Suction of air by a fan

I made very simple empiric test.

I had an air circulator dia 40 cm and a tachometer ( for R/C models ) and toilet paper.

I had the fan running at low speed about 1100-1200 rpm ( indicated ) and at full power at 1900-2100 rpm.

At 40 cm distance the suction was such that the 3 piece toilet paper strip was sucked into the prop at full 2000 rpm. From 50 cm the paper was leaning 10 cm to the prop direction.

Also at 1100 rpm at 40 cm the paper leaned 20-25 cm into prop...and from 50 cm only few cm:s.


Aeroplane fans like in MAX III will operate at 820 diameter faster than 3500 rpm ( in fact even 6200 rpm is not supersonic at that size )....so I have a feeling that there is no way a tailfeather could stall as the prop turns causing thrust greater than the prop:s drag as long as the prop is not further than 30 cm ( rather like 200-250 mm to be sure ) from the elevator / rudder combo. Slight disadvantage of turbulence by the 10% thick feathers can only make minor difference in speed or efficiency whether it is 250 mm or 400 mm away. OTOH it will make big difference as a distance increase from aerodynamical center which has to be compensated with tailfeather and in torque related situations with ailerons ( see the german designs had tailfeathers really close to the prop ).

I would very much like to see some windtunnel data on this prop suction caused powersteering feature. Not to be neglected in any case. I have a haunch a strong braking with the prop would cause less adequate steering caracteristics with pitch and yaw.

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