Birdman100
Well-Known Member
I just read Normans comment in a thread about VGs, evoking issues if prop is in separated flow. Think of it for a moment and it sounds completely logical - avoid positioning prop behind some obstacle. But then, thinking again it seems answer is not that simple.
First, we use to think that any part is bad to be placed in a separated flow; for the majority of the parts including wings and tail that is absolutely correct; after all aerodynamic surfaces need moving air to work properly, and that air is disturbed and has no appropriate velocity in separated flow. But at the other side situation is a bit different with props. Propellers are rotating bodies and they induce "their own" velocity by rotating. After all props do create static trust when there is no axial flow at all (plane still on the ground) while that is not the case with wings. In fact in the situation mentioned, the axial flow exists through prop disk, but it is induced by prop itself. Wouldnt the same happened if the prop finds itself in separated flow in flight?
Just to clear one thing. One could say, guided by intuition and subjective feeling, that the air is somehow "thinner" in separated flow region and that prop would produce less thrust because of that. I am not sure about thrust magnitude but am sure the all physical air properties (density, viscosity) are the same in separated flow. Air is continuum and there are no "gaps" in it, as far as we are dealing with low speed subsonic aerodynamics.
So, what do you think are the consequences of putting the prop in separated flow and why? Vibrations? Reduced thrust - if - why? Something other? Does anyone have some bad experiences flying in some awkward pusher with shaded prop?
In the pics shown (sorry for some technical issue with attachments, can not correct that) looks that props should be in a big trouble at high AoA, not to mention stall; but apparently these planes fly well...(?)
First, we use to think that any part is bad to be placed in a separated flow; for the majority of the parts including wings and tail that is absolutely correct; after all aerodynamic surfaces need moving air to work properly, and that air is disturbed and has no appropriate velocity in separated flow. But at the other side situation is a bit different with props. Propellers are rotating bodies and they induce "their own" velocity by rotating. After all props do create static trust when there is no axial flow at all (plane still on the ground) while that is not the case with wings. In fact in the situation mentioned, the axial flow exists through prop disk, but it is induced by prop itself. Wouldnt the same happened if the prop finds itself in separated flow in flight?
Just to clear one thing. One could say, guided by intuition and subjective feeling, that the air is somehow "thinner" in separated flow region and that prop would produce less thrust because of that. I am not sure about thrust magnitude but am sure the all physical air properties (density, viscosity) are the same in separated flow. Air is continuum and there are no "gaps" in it, as far as we are dealing with low speed subsonic aerodynamics.
So, what do you think are the consequences of putting the prop in separated flow and why? Vibrations? Reduced thrust - if - why? Something other? Does anyone have some bad experiences flying in some awkward pusher with shaded prop?
In the pics shown (sorry for some technical issue with attachments, can not correct that) looks that props should be in a big trouble at high AoA, not to mention stall; but apparently these planes fly well...(?)
Attachments
Last edited:
