I was watching a Science Channel show discussing how birds fly. It
discussed the rotation a birds wing undergoes at the end of its
The description was of the vortices this produces and how this
contributes to lift. No doubt this does occur, but I was reminded of
the way that a swimmer rotates his hand on the return stroke while his
hand is still in the water so as not to counteract the thrust he
I thought then this same idea could be used to produce craft capable
of hovering, as helicopters do. You would have a cupped surface moved
vertically downward producing a downward thrust, but on the return
stroke the surface would be collapsed so as not to counteract the
thrust already produced. I'll refer to the cupped surface as a canopy
whether of not it is stiff during the propulsion phase or flexible as
an actual parachute.
I'll use the formula for drag to calculate the thrust that would be
It is the drag is 1/2 times the density, times a reference area,
times the drag coefficient, times the velocity squared. Note that
usually the reference area in this formula would be the cross-
sectional area, as for instance for an airfoil. However, for a
parachute the reference area that goes in this formula is the surface
The power required to overcome drag is also given on this page as the
velocity times the drag. From this we see the ratio of thrust to power
required is 1/v, where v is the velocity. This means it would be more
efficient to move the canopy at lower velocities.
For thrust given in terms of newtons, and power in watts, this thrust
to power ratio would be in newtons per watt. This lifting capability
for the power required is sometimes given in terms of kilos lifted to
kilowatts of power or grams per watt. For this scenario and for the
velocity given in meters/sec, the numerical value of grams to watt
power would be about 100/v.
The efficiency of helicopters is in the range of perhaps 10 grams
lifted per watt. Then this proposed method would exceed the efficiency
of helicopters if the downward velocity of the canopies was less than
10 m/s, 36 km/hour.
If we made the downward velocity say 5 m/s the efficiency would be 20
gram/watt. Say we wanted to make a personal hovering device capable of
lifting a person. Let's suppose the total weight was 200 kg, including
the person, engine, canopies, support structure etc. Then we would
have to have the thrust be at least 2000 newtons. The density of air
at sea level is 1.22 kg/m^3. For a parachute, the drag coefficient Cd
is perhaps 1.5 to 2. The surface area of a hemispherical canopy is
2*Pi*radius^2. This would result in a radius for the canopy of about
3.2 meters or a diameter of 6.4 m. You could of course break the
surface area up into more than one canopy. Note that to reduce the
volume of the craft, it may also work to have several of the canopies
in a single column.
For an efficiency of 20 gram/watt and lifting capability 200 kg this
would require a power of 10,000 watts, about 13 horsepower, quite low
for a power plant capable of lifting a person.
I originally was thinking of this for heavy lift applications
however. A common heavy lift helicopter is the S-64 Skycrane. It can
lift a gross weight of 20,000 kg using 7,000 kwatts of power with a 22
meter diameter rotor. This is actually a weight to power ratio of only
3 to 1.
Suppose we wanted to get a 5 to 1 ratio using the same power. This
would require a velocity of 20 m/s in our scenario and a lifting
capability of 35,000 kg, or a thrust of 350,000 N. Then to get the
350,000 N thrust would require diameter of about the same size as the
helicopter rotor. But this would be able to lift almost twice as much.
To allow further lift during forward flight and reduce the forward
drag we may make the canopies in the shape of parafoils. Note that
parafoils create most of the reduction in descent speed from the fact
they produce lift, with a relatively small amount coming from the drag
they produce. Then it might be advantageous to use these moving in a
horizontal direction to generate the hovering effect. We might also
want to change the shape of the canopies depending on the direction of
flight, vertical, horizontal or a combination.
The idea might be usable for supersonic or hypersonic propulsion as
well. In this instance the canopies would not have to be moving at
supersonic velocity with respect to the surrounding air. Their
velocities would be at or a little more than that at which the ambient
air is moving rearward (with respect to the craft) in order to get the
- posted 14 years ago