The Trouble With Turbines

Small turboprops dont work well because they cant haul enough gas. Can Eclipse rewrite the rules?

One reason pilots get to be pilots is that they dislike crowd following. No buses for us, thanks, city, Greyhound or the airborne variety. We know these conveyances move people efficiently, but were wired to be in charge. And some of us are important and wealthy enough to afford realistic GA transportation.

Realistic means serious traveling machines, such as high-performance and pressurized singles on the low-end, luxurious piston twins in the mid-range and, increasingly, a taste for very expensive single-engine turbines, a market thats not quite burgeoning but is beginning to take root.

But theres a problem here. As pilots, we want the perfect personal airplane; one we can use any time the conditions are good enough for the airlines, with enough range, performance, payload and low enough costs to justify not riding the airborne buses.

Single-engine turbines-with few exceptions-dont really fill this bill for one simple reason: For as reliable and powerful as they are, turbines are hard pressed to match the fuel efficiency of the piston engines weve grown accustomed to, even if were not comfortable with the reliability of what comes out of Williamsport and Mobile.

The Long Struggle
If the GA industry ever faced a brick wall composed of the laws of physics, making turbines more efficient may be the most difficult to overcome. For designers, making personal airplanes truly practical is a constant balancing act governed by weight, range, reliability and weather. In this fight, GA piston engines provided limited options. Engines greater than 350 HP or so have proved problematical and played anemically in the market.

Twins offered power and redundancy needed for performance and to run systems such as radar, pressurization and known icing packages. Yet even in the sophisticated piston twins, the climb gradient on one engine is rarely more than a sobering 100 feet per mile.

With the dead weight of power split into two packages, we couldnt stuff four or five passengers into many twins and then carry enough fuel to get more than about 250 miles on an IFR day. It was simply a question of efficiency; hauling the fuel-sapped payload.

As singles became more sophisticated, the equation shifted. The Cessna T210 with radar and known icing certification got about as close to the goal of the perfect personal airplane as many could imagine. You could fill the tanks with 89 gallons of gas, fill the seats, carry some baggage and go 180 knots for about 800 miles in the flight levels with IFR reserves. Not bad, even if everyone aboard was sucking oxygen.

With dual vacuum pumps and alternators, reliability was acceptable, so long as the engine held together. Pressurization increased comfort but dinged the range and payload. (Nothing is free, much less cheap.)Single-engine piston development continues yet in the form of the Extra 400, which currently defines the upper end of piston capabilities. But the Extra is fighting a battle with useful load and a million dollar price tag, not to mention the overhanging promise that better turbines will render it obsolete, a dead end on the road to the perfect personal airplane.

While suitable piston singles are well under a million bucks-because most are bought used-worries about their ability to deal with weather and engine reliability cause more than a few pilots to seek a more capable airplane.

The Perfect Engine
Every new aircraft development has been spurred on by new engine development and in the personal aircraft market, the small turbine engine seemed perfect. With its incredible reliability and sheer power output, the turbine engine promised the potential of the perfect personal airplane, if only cost could somehow be contained.

The LearJet was truly the first attempt to do this. Small, fast and light, it debuted in 1963 and was designed to be a single-pilot airplane because turbines are generally easier to fly than pistons. But the 23 proved to be a handful and the FAA balked. While some were owner flown, a copilot was required. Other bizjets have followed but none really qualify as personal airplanes.

The next great hope for the owner-flown market has been the turboprop engine. Unfortunately, in trying to beat the cost demon, manufacturers and conversion shops have pushed against uncompromising laws of thermodynamics: the specific fuel consumption of a turboprop engine is higher than that of a piston engine and jet fuel weighs more than avgas. This is a fundamental fact of life in comparing pistons and turbines and thus far, it remains the showstopper for small airframes.

Putting some numbers into the equation, piston engines have fuel specifics in the .35 to .45 range, meaning they burn .35 to .45 pounds of fuel per horsepower per hour. Piston engines can be creatively leaned to produce good fuel specifics at a wide range of altitudes and power settings.

The best turboprops are in the .5 to .6 range and, depending on the model, tend to do worse at low altitudes and better at high altitudes. Turboprops easily overcome the power shortage of piston engines which gives better climb, more speed and thus more weather flexibility. So you might reasonably conclude that with its greater speed, the turboprop makes up for its thirsty fuel consumption by getting you there sooner.

But it doesnt work that way. The turbines high fuel consumption drives the airframe designer deeply into the weight, power and range coffin corner. Speed is no bail out.

And its not just a power/fuel equation, either. The size of the airframe is critical, since at the fuel specifics quoted here, the piston that burns 20 gallons per hour equates to a turbine that burns 35 gallons. With an 80-gallon capacity, the piston engine will run the tanks dry in a little over five hours while the turbine will do the same in under three hours. If the turbine cruised half again as fast-say 325 knots to the pistons 200 knots-the numbers might even out in the wash. But it doesnt and they dont.

Head to Head
Carrying the theory over into the real world, the six-place airframe that works for the Malibu, the 210 and the Bonanza just wont cut it with a turboprop up front, at least in terms of range and payload flexibility. You can have one or the other but not both.

Once the power is increased to the level that gives the desired performance, the rate of fuel consumption means the airframe cant carry enough fuel to let the airplane go around the block. If space is found somewhere in the airframe for the gas, the FAA-mandated 61-knot stall speed limit for singles and its effect on gross weight means that theres no way to carry a significant load in the cabin. Its an ugly reality.

The turbine conversions to the P210, Bonanza and Malibu are all weight or range limited. (The stall speed can be higher than 61 knots, but the FARs mandate that crashworthiness standards get much tougher, adding to weight.)

Take the six-place Piper Malibu/Mirage airframe, the basis for Pipers new turboprop single, the Meridian. In terms of overall range/flexibility, it offers more than turboprop conversions generally do. But thats not saying much.

Piper engineers did wonders finding a way to put 170 gallons of fuel into the airframe. With full fuel, its 500 HP PT-6 allows it to cruise at 262 knots over an IFR range of about 900 to 1000 miles; Teterboro to St. Louis in three hours on the proverbial no-wind day.

Yet with a pilot and three people, the range drops to 560 miles and probably less allowing for winter headwinds. That means even Teterboro to Chicago isnt doable without a fuel stop. Not so hot.

Lets look at an older Malibu for the same trip. With four people aboard, theres still room for full fuel, yielding a still-air range of 1200 miles, with generous reserves. Even allowing for wind, the St. Louis trip is an easy snooze.

It may take more flying time in the Malibu-although not much more-but the total trip time is the same and perhaps shorter because it can be flown non-stop. Apply these numbers to something like the Cessna 210 and the results are similarly favorable for the piston engine. Its the classic tortoise and hare, with the turtles better efficiency making for a quicker trip.

Another point of comparison is the JetProp DLX, a PT6A-34 conversion of the Piper Malibu. With 560 HP, its a faster climber than the Meridian but cruises about the same speed. It has similar payload but less fuel capacity, but also lower claimed fuel burn.

That doesnt help. With four people aboard, the JetProp has room for 95 gallons of Jet-A, which it consumes at a rate of 34 GPH. Even on a summer day with light winds, Teterboro to Chicago with four people wont be in the cards unless youre comfortable with slim fuel reserves.

We Dont Go There
The marketeers of single-engine turbines have an answer for this glaring weakness in their products: Our customers dont go from Teterboro to Chicago with four people, they go with two people and an overnight bag. Or they go 200 miles to the regional office with five colleagues and briefcases.

Fair enough. Except we dont really buy the argument. The true measure of an effective personal transportation airplane, in our view, is one that extracts as little range penalty as possible in favor of payload. In other words, when you need butts in the seats, it doesnt punish you.

We freely admit to a dearth of data on practices of pilots seeking the perfect personal airplane with regard to whether they carry passengers, and if so, how many. We have heard the arguments on each side and know that some pilots usually travel alone, or with one other person, making the weight limitations of the Meridian moot. Yet we also know that many of those looking for the perfect personal airplane want to take a family of four to six people, along with golf and scuba gear.

Some piston singles and twins do better at this than the turboprops do and for a lot less money. On the turboprop side, one solution to the problem is to simply build a larger airframe, use more power and have more room to store fuel, more useful load and the cost be damned.

That describes the Cessna 208 Caravan series. Originally marketed as a cargo hauler, Cessna found that individuals were buying them. Despite being pricier and slower than a Meridian, Cessna is now marketing the airplane to the owner-flown market.

Its speed is more in the T210 range, at about 180 knots, yet a pilot can fill up the 335-gallon tanks, have an IFR range of about 800 miles and still put a crowd in the cabin. And we do mean a serious crowd. Fill the tanks and theres still room for six people and all the bags they can carry.

The Socata TBM 700, at 6580 pounds for takeoff, is also in this class. It uses a 700 HP PT-6, carries 282 gallons of fuel, cruises only marginally faster than the Meridian at 245 knots (long range) and has an IFR range of about 720 miles with a pilot and three passengers. It also costs more than a Meridian, moving it away from our ideal personal hauler.

Still further away but probably the most capable of the single-engine turboprops is the 260-knot Pilatus PC-12. It takes full advantage of the economies of scale, with 1200 HP available, 402 gallons of fuel and the impressive ability to fly 1480 miles with IFR reserves while carrying a pilot plus three pax.

At 9920 pounds for takeoff, its also much heavier than both the TBM 700 and Meridian. Of the three, only the Meridian falls into a price range that we can reasonably consider for a personal traveling machine. But price notwithstanding, the PC-12 does what the smaller personal turbines dont; it combines range, speed and payload in almost unlimited variety.

For example, the PC-12 claims a useful load of nearly 4200 pounds so even with 402 gallons of fuel aboard-2800 pounds-theres still room for 1400 pounds of people and baggage. Call it six people and all the baggage in the world.

The developing JetCruzer-a single-engine pusher turboprop-shows no indication of being a breakthrough in technology although its difficult to determine its true abilities. Its advertising asserts a 300-knot cruise with IFR range of about 1000 miles while only carrying 240 gallons of fuel for a 1572 HP PT-6. We assume its derated, otherwise the numbers just dont add up. The airframe appears to be slightly larger than a TBM 700 but at a 6200-pound takeoff weight, its slightly lighter.

Nevertheless, we have our doubts that the JetCruzer will be able to go faster and farther than the smaller TBM 700 with 42 fewer gallons of fuel in the airplane. The past decade has shown that the canard design-which the JetCruzer has-is not aerodynamic magic. It just looks cool.

The Eclipse Challenge
Its possible to draw some general wisdom from the foregoing. Chief among them is that when the airframe is small, turbine powerplants are pressed to deliver both range and payload. When the airframe is large, they can manage it. When the airframe is huge-the PC-12-theyre winners, if expensive.

This would appear to be the central challenge before the Eclipse jet and others of its ilk. Ignoring the selling price, can a turbine airplane this small and light do what others have not?

The Eclipse is Malibu size, not King Air size, as is the PC-12. It claims a maximum range of 1600 miles and, at a speed of 355 knots, a 1300-mile range with four people aboard, performance that will flat out blow away the Meridian, the JetProp and the TBM 700, all of which cost more.

By way of comparison, the closest thing to the Eclipse currently on the market is Cessnas CitationJet CJ-1. At roughly $3.5 million, it reflects the reality that the bottom-end price for a blowtorch with pressurized cabin, radar and known icing has been something over $3 million in current dollars.

The CJ-1 cruises at 375 knots with a range of about 700 miles with a pilot and three people. It carries 480 gallons of fuel and its Williams jet engines each put out 1900 pounds of thrust. Moving up a little, the CJ-2 is larger than the CJ-1 at 12,375 pounds for takeoff, cruises at 408 knots and it has IFR range of about 950 miles with a pilot and three. The CJ-1 carries 480 gallons of fuel and the CJ-2 holds 586 gallons. (The Eclipse will carry 190 gallons.) How will Eclipse do what it claims for less than the price of a Meridian? If Eclipse respects the laws of aerodynamics it will need major breakthroughs in three areas.

Aerodynamically, the airplane will perform because its small. Cabin size is almost identical to the Meridian, something no one would mistake for roomy. (See Bill Lears comments on page 12 for what they said about his Daddys jet.) The Eclipse is smaller than all models of the LearJet and Citation.

Williams International is promising an 85-pound engine that will produce 770 pounds of thrust. Thats a thrust-to-weight ratio of 9. Increasingly, were hearing engineers opine that Williams will succeed despite the fact that the rest of its bizjet engines have a thrust to weight ratio of about 4.2.

As lighter, more heat-resistant alloys have been developed, engines became lighter, more powerful and more efficient. The fanjet provided a jump in fuel efficiency over the turbojet. Weve heard arguments that Williams claims dont pencil out but those arguments arent convincing because they assume no progress and ignore the Williams record.

Cessna was able to develop the CJ-1 and CJ-2 with performance head and shoulders over the Citation I and II because Williams developed more efficient engines.

Comparing the Eclipse to the CJ-2, we note that the Eclipse weighs a bit more than a third of the CJ-2. The Eclipse engines put out almost exactly one third the thrust of those of the CJ-2. With 190 gallons aboard, the Eclipse Jet carries almost exactly one third the fuel of the CJ-2. The Eclipse Jet is 13 percent slower than the CJ-2 and will go about 27 percent farther. Speed means fuel burn. Slowing a CJ-2 down to 355 knots would increase its range noticeably, perhaps to close in on what the Eclipse claims.

Assuming the engines do as promised, the numbers on aerodynamics and engine efficiencies are probably realistic. All Williams has to do is double its current thrust-to-weight ratio.

The toughest breakthrough, in our view, is the bane of every aircraft designers existence, weight. The airplane will fly at 41,000 feet requiring a correspondingly rugged pressure vessel. The associated environmental systems, avionics, de-icing, crashworthy structure and seats, landing gear mechanism, antiskid and all the systems that are on current six to eight-seat jets will have to go into the Eclipse.

However, Eclipse advertises an empty weight of only 2700 pounds, as compared with the 3400-pound empty weight of a Meridian or the 2400 pounds of an unpressurized Cessna 210. Yes, the engines are light. If Williams merely hits its historic 4.2 thrust-to-weight ratio, only 170 pounds gets added to the empty weight.

One measure of aircraft structure is the ratio of pounds of empty weight per foot of length. Eclipse promises 81 pounds per foot; the lower flying Meridian is 114 pounds per foot and the unpressurized T210 is 84 pounds per foot.

The CJ-1, which flies at the same altitude as the Eclipse, has a cabin thats less than 4 feet longer, an overall length less than 10 feet more, yet at 6600-pounds, weighs more than twice as much. Its ratio is 156.3 pounds per foot.

Either Cessna engineers have a serious problem with weight control or Eclipse is a bit optimistic. We cant help but wonder if Eclipse can put everything in that little package, keep it light and do so in a fashion that allows a mechanic to get access for maintenance.

A rule of thumb in aircraft development is that a new engine on a new airframe is a recipe for difficulty. Eclipse wants to put a new engine on a radically lightweight airframe while making a third breakthrough in cost of construction and customer support.

But for all the difficulty it faces, Eclipse has something other manufacturers-including the likes of Piper-dont have: Generous working capital. And as we all know, in the end, what makes every airplane fly is money. Lots of it. Thats what it may take to build the perfect personal airplane.

Also With This Article
Click here to view “Piston vs. Turbine Range.”
Click here to view “Checklist.”

-by Rick Durden

Rick Durden is an aviation attorney and contributing editor to Aviation Consumer.