|Oct 08, 2012, 09:46 PM|
Taming the beast - How to use turbulators
A while ago I built a Windrider/HK Mig-3. A wonderful bird to look at but it turned out to be a beast in the air. When flying at lower speeds it did just fall out of the air - tip stall without any warning, its name was Mig-3.
After some crashes and rebuilds I gave the Mig-3 to a colleague and forgot about it.
Well, almost - when I stumbled over a HK Mig-3 on eBay I know I had to try again and this time make it fly, and fly well.
Isolate the issue
The Migs tip stall issue comes from several causes.
One is the wing geometry, a wide wing in the center and quite small tips. While scale in appearance (and true to it - the original Mig had quite unpleasant stall characteristics) its always cause for alarm. Its problem is that on such a wing the tip tend to stall before the root. Several things can be done to prevent this from happening:
- reshape the wing - gives good results as the VQ Mig-3 shows but impossible to change later for an ARF bird (and besides it does not look scale anymore!)
- geometric twist, decrease incidence of the outboard leading edge by some degree - usuall works fine but also impossible to change later as well. The Mig-3 has no such increased incidence at the tips.
What is possible later is to raise both of the ailerons a few degrees. This reduces lift but also the tendency for a stall however only slightly.
- different airfoils tip / root. Likewise impossible to change for a ready-made plane.
Well, that leaves one more thing: Tubulators
Tubulators are devices (strips of tape/plastic) that are placed on the top of a wing causing the air to become turbulent. Why that? - because turbulent air has enough energy to keep attached to the surface of the wing while laminar flow does not. Laminar flow likes to separate and this causes the dreaded stall. The Mig also has a rather smooth wing surface and this cries out load: Stall!
Where to place a turbulator? At the front of the wing the airflow is still attached. Usually at stall conditions separation starts to occur at 30-50% of the depth (depending on airfoil and Reynold numbers) and the turbulator should be placed before that point (because after the separation it has no effect). So a good measure is about 20-40%. I decided to place it at 30% depth. Also only cover the outer aeras of the wing, in particular the region of the ailerons - you still want a stall to occur at the root. It only has to happen before the tip stalls!
What to use as turbulator? At this 30% point a turbulator should have a height of about 0.5mm (or even a tad higher). For testing use the thickest, ugliest, coarse tape you can find, the stuff you would never touch for any serious painting work and make a test first to make sure it does not glue to the paint job and rips it away on removal!
A better and nicer solution is a tailor made zigzag tape. Graupner sells a great zigzag-shape tape matching the needed height that fits perfectly.
Did it help? Boy - its a new airplane now! All the bad tip stall issues are gone. On stall it just tips the nose a bit and stabilizes again. Despite its weight (1.35kg, electric retract and door sequencer build-in) it now handles like a darling, not the beast it was before. Top speed seems to be a little lower (I guess due to higher drag) or so I believe - but I trade that in any day for better flight characteristics and improved low speed handling.
If you have an airplane that tip stalls give it a try. Its quick to apply and test and can make quite some difference.
Also check out for more background info: http://www.mh-aerotools.de/airfoils/
|Oct 08, 2012, 10:29 PM|
Quote: - geometric twist, increase incidence of the outboard leading edge by some degree - usuall works fine but also impossible to change later as well. The Mig-3 has no such increased incidence at the tips.
I think a decrease in incidence is the thing that usually works
Turbulators work well, no doubts about that.
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