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
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
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
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
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
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