- 25 Mar 2020
- Avionics for General Aviation
The approach is almost as old as powered flight: improve a proven airframe by grafting on a better powerplant. In aviation- the quest never ends to make airplanes better- more efficient- more capable- and always faster. For business turboprops- upgrading offers a path to such outcomes and better value than sticking with the old engines- as we have been outlining recently. In some instances- converting to a different brand engine or to a different type provides the same benefits.Back to Articles
More ‘Bang’ For The Buck:
Updating to propjet from piston benefits many popular light business aircraft.
The approach is almost as old as powered flight: improve a proven airframe by grafting on a better powerplant. In aviation- the quest never ends to make airplanes better- more efficient- more capable- and always faster.
For business turboprops- upgrading offers a path to such outcomes and better value than sticking with the old engines- as we have been outlining recently. In some instances- converting to a different brand engine or to a different type provides the same benefits.
And that’s what we examine this month: converting to turboprop power from a piston airplane - or from one propjet brand to another. While not something widely embraced- the customer-base has proven large enough over the years to support some long-term programs.
For decades back- aircraft engineers and designers recognized that putting aircraft in higher- thinner air offered the potential to fly faster. Higher altitude equals lower drag. That’s simple enough to grasp.
The problem that accompanies this idea stems from the fall-off in engine output as altitude increases. Lower air pressure reduces the efficiency of engine-inlet pressures- which cuts power output. Higher altitude equals lower power.
Early successes in countering the enginepower problem mated some form of compressing mechanism to feed the engine induction system air at pressures closer to sea level – or at least to pressures equal to cruisepower output. Superchargers- driven by the engine itself- and exhaust-driven turbochargers both have the benefits and their devotees. Both types of pressure-aspiration enhancements show up in piston aircraft across the years. A problem for them is the maintenance involved and the excess temperatures produced by the act of compressing the air.
Turbine engines suffer from the same power-loss phenomenon- but because of how they work they continue to provide strong power to altitudes that let them eke out the maximum in speed while driving the airplane.
An issue piston and turbines also share involves fuel efficiency. Both engine types propel their airframes with greater fuel efficiency up high than down low. While rare in piston aircraft- however- engine failures are statistically even lower in turbine aircraft- providing yet another element that helps compel the buying decision for some.
Finally- the time to overhaul for piston aircraft generally comes far earlier than in turbine engines- adding a significant attraction to the appeal of turbine engines.
For many a year- Jet-A sold for less than aviation gasoline- adding to the appeal of going to a jetprop engine. But that price differential no longer remains consistent or reliable. And it always ignored a couple of realities of turbine operations:
First- kerosene- Jet-A and the like boast heat contents far lower than gasoline - so all other things being equal- an airplane powered by Jet A needs more fuel to produce the same amount of work. Second- turbine engines produce their work differently and tend to need more fuel per horsepower because of how the engines work.
What this means is converting to a turbine from a piston with equal horsepower requires carriage and use of more fuel – in the 20 percent range- horsepower-for-horsepower. But typically the turbine engine makes far more power – particularly flying low – and doesn’t achieve its best economies until up high. Between these differences and other issues the difference in fuel needed may work out closer to 25-30 percent on a per-hour basis.
And that per-hour basis should be nicely offset to a per-mile degree by the higher true airspeeds available.
FINDING THE BALANCE
There’s something of a trick in this airplanedesign business and in few areas is that more apparent than when designing a powerplant change. Going to turboprop from piston involves some interesting balancing acts because of different aspects of the engine going in and the engine coming out.
For example- compared to the piston powerplants they replace- turboprop engines generally weigh less per horsepower delivered. As noted just above- the fuel requirements increase for same-distance trips- so somehow the converted aircraft needs to offer greater fuel capacity – and more fuel means accepting more fuel weight - a factor complicated by the heavier weight per gallon of Jet-A.
Work-arounds to these two factors have often involved longer engine mounts so the lighter engine works within the weight-andbalance needs. Tip tanks or some other auxiliary storage option- or using more of what may be unused existing capacity- can resolve the fuel issues.
But whatever the decision- the added load must also work within the weight-and-balance envelope. And while the more-powerful engine may offer the ability to lift more- a higher operating weight can introduce issues into the capacity of the landing gear- wings and other airframe structures.
AND THEN THERE’S APPROVAL…
Finally- once the developer of such an upgrade feels the issues have been worked out satisfactorily- the revised airplane must emerge from the rigors of earning a Supplemental Type Certificate.
In this process- the package must usually show that it first does no harm- or introduces no negative issues into the aircraft’s use. For a change as involved in a propjet-from-piston conversion- the testing and documentation required is formidable.
For all of this to pass an economic muster- the re-engined airplane needs to demonstrate serious performance and utility advantages and an economic argument with appeal to the market.
It’s for the combination of these factors- the complexity and expense- that many more have been attempted than have succeeded. And the ones to succeed and build a market generally enjoy strong- loyal followings from their customers.
An early target of propjet conversions back in the 1960s – when turboprop engines were coming of age – was an aircraft plentiful and well-regarded: the Douglas DC-3- arguably the world’s first successful airliner and a backbone machine for ops from Normandy to the Outback.
Several companies tackled such conversions over the years- among them Bassler in Oshkosh- Wisconsin- and Conroy Aircraft in Santa Barbara- California. Engines used were Rolls-Royce Mk. 510 and Dart turboprops- and the Pratt & Whitney Canada PT6 line coming into vogue.
Over successive generations of conversions- the DC-3 and its military variant- the C- 47- received new leases on life- thanks to the improved performance available: Higher speed- greater lift- better runway performance – all the points that hold appeal for pilots and operators- particularly those operating in remote areas where aviation gasoline remains rare to non-existent even today.
It was successes like those with the DC-3 that helped inspire developers to design turboprop conversions for a wide range of airframes with strong appeal to the owner-pilot business aviator.
Following- we’ll look at three of the betterknown and longer-running conversions on the market- recognizing that others exist – and that more will be coming in the future.
Among one of the stronger programs on the market- the JetPROP DLX is well along in its second decade of service- with more than 240 of this conversion delivered as of September 2009.
The base airframe converted provides the main foundation for the success of the JetPROP DLX: a strong single with a pressurized cabin. JetPROP- LLC developed the DLX around the long-running Piper Malibu/Mirage- sporting a 350-horsepower piston engine with dual turbochargers- providing breathing for its high-altitude abilities and cabin pressurization.
JetPROP got its DLX to market ahead of Piper’s launch of its turboprop remake of the Malibu (the Meridian) and they both compare closely in some areas.
The airframe modifications to the Malibu are largely limited to those required to accommodate the engine and its fuel needs that make it a JetPROP DLX. The Meridian received far more rework en route to its transition from the Malibu – even though they are quite similar in appearance.
Both the JetPROP and Piper turboprops employ the wildly popular PT6A turboprop engine from Pratt & Whitney Canada: the DLX got the PT6A-35 flat rated to 560shp; for the Meridian it’s the PT6A-42 flat rated to 500shp.
Compare that to the Malibu’s 350 maximum horsepower- which drops about 20 percent at cruise altitude. The performance difference between the DLX and the Malibu serves as the basic attraction for those 240- plus customers.
The Malibu- for example- manages about 210 to 220 knots true airspeed up at about FL230 while consuming about 22 to 24 gallons per hour. It needs about 2-550 feet of take-off space to clear a 50-foot obstacle and climbs at about 1-200 fpm.
The aircraft converted to the JetPROP DLX turns in between 250 and 270 knots true- can cruise as FL270- consumes about 30gph and needs 1-200 feet of runway to clear that same obstacle.
Neither the Malibu nor the JetPROP DLX win any contests for the full-fuel payload needed to cover their 1-000-mile maximum leg. The JetPROP offers about 360 pounds to the Malibu’s 598 pounds- with much of the difference due to the higher fuel capacity of the turboprop - 151 gallons versus 120 for the Malibu.
Interestingly- however- the JetPROP DLX compares favorably with the Meridian in most areas- including speed- range- maximum altitude and payload- at 350 pounds. The conversion costs for a Malibu owner makes the proposition a well-below-one-million way to own a fast- comfortable- flexible turboprop single. For the pilot without a Malibu- the cost can vary and still beat those of a new Meridian by several hundred thousand dollars – though the panel of a new Meridian may have the JetPROP devotee thinking about spending some more to gain similar technology and capabilities on top of the basic DLX package.
More information from www.jetprop.com
The honors for longest production run for a civilian airplane go to the Hawker Beechcraft Bonanza- which started life as the Model 35 in 1947. The 36 Model Bonanza introduced in the late 1960s is the foundation for the Tradewind conversion.
Tradewind picked the highly regarded- light and compact Rolls-Royce Allison 250- flat-rated to 420 horsepower- and since the Bonanza lacks pressurization- Tradewind focused on providing a performance increase with the Rolls-Royce Allison due to it's low fuel consumption at non-pressurized altitudes. It is also very useable at altitudes that require oxygen up to 25-000 ft.
Performance with the 250 turboprop engine marks a dramatic break from that of the piston-powered Bonanza. For example- climbing to the Tradewind Bonanza’s optimal altitude will take the Bonanza more than 15 minutes- while the converted airplane needs less than five.
Runway performance improves dramatically as you’d expect from the higher climb performance- and the speed difference and range potential both improve greatly- as well. The Tradewind Bonanza turns in 220 knots true at 15-000 feet- with range potential out to 900 nautical miles. That gives the airplane the ability to fly more than 1-000 statute miles in under 4.5 hours. In contrast- the Bonanza makes its 170-knot best speed at about 7-500 feet- and by 13-000 the speed available is off by more than 30 or more knots.
The advantage at altitude is what gives turboprops such appeal. By comparison- the Bonanza with its piston engine can cover only about 730 nautical miles in the same time needed by the Tradewind-converted Bonanza to fly 900.
The Bonanza does need considerably more fuel capacity- and Tradewind accomplishes that with a set of distinctive tip tanks with integral winglets for efficiency.
More information from www.tradewind-bonanza.com
NORTHWEST TURBINE- LLC:
The old Beech Aircraft Corp. produced what is arguably among the most-distinctive looking twins made when it brought out the pressurized A60 and B60 Duke. Northwest Turbine- a relative of JetPROP- LLC- did for the Duke what its corporate cousin did for the Malibu: give it the benefits inherent in a turbine engine.
The Royal Duke employs the D60 airframe produced between 1974 and 1982- and the conversion involves either the PT6A-21 or -35 engines- with new props- as per the other conversions covered here- and other changes to cover the engines’ fuel and monitoring needs.
New cowls- streamlined exhaust stacks- engine instrumentation- and other modifications give the converted Royal Duke significant performance potential: 292 knots true burning 67gph fuel at FL270.
The aircraft can cover 1-000 nautical- but a more-balanced real-world mission might read as follows: four adults- 110 pounds of luggage and capacity to fuel for more than 600 nautical miles with reserves. Unlike its singleengine kin- with full fuel the Royal Duke remains a genuine hauler- with 750 pounds of payload available. In fact- its full-fuel payload compares well with the TBM 700.
Interestingly- the difference in fuel consumed by the piston original making 235 knots and the Royal conversion’s best cruise amounts to a mere 10gph.
But the 3-600-hour engine overhaul cycle for the P&WC powerplants is more than twice as long as the Duke’s original engines.
The total conversion cost depends on the engine selected- with the PT6A-21 coming in about $100-000 less – and giving a Duke devotee the potential to own a turboprop twin that’s not a King Air for about $1.1 million... with new engines- props- cowls- support systems and engine instruments.
More information from www.royalturbine.com