# Could a moving film of gas reduce aerodynamic drag?

Source: Wikipedia

In this image there is depicted a system which creates, using the Coandă effect, a film of gas traveling along the surface of a body. If this object was moving through air in the "up" direction on the image, could aerodynamic drag be reduced since the gas at the boundary layer of the body would have lower velocity than the free stream air (in relation to the body)?

• Related: Will it reduce lift in proportion? – mins Dec 30 '18 at 13:46

No, it doesn't reduce drag through reducing boundary layer speed, but it does create thrust by creating a region of low pressure in front of the vehicle (I mean technically, if the entire vehicle produces thrust, it does reduce vehicle drag, but in either case the reason why that happens isn't because the boundary layer speed is reduced). If you mount the device like in the image, then the low pressure zone will be created. The pressure at the bottom of the vehicle is higher than on the top, so it will generate forward thrust (well, in the orientation of the image, upwards thrust). This is because a jet of fluid will entrain it's surrounding fluid (aka "pull the fluid along with the jet"), which will create a region of low pressure around the jet. There actually are real-life implementations of this concept -

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However, the reason why this isn't used in real life is because if it's low efficiency - there's other ways to generate that thrust, such as using a propeller, a turbofan engine, etc, that don't require nearly as much fuel to produce the same thrust.

Only if you somehow manage to reduce friction. This could be achieved by using a gas with a significantly lower viscosity than air. As long as a sufficiently thick film is maintained, it will produce lower shear stress than an air boundary layer would while maintaining flow speed at its outside, so the air flowing around the object will not be subjected to shear.

While hydrogen has less than half the viscosity of air, its low density will make it disappear quickly (on the other hand, the needed mass of hydrogen per volume is unbeatable). A mixture of propane and ethane looks promising to me, but I cannot remember to have ever seen experimental evidence for the feasibility of this concept.

A similar concept is used by fish which are covered in a slimy film which reduces friction with water. Also, the supercavitating torpedoes successfully use a film of gas bubbles to reduce friction. So I see no obvious reason why your idea would not work. However, the gas needed to be carried along will eat into the possible payload, so this looks most promising for short range.

For the system to reduce drag, it has to eject air faster than the speed of flight. Much slower and it looks to the oncoming air like a wider body and that tends to increase drag, counteracting any low-pressure effect.

Coanda's original idea was to use a similar system to create thrust by inducing a flow of ambient air alongside. He built a plane to demonstrate the effect in 1910. Although it failed to generate the anticipated thrust, it has gone down in history as the first jet plane ever built.

Placing the exit half way back along the surface to re-energise the boundary layer after it has slowed down is less demanding on the exit velocity, but it requires a high volume of air. Such blown wings have been tried and it does work, but the complexity, weight, bulk and cost of the system mean it is not worth doing.

• You think of rocket-like thrust generation. The question clearly does not: "the gas at the boundary layer of the body would have lower velocity". – Peter Kämpf Oct 16 '20 at 15:36
• @PeterKämpf The question title bears slightly wider explanation, in my humble opinion. – Guy Inchbald Oct 16 '20 at 16:25