Hi lukyduky81,
I've inserted text from "The General Aircraft Technical Knowledge, Third Edition" published in 2000 by The Aviation Theory Centre, New Zealand, Page 8-2 (Propellers). I've also put proper APA citation below to cover myself even further:
Another disadvantage of the fixed-pitch propeller is that, at a given throttle setting, changes in airspeed have an effect on engine rpm. With a decrease in airspeed, the propeller angle of attack is increased. Accordingly, propeller torque also increases resulting in a drop in rpm. Conversely, if the airspeed is increased with a constant throttle setting, the propeller angle of attack and torque reduces, resulting in an increase in rpm. In most cases where a steep nose-down attitude is adopted, the throttle will have to be fully closed to prevent an engine overspeed.
Hopefully that clears things up. IIRC all C172's have fixed pitch props but the same principle applies to Variable Pitch Props where the throttle and pitch are maintained. For those though the CSU will adjust for the increase/decrease to maintain rpm. So to the pilot it doesn't seem like the principles apply, but they do.
If that doesn't clear it up its triangle of velocities time!
Citation
Wagtendonk, W., Boys, S., Wagtendonk, J. (2000, January). The Commercial Pilot's Series:
General Aircraft Technical Knowledge. Third Edition. Aviation Theory Centre (NZ) Ltd. Tauranga, New Zealand