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An analysis of the effect of Feathers VGs on a 601 HDS wing.

 
 
First of all is a big thank you to Viktor in Switzerland for these videos and his work on this topic.  Viktor is a very enthusiastic and experienced HDS flier, and a curious and dedicated experimenter.  He’s meticulous in his work, and understands what he’s doing.  He got his first ‘Feathers’ VGs in August 2007, and after testing them has become a very strong supporter, leading to many conversions in Europe. 
 
Now every 601 HDS owner needs to pay close attention to these demonstrated results. 
 
Following is an analysis of the images and the features that need to be noted. 
 
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The first video was done with VGs only on the OUTER portion of the wing. 
 
 
 Tip!  When you've seen the video, click on the 'Back' arrow in the far upper left hand corner of the YouTube page to come back to this page.
 
This is a really good way to demonstrate the difference that VGs make to the airflow. 
 
Notice that outer portion of the wing, behind the VGs, doesn’t stall at all, and the airflow remains attached and smooth.
 
Notice that the inner portion of the wing, without VGs, stalls suddenly and the disruption immediately spreads over the whole portion.  But note that the disruption doesn’t spread to the portion of the wing behind the VGs.  
 
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The second video was done with VGs on ALL of the wing.
 

 

Notice that wing now flies at a much higher angle of attack, and you can hear the pilot calling the indicated air speed ....35.... 35.... , while the airflow over the wing stays attached and steady. 
 
  Notice that when the turbulence finally does trigger a stall, the disruption is confined to just the root of the wing and then dissipates immediately and doesn’t spread to the rest of the wing area. 
  

Conclusions
 
 
Easier Landings
This video dramatically demonstrates how it is that VGs give the soft, gentle stall that makes landings so much easier!  Imagine you are flaring for a landing and are still a bit too high when the wing stalls.  Wouldn’t you rather have the stall start by just ‘nibbling’ at the root rather than suddenly stalling most of the wing at once??? 
Settle on gently rather than come down with a thump!!
 
Then, when it comes to the very last of the hold-off, it sure is good to be able to pull that very high angle of attack, and settle on with the nose way high, at a much slower speed than without the VGs.  As Viktor has said, it turns his HDS into a STOL aircraft!
 
To be able to pull such a high angle of attack at such a low air speed, the horizontal tail will probably also need VGs to give assistance.  ie – ‘Tail Feathers’.
 
 
 
Better aileron authority
Notice that in both videos the airflow over the aileron is steady and smooth, and despite the low speed, there’s still aileron authority to handle the considerable turbulence on that day.  

 
 

Safer stall characteristics

Notice that even when the portion without VGs stalls, the nose doesn’t drop suddenly – just an increasing ‘mush’ which is easily and quickly checked by easing the stick and it’s flying again, with no sudden loss of altitude. 
 
There’s an old rule of thumb that’s not far off the fact – “...when a full stall occurs, the nose will drop to an angle below the horizon pretty much equal to the angle above the horizon at the stall...”.  Then the aircraft has to build up speed to re-attach the airflow  This will give up at least a couple of hundred feet of altitude, if not more, before pulling out and stabilizing.  With VGs the nose only has to drop enough for the VGs to get a ‘bite’ again and stabilize the airflow, usually recovered about horizontal, with very little loss of altitude.  That’s a critical safety issue if you happen to get too slow at low altitude!!
 
When I got my first VGs all the talk was of lower stall speeds and shorter landings, but it turns out that the most important benefit is SAFETY !
 
 
There were also predictions, and they still prevail, that VGs “....wouldn’t make much difference on high-lift wings....”  I’m sure glad that despite those predictions, I tried VGs on the 701 wing and discovered that they do make a tremendous difference!  Now it seems that they also make a tremendous difference on the HDS wing as well!  These aren’t predictions and loose claims, they’re discoveries that are now well proven. 
 
Viktor has done considerable work to determine the correct placement of VGs on the HDS wing.  Those instructions are at  the HDS page.  An HDS needs 80 VGs on the main wing and 80 under the horizontal stabilizer.
 

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