6B computers are marketed under different names (such as Jeppesen’s Slide-Graphic Computer), they are nevertheless the products of Philip Dalton’s ingenuity. It is unfortunate that he did not live long enough to appreciate the relative immortality of his creation.

Links to additional information on wind-triangle computers may be found on AOPA Online (