C170 Performance

[quintdxb]

I recently purchased a 1948 170. Can anyone supply cruise performance figures for this type as a comparison.

On the ferry flight to my home base the aeroplane achieved a TAS of around 95mph under the following conditions:
QNH 30.15 inches
OAT 74F
RPM 2400
Fuel burn was about 7,8gph.
Also of note, the Oil temp was close to the 225F red line and did not vary when mixture was made richer. (Qty was 7quarts) CHT gauge was unserviceable so no clues were available from the CHT. Oil pressure was constant at about 40 psi. (D100 oil used)
No wheel-pants/spats ar fitted.

I would appreciate any data from other 170 owners, in the meantime, I am waiting to hear if it has a cruise prop or a climb prop.

Many thanks,
Quintin

[3958v]

Congradulations on your purchase. To answer your question I would only be concerned about the oil temp. It seems quite high. I never see temps that high even when the outside temp exceeds 100F. I suggest you check to make sure the baffeling is tight around the engine and that the blast tube is blowing right on the oil temp probe. You can check your probe by inserting it in a pan of boiling water. should read 212. One more thing to look at is the size of the lip at the bottom rear of the cowling. I have the larger one about 2.5 inches wide vs 1 inch or so. I belive the larger one provides better air flow for cooling. There are other sources for high temps such as poor compresion or engine timing but I will leave that talk for some of the more experienced members to help explain. By the way how long a cross country can you take over there and do you have to have a flight plan for every flight? Bill K

[quintdxb]

Bill,

Many thanks for the informtion.

A bit of background. I work for an airline in Dubai, UAE, but spend about two months of the year in my home country - South Africa. That is where my 170 is based. The ferry flight was from the North East of South Afica to the Southern Cape Coast where I have a home. The filing of flight plans is only required between conrolled fields in South Africa although you may file a plan for any flight - an option I chose to take up on a long ferry flight in an unknown aeroplane in some pretty poor weather. The ferry flight took me about 9 hours of flying.

General/Sport aviation in South Africa seems to exist in spite of the Civil Aviation Authority, rather than because of it.

Unfortunately I do not have the serial number etc. for the 170 with me as I am back in Dubai. Apart from a few flying schools, there is VERY little general or sport aviation in the UAE.

The previous owner tells me that the oil temperature has always behaved in the manner described. Nevertheless, I would like to get to the bottom of the oil temperature problem. The baffeling looked pretty sound but I shall certainly investigate your blast tube and temperature probe suggestion.

The aircraft has the 1 inch lip at the bottom of the cowling.

Confirm your 170 produces similar cruise speeds. I know the manual for the 170A quotes a TAS of about 105mph under similar conditions.

Many thanks again,
Quintin

[doug8082a]

It's important that you get a baseline to judge these things by. As Bill pointed out, re: oil temp you'd be best served by confirming that the gauge is accurate. If you do a search here on the forums for oil temp you'll find that inaccurate gauges are more comman than you might think.

As for the cruise speed, I'd first confirm that the airspeed indicator is accurate - maybe do a few runs with a GPS for comparison. I'm flying a Cub right now that has an ASI that reads 10mph fast.

Once you've confirmed what your airspeed really is, check into what sort of prop you have, and the rigging of the plane. As Bruce can attest (speak up Bruce) having an aircraft out of rig can cost you 5-10 mph in cruise speed.

Congratulations on your new (to you) 170! You are really going to enjoy it.

[N2865C]

Welcome!

I went through exact same thing when I got my plane. First thing to check is the gauge. They often fail on the high side and mine was reading was reading 30 degrees high. You need to keep the water boiling as it takes a minute or two for the gauge to register. I think it is easier to remove the gauge and test it at home. Should be 212 (adjusted for altitude).
I had the seaplane lip (as described above) and could not see any difference after a couple of test flights with and without it. I suspect that it is most effective at lower speeds and high power settings. I took it off. YMMV.
If you check the TCDS for your engine you will see that the max. oil temp is 240 degrees if you use straight 50 wt, so you are still OK. Still, it's a warm fuzzy feeling to see your needle in the green part of the gauge.
If you search this site for oil and temperature (search all terms) you will find plenty of info.

My airspeed was also reading 90 - 95 when I picked up my plane (sounds like we got the same plane). I removed the line from the back of the airspeed indicator and blew the line out with compressed air... Voilà! I was cruising at 112.

You fuel burn is about right.

[Bruce Fenstermacher]

Doug you guys beat me to most of the information I would have to offer. Yes poor rigging can account for lost airspeed.

One thing I don't think was mentioned about the airspeed is you say you where at 2400 RPM. How do you know your tach is accurate. Mine is reads nearly 100 RPM high at the upper end of the tach range.

So given tach, airspeed and oil gage inaccuracies which are common in the field, your numbers seem about right and very normal.

That doesn't mean you don't need to check all these things as suggested so you know where your aircraft stands.

[iowa]

my temperature gauge runs
on the high normal side
especially in warm temperatures.
the cylinder head temp is not too high.
i've always wondered if this is normal
and many others have this problem.
iowa

[WWhunter]

The way the guys have suggested checking your oil temp gauge is good. I had a mechanic doing the annual on my plane and he told me I needed a new gauge as it wasn't working. After spending a couple hundred dollars on a new gauge and putting it in it read the same as the old gauge. One of the old style that only has a red line. Checked the removed gauge and it read fine.

[cfiatzph]

I believe with modern oils the FAA has a letter that says with the new specs that the oil keeps it lube properties another 35 degrees since the red line on the gauge is really a function of not really engine damage but the loss of viscosity that can happen with too high of a oil temp thus causing excessive wear. How are you compressions? (are you burning oil) I had a C-172 that did this (0-300-A). Short flights like 2.5 or less it would be ok. One time I flew 3.5 hours and by the time I was heading back 1.5 hours into the flight the gauge was at red line the entire time. The cause was oil blow by on the cylinders, so the combustion is actually heat soaking the oil after a few hours. Another thing to check is the accuracy of the gauge. I would put my bet on either a cylinder(s) with blow by OR a inaccurate gauge. Since then we have installed electric gauges that are MUCH more accurate than the old style gauge. Pull the sending unit out and the gauge and stick it in boiling water, it should read around 212, if it is much higher then part of the problem is the gauge.
With a new overhaul I have never seen my temps rise about 212 or so, they can run hot but it is ALWAYS like 209-215 consistantly. Also keep in mind that at high temps (real high temps not false gauge readings) that the oil pressure is going to drop a few PSI. With that high of a oil pressure I doubt you are really running that hot, once again high cylinder temps will start burning stuff but I believe (I am layman) that high oil temps will increase friction and wear on the engine ultimately not necc a serious concern. The engine could prob run at 300 F all day long but you would have a signifcantly reduced overhaul time. If the temps are true the cause needs to be found, are you oil fouling plugs? If you are that is going to contribute to a high oil temp very easily.

[GAHorn]

Bruce, Bill, Doug, John etc all offer good words on the oil temp situation.
Oil temps have been cussed/discussed here many times. This engine does not usually have an oil cooler per se, but it has oil cooling thru it's pushrod tubes as they drain oil back to the sump, and also from the relative wind as it's blasts the front of the sump below the prop-flange.
Actual high oil temps can be caused by high ambient temps, as well as some engine malfunctions such as exhaust riser gasket leaks (which blow hot exhaust onto the pushrod tubes) and leaky engine cooling baffles.
While I've heard it postulated many times that "blow by" or worn rings that allow combustion gasses to escape down into the sump can cause high oil temps....I'm dubious about that in any engine that can pass a compression test and still be considered airworthy. The engine manufacturer allows differential compressions as low as 45/80 and still consider the engine airworthy provided the loss is due to worn rings and not thru a valve,...so I doubt that "blow by" is a concern. (I have also flown engines which were definitely worn out in that fashion and they did not exhibit high oil temps.)
Certainly the oil temp gauge may be suspect. But even if it proves generally accurate, an oil temp reading of 225-F is no longer considered excessive. As mentioned previously, use of a straight weight oil of SAE 50 meeting MHS-24 (and modern aviation oils do meet that spec) provides a new redline oil temp of 240-F in ambient temps above 40 degrees Fahrenheit, according to Note 2 of the engine type certificate.
Inspect your cooling baffles and engine for general condition and security, check your gauge for accuracy, and don't worry if your oil temp is running 225 or even higher (up to the 240 limit, of course.)
As for the rest of your performance numbers, I agree with others that gauge accuracy is always an issue. Also parallax can play a part. For example, your observation of 2400 RPM.... was that with your face squarely in front of the gauge? OR was that from the pilot's seat? My own tach reads 50 RPM higher due to parallax, when viewed from the pilot's seat despite the fact my tach is accurate at cruise readings. (From the pilot's perspective it reads 2500 when it is actually indicating 2450. Therefore it's conceivable that your observed 2400 was actually 2350. ??

[quintdxb]

Many thanks for all the good information. I now certainly have enough areas to investigate regarding the high oil temperature.

To summarize:
I will definitely check the oil temp gauge as suggested.

There seems to have been a modification made with the baffeling. It involves a sort of "inner cowl" which is not to be seen in the photograph of the engine bay in the manual. As stated, the CHT gauge is unserviceable so I was not able to ascertain the performance of this "modification". It could well be resulting in poor cooling (including pushrod tube cooling). After checking the oil temp gauge and repairing the CHT gauge, the next job may be to restore the baffeling to original condition.

The readings stated above were recorded with a digital camera, the photograph being taken from directly behind each instrument, largely reducing any parallax error.

I will also check the ASI and plumbing as well as the tach for accuracy.

The aeroplane does not seem to use an excessive amount of oil and the compressions are 60, 75, 55, 70, 75, 70. The previous owner tells me that until a year ago they were all in the 70's when the airplane was being used more often, but that the oil temp has always behaved as described.

The plugs do not seem to be oil fouling.

The oil temp is constant throughout a flight of up to 3 hours.

Guys, thanks again for all the advice. I will be returning to South Africa in March when the airplane is due for an MPI (100 hour/1 year inspection) All the items will be checked at that time. In the meantime, the engine is being run every 2 weeks.

Can't wait to fly it again. Will report back in mid-April.

[doug8082a]

There seems to have been a modification made with the baffeling. It involves a sort of "inner cowl" which is not to be seen in the photograph of the engine bay in the manual.

Does it look like this (except not so rusty ) ??



If so, that is the normal baffling installation for the 1948-1952 model years. In 1953 Cessna went to a pressure cowl and a more conventional baffle arrangement.

[quintdxb]

Doug,

Yup, its similar to what is shown in the photograph. I will check the integrity of any baffeling in this installation but if this is standard, the problem may lie elsewhere.

Thanks for the photograph,
Quintin

[GAHorn]

Quinton, ...if you don't already have one, you simply MUST obtain an Illustrated Parts Catalog for your airplane. It answers all sorts of questions.

[bsdunek]

Doug,

Yup, its similar to what is shown in the photograph. I will check the integrity of any baffeling in this installation but if this is standard, the problem may lie elsewhere.

Thanks for the photograph,
Quintin

Note there are several internal baffles. The ones between the cylinders on older engines are sometimes missing or loose, which will cause higher temperatures.