How a Plane Works · Chapter 04
Why does every aircraft seem to have one natural speed? Why do airliners all cruise within a few percent of each other, and why did going faster than sound demand a different shape of machine entirely? The answers live in one U-shaped curve — the sum of two drags that pull in opposite directions.
· THE TWO DRAGS
Drag comes in two currencies. Parasite drag is the cost of shoving a body through air — skin friction and pressure — and it grows with the square of speed: fly twice as fast, fight four times the force. Induced drag is stranger: it is the bill for lift itself. At low speed the wing must work at a steep angle, tilting its lift backwards and stirring wingtip vortices — so this drag is worst when flying slowly and fades as speed rises. One cost rising, one falling: their sum has a bottom.
TAKEAWAYParasite drag rises with speed, induced drag falls — their sum is a U with one best speed at the bottom.
· BEST SPEED
The bottom of that U is where each kilometre costs the least effort — and nearly everything about an aircraft’s mission is tuned around it. A glider, built to spend nothing, has long thin wings that shrink induced drag and a best speed barely above a bicycle’s. An airliner’s U bottoms out near 900 km/h at altitude, which is why every airline flight you have taken cruised at nearly the same speed.
TAKEAWAYEvery aircraft is tuned around the bottom of its own U — that is its one natural speed.
· ALTITUDE
High up, the air is a fraction of its sea-level density, so parasite drag collapses — the same reason chapter 2’s jet engines love altitude while breathing it. That is the whole economics of the cruise at 11 km: thin air to slip through, cold air for the engine cycle. But thin air also lifts less, so the stall speed climbs with altitude while the maximum speed falls — the two edges of flight creep toward each other until, at the aircraft’s ceiling, they nearly touch.
DEMOThe turbofanFly it across Mach and altitude and watch its envelope.TAKEAWAYThin air is cheap to fly through but poor to lift with — altitude squeezes flight between stall and thrust.
· THE ENVELOPE
Plot every combination of speed and altitude an aircraft can sustain and you get a closed shape: the flight envelope. Stall fences the left edge, engine thrust and structural limits the right, the ceiling on top. “Pushing the envelope” is test-pilot language escaped into the wild. Supersonic flight redraws the map entirely — past the speed of sound the air stops politely parting and piles into shock waves, and only machines shaped for that world live there comfortably.
READConcordeThe airliner that lived past Mach 2.TAKEAWAYStall, thrust and ceiling fence in a closed map of speed and altitude — every aircraft lives inside its envelope.
FOR THE SELF-CHECK