In 1939, a group of scientists at Shell’s Wood River Research lab in Wood River, Illinois, got in a friendly argument as to whose car got the best gas mileage. They finally decided that the only way to settle the argument was to have a contest where everyone got a gallon of gasoline and whoever went the furthest won.
They called it a mileage marathon and continued the tradition until the lab closed in 1975. In later years they added modified and unlimited divisions to the stock rules. The unlimited allowed any modification to the vehicle except that the vehicle had to weigh as much as the original vehicle and had to have an engine that was originally available in the vehicle.
Over the years some employees went a little crazy in their modifications. Being a leading crazy, I set the world record in 1973 at 376.59 miles per gallon, which was listed in the Guinness Book of World Records.
So, what does that have to do with general aviation? The principals for improving gas mileage in a car also apply to aircraft. The main points are optimizing engine operation, minimizing weight, reducing aerodynamic drag, and improving operation parameters.
Optimizing Engine Operation
On the engine operation, the main thing is to make sure that the engine is up to par and all adjustments are correct.
The first item, of course, is timing and mag operation. If the timing is retarded, you will lose a significant amount of efficiency.
The same is true of a fouled spark plug. If you have a fouled plug, the cylinder may still be fired by the second spark plug, but the fouled spark plug will effectively retard the timing and reduce fuel economy.
This is fairly common on low compression 80/87 engines operated on 100LL. The spark plugs core tips in these engines run at a lower temperature, which does not burn off the lead. This results in lead deposits on the core, which can ground the spark before it can jump the gap.
If you have an 80/87 engine and run 100LL, you need to take steps to ensure that the engine is not misfiring or mal firing.
Another consideration for the engine is good compression on all cylinders. A leaking valve or broke ring will have a definite negative effect on fuel economy.
One of the other considerations is using multi-grade oil, which should improve your fuel economy 3% to 4%.
Minimizing Weight
Excess weight does not just refer to the pilot, but also to unused clutter.
Another thing is accessories, such as air conditioning. I know air conditioning is nice, but it costs you in three ways. First is when you buy the plane, second is from the drag on the engine, and third is due the increased weight of your aircraft.
Even electrical accessories are not free because they increase the load on your engine to recharge the batteries.
The main concern with weight is loading your aircraft with the proper CG. If your plane is improperly loaded, it will take additional power from the engine to maintain safe level flight.
Reducing Aerodynamic Drag
Aerodynamic drag is mainly ensuring such things as proper door gaps, etc. Other possibilities are wheel pants or STC mods that can smooth out your aerodynamic posture.
And, of course, speed is the biggest consideration. I know we fly to get someplace quickly. But running at 65% power will reduce fuel burn significantly over 70%.
Improving Operation Parameters
Finally, there is operation techniques. I have mentioned speed, but there is also reduced idle time, flying at constant speed and altitude, plus other items.
But the big factor here is leaning. Leaning can offer the biggest improvement, but also offers the biggest risk.
In an engine, the chemically balanced air fuel ratio is called stoichiometric. At this ratio, all of the fuel and air balance out and is usually around 14.7 pounds of air to one pound of fuel.
But this results in maximum combustion temperature and can lead to a burnt cylinder assembly.
The best power is usually around 10% rich of stoichiometric, which is needed for takeoff and climb out.
Unfortunately, the best fuel economy is about 10% on the lean side. Theoretically, we should operate the engine on the lean side during cruise.
The problem is that the fuel systems on aircraft engines are not perfect, so there is usually a wide variety of air fuel ratio for the different cylinders under varying conditions.
In the automotive world, I believe almost every car produced in the US since 1968 has operated lean of peak during cruise conditions, but they are now all liquid cooled, so burning cylinders is not a problem.
In aircraft, operating just one cylinder at peak can lead to a problem. If you choose to do that, you should have multi-point temperature read outs so that you can monitor all cylinders and not overheat any of them.
Will all this work make a huge difference in your fuel bills? Maybe not, but it should help.
I agree with rigging is No1, especially for decades-old aircraft. CG however should be as far to the rear as possible, that reduces drag (not in the middle).
I had added ballast to put the CG within a few inches of the rear limit on my Cessna, and it scared the cr#p out of me , with the elevator way too sensitive. I had to use 2 fingers on the yoke to not overcontrol pitch…
I’ll stay with about 25% back from the forward limit…that gets me about a 1 GPH reduction in fuel burn at 65% power.
BTW, we don’t fly aircraft to go slow.!
I agree with Ben Visser’s tips to improve fuel efficiency when flying airplanes. First, I lower my engine rpms on cruise flights between 2,250 to 2,350 rpms. Then, I use maximum engine lean which saves 1.0 gallon of fuel each flying hour compared to flying with a rich mixture. I don’t compromise high engine temperatures on my cylinders because I have an engine analyzer for all 4 of them on my Lycoming O-320 engine. I never let the CHT rise above 330 degrees F and the EGT above 1350 degrees F. This gives me safe temperatures for the cylinders and maximum lean that safely lowers fuel consumption. I have currently run out my Lycoming engine with 2,040 flying hours and my leaning procedure has been excellent the entire duration.
William A. Quirk, lll
Alaskan Pilot
I am curious….with the stated fuel conservation measures, how have the components of the exhaust system fared? Cracked/burned/excessively eroded pipes or muffler components?
Lean of peak and reduced power settings will reduce exhaust temps, so no affect on the exhaust system, other than I see a lot of lead buildup on the tail pipes…gray, crusty stuff….better there than on the exhaust valve stems and spark plugs.
BTW, The plugs are always clean without any lead showing.
Flight planning your route, altitude and time of day to maximize tailwinds and minimize headwinds doesn’t hurt either.
Per Cessna, the #1 thing to do is to be sure the rigging is correct, that the aircraft is flying straight, without corrective inputs that induce drag.
Another factor is setting the CG near the mid-point, to reduce the elevator induced drag.
The C175 that I fly is nose heavy with the psru, so I carry temp. ballast in the baggage compartment.
I agree with the other comment to lean at all power settings less than 75%, and at all altitudes.
bad advice as usual
listen to mike busch
lean of peak at all power settings beliw 65% is totally safe
also dont see why multi grade oil would save gas at cruise since 20w50 same as w50 at operating temp
How is Mr. Visser suggesting bad advice with regards to lean operation? Note,” the best fuel economy is about 10% on the lean side. Theoretically, we should operate the engine on the lean side during cruise.” Exactly what Mike Busch recommends,”lean of peak” operation. He simply states to be careful of temperatures,again, just as Mr. Busch would with his constant admonitions to have an engine monitor so as to track each cylinder independently.
Multi grade oil flows better requiring less engine power at all temperatures,any time the engine is running it is burning fuel,not just at “cruise” and so effects total fuel consumption.
I also reduce the the speed by reducing R P Ms this saves on fuel.