humptybump
Well Known Member
53VG was invaded briefly this afternoon. The air was heavy with moisture which provided a very clear visual representation of the left turning tenancy known as spiral slipstream ...
This phenom only happens when a skilled pilot feathers in precisely the right amount of power while monitoring the correct blade angle via the RPMs.
I think you posted the wrong video, Damon. But its a cool Extra 300 video anyway.
The tip vortexes (vortices?), assuming the airplane is moving in those shots, suggest something other than a spiral airstream hitting the rudder and empennage.
I am not saying that the spiral of accelerated air is not there, only that the tip trails don't actually show it.
In a way this reminds me of the argument about "slowdown" of air as it approaches the plane of the prop because of the fuselage or cowl behind it. Most agree there is higher pressure. So how can there be both slowdown and higher pressure? All the videos of smoke in wind tunnels, mostly of motorcycle fairings, suggest that there is deflection, but no slowdown.
I know that the P factor is evidence, but that gets mixed up with gyroscopic forces. Do we have direct evidence of air moving somewhat sideways?
I know that the P factor is evidence, but that gets mixed up with gyroscopic forces. Do we have direct evidence of air moving somewhat sideways?
The European says....
Why only 2 blades
The bold-red is mine.I experimented with putting dirty oil on the sides of the fuselage a couple of years ago to test this. No spiral that I could see. I did see evidence for downward air deflection on the aft fuselage though - a nice illustration that the horizontal tail has a negative angle of attack (as expected). The pattern looked the pretty much symmetrical on both sides.
The bold-red is mine.
If the air is being deflected downward then normal lift would be produced. Negative lift would require air deflected upward. Right?
I may not be explaining this very clearly. What I mean is that the force generated by the tail is directed downward. The horizontal stabilizer has 0 degree incidence (its parallel to the longerons), and the local airflow as indicated by oil streaks is directed slightly downward relative to the longerons.
Yep, I got that. So far, so good.
The theory as explained by Roncz at OSH11 was that the lift of a wing (up) is the result of deflection of a quantity of air down at whatever velocity thus producing a F=MA Newtonian action-reaction situation.
Since I expected the tail to be pushing down, I expected that the air under and behind it would be deflected in the opposite or up direction.
However, since my recent post I have been looking at old textbooks and thinking about it. In climb, the tail of an RV pushes down to raise the nose thus increasing AOA of the wing and so on. However, in high speed cruise, we need to lower the AOA which requires a neutral or even upward pushing tail plane. The nose, being heavier, wants to be lower on its own. But, don't we all trim the nose down as we get to cruise speed? The CG forward of the lift point, alone, may or may not be enough?
I know that my airplane's "natural" speed with neutral or zero trim is about 110 KIAS. If I want to go faster, I have to push the nose down which requires that the elevators be pointed down which pushes the HS up. The deflected air from the elevators would be downward and so would the air from the HS, but this is not negative "lift" at this point. Or so it seems to me as I scratch my head and re-think all my primary instruction from 35 years ago.
Your oil experiment is a wonderful example of practical curiosity. I commend you!
The tip vortexes (vortices?), assuming the airplane is moving in those shots, suggest something other than a spiral airstream hitting the rudder and empennage.
I am not saying that the spiral of accelerated air is not there, only that the tip trails don't actually show it.
In a way this reminds me of the argument about "slowdown" of air as it approaches the plane of the prop because of the fuselage or cowl behind it. Most agree there is higher pressure. So how can there be both slowdown and higher pressure? All the videos of smoke in wind tunnels, mostly of motorcycle fairings, suggest that there is deflection, but no slowdown.
I know that the P factor is evidence, but that gets mixed up with gyroscopic forces. Do we have direct evidence of air moving somewhat sideways?