What If Humans Could Fly?

The disk is doing a lot of work

Treat the wingspan as the diameter of an actuator disk. Hover power is then (mg)^{3/2} / sqrt(2 ρ A). A 70 kg person with a 10 m span already owes the air on the order of a kilowatt to hover — more than a human body can sustain. Stretch the span toward 30 m and the bill drops into the few-hundred-watt region where elite legs live. That is why the successful human-powered aircraft look like sailplanes that went wrong, not like angels.

Birds are not scaled people

A pigeon has a power-to-weight ratio and a wing loading that do not survive linear scaling to 70 kg. Muscle fraction, metabolic rate and the Reynolds number of the wing all move. Flapping at human size also wants a chest that we do not have. The comparison on this page is a power ratio, not an evolutionary pathway. If g were lower, the same formula would get kinder; the doubled-gravity experiment is the rude version of that remark.

Cruise is kinder than hover, still not free

Once you can exchange altitude for speed and use a high lift-to-drag wing, power can fall below the hover figure. That is the Gossamer-style loophole: do not hover, glide with a whisper of thrust. This model still shows hover power because it is the realistic 'stay there' number. It is not a flight-manual for an aircraft, and it will not certify a pair of backpack wings. Borderline is the right word; guaranteed is not.

Worked example: the aircraft that crossed the Channel

On 12 June 1979 the Gossamer Albatross, pedalled by Bryan Allen, flew 36.2 km from Folkestone to Cap Gris-Nez in 2 hours 49 minutes. It is the best real test of this page’s maths.

1. Its wingspan was 28.6 m and its take-off weight, pilot included, was 97.5 kg.
2. Put those into the hover formula on this page and it would need about 745 watts just to hang in the air.
3. In cruise, gliding forward on those long wings, Allen supplied about a third of a horsepower: roughly 250 watts, for nearly three hours.
4. Now try a 70 kg person with 10 m wings: hovering would take about 1,300 watts, more than four times the 280 W a trained person can keep up for long.

Human-powered flight is possible, but only with very long, very light wings, flying forward rather than hovering, and pedalled with the legs. Flapping your arms is not close.

What people get wrong

“With big enough wings, a person could flap like a bird.”

Our arm and chest muscles are far too weak for that. Every successful human-powered aircraft is driven by the legs, through pedals and a propeller.

“Birds just have stronger muscles.”

Birds are also light, with hollow bones and huge flight muscles for their size. Power needed rises faster than weight, so scaling a bird up to 70 kg does not work.

“Nobody has ever flown under their own power.”

The Gossamer Condor (1977) and the Gossamer Albatross (1979) both did, but at walking-to-cycling speeds of around 12 to 24 km/h, close to the water, in calm air.

Common questions

How we calculated this

Hover (induced) power P = (mg)^{3/2} / sqrt(2 ρ A) with ρ = 1.225 kg/m³ and A = π (span/2)². Muscle budget is 4 W/kg sustained, a round endurance figure, times body mass. If required power exceeds that budget the verdict is 'not with ordinary muscle, not hovering.' History of human-powered flight is cited as the existence proof that cruise with huge span is borderline-possible. No CFD, no flapping-wing aeroelasticity.

Assumptions and limitations

Sources

• NIST CODATA — standard acceleration of gravity

  gn = 9.80665 m/s².

• NOAA JetStream — layers of the atmosphere

  Tropopause height and the idea of a standard atmosphere.

• NASA Earth fact sheet

  Earth mass and radius for orbital and gravity-train numbers.

• Smithsonian National Air and Space Museum — Gossamer Albatross

  Channel crossing on 12 June 1979: 36.2 km in 2 h 49 min; wingspan 28.6 m; take-off weight 97.5 kg.

• NASA — Gossamer Albatross

  NASA flight tests of the backup aircraft and why its low power need mattered.

All IfSketch sources · Methodology