Originally Posted by
Pilot DAR
In the case of a CS prop piston engine governing, the torque and MP indication are directly related.
They are not directly related. MP is proportional to torque if RPM and air density are held constant. But we are talking about applying carb heat which changes the inlet air density. Therefore the reduced torque resulting from this reduced air density does not show up on the MP gauge.
for the O-470, MP is your only indication of torque if the prop is governing RPM.
The MP is neither a direct indication of torque nor is it the only indirect indication of torque. It is proportional to torque if the RPM and air density are held constant. With a CS prop, a change in torque will show up on the ASI (note, I am definitely not implying that all changes in airspeed result from a change in torque). Eg you apply carb heat > BMEP reduces > CS unit fines off the blades > airspeeed reduces.
Measuring torque is not what I am talking about. The point I am making is it does
not follow that application of carb heat results in a reduction of MP
due to the reduction of power, which is what you wrote. The reduction in torque resulting from the reduction of inlet charge density does not show up on the MP gauge. But it will show up on the ASI. And I am not saying that cycling carb heat on and off does not change the MP. I am saying it does not change MP for the reason you think.
The carb heat induction air path delibertely does not go through the air filter (in case your filter ices over - 'happened to me once). Correct, there is no ram effect when running carb heat hot, but then at Cessna 182 speeds, there's not much ram effect carb heat cold, through the air filter either. If your air filter is truly clogged, then yes, carb heat hot would seem to result in an increase of MP for the same reasoning.
Right, so back to my original point: the pressure drop between upstream infinty and the bell mouth of the carb is inevitably going to be different for the normal versus alternate air. This will impact the MP. And because, for the reasons I spelled out above, the reduction in air density is not indicated on the MP gauge, the primary reason for any observed change in MP when cycling between normal and alternate air is........the change in air pressure delivered to the carb resulting from the difference in those two induction paths.