Hmm, a question that might need a book.

Many airfoils develop an unstable flow over the upper surface "just before" that part of the wing stalls. That means for a start that we are dealing with low speed airfoils, or a airfoil operating at the very low point in its effective range.

Very often this instability is accompanied by a "bubble" with reverse flow on the wing surface usually somewheres just back of the high point.

This process is caused by the boundary layer slipping off the wing surface and then re-attaching itself. (If it does not re-attach then you have a full-blooded stall).

The idea of the turbulators is to make the boundary layer turbulent (hence the name). This makes it "sticky" and prevents the partial stall/bubble condition.

In some instances, turbulators have been used to actually generate the surface bubble.



OK having said that we obviously have two different groups.

The first group (preventing b/l detachment) are most frequently found in the front 20% of the wing. The most common on models are thread and "trip tape". The most effective are the "zigzag strip" but note that these are specialised and require some care in building.

The second group can be multi spar wings, step at the high point (tried extensively on ffhlg), and multi strip turbulators. Usually these are found between 30% and 50% C.


How to make?

Strip turbulators - generally less than .5mm high and less than 3mm wide.

Threads - anything from fine terelene thread to knitting wool (dope it on). Personally I dont like these.

The zigzag strip I have tried and it works but it is a B*** to set up. Made mine from 160gm paper for a 130mm chord. It was about 3/8" wide and the zigzag was about 1/4". Glue it on straight edge forward, zigzag back. Leave to dry thoroghly then carefully sand a taper into the paper so that it is wedge shaped. The zigzag edge should finish in a step.

<e> BTW take a look at the series of airfoils that Lissaman developed for Gossamer Condor. The upper surface is actually concave (not a reflexed airfoil). I understand that this feature was developed to encourage and capture the vortex bubble. Made these airfoils effective in a very narrow Reynolds No. and speed range.