A Pilot's Thrust Indication

[636_Castle]

Here's a question regarding thrust indication.

 

The A-10's flight manual states that since bypass air produces 85% of the engine thrust, engine fan speed should be the pilot's primary indication of thrust from the engines.

 

Obviously, bypass air completely bypasses the engine core, since it's so great in volume, and doesn't need to be ignited or funneled through the compressors.

 

In the normal procedures for the A-10, it says to look at the core RPM gauges, when making power settings with the throttle. Why is core RPM looked at to determine power, if 85% of the engine thrust bypasses the engine core? Why isn't fan speed looked at, when making power adjustments?

[Eddie]

Because your throttles effect fuel flow (not directly) and therefore core RPM, core RPM is also the primary indicator of engine health.

 

Fan RPM also varies with air pressure and temperature, so 82% Fan RPM (for example) may be maximum throttle on one flight, but on the next you might be hitting 85% Fan RPM at full throttle. Where as in both cases core RPM 98% (IRL in won't be as it varies from aircraft to aircraft, and even between engines) at full throttle.

[effte]

Fan RPM is your performance indicator, but core RPM tells you more about how the engine is doing quicker.