Question for Dave Robelen [Archive]

Chris

10-31-2001, 11:57 PM

Hi Dave,

I read the November articles and I must say, your Punkin II is just great. I will DEFINITELY be building one of these as soon as my latest Hawker Fury crashes ;)

One question though: I've noticed you use turbulator strips on most of your models. Can you expand on exactly what they do and how they benefit these types of models? I always thought laminar separation resulted in a wing stalling, yet these turbulators seem to cause this intentionally. Just a little confused.

Thanks.

Dave Robelen

11-01-2001, 05:35 PM

Hi Chris,
Very good question. There is a term "Reynolds number" which in our case is derived by multiplying the length (wing chord) X Velocity X air density. In the case of our models this number can be as low as 50-100 for an indoor endurance model, and as high as 1,00,000 for a 1/4 scale jobbie. This Reynolds number term describes the amount of energy in the boundary layer of air next to the surface of the vehicle. There is a range of values where the air just lacks the necessary energy to follow the airfoil curve without seperating from the surface completely. As it turns out the micro models that I have been designing usually fall in this range.
The application of a sharp edge spanwise disturbance near the leading edge disturbs (turbulates) the surface flow just enough to add energy to the main body of airflow and help it conform to the airfoil surface. There are a variety of situations where this approach is not helpful. First, the very thin curved airfoils as in the "Roadkill" series, the very low Reynlods no. indoor models, and models where the Reynolds no. is large enough for the flow to naturally follow the curve ( about 60,000-75-000).
Although I have only skimmed the surface, I hope this has been of some help.
Cheers, Dave Robelen