ANN Asks Hartzell

ANN Technical Special by Tim Kern

That Bombardier V-6: it didn't just happen, you know; and one of the major considerations in any propulsion system is the thingie that turns engine rotation into thrust. In the case of Bombardier Recreational Products' newest offering, that's a propeller -- and this new engine design, catering particularly to noise considerations of the future, required something... different.

Last week, we talked at length with Hartzell, one of the two principal prop makers involved with the development of the Bombardier Recreational Products V-6 engines, and asked the engineers just what considerations -- new and old -- came into play in the design.

The brand-new V-6 comes in two flavors: a normally-aspirated, 220hp version, and a turbocharged, 300hp package. Other than the obvious packaging differences required to accommodate the turbocharging, there are two big differences between the two engines: the horsepower, and the torque curves. Both play key positions in prop design, although, of course, the horsepower number is by far the greater factor.

Hartzell's Brian Meyer, Manager of Aircraft Applications Engineering, and Les Doud, Propeller Integration Engineer, gave us insiders' insight into the tradeoffs that are required in any design, and in the design of what is really a radical departure from common practice: a really slow-turning prop hub is a new deal in this part of the market.

A new deal, but not unique; and solutions to any new set of questions do exist. The trick is picking a starting point, and ending up where you want to be. As Mr. Meyer said, "We went to the 'corners of our existing database' -- it's not an area where go go all the time; but we've had some experience in this area." Mr. Doud added, "When you look at it, it's not too different from a design challenge that would come from a small diameter turboprop." He added that the basic designs have been around a while: "Look at a Conquest -- it's not all that different -- or the Meridian (pictured)."

"Noise," Brian noted, "is a major consideration. Bombardier Recreational Products is very sensitive to noise." In Europe, especially, the noise small aircraft make has become quite a political issue. In the States, too, people constantly cite "safety" as why they want to close local airports, even when the actual safety record is admirable; they're really expressing their animosity to noise -- the complainers just don't want to sound like they're whining. Noise is a big, and growing, issue -- and Bombardier, with its gearbox, went to attack it head-on.

That, of course, meant that the prop designers would have an unconventional set of parameters to address.

A low-rpm prop hub (2000 rpm is maximum on both V-6s; cruise will occur between 1550 and 1700) means, other things being equal, low prop-tip speeds. That is a gateway to low noise output. Slow-turning props are quiet; but, beyond a certain point, they're also not going to generate a decent amount of takeoff and climb thrust, efficiently, especially when prop-diameter considerations are put into the mix.

Brian explained things, from a theoretical standpoint: "Props are most-efficient around .8 Mach" at the tips. That's not all physics, though; some of it is tradition: ".62 Mach [the Bombardier target, on a typical-diameter, 78~80 inch diameter prop] presents a challenge, partly because of the proven efficiency of .8," where most of the emphasis has been focused, over the past half-century, in piston-engine prop design. Physics still drives the design, though, and it shows up, "...partly because of the high blade loading needed to generate adequate thrust at the lower Mach number." Then, as tip speed increases, not only does noise increase dramatically, efficiency starts to drop off. Slowly at first, then more noticeably: "Above, say, .9, efficiency really starts to drop off, due to shock losses."

How can we understand this? Les offered, "It's analogous to the lift on a wing of a J-3, vs a Lancair IV -- one design will work better, at any given speed.  The Lancair IV wing would require very high lift capability at J-3 speeds." In this case, think in the realm of 50 knots.

Where's the 'sweet spot?' That depends on what you're trying to achieve, but Brian noted where the noise-vs-efficiency curve starts to turn: "Low noise is really optimally achieved at about .75 Mach [tip speed]. Below that speed, the tradeoff between the thrust available and the reduction in noise hits some diminishing returns." Largely because of decades of development, thrust efficiency on typical props peaks at higher Mach numbers -- .8 to .9 -- and that range is pretty much out of the question with these engines.

The .62 Mach number that would occur in a 78~80 inch prop is well below even that .75 figure. Oh -- and as Les said, "There's also the idea that we want lighter weight." That brings into play another factor: blade width. "At lower Mach numbers," he explained, "you require more blade width and/or high-lift airfoil section properties; the airfoil sections we like to use are thin, for low weight and low drag at the higher tip Mach numbers. When you reduce the Mach number, the blade gets wider, and typically thicker." In other words, with the diameter limited by the design of the airplane, as the tip speed gets slower, you'll need to alter other variables: airfoil at various points along the blade, the blade's overall shape -- and particularly blade width. Think again of an airplane's wing: the U-2 and the F-104 started life as very much the same basic airplane -- except the wings are radically different, reflecting the radically-different missions of the machines.

Brian added, "The prop and the engine -- and the airframe -- are a system. This is particularly important when considering the retrofit (STC) market." However, "This is less a consideration in the fresh-sheet-of-paper approach. We can adapt to this engine -- we can adapt to existing [airframe] design -- we can customize a prop to accommodate a given application."

To the drawing board:

Les reiterated that a prop is just one part of a propulsion system; and a propulsion system is just one part of an aircraft. The entire system needs to be optimized for a task. "What I wanted to point out was, with a typical 78-80 inch diameter constraint, we have a design challenge," he said. "It [low rpm operation] requires a new design, and new approach. You can't just take one of today's typical props, stick it on there, and have it perform well." There is a whole set of things he can work with: "We can use diameter, blade width, airfoil sections, and planform." A typical GA airplane can't swing [a more-optimal 97" prop], so we play with geometry, number of blades, blade width, and airfoils."

Why not just make the tips wider? That's where the majority of the work gets done, due to the higher speeds at the tips, versus near the hub. There are multiple problems, though: bigger tips -- longer 'levers' -- require more support near the hub; and, the more blades, the less room there is for any one blade at the center of the prop, so the longer (front-to-back) the support has to be: that adds weight and creates drag (against rotation).

There's more: "While the structure is important, we need to stay compatible with the vibration characteristics; higher weight at the tip leads to a lot of other forces," Les explained.

Then, there's weight, again: Brian noted, "Of course, increasing the number of blades, or width, or cross-section, increases weight, and cost -- we have to balance that to get a good overall design."

This is no place for the backyard experimenter. Size, complexity, materials -- turning the horsepower of the new engines into thrust, usable at different airspeeds -- the equation becomes calculus, rather than algebra. "A constant speed prop is especially beneficial for airplanes that have a wider speed range," Brian reminded us, from a "Props 101" course we assumed he took.

What else is different?

Then, there's the powerband. A turbocharger is powered by engine exhaust gases. As the engine turns more rpm, exhaust gas volume and velocity rise, spinning the turbine wheel faster, which allows the turbo to produce more boost, which allows a loaded engine to turn faster, producing more power, which will spin the turbine faster, allowing more boost... What that means, though, is that the engine must be turning a respectable rpm, before the turbo's benefit can be realized. The engine must be free enough to get to some critical rpm, for the turbo to do any good. As Mr. Meyer noted, "With a turbo, that makes a variable-pitch prop even more useful, and requires a more-tailored blade design."

We asked if there is there a radical difference between the powerbands of the normally-aspirated 220 and turbo'd 300. Hartzell's Mike Disbrow, Senior VP of Marketing, Applications, and Customer support, made it sound easy: "They [Bombardier] didn't need to furnish us with complete power curve data. We looked at several points along the curves, where we needed to optimize: peak power, cruise, and a couple transitional points. " Brian confirmed, "The turbocharged engine will require a different blade planform and some other considerations, than the 220; not just because of the horsepower, but because of the power curve itself."

Is there any likely application for a fixed-pitch prop on this engine? Brian offered, "I just can't imagine anyone outside of perhaps a bush pilot with a very slow aircraft, wanting to use a fixed-pitch prop. If all these [Bombardier] guys spent all this time, money, and effort to come up with this system, it would behoove one to take advantage of the work that has been one." [Translation: "No."]

Hartzell's team looked at the results of the whole system: "The tradeoffs here are extremely important: weight, CG, moment of inertia -- you're thinking of a long list of tradeoffs -- costs, complexity, multiple blades, noise, engine wear," he reminded us.

What any design comes down to is tradeoffs.

This particular application, particularly because of the new emphasis on low noise, forced the designers to do without some traditional flexibilities, and explore some of the less-visited ones. When all the criteria -- low-rpm thrust (cruise), high-rpm thrust (takeoff and climb), efficiency at various speeds, ground clearance, complexity, cost, weight, CG of the finished airplane, inertial feedbacks, likely airframe applications, even future STC markets -- and noise -- are taken into account, the designers have plenty of variables to use. The good news is, the variables -- diameter, number of blades, planform, pitch range, and airfoils -- is a limited set; the thing that complicates the recipe is that some of the options -- particularly in planform and airfoil selection -- are quite open, and have to be used to compensate for the effective loss of others (most particularly, diameter).

That's why you let professionals design these things, and that's why Bombardier Recreational Products did, too.

[Special thanks to Hartzell's Les Doud, who prepared the drawings --ed.]

FMI: www.hartzellprop.com/engineering/sitelink_design_process.htm