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Gps speed under power less than at idle

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Hi guys,

Has anyone else noticed that their wing at a given trim setting flies faster with engine at idle descending than under power straight and level?


I tested this twice today and both times my gps speed increased by about 3km/hr when i went back to idle and began to descend compared with straight and level flight under power.


Any ideas on why?


I have 2 theories...


1. Increased drag at idle lowers angle of attack to the point of producing increased airspeed that more than compensates for increased drag,

2. Approx 7 degree recline on prop unloads the wing under power...so wing flies slower for a given weight under throttle because the vertical component of thrust due to the 7 degree recline carries some of your weight.


I dont think its entirely point 2 above...because 7 degrees as a percentage of 180 degrees i dont think equates to a gps speed difference of 3km/hr  (35 vs 38km/hr)


My calc says 7 degress is 3.8 percent however that only equates to 1.47km/hr change in speed.

Any other thoughts on this?

Edited by adamjedgar
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