Ask anyone who’s tried to wring more speed from an existing aircraft design, and you’ll learn that the task is very difficult. Hot-rodders have long been adding speed on cars and motorcycles by installing progressively more powerful engines, and that works great for machines that roll on wheels. Unfortunately, it’s not nearly as effective on airplanes.
Weight and aerodynamics aren’t terribly important on mere earth-bound conveyances, but they can make all the difference on flying machines. For airplanes, the trade-offs of a larger, more powerful engine may not be worth the effort. More horsepower usually means more weight and higher fuel burn, which can demand bigger tanks and reduce payload, unless you raise the gross, which reduces speed and demands more horsepower, which requires more fuel and higher weight… You get the idea.
The late Roy LoPresti, grand guru of all things aerodynamic, generally disdained power increases for more speed. Roy was my Yoda on anything aeronautical, and he used to tell me that if cooling drag didn’t increase, the standard formula for calculating speed increase with extra power was to expect the cube root of the percentage of horsepower increase in knots. Translation: If you had a 160-knot airplane, added 30 hp to a 300 hp engine (10%) and didn’t increase the cooling drag, you could reasonably expect to see a speed increase of about 2.16% (2.16 x 2.16 x 2.16=10). That’s an unimpressive 3.5 additional knots (160 x 0.0216=3.5).
The smartest possible course would be to combine aerodynamic improvements with more horsepower, and that’s exactly what Diamond Aircraft (www.diamondaircraft.com) has done on the new Diamond Star DA40 XL. Diamond recently introduced the result of its improvement campaign on what was already one of the best airplanes in its class.
The horsepower improvement is subtle but effective. Rather than recertify the airplane with a more powerful engine, Diamond contracted with Power Flow Systems (www.powerflowsystems.com) of Daytona Beach, Fla., to have a tuned exhaust designed specifically for the Star’s injected Lycoming IO-360-M1A.
Tuned exhausts can work wonders at recovering the horsepower that’s normally lost to an inefficient exhaust system. Power Flow has been building model-specific tuned exhausts for several years, concentrating first on airplanes powered by the 150/160 hp Lycoming O-320s and later on those fitted with 180/200 hp O-360s and IO-360s. The company has made a name for itself with exhaust mods designed for a variety of airplanes, primarily the Piper Warrior, Cessna Skyhawk, Grumman Tiger, Cessna Cardinal, Piper Archer and a number of 180 and 200 hp Mooneys.
There’s often a general skepticism about the effectiveness of STCs, but I have personal knowledge that Power Flow’s products work as advertised. Three years ago, I had my Mooney fitted with a Power Flow exhaust, and the aircraft experienced a dramatic difference in performance. I picked up 50 to 75 fpm in climb, saw CHTs and oil temps drop by as much as 15 degrees, and witnessed an impressive five-knot speed increase at 11,000 feet.
In the case of the Diamond Star, Power Flow’s general manager, Darren Tillman, told me his company tweaked the exhaust to reduce back pressure, and the final system wound up delivering about an extra 23 additional horsepower. Incidentally, that doesn’t mean the certified, 180 hp Star now delivers 203 hp. The airplane is still certified for a maximum 180 hp. The tuned exhaust merely recovers the horsepower normally lost to power-grabbing accessories such as starter, alternator/generator, etc.
The extra ponies provide an additional five knots cruise at 6,500 feet, and as much as seven knots at 10,000 feet. Power Flow will supply the more efficient exhaust to Diamond Aircraft for all future DA40 XLs, and the company will also market the system as an STC’d conversion for stock DA40s equipped with the composite MT-Prop. (Pilots flying behind the Hartzell-driven Star must stick with the stock exhaust for now; Hartzell is working on an STC for the Power Flow exhaust, but it will likely also require a new Hartzell prop.) Price for the aftermarket DA40 Power Flow exhaust (with ceramic pipe) is $5,190, plus labor.
The new Star also has an improved MT composite, scimitar, three-blade prop to help convert the additional Power Flow horsepower to thrust. The new prop is pounds lighter than the old one, and you can feel that during power-up. The new prop comes up to speed notably quicker.
There are another half-dozen less-perceptible improvements to the Star that make the XL a fully equipped, ready-to-fly, IFR aircraft. Diamond analyzed how pilots typically order the Star, and configured the airplane with most of what was formerly optional as standard, offering the DA40 XL at an all-up tab of $339,695. (One of Diamond’s newest offerings, the DA40 XLS, comes at a $334,950 price tag. Learn more in the Sidebar.)
To offset the weight of the improvements, Diamond has recertified the DA40 to a gross weight of 2,646 pounds, 111 more than the previous Star. As a result, useful load increases by about 70 pounds. Maximum landing weight remains at the old 2,535 pounds, so if you depart at gross, you’ll need to burn off about 19 gallons of fuel before you can return for landing.
Inside the cabin, Diamond added electrically adjustable rudder pedals on both sides as standard, AmSafe inflatable seat-belt restraints and an electric CO2 detector. The company also incorporated a Power Flow heater to deliver air 25 degrees hotter than before. Outside the airplane, Diamond fitted high-intensity discharge (HID) taxi and landing lights.
Standard avionics on the XL include the Garmin G1000 and GFC 700 autopilot. They include traffic, XM weather and GPS-based terrain warnings.
Diamond emphasizes that many of the features above could become available as retrofittable options. As mentioned earlier, the Power Flow Systems exhaust is already available on the STC aftermarket.
The big question is, what are the results of all this innovation? The Diamond dealer in my neck of the woods is USAero (www.usaero.aero) in Long Beach, Calif. Robert Stewart, USAero’s Diamond expert (he also dabbles in emeralds and turquoise), agreed to fly with me to demonstrate the new airplane’s talents.
Since the first Diamond Star hit the market in 2000, the little four-seater has been almost universally regarded as one of the most innovative singles in its class. With a composite design that has little hanging out to grab the wind, even the original Star offered excellent performance on minimal horsepower.
Standing on the XL’s wing provides perhaps the optimum view of the airplane’s slick aerodynamics. There’s nary a rivet or section line in sight, the overhead hatch fits so tight, you have to practically open a window to get it closed, and, oh yes, there’s even a back door on the left, something rarely (if ever) seen on a four-seat airplane. (The old Beech Sierra offered an aft right rear door, but at the time, Beech was laboring under the delusion that the airplane was a six-seater.)
Settle into the airplane’s plush leather interior and you can’t help but be impressed with the nearly automotive comfort. Diamond didn’t design the interior around the BMW 5-series or Audi A8, but the cabin still winds up being eminently comfortable, measuring 47 inches across in front, 45 inches in back.
That’s not to suggest you can top the tanks and fill the four seats with 680 pounds of people. Even with the gross weight increase, the test airplane sported a payload of only 850 pounds. Subtract 50 gallons of petrol, and you’re down to more like 562 pounds for people and their stuff.
I know it’s a song you’ve heard before, but that’s not really such an evil limitation, because most pilots buy at least two seats more than they need. Besides, if you really need to fly with a 680-pound string quartet (minus their instruments), you could leave 20 gallons in the truck and still have enough fuel for a 250 nm trip plus reserve.
Level at 6,500 to 7,500 feet with a full load, and you’ll see something between 148 and 150 knots, again not too shabby with only 180 hp under the bonnet. I actually saw 151 knots true on the day of my flight, though we were operating perhaps 200 pounds below gross.
Remember, however, that specific fuel consumption is immutable. The more horses you employ, the faster your airplane will go, but you’ll also burn more fuel. Diamond claims the Power Flow system makes that about 10 gph at 75%. That suggests an SFC of 0.444 pounds/hp/hr. With 50 gallons aboard, you should be able to range out an easy four hours plus reserve—about as long as most pilots are willing to sit in one place anyway.
“Realistically, the Power Flow exhaust is probably the major contributor to the improved performance,” says Stewart. “All by itself, it accounted for at least five knots of the higher cruise.”
Diamond’s max-cruise spec is 150 knots, and we were right on the numbers. That’s excellent performance for a 180 hp single with wheels in the wind. Granted that speed, you’ll run away from an Archer or Skyhawk, though you’ll still be at least five knots slower than a Cirrus SR20. Pull back to 55%, and you can see 130 knots on eight gph.
Personally, I’ve always felt the Star was the most aerodynamically innovative airplane in the class. That class, incidentally, consists of the Star, Cirrus SR20, Piper Archer and Skyhawk SP. (Arguably, you might include the Skylane, though it employs 50 more horsepower to cruise at roughly the same speed.) The Archer and Skyhawk have been around for at least four decades, whereas the Star and SR20 premiered less than 10 years ago.
Using Jane’s All-The-World’s Aircraft 2006/2007 or Diamond Aircraft’s Website, you can see how the numbers compare for the four contenders. It’s just about impossible to keep these comparisons from matching apples to grapefruit, but these are the best numbers we could assemble.
The new Diamond Star represents a definite improvement over the previous version, not just a rehash of the original airplane with new paint and a fancier interior. Now that Diamond has introduced the DA50 Super Star to compete head-to-head with the Cirrus SR22, Columbia and Mooney Ovation, the DA40 XL raises the bar in the midpriced, four-seat, fixed-gear class.
NEW From Diamond
Diamond Aircraft has introduced two new versions of its successful DA40 Diamond Star: The DA40 XLS, the top-of-the-line, loaded model with a new Platinum Interior, and the DA40 CS, a four-place, constant speed version, which can be custom tailored to best meet a customer’s personal requirements and budget.
The DA40 XLS retains all the advantages of the DA40 XL—great visibility, fuel efficiency and pilot-friendly handling characteristics—and now offers more room under a wider, higher canopy. Its Platinum leather interior features a long list of XLS exclusives, including a choice of leather colors, aluminum-framed genuine wood inlay accents, engraved and brushed aluminum trim, carbon-fiber sill plates and a distinctive metallic striping package. The WAAS-enabled Garmin G1000 glass cockpit features Garmin FlightCharts and SafeTaxi and other enhanced software functionality, including an ability to fly autopilot coupled procedure turns and hold entries, and to program Victor Airways into flight plans.
Additional Garmin functionality includes a full-screen engine monitoring page and wind vectors on the PFD. The DA40 XLS also comes standard with such safety-enhancing features as traffic (TIS), satellite weather (U.S. only), TAWS-B and a 406 MHz ELT. Additionally, you can upgrade the DA40 XLS with optional active traffic, ChartView, geosynchronized approach plates by Jeppesen and Diamond’s Premium Care program.
The DA40 CS shares all the strengths of the DA40 XLS, including standard Garmin G1000 glass cockpit, and it also enables customers to configure the airplane to their needs, starting at a competitive base price of $259,950. Customers can choose the attractive, durable fabric interior or upgrade to luxurious leather. Those looking for comfortable cross-country cruising can add the Garmin GFC 700 digital autopilot, extended luggage compartment, performance landing gear and extended range tanks. All these options make the DA40 CS a great choice for individual owners, particularly first-time owners, as well as for flight schools looking to add modern appeal and technology to their flight line. For more on Diamond’s newest offerings, visit www.diamondaircraft.com.
SPECS: 2007 Diamond Star DA40 XL