The International Cessna^® 170 Association

I discovered something interesting today - my tach has been reading 100 RPM low. I became supicious when I was burning about nine GPH at what I though was 65% power; it turns out I was running about 75%! I used my hand held electronic tach designed for model airplane use to check my tach.

Today, I flew a 322 nm out and back to Wilmington, NC with a stop on the way back at Lumberton, NC to check out an old 1959 310 for sale (yeah, I know but I can dream, right?) Total flight time was 3.2 hours at an average speed of 100 knots. Fuel consumed was 24.8 gallons, which works out to 7.75 GPH...this was at about 55% power at 7,500 eastbound and 55% at 2,000 west bound. This is the first time I've made this trip without re-fueling.

The point of all this number crunching is I'm trying to come up with a long-range cruise strategy. I use the power chart in the owners handbook, which seems pretty accurate, but I'm wondering if I'm missing any special tricks here. I have a single point EGT on the number two cylinder and run it 50 degrees ROP, with a touch of carb heat to help atomazation (it's a theory, anyway!) I figure optimum altitude to be between 5 to 7,000 feet zero wind, higher with a tailwind and lower with a headwind.

I'm welcome to any suggestions on maximizing the range of my 170, short of adding Flint or Javelin tanks Russ Farris

Russ,
I've looked at the book numbers too and have discovered that my 1950 C-170A performs pretty much by the numbers. The RPM I see at the fuel burns and speed you saw is ~2450.

Russ,
At 100 kts (115 mph) your fuel burn of 7.75 gph seems a little
high to me. My long hauls are generally from my home here in California to Nampa or Caldwell, Idaho, a flight of about 3+45
to 4+00 hours at about 7500 to 9500 ft. My average TAS is 115
mph and the average fuel burn is 6.8 gph. I lean to engine rough and then enrichen just barely to smooth at about 2450 rpm
on a 53" prop or about 2600 rpm on the 50" prop. The burn might be a little higher at lower altitudes, but all my long hauls seem to be over higher terrain out here in the west. I've been making this same route for a number of years now and it is almost always the same.

Russ, 7.75 GPH looks like an excellent fuel burn to me. Using a bit of carb heat to help atomization is interesting. Carb heat richens the mixture, but if you compensate by additional leaning... Have you been doing this for some time ?
Rudy

Russ, the 7.75 gph seems about right for 100 knots, averaged for the 2 quite different altitudes you mentioned. But please specify what rpm you're talking when you say 55 65 & 75 %. Usually percent is used when discussing jet power settings, eh? You're in the 170, please use terms us poor piston-pounders can understand.
I just remembered that the power charts give different rpms at different altitudes for the same percent of power. But I'd still be interested in what rpm at what altitude you used.
Now, doesn't manifold pressure tie into percent of power? With a c/s prop-equipped airplane, 20"/2400 is gonna be less power than 24"/2400, even though the rpm is 2400 in both cases.
So your 65% power rpm with (for example) a 55" pitch prop is gonna be different than my 65% power rpm with a 51. Seems like it'd be at lower rpm due to the higher MP. Right?

russfarris wrote:... I have a single point EGT on the number two cylinder and run it 50 degrees ROP, with a touch of carb heat to help atomazation (it's a theory, anyway!)

Russ,
Be careful about running 50 degrees ROP. At low power settings this may be OK, but the engine performance charts show that this is about where CHT peaks -- at higher power settings, running this way can reduce exhaust valve life because they won't cool as well.

Also, be careful about running with carb heat if you don't have a carburetor temperature gauge. In the northern latitudes during winter, we would be concerned that partial carb heat could be warming the carburetor up into the icing range. Maybe where you are this doesn't happen?

Best Regards,
John

The only time running with partial carb heat is beneficial to fuel performance is with less than a wide open throttle. When the throttle is wide open, any carb heat at all will decrease available power. Why? Because by definition the carb heat warms up the air....equalling higher density altitude (less dense air)...and therefore less Oxygen molecules available to any given engine displacement. (In other words....by running carb heat you're making the engine think it's summertime....with a resultant loss of power.)
Opening the carb heat butterfly also relieves the induction system of ram air effect, causing a loss of "breathing" for your engine.
Forget about improving atomization in a carbureted engine with partial carb heat. Run it full cold...or full hot...unless you have a carb air temp gauge and a willingness to achieve less than optimum power. IMHO

George is correct, think of what a turbo charger does. As for the fuel burning, it does fine in an engine at 20 or 30 below F. Even though trying to light gas with a match at those temps, is like lighting engine oil with a match.