VEERRY INNTTEERESSTING!!! Let's see now, we have the slipstream twisting around the fuse as it passes over the surfaces. Probably has pulses corresponding to the prop rpm times the number of blades on the prop, with diminished angular velocity between the pulses. Are we going somewhere? If we're going say 120 mph(176fps) and the engine is turning 2,400 rpm with about a 55 inch pitch, and assuming effective movement of 48 inches just for the sake of discussion, is it fair to say the slipstream should be winding around the fuse one complete circle every 4 feet? Of course not, because the slipstream would presumably be the vector sum of the forces acting through the prop disc, that is, the force perpendicular to the disc would be equivilent to the lift produced by an airfoil wing in constant-speed level flight, and the twisting moment would be derived from the combination of air friction and induced drag created from the propeller airfoil.

So, when I blow an oil cooler over the cascades at 11K and keep the fan turning all the way to touchdown, the 3 quarts of lost oil should produce a neat barber pole effect from the engine all the way to the tail. Streamer tape attached to the fuse should indicate the magnitude of the twisting moment. If the twist is the sum of the vectors, then is it in the ratio L/D of the prop? If it is any of these things, then it should be demonstrated by the oil slick or the streamer tape. If it's 20/1, the stream would still make about a half turn within the length of the fuse. BTW, we should also see more bugs on the left side of the vertical fin, if it is straight w/ the fuse and not canted, and for sure we should see more bugs on the bottom of the left wing, left stab and the top of the right ones, no?

Darned testing just doesn't want to add up to the assumed results. HMMM. If the slipstream is going to be turning like a horizontal tornado, we really need to identify the forces that will generate it and the mechanism that will make it behave as we assume it does. Fortunately, most of the net horsepower gets used up to create thrust perpendicular to the prop disc. A good number might be 80%, but you can feel free to substitute any actual tested numbers that contribute to the analysis. That leaves about 20% for various inefficiencies, including surface friction on the blades, noise generation, induced drag, etc. Some variables will change according to blade aspect ratio, prop airfoil variations, etc., but we might assume that in normal operation, only a small part of the horsepower might be available to produce a twisting slipstream.

Of the effects that can be demonstrated and have been tested, let's take a look at tip vortices, specifically as they might apply to rotating (prop)airfoils. We know that large air movements result from passage of heavily loaded airfoils, and we're constantly warned about prior passage of "heavys" even several minutes prior to our sharing the previously used airspace. Looking through the windows where things are upside down and you are low to the ground is not fun unless you are doing it on purpose. I digress. It seems logical that a heavily loaded airfoil, even though smaller would also generate significant tip vortices that would presumably behave in similar fashion to those generated by wings. This could be visualized as the turbulent interface between the stable surrounding air and the slipstream column. The demonstrated existence of vortices from the passage of any lifting airfoil combined with the other prop inefficiencies, would seem to leave very little engine horsepower available to twist the slipstream.

Let's see if we can twist the slipstream on purpose. What happens if we wind er up setting on the ground with the prop pitch flat, then pull the pitch to the feather position? Well now, looks like the prop airfoil went to critical pitch, stalled, then proceeded into an effective elevator perpendicular to the direction of rotation/airflow over the airfoil. Air movement went from almost none (only friction) through maximum efficient thrust to increased drag/decreased thrust, and finally to all drag/no thrust. In the full feathered position, it is definitely churning air and using lots of horsepower, but no thrust. If done in a static position, it appears that the air movement would be inwards towards the prop hub from the front and back of the prop disc, and outwards radially from the prop tips. In the full feathered configuration, the prop acts like a squirrel cage fan without the cage. BUT, it is interesting to note, there do not appear to be any major barber pole spiral airflows emitting from all this fuss.

Seems to me, we could demonstrate that twisted airflow on the ground with a tricycle gear airplane that sits in flight attitude. Just line up the surfaces, set everthing in a straight line, no offsets, fasten the gear to a bearing turntable with the center of the turntable on the cg and thrust/drag line of the airframe, set it at constant throttle and let er rip. Anyone wanna become famous?