Ian Twombly

Chasing efficiency

June 11, 2008 by Ian J. Twombly, Associate Editor

We had a spirited disucssion in the office a few days ago about reducing the cost of flying from a fuel-savings point of view. Everyone has their own ideas on how to reduce the overall cost of flying, but I was always under the impression that ideas were pretty homogenous on how to do it in relation to fuel. I was wrong.

While some of us thought that running lean of peak or adding mods was the answer, I’m personally more interested in efficient airframe designs. There’s a discussion right now on the AOPA forums about this topic and it’s been interesting to follow. I think it all started when one member asked what the most efficient airplane was. The mere fact that he was asking the question is amazing. Before avgas was $6 a gallon, the thought of buying for efficency was unheard of. Well, almost so.

Between 1981 and 1990 a forward thinking group of individuals ran a race of efficiency. The CAFE foundation rounded up interesting aircraft designs and raced them with a common metric of miles per gallon. If you assume a highly efficient stock airplane flies at around 15 to 20 mpg, it’s astounding that Gary Hertzler got almost 50 mpg on an 80-horsepower engine in his VariEze. Or that Mike Melvill (yes the space guy) got 21.6 mpg doing an average speed of 192 mph in a Rutan Catbird. It’s incredible stuff that leads one to believe the current fuel prices may be a good thing. Hopefully we’ll be looking at more efficient designs.


20 Responses to “Chasing efficiency”

  1. Al Jenkins Says:

    As we all know, speed costs energy (and money). I have gotten 110 KTAS at 8500 ft on 7.3 GPH and I can use auto fuel. That worked out to almost 15 statute miles per gallon (not bad for a plane built in 1950). To get 50 to 80 more knots with somewhat better mpg and four seats, I would have to spend ten times the value of my 58 year old Cessna 170A on such an aircraft. That money would buy a lot of fuel no matter how much it cost!

  2. David F. Rogers Says:

    G’day Ian,

    Well, it depends on how you define efficiency. If time is added
    to the equation, then you would want to design and
    fly at the so called Carson Cruise Speed which CAFE has in
    the paste used as a measure of aircraft efficiency.

    Ref. Carson, B. H. Fuel Efficiency of Small Aircraft,
    AIAA J. of Aircraft , Vol. 19, No. 6, pp. 473–479, June 1982.

    or go to the nar-associates.com website, click on Technical Flying, enter the site and look for the article on Efficiently Wasting Fuel.

    I typically get approximately 16 sm/gallon at 60% power running LOP at 8000 ft with a true airspeed of 160 kts
    in an E33A Bonanza. That is basically SUV mileage at
    three times the speed with somewhat less distance between
    point A & B.

    Dave Rogers

    David F. Rogers, PhD, ATP
    Professor of Aerospace Engineering (Emeritus)

  3. G.W. Trotter Says:

    The” Gas Miles Air Race of 08″ is on.

    This is an FAI efficiency triangle of about 700 miles.

    The EAA 280 is organizing the “The Gas Mileage Air Race of 2008″. If a CFI can legally give Dual Instruction in your flying machine, then your craft qualifies. VFR day rules. Prizes with Trophy’s. The most efficient in both Time and Fuel economy’s will win.

    Kept updated on eBay. Please enter your flying machine. I will enter the Quasar Lite LSA. This LSA uses 2 GPL @ 100, while carrying a 580lb payload. I welcome your comments and competition. Search anywhere on eBay for : Quasar Lite for details or email [email protected] or call 817/210-7907 cell

  4. George Strohsahl Says:

    If a single place aircraft is all you require and 100 KTAS or so is fast enough, then try the Carat A motor glider. On a recent 140 NM trip, engine running the whole time, I averaged over 67 MPG including start, taxi, T/O, climb, cruise, and idle descent. Should you find lift enroute and have the time to relax and really enjoy flying, soaring part of the route with the engine off sends the efficiency toward infinity.

  5. Barry Fisher Says:

    I am in the process of trading in my Mooney 231, a turbocharged airplane that cruises (block to block) using about 13 gallons per hour (ROP) at 163 kt, for a Mooney 201, which will fly at 10 gph at 159 kt adding almost nothing to time for a two to three hour flight. I give up flight at high altitude (above FL 160) which I have rarely used in the 10 years that I have owned this aiurplane. One of the prime motivations for this has been the increasing cost of fuel as well as the fact that most of my flights have been under the four hour duration.

    There is no doubt that this increase in efficiency without sacrificing time has been a prime mover in this decision. Fortunately, the Mooney 201 offers this efficiency and speed as well as comfort for me amd my wife.

  6. Roger Newcomb Says:

    The easiest and quickest way to increase the efficiency of any piston aircraft engine is thru FADEC systems for all fuel-injection engines and an automatic ignition and mixture control for all carbureted engines.
    I find it amazing that we’re still building engines using 80+ year old ignition and fuel systems technology. Let’s find a way to bring the costs of retrofitting with automated systems down.
    The car manufacturers have come up with all kinds of technology to improve engine efficiency (they don’t have to deal with the FAA!), so why not apply these to aviation powerplants?

  7. Jeffrey Moore Says:

    When I take my Comanche 260-B up to altitude (11,500 ft. to FL 200), lean for best power, and turn the prop down (usually 2000 RPM, but the book says it can go to 1800 RPM), I cruise at 135 to 145 KTAS depending on gross weight and burn 8 to 8.5 GPH. That’s 18 to 20 MPG.

    Climb up to 11,500 or so; set the prop at 2200 RPM; lean for best power; trim for level flight; and let it stabilize. Then turn the prop down to 2000 RPM and maintain altitude. If your airplane is anything like mine, the difference will be 3 to 5 KTAS, 1 to 1.5 GPH, and a quieter cabin. Plus there’s the added attraction of more actual flight time between overhauls, etc.

    My Comanche has tip tanks (120 gal. total fuel), so I can easily go non-stop from Fullerton to Chicago or Little Rock, and have done so several times. I check the high and low pressure areas, and find a path (usually a small deviation from the great circle route) and altitude that give me the fastest flight time. That usually means I’m flying higher going east to take advantage of the higher tail winds, and lower going west to get out of the higher head winds. And, since I carry oxygen, I have no problem getting over the Rockies and over much of the weather.

  8. Bill Stout Says:

    I too have good things to say about the Comanche, in my case the 180. It will give an easy 16 mpg withoud climbing to extreme altitudes or exotic leaning techniques. It is competitive with the 180 Arrow, 180 Mooney and Cardina RG. I like the low wing and I don’t like the Mooney baggage comaprtment, so it’s a Comanche for me. I also fly a B55 Baron, gets there in 2/3 the time at twice the gas! Guess it depends what you are trying to lift and what you are trying to fly over!

  9. Reg Francklyn Says:

    On recent trip from Colorado Springs to Dubios, WY, for aerial and ground photos, we flew a Cessna172 M with STOL kit(not the cleanest arplane around )averaging 13 mpg with prevailing headwinds and maybe 15-16 mpg on the way back.Time was about 4 hours(vs a 10 hour drive by car) for the 400MN flight. We paid close attention to mixture, leaning for peak temp whenever appropriate in cruise flight. I pay 60 per hour for the plane and the total was less than it would of cost to fly there commerical and charter a plane/pilot for the air work, not to mention the time and hassle of commercial flight to a remote area.. So in this case GA was cheaper and a lot more convenient! Plus with my instructor along, I got moutain time, cross county, and Actual IMC time. Can’t beat that for fun and profit!

  10. JT Patterson Says:

    I have increased fuel milage in both a Piper PA28-140 and a CT-39G Sabreliner by moving people and baggage further aft. It unloads the elevator and may even allow it to create lift like the wing. Energy not used to keep the wing’s angle of attack is converted to lift which is converted to higher speed by lowering the nose to keep the same altitude. Be sure that you do not try this for takeoff and landing. Just straight and level flight. After determining we were going to get to our destination with less than 45 minutes fuel we moved people and baggage aft in the CT-39 and were rewarded with eight an extra knots per hour which put us in legal range over a four hour period and we did not have to land to get extra fuel.

  11. Steve Ells Says:

    My stock 1960 Piper Comanche burns 10 gph and cruises at 132 knots, or 151 mph. I am planning to install flap and aileron gap seals, and check it again. Not bad for a 48 year old airplane.

  12. Glen Ramsey Says:

    Don’t know if your interested in RV’s I regularly get 5.8 GPH on my 2000 RV8 with a Lycoming 0-360 carburetor mags, and constant speed prop. 2250 RPM 22″hg 1400 deg ext. 140 kt @ 2000′ 160 kt @ 8000′.

  13. Symphony Pilot Says:

    I am very happy with the gas mileage on my 2002 OMF Symphony Aircraft. With a Power Flow exhaust and Blue Printed Performance Engine, I am getting 6.5 GPH at 100 kts, which is almost 18 miles per gallon. Sure my Harley gets better mileage, but not having to deal with interstate traffic between Wisconsin and Illinois makes it all worthwhile.

    Donovan Moore
    Spring Green, WI

  14. Vince Kreizinger Says:

    While everyone is attempting to increase their fuel economy by running their engines at a lower power percentage setting and running LOP, (I do both of those) I don’t see anyone talking about AirNav. I just flew my Cardinal RG from San Diego to Toledo, OH and bought fuel near Kansas City for $3.80. So while you can do the other stuff, don’t forget to buy your fuel where it makes the most sense. Saving a buck a gallon by flying to an airport near your flight path just makes sense.

  15. Paul Hamilton Says:

    I just finished a 2 hour flight for “fun” and burned 7 gallons of auto gas in my Light-Sport Aircraft(LSA). If you want to fly just for fun with the wind in your face consider a LSA trike. It is the difference between driving in a car and riding a mototcycle.

    If you want to stay with an airplane, consider the top of the line airplane LSA burning about 4 gallons per hour of autogas at 100 knots or 5 GPH at 120 knots. Modern technology LSA are about half the weight, more streamlined, and have more efficient propulsion systems than older designs.

  16. Ian Twombly Says:

    Great comments. The range of opinions in this string are about the same as in our office conversation. Good stuff.

    Dr. Rogers – thanks for chiming with a little more of the technical side. Interesting.

    George – Motor glider. A segment I hadn’t considered. With prices that match the LSA scale, they seem like a good option. Wonder why we aren’t seeing more…

    Vince – Good point about seeking out cheaper fuel, although I’ll put in a shameless plug for AOPA fuel prices http://www.aopa.org/members/airports/. Sorry I looked. FDK is up to $6.32/gallon. I wonder how much people are actually flying out of the way in search of cheaper fuel. Sounds like a subject for another blog.

  17. John Drago Says:

    My Bonanza burned more fuel than I was willing to buy and I only had 2 passangers in the back seat in a 3 year period. My Cessna 140 was fun but a distant mission was shall I say time consuming. The answer is a IO 360 Cont. pawered Swift. I lean to 9.2 gal at 6k and go 150mph indicated.

  18. Chris OCallaghan Says:

    My Schempp Hirth Ventus 2bx has a glide ratio of about 45:1 at 60 knots and around 30:1 at 90 knots. Last Sunday, I took an aerotow to 1500 feet agl, releasing into a thermal that lifted me to 5500 agl. The 265 hp Pawnee burned about 2 gallons during its 7-minute flight. I was in the air for 4h26m. About an hour was local soaring. The rest was a 210-mile cross-country flight at an average speed of 60.9 mph. Hardly fast, but great for sight seeing and decidedly satisfying.

    The numbers:

    Total flight time: 4h33m (glider and tow plane/two take-offs, two landings)
    Fuel used: 2 gallons of 100LL
    Cost of fuel: $12
    My total cost: $22.50
    Burn rate: 0.44 gallons per hour
    Efficiency: 105 sm per gallon
    Fun factor: Off the scale

    There are lots of choices available. If stick time is your goal, a glider is among the most cost-effective alternatives. Inside the cockpit it’s simple. It’s outside that’s complex. Of course, touch and goes are a challenge. Everything has trade-offs.

    Oscar Charlie

  19. Bob Wilson Says:

    If you want to know about fuel effeciency, they just had a fuel effeciency race near Colorado Springs last week. Rob Martinson flew his 0200 variez at about 135 mph and 2 gph for a 66 mpg flight. Gary Hertzler the other fuel effeciency gurue made 61 mpg. Robs e-mail is N

  20. Mike Perkins Says:

    The easy answer with aircraft we own right now, today, is to fly at a reduced power setting. A simple Excel spreadsheet reveals that:

    A 1965 C-172 can get either 15.1 or 17.8 mpg at 75% or 44% power respectively (no wind). That’s an immediate savings of 18%.
    A 1980 C-152 can either get 16.9 or 21.1 mpg at 75% or 46% power respectively (no wind). That’s an immediate savings of 25%.

    The same spreadsheet shows that with a 40mph headwind, fuel savings is still obtained by throttling back. Same airplanes in a 40mph headwind:

    C-172 gets 15.3 or 16.1 mpg at 75% or 44% respectively
    C-152 gets 8.3 or 8.5 mpg at 75% or 46% respectively

    Obviously the immediate answer is to Throttle Back. This isn’t theoretical or a future concept. This is right now, today.

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