Methods and apparatus for preventing or retarding fruit and vegetable freezing
This application is a continuation of application Ser. No. 716,325, Filed Mar. 26, 1985, which in turn was continuation-in-part of application Ser. No. 523,737 Filed Aug. 16, 1983,both now abandoned. 
    
    
     The damaging freezes which occur in the fruit and vegetable industries are a well-known occurrence. These freezes are generally known to be due primarily to the formation of atmospheric inversions over a fairly wide area. This condition normally occurs on clear, calm, cold nights and is characterized by an atmosphere in which air temperature increases with altitude. Unfortunately, and inversion is an inherently stable condition which tends to persist until wind velocities increase or until daybreak, at which time incoming solar radiation begins to warm the earth and trees. 
     The freezing of foliage and of the crop is purely a radiation phenomena, brought on by the inversion, in which the soil, foliage and fruit all radiate heat to space. The result is that the atmoshere cools from the ground upward. As a result, it is not uncommon to find temperature differnetials of 10° or more between the ground and 60 foot levels above the ground. 
     The existence of this warmer air mass at higher elevations represents a very large potential energy source which can be used to offset a grove&#39;s net radiation heat loss. 
     There have been many methods and apparatus proposed in an effort to solve this freeze problem. Perhaps, the best known and most time honored is the use of &#34;smudge pots&#34; scattered through the area to be protected. While these are somewhat effective, they fall far short of being the real answer to the problem. It has also been proposed to use wind machines to create a surface wind simulating that which, when it naturally occurs, reduces the freeze damage. Such machines are illustrated in Towt U.S. Pat. No. 2,481,702; Potter U.S. Pat. No. 2,836,932; Beckett U.S. Pat. No. 2,895,259; Graboswki U.S. Pat. No. 3,180,570, Coles U.S. Pat. No. 3,288,353; and Vertrees et al. U.S. Pat. No. 3,309,533. The difficulty with such machines is that they were useful only on very calm nights and were at best very inefficient because they simply mixed the cold air next to the ground with somewhat warmer air at a limited height above the ground so that the effective raising of the temperature of the operative air mass at plant level was very low. In order to overcome this problem some attempts were made to draw air down from a higher level through a vertical duct and distribute it horizontally along the ground as in Graham Pat. No. 2,807,120. Unfortunately, such machines were not strong enough or effective enough to accomplish the result sought. Another approach is that shown in Davies U.S. Pat. No. 3,003,282 which attempted to combine the approach of drawing higher level air down through a duct with the smudge pot approach by heating it at ground level and then blowing it horizontally across the surface. Such an approach is expensive and difficult to use. Still another proposal is that of Smith U.S. Pat. No. 3,067,541 which uses rotors having nozzles along the trailing edge for spraying fertilizer, pesticides, fog dispersal substances, smoke and heat to the air. This is very complex structure designed to disperse air widely above a localized section. 
     The present invention involves the transportation of large volumes of air from the higher, warm air levels existing during inversionary conditions down to the grove or orchard floor and spreading it out radially under the foliage canopies in the form of a &#34;horizontal wake&#34;. As this horizontal wake is warmer, and therefore lighter, than the air above it, and because of the friction imposed by the ground and lower foliage level, a portion of this wake tends to ascend through and between the trees as it spreads radially, transferring its heat to the foliage and fruit. 
     A hovering, or stationary, rotor operating in a horizontal plane derives the majority of its air from a distance one rotor diameter above the plane of the rotor &#34;disk&#34;. Furthermore, the maximum velocity which can be imparted to that air is twice the average velocity of the air passing through the rotor disk, this velocity occurring at a distance of one rotor diameter below the plane of that rotor. 
     The present invention is based upon the general precepts above, but more importantly, upon three characteristics of the horizontal wake which are unique to the rotor operating in &#34;ground effect&#34;, i.e. within one rotor diameter above the earth: 
     (1) the horizontal wake of such a rotor is composed of a very large number of vortices having a rectrograde spin, similar to a backward spinning tennis ball. This characteristic makes the wake tend to hug the ground, this tendency being opposed by the convective bouyance and friction effects noted above, and provides the horizontal range required to make the invention a viable frost-protection device; 
     (2) the extreme turbulance of the horizontal wake acts to greatly increase the heat transfer coefficient between the air of the wake and the foliage and fruit; and 
     (3) the horizontal wake is very thin and maximum air velocities occur within a distance 2% to 5% of the rotor radius above the ground. This phenomena acts also to give the air flow a useable range. 
     The use of the ground effect phenomena is a totally new concept in dealing with the frost problem, differentiating this invention from those of the past, none of which can be effective in severe conditions. 
     I provide an apparatus and method for protecting crops, especially treeborne crops, against freezing which provides a rotor having at least two blades, each of an air-foil section, a blade retaining member connecting said blades, said retaining member being bent upward to form a &#34;coning angle&#34; to satisfy the rotor&#39;s structural requirements, a retaining member clamp which is &#34;underslung&#34; and pivots around a horizontal axis in a rotor yoke, a vertical tower which is fixed to the earth to inhibit gyration, a vertical drive shaft journaled in a tower and fixed to the rotor yoke at the top of that tower, drive means drivingly connected to the drive shaft at the bottom of the tower, said tower extending substantially above the level of the trees to be protected, a distance sufficient to pick up air in the inversion area but within the ground effect zone, and to deliver it to the ground in a generally cylindrical mass until it approaches the ground, at which time expansion of the compressed wake causes it to turn outward and move radially along the ground. 
     The rotor radius, and thus the tower height, is a function of grove geometry. If, for example, 
     R r  = the rotor radius; 
     R w  = the radial distance form the rotor&#39;s axis of rotation to the nearest foliage; 
     h i  = the tree skirt height; 
     u o  = the average velocity of the air passing through the rotor disk; and 
     u w   the averag velocity of the air entering the grove or orchard under the foliage of the trees, which ideally is 2u 0 , then as the air entering the grove must equal in quantity that passing through the rotor disk, 
     
         pi×R.sub.r.sup.2 ×U.sub.o =2pi×R.sub.w h.sub.i ; 
    
     In a grove or orchard having a 25&#39;×25&#39; tree spacing, 
     3&#39; shirt heights and a skirt diameter of 20&#39;, R r  =√180, or 13.42&#39;. 
     Preferably the rotor is of a &#34;teetering&#34; design to avoid wing sensitivity caused by the differential lift created when one blade advances into a wind and the other retreats from it. The blade presently preferred is a Boeing VR7.1 blade. 
     The coning angle, imparted from the upward bend of the blade retaining member, is such that the vertical component of the centrifugal forces poses the lifting moment of each blade. As a result of this structure, there is no net lifting moment on the blades, all lift forces being translated to shear. 
     The undersling of the blade retaining member with respect to its teetering axis eliminates the undesirable moment caused by the Coriolus forces inherent in a coned rotor. 
    
    
     In the foregoing general description of the invention, I have set out certain objects, purposes and advantages of this invention. Other objects, purposes and advantages of the invention will be apparent from a consideration of the following description and the accompanying drawings in which: 
     FIG. 1 is a side elevational view of an apparatus according to this invention installed in an orchard or grove; 
     FIG. 2 is a fragmentary section of the tower base and gear box of FIG. 1; 
     FIG. 3 is an enlarged side elevational view of a presently preferred hub arrangement for the blades of this invention; 
     FIG. 4 is an enlarged elevational view at right angles to the view of FIG. 3; 
     FIG. 5 is a section on line V--V of FIG. 4; and FIG. 6 is a section on line VI--VI of FIG. 5. 
    
    
     Referring to the drawing, I have illustrated a tower 10 mounted on a gear box 11 fixed to a concrete pad 12 in an orchard. A vertical drive shaft 13 extends upwardly through the tower and is journaled at the top of the tower in bearing 14, in the middle, and at the bottom of the tower in bearing 15. Blade retaining tube 64 is fixed to a clamp 57 pivoting in yoke 50 on top of drive shaft 13. The blade retaining tube has a positive dihedral which carries airfoil blades 19. The lower end of the drive shaft 13 carries a bevel drive gear 20 which engages a bevel gear 21 at right angles thereto. Gear 21 is mounted on one end of a horizontal power shaft 22 which shaft extends out of housing 11 through bearings 23, 24. The other end of shaft 22 is connected to a drive means 25, such as an engine directly connected to the gear box. 
     The air is carried downwardly and swept along the surface of the ground radially beneath the trees in the fashion as shown in FIG. 1. Such an apparatus when combined with the waste heat from the drive engine has a capacity for providing a reasonable degree of frost protection over a wide area with temperature differentials. 
     In FIGS. 3 through 6, I have illustrated the presently preferred hub arrangement for the propellor blades of this invention. In this arrangement, I provide a yoke 50 adapted to be removable mounted on the top end of driven shaft 13 by bolts 51 which are threaded into holes 52 in a solid block 53 fixed on the top end of shaft 13. A key way 54 in the hollow base of yoke 50 is designed to accept a key 55 in head 53. A teetering head and blade retaining tube clamp 57 is pivotally mounted at one end 58 on a pivot pin 59 in yoke 50. The other end 60 of the teetering head and blade retaining tube clamp is provided with a removable clamp ring 61. Each of the other end 60 and clamp ring 61 are provided with half circle grooves 62 and 63 respectively adapted to receive and tightly clamp a blade retaining tube 64 between then by means of tightening screws 65 which extend through clamp ring 61 into tube end 60. The rotor blades 19 are bolted onto the blade retaining tube 64 at the desired angle to provide the necessary air velocity. Rubber bumpers 70 are provided on yoke 50 to absorb the shock of abrupt movement of teeter head 57 around its pivot. 
     In the foregoing specification I have set out certain preferred practices and embodiments of this invention, however, it will be understood that this invention may be otherwise embodiment within the scope of the following claims.