Curving it with Coanda

Nothing can clear a room faster than talking about helicopter aerodynamics. But certain concepts do seem to create lasting intrigue and can spark good conversation.

Sometimes you’ll find pilots using various hand gestures as a visual aid to assist them in describing theory and physics. If you’re really crafty, you can use a parlor trick or two to get your point across.

One of the more unique concepts that still tends to stir up interesting chatter is the Coanda effect, which describes the tendency of a moving stream of fluid to attach to and follow the curvature of a surface. It is a key component of the NOTAR anti-torque system found on MD Helicopters 520N, 600N and Explorer models (“NOTAR: More than What it Appears To Be,” R&WI, September 2012, page 26).

You might have seen the phenomenon highlighted in car commercials, as streams of smoke follow the contours of a vehicle in a wind tunnel. As it turns out, the curving effect has the added benefit of creating lift.

There are many ways to demonstrate this effect yourself.

Take a dollar bill. Pinch the short edge between your forefinger and middle finger, letting the bill drape down over the middle one. Now bring your fingers up to your lower lip in a fashion similar to smoking a cigarette so that the bill covers your chin. Gently blow a stream of air straight out over the top of the bill clasped in your fingers. The air blown over the top of the bill will actually curve downward, hugging the contour of the hanging bill. As this happens, the bill will rise up. Stop blowing and the bill falls again.

Another way to demonstrate the effect is to hold a spoon at the very tip of its handle, vertically downward. Turn on the kitchen sink and allow a small stream of water to run. If you slowly bring the backside of the spoon in contact with the water, the water will cling to the spoon, and you will also see and feel the spoon pull into the stream of water.

Now look at the tail boom on any NOTAR model helicopter. High-volume, low-pressure air is forced down the hollow inside of the circular tail boom by a transmission-driven, fenestron-type fan tucked up in its front end. In fact, original NOTAR testing used a fenestron taken from a Eurocopter Gazelle.

This air is vented out the right side of the tail boom through two slots that run its length. The vented air entrains or pulls the main rotor downwash in to follow the contour of the boom, keeping it attached down and around the boom further than normal — thus the Coanda effect.

Like the examples showed earlier, curving the airflow creates a suction (or lifting force) to the right, opposing up to 60% of the main rotor torque when hovering. The other 40% of anti-torque is created by venting the rest of the forced air out a rotating tail cone thruster at the very end of the tail boom. The pilot’s pedals control tail cone rotation, while also changing the pitch of the NOTAR fan blades to keep a constant air pressure inside the tail boom.

In forward flight, the rotor downwash no longer flows directly down over the tail boom. The Coanda effect diminishes, and anti-torque then primarily becomes the job of the vertical stabilizers.

While in service for many years, the NOTAR system provokes interesting conversations about how the Coanda effect works to produce lift. Whether you are supporters of Bernoulli’s principle or Newton’s laws, the physics behind the lift produced by the Coanda effect is ultimately the same.

Newton’s third law of action-reaction applies. The net forces that curve the air around the boom to one side generate reactive forces that pull the tail to the other. Bernoulli’s equations are actually derived from Newton’s laws and describe this net force by adding up the changing pressure distribution developed over the surface.

So demonstrate it and describe it any way you like. Just remember it’s much more effective to pronounce it properly. This avoids the possibility of it mistakenly being associated with that little gray marsupial that eats eucalyptus leaves. R&WI