SynchroLite_Stability

SynchroLite ~ Trim, Stability & Control - Stability & Control

Overview:

Theoretical notes re stability and control sensitivity related specifically to the SynchroLite

For general information, see:- OTHER: Dynamic - Trim & OTHER: Dynamic - Stability & Control

Notes:

There will be cross-coupling in the two rotor disks of the SynchroLite, but theoretically little of it should show up in the flight characteristics or the cyclic controls, because there will be self-canceling between the rotors. The UniCopter's advantage of no coning angle and therefore minimal cross coupling may not be that big an advantage. OR could the SyncroLite compound the problem by causing an oscillation between the two rotors?

Dragonfly Note: (to be eventually moved ~ or these pages changed to Dragonfly or SynchroLite II)

The two rotor heads have some mass and diameter and since they do not flap or lead/lag the pair may give some gyroscopic stability, similiar to the UniCopter.

Trim Analysis:

Stability Analysis:

Static Stability: See also:- OTHER: Dynamic - Stability - Static

Speed Stability: (Static Longitudinal Stability)

OTHER: Aerodynamic - General - Flapback is the main source of positive speed stability in conventional helicopters.

The horizontal stabilizer will be another source of positive speed stability

Angle-of Attack Stability:

XXX

Directional Stability: [rotation about the Z-axis]

A large enough vertical stabilizer should handle this. Note that too large a vertical stabilizer might produce an unstable spiral dive. See; Twin Vertical Stabilizers. This should be advantageous. It should also have the same effect as the Variable Vertical Fin Concept

Sideslip:

XXX

Dihedral Effect: (Static Lateral Stability)

OTHER: Dynamic - Stability ~ Dihedral Effect

The high fuel tank, and possible a high vertical plate in the X-Z plane may help.

Pitch-Roll Coupling:

XXX

Pitch- Sideslip Coupling:

XXX

Dynamic Stability: See also:- OTHER: Dynamic - Stability - Dynamic

In Hovering:

The following investigates dynamic longitudinal stability of a hovering helicopter. A lateral rotor type such as the FW-61 is considered in particular.

Hohenemser, K., Dynamic stability of a helicopter with hinged rotor blades, NACA Technical Memorandum 907, 1939, pp. 19.

In Forward Flight:

Short Period Mode:

Long Period Mode ~ Phugoid:

Dutch Roll:

Spiral Dive:

Lateral Stability:

xxx

In Autorotation:

Control Analysis:

Control Sensitivity:

Control Moment:

Tip-path-plane tilt, in the SynchroLite, will produce a moment about the helicopter's center of gravity.

For a comparison of the moment generated by a teetering rotor to that of an absolutely rigid rotor see. DESIGN: SynchroLite ~ Control - Flight ~ Control Moment

The ratio of control power to damping, which frequently tends to reach values that lead to major problems in the hovering characteristics of small low-inertia helicopters, is < 20 to meet MIL-H-8501. Increasing the blade inertia is one method of changing the damping. This is another reason for large tip weights, in addition to autorotation.

Notes re Rotor Torque and Pitching Momentum:

Stovepipe exhaust exits at tail with a downward angle of 30º. Ref: DESIGN: Exhaust # Kaman_Huskie. This means that about a third of the force is directed downward, to offset changes in rotor torque, particularly on the loss of the engine.

The Huskie has a 1/2 'V' angle of 13-degree. The SynchroLite has a 1/2 'V' angle of 12.5-degrees. Sine (13º) = 0.225 and sine(9º) = 0.216, therefore the SynchroLite has, proportionately, (0.216 / 0.225) = 96% of the Husky's rotor torque induced pitch momentum, because of the more vertical masts.

There may be a reason to deflect the SynchroLite's exhaust downward. Since the tail boom will have a taper to it, perhaps some of the efflux could be discharge downward through slots in the bottom of the boom. This extra lift may have to be offset by a slightly greater downward pitch in the horizontal stabilizer.

The gyroscopic effect of the two rotors might help resist the downward pitch upon the loss or reduction of engine power; and this should be good.

Working Papers:

DESIGN:

MAKE:

Items:

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Last Revised: May 13, 2008