CofG

It's also essential to establish that the CofG of your aircraft is correct before installing VGs. 

And here we're talking about the actual loaded CofG as it is for flight right now, not just the empty aircraft, or calculations way back when it was built. 

I keep running across aircraft owners seeking to use VGs to correct defects in handling which are really due to CofG problems - some due to additions over time - some obviously could never have passed a proper weight and balance at any time!  (Auto engines need special attention...)

It's a trap, that most aircraft will fly fine under power even if the CofG is way out, but lose that power and reality bites - either can't get the nose up for a landing flare, or even worse, can't get the nose down in a stall .......

 

I know that very few aircraft owners ever do a weight and balance other than when it was built, but it sure is worthwhile exercise - now is a good time to do it!

 

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Instructions for Installing Feathers VGs on your aircraft.

 

First it’ll be necessary to apply the special adhesive to the VGs.

This adhesive comes as a thin sheet in a sandwich between two backings.  It’s a solvent-free, acrylic adhesive that won’t damage any paint surface.  Once firmly pressed onto a clean surface it bonds really well, but still has flexibility.  Sudden impacts will not knock it loose, but it can be peeled away by pulling steadily on the VG and sliding a razor blade under.  To remove any remaining adhesive from the surface, just rub with your thumb and ‘roll’ it away in a little rubbery ball.  It’s a perfect adhesive for this purpose!

 

First wipe the bases of all the VGs with methylated spirits, also called denatured alcohol or methyl hydrate.

With a sharp trimming knife, slice the adhesive sheets lengthwise into 25mm (1in.) wide strips.

Peel back the shiny-side backing of the strip as you place the VGs side-by-side across the strip, and press them firmly onto the adhesive.

 

 

With the sharp trimming knife, cut around each VG, right through the backing under the adhesive.  Those little, curved fingernail scissors are also ideal for this.

Tip – It’s a bit fiddly for heavy fingers, but young females are really adept at it – try to make a deal.......

Rub the backing firmly with a fingernail to press the adhesive onto the VG and expel any air bubbles. 

 

If you are mounting the VGs temperarily to find the best position, cut the adhesive in a 10mm (3/8") strip and stick the VGs across that strip, then just cup straight across between them - lots quicker and easier than cutting all around, and holds on the wing well enough for testing.

 

The ideal temperature for applying this adhesive to the wing surface is between 60°F(15°C) and 100°F(38°C).  If the temperature is below 50°F(10°C) the adhesive may not 'flow' properly for a good bond.  Use a heat gun or hair dryer to carefully warm both the wing surface and the VG.

 

 

Determining the correct position for the Feathers VGs

The VGs need to be positioned such that they have the best ‘bite’ at the airflow when the wing is at the stall angle of attack.  If they are too far aft they risk being buried in the thickening boundary layer and the start of separation, so they lose effectiveness at stall.  If they are too far forward they are reputed to slightly increase drag at cruise.  So, with the wing positioned at the stall angle of attack, the VGs should be slightly forward of the highest point of the airfoil.

 

Historically, the margin is considered to be 8-12% of wing chord back from the leading edge to the highest point of the VG, with most experience settling on 10% as the most appropriate.  There’s lots of talk around the hangar fliers of finding the ‘sweet spot’, but my experience doesn’t show any dramatic difference at any one point.  My testing indicates that 12% is too far back, so that at a very high angle of attack the VGs can become ineffective.  From 7-10% I could detect no difference in stall performance at all, and couldn’t measure any change in drag at cruise speed. 

 

Those figures are to the highest point of the VG, but since it's easiest to measure to the front tips of the VGs, I recommend starting with the tips at 7% of chord aft of the leading edge - this puts the highest point of the VG at about 9%.  We use the measurement to the tips of the VGs to mark the line on the wing because it's easier to place the VGs accurately to this line.  If you try other positions and get significant results with your aircraft, please let us know so that others can benefit.

 

There are specific measurements for the Zenith 701, 801, and 601 aircraft on their respective pages in the main menu.

 

 

  

Preparing the wings.

Wash the wing thoroughly with detergent (not one of those automotive ‘wash-and wax’ products).

Measure the chord of the wing from the front of the leading edge to the aft edge of the ailerons or flaps.  In the case of detached flaperons, measure to the aft edge of the flaperons. 

If the wing is tapered then measure both at the root and the tip. 

Tip - To get a vertical reference on the rounded leading edge, use a carpenter’s level, or even handier, sight to the vertical lines in the steel of most hangar walls, or to a vertical post in the background. 

Calculate 7% of each the those measurements. 

Measuring from the leading edge, mark those 7% points on masking tape at the ends of each wing.  

Wipe a zone along that line with the denatured alcolhol.

Stretch a chalk line over those marks and ‘snap’ a line onto the wing.

Placing the VGs on the Wing

Starting 50mm (2”) from each wing tip:

Remove the backing from the adhesive on a VG.

Set the first VG in a notch of the 60mm template with the nose angled out toward the wingtip. 

Carefully touch the very tip of the VG to the wing surface right behind the line. 

Guided by the angle of the template, lightly set the base of the VG onto the wing surface.

 

 

Double check the  alignment, and if correct, press the VG firmly onto the surface.

Rub each side of the base firmly with thumbnail or a popsicle stick to press out any air bubbles.

With the template secured by the first VG, use the remaining notches to set two more VGs in place.

 

 

Move the template so that the end notch is secured by the last VG already set, and set two more.

Continue to place 15 VGs at the 60mm spacing  =  900mm (3ft) from the wingtip.

(This closer spacing near the wingtip is to ensure that the tip does not stall first.)

 

Do the remaining portion of the wing with the 90mm spacing template.

 

120 VGs is enough to do 9.9 metres (32 ft) of wing at this spacing.

 

If a VG happens to land on a rivet or a seam, displace it just enough to miss the obstacle, then continue the regular spacing.

 

 

Placing VGs on the Horizontal Stabilizer

Mark a line on the underside of the horizontal stabilizer, 100mm (4”) in front of the gap between the stab and the elevator.

Using the 30mm spacing template, set a row of VGs along that line, starting from the outside.

 

If the surface of the horizontal stabilizer is flat, the curved base of the VG will contact only at he tip and the heel.  Rub the VG on a piece of sandpaper laid on a flat surface to adjust.  Don't try to completely flatten the base - just make flat areas at the tip and heel, the adhesive will do the rest.

 

 

For a Wing with a swept-back leading edge

Since the VGs need to be oriented with respect to the airflow rather than the leading edge, a slightly different procedure is needed with a swept-back leading edge.  The VGs are effective at angles from 10º to 20º, so setting them at 15º allows for best margin of variable airflow.  There will be very little spanwise flow this far forward, so we can orient the VGs to a reference line at right angles to the centre line of the aircraft.

 

Snap a chalk  line 7% back and parallel to the leading edge as before.

Snap a couple of spanwise reference lines at right angles to the aircraft centre line.

 

Cut the templates completely across at the end slots.

  

 

 

With the VG in the centre slot, and the VG tip on the 7% line, orient the template so that it is parallel to the spanwise reference line rather than the leading edge, with one end of the template against the fin of the last VG already placed.

 

Flip the template over, end for end each time, to match the angle of the previous VG.

 

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Painting Feathers VGs

If you wish to paint your Feathers, it is best done before they are installed on the wing and before the adhesive is applied to them.  Even on a new aircraft it’s much better to first paint the wing without the VGs on, paint the VGs separately, then stick them on.

 

Staple a strip of masking tape, sticky side up, onto a length of wood.

 

Place the VGs inline along this tape, so that you can paint both sides easily.

 

Keep in mind that the Feathers are flexible, so you won’t want a thick layer of brittle two-pack paint.  Keep the paint layer thin, and without hardener if possible.

 

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Certified Aircraft

Please note that Feathers VGs are not approved for certified aircraft. 

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Note – Be very careful when refuelling wing tanks –

Polycarbonate is very tough stuff, but fuel destroys it!

 

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