Last Updated : 14 February, 2007
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The following question came from Steve Kerry "" )

Best airfoil for parkflyers/ Jedelsky Wing?

    Would anyone care to venture an opinion on the importance of Re at such low numbers? Slowfly and parkfly is a pretty extreme case, is an accurate airfoil more critical or less?

From : Don Stackhouse

That depends on what you're trying to do. If you have any specific performance and/or handling goals in mind, in my experience it's MORE critical.

BTW, for those of you not familiar with Reynolds number, it's simply a numerical measurement of what we modelers often refer to as "scale effect". It quantifies how large the aircraft appears to be from the point of view of the air molecules, and vice-versa. Air molecules that look like ping-pong (table tennis) balls to a large airplane might look more like soccer balls or basketballs to a small model. The job of tunneling your way through a pile of ping-pong balls is different than tunneling through basketballs, and requires some changes in your strategy.

Numerically, Reynolds number is the airspeed times some characteristic dimension (chord for a wing, diameter in the case of flow through a pipe) times the air density, divided by the air's viscosity. At sea level standard-day conditions, multiply 778 times the speed in mph times the chord in inches to find Reynolds number.

At more typical higher Reynolds numbers ("Re"), the boundary layer airflow (that thin layer right next to the surface where the air is transitioning from the speed of the aircraft's surface to the speed of the freestream flow) likes to transition from laminar (flowing in smooth layers, with little or no mixing between them) to turbulent (swirling and churning within the boundary layer, with lots of mixing going on). Turbulent is thicker and has more drag than laminar flow, which is why full-scale sailplanes go to so much trouble to preserve laminar flow as much as possible.

However, at low Re's, the boundary layer flow would rather be laminar. In fact, even if you add turbulators to try to force it to become turbulent, it will try to return to laminar flow. And this is where the great drawback of laminar flow raises its ugly head.

Laminar flow does not stay attached very well, not nearly as well as turbulent flow does. After the highest point on the upper surface, where the airflow begins to slow down and the pressure starts to rise, the air sees what's called an "adverse pressure gradient". In other words, the static pressure in the region the air is flowing into is higher than in the region the airflow is in right now. In effect, it's like water trying to flow uphill. All the airflow has available to fight its way into this rising static pressure is its own kinetic energy, and it's continuously losing that through skin friction. If it's laminar, then it isn't getting any fresh kinetic energy from the faster layers above it, so the closer to the trailing edge it gets, the weaker it gets. If it gets weak enough (i.e.: not enough kinetic energy to fight the rising static pressure any more), it can no longer follow the surface. The airflow separates, causing a loss of lift and a big increase in drag.

You can try to turbulate it, so the weakening layers near the surface get infused with fresh energy from the layers above (which is why a turbulent boundary layer can stay attached better). However, if you turbulate it in the wrong place, or try to impose too great an adverse pressure gradient on it, it may still separate.

The same conditions can exist on the lower surface at low angles of attack, especially on undercambered airfoils, beginning usually near the leading edge of the undercambered portion. The resulting drag at low angles of attack will brutally murder any hopes of having decent high speed performance. On a sailplane, which needs to be able to penetrate and launch at higher speeds, and on any model that is expected to be able to fly fast (such as while penetrating into the wind, or to carry enough energy to make it over the top of a loop, etc.), this can be a killer. OTOH, if you WANT a model that has a built-in "speed limit" for some reason, you might want to deliberately design for this condition on the lower surface above some particular speed.

The problem is that the operating condition and manner in which these separations occur is very sensitive to each individual model design. You can't really pick a "best airfoil" outside of the context of a specific location (the conditions and operating requirements at a wingtip can be VERY different from the conditions at the wing root) on a specific model design with a specific mission profile and operating envelope. If you are not VERY careful, you could have all sorts of quirky localized separations popping up at various points on the airplane at various operating conditions, causing quirky trim changes and inconsistent handling, or messing up performance.

In my experience, this becomes an increasingly worse issue as the Re gets lower. By the time you get to the Re's typical of indoor models and Mosquito-class RCHLG's ( typically around 15K to 60K for most operations), there is often a narrow window of thicknesses and cambers where you can get good performance and handling, especially if the model has to fly at more than one airspeed. Too much OR too little camber and thickness, and/or the wrong distribution of each of those along the airfoil, can result in a model that just doesn't do what it's supposed to. It reminds me of the infamous "coffin corner" the Lockheed U-2 pilots have to deal with at high altitudes; there's a very narrow range (4 knots of airspeed in the case of the U-2, a percent or so of the optimum cambers and thicknesses for our low-Re model designs) you have to stay within. Get outside of that range, either too much or too little, and the situation can get very ugly, very fast. And don't forget, the airfoil that works well on one particular model might not work if you scale that model up or down in a little in size, especially at very low Re's.

Of course you can always just use your shoe shape for the airfoil, and stuff a bigger motor and more batteries into the model until it HAS TO FLY. Anything will fly if you put a big enough motor on it. However, most of the folks I know of who are into electric flight appreciate designs that use a little more finesse than that!

Don Stackhouse
DJ Aerotech

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