Vortex Generators
11-26-2007, 02:11 PM
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Vortex Generators
Has anyone tried vortex generators?
I have a 72" Cap 21 ARF thats be sitting in a box for alot years. Everytime I hear about that bird most mention the Cap wants to snap upon landing. My thoughts are while its best to fly that type of airplane on the runway to preclude a stall. Using vortex generators maybe a possible solution to delaying a tip stall upon landing.
I have a 72" Cap 21 ARF thats be sitting in a box for alot years. Everytime I hear about that bird most mention the Cap wants to snap upon landing. My thoughts are while its best to fly that type of airplane on the runway to preclude a stall. Using vortex generators maybe a possible solution to delaying a tip stall upon landing.
11-26-2007, 04:11 PM
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RE: Vortex Generators
Vortex generators, turbulators, invigorators... they all do the same thing. Induce some turbulence into the boundry layer before it can separate on it's own. Adding that energy to the boundry layer tends to delay the formation of separation bubbles and delay the stall if done right.
However in this case it'll most likely also make the model harder to snap cleanly for maneuvers that require a hard stall. It's definetly a two edged sword.
Easiest way to deal with landing stalls would be to add turbulator strips along the leading edge of the outer half or third of the wings. With luck and some experiementing you'll find a spot and size of turbulator that will only reduce the landing stall risk without ruining the desireable hard snap for maneuvers. You'll need to play with thickness and %chord placement to find the sweet spot.
Automotive trim stripe tape layered on two or three or 4 layers thick makes a nice way to do this and vary the thickness. The risk will be that you'll need to first fly the model stock to find out what is good and what is bad about it. Try the low speed "landing" stalls a good 3 mistakes high...
Then start playing with turbulators with varying thickness and chord placement. I'm guessing a little but what you're looking for is the smallest and furthest back that'll help the landing. By keeping them as far back as possible it SHOULD still let the model deep stall easily when you haul on the sticks for a snap maneuver. I'll guess that this sweet final placement will probably be the outer 30 to 40% of the span and placed back at around the 20 to 25% chord point or just at the high point if it's one of the airfoils with a very forward max thickness point. It'll probably need to be roughly 1/32 to 3/64 thick to do the job and perhaps 3/16 to 1/4 wide. Narrower is supposed to be better.
However in this case it'll most likely also make the model harder to snap cleanly for maneuvers that require a hard stall. It's definetly a two edged sword.
Easiest way to deal with landing stalls would be to add turbulator strips along the leading edge of the outer half or third of the wings. With luck and some experiementing you'll find a spot and size of turbulator that will only reduce the landing stall risk without ruining the desireable hard snap for maneuvers. You'll need to play with thickness and %chord placement to find the sweet spot.
Automotive trim stripe tape layered on two or three or 4 layers thick makes a nice way to do this and vary the thickness. The risk will be that you'll need to first fly the model stock to find out what is good and what is bad about it. Try the low speed "landing" stalls a good 3 mistakes high...
Then start playing with turbulators with varying thickness and chord placement. I'm guessing a little but what you're looking for is the smallest and furthest back that'll help the landing. By keeping them as far back as possible it SHOULD still let the model deep stall easily when you haul on the sticks for a snap maneuver. I'll guess that this sweet final placement will probably be the outer 30 to 40% of the span and placed back at around the 20 to 25% chord point or just at the high point if it's one of the airfoils with a very forward max thickness point. It'll probably need to be roughly 1/32 to 3/64 thick to do the job and perhaps 3/16 to 1/4 wide. Narrower is supposed to be better.
11-27-2007, 09:38 AM
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RE: Vortex Generators
Bruce,
The turbulator in tape application would be a easy way to do the testing. Thanks!
Kind of like the look a of vortex generators though
It occurred to me that applying a low wing fence outboard towards the wingtip it may provide similiar results. I'm sure it can be tested with a stick-on application as well.
The turbulator in tape application would be a easy way to do the testing. Thanks!
Kind of like the look a of vortex generators though It occurred to me that applying a low wing fence outboard towards the wingtip it may provide similiar results. I'm sure it can be tested with a stick-on application as well.
11-27-2007, 12:55 PM
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RE: Vortex Generators
The only sure cure for the CAP Snap is get a different make of airplane.
Adding -anything- to the outboard wing of a CAP will make a bad situation worse!
The wing-root mounted sharper leading edge devices commonly used on many small planes are a much better choice than doing anything else to the wing.
And in reality, these work by stalling that portion of the wing first, creating turbulence and noise the pilot can hear.
On a model, the noise wouldn't be observable.
For a CAP, the safest thing is never let the speed get low. Fly it faster than a similar Extra or Edge.
Adding -anything- to the outboard wing of a CAP will make a bad situation worse!
The wing-root mounted sharper leading edge devices commonly used on many small planes are a much better choice than doing anything else to the wing.
And in reality, these work by stalling that portion of the wing first, creating turbulence and noise the pilot can hear.
On a model, the noise wouldn't be observable.
For a CAP, the safest thing is never let the speed get low. Fly it faster than a similar Extra or Edge.
