Hybrid drive powered lift platform

An air-based thrust recovery system for use in a powered lift platform utilizing fixed-pitch propellers is provided. The system utilizes an air compressor coupled to the drive motor of the platform and an on-board, high pressure tank for air storage. The fixed-pitch propellers are coupled to the primary drive system with over-running clutches, thus allowing the thrust recovery system to rotate the propellers faster than if driven solely by the primary drive system. Preferably the propellers are surrounded by air ducts. The thrust recovery system uses individually, or in combination, cold air jets mounted at the tips of the propellers, a circulation control system which blows out the leading or trailing edge of the propellers, and one or more perforated tubing segments surrounding at least a portion of the platform's air ducts.

FIELD OF THE INVENTION

The present invention relates generally to powered lift platforms and, more particularly, to an apparatus for augmenting the lift capabilities of the primary drive system in a dual propeller platform.

BACKGROUND OF THE INVENTION

A variety of propeller based lift platforms have been designed over the years which use two or more propellers coupled to a single engine. Typically the propellers are ducted, thus increasing propeller efficiency, decreasing noise and eliminating the dangers associated with exposed blades.

In general, ducted propeller platforms do not possess autorotation capabilities. In order to achieve at least a level of fail-safe operation, such platforms often employ redundant engines thus allowing the platform to overcome the failure of a single engine. Unfortunately as such an approach does not provide complete system redundancy, the failure of any of a variety of other components within the drive system will still lead to the catastrophic failure of the platform.

A typical ducted platform uses fixed-pitch propellers, thus simplifying the overall design. As a result, however, such a platform must use an internal combustion engine since turbine engines, although more efficient, suffer from spin-up lag. Although the inherent lag in turbines can be overcome using variable-pitch propellers, this defeats the mechanical simplicity and the weight savings offered by the used of fixed-pitch propellers.

Another issue confronting ducted propellers is their susceptibility to upsets due to sudden wind gusts.

Although a variety of fixed-pitch propeller platforms have been designed, these platforms typically require the use of internal combustion engines, provide limited fail-safe operation, and are susceptible to wind gusts. The present invention overcomes these limitations.

SUMMARY OF THE INVENTION

The present invention provides a thrust recovery system for use in a powered lift platform utilizing fixed-pitch propellers. The thrust recovery system of the invention is a compressed air-based system which enhances the lift characteristics of the platform with minimal weight penalties. The system utilizes an air compressor coupled to the drive motor of the platform and an on-board, high pressure tank for air storage. The fixed-pitch propellers are coupled to the primary drive system with over-running clutches, thus allowing the thrust recovery system to rotate the propellers faster than if driven solely by the primary drive system. Preferably the propellers are surrounded by air ducts.

In one embodiment of the invention, the thrust recovery system uses cold air jets mounted at the tips of the propellers, i.e., tip jets, to rotate the propellers. The tip jets on each propeller can be under the control of a single valve, thus providing an extremely simple design, or under the control of multiple independent valves, thus providing a means of achieving single axis attitude control. In addition to the tip jets, the air-based system can be coupled to a circulation control system comprised of a plurality of holes within either the trailing edge or the leading edge of the propeller blades, thus providing a means of increasing the coefficient of lift. As with the tip jets, valves can be used to allow either simultaneous control, or independent control, of the circulation control system contained within each propeller. Alternately, or in addition to the tip jets and the circulation control system, the air tank of the thrust recovery system can be coupled to one or more perforated tubing segments surrounding at least a portion of the platform's air ducts, thereby providing a means of overcoming the effects of sudden wind gusts.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

FIG. 1is a simplified view of a dual propeller powered lift platform according to the prior art. In this figure, as well as those that follow, the required control system is not shown as there are a variety of suitable control systems that are well known by those of skill in the art and which can be used with the prior art platform as well as the invention.

As shown, platform100utilizes a pair of fixed-pitch propellers101/103that are counter-rotating to offset yaw effects. Propellers101/103are coupled to an engine105via a transmission107and a pair of gearboxes109/111(e.g., ninety degree gearboxes). If desired, propellers101/103can be enclosed within a pair of aerodynamic ducts201/203as shown inFIG. 2. Ducts201/203increase the efficiency of propellers101/103, thus increasing the lift of the platform for a given propeller diameter and engine power. Additionally, by eliminating the exposed blades of the propellers, the ducts substantially increase the safety of the platform while decreasing the noise generated by the platform. Thrust is controlled by rapidly varying the rotational speed of the propellers.

There are a number of disadvantages associated with the platforms shown inFIGS. 1 and 2. First, failure of engine105, transmission107or gearboxes109/111will lead to the catastrophic failure of the platform. Even assuming a low altitude application, on the order of 25–100 feet above ground level, such failure would undoubtedly harm any on-board personnel as well as damage the platform and any platform payloads. Second, due to the use of fixed-pitch propellers, a turbine engine cannot be used in the drive system since the system is incapable of compensating for engine lag. Third, lift platforms of this design are susceptible to upsets due to wind gusts, a problem exacerbated by low altitude applications in which there is limited time for correction prior to ground impact.

FIGS. 3 and 4illustrate one method of achieving fail-safe operation. As shown, in addition to the drive system ofFIG. 1, a second, redundant drive system is provided. The redundant system couples a second engine301and a second transmission303via gearboxes109/111to fixed pitch propellers101/103. If desired, the propellers can be enclosed in ducts, i.e., ducts401and403as shown inFIG. 4. Although the platforms shown inFIGS. 3 and 4supply drive system redundancy, and thus a level of fail-safe operation, such redundancy comes at a significant cost in weight due to the second motor and transmission.

Air-Based Thrust Recovery System

FIGS. 5–14illustrate a variety of embodiments of a thrust recovery system, each of which utilize an on-board air compressor/air storage system. The air compressor/air storage system of the invention provides a secondary power source with minimal additional weight. As described in more detail below, the secondary thrust recovery system can be used to provide fail-safe operation and/or rapid propeller spool-up and/or gust alleviation.

FIGS. 5 and 6illustrate an embodiment of the invention which achieves fail-safe operation without the weight penalties required by including a redundant mechanical drive system as previously described. Platform500includes a standard fixed-pitch, dual-propeller drive system as previously described. Additionally, engine105drives an air compressor501which supplies compressed air to air tank503via supply line505. Compressed air from air tank503is supplied to cold air jets507integrated into the tips of propellers509/510via supply lines511. Preferably supply lines511are integrated into propeller drive shafts513and ducted through the bodies of the propellers. Valve517provides simultaneous control of the jets integrated into both propellers, providing a means of activating jets507as well as controlling the amount of air exhausted through the jets. Platform600is the same as platform500except for the addition of air ducts601/603.

In use, after a failure in the primary drive system (e.g., engine105, transmission107or gearboxes515) is detected, the secondary, air jet based system is activated in order to rotate propellers509/510using tip jets507. The rotational velocity of the propellers, and thus the thrust of each propeller, is determined by the amount of air exhausted through the jets which is controlled by valve517. Although the secondary, air jet based system is capable of providing the same thrust as the engine (i.e., engine105), due to the limited capacity of compressed air tank503, this thrust can only be supplied for a short period of time. Thus the air jet based system is designed to safely land the platform from an expected flight altitude on the order of 100 feet above ground. It will be appreciated that as the platform approaches the ground, the overall weight of the platform decreases due to the expenditure of air from tank503, thus yielding increased control authority during landing.

As shown inFIGS. 5 and 6, the thrust recovery system of the invention also includes an over-running clutch519added to each propeller gearbox, thus allowing the propellers to rotate faster than drive shafts513. There are several benefits to this configuration. First, if there is a failure in the primary drive system, i.e., engine105, transmission107or either gearbox515, the over-running clutches insure that the secondary drive system, e.g., the air jet system, can continue to rotate the propellers at the desired rotational velocity. Second, this configuration allows a turbine to be used for engine105since the air jets are capable of rotating the propellers during engine spool up. As turbine105and coupled drive shafts513reach parity with the tip jet assisted propellers, the output from the tip jets can be gradually reduced until they are no longer required. Thus the invention allows the platform to utilize higher performance and more reliable turbine engines without using variable pitch propeller blades. It will be appreciated that the invention is not limited to the inclusion of over-running clutches within the gear boxes as shown. For example, an over-running clutch coupled to transmission107will provide similar capabilities.

The system shown inFIGS. 5 and 6not only provides fail-safe operation and the ability to use a turbine engine, it does so with minimal weight penalties. For example, assuming a ducted system in which the ducts are 5 feet in diameter and the overall platform is approximately 6 to 7 feet in width and 12 to 14 feet in length, a a realistic mission weight for such a platform is 750 pounds (e.g., airframe—210 lbs., drive train excluding engine—90 lbs., engine (installed)—140 lbs., air compressor system—40 lbs., air in tank—20 lbs., fuel—50 lbs., and payload—200 lbs.). Utilizing a 100 horsepower 2-stroke aircraft engine, the static thrust for a ducted, dual-propeller platform of the above duct diameter is approximately 820 pounds. Accordingly, during flight the platform has approximately 9% additional thrust available for control authority.

FIG. 7illustrates an alternate preferred embodiment based on the platform shown inFIG. 6. In this embodiment, however, a pair of valves701/703couple the compressed air system to the tip jets on propellers509/510, respectively. Valves701/703provide independent control of the amount of air exhausted by the tip jets contained on propellers509/510, thus providing a simple means of controlling the attitude of the platform in one axis by independently controlling the rotational velocity of the two propellers. Preferably the platform includes air ducts601/602as shown.

FIG. 8illustrates an alternate preferred embodiment utilizing the compressed air secondary system of the invention. In this embodiment, in addition to the tip jets, the compressed air system is coupled to a plurality of small holes801located either in the trailing edge or the leading edge of each propeller blade. Preferably holes801are approximately 0.020 to 0.30 inches in diameter and are located within the region of 15 to 25 percent axial chord upstream of the trailing edge. Coupling air tank503to holes801increases the coefficient of lift via circulation control, thereby increasing the level of thrust control. As use of circulation control also increases the coefficient of drag, preferably tip jets507are used to compensate for the additional load placed on the drive system. In the embodiment illustrated inFIG. 8, a single valve803controls air flow to both propellers as well as both air based system, i.e., tip jets507and circulation control holes801. The embodiment shown inFIG. 9is the same as that shown inFIG. 8, except for the addition of ducts901/903.

Although the air systems of both propellers can be operated simultaneously using a single valve as illustrated inFIGS. 8 and 9, dual valves1001/1003as shown inFIG. 10allow independent control of the air flow to each propeller. As a result, and as described relative toFIG. 7, dual valves provide the platform with single axis control. Although the platform ofFIG. 10is ducted, it will be appreciated that ducts901/903are not required.

In order to maintain as simple of a mechanical design as possible, preferably the air supply lines contained within the propellers are used both with the tip jets and the circulation control system. If, however, independent control of the two air-based systems is desired, separate supply lines can be used. Although it is possible to use a single valve with the supply lines to the tip jets for both propellers, and a second valve with the supply lines to the circulation system for both propellers, preferably independent control of both air-based systems and both propellers is provided as illustrated inFIG. 11. As shown, air line1101to propeller1103is split into two lines,1105and1107, the first air line (e.g., line1105) feeding tip jets1109and the second air line (e.g., line1107) feeding circulatory holes1111. Separate valves1113and1115on lines1105and1107, respectively, provide independent control of the two air-based systems for propeller1103. Similarly, valves1117/1119on air lines1121/1123, respectively, provide independent control of tip jets1125and circulatory holes1127of propeller1129. This embodiment provides independent control of the two propellers as well as independent control of the two air-based systems for each propeller. Other variations are clearly envisioned by the inventor, such as (i) four valve systems in which one valve controls total airflow to one propeller, a second valve controls total airflow to the second propeller, and a three-way valve per propeller controls distribution of air flow to the two air-based systems; (ii) three valve systems in which one valve controls air flow to the tip jets on both propellers, a second valve provides circulation control of one propeller and a third valve provides circulation control of the second propeller; and (iii) dual valve systems in which one valve controls air flow to the tip jets on both propellers and the second valve provides circulation control for both propellers.

In addition to using the air-based system of the invention to provide system redundancy and control via tip jets and circulation control, the air-based system can also be used to provide gust alleviation in any embodiment using air ducts (e.g.,FIGS. 6,7and9–11). More specifically, by selectively blowing air from air tank503over selected duct segments, the effects of wind gusts can be reduced.FIG. 12illustrates one such system based on the tip jet system shown inFIG. 7. As shown, air from air tank503not only feeds tip jets507via lines511, but also feeds air through line1201into perforated tubing1203located around the periphery of air ducts1205. At least two valves1207/1209are used to control which air duct tubing is coupled to tank503, as well as the amount of air to be emitted through the tubing's perforations. Preferably the perforated tubing surrounding each duct is divided into segments, for example four segments, the amount of air flowing through each segment under the control of an independent valve.FIG. 13provides a bottom view of one of the ducts1205. As shown, the perforated air duct tubing is divided into four segments,1301–1304, each coupled to air line1201via independent valves1305–1308, respectively. It will be appreciated that each air duct tubing surrounding the individual ducts can be divided into fewer, or greater, numbers of segments, depending upon the desired level of control. It will also be understood that the gust alleviation system, i.e., the perforated tubing surrounding the air ducts, can not only be used with the tip jets as illustrated, but also with the combination of the tip jets and the circulatory control system (e.g.,FIG. 14).

It will be appreciated that the figures are meant to illustrate the primary elements of the invention and that variations are clearly envisioned by the inventor. For example, the air compressor can either be a stand-alone device as shown, or integrated within the engine. Accordingly, the disclosures and descriptions herein are intended to be illustrative, but not limiting, of the scope of the invention which is set forth in the following claims.