VGs? Who has them and do you like them?

23 posts in this topic

Posted

I have been toying with the idea on installing VGs on the tail and wing on my Avid.  Who has them and what are you thoughts?

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Posted (edited)

I have heard mixed reviews on the STOL wing without the solid leading edge. Placement is critical. I have some pretty good data from JG of STOLspeed if you are interested. I have had a set on my bench but haven't gotten around to installing them.

 On the bottom of the horizontal it made a very noticeable improvement on my plane. Worth every bit of the $50 and hour to install them. Seal up your elevator gap with tape while you are at it. I would get them straight off the STOLspeed website. He's in Australia but ships quickly for normal shipping prices. 

Edited by C5Engineer

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Posted (edited)

Here's an old thread from the install on the tail. 

 

Edited by C5Engineer

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Posted (edited)

Thanks Joey!  You and I now have the same tail now and I will use the other thread to mount mine the same.

I have the Stol HH wing without the solid leading edge too and I have heard mixed reviews as well.  Ill get a set of 50 for the tail now.

Edited by thebriman

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Posted

I have not installed them yet, But what I gleaned from discussions is that they are not as effective as they could be due to the scalloped forward wing area. My Mod 1 has some areas of non scalloped due to wing tanks and locker plus the builder used aluminum in some areas then used the extra false ribs to reduce the scallop in the other spaces. I also see that due to forward C.G. and shorter distance to tail on Mod 1 the elevator loses effectiveness and you can not explore the benefits of the V.G.s. This I believe is why Joey installed a larger elevator.

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Posted

There has been a lot of discussion on this.  % of wing cord etc.  There was a line of thought that the flaperons should be included in the measurement as well but I don't think so.  I did and I wish I had put them on temp as some people did and not permanent from the get go.  I would put them around 4" back from the leading edge to start with using double sided tape so you can play with them and get them in the perfect spot for your plane.  There are so many builder variations in washout etc that its hard to say use this measurement and you will be perfect.  I still want to play with putting them on the underside of the leading edge to try and get rid of the speed wall that the undercambered ribs have.

 

:BC:

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Posted (edited)

I will repeat this for the new guys:  It depends on what you are trying to do with the VGs - If  you are looking for more lift and better flaperon response at slow speed, put them on top of the wing -

If you are wanting faster cruise from the undercambered wing, one guy reported that he got a 10 mph cruise increase using them UNDER the wing.   This letter is in our Files and Forms posts.

I don't think anyone has tried putting them both on the top and bottom of the wing - any volunteers?

EdMO

Edited by EDMO

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Posted

 

I don't think anyone has tried putting them both on the top and bottom of the wing - any volunteers?

EdMO

I'll try anything once...

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Posted

I placed them on the wing top on my MK IV at the recommended position (StolSpeed) and I didn't get any reduction in stall speed but it did increase the angle of attack at which the wing stalled.  That was not very helpful, making the plane wanting to land tail first.  I have removed most of the VG's but left them on the last two feet of each wing thinking that it may help reduce the effect of the excessive washout that was built into the wings (plans built).  I don't know if it has had any effect with that.  I do plan to try installing the VG's more to the rear and see if I get better results but have not done so at this time. To be continued sometime in the future.

Paul

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Posted (edited)

The people at Kitfox have done exhaustive testing with VGs from many different manufacturers.  Changing % of chord, spread, angle, etc, etc.  They have found that their benefits on the main wing are negligible at best, and nonexistent at worst.

Remember people, the reason for VGs.  They excite the air over the wing giving authority to the flow at higher angles of attack.  This mostly manifests itself in better flap and aileron effectiveness at slow speed.  What is different about our planes?  Fokker style flapperons.  These are already in the high pressure air below and behind the wing.  

Sure, maybe if the top of the wing were not scalloped by ribs and fabric there may be some benefit to VGs keeping the flow attached at higher angles of attack.  But what would that get you?  Maybe drop the stall break from 35 to 30 mph?  That will shorten your ground roll by what, 25'?  If you really need another 25', you need a helicopter, not an airplane.

 

 

IMO:  VGs for an Avid or Kitfox = waste of money.

 

(Benefits for use on the underside of the horizontal and sides of the vertical tail notwithstanding.)

Edited by Av8r3400
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Posted (edited)

There you go Joey,  might as well put the VGs on the underside of the wing to get rid of the bubbles and go for more cruise...Maybe under the wing, they will also affect flaperons a little too?

EdMO

Edited by EDMO

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Posted (edited)

i'm not in for VG, imho they work for faster planes. I'm sure the vortex is very small at the speeds we want to stall, and they are very expensive for what they are. If i would try to put them, i'd certainly go build myself those little plastic things.

The first thing (free) to do is close the gaps of the tail: rudder and elevator create depression on tail, those huge gaps reduce it a lot.

Edited by efil 01

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Posted (edited)

I flew my plane, Avid Mark IV, Extended Aerobatic (HH) Speedwing for a few hours without the VGs.  Then I put VGs on the outer 66% of each wing at 4.5" back from LE as measured along the chord, then stepped forward to 4" where the flapperons end to the tips.  I also put VGs 4" forward of the hinge opening on the underside of the horz stabilizer. All VGs are spaced at 60MM

Keeping in mind these are seat pants observations, Here is what stands out to me:

Before the VGs I had to modulate power all the way to the grass.  If I cut power and tried to hold it up in ground effect, it would just plop down.  On take offs, soft field (holding the nose wheel up) was impossible.  Take off speed was ~60-65MPH IAS (1/3 flaps)

After the VGs: Now I cut power several hundred feet above and ahead of the threshold (right after turning final) and have to WAIT for energy to bleed off as I ease the stick back in ground effect.  The landings are much greasier now.  (Perhaps I'm getting better at flying the plane too).  On take offs, I have the nose wheel up before the ASI starts to register movement.  The plane leaves the ground at ~50mph IAS (1/3 flaps)

Overall, Landings feel different.  I'm still relearning where to cut power so that I don't glide halfway down the field before running out of elevator.  The first time I came in with the VGs, I was surprised by the glide and about the time trees at the end of the runway were starting wave me back on the power it settled softly into the grass.  My preference for landing is to stay at 80-100% pattern altitude until I start turning short final.  With the VGs I've been adjusting base & final out further or slipping it down faster (more fun!)

Bottom line, I feel better about the plane landing and taking off.  The change in handling put a huge smile on my face too!

Edited by Knuckledragger
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Posted

I put them on the underside of my horizontal about 2" in front of the elevator hinge and it was a big improvement in elevator authority.  I put them on the top of my HH wing and included the flaperon in the chord so they are about 7" back from the leading edge (I agree with Leni, probably too far back).  I could not tell much improvement in stall speed but surprisingly had improved roll authority.  Well worth it in my MKIV Avid with very limited ability to move stick left with heavy clothes on.  My Avid plus has a lot more room but I still like the added roll authority.  I later tried them on the underside of the outer 2' of the wings at about mid chord and found no noticeable affect.  I then tried them on the underside of the wings just at the transition from the spar to the rib to see if it would give me some extra speed but found no noticeable difference.  I pulled all of them from the underside of the wings  but left them on the tops.

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Posted (edited)

SuberAvid,  try moving the VGs closer to the leading edge of the wing.  My understanding is ~7% of chord is a good starting point, which is ~4.5".

 

Rob

Edited by Knuckledragger

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Posted

Hi Knuckledracker and SuberAvid,

I am interested by your VGs intalling. Can you make same pctures from the VGs on the wimgs and elevator?

Thank you

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Posted

Hi Guys, 

We have installed about six sets of VG's with varying results. I agree wholeheartedly with the statement ...it depends what you are looking for. PLACEMENT: Last set went onto a Kitfox 4 in a Stagger Fashion and it yielded the best improvements yet. Photos and a little write up in the KF4 section of this forum. Thinking why they worked better in stagger is probably the fact that 50% are fore and the other 50% are aft, hence as the wing angles the effect is in a broader range of effectiveness. Just my guess, but about the only way I can explain the stagger findings (1) vs the straight (5) installations. 

 I'd be tempted on the next set to go with the stagger again, this time put the front edge of the VG near the front% of cord and the tail edge of the rear ones near the aft% of cord as recommended. Flaperons don't count as part of the wing. Last I looked they seemed like control surfaces to me. 

Lessons learned; Unless someone has the pretty much the exact same bird, W&B, rigged the same the results will vary from plane to plane even on the same type, model. That much we figured by now. Looking forward to hear about your findings...

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Posted

Hi Guys, 

We have installed about six sets of VG's with varying results. I agree wholeheartedly with the statement ...it depends what you are looking for. PLACEMENT: Last set went onto a Kitfox 4 in a Stagger Fashion and it yielded the best improvements yet. Photos and a little write up in the KF4 section of this forum. Thinking why they worked better in stagger is probably the fact that 50% are fore and the other 50% are aft, hence as the wing angles the effect is in a broader range of effectiveness. Just my guess, but about the only way I can explain the stagger findings (1) vs the straight (5) installations. 

Have you done any tuft testing on the Kitfox? It seems with a video camera and some time you could point to which one of the VG sets (fore or aft) are working better towards the stall speed. Just a thought.

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Posted

Hello friends,
I have 120 small VG's bought us so now the right place for the attachment. From Aircraft Spruce: Link to www. stolspeed.com

Since some of you have already done experiments I would gladly provoke.
I have a MK4 Speedwing. The wings measure from the front edge to the rear edge 104cm (measured on the bottom.)

Where should I start (measured from the front edge) with the VG's? What is the distance between the VG's you have chosen?
The pictures with the offset VG's on the KF4 I have seen. Can not imagine, however, that is the good.

It would be best if someone had mounted VP's at Speedwings.

And the next question would be where and to what distance the VG's at the bottom of the elevator? From the front edge to the flap it is 23 cm.
Can the VG's simply be glued to the cover? Since no pressure can be exerted during gluing.

The best thing would pictures  be to help me.
Thank you very much.P1140804.thumb.JPG.ba7a44a9058950bd711ccP1140795.thumb.JPG.1831a902e3ddb4ea67d16

 

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Posted

Hello Maddin, 

This is a 2 person job, so bring a helper.

Place a Drywall square/Carpentry Square (with extension) on the bottom of the wing (long side) and up tight against the leading edge (short side). Measure parallel to the bottom edge the proper distance. If I recall the VG placement is 8% - 12% (?) of cord, in your case 8.3cm to the front edge of the front VG's and 12.5cm to the rear edge of the rear VG's. That's how we placed them on the Kitfox 4.

String 2 lines, use black sewing thread and tape in place. Make sure you do this inside and the string is very tight. Place your VG's in the proper Guide Paper and alternate every 4 VG's. Dont be afraid to press down on the VG's when you glue them and make sure its not too cold out either. 

Do a test flight prior and note your temperature, speeds, rpm's, climb rates, etc. Place the wing VG's first and fly the aircraft and note the new parameters. We are looking forward to the report. Tails section VG's; Describe the problem and why to you need them? I remain fully convinced on the 701 Tail VG's but dont think the Kitfox 4 needs them. Like mentioned in a previous post, it all comes down to CofG, Center of Lift on the wing, load distribution, engine weights, H-stabilizer settings, and the list goes on. In either case only do one upgrade at the time and note the differences. 

If you have any doubt with the enclosed please refer back to the original VG instructions for your aircraft, as "staggering" is an unproven installation of VG's. We can only second the experience as encountered by a previous Kitfox 4 owner for the staggered installation and neither discourage nor encourage any such attempts on similar or different airfoils/aircraft.   

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Posted

I'm also interested in vg's for my mk4 with speed wing (23'11.5") . I remember putting them on a Excalibur with clipped wing but don't recall the results . I wouldn't mind a little more speed . I'm at 95-100 mph indicated but like anyone I want more speed without braking the bank . Maybe vg's , wheel pants and that round cowl I could get a little more speed ?

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Posted

The round Kitfox cowl is far more draggy than the Avid Mk4 cowl.

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Posted

I can't speak for the speeding but when I installed them on my heavy heavier wing at 10% cord I believe it was too far back. I did make for a more gentle stall, meaning it was even softer than before but didn't see much difference in stall speed. Due to the wing I think it would have worked better at 8% instead. However I never got to test that theory. The other difference was that my wings had the extruded plastic leading edge in place changing the airfoil slightly. 

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