All in a Semester's Work
The Making of An Aircraft Towbar

John Nolfi is a pilot who owns his own single-engine Cessna 172, teaches a design engineering class at Purdue University, and is over 50-years-old. As such he is well aware of the difficulty an aging pilot population has pushing and pulling airplanes around hanger and ramp.

"Small, general aviation airplanes are pushed into and out of the hangar, and when yours is sitting on the ramp you may have to pull it out of a spot to start it up," says Nolfi, who acknowledges that "people my age have trouble pulling their planes around as easily as when we were 25."

It was to make life easier for himself and his fellow aging pilots that Nolfi assigned one of his recent capstone design classes the task of designing and building a motorized aircraft towbar. He also felt it was a perfect project because it included electrical, electronic, and mechanical systems, all of which the modern ME must be familiar with.

Nolfi organizes his classes using a business model, which means that the students must not only design and build a product, but keep their eyes on the marketability of the product as well. Right off the block the class of 23 students had to conduct a survey to determine if there really was a market for such a towbar. They also looked into the type of airplanes most pilots owned, what they would like in a towbar, and how much they would be willing to spend for such a device.

The students found the local airport, during a Purdue football weekend, to be a fertile ground for surveying private aircraft owners. They also called a number of airport operators around the country, and collected data from the Aircraft Owners and Pilots Association.
 
ME seniors Tim Antcliff (right) and Dave Hostetler keep a close watch as the towbar their class designed lifts the  nose wheel of a Cessna 152.

"Their analyses demonstrated that the average person who owns an airplane in the U.S. is about 50-years-old and that a lot of them are pilots who can't tow an airplane by themselves anymore," said Nolfi. The surveys also helped them develop design criteria, which included requirements that the device have ambidextrous capability, that it should be usable by both men and women, and that it should sell for about $1,000.

The class then created several towbar concepts, one of which was selected for further development. At this point the students divided into structure, propulsion, and manufacturing teams. "Each team fed the entire organization with detailed specifics," said Nolfi. Then came critical design review, during which the students made formal drawing presentations. Necessary changes were made, and the build cycle began.

The class then spent about four weeks in the machine shop using lathes, mills, and computer numerically controlled machines to make all of the parts. "They did all of the machining, and all of the electronics, nothing was subbed out," said Nolfi.

As is frequently the case in the industrial world, the project was not without it problems. For one thing, two electronic motor controllers burned out before the students realized they had the wiring backwards. They also learned that what they learn in class is not always applicable to the commercial world. When it comes to sizing motors and gears, for example, students are usually taught to err on
 the conservative side, which, according to Nolfi, is "more industrial than commercial. They learned that we should have been designing a consumerized product, which is lighter duty, easier to use, and less expensive."

Not The First
One problem the students confronted was that theirs would not be the first motorized towbar on the market. But, as Nolfi explains, existing devices "lock onto the nose wheel strut of an airplane, and…they don't turn very well-they scuff the ground." As a result, "Sometimes you have to pick the towbar up and set it down at an angle to make the turn." Existing devices also generally sell for between $750 and $3,000, said Nolfi.

To beat the competition, the students created a towbar with more features that could conceivably be manufactured for less money. Their final design gets around the scuffing problem by scooping the nose wheel of the airplane off the ground, making it easy to turn. To be able to do so requires the lifting of some 3,000 pounds, which is accomplished through the use of a jackscrew, a device frequently used to move aircraft control surfaces because it is efficient, and requires minimum space.

The six-foot long, two-foot wide towbar utilizes tricycle running gear, and is powered by two, 12-volt batteries. A "T" handle has a built-in potentiometer and a toggle switch. Rotating the potentiometer causes it to move forward or backward depending upon the position of the toggle.

"To get features that the student built towbar has, one currently needs to pay about $12,000," said Nolfi, who hopes to turn the students' "proof-of-concept" design into a commercially viable version that is "lighter, easier to use, and smaller."