by Paul Bertorelli
With a perfectly straight face and as sincere a smile as could be expected at 6:30 a.m., the counterwatcher at Signature Flight Support honestly thought we should be sanguine about paying $3.10 a gallon for avgas. That includes our 50-cent weekend discount!
Well, whoopdy-do. Splash some water on our fun meter; its about to overheat. While recovering from the raw thrill of paying $102 for 30 gallons of gas, were left to wonder why aircraft owners put up with both high fuel prices and shamefully inefficient engines and airframes. Surely, there must be a better way. But are airplanes really that inefficient compared to a truly piggish SUV, say a Cadillac Escalade sucking up a gallon to go a mere 16 miles at highway speeds? After all, in the Caddy, thats 3.7 GPH; not many airplanes are so miserely; a few burn that much gas getting to the runway.
Actually, on a miles-per-gallon basis, airplanes can be more efficient than SUVs for one overarching reason: they go faster and if economy is the pilots sacred grail, he can get there by going just a little slower and manipulating the mixture for best efficiency.
On the other hand, some airplanes are more efficient than others due to basic engine and airframe efficiency. Whats true of cars is also true of airplanes: slower and lighter is more efficient than faster and heavier.
Within the same class of airplanes, some are more efficient than others, sometimes dramatically. The Mooney Ovation and Lancair 300 are more efficient than the Cirrus SR22, even more so at economy cruise speeds.
One unavoidable truth is that to go really fast, efficiency goes out the window, although again, some airplanes are better performers than others. When the throttle goes forward, the Mooney TLS Bravo and Cirrus SR22 are appalling gas hogs while the Cirrus SR20 and Mooney Ovation are relatively miserly fast flyers, as is an aftermarket turbonormalized Bonanza with balanced fuel injectors.
The good news/bad news is that while the auto industry has improved fleet fuel economy in passenger cars over the past two decades, the GA industry hasnt bothered. However, with electronic ignition, balanced fuel injectors and emerging diesel technology, there are at least some options available for increased fuel economy, even at higher speeds, a faint ray of hope when circumstances force you to fuel up at Signature.
Calculating vehicular efficiency accurately is complex business but theres a simple way to look at it: under like conditions, how far can you go on a gallon of gas and/or whatll it cost? How much stuff-occupied seats, really-can you carry and how far?
Our Mooney 231-an efficient airplane in its own right-has a Shadin Mini-Flow with a curious but dubiously useful feature: it measures nautical miles per gallon based on current fuel flow and groundspeed. Although this feature isnt particularly useful, it dramatically illustrates what we already know about airplane efficiency. For a given engine displacement, power settings and lean state rule basic fuel flow and wind rules the overall miles per gallon performance.
On a recent trip, for instance, we flew the same 70-mile leg in both directions with wind conditions unchanged. On the upwind leg, the Shadin pegged our fuel economy at 12.2 NMPG at 150 knots TAS, into a 15-knot headwind for a groundspeed of 135 knots. On the return leg an hour later and at the same power setting, the groundspeed was 166 knots for an economy of 15 NMPG, a bit better than the Escalade when normalized for statute versus nautical miles but not especially impressive as airplanes go. On a longer leg, with a blistering 40-knot tailwind and running lean of peak in the high teens, we were zipping along at 190 knots groundspeed on 8.8 gallons. NMPG: 21.5 or the equivalent of 24 MPG on the open road. Try that in a Cadillac at 220 MPH.
With the wind, altitude, power and lean state as such transformational variables, we thus need to measure efficiency with a set of constants. For this exercise, were using no-wind conditions, claimed true airspeed from POHs and 6000 feet for normally aspirated airplanes and 15,000 feet for turbocharged models.
Weve examined a best-case economy scenario-55 percent power setting-and a go-fast scenario at 70 to 75 percent percent power. We used the POH leaning recommdations; if the charts show lean of peak, thats what we used.
While brake specific fuel consumption (BSFC) is a measure of engine efficiency, were using nautical miles per gallon (NMPG) as a measure of how efficient the engine and airframe are together. This blunt approach reveals such sins as how poorly (or well) the manufacturer attended to aiframe and cooling drag reduction.
Faster: More Gas
In the fantasy world of aircraft ownership, many a would-be buyer has advanced the argument that owning a faster airplane will actually be cheaper because you get there in less time and thus you burn less fuel. And that doesnt count the value of your own time which is, of course, immeasurable.
Airplane physics have always blown a hole in this reasoning, something which most of us know but are too timid-or deranged-to admit. Going faster requires more fuel because it requires a bigger engine to make more power.
And the faster it goes, the greater the drag, requiring yet more power and fuel. Light aircraft are a balancing act between power, weight of the engine and airframe, drag and enough space to carry sufficient fuel for acceptable range. Some aircraft makers have juggled the variables better than others but there are few widespread recent efficiency gains to crow about.
If William Pipers J-3 Cub can be considered the genesis of modern light aircraft, efficiency has tumbled downward as a function of increased speed. The basic laws of aerodynamics dictate that this is no surprise. A J-3 cruises about 70 to 75 knots for an efficiency of 17 MPG, a cheapskates dream. By the late 1950s, a Cessna 172 would go 105 knots on twice the gas, for a rating of 12.75 NMPG for not much gain in speed. Sixty years after the J-3 appeared, a Cirrus SR20 does 150 knots on 10.5 GPH, lean of peak for an economy of 14 NMPG.
With few exceptions, engine and airframe manufacturers havent done much to pick off what efficiency gains are available for the taking, chiefly cooling and airframe drag reduction, better induction and fuel delivery systems and improved ignition. One reason for this is that aircraft buyers havent demanded efficient airframe/engine combinations but have embraced the idea that shoving ungodly volumes of gas through an engine to go fast is just the price of being an aviator.
Even in Europe, where avgas at $6 a gallon is a bargain, efficient designs-chiefly from Diamond-have emerged only recently. The Diamond Katana and Star are models of efficiency but the French-designed Socota TB20 is one of all-time guzzlers, burning 14 GPH to go 150 knots. (The Diamond DA40 goes almost as fast on 10 GPH.)
The Star is one of the few examples of a new design that delivers better efficiency. But even in the past, there have been other impressive efforts, including the Mooney 201 and Pipers Malibu, which was intended to be flown lean of peak EGTs to deliver on Pipers range goals. By necessity, that made it more efficient but you have to crunch some numbers to appreciate how efficient it is. The Continental-powered Malibu has the same efficiency rating as the thrifty Mooney Ovation yet is 15 knots faster. Not bad.
In one of those inexplicable reversals GA is so adept at, Piper messed up a good thing when it switched engines from the Continental TSIO-520-BE to the Lycoming TIO-540-AE2A. More power, yes, and a bit more speed and climb rate. But the Mirages efficiency tanked from 12.9 MPG to 11.3 MPG, something that adversely effects the airplanes range, occasionally compromising with a fuel stop any advantage of climbing to go faster over a great distance.
Interestingly, even when slowed to a pokey efficient cruise speed at 55 percent power, a modest airplane is capable of at least twice the speed of an Escalade. At those power settings, the Escalade dwells at the bottom behind all of the airplanes weve examined. When the airplanes go fast, the Escalade leaps ahead in efficiency, finishing in the top 10 against the airplanes.
Nibbling at Drag
Buyers and owners complain that engine manufacturers havent done much to improve performance and reliability for the past half century and implicit in that complaint is that engines havent gotten more efficient. But this is only partly true. At their very best, piston aircraft engines can be suprisingly efficient, with best defined as good induction systems, fuel flow thats uniform from cylinder to cylinder-that generally means fuel injection-and the capability to be leaned aggressively with no roughness. Lean-of-peak operation is also an efficiency plus.
Recently introduced electronic engine controls-Continentals PowerLink FADEC being the current leader-promise improved fuel efficiency. But thus far, these are getting a bland reaction in the market because theyre expensive, complex and dont seem to offer much benefit other than incremental improvements in fuel efficiency. Buyers may be saying buying a $10,000 FADEC that saves a gallon an hour isnt a good deal.
With engine efficiency somewhat stunted by both market and technical limitations, the best way to improve fuel economy is to push less air around unecessarily; to shave drag. Again, results are mixed and most of the impressive gains are coming out of the aftermarket in the form of speed mods.
These tend to be expensive add ons that dont always deliver the claimed performance. The rule of thumb is that aftermarket speed mods cost about $1000 per knot of additional speed. On a 160-knot airplane, youll thus spend $5000 for a 3 percent gain in speed and perhaps a bit less than that in improved economy.
Impressively, some manfuacturers have nailed good economy right out of the box. Note that the Cessna 100 series–150 through 172–do quite well and newer models show a slight improvement. But theyre slow as slugs. Not slow is the Mooney 201, however, which turns in impressive economy at 150 knots plus. The late Roy LoPresti was rersponsible for the drag reduction magic during his tenure at Mooney, rejiggering the 145-knot F-Model Mooney into the 155-knot plus J-Model 201. As high-performance singles go, the 201 isnt the fastest but its one of the most efficient engine/airframe combinations, finishing well in our survey.
In the Ovation, Mooney increased the speed over the 201 by 10 to 15 knots with little hit on efficiency. In the TLS, it did the Malibu-to-Mirage trick; efficiency cratered in favor of raw, gas guzzling speed. The TLS is dead last in economy when flown fast.
Surveying the current crop of new-age aircraft, the Diamonds, the Cirrri and the Lancair Columbia, we find a mixed bag. The Diamonds-both the DA20 and DA40-are efficiency standouts. The Star of Stars, so to speak, is the Thielert-diesel powered DA40TDI which turns in an astonishing 37 NMPG in economy cruise. At higher speeds, the TDI finishes behind the Rotax-powered Katana, while the gasoline-powered Star essentially ties with the Cirrus SR20 at about 13 MPG. The Cirrus SR22 plummets to the bottom at 9.7 MPG, both a surprise and a disappointment, in our view.
Does aircraft efficiency really matter? Most owners seem to say it doesnt while at the same time complaining about burning too much gas to go too slow. Further, twin sales usually go soft when gas prices spike, suggesting that would-be owners have a threshold of price pain defined by a complex combination of speed and load carrying capability. With the age of $4 avgas upon us, the pain may become more acute.
A twin-engine aircraft makes an SUV look like a Green Partys dreamboat ride but the payback is that they carry more people and stuff, if you need that sort of thing. Further, theres the perceived safety of a second engine. If you compare the efficiency of a Continental-powered Malibu with something like a Cessna 340, you can readily see the price Delta of the safety margin an extra engine provides and then decide if its worth the extra bucks.
We see two ways to improve efficiency. One is to use the sophisticated aerodynamic computer modeling available to chip away at drag, especially cooling drag. The second is to improve the basic fuel efficiency of available powerplants.
We see more of the latter than the former. Although Lancair, Mooney and Piper appear to have paid attention to cowling design, we still think theres a way to go in this area in terms of both airflow and proper baffling. On the airframe side, Diamond appears to have done the most with drag reduction, obtaining respectable speeds from low horsepower engines.
In the engine arena, the results have been less impressive. General Aviation Modifications, Inc. has made great strides with its GAMIjector balanced fuel nozzles which seem to work wonders on many Continental engines but with less uniform results on Lycoming engines. GAMI is developing an electronic ignition system that may yield further economy gains and Continentals PowerLink FADEC is already on the market, but not yet enjoying brisk sales. (See September, 2003 Aviation Consumer.)
Diesels may offer the most promise for meaningful economy gains. Again, Diamond is leading the way with the Thielert engines in both its single-engine Star and soon-to-be DA42 TwinStar. The SMA diesel project-Maule and Cirrus-are close behind. Diamonds Thielert-powered DA40 TDI blows away the closest competitor with its 37 MPG economy. The airplane isnt as fast as the gasoline-powered version, climbs a little slower and diesel engine durability remains a question mark, but Diamond has clearly staked out the possibilities. We like this trend and hope that it continues.
Also With This Article
“Economy Winners and Losers”
“Aircraft Economy: From Best to Worst”
“Welcome to $4 Avgas”
“Its Not Just the Engine”