Autogiro Company of America v. The United States

*395OPINION *

The Autogiro Company of America, a Delaware corporation owning all patents involved in this litigation, sues, under 28 U.S.C. § 1498,1 to recover the “reasonable and entire compensation” for the Government’s allegedly unauthorized use of its patented inventions. Only the question of liability is now before the court. The determination of any amount of recovery is reserved for further proceedings before the trial commissioner.

Plaintiff initiated suit in this court on September 21, 1951, with a Petition Pending Motion for Call which claimed the infringement of twenty-six patents. The petition was amended on March 10, 1954; twelve patents were dropped from the suit and six were added. Four years of pre-trial proceedings reduced the number of patents to sixteen. During that period, fourteen witnesses appeared, over one thousand exhibits were presented, and almost fifteen thousand pages of transcript were taken. The trial commissioner issued a 232-page report containing 415 findings of fact. Fifteen of the sixteen patents in suit and eighty-five of their eighty-six claims in suit were found valid and infringed by various Government structures. One patent and its one claim were found not infringed.

Plaintiff has not excepted to any findings of fact. Defendant has excepted to findings concerning the fifteen patents found valid and infringed. We find claims of eleven patents to be valid and infringed.

Patent Infringement

Like many patent infringement suits, this case presents no novel legal questions. Absent are any issues not found in the run-of-the-mine case. This does not mean that the law involved should go unstated. The complexity of the inventions in suit, the length of the trial, and basic disagreement of the parties on legal issues are considerations that dictate a recital of this court’s law of patent infringement.

I

The Patent Act of 1952, 35 U.S.C. § 1 et seq., which applies to all patents granted on or before January 1, 1958,2 is the controlling law in this case. No previous patent act contained a section on infringement. Congress had always allowed the courts to settle the issue without any legislative guidelines. Rich, Infringement Under Section 271 of the Patent Act of 1952, 21 Geo.Wash.L.Rev. 521 (1953). Section 271(a)3 which covers the type of infringement alleged here was not inserted in the Act to clarify any legal problems, but only as a codification of existing judicial determinations. As one commentator has stated: “We got along without it for 162 years and we could again. Its omission would change nothing.” Id. at 537.

The claims of the patent provide the concise formal definition of the invention. They are the numbered paragraphs which “particularly [point] out and distinctly [claim] the subject mat*396ter which the applicant regards as his invention.” 35 U.S.C. § 112. It is to these wordings that one must look to determine whether there has been infringement.4 Courts can neither broaden nor narrow the claims to give the patentee something different than what he has set forth.5 No matter how great the temptations of fairness or policy making, courts do not rework claims. They only interpret them. Although courts are confined by the language of the claims, they are not, however, confined to the language of the claims in interpreting their meaning.

Courts occasionally have confined themselves to the language of the claims. When claims have been found clear and unambiguous, courts have not gone beyond them to determine their content. Keystone Bridge Co. v. Phoenix Iron Co., supra, 95 U.S. at 278, 24 L.Ed. 344; Borg-Warner Corp. v. Mall Tool Co., 217 F.2d 850 (7th Cir. 1954); Zonolite Co. and Insulating Concrete Corp. v. United States, 149 F.Supp. 953, 138 Ct.Cl. 114 (1957). Courts have also held that the fact that claims are free from ambiguity is no reason for limiting the material which may be inspected for the purpose of better understanding the meaning of claims. Warner & Swasey Co. v. Universal Marion Corp., supra, 237 F.Supp. at 737.

We find both approaches to be hypothetical. Claims cannot be clear and unambiguous on their face. A comparison must exist. The lucidity of a claim is determined in light of what ideas it is trying to convey. Only by knowing the idea, can one decide how much shadow encumbers the reality.

The very nature of words would make a clear and unambiguous claim a rare occurrence. Writing on statutory interpretation, Justice Frankfurter commented on the inexactitude of words:

They are symbols of meaning. But unlike mathematical symbols, the phrasing of a document, especially a complicated enactment, seldom attains more than approximate precision. If individual words are inexact symbols, with shifting variables, their configuration can hardly achieve invariant meaning or assured definiteness.

Frankfurter, Some Reflections on the Reading of Statutes, 47 Col.L.Rev. 527, 528 (1947). See, also, A Re-Evaluation of the Use of Legislative History in the Federal Courts, 52 Col.L.Rev. 125 (1952).

The inability of words to achieve precision is none the less extant with patent claims than it is with statutes. The problem is likely more acute with claims. Statutes by definition are the reduction *397of ideas to print. Since the ability to verbalize is crucial in statutory enactment, legislators develop a facility with words not equally developed in inventors. An invention exists most importantly as a tangible structure or a series of drawings. A verbal portrayal is usually an afterthought written to satisfy the requirements of patent law. This conversion of machine to words allows for unintended idea gaps which cannot be satisfactorily filled. Often the invention is novel and words do not exist to describe it. The dictionary does not always keep abreast of the inventor. It cannot. Things are not made for the sake of words, but words for things. To overcome this lag, patent law allows the inventor to be his own lexicographer. Chicago Steel Foundry Co. v. Burnside Steel Foundry Co., 132 F.2d 812 (7th Cir. 1943); Stuart Oxygen Co. Ltd. v. Josephian, 162 F.2d 857 (9th Cir. 1947); Universal Oil Products Co. v. Globe Oil & Refining Co., 137 F.2d 3 (7th Cir. 1943), aff’d 322 U.S. 471, 64 S.Ct. 1110, 88 L.Ed. 1399 (1944).

Allowing the patentee verbal license only augments the difficulty of understanding the claims. The sanction of new words or hybrids from old ones not only leaves one unsure what a rose is, but also unsure whether a rose is a rose. Thus we find that a claim cannot be interpreted without going beyond the claim itself. No matter how clear a claim appears to be, lurking in the background are documents that may completely disrupt initial views on its meaning.6

The necessity for a sensible and systematic approach to claim interpretation is axiomatic. The Alice-in-Wonderland view that something means whatever one chooses it to mean makes for enjoyable reading, but bad law. Claims are best construed in connection with the other parts of the patent instrument and with the circumstances surrounding the inception of the patent application. Doble Engineering Co. v. Leeds & Northrup Co., 134 F.2d 78 (1st Cir. 1943). In utilizing all the patent documents, one should not sacrifice the value of these references by the “unimaginative adherence to well-worn professional phrases.” Frankfurter, supra, at 529. Patent law is replete with major canons of construction of minor value which have seldom provided useful guidance in the unraveling of complex claims. Instead, these canons have only added confusion to the problem of claim interpretation. Doble Engineering Co. v. Leeds & Northrup, supra, at 84.

II

In deriving the meaning of a claim, we inspect all useful documents and reach what Justice Holmes called the “felt meaning” 7 of the claim. In seeking this goal, we make use of three parts of the patent: the specification, the drawings, and the file wrapper.

Specification. — Section 112 of the 1952 Patent Act requires the specification to describe the manner and process of making and using the patent so that any person skilled in the patent’s art may utilize it. In serving its statutory purpose, the specification aids in ascertaining the scope and meaning of the language employed in the claims inasmuch as words must be used in the same way in both the claims and the specification. U.S.Pat.Off. Rule 75(d).8 The use of the *398specification as a concordance for the claims is accepted by almost every court, and is a basic concept of patent law.9 Most courts have simply stated that the specification is to be used to explain the claims; others have stated the proposition in different terms, but with the same effect. The following expressions are indicative of this latter approach: (1) «* * * [A] patentee’s broadest claim can be no broader than his actual invention.” Kemart Corp. v. Printing Arts Research Laboratories, Inc., 201 F.2d 624, 629 (9th Cir. 1953), (2) “ * * [R]ecourse must be had to the specifications to see how far the means there disclosed correspond with those made by the defendant.” Perry v. United States, 76 F.Supp. 503, 505, 112 Ct.Cl. 1, 30 (1948); Grubman Engineering & Mfg. Co. v. Goldberger, 47 F.2d 151, 152 (2d Cir. 1931), (3) “[W]hile the specification may be referred to in order to limit the claim, it can never be made available to expand it.” Hanovia Chemical & Mfg. Co. v. David Buttrick Co., 39 F.Supp. 646 (D.Mass.1941), aff’d 127 F.2d 888 (1st Cir. 1942).

The specification “set[s] forth the best mode contemplated by the inventor of carrying out his invention.” 35 U.S.C. § 112. This one embodiment of the invention does not restrict the claims. Claim interpretation must not make use of “best mode” terms inasmuch as the patentee need not guard against infringement by listing every possible infringing device in the specification. Adams v. United States, 330 F.2d 622, 165 Ct.Cl. 576 (1964), aff’d 383 U.S. 39, 86 S.Ct. 708, 15 L.Ed.2d 572 (1966); Grant Paper Box Co. v. Russell Box Co., 154 F.2d 729 (1st Cir. 1946), cert. denied 329 U.S. 741, 67 S.Ct. 79, 91 L.Ed. 639. But where the specification does not refer to an embodiment or a class of embodiments in terms of “best mode,” such reference may be of value in claim interpretation. This would be where the patentee describes an embodiment as being the invention itself and not only one way of utilizing it.

Drawings. — The patent may contain drawings. 35 U.S.C. § 113. In those instances where a visual representation can flesh out words, drawings may be used in the same manner and with the same limitations as the specification. Permutit Co. v. Graver Corp., 284 U.S. 52, 52 S.Ct. 53, 76 L.Ed. 163 (1931); Cameron Iron Works, Inc. v. Stekoll, 242 F.2d 17 (5th Cir. 1957); Binks Mfg. Co. v. Ransburg Electro-Coating Corp., 281 F.2d 252 (7th Cir. 1960); cert. dismissed 366 U.S. 211, 81 S.Ct. 1091, 6 L.Ed.2d 239 (1961); Foxboro Co. v. Taylor Instrument Companies, 157 F.2d 226 (2d Cir. 1946), cert. denied 329 U.S. 800, 67 S.Ct. 494, 91 L.Ed. 684 (1947).

File wrapper. — The file wrapper contains the entire record of the proceedings in the Patent Office from the first application papers to the issued patent. Since all express representations of the patent applicant made to induce a patent grant are in the file wrapper, this material provides an accurate charting of the patent’s pre-issuance history. One use of the file wrapper is file wrapper estoppel, which is the application of familiar estoppel principles to Patent Office prosecution and patent infringement litigation. The patent applicant must *399convince, the patent examiner that his invention meets the statutory requirements ;10 otherwise, a patent will not be issued. When the application is rejected, the applicant will insert limitations and restrictions for the purpose of inducing the Patent Office to grant his patent. When the patent is issued, the patentee cannot disclaim these alterations and seek an interpretation that would ignore them. He cannot construe the claims narrowly before the Patent Office and later broadly before the courts.11 File wrapper estoppel serves two functions in claim interpretation; the applicant’s statements not only define terms, but also set the barriers within which the claim’s meaning must be kept. These results arise when the file wrapper discloses either what the claim covers or what it does not cover.

The file wrapper also has a broader and more general, use. This is its utilization, like the specification and drawings, to determine the scope of claims.12 For example, the prior art cited in the file wrapper is used in this manner. In file wrapper estoppel, it is not the prior art that provides the guidelines, but the applicant’s acquiescence with regard to the prior art. In its broader use as source material, the prior art cited in the file wrapper gives clues as to what the claims do not cover. Westinghouse Electric & Mfg. Co. v. Formica Insulation Co., 266 U.S. 342, 45 S.Ct. 117, 69 L.Ed. 316 (1924); Remington Rand, Inc. v. Meilink Steel Safe Co., 140 F.2d 519 (6th Cir. 1944). Moto-Mower Co. v. E. C. Steams & Co. Inc., 126 F.2d 854 (2d Cir. 1942).

Ill

The use of the various parts of the patent to determine the meaning of the claims is only half the process of determining patent infringement. The other half is “reading the claims on the accused structures.” If the claims read literally on the accused structures, an initial hurdle in the test for infringement has been cleared. The race is not over; it has only started. To allow literality to satisfy the test for infringement would force the patent law to reward literary skill and not mechanical creativity. And since the law is to benefit the inventor’s genius and not the scrivener’s talents, claims must not only read literally on the accused structures, but also the structures must “do the same work, in substantially the same way, and accomplish *400substantially the same result.” Dominion Magnesium Ltd. v. United States, 320 F.2d 388, 396, 162 Ct.Cl. 240, 252 (1963). This approach oí making literal overlap only a step and not the entire test of infringement has been consistently applied by the courts since Westinghouse v. Boyden Power Brake Co., 170 U.S. 537, at 568, 18 S.Ct. 707, at 722, 42 L.Ed. 1136 (1898), where Justice Brown stated:

* * * The patentee may bring the defendant within the letter of his claims, but if the latter has so far changed the principle of the device that the claims of the patent, literally construed, have ceased to represent his actual invention, he is as little subject to be adjudged an infringer as one who has violated the letter of a statute has to be convicted, when he has done nothing in conflict with its spirit and intent. * * * 13

If the claims do not read literally on the accused structures, infringement is not necessarily ruled out. The doctrine of equivalence casts around a claim a penumbra which also must be avoided if there is to be no infringement. It provides that a structure infringes, without there being literal overlap, if it performs substantially the same function in substantially the same way and for substantially the same purpose as the claims set forth.14 Equivalence is the obverse of the discounting of literal overlap. The latter is to protect the accused; the former to protect the patentee. The rationale behind equivalence was set forth by the Supreme Court in Graver Tank & Mfg. Co. v. Linde Air Products Co., supra, 339 U.S. at 607, 70 S.Ct. at 856, 94 L.Ed. 1097:

[T]o permit imitation of a patented invention which does not copy every literal detail would be to convert the protection of the patent grant into a hollow and useless thing. Such a limitation would leave room for — indeed encourage — the unscrupulous copyist to make unimportant and unsubstantial changes and substitutions in the patent which, though adding nothing, would be enough to take the copied matter outside the claim, and hence outside the reach of the law. * * *

Checking the subordination of substance to form and not depriving the inventor of the benefit of his invention cannot be standardized. The range of equivalence varies with each patent; however, some general guidelines can be drawn. One important guide is whether persons reasonably skilled in the art would have known of the interchangeability of an ingredient not contained in the patent with one that was. Id at 609, 70 S.Ct. 854. Another guide is the notion that pioneer patents are to be given wider ranges of equivalence than minor improvement patents. Pratt & Whitney Co. v. United States, supra; Aluminum Co. of America v. Thompson Products, Inc., 122 F.2d 796 (6th Cir. 1941). This statement is less a canon of construction and more a shorthand expression for the dictates of the law and the patents themselves. The doctrine of equivalence is subservient to file wrapper estoppel. *401It may not include within its range anything that would vitiate limitations expressed before the Patent Office. Exhibit Supply Co. v. Ace Patents Corp., supra; Smith v. Magic City Kennel Club, Inc., supra. Thus a patent that has been severely limited to avoid the prior art will only have a small range between it and the point beyond which it violates file wrapper estoppel. Similarly a patent which is a major departure from the prior art will have a larger range in which equivalence can function. The scope of the patents also influences the range of equivalence. A pioneer patent which occupies symbolically a six-inch circle will have three inches of equivalence if its range is fifty percent. An improvement patent occupying a two-inch circle has only one inch of equivalence with the same range. Thus with relatively identical ranges, the scope of the patent provides the pioneer patent with absolutely a larger range of equivalence.

IV

In summary, the determination of patent infringement is a two-step process. First, the meaning of the claims in issue must be determined by a study of all relevant patent documents. Secondly, the claims must be read on the accused structures. In doing this, it is of little value that they read literally on the structures. What is crucial is that the structures must do the same work, in substantially the same way, and accomplish substantially the same result to constitute infringement. This is the general approach which this court uses to determine the infringement of all the patent claims properly before it in this case.

The Patents

The patents in suit are concerned with rotor structures and control systems on rotary wing aircraft. The two principal types of rotary wing aircraft are the helicopter and the gyroplane (also referred to by plaintiff’s trademark Auto-giro).15 The autogiro has both fixed wings and rotary blades, and resembles a cross between an airplane and a helicopter. The helicopter has only rotary blades. The autogiro is propeller-powered ; the helicopter, rotary blade-powered. The autogiro cannot achieve under normal circumstances a vertical take-off like the helicopter with rotary blade power; it must taxi on takeoffs like the airplane with propeller power. The two types of aircraft do have several common characteristics. They both rise and descend and control horizontal movements through the operation of the same control systems — respectively, the collective and cyclic pitch control system.16

I

Stanley Patent No. 2,302,-068. — Neither party has excepted to the trial commissioner’s finding of fact on this patent and its single claim; therefore, it is adopted as part of this opinion. The '068 patent17 is concerned with rotor blade construction. Since they have routed and not apertured rotor blades, the accused structures, the Vertol HUP-1 and the Vertol H-21B helicopters, do not infringe the one claim of this patent.

II

Court of Claims Rule 58(c) requires that “ * * * [appropriate references shall be made at the end of each *402exception to the parts of the record relied upon in support thereof.” Otherwise, in determining the merit of the exceptions, the court would be required to go through the entire record without benefit of a baedeker. Such a task relegates the court to making the parties’ case rather than deciding it. Thus it has been strict in enforcing Rule 58(c). For example, a party has not been allowed to set forth only its own extended narrative version of the facts with an appendix of' its own proposed findings of fact. Hunt and Willett, Inc. v. United States, 351 F.2d 980, 168 Ct.Cl. 256 (1964). See, also, Societe Anonyme Des Ateliers Brillie Freres v. United States, 160 Ct.Cl. 192 (1963); Schmoll v. United States, 63 F.Supp. 753, 105 Ct.Cl. 415, cert. denied 329 U.S. 724, 67 S.Ct. 69, 91 L.Ed. 627 (1946).

For eight of the patents and their claims, defendant has excepted to the trial commissioner’s opinion by only listing the numbers of the findings of fact with which it finds fault. No briefs have been submitted in support of these exceptions. No references to the trial record or exhibits have been made to underpin the allegation that the trial commissioner erred in his findings. This method of excepting is a hollow gesture and has the same effect that failure to except has. See, Harrington v. United States, 161 Ct.Cl. 432 (1963); Batchelor v. United States, 169 Ct.Cl. 180, cert. denied 382 U.S. 870, 86 S.Ct. 147, 15 L.Ed. 2d 109 (1965). This failure to meet the burden of Rule 58(c) results in this court’s adoption of the trial commissioner’s findings of fact on the following patents:

Larsen Patent No. 1,918,157 — The ’457 patent relates to rotary wing aircraft sustaining rotors and particularly to improved blade damper mechanisms for controlling the pivotal movement of rotor blades about their vertical pivots. Claims 9, 12, 13, 14 and 18 of this patent are infringed by the Vertol HUP-1 helicopter and particularly the construction of its fore and aft rotors.

Cierva Patent No. 1,991,165— The ’465 patent relates to the mechanism for folding the rotor blades of rotary wing aircraft equipped with vertical pivots and to the mechanism for controlling the movement of the blades about these pivots. Claims 1, 5, 6, 7, 10 and 13 are infringed by the Vertol HUP-1 helicopter and particularly the construction of its fore and aft rotors.

Larsen Patent No. 1,990,291.— The ’291 patent relates to a method for adjusting the effective incidence of flappingly-pivoted rotor blades in their tracking. Claim 4 is infringed by the Hiller H-23A helicopter and particularly its sustaining rotor construction, the Vertol HUP-1 helicopter and particularly the construction of its fore and aft rotors, and the Bell HTL-4 helicopter and particularly its sustaining rotor construction. Claim 5 is infringed by the rotor blade tracking procedures of the Vertol HUP-1. Claim 6 is infringed by the rotor blade tracking procedures of the Hiller H-23A, the Vertol HUP-1, and the Bell HTL-4.

Larsen Patent No. 2,151,215.— The ’215 patent relates to displaceable droop support mechanism for sustaining rotary wing aircraft rotors. Its purpose is to prevent rotor blades from striking the ground or the aircraft when the rotor is either not rotating or rotating at substantially less than normal flight speed. Claims 1, 2, 3, 5, 6, 8 and 9 are infringed by the Kaman HOK-1 helicopter.

Campbell Patent No. 2,389,836. —The ’836 patent relates to aircraft with twin rotors arranged side-by-side so that when viewed from above, the left hand rotor rotates counterclockwise, and the right hand rotor rotates clockwise. Claim 1 is infringed by the Kaman HOK-1 helicopter.

Campbell Patent No. 2,321,572. ■ — The ’572 patent relates to aircraft with side-by-side rotors and flappingly-pivoted blades. Claims 8, 9, 28 and 29 are infringed by the Kaman HOK-1 helicopter.

Campbell Patent No. 2,311,966. —The ’966 patent relates to the control *403of aircraft with side-by-side rotors and flappingly-pivoted blades. In particular, this is the turn control which functions by having the rotors so positioned to provide for differential shifting of the rotors’ lift lines both fore and aft of the aircraft’s center of gravity. This allows the craft to turn or bank in a certain direction. Claim 1 is infringed by the Kaman HOK-1 helicopter.

Bennett Patent No. 2,3UU,967.— The ’967 patent relates to the control of aircraft with side-by-side rotors and flappingly-pivoted blades. In particular, this is the longitudinal and lateral attitude control which functions by the conjoint tilting of the lift lines of the rotors both fore and aft and laterally of the craft. The ’967 patent also relates to the rotor phasing which minimizes the bouncing effects which rotors experience in translational flight. Claims 1, 2, 3, 4, 13 and 18 are infringed by the Kaman HOK-1 helicopter.

Ill

Findings of fact for the following patents were properly excepted to either in whole or in part:

Cierva Patent No. 1,9U7,901.— The trial commissioner found Claim 3 infringed by the Vertol HUP-1 helicopter and particularly the rotor construction of its fore and aft rotors, the Hiller H-23A helicopter and its sustaining rotor construction, and the Bell HTL-4 helicopter and its sustaining rotor construction.

The claim. — Claim 3 is a combination claim containing several elements, each considered to be essential and equal to all others. Aro Mfg. Co. v. Convertible Top Replacement Co., supra, 365 U.S. at 345, 81 S.Ct. 599, 5 L.Ed.2d 592. For there to be infringement, it is necessary that every element or its substantial equivalent be found in the accused structures. E. g., McCullough Tool Co. v. Well Surveys, Inc., 343 F.2d 381 (10th Cir. 1965), cert. denied 383 U.S. 933, 86 S.Ct. 1061, 15 L.Ed.2d 851 (1966); Texas Co. v. Anderson-Prichard Refining Corp., 122 F.2d 829 (10th Cir. 1941); Alex Lee Wallau, Inc. v. J. W. Landenberger & Co., 121 F.Supp. 555 (S.D.N.Y.1954); Vollink v. Holland Celery Planter Co., 33 F.Supp. 203 (W.D.Mich.1938), aff’d 112 F.2d 576 (6th Cir. 1940). Therefore, only one element of the claim is to be discussed. This is the element which states:

In an aircraft, a sustaining rotor construction having blades mounted for movement with respect to an axis member and so proportioned that, under the influence of air currents, the blades have an average autorotational speed at the tip substantially in excess of the maximum flight speed of which the craft is capable. * * *18 [Emphasis added.]

The claim, on its surface, defines “so proportioned” in terms of function or result; however, a reading of the term in light of the patent documents shows “so proportioned” to include a means of accomplishing its function. See 35 U.S.C. § 112. The specification shows that the proportioning is a relationship between rotor blades and a fixed wing. In the beginning, it states that:

* * * the present invention contemplates certain novel features of proportioning and disposition of the blades and blade system, and the relation thereof to the fixed lifting surfaces * * *. ’901 Specification 1, Is. 46-50.

The invention is later described in the following manner:

According to the most complete development of the invention, however, I contemplate combining, in the same construction, fixed wings of a certain positive incidence in conjunction with a rotor having freely swinging air-rotated blades, each of which blades is itself set at positive incidence with relation to a no-lift setting on the axis * * *. Id at 2, Is. 3-9.

*404In a section which discusses embodiments of the ’901 patent, the specification expressly discusses proportioning. The embodiments referred to in this section are not means of proportioning the rotor blades, but various relationships between the rotor blades and a fixed wing. The rotor blade-fixed wing relationship is constant throughout this section. No other relationship is suggested, only various ratios of this relationship.19

Plaintiff contends that “so proportioned” refers to the construction of the individual blades. It cites specification references to “a thin or medium-thick section of substantially fixed center of pressure” (id at 3, Is. 46-48) and a blade “of uniform symmetry from end to end of its effective lifting portion” (id at 3, Is. 28-30). Neither of these references was made in the company of the term proportioning. Furthermore, these references are to embodiments inasmuch as the specification states that “[t]he foregoing are cited to show that different types of blade construction may be employed in carrying out the present invention.” Id at 3, Is. 51-53.

The file wrapper does not compel a different interpretation of the claim. Claim 3 was initially rejected three times on the strength of the prior art. Referring to its rejection, along with other claims, the applicant remarked about “the entirely novel cooperative relationships of the rotor and the particular type and arrangement of fixed wing employed * * '901 File Wrapper 124. The novelty of this relationship between rotor blades and fixed wings is recited in other parts of the file wrapper dealing with the inadequacy of the prior art to meet the spirit of his claims. See, e. g., id at 46, 60 and 82.

The above interpretation of “so proportioned” does not ignore the concept of claim differentiation which states that claims should be presumed to cover different inventions. This means that an interpretation of a claim should be avoided if it would make the claim read like another one.20 Claim differentiation is a guide, not a rigid rule. If a claim will bear only one interpretation, similarity will have to be tolerated.

Claims 4-7 of the ’901 patent refer to a substantially fixed aerofoil (i. e. wing). And Claims 8-17 refer to a relatively fixed lifting aerofoil surface. Interpreting Claim 3 to refer to a fixed wing would not render any of those claims redundant. Claims 4 and 5, the ones with which 3 has the greatest similarity, are markedly different. Claim 3 refers to “maximum flight speed of which the craft is capable;” Claims 4 and 5 refer to the “translational flight of which the craft is capable.” Also, Claim 3 does not have the specificity of these claims — for example, their limitation to an aircraft

*405“with a substantially fixed aerofoil of an effective area approximately 50 percent to 100 percent of the area of the rotative blades.” These differences are not to be undervalued because of their slightness since claims covering the same invention in varying degrees of breadth or scope usually have only slight differences.

The accused structures. — The Vertol, Hiller, and Bell helicopters found by the trial commissioner to infringe Claim 3 do not have a fixed wing; therefore, they cannot have their rotors proportioned in the manner called for by the claim as explained by its supporting documents. Since they do not operate in substantially the same way as the claim requires, they do not infringe it.

Prewitt Patent No. 2,380,581. — The trial commissioner found: (1) Claim 28 infringed by the Hiller H-23A helicopter, and (2) Claims 29, 36, 37, 38 and 42 infringed by the Hiller H-23A and the Bell HTL-4 helicopter. Infringement by these helicopters included their collective pitch systems having both manually and centrifugally actuated pitch governing mechanisms.

The claims. — Each claim contains an element which defines a nonmanual mechanism for regulating the rotational speed of rotary wing aircraft rotors.21 Such changes are achieved through the use of movable weights mounted in each rotor and attached to the collective pitch control system. As the rotor speeds up, the concomitant increase in centrifugal force causes the weights to move outboard. Such movement increases the pitch angle of the rotor blades and slows the blades by presenting a greater blade surface to the air flow.

In listing the patent’s purposes, the ’581 specification states at 1, col. 2, Is. 1-5: “A further object is to provide governing means in cooperation with my blade incidence control which will tend to maintain constant rotational speed of the rotor.” Since the term “governing means” is only used in the specification in this way,22 it is proper to presume that it is being used in the same way in Claims 29 and 37. op. cit. supra at footnote 8. The other claims (28, 36, 38 and 42) use the term “centrifugal means.” Since it is centrifugal means which produces the governing means, these terms are interchangeable. See ’581 Specification at 2, col. 2, Is. 10-17.

Plaintiff contends that only Claim 28 provides a means for governing rotational speed of the rotor. It claims that centrifugal means and governing means are not the same. To obtain governing means, it says that there must be centrifugal means plus a resilient means and that only Claim 28 provides this combination.23 This combination is only *406a preferred embodiment in that the resilient means (usually a spring) is not necessary for the maintenance of a constant rotational speed; it only acts as an aid to such an end. id at 4, col. 1, Is. 8-30. Without the resilient means, the centrifugal means will still “tend to maintain constant rotational speed of the rotor.”

In prosecuting Claims 28, 29, 36 and 37 of the '581 patent24 before the Patent Office, the applicant based their validity on novel non-manual governing means which tended to maintain constant rotor speed.25 After the claims had been so narrowed, the Patent Examiner allowed Claims 28, 29 and 36; Claim 37 was rejected for not distinguishing the prior art. It was then allowed when amended to be in accord with its com- . panion claims. ’581 File Wrapper at 73. After the allowance of the ’581 patent, Claims 38 and 42 were entered under Patent Office Rule 78 26 which permits the entry of additional claims after the allowance of the application. This rule is not intended to provide a way for the continued prosecution of