First cross country...a few engine ops questions

[Pavewlc]

Hi folks,
I flew my first cross country today after being cleared solo last weekend. Overall the trip was eventful, despite my not so good landing(s) in a 15 mph crosswind (where was my IP when I needed him!?).
Here's the basic numbers for the trips:
Altitiude: 3,000
Lenght: 2.5 and 0.5 (3hrs total)
Fuel burn: approx 8.5/hr
Lean mix: didn't even try it
OAT: 65 -70 F
Tailwind the whole way
Tach: between 2300-2450


so now my questions:
1) Why does the fuel feed faster from the right tank? The right tank got down to a little less and 1/4 remaining when the left had over 1/2. I got nervous and landed. My math said no worries, but I was unsure.

2) Engine oil temp ran in the top 25% of the gage the whole way. I tried to adjust altitude to get cooler air but it got worse. Never red line, but up there. I've heard "they run hot" but is this correct.

3) On post flight inspection the bottom cowleing from the stacks back had a gray dust (film?). The exhaust pipes were coted as well. I didn't notice this on the 8 previous hours of flight, but most of that was pattern work, not strait n level droning.

4) On take off today I actually looked at the Tach with full throttle. It showed 240/2500 (ish) but not 2700 (that's in the red). Normal, right? It should just touch the yellow range on T/O.

Not sure of the oil burn yet but I'll check it before I go the reverse route on sunday.

Thanks for any tips, techniques and comments.

Cheers,
lee

[GAHorn]

Well, it all looks normal except:
The fuel burn... But that makes sense since you didn't lean it. NOrmal is about 8 gph.
The takeoff rpm: but you don't say what prop you have, or if you've actually checked your tach accuracy. Normal , with the standard prop is 2230-2330.
Recommended cruise rpm is 2450.

Oil temp: If you are using SAE 50 oil (100 grade) then the oil temp is allowed to go all the way up to 240. It sounds as if you have the original green arc/red line at 225 guage. If that's so, .... then you're doin' just fine.
The "apparent" faster consumption from the right tank is normal. This has been cussed/discussed on previous threads. Do a search, but the bottom line is: It's only an indication situation. Both tanks feed equally, but the left tank is being re-filled from the right tank via the overhead vent line...until both tanks have dropped down a bit. It's an idiosyncracy of the system. Don't worry about it. Run it on BOTH and be happy.

[Pavewlc]

Thanks for reassurance, as a helo guy I'm always a little worried something is about to happen.
I'm usuing the oil you outlined above. The prop question is a good one. I don't think I've actually looked.
"Put it on both and be happy.." you bet!!

So the right tank is feeding the left....when the right goes dry will I keep flying? I assume so. I've done some searches on the forums, but what's the lowest the right will get as it feeds the left...zero?

Thanks again,

Lee

[GAHorn]

I'm glad you "read more better" than I write lately.
I edited my previous msg to say what I meant. (left tank is refilled from right tank... but at least it's both technically and actually an airframe part)
Yes, it'll feed down until the right tank indicates about 1/4 lower than the left, then the suction/siphon action will stop and they'll both drop equally, until the left tank is down to about 3/8 and then it'll feed faster than the right until they both read zero. (Yes. I've seen it happen. No. I don't want to talk about it.)
(Well, actually I guess I should. I was on a long x-country and reached a planned fuel stop, and decided to orbit overhead in a test of unuseable fuel. When my gauges no longer would "slosh" when I kicked rudder, I landed ...with lots of altitude/flat attitude, etc. ... and refilled. According to the pump, it took 37 gals. exactly. There was a small amount in the tanks remaining, but was not easily measureable, as it sat at the rear of the tank in the 3-point attitude, and only wet the bottom of my dipstick, but I was convinced Cessna knew what they were doing when they claimed 42 gals total, 37 useable, and that both tanks feed the engine equally, despite early indications due to tank venting/interaction issues.)
I also suspect that if one could observe it, the fuel, as it descends the line toward the engine, swirls in a clockwise direction...but that curiously, it reverses itself and flows counter-clockwise when one crosses the equator.)

[Bruce Fenstermacher]

Lee

George has written about the uneven fuel usage several times with explanations as to why which I can not dispute.

It is my practice never to run a tank dry but I will get it close. I manage my fuel during longer flights by manually switching from Both to Left tank to attempt to keep the fuel levels about even. I don't have a problem doing this because my first aircraft was a Cherokee which doesn't have a Both position and one has to change tanks as a matter of necessity.

At about 1/4 tanks I put it on Both and leave it because any further fouling with the fuel control isn't worth the effort and I'm usually thinking about landing and want it on Both for landing.

I like the fuel amounts in each tank level because it keeps my plane in better trim as far as a heavy wing condition goes. Other than that I just feel better having fuel in both tanks rather than one.

Might be a helicopter thing I don't know. After all we helicopter pilots are revolutionists.

[cpolsley]

Lee

My 170 is a 1953 B model with the vented fuel cap on the right wing. My right tank will run lower than the left by about 1/4 tank as the others have suggested theirs will. The right tank will indicate completely empty, no needle bounce at all. The left tank still indicates about 1/4. When I top off with fuel the right tank will take 18 to 19 gal which would indicate it's completely empty of usuable fuel. It tends to make one a little nervous but in the 21 years I've owned the airplane its never be a problem. With the 1/4 left tank and the right empty there is still 8 gal remaining.

Keep track of your flight time and gallons used and you'll feel more comfortable with each flight. My airplane from Ohio to Alaska and back averaged about 7.8 GPH.

Enjoy your airplane the 170 is a great tail-dragger

Chris

[CraigH]

My bird has the same fuel flow phenomenon, and BOTH of my caps are vented. Seems to be a universal truth amongst 170's.

[Pavewlc]

Hi everyone,
Well I flew back today, all the more confident in the aircraft and my ablilites as well. I completely understand the fuel now, thanks for the great advice. I don't think I'm so confortable yet with letting the right go to zero, but I'm a very short way down the long road of aircraft ownership.


The oil situation still has me puzzeled. Today I was at 2500 hundred with the needle in upper 25% of the green. I climed to 4000 and leveled off. Needle went to just touching the red and stayed. I also tried leaning the engine and it went well into the red and stayed. I didn't let it stay long! I went full rich which brought it down some, but still in the red. I went back to 3000 and it came into the green.

1) Why does oil temp go up with altitude! Seems very backwards.
2) How do I lean the engine if it goes way into the red?

I've got mostly turbine time in my life, so this is a little odd for me. But my experiance with MG's is starting to pay off...just glad we don't have Lucas parts.

Cheers,
Lee

[N2865C]

When I purchased my 170 2 1/2 years ago I had the same high oil temp readings. It ran just below the red line and at or just above red-line with OAT's over 85. The previous owner had spent a lot of time and money trying to fix it. After spending a year worrying about it and trying to figure out what to do I finally replaced the oil temp gauge. My oil temps dropped 25 - 30 degrees. Apparently it is common for them to fail by giving you high readings. I checked my old and new gauges in boiling water when I replaced it. Now I spend my time worrying about how to get my oil temps up high enough to burn off the moisture........ You just can't win

If you do a search for oil and temperature there has been a lot of discussion on this.

[cpolsley]

Lee

I recall the 170 Book having a lot of oil temp info and if your new to the 170 it's packed full good 170 info. This forum also has info on the oil temp issue.

Chris

[Bruce Fenstermacher]

Lee
One of the things the old threads will point out is the actual oil temperature limits which your oil temp gauge markings may or may not reflect. I'll bet your redline is at 225° Also as mentioned your temperature gauge probably isn't showing the actual temperature anymore.

I presume you're aware of the 240 -degree redline on our engines when used with SAE 50 wt oil? (See the engine type certificate Note 1.)

NOTE 1. Maximum permissible cylinder head, barrel and oil inlet temperatures, 525°F, 290°F, and 225°F, respectively.
Maximum oil inlet temperature limit is 240°F, when using Spec. MHS-24 oil SAE No. 50 above 40°F ambient and SAE No. 30 or 10W30 below 40°F ambient.

[GAHorn]

...1) Why does oil temp go up with altitude! Seems very backwards. ...

Cheers,
Lee

As altitude increases OAT decreases due to the fact that the less-dense air consists of air molecules farther apart. The molecules are less able to transfer heat to each other. This helps our TAS, of course, because fewer molecules impact the airframe to cause drag.
The same phenomena also means fewer molecules impact our engine's cooling fins and therefore less heat is carried away. Don't worry about your oil temp if you're still equipped with the old 225-redlined gauge, and if it's only ON the redline. You're fine (presuming the gauge is accurate. One way I've checked my own gauge is to use a long-probed BBQ thermometer, bought at Academy for less than $5, and stick it down the oil gauge rod/dipstick tube immediately upon landing and comparing the reading with the cockpit gauge.) A good way to check calibration is to submerge the probe/thermometer in boiling water. Keep it off the sides/bottom of the pan. Water boils at 212-F at sea level, close enough to call it 210.

I have also occasionally used the fuel valve to balance fuel, especially if I've got the airplane loaded oddly with two on the left and one pax on the right, etc. But keep in mind that if your airplane is equipped as most of the fleet is, that only your right tank cap is vented. If you are operating on the Left only, the only venting available is the "gooseneck",... which if plugged can lead to fuel starvation despite fuel being in the tank. (I have vented caps on both tanks also and notice no change in behavior of the fuel system.)

[Bruce Fenstermacher]

In my efforts to be "Just like George" , both my fuel caps are vented as well. I'd also like to point out that to be more precise with just the right cap vented if you have a clog down in the gooseneck past the T fitting on the left side of the vent line, then your left tank won't be vented. If the clog occurs in the gooseneck itself the left tank will be vented through the right tank fuel cap vent.

[cessna170bdriver]

As altitude increases OAT decreases due to the fact that the less-dense air consists of air molecules farther apart.

It's only a very minor point in this discussion, and it is not my intent to start an argument, but the anal engineer part of me thinks a bit more explanation is in order . The air is not necessarily cooler due to the fact that it’s less dense (molecules are further apart). In fact less dense air can be warmer, as in inversion layers. The phenomenon George is talking about is the "Adiabatic" (constant heat) Lapse Rate, which only applies to a parcel of air in the process of rising, and therefore in the process of expanding, like letting compressed gas out of a closed container. This is due to the fact that as a constant amount of heat occupies a larger volume (expansion), the temperature will decrease. The cooling rate due this phenomenon is just under 5.5 degrees F/1000 feet in dry air.

In an atmosphere with little or no vertical movement, the air at higher elevations is cooler mainly because it’s further from its heat source, the earth. This phenomenon is called the “Standard” Lapse Rate, and runs just over 3.5 degrees per 1000 feet (especially in the lower portions of the atmosphere where 170’s can go). The fact that density decreases with altitude is mostly attributed to its lower pressure.

The part about less dense air (regardless of how it got that way) having less cooling CAPACITY is absolutely correct. The amount of heat a given volume of air is capable of carrying away from the engine depends much more on its density than its temperature

Miles

[lowNslow]

The part about less dense air (regardless of how it got that way) having less cooling CAPACITY is absolutely correct. The amount of heat a given volume of air is capable of carrying away from the engine depends much more on its density than its temperature

How about in a dynamic situation? i.e. if you have the same indicated airspeed at different altitudes, wouldn't the cooling be the same? If so wouldn't it be more accurate to say that cooling varies with indicated airspeed?