Originally Posted by
Pilot DAR
And, density of the air going into the carb. Heating the air, will reduce its density.
...that will not reduce the manifold pressure. Each component in the induction system has a pressure loss Δp = kρvē, where k represents an experimentally determined coefficient. If density goes down, the pressure loss goes down therefore manifold pressure goes up. When switching to alternate air, the MP will go down if the pressure supplied to the carb is lower than the normal supply. It will not go down just because the density was reduced by carb heat, the opposite will in fact occur.
If you have selected carb heat hot, then cold again, and had a net increase of MP, you had carb ice, and melted it away. The RPM remained the same the whole time, as the propeller governed as intended.
I hope everyone knows that but it is not what the OP was asking about. He specifically stated that in conditions where carb ice was not present the MP rose significantly and immediately upon application of carb heat. The obvious thing to suspect here is that the normal air system is restricted by other than icing. Other more obscure possibilities exist. For instance an exhaust leak into the alternate air supply.
On a fuel injected engine, there is no carb heat control. There may be an alternate air control, which if used, should have no effect on MP, as the alternate air would be the same density, just a different source.
You probably wouldn't argue that changing configuration of the wing has no effect on coefficient of lift if air density remains constant, so it doesn't make much sense to apply the argument to changing the configuration of the inlet system either.
If you change the RPM by moving the propeller control, the MP will increase with RPM decrease, as with the increased blade angle of lower RPM, the engine has to produce more power to turn the more coarse prop.
The engine does not have to produce more power. MP increases due to the decreased mass flow rate resulting from the lower RPM. This same effect is seen when exercising the prop during the run-up.
The power output goes down when you reduce RPM. (Note: I am not saying you cannot achieve a higher power setting at lower RPM by increasing the MP with the throttle). If you select lower revs with the prop control, the new torque resulting could be more, less, or the same as the initial torque. It depends where the initial and final RPMs are on the engine torque curve. Blade drag varies with the square of RPM, so it follows that lower revs require more pitch for a given torque, but it does not follow that the torque will be higher than before, nor does it follow that the engine has to produce more power.
You said in your first post:
if you do anything which reduces power on the whole, the MP must reduce, as the RPM won't
That is provably wrong. You could, for instance, enter a cruise climb and hold MP constant by adjusting the throttle but the power would still reduce as you climbed. The MP does not have to reduce, the reduction in density and mass flow rate alone can explain the reduction in power.