Air propeller

A propeller is used to displace a vehicle at a reduced speed but with a high slip of approximately at least 60% and has a quality coefficient of at least 70% for 100% slip. The ratio of the diameter of the propeller to its pitch is approximately 1.18 to 2.9 times the value of the quality coefficient, and the blade width of the propeller is 0.09 to 0.19 times the pitch of the propeller. The propeller pitch is substantially constant and the aerodynamic center of pressure is located at a distance from the propeller axis of rotation equl to approximately 3/4 of the propeller radius. The radius of the curvature of the leading propeller edge, when seen in cross section, is to approximately 1/4 to 1/3 of the greatest blade thickness. In addition the angle of the blade inclination decreases continuously as the propeller diameter increases and the blade thickness is greatest in the leading third of the blade width, relative to the normal rotation direction of the propeller.

FIELD OF THE INVENTION 
The present invention relates to an air propeller. More particularly this 
invention concerns such a propeller for a slow vehicle where the propeller 
operates with high slip of at least 60% and a quality coefficient or 
figure of merit of at least 70% for 100% slip. 
BACKGROUND OF THE INVENTION 
When an air propeller is itself driven and is used as the propulsion system 
for a watercraft, helicopter, or land vehicle it is a so-called negative 
propeller. In such usage it is converting the mechanical energy applied as 
torque about an axis into relative axial thrust between the propeller and 
the fluid mass surrounding it. When used to drive a generator, as for 
example in a system for exploiting wind energy, it is termed a positive 
propeller and in effect converts the axial thrust of the surrounding fluid 
mass into rotation of the propeller shaft. 
Negative propellers are particularly suitable for moving watercraft on 
small, shallow inland waterways, rivers, canals and the like, since these 
channels are usually so shallow that use of a standard submerged propeller 
or paddlewheel would be impossible. In addition such small-draft vehicles 
create little wake and therefore do little damage to the banks of the 
waterway. Thus large areas of economic importance, which due to reduced 
water depth of the rivers over stretches of many thousand of kilometers 
are not normally navigable or are navigable only by uneconomical screw- or 
wheel watercraft, become passable to watercraft. Furthermore such a 
negative propeller is the ideal drive system for a slow land vehicle which 
must move over terrain where traction may be very low. Positive propellers 
are used to drive generators intended to overcome the energy shortage. 
Most current propeller work is done in airplane propellers, as such must 
operate with the greatest possible efficiency to economize fuel. The 
quality coefficient is not important for such airplane usage. When such 
propellers are used to drive either as positive propellers to drive a 
generator continuously or as negative propellers to move a landcraft or 
watercraft, the slip must frequently be much more than 60%, that is the 
relative displacement of the propeller and the fluid mass it is in will 
only be six-tenths that it would be if the propeller operated with no slip 
like a screw in a piece of wood. In this case the figure of merit or 
quality coefficient must be as great as possible, and the efficiency of 
the propeller is of secondary importance. This is also true in a 
helicopter where 100% slip is required for hovering, even if some 
efficiency is needed when the propeller is tipped to move the vehicle 
horizontally. 
Since, however, propellers are invariably designed with efficiency as the 
primary consideration, propellers are not generally used in these 
high-slip applications like moving land and watercraft at slow but varying 
speed against varying resistance and driving a generator continuously but 
at very low speed from a propeller subject to winds of widely varying 
speeds. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to provide an improved 
air propeller. 
Another object is the provision of such an air propeller which overcomes 
the above-given disadvantages, that is which is particularly suited for 
use with high slip. 
SUMMARY OF THE INVENTION 
The propeller according to the invention is used to displace a vehicle at a 
reduced speed but with a high slip of approximately at least 60% and has a 
quality coefficient of at least 70% for 100% slip The ratio of the 
diameter of the propeller to its pitch is approximately 1.18 to 2.9 times 
the value of the quality coefficient, and the blade width of the propeller 
is 0.09 to 0.19 times the pitch of the propeller. 
According to this invention the propeller pitch is substantially constant 
and the aerodynamic center of pressure is located at a distance from the 
propeller axis of rotation equal to approximately 3/4 of the propeller 
radius. The radius of curvature of the leading propeller edge, when seen 
in cross section, is approximately 1/4 to 1/3 of the greatest blade 
thickness. In addition the angle of the blade inclination decreases 
continuously as the propeller diameter increases and the blade thickness 
is greatest in the leading third of the blade width, relative to the 
normal rotation direction of the propeller. 
The radius of curvature of the leading edge when seen in cross section 
according to the invention is greater than the radius of curvature of the 
trailing edge and the radius of curvature of the trailing edge is smallest 
in the area of approximately the outermost third of the propeller. 
Furthermore the radius of curvature of the trailing edge has always 
approximately the same value in the area of the outer half of the 
propeller. 
In accordance with a further feature of this invention the shape of the 
suction surface when seen in a cross section is an essentially straight 
line in the area between the region of greatest blade thickness and the 
trailing edge. Thus the total effective suction surface is increased. This 
is important especially in cases where low rotational speeds of the 
propeller are employed. 
The tests carried out with the propeller according to the invention have 
yielded remarkable results. A vehicle having a distance between axles of 
3.0 m, a 500 cm.sup.3 engine and a propeller according to the invention 
with a diameter of only 1.5 m at a head wind of 50 km/h developed a travel 
speed of 70 km/h and took a 5-10% slope with a speed of 50 km/h without a 
running start. Further, a tugboat with an engine of 100 HP and a 5 m 
diameter propeller according to the invention was able to tow a deadweight 
barge of 650 metric tons at a dead-water velocity of 7.5 km/h. Research 
activity carried out in this connection has shown that in the case of 
deadweight barges with a cargo up to 200 metric tons propeller propulsion 
according to the present invention is superior and more economical than 
propulsion by a standard submerged screw. Thus for instance a 
self-propelled craft with a cargo with 150 metric tons and an 80 HP 
engine travels with a dead-water speed of 11 km/h when using a 5 
m-diameter air propeller according to the invention. 
Thus it can be concluded that with the propeller according to the invention 
a considerably higher pushing force or thrust can be achieved than with an 
airplane propeller of comparable size. In addition, due to the 
construction of the propeller according to the invention a considerable 
propeller velocity results, so that the traction power of the propeller is 
only minimally influenced by strong headwinds, that is by a negative wind 
stream. 
This invention makes possible, for instance in the case of application of 
the propeller to land vehicles, the elimination of a number of mechanical 
components (drives, gear-boxes, differentials, cardan axles, universal 
joint shafts, etc.) and thereby simplifies the construction and reduces 
the weight of the vehicle. In spite of the relatively small dimensions of 
the propeller, the vehicle can travel on almost any difficult terrain and 
is not influenced by head winds. In cases where the propeller according to 
the invention is used on helicopters or other vertical take-off vehicles, 
the propeller can provide improved climb power and horizontal speeds due 
to the high figure of merit.

SPECIFIC DESCRIPTION 
As seen in FIGS. 1 through 3 the propeller according to this invention is 
centered on and rotatable about a propeller axis 1 and has a leading edge 
E extending almost perfectly radially of the axis 1 and a trailing edge A 
generally parallel to the leading edge E. The angle b formed at the outer 
end of the propeller half is between 40.degree. and 55.degree.. 
FIG. 3 shows the chord or width B of the blade measured in the area of the 
aerodynamic center of pressure and the thickness Bs, the latter being at 
its highest value in the leading third of the blade starting from the 
leading edge E. The propeller blade forms an angle a with a plane running 
perpendicular to the axis 1 and has a diameter D (not shown) equal to 
twice its radius r.sub.2. The pitch which is the theoretical displacement 
of the propeller during one rotation presuming that there is no slip. 
The above-mentioned quality coefficient or figure of merit N is the the 
power ratio of the propeller when stationary with a slip of 100% and the 
efficiency is the propeller power ratio when in full motion. The slip is 
the relative deviation between the actually reached and the theoretically 
reachable speed and corresponds to the difference between the theoretical 
and actual speed, inversely proportional to the theoretical speed. The 
effective quality coefficient N can be calculated according to the 
Bendeman formula: 
EQU N=S/(2p/gFL.sup.2).sup.1/3 
where 
S=the effective thrust in [N] or [kg.m.sec.sup.-2 ], 
p/g=the mass of 1 m.sup.3 of air [kg.m.sup.-3 ], 
F=the propeller disk area in m.sup.2, 
L=the power input in [W] or [kg.m.sup.2 sec.sup.-3 ]. 
The efficiency coefficient .eta. is equal to: 
.eta.=S.v./R 
where 
S=the propeller thrust in [N] or [kgm.sec.sup.-2 ], 
v=the speed of the vehicle in [m/sec] and 
R=the power input of the propeller in [W] or [kgm.sup.2.sec.sup.-3 ], 
where 
kg means kg mass and 
N means Newton and 
W means Watt 
m means meter 
sec means second 
According to the invention the proportions in a propeller must be selected 
so that the ratio of the propeller diameter D to its pitch H is 
approximately between 1.18 to 2.9 times the quality coefficient N and the 
blade width B of the propeller corresponds to 0.09 to 0.19 times the pitch 
H. When using the propeller according to the invention to drive a barge or 
the like and also for any other vehicle with low speed and high slip, the 
proportions are at the upper limit of the diameter/pitch ratio. The lower 
ratio range is more suitable when the vehicles are supposed to move 
quickly, and also when they are supposed to move slowly but continuously. 
The ratio of the blade width B to pitch H is important inasmuch as the 
speed increases with increasing blade width. The construction of the 
propeller according to the invention creates the advantage that even with 
considerable decreases of the slip as a result of head winds the static 
thrust force does not decrease correspondingly. 
Since there is high slip even when moving against a head wind, going 
uphill, or for other reasons working against an impediment which normally 
cause a considerable reduction of the slip, the propeller according to 
this invention can be fairly small. Up to now, in order to maintain the 
tractive power in conditions of reduced slip and reduced speed it was 
necessary to substantially increase the diameter of the propeller. Since 
the other dimensions had to be increased also, this lead to quite 
unfavorable ratios of diameter to pitch and blade width to pitch. 
The propeller according to invention and shown in the drawing has 
proportions such that the aerodynamic center of pressure is located at a 
distance from the center of the propeller which equals approximately three 
quarters of the propeller radius r.sub.2. 
As seen best from FIG. 3, the radii of curvature of the leading edge E 
correspond to approximately a quarter to a third of the greatest blade 
thickness Bs. Seen in cross section, the radius of curvature of the 
leading edge E is bigger than the radius of curvature of the trailing edge 
A, and the radius of curvature of the trailing edge A is smallest 
approximately in the area of the outermost third of the propeller. Further 
the proportions are so selected that the radius of curvature of the 
trailing edge A is fairly constant in the area of the outer half of the 
propeller. When the propeller is shown in cross section, the outline U of 
the suction surface in the area between the greatest blade thickness BS 
and the trailing edge A is an essentially straight line, as shown in FIG. 
3. 
In the following an embodiment of the propeller according to the invention 
for the propulsion of a ship is given. A drive motor with an output of 116 
HP and 3400 RPM is used to drive this propeller. A stepdown transmission 
is employed between the motor and the propeller so that the drive speed of 
the propeller is 1100 RPM. 
For this application an axial hub length of 200 mm, a diameter of 3500 mm, 
and a ratio of D to H of 2.12 was selected. Under these conditions and 
taking into consideration the fact that B/H=0.09 to 0.19, meaning that 
when H=1650, B=148.5 to 313.5, the data of the propeller calculated in the 
common manner were the following: 
TABLE 
______________________________________ 
Blade width 
Project'n 
Blade thickness 
Radius 
Blade angle 
mm mm mm 
mm .degree. 
' B P B.sub.S 
______________________________________ 
0 -- -- -- -- (177) 
400 33 18 262 219 92 
600 23 37 240 219 72.5 
800 18 11 232 219 57.5 
1000 14 44 227 219 46 
1200 12 12 224 219 36.5 
1300 11 25 224 219 32.1 
1400 10 37 223 219 29 
1600 9 19 221 218 21.5 
1700 8 50 115 114 10 
1750 -- -- -- -- -- 
______________________________________ 
In another embodiment the propeller had a diameter D of 700 mm, a pitch H 
of 393 mm, a blade width B in the center of 55 mm, and a hub height of 45 
mm. Measurements with this propeller have indicated that at 500 RPM a 
specific thrust of 53 N/HP is reachable and the propeller at 1 HP input 
can develop a thrust force of 20 N. These measurement readings already 
show that in the case of a larger propeller with lower number cf rotations 
an essentially higher value of thrust can be reached than an airplane 
propeller 10 N/HP).