1505

Item 1505

OTHER: Helicopter - Inside - Interleaving - Pros & Cons vs. Tilt-rotor

Tilt-rotor Interleaving

Advantages:

At first blush, this configuration would appear to be superior to V-22 in all aspects, except perhaps for maximum forward speed.

Hover: With Active Blade Twist (ABT) and Higher Harmonic Control (HHC) at the root, the Figure of Merit should be superior to that of current rotorcraft.

Forward Flight: The Tiltrotor configuration has the advantage of less drag in forward flight. However, the Interleaving configuration has propeller(s) that are optimized for forward flight, whereas the V-22 proprotors are a compromise between propeller and rotor. In addition, the Interleaving configuration could implement a future turbofan+pto engine for even faster forward speeds, which the Tiltrotor cannot do.

Transition: The transition of this configuration between hover and cruise should be benign, whereas it has been said that if the tiltrotor was to loose power during transition the results would be fatal. See 'Concerns:' below.

The Future: It is extremely likely that future rotor blades will incorporate Active Blade Twist, Higher Frequency Individual Blade Pitch Control, and Reverse Velocity Utilization. These should significantly improve the Lift/Drag ratio of the rotors. In turn, this allows for smaller rotors and therefore reduced drag.

Note that Sikorsky's work on a Variable Diameter Tiltrotor may result in a marginal improvement to the Tiltrotor configuration in the future.

Disk Area:

Drawing:

The Interleaving disks can be significantly reduced in size, and the stagger reduced, while still provide a lower disk loading than the Tilt-rotor can.

Tilt-rotor vs. Interleaving:

Two serious disadvantages of the Tilt-rotor are its small lifting capability, and its large rate of descent during autorotation. Both of these shortcomings result from the small disk area, which in turn necessitates a high disk loading. Sikorsky has a number of patents on a variable diameter tilt-rotor, however the high disk loading will probably be an unavoidable limitation of the configuration.

These disadvantages do not exist on the Intermeshing configuration. For an equal stagger, the Intermeshing configuration has 521.5 / 127.3 = 4.1 times the disk area of a comperable tilt-rotor. This represents a significant increase in the payload capability and a significant decrease in the descent rate during autorotation.

The nondimensionalized radius of disk of Tilt-rotor is 11/ 46.58 * 19.04 = 4.5 to match the nondimensionalized stagger of the Intermeshing. Therefore the total area of Tiltrotor 2 * (pi * 4.5²⁾ = 127.3

The downwash on the fuselage of the interleaving will be significantly reduced due to the inclusion of Active Blade Twist and Advancing Blade Concept.

On the Interleaving, the spar, which connects the rotors to the fuselage, will be sheathed in an airfoil. This airfoil will rotate about the spar. Therefore during hover its drag is significantly less than the Tilt-rotors wing. During cruse the airfoil will have an +/-8º positive pitch and it will contribute to lift, while not experiencing any meaningful downwash from the retreating blades. The lift will not be as efficient as the wing but it will still be an improvement for the rotorcraft. Prouty has said; "The overall airplane lift-to-drag ratio can be 10 to 30, depending on the configuration, whereas the maximum a helicopter can do is 4 to 6."

Tilt-rotor vs. Interleaving, Intermeshing & Coaxial, during Forward Flight:

Lift: The wing of the tilt-rotor will probably be more efficient than the rotors of the three ABC configurations.

Propulsion: The propulsors of the three ABC configurations will probably be more efficient then the prop-rotors of the tilt-rotor.

In addition

Tilt-rotor vs. Intermeshing:

The Intermeshing configuration has 344.5 / 127.3 = 2.7 times the disk area of a comperable tilt-rotor.

Intermeshing vs. Interleaving:

The Interleaving configuration has 521.5 / 344.5 = 1.5 times disk area of a comperable Intermeshing.

Supporting Information for the Above Statements:

Calculation used for obtaining the area of overlap: The overlapping area is only counted once. (Angle is in Radians.)

A = 1/2 (R² (θ - sinθ)) = 1/2 (10 * 10 (2.891 - sin(2.891))) = 1/2 (100 (2.891 - 0.050)) = 1/2 (100 * 2.841) = 142.05

The following table and notes uses nondimensionalized numbers for the comparison. The shaded values are the actual dimensions of V-22, in feet.

A single rotor and a comperable Intermeshing configuration are also added for interest.

	Individual Twin Disk:	Intermeshing Twin Disks:	Interleaving Twin Disks:	Tilt-rotor V-22
Radius of Individual Disk:	10	10	10	10 [19.04 ¹]
Stagger:	~	2.5	11	11 [46.58 ¹]
Total Area of Disks:	314.3	628.6	628.6	127.3 [9114.8 ¹]
Area of Overlap (Circular Segments):	~	284.1	107.1	~
Area Excluding Overlaps:	260.75	344.5	521.5	127.3 [9114.8 ¹]
Total Lateral Span:	20.0	22.5	31.0	20 [84.55 ¹]
Total Longitudinal Span:	20.0	20.0	20.0	9 [76.16 ¹]

Additional Advantages:

OTHER: Aerodynamic - Rotor Disk - Dual Configuration - Interleaving ~ Advantages

Concerns:

The inability to fold the rotor blades due to the extreme rigidity of the rotors. Swinging the complete rotors back and inward will reduce the footprint somewhat.

"At stage lengths of about 200 n. mi., where the maximum number of revenue flights occur, rotary wing cruise speeds of about 240 knots are required to produce the same block speeds as existing jet transports." ~ from the 1977 report, 'A review of Advanced Rotorcraft Research'.

This should be obtainable from today's technology, particularly if active blade twist and individual blade control were to be implemented.

However to achieve faster speeds, and therefore a longer range, reverse velocity utilization or some other type of configuration such as perhaps the AeroVantage will be required.

Last Revised: August 1, 2008