Additionally, ducted fans are more efficient, have better disc loading qualities and produce more thrust than do helicopter rotor blades of equal blade area. A more current testament of the advances in ducted fan technology as to the weight carrying capability of ducted fans is the Solo Trek XFV, seen above, currently being developed jointly by NASA and Millennium Jet Inc. of Santa Carla, California.The Solo Trek lifts 850 pounds of gross weight vertically employing two  small 32 inch ducted prop fans. The 32 inch ducted fans are of the same  type used in the smallest HOVTOL UAV for applications such as law enforcement.

The technical data provided herein regarding ducted fans and shrouded propellers was derived from the following research as well as Airborne Autonomous Systems' own research:

• Confidential Research study performed by Hiller Aircraft Company in 1957 for the Office of Naval Research - Contract No. 2199 (00) [Declassified]
• US Army Transportation Research Command, Fort Eustis, Virginia - Technical Report No. 63-21
• Results of Wind Tunnel Test of a Full Scale Wing Mounted Lift Fan - By General Electric Company - US Army Contract No. EA 44-177-TC-584
• CAL/USAAVLBS Symposium Proceeding, Volume 2 - Propulsion and Interference Dynamics, Volume 3 - Aerodynamic Research On Boundary Layers, Dated 22-24 June 1966
• Variable Geometry Shrouded Propeller Test Program, a Hamilton Standard Test Report by Harry S. Wainauski
• AIAA Fifth Annual Meeting and Technical Display, AIAA No. 68-994 - Shrouded Propellers - A Comprehensive Study Dated October 1968
• Theoretical Study of Ducted Fan Performance - NASA CR-1494, January 1970
• Variable Pitch Ducted Fans for STOL Transport Aircraft by R.M. Denning, Rolls Royce, 1971 - ASME Conference 1972 (ASME 72-GT-61)
• AIAA-86-1474 - Turbofan and Propfan as Basis for future economic Propulsion Concepts - June 1986, Huntsville, Alabama
• Axial Flow Fans Book by R.A. Wallis
• Ducted Fans for Light Aircraft - Analysis, Design, Construction, by R.W. Hovey June 1986
• Design and Testing of a Ducted Tail Rotor Concept Demonstrator for a Model 222U Helicopter - 22nd European Rotorcraft Forum, Brighton, United Kingdom, September 17-19, 1996 by J.R. Andrews, R.G. Riley and C. Rahnke

ADVANTAGES OF DUCTED FANS AND SHROUDED PROPELLERS OVER FREE AIR PROPELLERS AND ROTORS

The aforementioned studies demonstrate that ducted fans or shrouded propellers have five primary advantages over free air props or rotors they are:

• (A)  Free props / rotors must have about 1.5 times larger diameters to produce the same static thrust from the same horsepower because of thrust loss at the blade's tips.
• (B) Reduced diameters of ducted fans or shrouded props permit higher shaft speeds, which further increase thrust potential over larger free props and rotors.
• (C) Ducted fans and shrouded propellers are acoustically stealthier as they are inherently quieter than free props and rotors.
• (D) Ducted fans and shrouded propellers when mounted horizontally in a fuselage for vertical lift are considerably stealthier for both thermal and radar signatures as the spinning blades are concealed.
• (E) Ducted fans and shrouded rotors are less dangerous to ground personnel and other objects in close proximity to their spinning blades because of the duct or shroud.
• ANOTHER ADVANTAGE - HIGHER DISC LOADS ARE POSSIBLE

Government and industry research has demonstrated that ducted fans and shrouded propellers have better disc loading qualities than do free propellers and un-shrouded helicopter rotor blades of equal blade area.

For an analogy on ducted fan disc loading using a 2-inch by 4-inch by eight-foot board to represent a helicopter blade, if a 500-pound weight is placed in the center of the board it would break. If the same board was cut into four equal pieces and stacked on top of each other at right angels to represent a 4 bladed-ducted fan of equal blade area and the same 500-pound weight is placed at the center; it would not break the stack of boards. The latter analogy of the shorter stacked boards is representative of the enhanced disc loading quality of a ducted fan compared to that of a single board representing the helicopter rotor blade disc loading analogy.

AN ANALOGY OF HIGHER THRUST AND EFFICIENCY FOR THE SAME BLADE AREA

According to government and industry research ducted fans and shrouded propellers are significantly more efficient and produce more thrust per blade area than do free air propellers or un-shrouded helicopter blades of equal blade area. One reason for this is that there is no thrust loss at the tips of the blades. Therefore, the blade diameter can be shorter. Because the blades of ducted fans or shrouded propellers can be made shorter they can be spun much faster which is possible as a result of their shorter blade length. Thus, these two factors combine to give ducted fans or shrouded propellers greater thrust and efficiency over that of free air props or helicopter blades of the same blade area.

While a free air propeller's efficiency may drop due to reduced radius this loss in propeller efficiency is exponentially offset by increases higher disc loading capabilities and higher thrust afforded by the shroud or duct and the higher RPM potential. Therefore, the efficiency of a shrouded propeller or ducted fan whose rotor or propeller is of the exact configuration as that of a given free prop or rotor configuration, when the duct or shrouded is added to that configuration the thrust is significantly greater than that of the free prop or rotor without the shroud or duct where disc loading remains unchanged. What this means is the advantage that a ducted fan or shrouded propeller affords is a much smaller radii is possible for a given thrust requirement over that of a free prop or rotor.

A second analogy to demonstrate a ducted fan's or shrouded propeller's greater efficiency and thrust over that of a free prop or helicopter blade of equal blade area would be thus: If we take the same board in the previous analogy and the eight foot board was cut in half and used as oars for a boat with each oar being four feet long to represent a helicopter rotor application; and likewise, in comparison to represent a ducted fan, for example, we would use, as oars, four two foot long boards. The boat of the ducted fan analogy with the four, two foot, oars will go faster than the two four foot oared boat of the helicopter analogy because more oars (blade area) are being used in the ducted fan boat analogy. Also, because the oars are smaller on the ducted fan boat analogy the repetitions of the oar strokes can be more rapid as compared to the two four foot oars of the helicopter boat analogy.

COMPARING THE POTENTIAL OF A HOVTOL DUCTED FAN OR SHROUDED PROPELLER UAV TO THAT OF THE NEW NAVY FREE AIR ROTOR HELICOPTER VTUAV

Taking the Navy's new VTUAV, as an example for comparison purposes to the HOVTOL, the Navy's VTUAV, dubbed the "Fire Scout", is an unmanned helicopter with a three bladed rotor diameter of 27.5 feet or 594.5 square feet of disc (rotor) area. As any vertical flying aircraft requires 1.25 pounds of thrust per pound of gross weight and the Fire Scout's maximum take off weight is approximately 2,550 pounds it can be easily calculated that the thrust produced by the Fire Scout's 3 bladed 27.5 foot diameter is around 3,200 pounds of thrust with a 480 horse power engine. As with any helicopter up to 30% of its horsepower is typically consumed by the helicopter's tail rotor. If we took the same 480 horse power engine and placed it in a HOVTOL configuration with two 8 foot diameter shrouded rotors you would get more than 3,200 pounds of thrust for vertical lift. Also, because this amount of thrust would only be needed during VTOL and hover operations the HOVTOL would have a much greater range, payload capacity and higher speed as the HOVTOL's vertical lifting rotors would be clutched out while flying horizontally like a conventional aircraft. It should be noted that a HOVTOL equivalent of the Fire Scout would not have the aircraft's fuselage in the way of the downward slipstream of the vertical thrust and it would not waste horsepower and fuel on a tail rotor. These advantages equate to greater range, speed, safety, payload, and acoustic, thermal, and radar stealthiness over that of the Fire Scout comparison.

EVEN MORE ADVANTAGES

Three more important reasons for using ducted fans or shrouded propellers in VTOL UAV's or manned applications is that: (1) They require less landing space than helicopter or tilt rotor aircraft do, and (2) ducted fan VTOL aircraft are safer because they can hover close to objects and people without exposed rotor blades posing a danger. (3) Ducted fans do not produce a blast of scorching flames and heat as jet engines do when they are employed in VTOL applications whether manned or unmanned. These latter two points are very important considerations for a busy Naval flight deck while the ship is pitching and rolling in heavy seas where the pilot has limited situation awareness.

If shrouded propellers are more efficient and ducted fans have better disk loading qualities, and both produce more thrust than free props or rotors then why are they not used in aviation very much?

First, there is the cost of adding a ducted fan or shrouded propeller to existing aircraft design configurations, which have essentially remained unchanged with very few exceptions since the Wright Brothers, except for helicopters, whose limits have not only been deduced but whose limits have also been reached.

Second, there is the issue of aircraft configuration changes that do not necessarily lend themselves to being adaptable to current aircraft design axioms taught by Colleges and Universities and employed by the aviation industry today.

Third, depending upon the configuration, ducted fans, and to a lesser degree shrouded propellers, build up a boundary layer of air in front of the duct when a ducted fan is used for horizontal flight above 150 knots of airspeed. This limits the forward horizontal airspeed of an aircraft that employs ducted fans for "horizontal" propulsion.

Depending on the application the HOVTOL UAV utility patent can use, for vertical lift, either ducted fans for smaller aircraft designs or shrouded rotors for larger aircraft. It is not necessary to ascend vertically at speeds greater than 30 knots. Therefore, it stands to reason that the HOVTOL aircraft configuration lends itself to employing either ducted fans or shrouded propellers economically for vertical flight without rotating it's propulsion system for horizontal flight, VTOL transitions or for hovering.

The patented HOVTOL UAV configuration is programmable. It is not limited to employing just ducted fans or shrouded propellers for "horizontal" flight. For instance, the utility of the programmable configuration of the patent allows for employing delta wings with single or multiple jet engines for faster forward air speeds while still employing ducted fans or shrouded propellers for vertical lift for VTOL and hover flight operations. The programmable concept allows for a wider range of configurations of engines and wing designs for various missions from higher payloads, to faster airspeeds or for stealth.

The following are several examples of HOVTOL possible wing configuration changes without making major changes to the overall aircraft's fuselage.

VISUAL COMPARISON OF A DUCTED FAN WITH A FREE AIR PROPELLER

The following is a simplified graphical representation of the thrust comparisons determined by government and industry research. As can be seen below in the free prop drawing, (D-1) represents a lower volume of thrust than that of the ducted fan's higher volume of thrust (D-2) even though the propeller radius is the same for the ducted fan and free prop.

COMPARING DUCTED FANS WITH SHROUDED PROPELLERS

There are many variations of ducted fans and shrouded propellers. While both ducted fans and shrouded propellers increase thrust compared to free props or rotors there are some significant differences between the two concepts. Typically ducted fans are smaller, have higher RPM rates, greater disc loading abilities and produce more thrust per blade area than do larger, slower RPM rated shrouded propellers but they are less efficient. Ducted fans can produce from 30% to 50% more thrust over a free rotor of the same blade area whereas a shrouded propeller can produce 15% to 25% more thrust with less disc loading capacity but with a higher efficiency. This means that a ducted fan's or shrouded propeller's design, construction and application are major considerations in determining what is desired relative to the application. For example, if the mission requires hovering all the time and space or size is not a factor then a more efficient shrouded propeller should be chosen. If however, hovering is a smaller percentage of the overall mission requirement then a less efficient smaller but higher thrust ducted fan would be chosen. Therefore, when to employ a ducted fan or shrouded prop is relative to the mission requirements. There is no definitive demarcation line between where a ducted fan becomes a shrouded propeller.

In either case the bottom line is that a shrouded propeller or rotor and a ducted fan will always out perform a free prop or rotor. The challenge, therefore, is not so much in employing ducted fans or shrouded propellers in existing aircraft design axioms as it is to develop feasible and practical aircraft designs that can take advantage of what ducted fans and shrouded propellers have to offer.

EXAMPLES OF DUCTED FANS VERSUS SHROUDED PROPELLERS

Shown below on the left is the Piasecki VZ-8, which would be an example of a shrouded rotor application as compared to the Moller Aerobot, below at the right, as an example of a ducted fan. Shrouded rotor shrouds are typically much shallower than a ducted fan's duct. A shrouded propeller is much larger, turns at lower RPM rates, and is more efficient than ducted fan rotors that turn higher RPM rates and have smaller diameters. In either case both ducted fans and shrouded propellers will produce more thrust with better disk loading qualities than their free propeller or open rotor counterparts.

EVEN SMALL VARIABLES IN DUCTED FAN OR SHROUDED ROTOR DESIGN CAN PRODUCE LARGE VARIANCES IN PERFORMANCE

There are many variables to consider in designing ducted fans or shrouded propellers such as the number of propeller blades, horsepower available, RPM, blade design, duct design among other factors. Variations in these design parameters can produce wide variations in performance. For example, the Hiller VZ-1 compared to the Sikorsky CYPHER, both shown below, are very similar in rotor diameter, they both employ a duct or shroud and both use coaxial rotors. However, the Hiller VZ-1, below left, carried two engines and the payload weight of a man whereas the CYPHER, below left, has one engine and can only carry payloads of between 30 and 50 pounds depending on its mission, range or endurance requirements.

In another example in the wide variation of design and applications of ducted fans and shrouded propellers can be seen in the differences between a 38" shrouded propeller application of the new SoloTrek Personal Transportation Vehicle, shown below at left, and the 1950's vintage, full sized, manned aircraft employing 48" ducted fans below at right.

DESIGN ENHANCEMENTS THAT CAN FURTHER INCREASE THRUST

There are a number of ducted fan or shrouded propeller design attributes that can further enhance thrust as well as lift in VTOL applications by synergistically optimizing the relationships of the following factors:

Matching blade geometric configurations of the number of blades together with blade twist, pitch angle, airfoil shape, tip speed among other factors congruently with an optimum engine horsepower and engine RPM. For example, by increasing the number of blades from a single blade propeller to a five bladed propeller and increasing engine horsepower and RPM will significantly increase thrust without increasing blade or duct diameter. This has the same effect as increasing the blade diameter with a corresponding increase in horsepower in conventional free prop designs.

Ducted fan or shrouded propeller blade tip clearance with respect to the duct's or shroud's inner wall can have a significant impact on a ducted fan's or shrouded propeller's performance. As even metal propellers can grow (expand) at high RPM's a close tolerance on blade tip to inner wall clearance can be very difficult to maintain. However, the closer to the inner wall the blade tip is in operation the better the efficiency of the fan. By placing the blade tip into an annular grove designed into the side wall of the duct or shroud not only eliminates blade tip clearance issues but it increases thrust, as the blade's tip clearance relative to the inner wall is always zero regardless of the blade's tip clearance inside the grove.

Another design consideration to increase thrust in a ducted fan or shrouded propeller is based on Theodorsen’s "Theory of Propellers" which states that the thrust of a ducted fan with an inlet diameter greater than the propeller's will be equally divided between the pressures on the rotor and pressures on the duct. Therefore, by optimizing the duct's or shroud's inlet and inlet lip radius relative to the diameter of the propeller's can produce increases in thrust without changing propeller configuration, engine horsepower or RPM.

Yet another means of increasing thrust in ducted fans or shrouded propellers without increasing propeller configuration, engine horsepower or RPM is by making the duct or shroud in an hourglass shape where the inlet and exhaust radius is larger than the propeller's where the exhaust's radius is smaller than the inlet's radius. This configuration has a much smaller but similar effect as that of a jet engine where a large air mass is axially impelled through a large inlet and then compressed by the propeller at the narrowest point of the duct or shroud. The resultant expansion of the compressed air is allowed to expand aft of the propeller thus adding to the thrust.

WHAT IS NEEDED

Today there are no known publicly or commercially available design software, CAD or Catia applications that stand alone to aid the designer in developing a specific shrouded propeller or ducted fan design with enough accuracy sufficient to preclude educated trial and error development techniques which are expensive and time consuming. What is needed is a Catia based CAD design program where full exploitation of the technology can be tested without the expense and time involved in trial and error research and development. However, this may be about to change. In July of 2001 the US army started accepting proposals for a SBIR initiative aimed squarely at developing a software tool to aid designers in this respect. The reference for those interested is US Army SBIR - Topic A01-080, "A Preliminary Design Tool for Shrouded-Fans". Just how extensive, reliable and applicable this tool ends up being will remain to be seen. It is hoped that enough foresight will be brought to bear to require the software to cover the gamut of fan design possibilities from shrouded propellers and rotors to ducted fans as one could only hope that industry will know the significant differences of these "black arts". For information regarding the status of the design tool referenced above the technical point of contact is: Mr. Tom Maier, Program Manager, (650) 604 - 3643, Email: tmair@mail.arc.nasa.gov

EXAMPLE OF MODERN DAY APPLICATION OF DUCTED FAN DESIGN

Seen above is the latest next generation fighter aircraft, dubbed the Joint Strike Fighter (JSF) by the government. Lockheed Martin, which just won the competition against Boeing for the coveted 30+ year contract, is to produce the aircraft for several US Military branches and foreign countries. As can be seen below the fighter has a single ducted fan behind the pilot's seat. This ducted fan generates more than 18,000 pounds of thrust.

The HOVTOL invention and its potential variants, like the SolloTrek personal transportation vehicle and the Lockheed Martin X35B Joint Strike Fighter, are modern day examples of new applications of a once abandon, time proven but little known, 1950's, "black art" technology.