In the simplest terms, a helicopter’s rotor blade is a wing that generates lift by flying in a circle. But the similarity between a wing and rotor pretty much ends at the airfoil because the forces acting on each of them is vastly different. Imagine flying an aircraft that is constantly trying to shed its wings through the centrifugal force of normal operations. From building airplanes small and full-scale, I know how the skeleton of the wing deals with the forces of flight when fixed in one position. But when it comes to wings that fly in a circle, my understanding is destitute. Enstrom Helicopter Corporation, which for 60 years has been building piston and turbine helicopters in Menominee, Michigan, just up the coast from Green Bay, said they could fix that.
On the wall outside the Leland Burdue Training Center on the second floor of the Enstrom factory at the Menominee-Marinette Twin Country Airport (MNN) are two rotor blades. It’s clear that like the first propellers, the first rotor blades were carved out of wood by artisans of the drawknife and spoke shave. This long, wooden aerodynamic blade probably lifted one of Rudolf “Rudy” Enstrom’s prototype helicopters to a hover sometime in the late 1940s or early 1950s. “We’re really not sure,” said Dennis Martin, director of sales and marketing. “A family member found this in a barn [after Rudy passed on September 25, 2007], but we’re pretty sure it flew” on one of his early prototypes, which employed a two-bladed teetering rotor system.
Tool marks are visible beneath the worn black paint on the yellow-tipped wood blade. Beneath it is a seamless black-and-white striped metal blade. It is one of a trio that creates the fully articulated rotor system on the piston-powered F-28F and 280FX and the Rolls-Royce turbine-powered 480B. The rotor systems are essentially the same for all models, and together the main rotor systems have logged 4 million flight hours without a catastrophic failure.
Building Blades
The wood and metal blades have two things in common. Both are 16-feet long give or take, and artisans make both. Working what looks like an orbital sander loaded with a fine abrasive that seems to be polishing the metal is the leader of Enstrom’s blade shop, Ken Clark. Asked how many blades he’s built, Ken furrowed his brow for a second. “I’ve been here 32 years, more than half my life,” he said. Founded in 1959, Enstrom has been in business for 60 years, “So about half the number ever made.” That would be approximately 3,200 blades, with a birthrate of 12 per week.
“I have a couple of guys who work with me, and we make the tail rotor blades, too,” Clark said. When called for, another couple of workers join the crew. With 150 total employees, most of Enstrom’s technicians are cross-trained in several departments, and the blade shop is one of the more demanding studios. “This is an art,” said Clark. “This is nothing anyone is going to teach you in school. To take a guy fresh, it’ll take about seven years to teach him everything.” His apprentices have been working with Clark for about two years, and like many of the skills Enstrom’s artisans employ, their education is on the job.
Given the forces involved, I expected a more complex design. But Enstrom blades are built around an extruded D spar leading edge. Two sheets of 2024 are bonded to the recesses on the top and bottom of the spar, and at the training edge. “There are no ribs, no honeycomb, they are hollow all the way through,” said Martin, but the blade’s interior is epoxy primed to prevent corrosion. There are some doublers at the blade’s root, where the grip that connects it to the hub is bonded in, added Clark. He’s never counted the steps involved in building a blade. “It doesn’t matter; it’s got to be done either way.” The most challenging part of the process is, however, setting the tip cap rivet. “You’re almost done with the blade, and one wrong hammer—and it’s scrap.”
All of Enstrom’s metal blades have been built in the same fixture, which holds the pieces in place, forms the fully symmetrical airfoil, which includes a 7-degree twist down near the tip, and electrically heats the bonding seams and the entire fixture, with a box that encases it once all the pieces are in place. It takes an hour to warm up, it spends an hour at the perfect adhesive bonding temperature, and it takes an hour to cool. The twist, Martin explained, comes into play during an autorotation, a helicopter’s engine-out glide. Air passing through the center of the rotor disk turns the blades, and the outer portion provides the lift.
The relatively simple design blade wasn’t its only surprise. With so many rotating parts working in critical concert, their lifespan counts the hours of operation. Asked how long a rotor blade lives, Martin smiled: “97,500 hours. Effectively, they are on condition. There’s no calendar life, no hour life. They will last as long as they are maintained. The oldest calendar set that I’m aware of has been flying since 1973. The highest time I’m aware of is 22,000 hours. If you take care of the blades, keep them clean and corrosion free, they’ll last forever.”
Another question Martin often hears, he said, was about composite blades. He keeps his answer in a green, four-drawer file cabinet in the corner of the blade shop, in a drawer labeled “Broken Blades.” Halfway expecting some Harry Potter magic to produce a 16-foot blade, Martin instead pulled a deformed tail rotor blade out of the drawer. “This guy had a bad day,” he said. “But he still had something back there doing work for him. He put the helicopter back on the ground safely.” While the aluminum was bent and cracked, the bonding adhesive was unbroken. A composite blade would shatter and shred itself to an ineffective stump.
Blade Matching
“Everyone thinks you just throw the pieces in the fixture, and it’s magic,” said Clark. “But there’s a lot you have to do to make sure it comes out right.” Quality control begins before the pieces get near the fixture. Enstrom helicopters have mechanical controls, so they are sensitive to blade balance. “If the blades have different weights, they will fly differently, so we weigh every spar and rout the inside of the D to equalize the weight. After we build them, we match them in sets of three,” said Martin.
The birth certificate of every blade is the record of several hundred measurements and a profile of the entire blade. This data is fed to a spreadsheet that creates a chart for every set of blades. Call it a family born of a common fixture. “We keep this information forever,” said Martin, “If a customer needs a new blade or two, we look at that chart [for the family of blades delivered with his helicopter] and send replacements with matching numbers. Usually, they track very well. But if they don’t the customer sends them back and we send another one.”
Rotor blades are not the only components Enstrom builds from scratch. It’s easier to itemize the components it does not build: engines, avionics, and instruments. “We don’t have a foundry to cast parts like the main transmission housing and tail rotor gear boxes,” said Martin, “but we have four CNC vertical machines and three turning centers, so we do the final machining. We also have a CNC router for cutting metal, and CNC press brakes for bending it.” Because it is a critical component, Enstrom has two firms that precision grind the hollow main rotor masts to 5/10,000th of an inch. One is in Traverse City, Michigan, and the other (which made parts for the space shuttle) is in Green Way, Wisconsin. Without doing the math, Martin estimated that 99 percent of the Enstrom is made in America, and most of the handful of foreign vendors is Canadian.
One of them extrudes the rotor blade D spar. “Spar thickness is critical to the blades’ performance, and we can tell when the company’s die starts wearing out because the blades fly differently on the helicopter,” said Martin. “So call them up and say that it’s time to refresh the die.” —Scott Spangler, Editor
I was with the original group (including Jack Christenson) that visited Rudy’s place and saw him fly the prototype. I can help with some early history if you’d like to hear it.