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push vs pull vis a vis rudders
On Wed, 31 Mar 2004 10:20:59 +0100, "JimB"
wrote: ..... I fixed a card to a bit of wire which I could bend at various angles to the card (rudder angle). I then hung the card upstream of the fan so that it could pivot only along the fore and aft axis (above the fan) and again so it could pilot only along the lateral axis, and again so that the rudder could rotate around the vertical axis of its front post. Smoking fag ends came later, with a rigid mount. Fan was turned on. Forces were observed by noting the degree of card deflection around the relevant hinge. There was a net force on the rudder, primarily exerted towards the fan. It has little lateral component, but lots of fore and aft component. Smoking fag ends showed strong non-linear flow when the rudder was deflected, but the net flow is an s bend zig-zagging around the rudder towards the fan and two carpet burns. Smoke speed dropped markedly with distance away from the fan. The rudder kicked hard over (either way) when allowed to pivot around its forward vertical axis. Within the limitations of my crude experiment, rudder kick is probably caused by the net effect of the fore and aft component of force, not a lateral component. I think this explains the rudder kick I've witnessed in astern in some boats engaging astern gear, and probably explains Derek Rowell's observation that the rudder rotates when allowed to (If I understood his experiment design right). However, the zig-zagging airflow proves to my satisfaction that the rudder may not create a net lateral force, so I'm stuck with the revelation that the yaw effect that my old skipper demonstrated to me was wind, mirrors, inertia, prop walk and my hero worship. Ah well. But now I've got to get rid of those carpet burns before the wife comes home. JimB Asking for explanations from experimental rigs is the royal road to progress. Congratulations! Couple of your observations bear talking about. If a hinge surface is hinged more than about 1/4 aft of its present leading edge it is unstable in the fluid flow. ('rudder kick') If a surface *is* hinged about 1/4 from the leading edge, it can still break into oscillations which are quickly destructive, unless the mass is balanced closer to the hinge line. If a FLAT surface is inclined slightly ( 20 degrees) to the fluid flow, the flow over the 'upper' surface is faster and provides lower pressure than the flow over the lower surface. The streamlines do not follow the (flat) surface of the test article (of course!), they kick up in a smooth curve over the top. This applies to an airfoil flown upside down too. The streamlines look similar to the streamlines over a right way up foil, but less efficient and with lower pressure difference from top/bottom. It is not necessary for a lump of fluid dividing past the foil to join up again after it has passed.. When providing lift, the lump of fluid does not join up again, in fact. Brian Whatcott |
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