4 ms·
I would expect the jet stream not to end abruptly, but there is likely a gradient you have to cross before reaching "still" air. When leaving the stream, airspe
by MayeulC 8y ago
I would expect the jet stream not to end abruptly, but there is likely a gradient you have to cross before reaching "still" air. When leaving the stream, airspeed will increase, because of inertia, and so will drag. This will quickly bring the plane back to its cruising speed.
I had a debate with a friend on this one the other day, but I am pretty sure something like 80% of the engine power is there to combat drag, which increases with v² (if you discount lift from that, otherwise it' obviously 100%).
- aidenn0 8y agoWhen cruising, in level flight by definition, 100% of the engine power is there to combat drag, right? That is if the thrust vector of the engine is 100% level and the airspeed is constant, then 100% of the engine power applied is offset by drag.
- ars 8y agoDepends on how you define drag. If you define it as friction due to air against the plane then no, because a lot of the drag is due to sending air downward to keep the airplane up. The distinction is that, that that part of the drag is impossible to reduce, while friction you can work to reduce. That's what the GP meant by "discounting lift".
- aidenn0 8y agoThere are two types of drag, parasitic drag and lift-induced drag, but they are both drag.
- astrodust 8y agoSome of that thrust has to be used to generate lift.
- aidenn0 8y agoIf the engines are level, then none of the thrust is generating lift; the wings generate the lift (and with it some corresponding drag).
- marvin 8y agoThat's just semantics; you can split the drag generated by the wings and fuselage into different components. Of course, 100% of a constant-speed plane's thrust is necessarily used to counter all the other forces acting on the airplane :)
- astrodust 8y agoThe engines provide the energy to keep the plane flying. It must overcome two forces: Drag and gravity. The energy requirements are split between those two. If the plane is not generating lift then it's on the ground or about to end up in the ground.
- aidenn0 8y agoAfter some clarifications from replies to my sibling post, the answer is "definitely more than 50%" because the lift-induced versus parasitic drag curve is at its minimum when the two are equal, but the efficiency curve is a relatively flat-bottomed bathtub shape, so the cruising speed is selected to be faster than where this point is as you get significant reductions in travel time for relatively small reductions in total efficiency when you are near the 50% point and increase airspeed.