Ez2cDave
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Copying Nature - Feathers On a Glider Wing to Reduce Drag and Soften Stall
Dave F.
"Feathers On a Glider Wing to Reduce Drag and Soften Stall"
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Dave F.
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Wondering if this is the same as using turbulators?
Ez2cDave
Well-Known Member
That's an interesting question . . .
A turbulator serves to trip the airflow to reduce drag, but I don't think it affects stall characteristics.
The "feathers" appear to have a large affect on stall characteristics, virtually eliminating it.
I wish the maker of the video had done some "before & after" duration testing.
Dave F.
A turbulator serves to trip the airflow to reduce drag, but I don't think it affects stall characteristics.
The "feathers" appear to have a large affect on stall characteristics, virtually eliminating it.
I wish the maker of the video had done some "before & after" duration testing.
Dave F.
Ez2cDave
Well-Known Member
I found this . . .
https://www.chemeurope.com/en/encyclopedia/Turbulator.html
QUOTE :
When air flows over the wing of an aircraft, there is a layer of air called the boundary layer between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the negative pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point.
These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.
In gliders the turbulator is often a thin zig-zag strip that is placed on the underside of the wing and sometimes on the fin. The DG 300 glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.
For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the furthest for a given height.
END QUOTE :
Dave F.
https://www.chemeurope.com/en/encyclopedia/Turbulator.html
QUOTE :
Turbulator
A turbulator is a device for improving the flow of air over a wing.When air flows over the wing of an aircraft, there is a layer of air called the boundary layer between the wing's surface and where the air is undisturbed. Depending on the profile of the wing, the air will often flow smoothly in a thin boundary layer across much of the wing's surface. The boundary layer will be laminar near the leading edge and will become turbulent a certain distance from the leading edge depending on surface roughness and Reynolds Number (speed). However there comes a point, the separation point, in which the boundary layer breaks away from the surface of the wing due to the magnitude of the negative pressure gradient. Beneath the separated layer, bubbles of stagnant air form, creating additional drag because of the lower pressure in the wake behind the separation point.
These bubbles can be reduced or even eliminated by shaping the airfoil to move the separation point downstream or by adding a device, a turbulator that trips the boundary layer into turbulence. The turbulent boundary layer contains more energy, so will delay separation until a greater magnitude of negative pressure gradient is reached, effectively moving the separation point further aft on the airfoil and possibly eliminating separation completely. A consequence of the turbulent boundary layer is increased skin friction relative to a laminar boundary layer, but this is very small compared to the increase in drag associated with separation.
In gliders the turbulator is often a thin zig-zag strip that is placed on the underside of the wing and sometimes on the fin. The DG 300 glider used small holes in the wing surface to blow air into the boundary layer, but there is a risk that these holes will become blocked by polish, dirt and moisture.
For the aircraft with low Reynolds numbers (i.e. where minimizing turbulence and drag is a major concern) such as gliders, the small increase in drag from the turbulator at higher speeds is minor compared with the larger improvements at best glide speed, at which the glider can fly the furthest for a given height.
END QUOTE :
Dave F.
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It seems that the "feathers" actually function as a turbulator (see sentence 3 in intro paragraph here): https://www.researchgate.net/public...foil_at_Low_Reynolds_Number_in_Turbulent_Flow.
They decrease the speed at which a stall condition happens (and if properly applied, can reduce or eliminate that visible "porpoising" behavior that can mess up an otherwise ideal glide)...exactly what the "feathers" appear to be doing.
They decrease the speed at which a stall condition happens (and if properly applied, can reduce or eliminate that visible "porpoising" behavior that can mess up an otherwise ideal glide)...exactly what the "feathers" appear to be doing.
Ez2cDave
Well-Known Member
Eric,
Hmm . . . I wonder how much additional mass / drag they add ?
From that video, higher drag / mass might be offset by glide performance and "stall-canceling" of the "feathers".
Thoughts ?
Dave F.
No need for turbulators on feathered friends model rockets. Just sick three canted E12 motors in it's rear and press the button. 
Crawf56
Pig Soooiiieee!!
Turbulator on a T25 Centurian: https://www.rocketryforum.com/threads/estes-t25-centurian-rebuild.138486/page-3
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I think he discovered a novel way to add the benefits of a turbulator, but it definitely adds unnecessary mass and drag. He could have gotten by much easier and more effectively with turbulator tape that's easy to source and made for just that purpose. https://www.iflytailies.com/store/wing-accessories/turbulator-tape/
Ez2cDave
Well-Known Member
On the subject of Turbulator Tape, is there a "formula" or calculation to determine where to place it ?
Dave F.
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Could find no definitive online calculation as to turbulator placement, and it seems to vary even among the free-flight forums. Worse, the precise location varies according to the airfoil shape/geometry and changes according to whatever the life coefficient is at any given moment. https://www.rocketryforum.com/threa...ce-drag-and-soften-stall.173700/#post-2299274
An all-encompassing rule of thumb seems to rely primarily on experiential opinion.
E.G.:
"15% back from the leading edge". https://www.rcgroups.com/forums/sho...erator-Experimentation-and-Design-Tips/page20
"25% of the main wing chord " George Gassaway
Placement seems to be just aft of the "high point" of a standard (Clark-Y) airfoil, i.e. somewhere just behind 25% from the LE (standard "high point"). Unfortunately just exactly how much aft would depend on wind tunnel tests. That's far too difficult for simple hobbyist/sport fliers, and probably too much even for a NAR or FAI/WCSM contestant.
Agreed that some kind of generalized rule of thumb sure would be handy when faced with a stalling problem where you would rather not add more weight to the front of the glider to address it. Maybe someone on the Forum with an engineering background could chime in?
An all-encompassing rule of thumb seems to rely primarily on experiential opinion.
E.G.:
"15% back from the leading edge". https://www.rcgroups.com/forums/sho...erator-Experimentation-and-Design-Tips/page20
"25% of the main wing chord " George Gassaway
Placement seems to be just aft of the "high point" of a standard (Clark-Y) airfoil, i.e. somewhere just behind 25% from the LE (standard "high point"). Unfortunately just exactly how much aft would depend on wind tunnel tests. That's far too difficult for simple hobbyist/sport fliers, and probably too much even for a NAR or FAI/WCSM contestant.
Agreed that some kind of generalized rule of thumb sure would be handy when faced with a stalling problem where you would rather not add more weight to the front of the glider to address it. Maybe someone on the Forum with an engineering background could chime in?
Ez2cDave
Well-Known Member
Yes, I agree . . .
There has to be some method to determine proper placement, short of wind-tunnel testing or "endless "place & try" efforts with no true way to measure the results accurately. Flight duration, in and of itself, would not be sufficient, due to variables in flying conditions ( thermals, sink, etc. ).
A pic of an RC plane with Turbulator Tape installed is below. ( from your link )
Dave F.
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Ez2cDave
Well-Known Member
Eric,
I found this information from your link.
https://alofthobbies.com/products/turbulator-tape-8mm-clear
I found it interesting to note the increase in drag, at high speed, ( during Boost, for us ) and reduced drag at low speed ( gliding ).
Dave F.
I found this information from your link.
https://alofthobbies.com/products/turbulator-tape-8mm-clear
I found it interesting to note the increase in drag, at high speed, ( during Boost, for us ) and reduced drag at low speed ( gliding ).
Dave F.
Ez2cDave
Well-Known Member
A cool video about "Vortex Generators" ( powered aircraft ).
Watching the airflow is very interesting ( stall conditions ).
Dave F.
Watching the airflow is very interesting ( stall conditions ).
Dave F.
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My $0.02. It is impossible to tell what is going on in the video because it is an RC HLG, and I can not see the control inputs. I suspect the feather thing is an example of controlled separated flow and not simply a turbulator.
Viscosity is the very property that allows airflow to stay attached to a surface. A laminar boundary layer has less skin friction drag than a turbulent boundary layer, but will more readily separate from the surface, often resulting in increased drag, reduced lift. and ultimately stall. So, in airfoil design it is desirable to extend the laminar boundary region as far as possible and then force a transition to turbulent before separation occurs. This is best done by designing in a well placed adverse pressure gradient ramp. Of course it is difficult to do since airfoils must perform well over a wide range of angles of attack and airspeed. When the design gets to the actual flying stage, one may find that it is not quite right. A turbulator can often be applied as a band-aid to fix the problem, but with a slight increase in drag. In extreme cases, vortex generators can be applied to energize the flow and avoid flow separation. It is good to have turbulators and vortex generators in you bag of aerodynamic tricks, but you would prefer to have the time to redesign the airfoil.
The airfoil in the illustration above seems to be in zero lift condition, making it useless. An interesting example is the sailplane which spends most of its flight at two conditions. When in lift, you fly at minimum sink speed and angle-of-attack, and when moving to the next thermal or updraft you fly at max L/D. You can design the airfoil to maximize the laminar flow region on one side of the airfoil at one flight condition, and maximize it on the other side for the other flight condition.
You can extend these techniques to 3D flows, such as keeping the flow attached to a boat-tail. IF you look closely at the original B1 bomber in the Airforce Museum at Dayton, OH, you will see a ring of vortex generators around the tail cone. In the redesigned B1b, the vortex generators are gone.
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