What could be the most efficient propulsion solution/design for an aircraft (weighting less than 25 kg) designed only to hover (at a height of 100 m) ? No horizontal flight intended other then recovering the hovering position in case of atmospheric disturbances. Is ducted fan an option?
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3$\begingroup$ I would say hanged on a rope 🙂 $\endgroup$– sophitCommented Apr 30 at 12:04
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3$\begingroup$ Mounted on a pedestal. $\endgroup$– FreeManCommented Apr 30 at 13:21
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4$\begingroup$ Who needs propulsion go for a Tethered balloon $\endgroup$– DaveCommented Apr 30 at 15:11
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$\begingroup$ Or untethered. $\endgroup$– Camille GoudeseuneCommented Apr 30 at 19:58
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1$\begingroup$ A bit of more context (altitude, payload, endurance, ...) might help in giving you a better answer $\endgroup$– sophitCommented Apr 30 at 21:28
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3 Answers
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This is the most efficient type of aircraft designed only to hover. Tethered, sealed helium balloon. Unlike a hot air balloon, it doesn't require any propulsion, and altitude control is possible by inflatable air bags or tether tension. Some helium still leaks through the seams and needs to be topped up twice a year.
Anything else is comically wasteful for the purpose, including a helicopter.
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If this is for a station-holding drone, for minium power you probably would go with a coaxial helicopter with the longest rotor blades you can live with, to get the highest L/D from the rotor (imagine two 40:1 sailplanes flying in a circle, vs two stubby wing 8:1 airplanes flying in a circle, both with the same weight and wing area; the gliders will need way less power).
At some point really long rotor blades become a problem for other reasons, and you'll have to find a sweet spot that provides decent control with minimum power.
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The key here is disk loading ($T/A$). For most efficient hover, you want as low a disk loading as possible. Small thrust (but must equal weight) and a huge disk.
When throwing a gas to make thrust (propellers, jets, and rockets), thrust is
$T=\dot{m}\,\Delta V$
I.e. the mass flow times the amount you increase the velocity of the air.
$\Delta V = V_j-V_0$
The mass flow is:
$\dot{m}=\rho\,A\,V_j$
I.e. the product of the density ($\rho$), the area of the jet ($A$), and the velocity of the jet ($V_j$).
For an equal amount of thrust, you can either accelerate a lot of air (large $\dot{m}$) or you can accelerate the air a lot (large $\Delta V$).
As it turns out, it is always most efficient to accelerate a large amount of air as little as possible (large $\dot{m}$, small $\Delta V$).
In hover, $V_0$ is zero, so, to keep $\Delta V$ small, you don't want a huge $V_j$. So to maximize $\dot{m}$, you must maximize $A$.
So, we see that disk loading $T/A$ is the important parameter.
