On 15 Mar, 17:18, sisu1a wrote:

Hmm, I'm of the understanding that we use Pawnees because they are so
lightly wingloaded (relative to other tugs) and have such good power/
weight ratios when not full of bug juice and spray gear. I thought
this is also what allows them to happily fly too slow for our tastes
as well. While a Pawnee is perfectly content tugging at 55mph, I'm
not.

The vortex strength is inversely proportional to the airspeed. For a
free vortex, the lift per metre is the vorticity times the free stream
velocity. For an aircraft, the vortex strength is therefore
approximately weight / (airspeed x span).

PS. the propwash to wingwash ratio should be pretty easy to figure
out. The main wing has to support the entire a/c (couple thousand
pounds) while the little spinning wing only needs to provide thrust.
(couple hundred pounds?) *Which do YOU think is dominating the scene?

Each blade of a Pawnee propeller is about 1m long. Each Pawnee wing is
about 5m long. With the engine at 3000rpm, the tip velocity will be
about 300m/s, which is about 150kt. 200bhp (150kW) at 60kt (30m/s) is
5kN. The maximum takeoff weight of a Pawnee is about 12.5kN.

So ... 0.4 times the force, 0.2 times the span, 2.5 times the
airspeed ... the propeller vorticity will be around 80% of the wing
vorticity. Ball park.

Then you have to remember that the effects of the vortex shedding are
felt, mostly, in a cylinder of about twice the diameter of the span,
and that the air velocity is inversely proportional to the distance
from the vortex. The propeller's vortex street is going to be about 4m
across, the wing's about 20m across....

Finally, there is a danger of confusing two things here. The vortex
wake of a lifting surface is *not* the same as the turbulent wake.
It's bigger and lasts longer.

Ian