enginehistory.org

[jjuutinen]

How much greater cooling air flow did the R-2800-C require per hp in comparison with the Centaurus? I am asking for the max.allowed CHT for the R-2800 is 248 deg C for 30 min and 232 deg C continuously. The Centaurus 18 (Sea Fury) has a 5 minute limit of 310 deg C and 300 deg C continuously, even at lean mixture. This must have significant effect on air flows required. Another interesting point is how everybody else fitted spinners, often very big ones, to their radial engined aircraft while the US avoided spinners. Why? Someone has excused this with added maintenance caused by spinners, but that sounds a lame excuse indeed. What is also surprising how long the US stuck to those circumferential cowl flaps instead of sliding gills like in the Sea Fury or earlier in the La5/La-7. After all, the former had much greater drag (read any pilot manual on it), often harmed visbility over the nose and mostly caused substantial trim changes on closing or opening.

[szielinski]

As with any heat exchange process, the higher the temperature, the greater the Joules/sec (Watts) exchanged.

This is analogous to voltage in an electrical circuit (electrical 'pressure' vs. 'thermal' pressure).

Normally you would want the temperature to be as high as possible in a device to get the maximum heat transferred to the coolant.

While the above is true, the poppet valves, valve seals & rocker box seals in an R-2800 would tolerate much less thermal abuse than a nice wet sleeve with the entire junk head covered in finning (even though some of it is a bit shrouded).

I can't give you any numbers, but the other aspect to this issue can be related to electrical devices also. If you look at power transistor specs for similar devices in different enscapulations, you will see that the junction temperature is very similar (same silicon chip size), but the maximum outside case temperature is higher for the case with more metal (eg TO-3 compared to thinner TO-220 casing).

Thus in engines, the combustion temperature is similar, but the temperature drop along the way to the outside of the system varies, to the point where it might be high enough to do damage to thin or non-metal parts.

[hfriedman]

Were max CHTs set to protect the valves and perhaps other parts from the direct effects of heat or were they set primarily to avoid detonation and pre-ignition?

[szielinski]

Both, I would have thought (as well as other diagnostic functions).

I suppose a sharp rise in CHT that you DIDN'T knowingly cause would indicate detonation as the boundary layer would be stripped and the metal would lose that protection/insulation and heat up.

Conversely a rise in CHT that you DID cause (by working the engine hard) would be a guide as to how long the top end would last before needing mechanical attention.

There was an article in TM about ignition analysers and those devices make use of CHT analysis also.