CHT question

[woodmw]

I have a CHT gauge with the thermocouple attached to the lower spark plug on #2.

Question #1:

If you have BC-12D, A-65, and CHT gauge, what temps do you see?

The reason I ask is that after changing props, I have a change in CHT readings. With metal McCauley prop, CHT was 400^F. Now, with Sensenich wooden prop, CHT is 450^F. During prop change, cowlings and nose bowl were removed, and plugs were removed, cleaned, and reinstalled. I have checked baffles and seals, nothing seems to be bent, broken, missing, or damaged. At least more than normal. Found no red rags or pesky blue shop towels where they shouldn't be, and no new dirt dauber nests in the cooling fins.

So, Question #2:

Could the shape or profile of the wooden prop be different enough to cause a change in CHT readings?

[3Dreaming]

Re: CHT question

I would say the answer to #2 would be yes. The wood prop is less efficient that the metal prop, and likely more so especially in the hub area.

[flyguy]

Re: CHT question

It is also possible that with the cleaning and re-installation of the washer type thermocouple that you may have better heat transfer to the thermocouple bead. That is in addition to the poorer movement of air near the hub as mentioned above.
DC

[3Dreaming]

Re: CHT question

It is also possible that with the cleaning and re-installation of the washer type thermocouple that you may have better heat transfer to the thermocouple bead. That is in addition to the poorer movement of air near the hub as mentioned above.
DC

The way I read it he had the increased CHT, and then started trouble shooting.

[senior]

Re: CHT question

My gauge on a 85 reads just under 400F,
I would also suggest as pointed out above, the wood prop is probably not as efficient nearer the hub.

[barnstmr]

Re: CHT question

The axial flow into the air inlets could be lower due to 2 or more things related to decreased prop efficiency.
1. The relative blade shank profile of the wood is probably a lot more bulbous near the hub than that of the metal prop... thus greater drag/lower thrust generated in that section of the blade... thus less air flow and greater disturbance of the air in that section of the blade.
2. Lower efficiency means higher angle of attack to achieve the same rate of climb... thus less pitot-effect (ram air effect) into the inlets.

But 50 degrees is a big shift. Such a jump in temperature could be related to fuel, which is the other main coolant for upper cylinders (next to air flow). You may be seeing the combined effects of the above in combination with an already lean condition at full throttle. Be sure to check the fuel lines for restrictions, the carburetor adjustments, the air box for proper function... or any other thing that could cause lean operation.

[3Dreaming]

Re: CHT question

One other thing is the wood prop could be putting more load on the engine. A fellow here the other week did some prop test with his airplane. 3 different props in 1:30 minute time. It was interesting the see the manifold pressure differences between props at the same RPM levels. The differences were bigger than I would have thought.

[woodmw]

Re: CHT question

Thanks for the responses and ideas. Just to clarify, the plug cleaning took place during the prop change. Whenever work is done on the engine, I pull a plug from each cylinder. Makes it easier and safer. I have had mags come ungrounded. Pulling the lower plugs keeps things from falling into the cylinders. I just took the opportunity to clean them.

The flights with the wooden prop have all been below 2500'. OAT is in the 40's-60's. Straight and level after initial climb out. The CHT gauge has been in the plane for some time, and the plugs removed and reinstalled numerous times, so I think we can rule out the heat transfer to the thermocouple.

The annual runs out this weekend, so we will be doing more investigating during the inspection. I think swapping back to the metal prop for a test flight could prove interesting.

[barnstmr]

Re: CHT question

For what it is worth, here is a general description of the FAA powerplant cooling test criteria for a single engine

1. Takeoff and climb to a safe altitude and establish cruise power until the engine temperatures stabilize.
2. Set Power to the approved Maximum Continuous power setting and establish climb at the best rate of climb speed (Vy). This includes normal mixture setting.
3. Monitor and record Pressure Altitude (29.92 in. Hg), OAT, and engine temperatures (oil T and CHT) at regular intervals (usually every 10 to 30 seconds or so)
4. Continue climb until the engine temperatures peak and then continue another 5 minutes while recording data. (Maintain mixture as per normal procedures).
5. End test and then correct the data to Hot Day (100 *F) conditions. Here is the correction formula...

...Corrected temperature [*F] = measured temperature [*F] + 1.0 (100 *F – 0.0036 [F/ft] x pressure altitude [ft] – true OAT [*F])

Edit: additional comments...
- Discontinue test if temperatures exceed redline by more than 5%
- Test fails if any of the corrected engine temperatures exceed redline
- The correction formula is conservative, so the colder the test day... the greater the likelihood of failed results. i.e. if you test on a 60 degree day and the indicated temperatures look good... you might be surprised that the correction may take you over redline. This is why it is best if you can test in actual hot-day conditions.

[woodmw]

Re: CHT question

Meant to get back to this thread some time back. Just got sidetracked. Problem solved. Leaky exhaust. As the exhaust loosened, more gases were directed at the spark plug, hence the CHT thermocouple. CHT temp continued to rise. Pulled plugs at annual and saw exhaust stain on plug. Concerned about gasket integrity, and whether or not gases might have cut cylinder or exhaust flange, we removed baffles and nuts, lowered the exhaust a smidgen, were able to change out 3 studs, and install new gaskets. I now have a few modified tools, some broken ones, a new set of 1/4 drive flex sockets, and a wish list for Luscombe exhaust. And a vocabulary of words I didn't know I knew.

CHT now below 400^F in cruise.

[astjp2]

Re: CHT question

When you get the luscombe exhaust, order a new gascolator you can mount on the firewall because the exhaust hits it in the existing location. If you want some hard line, I can send you 3 feet of stainless and then you need a new hose from the gascolator to the carb.....Tim

[woodmw]

Re: CHT question

Thanks for the info and offer Tim. Current exhaust is in very good condition. Think I'll wish for Luscombe, but live with Tcraft.

[jstall]

Re: CHT question

The axial flow into the air inlets could be lower due to 2 or more things related to decreased prop efficiency.
1. The relative blade shank profile of the wood is probably a lot more bulbous near the hub than that of the metal prop... thus greater drag/lower thrust generated in that section of the blade... thus less air flow and greater disturbance of the air in that section of the blade.
2. Lower efficiency means higher angle of attack to achieve the same rate of climb... thus less pitot-effect (ram air effect) into the inlets.

But 50 degrees is a big shift. Such a jump in temperature could be related to fuel, which is the other main coolant for upper cylinders (next to air flow). You may be seeing the combined effects of the above in combination with an already lean condition at full throttle. Be sure to check the fuel lines for restrictions, the carburetor adjustments, the air box for proper function... or any other thing that could cause lean operation.

TERRY

On a different note: in the thread concerning strut angle if you look at the last of the postings in here you will see where members discussed heating and bending the upper
tang of the front strut to the correct angle +/- 2 degrees?? would you feel that that this
could be considered a safe proceedure if you used an accomplished a&p or IA with welding experience to accomplish this?? I have a set of factory Harry Ingram junk that were cut at -0- degrees and I need 5.5 on my installation. I really never considered this but one member said that Alaska Airframes did just this to correct one of there builds to fit his airplane.??

JS
Alva, OK

[3Dreaming]

Re: CHT question

jstall, Terry hasn't checked in here since October 10th. You would do best contacting him through his business email.

[PA1195]

Re: CHT question

I doubt Airframes bent an existing strut with heat to affect a change in end angle. Dick Smith would know but my guess is that they built a second P/N with increased offset. FAA regs and all that but could be wrong as usual.

Gary