Tire drag

[KS170A]

Interpolated data from the 170-B performance, standard engine/prop indicates the following altitudes vs fuel burn at recommended cruise power of 2450 RPM, LEANED:

Sea Level- 9.2 gph
2500 MSL- 8.3 gph
5000 MSL- 7.4 gph
7500 MSL- 7.0 gph
10K MSL- 6.5 gph

This is in level cruise and therefore does not take into account takeoff/climb data. I suggest adding 1 gph for every hour of flight to be conservative.

My own practice is to climb to 7500 MSL (plus/minus 1K westbound) for flights over one hour. I cruise 2450, leaned, and flight-plan 8 gph, but actual results are dependably 7.8 gph and 104 kts (117 mph) TAS.

I installed a JP Instruments FS450 indicator in my O-300A equipped 170. The flows, when leaned for altitude, matched this table very closely.

[Bruce Fenstermacher]

Richard I've never leaned aggressively because I could never tell by the tach when the engine was at peak and then started to drop. So I just lean by pulling the mixture out about the same which is to my first knuckle on my index finger (about an inch). I know the engine is running leaner because I see a rise in the egt and cht but I know it's not peak because I've tried to lean to peak a few times and the egt and cht will go higher. I'm satisfied that I'm not running full rich nor to lean of peak.

If I had lots of spare change I'd install a 6 probe egt, cht, a digital tach and a vernier mixture control. Then I'm sure I could adjust it just right.

Well I don't have a standard prop as it is a 54 inch pitch and I ran both these flights at higher rpm and in many cases low altitude. Add a few long high power climbs with little cruise time and add a little extra drag on top of that. Looking at the performance charts and adding these factors I think it very conceivable to get the fuel burns that I did.

What I'm trying to do is relearn how to fly while sipping gas rather than gulping it and large tires must add drag and can't help. I've broken down and will be buying Blue Leader's 7 x 6:00 he had for sale so we will see if anything improves for my trip to this years convention.

[170C]

Bruce, my memory isn't as good as I would like it to be, but I can't remember what color your plane is. If its like George's, got some red on it, then that would probably account for high fuel consumption to achieve that 110-115 mph cruise speed at 2500/2600 rpm's

[Bruce Fenstermacher]

Bruce, my memory isn't as good as I would like it to be, but I can't remember what color your plane is. If its like George's, got some red on it, then that would probably account for high fuel consumption to achieve that 110-115 mph cruise speed at 2500/2600 rpm's

It is maroon and silver in a later '54 scheme I think. Maroon is appearantly close enough to red to add additional drag.

[hilltop170]

Bruce-
I understand your conservative approach to leaning. I flew 1715D for many years without any EGT info. Now that I have it, the standard old leaning method I used to use yields results very close to what I get if I go by the numbers on the analyzer. I pull the mixture out until the engine runs rough then push it back in just until the engine runs smooth. It typically ends up between 7.2 and 7.8 gph at 3000'- 5000'msl. Those are instantaneous cruise numbers which do not take climb into account but dividing gallons by hours at fill up doesn't change it much. Obviously your results may vary.

[blueldr]

Any time you are using a power of down around 70%, which is about a normal cruise setting, you can't "over lean" the engine. If you lean it off to where the engine starts to stumble, you're well below peak power, the hottest condition. Richening it up to where it just smooths out puts you on the lean side of peak and that's where it's the most economical for the power produced. Over leaning damage will only occur at power settings above about 75% and then only if you're too close to peak to where the temperatures get damagingly high. If you're running at 70% or lower you could probably run at peak power without overheating any part of the engine. Most naturally aspirated engines will not produce above 70% at about 7000 ft. MSL.

[GAHorn]

Any time you are using a power of down around 70%, which is about a normal cruise setting, you can't "over lean" the engine. If you lean it off to where the engine starts to stumble, you're well below peak power, the hottest condition. Richening it up to where it just smooths out puts you on the lean side of peak and that's where it's the most economical for the power produced. Over leaning damage will only occur at power settings above about 75% and then only if you're too close to peak to where the temperatures get damagingly high. If you're running at 70% or lower you could probably run at peak power without overheating any part of the engine. Most naturally aspirated engines will not produce above 70% at about 7000 ft. MSL.

Even if you're running the high-blower?

[blueldr]

George,

If you're running in high blower, you somehow or other got in the wrong airplane.

[N4005V]

Are you using Tach hours or stopwatch hours for the fuel calculation? That could make a big difference.

[Bruce Fenstermacher]

Yes the tach hours would make a big difference. For both of the flights I used the time logged by the Garmin 496 which starts and stops flights based on airspeed. On the second flight the hours were backed up and extremely close to what my wife had logged using her watch. I have no doubt some of the extra was time spent taxiing but not that much to bring the numbers down.

Besides I couldn't have taxied any less and so the gph is an accurate accounting of what it takes to fly the trip we did.

[markeg1964]

I was just going to ask the same question. I keep all my mileage records based on tach time to be a bit on the conservative side wile flight planning. Around 90% of my flights are at 2,000 feet or below with 8 x 600 tires and I typically burn a little over 8 gph – I also fly a bit slower as my tach reads a bit fast and I’m not usually in a hurry. I have recorded 10 gph when flying with my wife (tach time) at 2600rpm (true).

To me it is simple. The 170 looks cooler with the 8s and who wants to get there sooner and spend less time in the air

[Bruce Fenstermacher]

Mark, I agree I like big tires. But to be honest I'd rather take off with big tires looking cool and then zoom to my destination at about 300kts and a gallon or so of gas, then slow down and do a couple of trips around the pattern doing touch and goes and looking cool with big tires.

I happen to be lucky enough or unlucky enough perhaps, to fly little aircraft at low altitude for a living. I have no interest in straight and level flight. For me straight and level flight is one of those necessities of a trip that comes between the take off and landing. But I certainly understand your viewpoint.

[GAHorn]

Guys who have big tires must be trying to make up for something.

[jrenwick]

Guys who have big tires must be trying to make up for something.

Yeah -- soft earth.

[blueldr]

Did you ever noticethat the guys that have those "teeny tiny tires" on their arplanes do go faster? About 120 ; but then they have to get out their whiz-wheel to see how far they went in two hours.