Can aircraft mods improve fuel efficiency?

June 5, 2008

Pilots, along with anyone else who is propelled by an engine, are looking for ways to mitigate the sting at the pumps. So, can you just buy a silver bullet to save your aching fuel credit card?

Common sense tells us it’s not that simple, but smart pilots have learned to continually investigate the options. Let’s take a look at three areas: Speed improvement on the same fuel flow for less time en route; power enhancement (throttle back for reduced fuel flow at the same speed); and futuristic options that are now decidedly closer to reality.

Speed improvements

There are many drag-reducing aerodynamic enhancements for speed increases. Inexpensive items include gap seals for flaps, ailerons, and other control surfaces. Aerodynamic wheelpants, gear fairings, different exhausts, landing light covers, and even wingtip shape changes can add modest speed. All of these items can work together to add up the extra knots.

The basic math should be that each 1-percent increase in cruise speed (let’s use 100 knots cruise, so 1 percent is 1 kt or 1.15 mph) will save you 1 percent in time on the same length being traveled. Burning 7.5 gal/hr equals burning .125 gal/min. Each 1-percent savings in time would be saving 0.6 minutes (0.6 min x .125 gal = 0.075 gal saved). Multiply the gallons by $6 of avgas, and the savings total $.45/hr. That may seem like just nickels and dimes, but that’s “per hour for each knot” (1 percent) of extra speed.

Lets say our flap and gap seals give a combined 3 knots (caution: that’s 3 percent on our 100-kt airplane, but that’s only 2 percent on your 150-kt plane, and 1 percent on your 300-kt machine). Now, three times that $.45 savings, times the average 100 hours per year pilots fly, would equal $135 in savings for the year. If gap seals cost $500, they would pay for themselves in 3.7 years at current avgas prices.

Engine and propeller mods

The second area of consideration would be some engine or propeller improvements (under either a supplemental type certificate [STC] or a field approval modification) that would produce more power, thereby enabling you to ease that throttle back to save fuel while maintaining your former airspeed. Lets look at these ideas in the table below for a Cessna 172.

Idea Fuel saved Cessna 172 cost/payback
Tuned exhaust system. Improved engine efficiency and power. Not higher hp rated but generates more power at lower cruise rpm. Reports of up to 30-hp increase save 100-plus rpm at same airspeed. Cessna POH shows around 1 gal/hr saved. Power Flow product says up to 2.2 gal/hr. $6 fuel and 1.5 gal/hr. Flying 100 hours could save $900. Mod cost $4,000. Payback 4.4 years. Note: If you flew 38/hr month, this would pay out in 1 year.
Upgrade to high-compression pistons: 150 hp rating becomes 160 hp. Throttle back, saving maybe one-third gal/hr. $2/hr on gas saved over 100 hours equals $200 per year. Cost: Nothing extra if you had to overhaul or top-overhaul anyway.
Re-pitch your prop for more speed; check the setting for most cruise pitch allowed on its FAA type certificate. Let’s say you could get 5 mph on our 125-mph Cessna 172 example. That’s a 4-percent improvement. Using our earlier math, 4 percent time saved would equate to 2.4 min/hr. Burning 8 gal/hr equals .133 gal/min, and we’re saving those 2.4 min or .319 gal/hr. .319 gal/hr @ $6 = $1.91 per hour, cash saved. Times our 100 hr/year is $191. Cost to re-pitch and overhaul a fixed-pitch prop might be $350. Payout is 1.83 years. Some shops might re-pitch w/o overhaul, so ask. Drawback: slower takeoff and climb.
Investigate other propeller-type options like constant-speed operation. Cessna/Pipers with 180 hp and fixed-pitch props could swap to constant speed. Note: Older classics may have option for cockpit adjustable prop. Do your homework. An adjustable or constant-speed prop forces the engine to work harder with the pilot able to select lower rpm for a lower fuel burn. Savings could approach 30 percent or 3 gal/hr. That’s 3 gallons times $6 or $18/hr and thus, $1,800 per year. Cost might be around $7,000 for a payback at 3.89 years. Additional advantage of improved takeoff and climb rates.
K&N type or foam type air filter. Better flow of intake air means more power. All pilots know the Mooney system of filter bypass adds 1 inch of free manifold pressure, so any flow improvement is worthwhile for the modest price.
Turbonormalizing (if you fly at higher altitudes). Higher power available at altitude should enhance speed improvement at a greater percentage than fuel increases. Lots of homework to do, but you know how to work the speed vs. time saved vs. fuel-flow numbers by now!

The FAA’s Web site officially lists all STCs. On the left margin, click “by make,” and then the alphabet letter across the top of the page for your aircraft manufacturer [example: “C” for Cessna or “P” Piper]. Search separately, “T” for Teledyne Continental, or “T” for Textron Lycoming. Select your particular aircraft (or engine) model in detail (172M). Popular models may have nearly 200 STCs available, so take your time in scrolling through the listings. You may have to check under the model listing for the basic 172 (no letter) as well as the 172M from our example, as some STCs approved for every 172 model may be placed there. Click any listing to see more detail. It might not be all you need, but now you have information for further research.

New technologies

Diesel engine conversion was U.S. STCed for many Cessna 182s (including Piper Dakotas in France) in 2006. It’s modestly expensive, but with a 30 percent or more fuel savings, a pilot about to overhaul his 100LL engine should take a look. Even though factory-new Cessna 172 turbodiesels are on hold with the German Thielert Company’s recent bankruptcy, the aircraft manufacturer Diamond Aircraft has said it is ready to introduce its own line of diesels.

The Wall Street Journal recently reported that the Air Force B-1 cross-country flight test on synthetic jet fuel paid $4.62 per gallon. All of a sudden this price looks very good for the diesel option above.

In April, the FAA was asked to develop and accept electric motor standards for light sport aircraft within the FARs. U.S ultralights flew on electric motor/lithium ion batteries in May 2007; the French flew an 800-pound single-seater in December; and Boeing just flew a two-seat European production motorglider by electric motor/fuel cell in April.

Each of the considerations discussed in this article can have some impact in reducing your cost of flying. Whether you’re thinking of small modifications, a more expensive improvement, or going diesel as soon as the technology is readily available, let us be of assistance. Give AOPA’s Pilot Information Center a call at 800/872-2672 as you work through your ideas.