I recently developed a sudden interest in flying. I'm wondering whether a pair of small ducted fans could lift a person off the ground.
Lets say the ducted fan is 15cm in diameter. What would be the most thrust a fan like that could produce?
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Sign up to join this communityI recently developed a sudden interest in flying. I'm wondering whether a pair of small ducted fans could lift a person off the ground.
Lets say the ducted fan is 15cm in diameter. What would be the most thrust a fan like that could produce?
A 15 cm diameter fan could maybe lift 4 kg of mass with a tip speed of 0.6M.
By increasing the power and the blade chord length (in other words, increasing the solidity of the rotor), this may be increased to maybe 6 kg or more, but a single fan would probably never lift more than 10 kg mass.
The thrust of a rotor is $C_T \cdot \rho \cdot \text{Area} \cdot \text{tip speed}^2$.
Assuming $C_T$ of $0.03$, the calculation shows $40~\mathrm{N}$ ($=4~\mathrm{kg}$) thrust.
Disc area is one of the most important parameters, even a 30 cm disc would generate 4 times the thrust. 60 cm would make almost 65 kg.
Apparently, a 50 cm diameter disc can possibly generate 45 kg thrust on its own. So two of them could carry and maybe even lift-off a person equipped with this backpack (the person + the system must be less than 90 kg).
One design required sixteen 20-inch (~50 cm) propellers to lift a person.
These were unducted but I doubt that any 15cm ducted fan produces more thrust than eight 50cm propellers can.
See Autonomous human transport for details of how the designer calculated thrust.
He used Thrust (pounds) = R2D4Tc where Tc is an empirically measured constant for which he had a value of 2.7734 x 10-12. R is RPM, D is diameter (inches).
I imagine max RPM might be limited by the need to keep the fan tips subsonic (e.g. < M0.5).
Note that thrust is shown as depending on the fourth power of diameter, sixteen 50cm propellers will therfore produce about 1000 x the thrust of two 15 cm propellers of the same design at the same RPM.
It seems you need to be careful when comparing ducted fans with propellers. Using higher RPM to compensate for smaller diameters results in lower efficiencies (you need bigger motors).
Small diameter, high disk loading ducted fans are often conceived to allow the use of a high rpm engine running a direct drive propeller. While these highly loaded fans (if properly designed) will be more efficient than a free propeller of the same diameter, they typically won’t match the efficiency of a larger free propeller (of much lower disk loading)
The $150000 Martin Jetpack uses two ducted fans powered by a 2-litre two-stroke engine of 200hp (~150000 watts?). The fan diameter looks much larger than your 15cm. The width of the machine is given as over 2 meters so I'd estimate the fan diameter is close to 80 cm.
The company website doesn't say how they calculated thrust. From their use of larger diameter fans I'd guess there are reasons that 15cm fans are unsuitable.
An Apache AH-64 has a rotor that's about 100x times the diameter, so the swept area is 10000x larger. It can lift around 10000 kg, which means your ducted fan would lift about 1 kg. You'd need 2 fans with approximately one meter diameter.
Here are technical data for existing 15 cm EDF from Schübeler Technologies GmbH https://www.schuebeler-jets.de/en/products/hst-en Technical Data DS-130-DIA HST® with DSM 7857-470:
Inner shroud diameter: 152 mm Fan swept area: 130 cm² Weight incl. motor, wires, connectors and Secure Fan Fix: 1750 g Static Thrust Range: 135 – 175 N Thrust range: 92 – 105 m/s Exhaust speed range: 17.500 – 20.000 rpm Input Power: 8,0 – 12,0 kW Allowed battery: 12 – 14S 14000 mAh Overall efficiency:76 – 74%
Regards blue
Thrust (in lbs) = 9.35(horsepower x diameter of ducted fan in feet)2/3 [power of 2/3]
This is the formula I recall from the book.
Assuming you design your fan for high static thrust:
for a 6 inch diameter to produce say 300lbs of thrust (lifting man and 100lb machine) you will need a 360hp powerplant.