Monday, September 7, 2009

Leaner and Greener?

The trip from KBDR down to VKX on Friday afternoon did not provide me with very much weather to talk about. Most of the way down it looked about like this:

With a rare easterly tailwind giving me good ground speed, I was put in mind of the old-timers advice to "fly slow in a tailwind and fast in a headwind." The idea is to get the headwind over with as soon as possible, giving it minimum time to delay your progress, and to throttle back and save fuel when a tailwind is available to do some of the work.

I must confess that I've gotten a bit lazy with respect to cruise power settings for N631S. I normally file for 7,000 or 8,000 feet MSL on my longer trips and set up with wide-open throttle (giving about 20" of manifold pressure), 2,400 RPM and a fuel flow of 11.8 to 12.0 gallons per hour (GPH). According to the Pilot's Operating Handbook (POH) that yields about 70% power and a true airspeed of 137 knots. The airplane seems happy that way.

On Friday I decided to experiment a little. I began by reducing the propeller speed to 2,200 RPM. The Outside Air Temperature at 8,000 MSL was 52F. At left, you can see the manifold pressure gage and the tachometer. According to the POH, at that altitude and RPM with about 19 or 20 inches of manifold pressure, I ought to be able to lean out the fuel flow to around 10.0 GPH and get 58% power and a true airspeed of 129 knots.

The picture to the right is a bit blurred but it shows the Shadin fuel flow computer indicating a consumption of 9.8 GPH. And take a look at the temperatures shown on the JPI engine monitor! A that moment it is indicating a Cylinder Head Temperature (CHT) for the #5 cylinder of 357F. That is about 20 or 25 degrees cooler than what I normally see in cruise. Cooler CHT's are decidedly a Good Thing!

But what was this costing me in airspeed? Before I started fooling around I was getting my usual 137 knots True Airspeed (KTAS). Now, having gone from 70% power to about 60% power, I was getting 131 or 132 KTAS. (The picture at left shows 115 KIAS (that's indicated airspeed), but by aligning the altitude and OAT scales on the movable True Airspeed ring, I can read off KTAS on the outer scale. It is reading just about 131.

So, I was giving up about 6 knots of airspeed to save 2 GPH of AvGas. Looked at another way, I had gone from 11.6 nautical miles per gallon to 13.2 NMPG...an improvement in fuel economy of 11.4%. (Since 1 NM = 1.15 statute miles, I was getting 15.2 statue MPG. I know people with SUV's that don't get that!)

Of course, airspeed is nice but it's ground speed that gets you home. The picture here shows the GNS-530W telling me my speed over the ground was 129 knots. On that course, the wind was having little effect. (When I made the turn to the west at SBJ I picked up a couple of knots courtesy of the easterly breeze.)

Not counting time-to-climb, N631S and I were about 2 hours en route. If I'd burned 2 GPH more fuel, I'd have gone about 6 knots faster. That works out to an extra 5.5 minutes for the trip. In return I saved about $15.00 on the fuel bill. Perhaps just as importantly, I reduced the fuel burn by about 25 pounds, and that's a lot of CO2 not added to the atmosphere.

General aviation may not be the "greenest" of activities, but I have now seen a way that I can reduce the environmental impact of my own flying. I'll still fly fast in a stiff headwind, but in more benign condtions I believe that I will start sparing the throttle.

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