Originally Posted by
Gaseous
I would presume that the limit is temperature related, either oil or CH temp will go overlimit under test conditions if take off power is used for more than 5 minutes. In other applications these engines have different or no limits. The MAP which allows 5 minutes of operation will probably have been found experimentally on a fully instrumented engine. MCP keeps everything in limit under test conditions.
Any better theories anyone?
I'd only add a couple of points,
1) how fast do you really want to go in a R44, i think you'll find MC gives you an airspeed of a fairly quick clip,
and 2) after tooling around LL for quite some hours in '47 J models where a fully clothed engine compartment runs equally if not hotter than the 44 engine bay and where fuel vaporisation was a BIG risk, even without the standard method of no cowls, i'd suggest backing off a bit more in anything above 30 degrees ambient.
Where i work its been up to 45 and 46 c free air temp on several days of late, admittedly a bit unusual.
For sure the J models we used were turbo charged not just gravitry fed but i'd still think about it, any sort of a cough could herald fuel starvation.
We used to have as critical go / nogo, the seviceability of the aux furel pump on the J's. Thing is they blow up very frequently if left on, so we would operate on a critical CHT above which you could oft observe the fuel pressure starting to fluctuate about three or four seconds before the engine might stop. At the critical temp, aux pump - ON. The switch was right by your left index finger, amazing how observant one was of the CHT and FP.
Some turbo charged '47's were fitted with a fuel pressure sensor coupled to the aux pump which meant that the aux pump cut in if set on the auto position when fuel pressure went below a set pressure, usually 6 psi. Of course you must first set the engine driven puimp at 7 - 8 psi. It was a very good mod.
Fuel pressure below 4 psi can be injurious to heart and health.
cheers tet