1108

Item 1108

OTHER: Rotor Concept - Reverse Velocity Utilization - General & Miscellaneous

Explanation Using Independent Root & Tip Control for Utilizing the Reverse Velocity:

The root of the retreating blade during forward flight will be experiencing reverse airflow. At speeds below mu (tip speed ratio) = 1, the tip of the retreating blade will be experiencing conventional airflow. This means there is a point on the blade that is experiencing no airflow. This point (r_0V) will move along the span of the blade when the craft's forward velocity changes. If the forward velocity increases, the point of no airflow will move toward the tip (mu increases). If the forward velocity decreases, the point of no airflow will move toward the root (mu decreases). The pitch at the tip of the retreating blade is positive (upward) and the pitch at the root is negative (downward). At some point on the span of the blade the pitch must be zero. This results in the angle of attack being positive (i.e. providing lift) at the tip and at the root. The purpose of the independent root and tip pitch controls is to move the location of the 'point of zero angle of attack' (r_0α) along the span in an attempt to make it coexists with the ''point of no airflow' (r_0V). This results in improved angles of attack on both sides of the 'points'.

The 'point of zero angle of attack' (r_0α) is moved by rotating the root and pitch the same amount in the same direction.

The lift is increased by increasing the positive twist.

The angle of attack, the air velocity and the thrust will increase as the spanwise distance from this no-airflow location increases; in both directions. See sketch.

General Information:

The location on the span of the retreating blade at azimuth 270º where the horizontal air velocity is zero = μ * R.

A linear twist from root to tip will result in θ_Root * (R - (μ * R)) = θ_Tip * (μ * R).

With negative pitch and reverse airflow, such as experienced at the root end of a retreating blade, the blade will provide lift. The airflow across the blade will be from trailing edge to leading edge and this will result in a higher drag. This drag may not be a serious concern since it is on the retreating side of the disk and is helping to rotate the rotor.

As the forward velocity moves from μ = 0 to μ = 1 the location on the blade at azimuth 270º where the horizontal velocity is zero will move from root to tip.

I suspect that at high μ values the greater drag on the retreating side, because of the reverse airflow over the airfoil, would result in a slow self-sustaining rotation.

The rotor disk will be pitched downward as the forward velocity increases because the intent is to have the rotor contribute a little to the craft's forward velocity. This will increase the diameter of the region of reverse velocity slightly, but it is too little to be of any significance.

The optimal profile of the blade at various locations along it's span will depend on the design mu at the design cruise speed.

Intermeshing Configuration:

Stepniewski's Concept:

The drawing on page 51 of Stepniewski's 'Quiet Rotorcraft' report shows a retreating blade where just about all of it's span is in negative lift (μ = 0.8). This negative lift must be offset by consuming some of the positive lift on the advancing blade. Ref. ~ The retreating blade on the UniCopter would go fully negative at +/- 300 mph.

UniCopter:

There is the consideration for designing the UniCopter around the Lycoming AEIO-320 aerobatic engine. This engine will allow inverted flight; as will the Zoche ZO 01A engine. For inverted flight, negative pitch is mandatory. This brings forth the idea that negative pitch on the retreating blade, during conventional fast forward flight, may assist in the overall lift and thereby reduce the [power / maximum forward speed] ratio.

From FORM: Tangential Velocity: At 480 RRPM, 270º azimuth, and air speed of 150 knots (171 mph), the location of zero air flow on the blade is a 5.0 ft radius.

When the blade is at 270º azimuth and at it is over the centerline of the craft (span = 1.25' r) the airflow across the blade will be 78 ft/sec during hover and -141 ft/sec at 150 mph. There is no airflow across the blade at 53 mph. The implication is that the reverse airflow at forward speeds is more important than the conventional airflow during hover. This raises the question of whether the blade profile should be reversed in this span location, or that the 'leading' and the 'trailing' edges be identical.

The negative pitching moment about the feathering axis on the retreating blade will be extremely large.

The greater drag on the retreating side may be sufficient to maintain rotor rotation. It might create too much rotation or too little but since it is coupled to the engine and the variable pitch ducted fan (Is it?) Don't forget overrunning clutch) its rpm will be controlled. This may mean that in fast forward flight all the engine power goes to the ducted fan. In hover all the power will go to the rotors, since there is no tail-rotor.

Interleaving Configuration:

Nemesis:

This may be an ideal configuration for fast VTOL heavy/medium lift transportation. It will obviously be better than the flawed Sikorsky's proposed single rotor RVR. It should be better than Bell's four-prop tilt rotor concept, because at cruse speed the fuselage will be located under the retreating blades, which is the area of the rotor disks with minimum downwash. The location of zero downwash is at the perimeter of the reverse velocity regions. The downwash will start increasing when moving outward and when moving inward from this perimeter.

Another advantage is that unlike the tiltrotor there will be a benign transitional phase between hover and fast forward flight.

For smaller craft, which have 1 to 7 seats, the intermeshing configuration (UniCopter) may be the best configuration.

For more information see;

Specific: OTHER: Helicopter - Inside - Interleaving - Enhanced Dihedral Rotors

General: OTHER: Aerodynamic - Rotor Disk - Dual Configuration - Interleaving

Blade Profile at Root:

On page 190 of 'Open Airscrew VTOL Concepts' a symmetrical (lead edge & trailing edge) is shown. The craft is the HV-2A "Vertiplane". This gyrocopter used a 2-blade stopped rotor in conjunction with a lower wing. Have hard copy and stored on computer as 'Stepniewski and Tarczynski.pdf'

The UniCopter is intended to have a 'slowed rotor' not a 'stopped rotor' In other words, the rotors are always rotating. This strongly suggests that the airfoil root should not be symmetrical for and aft. The airfoil shape should be such that it seriously favors the advancing side of the disk, where the velocity over the airfoil is much faster. For more on the blade see; OTHER: Rotor Concept - Reverse Velocity Utilization - Reverse Velocity Blade.

Advantages of Independent Root & Tip Control to Reverse Velocity Utilization:

OTHER: Rotor Concept - Active Blade Twist [ABT] ~ Advantages

UniCopter ~ Control - Flight - Independent Root & Tip - Blade Root ~ Reverse Velocity

Calculation re Reverse Flow Drag Effect:

OTHER: Aerodynamic - Drag - Profile ~ Reverse Flow Drag Effect:

Other Pages at This Site:

Reverse Velocity Pages:

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Related Pages:

OTHER: Rotor Concept - Improved L/D Ratio - Active Blade Twist.

Outside Information:

Principles of Helicopter Aerodynamics: [Source ~ PHA p.295]

The last paragraph in Section 7.10.6 discusses the problem of reattachment with reverse flow. Leishman mentions the relatively long time required for the air to readjust, however this problem should diminish somewhat as the slowed-rotor concept is implemented.

At mu = 1:

Root Profile of Blade: To get a sense of the relative forward to reverse velocities at root (0.1R). M = 0.828 (900 fps), mu = 1, [tip speed = 450 fps, forward velocity = 450 fps (267 kts)] ; using Access 'Tangential Velocity' Form.

Azimuth: (deg)

0º

30º

60º

90º

120º

150º

180º

210º

240º

270º

300º

330º

Tangential Velocity: (fps)

+45

+270

+435

+495

+435

+270

+45

-180

-345

-405

-345

-180

Total of forward values = 1,995. Total of negative values = 1455.

The maximum-cruise-speed ratio of mean forward velocity to mean reverse velocity = 58% of the time experiencing forward velocity and 42% of the time experiencing reverse velocity. These percentages should then be adjusted by the ratio of the mean times and speed spent from hover to maximum-cruise-speed; say 50/50. Then the profile of the blade should be shaped to suit this final ratio, which in this case would be ((58/2)+50)% and ((42/2)+0)% = 79% of the time in forward velocity and 21% of the time in reverse velocity.

In addition, drag may not be all that detrimental since it helps to rotate the rotors. However, this drag will create a disturbance ahead of the propeller.

This was written before the section immediately above. Move to 1485.html??

Design the tip solely for compression (advancing side) and totally disregard tip stall (retreating side).

Consider locating the pitch axis at 27% of chord at the tip and at 45% of chord at the root.

The tip will not be experiencing reverse velocity, therefor there is little need to take it into consideration.
The root will be experiencing reverse velocity. The velocity will be slightly greater on the advancing side then the retreating side. In addition, the intent is to control the root pitch by power, therefor there should be little chance of flutter.

Determination of the ratio of forward-velocity generated lift to reverse-velocity generated lift at various advance ratios. Note that the Mach number is set at 0.828 (900 fps) at 90º azimuth for advance ratios.

Record the maximum achievable forward velocity at each mu for a Mach number of 0.828 (900 fps) at the advancing tip at 90º azimuth.

Then calculate the total +/- Lift ratio between the reverse velocity and the advancing velocity regions.

Use;

For an airplane wing: L_W = (ρ / 2) * V² * S * C_L . Where S is the area of the wing.

Assume that ρ and C_L are constants

S is the total blade area of blade in each of the two regions.

'V^2' (actually just 'V') will be the average of the sum of the square of the evenly distributed velocity points in each of the two regions.
The Lift Ratio of L_{W REV}:L_{W REV}will be considerably greater than the Blade Area Ratios calculated below, with the exceptions of; stopped rotors, and unreasonably high integer mu values.

For an example of this see; OTHER: Helicopter - Inside - Single Rotor - Rotor X Wing - Improving Lift During Transformation:

Forget doing the final calculations below for now.

This might causes one to reconsider the validity of utilizing the reverse airflow, particularly if the implimentatation results in any penalty to the lift that is derived from the forward airflow.

In other words WHEN ONLY LOOKING AT LIFT, the cruise speed of the craft may have to be close to 400 mph to justify reverse velocity utilization. Ref. Osprey maximum speed is 277 knots (316 mph).

However, when also considering the drag resulting from reverse airflow over a CONVENTIONAL (albeit feathered) blade, I suspect that the cruise speed will be quite a bit below 400 mph, IF a good design of Reverse Velocity Utilization can be found.

July 8, 2007 ~ I suspect that for mu =0.75 (263 mph) the optimal rotors will have; Advancing Blade Concept, 'Absolute' Rigidity, Slowed Speed & Wide Chord, Active Blade Twist w/ robust root control and pitch axis at +/- 33% of chord. The blade's profile will not even consider Reverse Velocity, with the possibility of some very minimal consideration at the root. The higher profile drag on the root of the blades, when they are near 270º, will contribute to rotating the rotors.

At mu = 0 (hover). Forward velocity = 0 mph

The total blade area in the reverse air flow is 0% and the area in the forward airflow is 100%