[OLD PRAIRIE RED NECK]

Originally Posted by Singleminded

Great idea but, I'm lucky to have a fellow club member who has a 3-D printer and has already made up a dozen or so 'samples' that are going to be very close to the VG's on the CZC! So, I'll have a couple of different sizes to experiment with.....!

Pictures please and keep us posted as I am interested in trying some too!

[OLD PRAIRIE RED NECK]

Originally Posted by Lazy Ace

For what its worth the university SAE group did some extensive research on using the vortex generators on their Aerodesign project and found there was little to be gained at model sizes and speeds. Not sure of their data but I am sure if there was anything to gain we would surely seen them in use at these competitions.

Dennis

Any chance you could find and post that info please?

[Lazy Ace]

Originally Posted by OLD PRAIRIE RED NECK

Any chance you could find and post that info please?

No that was years ago 2005 when we first started. If I remember correctly it had something to do with the speed, the Reynolds numbers and the boundary layer. Wing tip fences and winglets work as most of the teams use something along this line. Cannot remember any vortex generators in the time I was involved.

One year they even used a Gurney Flap

I have included a link to a paper done for model airfoils it long winded but there are references to a type of vortex generator that worked on RC wing profiles. I seem to remember somewhere the near the same effect could be had by placing a zig zag strip similar to pinking tape at the same place on the wing. Not quite as much added effect as the brass strip but a lot less drag if I remember correctly.

See pages 176, 187

http://m-selig.ae.illinois.edu/uiuc_...il-Data-V1.pdf

[Singleminded]

Originally Posted by OLD PRAIRIE RED NECK

Pictures please and keep us posted as I am interested in trying some too!

Will do once I get things better organized!

[Lazy Ace]

Just remember as Gord has said earlier in the thread that with vortex generators the low speed stall will most likely be more severe and abrupt. After all their purpose was primarily to increase payload.

[Singleminded]

I haven't experienced that with my CZC yet....it exhibits really forgiving low speed characteristics and seems to float stably even down to what would normally stall any of my other planes! I will experiment with a Piper Pawnee first without VG's and later on with....should be interesting to see what changes the VG bring to the low speed characteristics!

Gord

[kip51035]

How they work, the basic idea,

Picture a speed boat on the water the faster it goes the narrower the wake pattern. When it slows down the wake pattern becomes very wide. Well that is what the vortex generators do. They create a narrow rotary wind on top of the wing and as you slow down it widens out and produces a a progressive stall. They found that on perfectly smooth wings the stall was a random pattern and changed shape violently. Some of the swept wing pattern ships would snap roll on landing approach without any warning. Many of the early jet fighters had wing fences on them to help control this. Some of the vortex generators are shaped like a small under-camber airfoils, some are simply a piece of 90 deg. aluminum set at a 10 deg. angle to the airflow and some are just a triangle bump set slightly ahead of the high point on the wing.

[Lazy Ace]

Originally Posted by Singleminded

I haven't experienced that with my CZC yet....it exhibits really forgiving low speed characteristics and seems to float stably even down to what would normally stall any of my other planes! I will experiment with a Piper Pawnee first without VG's and later on with....should be interesting to see what changes the VG bring to the low speed characteristics!

Gord

Keep in mind that vortex generators only work if strategically placed on the individual wing. This is accomplished through testing and is specific to each aircraft. True vortex generators do not follow the cord but are at angles to the liner airflow over the wings so they cause miniature vortices within the boundary layer.



If your home made vortex generators are liner in nature and tend to follow the cord of the wing or are to large they may be a form of stall fence that straightens or directs the airflow over the airfoil in high alpha or stall conditions. As you can see from the video the vortex generators are very small so they remain in the boundary layer and not at all aligned with the airflow over the wing. To translate these down to model sizing and proper placement would be extremely difficult.



Following is a quote on stall fences.

Stall Fences

Stall fences are another device to prevent wing tips from stalling before the roots. They are thin plates which project up from the wing and lie parallel to the aircraft’s axis of symmetry. Without these fences, a span wise airflow along wings causes the boundary layer to thicken toward the wingtips, especially on swept-wing aircraft. This results in early boundary layer separation at the wingtips and loss of aileron control. Fences block span wise airflow, preventing boundary layer buildup over the ailerons and thus improving stall characteristics.

Stall fences are primarily found on swept-wing aircraft like business jets and fighters, but are also found on straight-winged general aviation aircraft. The Eagle 150 has a unique design featuring a main wing, a forward wing, and a horizontal stabilizer. Each side of the main wing has a stall fence, which according to the manufacturer “redirects the airflow to the ailerons, creating a re-energizing effect. This allows the pilot or student pilot full control at minimum speed and even at the point of stall”.

Some aircraft have aerodynamic stall fences. These are not the typical vertical plates, but instead other devices which create the same effect. The Questair Venture kitplane has a small vertical slot on the leading edge of each wing; the airplane looks like it ran into a bandsaw. At high angles of attack, air flowing through this slot creates a trailing vortex that acts like a stall fence. NASA found that this “rooster tail” of air creates turbulent, high pressure air that can “impede the spanwise advance of the stall” (Cox). The Cirrus SR20 and SR22 have a similar feature. According to the manufacturer, “At high angles of attack, two distinct leading edges create an aerodynamic stall fence that serves as an airflow barrier, preventing the stalled inboard portion of the wing from affecting the outboard portion still producing lift”.

Whether the stall fence is a physical plate or an airflow barrier, this device combats the progression of a stall across a wing and helps keep air flowing over the ailerons.

Watch the whole video sorry about the commercials in the middle.

[YOUTUBE]www.youtube.com/watch?v=_y71Q4QtsT4[/YOUTUBE]

[OLD PRAIRIE RED NECK]

Originally Posted by Lazy Ace

For what its worth the university SAE group did some extensive research on using the vortex generators on their Aerodesign project and found there was little to be gained at model sizes and speeds. Not sure of their data but I am sure if there was anything to gain we would surely seen them in use at these competitions.

Dennis

So - these were cargo competitions then. I suppose that would explain why the VG's didn't help! They would be looking for more lift at cruising speed rather than lower stall speed and increased control response at high angles of attack. In fact they probably decreased the performance of the wing due to the extra drag created at higher speeds. Makes sense.

I think that the placement of the VG's fore and aft would relate directly to the angle if attack that your plane normally stalls at. The higher the normal stall angle the closer you would place them to the leading edge.

As far as size of the VG's, model vs full sized, it would not be a direct scale as the air density will not change when you fly your model. Therefore the model ones will look too large to get them to maximum effectiveness. Just a gut feeling on my part.

Good informative video by the way!

[Lazy Ace]

Originally Posted by OLD PRAIRIE RED NECK

So - these were cargo competitions then. I suppose that would explain why the VG's didn't help! They would be looking for more lift at cruising speed rather than lower stall speed and increased control response at high angles of attack. In fact they probably decreased the performance of the wing due to the extra drag created at higher speeds. Makes sense.

I think that the placement of the VG's fore and aft would relate directly to the angle if attack that your plane normally stalls at. The higher the normal stall angle the closer you would place them to the leading edge.

As far as size of the VG's, model vs full sized, it would not be a direct scale as the air density will not change when you fly your model. Therefore the model ones will look too large to get them to maximum effectiveness. Just a gut feeling on my part.

Good informative video by the way!

Actually its very much a low speed short take off situation. Limited motor size a .65 on a 14 ft span and 200 foot take off restriction. You can only hope to achieve about 5 to 8 percent reduction in stall speed for properly placed vortex generators. At our model speeds what does that equate to 3 mph best case slower stall speed at regular 40 mph landing speed? I couldn't tell the difference of 3 mph one way or another.

On the other hand a Gurney Flap on the small wing out performed the vortex generators and it was easy to install. A percentage of the cord along the trailing edge.

Fences were much more effective at the low speeds of models as it kept the air flowing over the ailerons and tail feathers deep deep into the stall. So much so one could be coming down at a 45 degree angle in a full stall without dropping a wing with nothing left for a flair. I watched it happen a few times not pretty with 40 lbs of lead on board.

Dennis