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LAMINAR FLOW, BY ACCIDENT!
During the 1930’s a self-taught aerodynamicist named David R. Davis went to the trouble of patenting an airfoil design, which he called the “Fluid Foil” (US Patent #1,942,688). He considered his design special because it exhibited lower drag than most other common airfoils, but he wasn’t sure why. Fortunately for Mr. Davis, the Consolidated Aircraft Company was looking for a marketing trick to make their new aircraft stand out from the competition; a unique low-drag wing was just the ticket. After verifying the low-drag performance of the Fluid Foil, Consolidated licensed the airfoil patent from Mr. Davis in 1937. Fluid Foil eventually found its way into the wing design of the B-24 Liberator bomber during WW2. (Ref: Vincenti, 1990)
Without knowing it, Mr. Davis had inadvertently invented the first airfoil to achieve low-drag through encouragement of a laminar boundary layer; the rarely seen smooth airflow that briefly exists before the higher-drag, turbulent boundary layer takes over.
Consolidated Aircraft went on to build over 19,000 of the B-24 bombers, putting it ahead of even the venerable B-17 in production count. Although many people consider the P-51 Mustang to be the first aircraft to use laminar flow airfoils, the truth is that the B-24 was the first, albeit accidental, aircraft to use laminar flow airfoils. The true significance of the P-51 wing was that it was the first to intentionally use the new scientifically developed NACA 6-series laminar flow airfoils.
As interesting as these historical facts are, it’s even more amazing to learn that neither of these airfoils actually produced much usable laminar flow when integrated on real world aircraft. In fact, they were just as turbulent as every other plain vanilla airfoil out there. We can forgive the designers though because all of their data came from finely polished wind tunnel models built to exacting contours. When they tried to reproduce the same contour and finish with sheet metal, rivets, bucking bars, hammers, and a War raging, it just did not function. Add mosquito guts to the wing leading edge and they had little chance of establishing much of a laminar boundary layer at all.
I don’t mean to downplay the development of laminar flow airfoils on metal aircraft. Analytically, it was a significant leap for the early engineers to “backward” solve the formulas used for analyzing airfoils. Rather than taking surface coordinates and calculating the resulting pressure distribution, the NACA personnel figured out how to take the desired pressure distribution and back out the airfoil coordinates!
Due to modern day construction methods, stiff composite materials, and improved laminar flow airfoil designs, there is renewed interest in the use of laminar flow airfoils in general aviation. Most modern racing aircraft (such as the decambered NASA NLF used on Nemesis NXT) use some type of laminar flow airfoils; often modified for proprietary purposes.
Laminar Or Not?
It may surprise you to learn that all airfoils have some laminar flow; even the airfoil used on the Wright Flyer. Granted, the laminar flow only lasted a few percent of the chord length for the Wrights, but there was some laminar flow. This brings up the question of how to classify airfoils. Laminar or not laminar? As it turns out, the answer is subjective.
Generally speaking, for an airfoil to be considered a laminar flow airfoil, it must have a favorable pressure gradient that extends past 30% of the chord length. Laminar boundary layers are sensitive beasts and prefer to have the surface pressure continuously dropping as they march downstream from the leading edge. When the surface pressure stops dropping and begins to increase, the smooth laminar flow becomes turbulent, fighting its way all the way back to the trailing edge. It’s easier to fall down a hill than walk back up it again.
Figure 2 shows a series of very common airfoils and how much of their chord length will experience favorable pressure gradients (i.e. laminar boundary layers) under ideal conditions. It is important to understand that extensive laminar flow is usually only experienced over a very small range of angles-of-attack, on the order of 4 to 6 degrees. Once you break out of that optimal angle range, the drag increases by as much as 40% depending on the airfoil.
Look closely at the airfoils in Figure 2. The laminar designs exhibit extensive laminar flow (past 50%). They generally have sharper noses which can result in a more unpredictable and sharp stall. However, the most obvious trait is the rearward placement of the maximum thickness. If you look at a wing edge-on and notice that the maximum thickness is far back, you can bet that the airfoil is a laminar flow airfoil. I recommend that you look at a Piper Tomahawk wing edge-on; you’ll discover right away that it uses a GAW airfoil.
The Quest for Low Drag
To understand where the great yearning for laminar flow airfoils comes from, we need an experiment. Imagine that we point a sheet of plywood into a 70 mph air stream with no angle between the chord length and the relative wind (zero degrees angle of attack). If we could magically force the boundary layer to stay 100 % laminar from leading edge to trailing edge, the frictional drag force would be roughly 0.6 pounds (.3 Kg). Now, if we flipped a switch to make the boundary layer completely turbulent, the frictional drag force would jump to almost 3 pounds (1.4 Kg), a net rise in drag of nearly 500%.
As we can see from our plywood airfoil example, laminar boundary layers result in much less friction to a wing surface than turbulent boundary layers. Remember that real world wings have a mixture of laminar and turbulent boundary layers so the actual gains are on the order of 40 to 50 percent. The ultimate goal of a laminar flow airfoil is one where we try to maximize the laminar boundary layer while minimizing the turbulent boundary layer without making the whole thing too overly sensitive to surface finish.
Consider the builder’s ability to control the wing contour during construction and flight. The surfaces of metal airplanes tend to “oilcan” during flight and this can change the contour enough to trip the boundary layer.
When using composites, it’s important to keep close tolerances on the airfoil contour. Contour control of a surface isn’t just a step-height allowance; it depends on the chord length that it occurs over. Aluminum? Forget about it.
Speaking of surface finish, I’ve heard stories of sailplane flyers actually scuffing the gloss off their wings chord wise from leading edge to trailing edge with 600-grit sandpaper. If roughing up a surface reduces drag, that typically means that the boundary layer was blowing off prematurely or had laminar bubble issues; roughing up the surface helps both of those situations (in some ways, it is similar to why dimples reduce drag on a golf ball). Those problems are usually only experienced at very low Reynolds Numbers (small chord wings flying at either slow speeds or high altitudes).
Stories of Indy racecar guys rubbing baby powder on their cars to make it more “slippery” has been circulating in the pits for years. It may have been smoother to their fingertips, but not to the air molecules! Traditionally, a very smooth, clean, and highly polished surface will always result in lower drag numbers. Wax it, don’t powder it!
Designing the Perfect Airfoil
You may be thinking the same thing that NACA engineer Eastman Jacobs thought back in the 1930’s. Why not design airfoils that only produce laminar boundary layers? That way, you could have ultra low wing drag! Let’s take a look at the numbers.
We can quantify the reduction in drag due to laminar boundary layer development. Figure 3 shows the reason why engineers have chased after laminar flow airfoils for so long. This graph compares the drag polars of two airfoils. One is for a typical airfoil (NACA 2415) and the other is for a laminar airfoil (66-415). For the latter airfoil, we see that the drag coefficient drops noticeably between a lift coefficient of roughly 0.25 and 0.5. Your goal, as a designer, is to make sure that your desired lift coefficient falls somewhere in that drag bucket. (See arrow in Figure 3)
Let’s briefly recall what a boundary layer is from Part #4 of the Airfoil Primer. Even the smoothest surface looks like a mountain range when viewed on a microscopic scale. As air flows past these surfaces, some of the molecules that try to maneuver through the miniscule peaks get stuck and donate their energy to the mountains themselves. These molecules of air that were originally moving with the speed of the oncoming air flow are halted and brought to zero velocity right at the surface! In engineering, this is called the “no-slip” condition. On a larger scale this effect is felt as a friction force tugging at the wing surface.
We can break it down even further. When the boundary layer begins forming at the leading edge, it is flowing smoothly with each microscopic layer of air flowing easily over the other like a deck of playing cards sliding over one another. This portion of the boundary layer produces very little drag force, but unfortunately it only lasts until the air racing across the airfoil begins slowing down. With non-laminar airfoils, this typically happens within five to twenty percent of the chord length. At that point, the laminar boundary layer will begin mixing with outside air and becoming filled with small eddies. These so-called turbulent boundary layers are usually quite stable, but produce higher drag than the laminar boundary layers do.
Prior to now, you’ve learned that all laminar boundary layers grow up to become turbulent boundary layers. When operating at very low Reynolds Numbers (less than 300,000 for example), this transition to turbulent sometimes does not occur. The boundary layer occasionally explodes away from the surface never to be heard from again. Sometimes it immediately reconnects forming a much thicker turbulent boundary layer than normal. The region between the laminar separation and the turbulent reconnection points look like a bubble and is often called a Laminar Bubble. If the laminar bubble fails to reconnect, the boundary layer leaves the airfoil at that point and the wing flies around in a semi-stalled condition. This is very bad. There have been a few rare cases where airfoils utilizing extreme laminar flow have been so sensitive that even raindrops caused the boundary layer to become unstable and blow off the surface causing a stall.
You may have seen radio controlled airplanes with zigzag tape on the upper surface of the wing to combat these low Reynolds Number problems. Those pilots are taking matters into their own hands and forcing that sensitive laminar flow to trip itself into a turbulent boundary layer before separating. After all, a draggy turbulent layer is better than separation and stall. Some folks have used this trick to get their radio-controlled airplanes to carry more weight than normal during cargo-carrying contests.
Luckily, this tendency to go from laminar directly to separated occurs less often as the Reynolds number is increased.
Key points to remember about boundary layer development:
The Final Laminar Twist
In yet another plot twist regarding laminar flow airfoils on metal aircraft, they turned out to be excellent performers for high-speed aircraft. High-speed as in jet-aircraft. And it had nothing to do with laminar boundary layers; rather it was a function of moving the minimum pressure location significantly behind the leading edge. This resulted in an increased critical Mach number, which allowed jet-fighters to go a little bit faster by minimizing supersonic drag over the wings (even a subsonic airplane can experience pockets of supersonic airflow on top of the wing due to local accelerations).
You probably don’t have a jet engine though. So how can you make good use of laminar flow airfoils? First of all, if you’re building a sheet metal wing and won’t be flying past Mach 0.6 (about 450 mph), then don’t bother with extreme laminar flow airfoils. Conventional NACA airfoils will work just fine for your purposes. Van’s Aircraft has used the NACA 5-digit series very effectively on their RV models.
However, if you are building a stiff composite wing, you may want to use a NACA 6-series or one of the more modern NASA natural laminar flow airfoils. Just be sure to keep those leading edges clean.
The next time you visit Oshkosh, Sun-N-Fun, or the Reno National Championship Air Races, look at the wings edge-on and try to guess if they are using a laminar flow airfoil. Ask the pilot about it; they will appreciate that you noticed.
That's the end of the Dreese Airfoil Primer. Let me know if it has helped you. Or if you have questions, feel free to contact me .
Note: All aerodynamic data and profiles were produced by DesignFOIL
About John Dreese:
John Dreese is a graduate of the Ohio State University, having earned both a Bachelors and Masters Degree in Aeronautical Engineering there during the 1990's. An aerodynamicist by training, John has spent over a decade working with all aspects of wind tunnel testing. Since 1996, he has been developing airfoil geometry generation & analysis software. After completing his Masters Degree, he worked in the exciting world of experimental aviation. Since then, he's worked on many interesting projects. John is a licensed pilot, a member of the EAA, and flies in brutal RC combat competition.