Tacho time question
 

Hi

I have a question about tacho time.

My understanding is that the tacho time ticks proportionally to the RPM - hence if you are cruising in a typical club C172 or PA28 (and yes, mine rents by the tacho time!), then increasing from 2,200 RPM to say 2,500 RPM in the cruise makes the tacho time tick ~13% faster.

However, if you look at the POH cruising data, e.g. for a C172, the increase in speed (e.g. 4,000 feet, standard conditions, perfect plane, right leaning etc you'd go from 100 to 115 kn, speed increase of 15%) outweighs the increase in RPM and your tacho hour per distance travelled would be lower for the higher power, or at least very similar.

Your fuel consumption will obviously be higher, but if you are charged on tacho time, this would imply that you get a cheaper trip per mile if you crank up the power. Is that correct? Seems a bit counter-intuitive.

Our group charge on tach reading, wet. I notice quite big differences in cost per logged hour, depending on whether I run the O200 at 2200 or 2500 cruise.
Track covered will vary depending on wind - 80kts +20kt tailwind or 120 kts - 20 kt headwind. The 120kts might be more economical with the headwind.