Why wouldn't some blown flap or blown wing device (suppressing propeller or fan or jet exhaust from aircraft design) produce sufficient thrust needed for takeoff and cruise flight, opposed to "only" providing stol capabilities at low speed?
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2 Answers
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In a nutshell, it's the inefficiency of expelling air out a long skinny nozzle which results in massive energy losses due to friction. In other words, too much duct surface area relative to the amount of total air volume.
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In the top diagram the blown air is not curved downward, which is what lift is. Digging deeper into the subject, it seems for it to work in cruise, the trailing edge needs to be blunt and the blowing to be internal.
The blunt trailing edge concept allows a direct control of the flow direction leaving the airfoil trailing edge and hence, circulation control. Unfortunately, one needs continuous blowing during cruise with a blunt trailing edge, in order to keep cruise drag low.
(Fundamentals in Coanda Flap Design)
The internal source for a blown wing on a jet transport would typically be from engine bleed, but engine bleed is not cheap. With all anti-ice devices turned on in-flight, it is already flight-level-limiting unless the plane is light, for example. The solution would be more powerful engines, but fuel consumption and weight go up with thrust.
The added step of blowing inherently reduces efficiency. Blown flaps make sense in takeoffs and landings because typically there is more thrust than needed, plus the benefit of the thicker atmosphere.
