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Turbines!

Is it turbine time for homebuilts?

By Vance Jaqua, reprinted from Contact! Magazine, February 2005

 

In the upper power regimes of commercial and military aircraft, the gas turbine variants have taken over. Does this mean that turbine power is ready to dominate the amateur builder field as well?

If you are building a 600 hp high-performance, high-speed ship, the resounding answer is yes. The wide availability of the Walter turboprop engines, surplused from the fleets in the ex-communist bloc countries, have made this option a bargain you cannot refuse. Although thirstier than piston equivalents, the lower initial cost and light weight are major selling points. However, in the 200 to 300 hp region, where most of the builders live, the picture is not nearly so rosy; the road appears littered with snake-oil salesmen.

Some of you may have seen in various articles the glowing claims for conversions of the Solar Titan–based auxiliary power units and jet starters. Again, the surplus market seems well supplied with numerous versions of these units at attractive prices. The most numerous applications are showing up in the form of small helicopter installations. There is high potential in this market, where a high power-to-weight ratio is a premium feature. The rather greedy fuel consumption of these units is relatively acceptable as a rational trade for the reduced weight and the promise of reliability and low maintenance issues under continuous high power settings. Fuel-limited flight duration is generally not a major goal for this type of aircraft. The pioneer in homebuilt helicopters, B.J. Schramm, was in the process of acquiring a supply of these units for conversion to helicopter use, but his tragic death will leave a void in this effort.

For the typical, RV, Long-EZ, and similar craft builder, the turbine picture is really quite bleak. In spite of some vendor claims, the maximum power capability of most of the Solar Titan–based units is about 130 hp, with a few examples as high as 160. A turbine does not lend itself to usual hot rod tactics. Two limiting characteristics are flow rate and turbine inlet temperature. The margin between efficient power generation and overdriven turbine temperature is very narrow and abrupt. The difference between thousands of hours and a few minutes or even seconds of turbine life can be as little as 100 degrees. Official manufacturers stated that specific fuel consumption for these units is typically 1.3 pounds per horsepower hour, or almost three times worse than a well-tuned piston engine.

History
The concept of gas turbines has been around for quite a long time. Actually one might argue that the windmills used for ages for pumping and milling are really a form of gas turbine. Heat energy from the sun creates the high and low pressure regions across the landscape, providing the wind from which power may be extracted. However, the modern history of gas turbines is primarily based on the development of jet engines in the WW-II period. Though attributed to Sir Frank Whittle in England, gas turbine work was proceeding in many places, with the Germans achieving the first operational combat aircraft in the air. These early jet engines have evolved into a major powerplant source in current times. There is hardly a power requirement that is not being met in some manner with a gas turbine.

Probably the first homebuilt aircraft to be powered by turbine was the Rover-powered Hot Wot. A popular home-constructed aircraft in England was a scaled down replica of the de Havilland Moth. One of these wooden biplanes was fitted with a small Rover gas turbine engine.

Hot Wot
Rover-powered Hot Wot, which is a version of the Currie Wot, a popular homebuilt aircraft in England. G-APWT first flew in 1960 with an air-cooled Walter Mikron engine. A year later it was fitted with the gas turbine that, although rated at only 5 hp less than the Mikron, was disappointing enough to be removed less than two years later; the Walter was reinstalled. In 1974 it was shipped to the United States and N-numbered. After a long restoration it’s reported to have flown as recently as 2008.

While Rolls-Royce had been charged with the development of the large turbines, Rover was given the task of developing smaller units for various uses. One of these engines was also fitted to a series of automobiles, producing probably the first gas turbine–powered cars. The Hot Wot was no barn burner with flashing speed, and the primary motivation for the installation was “because they could.” Modest power and speed were provided but at the cost of excessive fuel consumption.

The argument between turbines and piston engines has been going on for a long time. My own personal experience dates back to 1955 when I was a green, young engineer at General Electric’s Aircraft Gas Turbine Division. We had a newsletter by and for the young “test engineers,” as we were referred to in those days. One of my coworkers wrote an article predicting the imminent replacement of the piston auto engine with a gas turbine, using this lead-in; one of the projected breakthroughs was expected to be a ceramic turbine in about five years.

Although I was also in the turbine design business, I felt compelled to come to the defense of the classic piston engine. So I penned the response that followed this heading.

Pinwheels

I even went way out on a limb, predicting that the piston engine would remain supreme for auto use for at least 10 years—wow! To keep things in perspective, this was the year the Chevy small block was introduced, and ceramic turbines were still at least five years away.

Put-Put

Efficiency – Specific Fuel Consumption
Piston engines generally have a “sweet spot,” a combination of manifold pressure (throttle setting) and rpm. The chart below is from an earlier auto engine, where full throttle richening of the mixture is used to permit high compression ratios.

Chart
Earlier auto engine chart, showing full throttle richening of the mixture used to permit high compression ratios.

As you can see, backing off the throttle slightly at about 2500 rpm moves operation into the best specific fuel consumption area. This characteristic is exploited with overdrive in automobile applications for improved gas mileage and is also the principle applied during high manifold pressure, low rpm (over square) cruise for piston-powered airplanes. Gas turbines have no sweet spot. When you reduce power with a turbine, the fuel consumption is not reduced an equal amount, the specific fuel consumption being greater at low power settings.

The actual thermal cycle of the gas turbine (the Brayton cycle) is more efficient than the Otto (gasoline engine) or the Diesel cycle, for the same pressure ratio, but limitations of the pressure ratio and maximum practical turbine inlet temperature have resulted in rather low efficiency for actual devices in most applications. However, in large installations for stationary, marine, and large commercial aircraft, the use of complex active cooling systems for the turbine blades, coupled with multiple compression stages and exotic alloys, has led to specific fuel consumption competitive with Diesel power. With smaller and simpler (read “affordable”) units, the fuel economy numbers are pretty dismal. Efficiency for any gas turbine is at its peak while producing maximum rated power. Reduced power operation (equivalent to throttling) is provided by reducing fuel flow, so power generation is reduced by dropping turbine inlet temperature, reducing thermal efficiency.

Compressor efficiency is also lower at reduced rpm, and the parasitic losses remain at high levels. It’s not unusual to require half of maximum power fuel consumption to maintain idle (no useful power output) conditions. Operators of smaller turbine-powered aircraft will frequently completely shut down engines during runway holds.

Scale and development level are both major drivers in the fuel economy picture. In general, smaller and older designs will have poorer efficiency. Some relative examples are:

Engine

Shaft HP

Rated Cruise

Allison 250

317/500

0.68/0.59    0.73/0.66

Walter 601

700

 0.65

Avco LPT101

650

0.55

Pratt PT6

500/1020

0.65/0.56    0.67/0.58

Allison T58

4000/5000

0.53/0.50    0.54/0.52

Solar 65

60-130

1.3 (Pessimistic?)

Newer And Bigger Is Better
Note that for some models, we have old/new ratings. For example, the Allison 250 (now being produced by Rolls-Royce), which is an old design and has gone through numerous “dash number upgrades,” went from 317 to 500 hp, while specific consumption has dropped to 0.59 pounds per horsepower hour (which is a very livable number). Note also that cruise specifics are in all cases poorer than rated operation. The Allison has been applied to numerous successful applications. The primary down side of this unit is the high price, something over $200,000 the last time I checked. The low-cost Williams units, turbofan and turboshaft engines, were tightly coupled to the early Eclipse business jet airplane program which dropped them from consideration. The stated numbers for the Solar family of auxiliary power units are all given as 1.3 to 1.25, which, I believe, are pessimistic ratings, but even at more optimistic levels these are fuel hungry, underpowered devices.

As mentioned earlier, the relatively low price of the Solar Titan family of gas turbine auxiliary power units on the surplus market has led to numerous efforts to convert them for small aircraft applications. Helicopter usage has been fairly successful; good power-to-weight ratio and perceived reliability have outweighed the heavy fuel consumption. Fixed-wing applications have flown with generally meager results. A few vendors are actively trying to market such a product. Demonstrated performance has been disappointing in spite of the usual optimistic claims. The maximum speed and performance results are limited by the modest power available from these units, and fuel consumption has been predictably high.

One vendor has published some static thrust data points with fuel consumption figures. Static thrust is generally regarded as unusable for determining engine power, but with prop diameter known, one can make a pretty good estimate of engine shaft horsepower based on the air horsepower of the resultant air flow mass and velocity and estimates of typical prop pumping efficiency.

The Data Supplied

Computed Performance

Thrust LBF

Gal/HR

Estimated actual BHP

Specific fuel LB/HP/HR

300

10.7

52

1.37

375

11.6

72

1.08

450

13.4

93

0.96

525

14.9

115

0.90

600

16.6

143

0.078

 
These computed numbers support my feeling that the stated 1.3 pound per horsepower hour from the Solar data is pessimistic, but still much poorer than the vendor claims. I am not sure which dash number Solar T-62 was used, and I suspect that the higher power value was obtained with the turbine inlet temperature at or above the limit. The improved specific fuel consumption at higher power level is a typical turbine characteristic. The vendors frequently claim that with sophisticated injectors and atomization the performance or efficiency will be vastly improved, but the combustion process is not the limiting item in these units.

The flight performance of the vendor’s prototype installation reflected these more modest power estimates. The reported performance was typical of an engine the size of the Lycoming O-235, with over twice the fuel consumption. This year at EAA AirVenture Oshkosh (2004) the same people were there, only with a new name, and the claims were shakier than ever. They are actually claiming a 300 hp version now, and they still are not offering dynamometer data, and were promising an October 2004 delivery. Rumors of lost deposits and unfilled promises of delivery are starting to surface.
 
Altitude
You often hear it said, “If you fly at altitude they get a lot more efficient.” Well, that’s not really true. The actual efficiency of the turbine engine is reduced at altitude, such that the specific fuel consumption per horsepower is actually poorer at higher altitudes. The fuel consumption versus true airspeed is indeed improved, but just as with a piston engine, the power required to fly at that speed is reduced (by the reduced drag at true speed). The below charts show an estimated comparison between two identical airframes (RV-4 class performance level), one with a converted turbine and one with a conventional aircraft engine of similar maximum power.

RV-Solar

RV-Std Eng

The predicted miles-per-gallon equivalence with the Lycoming engine at a conservative 0.5 lb/hp hour is better than most mid-sized automobiles, approaching 25 mpg. The most optimistic prediction with the converted turbine is roughly twice the consumption.

Notice that the best overall miles per gallon would be a line tangent to the sea level curve, but that would be slower than most impatient pilots would tolerate. In this case it would be about 110 mph true speed and about 55 hp. With a piston engine and a controllable pitch propeller, this can put the engine in that sweet spot for outstanding economy. Indeed, this is the actual tactic that was part of the Voyager plan (high and slow). The turbine engine, on the other hand, becomes very inefficient near half rated power and would deliver crummy mileage even there. As you go up in altitude, that tangent point falls on a more effective true speed, with a very acceptable minor loss in miles per gallon. So while the old belief that you always get better gas mileage at altitude is not really true, it’s still a very sensible way to operate your plane.

Twin Regenerator Gas Turbine
Main Components Of The Twin Regenerator Gas Turbine
 (A) accessory drive (B) compressor (C) right regenerator rotor
 (D) variable nozzle unit (E) power turbine (F) reduction gear
 (G) left regenerator rotor (H) gas generator turbine (I) burner
 (J) fuel nozzle (K) igniter (L) starter-generator (M) regenerator drive shaft
 (N) ignition unit

Opportunity Lost
The lure of the market for a gas turbine–powered car (Hey! Look at me! I have a jet car!) led to development of regenerative systems to improve fuel economy. The most well-known turbine car was produced by Chrysler about 50 years ago, and they utilized what is often called the side-wheeler heat exchanger system. This system passed the exhaust through a mesh of metal tubes in the slowly rotating side wheels. These hot tubes were then rotated into the flow system between the compressor exit and the burner inlet, preheating the air to reduce the amount of fuel burn to heat the turbine inlet gasses.

Chrysler

This concept worked well enough that Chrysler fielded a small group of cars and loaned them to potential customers for real-life road experience. Potential production costs sank the program, and most of the fleet was scrapped to avoid tax penalties.

Twin Regenerator Gas Turbine
Another view of the Twin-Regenerator Gas Turbine.

About 20 years later, General Motors was on the brink of offering a similar but more refined turbine for a Camaro class sporty car. Design point specific fuel consumption was better than the current V8, but low speed operation was still rather poor. Emissions were outstandingly low, and the predicted driving cycle economy was in the range of roughly 18 mpg highway and 10 mpg city - rather poor, but saleable for the class, except the specter of the Corporate Average Fuel Economy (CAFE) standards promised heavy fines; the program was dropped. Although rather bulky, this would have been a great powerplant for the smaller, general aviation and sport aircraft. The fuel consumption would have been competitive with existing aircraft piston engines, and the smoother operation and longer potential life would have been a significant advantage.

Model Turbines and Jets
If you have been following radio-control model activity, you have undoubtedly seen and marveled at the proliferation of model aircraft turbojets. Available thrust levels have grown, and at least one man-carrying plane (a Cri-Cri) has flown using jet engines from the model aircraft field. Prices remain fairly high but have been steadily declining while becoming more sophisticated. Recent ads suggest that they are approaching the $100 per pound of thrust cost level. However, the rules of scaling continue to limit specific fuel consumption to painfully high levels. If you thought the converted Solar auxiliary power units were thirsty, just convert the ratings of these models to the pounds per thrust hour units. With the higher thrust levels, novelty airplanes, and perhaps self-launching sailplanes become viable, emulating the famous Baby Mamba built by Max Dreyher and shown in the picture below.

Max Dreyher

Max was a definite pioneer in the small jet engine field and produced this beautiful example of machinery/fine art, well before the radio-controlled jet market existed.

Max

The mission this unit was designed for was the self-launching of a small sailplane. Specific fuel consumption was high and the thrust minimal for this task—but it worked. Power-off drag of a small jet like this is very low as compared to even a feathered prop.

Again at Oshkosh (2004), a maker of model and remotely piloted vehicle jet engines showed a prototype of a high bypass turbofan of 650 pounds thrust and a thrust specific fuel consumption below 0.5 lb/lbf/hour. This would make for a very credible small jet airplane, but that is still a takeoff consumption of nearly 50 gallons per hour. Predicted selling price was estimated at $50,000, and although that may sound high, it’s well below the $100 per pound thrust value mentioned earlier. The performance of a well-designed small two-place using an engine such as this would easily exceed that of the famous Bede Microjet. However, the fuel consumption would still be pretty outrageous, being in the same class as a high performing piston engine twin.

HPX 650

Conclusions
In the meantime, if you want to look like, sound like, and smell like a jet, it is possible. You might end up spending money for fuel like a jet but likely disappointingly short of flying like a jet. At 500-plus hp, where you are already in the “If you have to ask, you can’t afford it” class, there is a lot of surplus hardware out there that can feed those urges. But for most of us, the best advice is to keep our hand clutched firmly over the wallet and keep the BS filter in the tight and fine mode.

In memory of our dear friend Vance Jaqua (1929-2006), who gave much of himself for the advancement of experimental aviation.

 
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