1310

Item 1310

DESIGN: UniCopter ~ Control - Flight - Independent Root & Tip - Autorotation Yaw Control

Abstract:

Coaxial and most Intermeshing helicopters use differential collective to effect yaw. When the craft is in autorotation, the pedals must be reversed to maintain the same rotational direction of yaw. Kamov and Kaman have a linkage changeover so that the pilot still uses right pedal to effect right yaw. It appears that the linkage changeover is dictated by the position of the collective lever and this may present a minor problem during landing, when the collective is raised to arrest the descent.

Should the main rotors of the helicopter be equipped with Independent Root & Tip Control (Active Blade Twist), it appears that a reduction of the root pitch on all rotors will give the blades a positive twist. This means that the rotors will operate like a gyrocopter,, which is always operating in the autorotation state. The advantage of this is that the stall region of the driving rotor will increase slower and therefore a larger differential collective can be applied.

If the rotor also has Reverse Velocity Utilization then seriously consider OTHER: Flight Dynamics - Rotor Concept - Yaw Control for Twin Rotor Configurations.

Description of Existing Rotors: Should this section be moved to OTHER: Aerodynamics ~ General - Autorotation

Individual Main Rotor:

The root portion of the disk (0.25R to 0.25R approx.) is the Stall region. It operates above the stall angle of attack and causes drag, which slows the rotation of the blade.

The mid region of the disk (0.25R to 0.70R approx.) is the Driving region (power to rotor). The aerodynamic force in this region is inclined slightly forward of the axis of rotation, resulting in a thrust, which accelerates the rotation of the blade.

The tip region of the disk (0.70R to 1.0R approx.) is the Driven region. (consumes power). The aerodynamic force in this region is inclined slightly behind the rotating axis, resulting in a drag force, which slows the rotation of the blade.

The RRPM (Ω ) is self-maintaining, when the collective is fully or nearly fully down. I.e. an RRPM increase will move the Drive/Driven boundary inward and this will automatically decrease the RRPM. Conversely, an RRPM decrease will move the Drive/Driven boundary outward and this will automatically increase the RRPM.

Manually raising the collective (Θ) will provide a temporary additional lift by increasing the induced velocity (v_i) and thereby making the inflow angle (Φ) less negative. This will consume some of rotational inertial in the rotor, which will thereby reduce the RRPM. This in turn will move the stall/driving boundary and the driving/driven boundary outward thereby increasing the stall region and the increasing (or at least moving) the driving region but decreasing the driven region. The rotor will now have established a slower RRPM. The slower RRPM will result in a lower profile drag but the stall region will be larger.

For more information see [PHA, section 5.4], & [AH, chapter 6] & [HT, section 3.2]

Coaxial and Intermeshing Main Rotors:

At all times the RRPM of the two rotors is the same and they always cross at the same azimuths. This is because they are mechanically interconnected.

During powered flight, the application of right pedal increases the collective on the CCW turning rotor and reduced it on the CW turning rotor. The net thrust of the two rotors remains the same but the increased torque transmitted to the CCW rotor will be resulted by the craft yawing CW.

During autorotation, the linkage from the pedals to the rotors is reversed. It appears that the reason for this is as follows;

The application of right pedal will increase the collective on the CW turning rotor and reduce it on the CCW turning rotor. The Driving region on the CW turning rotor will move toward the tip and therefore decrease its Driven region, but increase its Stall region. The Driven region on the CCW turning region will move toward the root and therefore decrease its Stall region and increase its Driven region. Therefore the CW turning rotor is doing the driving and it will want to speed up, whereas, the CCW turning rotor is driven and will want to slow down. This should cause the craft to yaw CW.
[My opinion]~ All this is fine on a single rotor where 2 or 3º of change to the collective will shift the regions marginally along the blade. On a coaxial configuration the change of collective must be significantly greater that 2 or 3º to cause a desired rate of yaw.

A small increase the collective on the CW turning rotor will increase its driving ability but a high increase in collective on this rotor will increase the stall region to the point that this stall region is consuming the torque from the driving region, and little or none is being transferred to the CCW turning rotor.

Description of Concept: REQUIRES MUCH MORE THINKING

Autorotation on Coaxial and Intermeshing Helicopters with Independent Root & Tip Control:

The rotors have separate, 1P root pitch controls and 1P (or 2P HHC) tip pitch controls. During autorotation, all the blades will be given a positive twist. This will slow the growth of the Stall region on the driving rotor as the collective is increased on this rotor. This, in turn, should mean that the rotors can be subjected to a larger differential collective, before the drag of the stall region on the driving rotor start to 'rob' an excessive amount of the torqe which is intended to go to the driven rotor.

Additional Related Information:

OTHER: Aerodynamic - Rotor Disk - Dual Configuration - Coaxial ~ Yaw

OTHER: Flight Dynamics - Rotor Concept - Yaw Control for Twin Rotor Configurations

Back to top | SynchroLite Home Page | UniCopter Home Page | Nemesis Home Page

Initially displayed: March 29, 2004 ~ Displayed on PPRuNe: March 29, 2004 ~ Last Revised: February 13, 2007

The above utility invention is openly and publicly disclosed on the Internet to negate an entity from patenting it, to the exclusion of all others whom may wish to use it. ~ Reference patent law 35 U.S.C. 102 A person shall be entitled to a patent unless - (a) the invention was known ... by others in this country, ..., before the invention thereof by the applicant for patent.