![]() ![]() This situation involving laminar flow would often be examined using Bernoulli’s equation, but this equation cannot capture the limits of laminar flow and onset of turbulence at high Reynolds number. The limit will also vary with attack angle at high attack angle, the amount of drag acting on an aircraft could suddenly decrease as flow separation occurs and turbulent flow begins. This just happens to be the critical Reynolds number for a flat plate, so we would expect a similar limit for a curved airfoil. turbulent flow varies for different aircraft, but the typical limit should be approximately when the Reynolds number reaches 500,000. When this occurs, drag suddenly increases and greater thrust is required to maintain the speed of the aircraft. Just like in other systems involving fluid flow, the flow across an airfoil will eventually break and become turbulent. lift), a wing that generates excessive turbulent flow may need to be redesigned to allow faster cruising velocity without creating excessive drag due to onset of turbulence. Depending on the required velocity, attack angle, and efficiency (drag vs. This all means that, for two different wing designs, one wing may allow laminar flow while the other allows turbulent flow, even though the fluid flow parameters (flow rate, density, length scale) are the same for both systems. Ensuring laminar flow avoids this problem and the increased turbulence acting on the aircraft. The drag force eventually recovers and drag begins increasing as the craft’s speed continues increasing. This is quantified using the Reynolds number, as discussed below.Īvoiding the drag crisis: As the transition between laminar and turbulent flow occurs, a phenomenon known as the drag crisis causes a momentary drop in drag coefficient and drag force. Higher speed: If the limit on laminar flow is higher, then the aircraft can generally sustain a higher top speed without turbulence than an aircraft with a lower laminar flow limit. As a result, the aircraft may be limited in its top speed due to a drastic increase in drag during flight. Why is laminar flow preferred during flight? The answer relates to two of the four primary aerodynamic forces acting on an airfoil and the aircraft as a whole:ĭrag in the boundary layer: Once the flow crosses into the turbulent regime, fluid flow in the boundary layer becomes turbulent and creates additional drag during flight. Regardless, we would prefer the flow rate across the aircraft to always be laminar. What exactly constitutes a “low” Reynolds number depends on the shape and roughness of the wing, as is briefly discussed below. At low Reynolds number flows, the airflow is laminar and sets up a boundary layer along the surface of the wing. Thrust exerted by the aircraft can create additional lift and drag due to skin friction along the surface of the craft, and particularly along the airfoil. Why Laminar Flow Over Airfoils?ĭuring flight, airflow across the wing of an aircraft creates drag and lift. ![]() We’ll discuss these aspects of laminar flow over airfoils and examine the limits of laminar flow across airfoils in this article. The reasons for this are varied, but they relate to the main aerodynamic forces acting on an aircraft, and particularly on the airfoil itself. Together, these are the main determining factors used to design aircraft to reach engineering and performance goals once an aircraft is put into operation.ĭuring flight, an aircraft would prefer to encounter laminar flow. The flow regime is important in aircraft design and engineering, as it determines the amount of lift and drag acting on an airfoil. On an aircraft, an airfoil is the structure responsible for moderating lift and drag created by oncoming airflow. Typical laminar-to-turbulent flow transitions over airfoils occur at Reynolds numbers of approximately 500,000.Īir traffic wouldn’t be possible without laminar flow and airfoils. When airflow transitions to turbulent at high Reynolds numbers, the result is greater drag. Airflow over airfoils generates lift and drag, and the amount of lift depends on the flow regime.
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