What is engine roughness caused by and when is it most apparent?

What is engine roughness caused by and when is it most apparent?

• A. Engine roughness is caused by fuel quality and is most apparent at idle
• B. Engine mechanical wear and at moderate RPM
• C. Propeller imbalance and at high RPM
• D. Cold weather operation and during takeoff

Answer: (C)

Engine roughness that escalates with increasing RPM points to an imbalance in the rotating part, most commonly the propeller. The propeller is a large, heavy rotor; even a small mass difference between blades or a damaged blade creates an unbalanced condition. As RPM increases, the unbalanced forces grow, producing more vibration that you feel as roughness, especially when you apply takeoff power. At idle, the rotor isn’t spinning fast, so the vibration is much less noticeable. Other causes listed—fuel quality, general mechanical wear, or cold weather effects—can cause rough running but don’t typically produce a vibration that worsens so clearly with RPM or is most evident specifically at high power settings.

Engine roughness that escalates with increasing RPM points to an imbalance in the rotating part, most commonly the propeller. The propeller is a large, heavy rotor; even a small mass difference between blades or a damaged blade creates an unbalanced condition. As RPM increases, the unbalanced forces grow, producing more vibration that you feel as roughness, especially when you apply takeoff power. At idle, the rotor isn’t spinning fast, so the vibration is much less noticeable.

Other causes listed—fuel quality, general mechanical wear, or cold weather effects—can cause rough running but don’t typically produce a vibration that worsens so clearly with RPM or is most evident specifically at high power settings.