Originally Posted by me
Why not both?
Let's consider a straight (non-bypass) jet engine with thrust reverser. Let's assume that the reverser buckets can be set in four positions and deflect the exhaust gas stream without loss of momentum (hypothetically). In the first position the buckets are stowed around the jet pipe: forward thrust mode (zero deflection). In the second position the buckets intercept the exhaust gas stream and divide it in two 'plumes', deflected 60 degrees from the x-direction, still producing forward thrust, but less. In the third position the buckets deflect the gas stream 90 degrees from the x-direction for zero forward thrust. Finally, 120 degrees deflection from the x-direction results in negative thrust, i.e. drag, or thrust for power-back from stand-still. Then we could have for example the following thrust or drag forces:
Deflection (degrees) ........... 0 .... 60 .... 90 ... 120
Gross thrust ................. 100 ... 100 ... 100 ... 100
X-component of gross thrust .. 100 .... 50 ..... 0 ... -50
Ram drag ...................... 40 .... 40 .... 40 .... 40
Net thrust .................... 60 .... 10 ... -40 ... -90
regards,
HN39
P.S. Just in case you don't have your slide rule handy: cos 60 = 0.5 and cos 120 = -0.5
P.S.2 As said earlier, the ratio between gross thrust and ram drag changes with airspeed and power setting.