#todo :: Update this, should be that induced drag is from downwash of wingtip vortices, not deflection of relative wind # Key Takeaways - Induced drag exists whenever lift is created. It is the portion of lift which is directed backwards. - Induced drag increases rapidly at slower airspeeds. - Ground effect reduces induced drag, making the airplane perform better. # Details ## What It Is There are two main forms of drag on an aircraft: [[Parasite Drag]] and [[Induced Drag]]. - **Parasite drag** is drag caused by the friction of air moving over the aircraft structure. - **Induced drag** is a byproduct of generating lift. It can be thought of as the rearward component of lift. Where parasite drag is most pronounced at faster speeds, induced drag is most pronounced as slow speeds. ## What Causes It An airstream flowing over the wing generates lift perpendicular to the relative airstream. When air passes around a wing generating lift, that relative airstream is deflected downward. (**Note**: Start video at 53 seconds.) <iframe src="https://commons.wikimedia.org/wiki/File:Flow_over_aerofoils.webm?embedplayer=yes" width="320" height="240" frameborder="0"></iframe> <cite>Holger Babinsky, University of Cambridge, CC BY-SA 3.0, via Wikimedia Commons</cite> That deflected airstream, or '[[Downwash]]', has the same effect as bending the lift vector rearward, creating a rearward lift component. This is induced drag. ![[Induced Drag Due to Downwash.jpeg]] A downwash exists anytime lift is generated, and so ***anytime lift is generated there is induced drag***. ## How It Impacts Flying ### Cruise Speeds At high speeds, induced drag drops significantly and is a negligible component of total drag. ![[Drag vs Speed.jpeg]] ### Slow Flight On the flip side, ***induced drag increases rapidly as airspeed decreases.*** From [[PHAK Ch5]]: > the lower the airspeed, the greater the AOA required to produce lift equal to the aircraft’s weight and, therefore, the greater induced drag. The amount of induced drag varies inversely with the square of the airspeed. This is why the aircraft requires so much power to maintain slow flight, and why it can actually fly faster with less power as you accelerate to a speed with less induced drag. ### Flight in Ground Effect [[Ground Effect]] changes the airflow of the downwash, since the hard runway surface does not allow the airflow to bend down as much. This means the lift vector does not tip backward as much, and there is less induced drag. ![[Difference in Induced Drag at Altitude vs Ground.jpeg]] This improved performance in ground effect is why it's important to rotate and climb out at the [[POH]] recommended speed during takeoff. Otherwise, once you leave ground effect, the performance loss may cause the airplane to settle back on the runway. Ground effect can also cause the airplane to keep floating as you approach to land -- especially if you approach with more airspeed than the POH recommends. ## How Wingtip Vortices Contribute Wingtip vortices are created by the higher pressure air below the wing rushing around the end of the wing to the lower pressure air above the wing. This creates a vortex. ![[Wingtip Vortices.jpeg]] Wingtip vortices increase downwash, therefore further tipping the lift vector backwards and increasing the rearward component of lift. This rearward component of lift is induced drag, meaning wingtip vortices increase induced drag. Wingtip vortex development is also interrupted in ground effect, further contributing to the reduction in induced drag near the ground. # Additional Resources - [[PHAK Ch5]] - [[Induced Drag and Ground Effect]] - [[The Secret of Flight 8 - Induced Drag]] ## FAA Definition ***Definition*** :: Drag caused by the same factors that produce lift; its amount varies inversely with airspeed. As airspeed decreases, the angle of attack must increase, in turn increasing induced drag. ***Source*** :: [[PHAK Glossary]] #glossary #concept