Steve_B
70cc twin V2
Actually conventional wisdom is that the wash from the prop forms a contracting stream tube, at least for the first few inches from the prop: http://www.mh-aerotools.de/airfoils/propuls4.htm
To Vortex Generate, or Not?
- Thread starter Jungle
- Start date
Actually conventional wisdom is that the wash from the prop forms a contracting stream tube, at least for the first few inches from the prop: http://www.mh-aerotools.de/airfoils/propuls4.htm
njswede
150cc
Hmm... OK, that kind of makes sense when I think of it. So I guess a larger portion of the wing is stalled then.
Jungle
50cc
I can see how using the term stalled an maintaining good airflow is somewhat conflicting. So I guess it comes down to even if there is good airflow then when does it drop in pressure to where the wing can not carry the weight with what's provided. Hehe. Most 3d is high angle of attacks and the reliance of using the motor to stay in the air so I guess we also should decided if we are going to test it's benefits in 3d or benefits in angle of attack and stall prevention. ??????
RCAddict16
70cc twin V2
NJSwede- just make sure the tufts are evenly spaced along the wing or you will ge inaccurate results. Just do 3 lines perpendicular to the chord of the wing to get an even result.
njswede
150cc
I believe it's because the "cork screw" twirling motion of the air makes it meet the wing surface at a higher speed than airspeed alone. And the "stickiness" of the air, i.e. the tendency to form a boundary layer is higher the faster the air moves.
Here's a picture that kind of explains it:
Steve_B
70cc twin V2
You might regret getting me started on the subject of aerodynamics
To understand how VG's work you need to think about how the boundary layer (that's the thin layer of air next to the wing surface) moves. In smooth laminar flow the boundary layer slides smoothly over the wing surface. When the plane flys slowly the air doesn't have much momentum so because the air is viscous, and there is drag as it moves over the wing surface, and also because it moves from low pressure on the front part of the wing to higher pressure near the TE, the boundary layer runs out of momentum, slows down and builds into a thick sluggish layer or may stop altogether in a static 'bubble' of stationary air, aka 'laminar separation bubble'. The faster moving air above the boundary layer is forced away from the wing surface by the thickening boundary layer and eventually 'breaks away' altogether.. i.e. the wing stalls.
Vortex generators work like tiny wings. They stick ip through the thin boundary layer near the leading edge and into the faster moving freestream air. They are set at a high angle of attack to the airflow and so form a vortex at their tips just like the tips of wings. This vortex in the fast moving freestream 'mixes' with the slow moving boundary layer and 'pulls' it along, preventing or delaying the boundary layer slowing or stopping, this delays separation and stall.
Sorry if I'm geeking out a bit I did say you might regret getting me started....
One other thing to consider is that wings still make lots of lift after they stall. Many seem to think that when a wing stalls all the lift goes away but that's just not the case. A typical wing might stall at maybe 10-15 degrees and the lift will fall by maybe 20%, but if you keep increasing the angle of attack then the lift will start to increase again. By the time the wing reaches 25 or 30 degrees then it's making as much lift as it did just before it stalled. By the time it gets to around 45 degees then it will be making significantly more lift than it ever did before stalling, before it decreases again as you go past 45 degrees.
So why do planes fall out of the sky when they stall and why dont normal planes fly at 45 degrees angle of attack...? Drag is the answer. When a wing stalls drag increases enormously, so the plane slows down. Slowing down decreases lift and this coupled with the drop in lift immediately after stall sees the plane fall from the sky. Drag is also the reason most planes dont fly at 45 deg, most planes simply would not have the thrust needed to overcome the huge drag produced at that angle of attack.
Here's a graph showing lift through a full 180 deg airfoil rotation, note the peak lift occurs around 40- 45 deg AoA.
Steve
To understand how VG's work you need to think about how the boundary layer (that's the thin layer of air next to the wing surface) moves. In smooth laminar flow the boundary layer slides smoothly over the wing surface. When the plane flys slowly the air doesn't have much momentum so because the air is viscous, and there is drag as it moves over the wing surface, and also because it moves from low pressure on the front part of the wing to higher pressure near the TE, the boundary layer runs out of momentum, slows down and builds into a thick sluggish layer or may stop altogether in a static 'bubble' of stationary air, aka 'laminar separation bubble'. The faster moving air above the boundary layer is forced away from the wing surface by the thickening boundary layer and eventually 'breaks away' altogether.. i.e. the wing stalls.
Vortex generators work like tiny wings. They stick ip through the thin boundary layer near the leading edge and into the faster moving freestream air. They are set at a high angle of attack to the airflow and so form a vortex at their tips just like the tips of wings. This vortex in the fast moving freestream 'mixes' with the slow moving boundary layer and 'pulls' it along, preventing or delaying the boundary layer slowing or stopping, this delays separation and stall.
Sorry if I'm geeking out a bit
I did say you might regret getting me started....One other thing to consider is that wings still make lots of lift after they stall. Many seem to think that when a wing stalls all the lift goes away but that's just not the case. A typical wing might stall at maybe 10-15 degrees and the lift will fall by maybe 20%, but if you keep increasing the angle of attack then the lift will start to increase again. By the time the wing reaches 25 or 30 degrees then it's making as much lift as it did just before it stalled. By the time it gets to around 45 degees then it will be making significantly more lift than it ever did before stalling, before it decreases again as you go past 45 degrees.
So why do planes fall out of the sky when they stall and why dont normal planes fly at 45 degrees angle of attack...? Drag is the answer. When a wing stalls drag increases enormously, so the plane slows down. Slowing down decreases lift and this coupled with the drop in lift immediately after stall sees the plane fall from the sky. Drag is also the reason most planes dont fly at 45 deg, most planes simply would not have the thrust needed to overcome the huge drag produced at that angle of attack.
Here's a graph showing lift through a full 180 deg airfoil rotation, note the peak lift occurs around 40- 45 deg AoA.
Steve
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Anthony7820
70cc twin V2
I used them on my 71 Slick 10% back from LE. Spaced 2 inches apart. They are 10mm long. They are on top and bottom of every LE.
After hours of putting's them on I went out for a test flight. I also switched to Futaba at the same time. I believe either the VG's or the Futaba made a difference, that being said, I would have to fly the same plane and setup back to back with and without VG's to tell of any huge differences. I talked with Joe about this and he said I think we trend to forget what they do, and if you took them off you would notice what they did while they were on.
Plus they look cool
After hours of putting's them on I went out for a test flight. I also switched to Futaba at the same time. I believe either the VG's or the Futaba made a difference, that being said, I would have to fly the same plane and setup back to back with and without VG's to tell of any huge differences. I talked with Joe about this and he said I think we trend to forget what they do, and if you took them off you would notice what they did while they were on.
Plus they look cool
njswede
150cc
You might regret getting me started on the subject of aerodynamics
Actually, I deliberately pushed your aerodynamics buttons hoping this would happen. I always end up learning something I didn't know before! Keeps the geekiness coming!