US2385662A - Deicing means - Google Patents

Description

B. voNNEcsU-rv DEICING MEANS Filed July 3, 1943 sept. 25, 1945.

IIIIIIIIIIIIIIIIIIIIIIII "5\\\\\\\\\\\\\\\\\\`\\\\v Patented Sept. 25, 1945 DEIC'ING MEANS Bernard Vonnegut, Boston, Mass., assigner to Research Corporation, New

poration of New` York Application July 3, 1943, Serial N0. 493,386

1.04 Claims- The present invention relates to the deicing of aircraft propellers and other exposed rotating parts of aircraft that are subject to icing during adverse flying weather.

The problem of preventing dangerous accumulation of ice on airplane propellers under icing conditions is particularly serious, as the ice which formsv o n the blades not only impairs the aerodynamic performance or lift by unfavorably modifying the blade shape, but also severely unbalances the propeller because of the building up and throwing olf of heavy icey collections at random from one oranother of the blades.

While several methods of ice removal from propellers have been used heretofore, none is particularly satisfactory. t t charge an anti-freeze solution over the blade whenV icing is encountered. Another methodV involves coating the blades before take-01T with a substance containing all anti-freeze. A third method involves heating the blades, either by electricity o r by exhaust'gases. Any system employing the latter method is naturally heavy and complicated, While the other methods provide only brief protection. The present invention has as an object the provision of propeller deicingy means for aircraft Which is highly effective in preventing dangerous accumulations of ice on the blades, is vcompletely automatic in operation, requires no heat or anti'- freeze solutions, isk light in weight and low in cost. More specically, the invention is concerned with the provision of means for automatically causing the ice to be thrown oifby centrifugal force before the ice builds up to a thickness that seriously impairs the lift of the blade or causes appreciable propeller unbalance during shedding. This result is achieved by the provision of means which substantially reduces the force required to remove the ice, so that the centrifugal force of even a thin coating of ice onthe spinning bladesissumcient because the incerto Shea When ice .forms .01.;1 Yany surface, it `has a Verystrone force Qf adhesicnlor most surfaces. the adhe- `ne methodisto dission is such that ashear stress of well over one V hundred pounds per square inch is necessary to break the bond. If the ice forms oneI rigid surface .or .on sheet material that .is firmlyfxed to.

arigidsurface-,any force applied` tothe ice will betransmitted uniformly to the rigidv surface."-y

YIf, however, the ice forms on a sheet material that is more easily stretched than, ice, then any will appear as a con-V force applied to the ice castrated ,strcssatthe ,edge of the ice. .Whil'ethe 55 Xerk, N.` Y., a' cerf total force applied may be relatively small, nevertheless because of t-he concentration of the shear stress at the edge of the ice, the stress in pounds per square inch at suchzone may be well over that required to break the bond, and in such case the ice in that zoneis freedv from the material, with the concentrated shear stress then appearing at the edge of the ice that remains adhering.

The present invention Ymakes use of this principle by applying to the propeller blade, or to p0rtions of the blade surface most susceptible to icing, thin sheet materialV more stretchable than ice, so that the ice may form on this stretcha-ble sheet' material rather than on the surface of the blade itself. The sheet material is arranged to move andstretch, andY under the influence of centrifugal force such movement and stretching occurs as the sheet material is loaded with increasing Weight of ice. The result of the stretch-u ing movement is to apply concentrated shear stress to the ice at the edges of the formation, causing the ice to free itself progressively from the material until entirely removed. Because of this` concentration of shear, the forces necessary to remove the ice are only a small fraction of the forces requiredwhen ice is formed on a rigid surface, and the result -is that the ice is thrown olf by centrifugal force before a dangerous thickness is built up- Furthermore, the deicing means is constructed andarrangedto cause the ice to be shed in relatively smallfragments independentlyv from a plurality of separate sections ofthe blade surface. This not o nly minimizes the possibility of damage to the aircraft from impact of the flying ice fragments, but also results, during flight under icing conditigns, in the ice being shed from theA propeller as a whole in a more or less continuous succession o f small fragments, with consequently but little effect on propeller balance as the shedding Rlceed The invention is likewise concerned with improvements in deicing means of the type which employs stretehable elastic sheet material around theleading Ledge portions of airfoils and other members which give rise, in their immediate vicinity, to non-uniform pressure distribution in the nature of aerodynamic lift More specifically, the invention provides a means for controlling or at :least minimizing the tendency of such nen-uniform pressure distribution or lift in the vicinity of the deicing means to disturb or displace thesh'eet material.

In the drawing illustrating the invention, Fig. 1 is a View of .apropeller blade having stretchable .4r-,and 5, the

- thin and the leading f the deicing sheet sheet material applied to the leading edge Portion of the blade in accordance with the invention;

showing a'modied form of deicing means on a blade having a relatively sharp leading edge; Fig. 4 is a view of one form of stretchable sheet deicing means before attachment to 'a propeller blade; Fig. 5 is a sectional scale of a portion' of the material shown in Fig. 4; Fig. v6 is a view of a slightly modifiedrform, takenin section similar to Fig. 5; Fig. 7 is an enlarged detail of one form of venti; Fig. 8 illustrates an alternative constructionA of deicing means prior to attachment to the blade; Fig. 9 is a sectional View thereof; and Fig.' 10 is a diagram illustrating the manner in which the stretchable material causes a substantial concentration of shear stress v,at the edge of the ice under the influence of a force applied in the plane of the surface.

View on an enlarged y Y prevent stretching In the carrying out of the invention, the

stretchabley sheetvmaterial is applied to the propeller blade around the leading edge and over adjacent portions of the 4thrust and camber surfaces, indicated at l2 and I4 respectively, so as t0 cover the blade for roughly per cent. of its chord, this-being the vzone most susceptible to seriousV icing. The deicing means may be secured to the blade byr cementing with a cement appropriate to the sheet material used, which material maybe thin sheet ,rubberv or similar stretchable elastic material. In general, it is considered preferable to provide a backing piece or sheet which is secured to the propeller blade, the active iceshedding material being carried by the backingk piece.A i 4 Thus, in the embodiment illustrated in Figs. backingy piece is indicated at i6 and theouter or deicing sheet material at I8. The backing layer is preferably though not necessarily of the same material as the deicing sheet, but somewhat thicker, to provide aresilient backing for the thin deicing material and thereby improve `the abrasion resistance thereof. In certain applications, especially wherethe blade .is edge relatively sharp, it'may be advisableto employ as the backing layerra molded member, such as indicated at l1 infFig. 3, shaped to provide for the de icing layer a radius of curvature around the leading edge not greatly'V different from that of the blade itself. Y

While the backing layer is preferably cemented to the blade over the entire surfaceofthe strip,

I8V is secured to layer only along its edges 2,0 and transversely at intervals along narrow lines indicated by dotted lines 22 to provide a Y plurality .of individually stretchable units or areas. The spacingbetween transverse bonds preferably varies with the `distance fromv the innerwend of the blade,` due to the variation in centrifugal force with radius for any given angular velocity. YIn the average blade, the centrifugal force varies by'a factor of labout five Afrom shank to blade tip, hence the radial dimensions of theindividual areas'r shouldV theoretically vary by about the same factor. However, it may be the material somewhat from shank yto tip, in view of the w-idevariationsin,blade width and thickness, and hence the radial dimensions'of the individual units may vary by some other smaller factor. Y `During the rotation of the propeller, the mathe Abacking.

desirable to Vary the width of! terial is subjected to centrifugal forces tendin to stretch the material outwardly along the surface of the blade and under certain conditions to cause wrinkling ofthe material. except when the blade is set for zero angle of attack, the deicing material is subjected to pressure distributions which may tend to lift the material awayfrom the blade.V The 'tendency toward wrinkling may be avoided by applying the strips to the blade while the material is under tension longitudinally of the blade sufficient to Y i v to the point of wrinkling under normal operation with the material free of ice. Tensioning thematerial chordwise of the blade duringY application inhibits lifting of the material away from the blade. A convenient method of applying the material to the blade under tension involves soaking the deicing strip in benzene or similar hydro-carbon to cause swelling of the elastic material. The material is applied to the blade while in swollen condition, so that upon evaporation' of the swelling agent, the material shrinks and is thereby tensioned. I

VTo minimize frictional'l yfects between the outer material and its supporting surface, such as the .backing material, it isdesirable, particularly where initial chordwise tensioning is to be employed, to lubricate the vcontacting surfaces of the material with graphite, talc, glycerine .or other lubricant suitable for the elastic lvmat'erial.

. An alternativelmethod of minimizing the' effects of reduced external pressures or lift on the ideicing material is to create ya-partial vacuum'be'- neath the'material sov that even'though'the pressure on the outer'sur'face of the', material drops substantially, nevertheless thepressure beneath the materialis at least as low,'so thatthe material is not distended or otherwise disturbed by .entrapped air beneath the material. f l

An effective method of partially vacuating beneath any or all of the individual areas of stretchable material, and.V one which is applicable to deicing devices of y'the stretchable or elastic'type, whether installed on propeller bladesandv operable by centrifugal force, or on wings'. or other airfoils Vand operatedby forces other than centrifugal force, makesv use 4of the Ynon-uniform pressure distribution around an airfoilg' By providing a seriesV of vents Vin the 'deicing` material beginning adjacent thele'ading edge and continu ing at intervals back along the ',c'amber surface, as indicated at 24 in Fig. 2,it is possiblelto insure that' over the range of angles. ofattack fofthe'V blade, one or another ofthe vents will be within'V or very close to the 'zone of lowestpressur'e arour'id'Y the airfoil. 'These vents" are 'preferably' in' the natureV of one-way' valves, `being merely small punctures or slits vin theV material and therefore normally closed by the elasticrcontraction ofY said, material. The vents may conveniently be formed by making a narrowslash the material oblique tothe surface, as shown inenlarged detail in Fig.

7. When the vents are employed in propeller Y amount to several pounds per square inch below atmosphere. If this drop carriesthe pressure externally of theV deicing material .belowthat ofthe pressure within, Ythen the `vent which isfin lthe zone of lowest outsidev pressure Awill open-to permit outward ow andY consequent `.equaliza'tionfof preissure."v The other vents, beingisubjected'to 'pres'-J VIn addition,

agences sure. differences.. the. opposite direction', clased.. .1.. y

.es the result 9ftlle prevsicn .0f a plurality cf vente having. cneryvay. llcw.. characteristics. it is evident. that the pressure bene thedeicine material may be automatically maintaiuedat. a value law. enough to. prevent distension. of thematerial clue t0. aerodynamic. lift, yet `not evacuated to a Peint.. such that lateral. et Surface. mcvemeets; cf

thematerial. under the influencent centrifugal f tierce. are uudulyrestrcted'by. friction.

Instead cf employing a eenstructlcu in which the deielng surface consists. ci a plurality f areas each secured entirely around its boundaries, a cenetructicn Such as indicated iu. Figs... 8. and 9 Isley be. utillzedfwherein the-,cuter layerconsists ci units.. er areas which are-.Secured alone. certain edges, and left .free along .other.edees,;.senerally tler radiallycuter edges. certain cases as. in. the. illustrated emr-@dime t the .outer sheet may be made up cf individual .sheets 2li cr the stretchable, material.y arraueecl te cverlap. 11.1 the .marlllec c f Scales,r f cr example These sheets. are secure-d to the backing layer 3.0. Lalcug therslue and. inner edges, the outerffedge of each sheet being freef s in the other embodiments, the deicing strip may be applied to the propeller either with or Without initial chordwise tensioning. If it is desired to provide somewhat increased thickness of material over the leading edge of the propeller blade, so as to enhance the abrasion resistance of the deicing strip, the strips may be wider in their central portion, as by curving the outer edges 42, to.,increase the amount of overlap in this zone.

The manner in which the stretchable material operates to shed the ice under the influence of centrifugal force before any appreciable thickness has accumulated is illustrated in Fig. 10. Assume 36 to represent a portion of a sheet of stretchable material, secured at 38 to a rigid surface, while 40 indicates a formation of ice on the sheet material. Under the influence of centrifugal force acting in the direction of the arrow 43 and which increases as the ice builds up, the distribution of shear stress at different points along the interface is indicated in general by the curve 44. The ice formation 4D, being relatively nonstretchable as compared with the material 36, -is in effect supported only along a line of contact opposite the line of attachment of the stretchable material to the rigid surface. Thus the total mass of the ice formation 40 contributes to the shear stress at this point an-d consequently, before any appreciable thickness of ice can build up, the magnitude of the stress reaches a value, represented by the ordinate OA, sufficient to break the bond at the interface.

With the bond between ice and stretchable surface broken along a line nearest the line of attachment of the sheet material to the rigid supporting surface, the shear stress set up by the remaining mass of ice is concentrated along the innermost line, radially considered, at which any bond remains between ice and sheet material. The shedding process thus may be considered to take place progressively, starting from the inside edge of the area being considered. As a matter of fact, the shedding, once started, usually occurs so rapidly as to appear to take place simultaneously over the entire area. Under steady icing conditions, any particular area will shed at fairly uniform frequency, when a thickness of the order of one sixteenth of an inch of ice has built up, but of course different areas will shed at different frequencies, so that generally the A material exceeds the total. massief. theflzlasle v.at .aar cuetilue ueeeectlluetuate arpreceblyf i Y.

A comparison of the relative of A ice Whiclialust be buislt uic..v ce pelli eu., .stretchable .Surfaces before' shedding talses. glace uecler the .influence of centrifugal farce may be gathered ftcm the'uplcei cart cf ,FlaV 1.0. ,Theareauuflec the 'curve f4.4. represents the class. cl ice that muet. accumulate betere ehedulue. takeerlaee. irc. stretchable surface.,A tue area.. cl. rectangle @ABC represents the. macs. cf; ice, that. muet build uu betere shedding Will cecur ucm a. rigid. surface of corresponding area. That is, Without con tfatlcu. cf alleati 'Stresa the ice .thickness must at.

atthemterface. j Y, t .While certalucsecllic cmlccflimeets-cf taciu- Veutlcu have. been rsllcwfu and; described, @essere for the' purpose of illustrating the invention and are toA be considered inno way a limitation .of ir'lventionto` the particular configurations and are rangements set forth.

Iclaim:

1`- A propeller. blade. having deicingxneans com` prlslue thlri Sheet material mcrestrtcliable than ice disposed around thev leading edge of the blade and covering at least a portion of the thrst and camber faces of the blade, said material being secured marginally along lines radially of the blade and at intervals transversely to provide a plurality of independently stretchable ice intercepting surfaces adapted under the influence of centrifugal force on the ice to shed said ice through the action of progressive concentrated stress in shear.

2. A propeller blade having deicing means comprising thin sheet elastic material disposed around the leading edge of the blade and covering at least a portion of the thrust and camber faces of the blade substantially from shank to tip, a backing strip intermediate the elastic material and the blade and secured to the blade, said sheet material being secured to the backing material along lines which define a plurality of individual areas of sheet material, the material being free to stretch intermediate the secured portions, and means for partially evacuating the areas intermediate sheet and backing material comprising a plurality of vents disposed in the sheet material throughout a Zone which includes the regions of minimum air pressure adjacent the blade over the range of the angles of attack Within which the blade operates.

3. An airfoil having deicing means comprising elastic material more stretchable than ice disposed around the leading edge of the airfoil and covering at least a portion of the camber surface thereof, appreciable areas of the material being free of direct adhesion to adjacent surfaces beneath, means for venting beneath said unsecured areas to minimize the effect on said material of varying external pressure drops, said means comprising a plurality of normally closed apertures in the material adapted to open to permit pressure equalization when the pressure Within the external pressure, said vents being disposed in the material throughout a zone which includes the regions of minimum air pressure adjacent the airfoil over the range of angles of attack Within which the airfoil operates.

4. A propeller blade having deicing means comprising thin sheet elastic material closely conforming to and covering the leading edge portion of the blade, said material being secured at sheer' stress @if magnitude 'ce terreau spaced points to permitthe'material to stretch outwardly along the blade under the influence of centrifugal force.Y f Y p Y l 5. A propeller blade having deicing means comprising a stretchable covering vof-thin sheet elastic material closely conforming to the blade contour, said material being disposed in independently vstre'tchable marginally secured units along substantially the full blade length.'

g v6. AA propeller blade havingdeicing means-comprising a stretchable covering of thin sheet elastic material closely conforming to the Vblade contour and covering atleast the leading edge portion of the blade substantially the full length thereof,

Y said fniaterial being disposed in independentlyV fstretchable marginally secured units of decreas.

ing area from shank to tip of blade.

7. A propeller blade having deicmg means comprisnga stretchable covering vof thin sheet elastic material closely conforming to and covering'at least the leading edge portion of the blade substantially the full lengththereof, said material being disposed in tensioned conditiony in independently stretchable marginally secured units. 8; A propeller blade having deicing means comp rising a s tretchable covering of thin sheet elastic 9. Deicing means for a propeller blade com-'- pris'ing a flexible backing strip adapted to cover the leading edge -of a blade substantially the fullY length thereof, thin sheet elastic `material secured tothe backing strip along side edges and at spaced intervals transversely to provide a plurality of independently stretchable ice-interceptingy surfaces; Y

10. yDeicing means fora propeller blade com-` prising a flexible backing stripV adapted tovcover the leading edge portion of av blade substantially the full length thereof,.. thin sheet elastic'materialsecured to the backing strip'along side edges and at spaced intervals transversely to fform-a plurality of independently stretchable ice-intercepting surfaces, said sheet material having vent'- ing apertures extending therethrough to the backing material beneath. f v

BERNARD V'ONNEcfUfl'.VV

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