B326

OTHER: ~ Flight Dynamics - Stability

Overview:

	General information related to intermeshing helicopters. Plus some information on Side-by-side
	Specific Information:	Related to the SynchroLite.	Related to the UniCopter

Symbol Definitions

Intermeshing Direction of Rotation:

Flettner build two craft with reverse rotation. " No test reports have survived regarding these test and it remains unclear how many Fl 282s were evaluated with reverse rotation. It appears that this flight configuration offered improved lateral stability and was, as such, of some benefit in executing turning maneuvers, though at the expense of longitudinal stability This was therefor a secondary rather than a primary benefit. .......... The benefits offered were by no means sufficient to compensate for the more desirable handling characteristics of a normally configured machine.

Interestingly enough, none of the post war manufacturers who utilized intermeshing rotors sought to replicate these tests." ~ [Source ~ HTR p.87]

More: DESIGN: Unicopter - Rotor ~ Direction Rotation

Static Stability:

Hovering:
?? Consider the exclusion of the horizontal stabilizer.

Speed Stability: (Static Longitudinal Stability): [transition along the X axis],

?also? (Static Lateral Stability): [transition along the Y axis]

The objective is a moment that will be nose-up (positive) with increasing speed and nose-down (negative) with decreasing speed. Pilots prefer a level of speed stability that is slightly positive.

The parasitic drag of the fuselage and landing gear provides negative speed stability. The flap-back of the rotors on the SynchroLite and the pre-cone of the rotors on the UniCopter will contribute to positive speed stability. In addition, the horizontal stabilizer can provide positive speed stability, but at the cost of lift.

Ref: High Center of Gravity and Stability, such as DeLackner:

"In forward flight, air passing through the rear portion of the rotor disk has a greater downwash angle than air passing through the forward portion. The downward flow at the rear of the rotor disk causes a reduced angle of attack, resulting in less lift. Increased angle of attack and more lift is produced at the front portion of the disk because airflow is more horizontal. These differences between the fore and aft parts of the rotor disk are called transverse flow effect." [Source ~ RWP4 ch 9]. Is this similar to Rotor Upwash?

The NASA Paper 3675, 'A survey of theoretical and Experimental Coaxial Rotor Aerodynamic Research', on page 18 says "Increasing advance ratio causes the upper rotor Wake to be 'swept back'. This results in more of the lower rotor being exposed to clean air." It can probably be assumed that this activity will apply to the intermeshing configuration and will therefore assist with speed stability.

Angle of Attack Stability: (Maneuver Stability): [rotation about the Y axis]

The phase angle is close to 0º. In other words, the maximum pitch is 90º later than that of a teetering rotor.

The tip path plane of the rotor and the horizontal plane of the fuselage are 'always' coplanar.

Note: For twin-rotor craft with 'absolutely' rigid rotors the horizontal stabilizer may not be necessary. See UniCopter ~ Trim, Stability & Control - Stability

This section is far from being complete

Maneuver Stability: Probably the wrong expression: [ A_H / A] [H.S area / Effective disk area]

Pitching moment per unit angle of attack change. [ M_α ] [ft-lb / radian]

The value to meet MIL-H-8501must be < 300.

[Information ~ RWP1 p.529]

http://www.monmouth.com/~jsd/how/htm/aoastab.html

- Angle of Attack Stability - Decalage.

Directional Stability (Weathercock): [rotation about the Z axis]

OTHER: Helicopter - Outside - Intermeshing - Kaman - H-43 Huskie - Yaw

For ideas related to the intermeshing configuration see;

OTHER: Flight Dynamics - Control - Twin Vertical Stabilizers This looks like the better of the two.

OTHER: Flight Dynamics - Control - Variable Vertical Fin

Dihedral Effect (Static Lateral Stability): [transition along the Y axis]

Sideslip:

Synchropter Directional Stability Information: from 'Stability and Control of Rotary Wing Aircraft:

In the closely spaced intermeshing rotors of the synchropter configuration, any sideslip velocity through the cocked rotors creates yaw moments. A side velocity as from a gust, yawed flight, or sideslip creates a flow down through one rotor and up through the other rotor. When the blades are set at high angles of attack, an upward velocity creates an increase in rotor torque and a downward velocity through the rotor creates a decrease in torque. Because the appositely turning rotors are geared together, this torque is additive arid manifests itself as a yawing moment on the ship. In a synchropter where the outboard tips are moving aft (like a breast stroke swimmer), this yaw moment acts in a stabilizing manner and the helicopter has good directional stability.

When the blades are set at autorotative angles of attack, downward flow through the rotor tends to retard the rotor and, conversely, upward flow tends to accelerate the rotor. Therefore, in autorotation of a breaststroke type synchropter the yawing moments created by side flow through the rotors creates an unstable yaw moment which must be corrected by the addition of more fin area.

[Source ~ SCR p.28]

UniCopter Directional Stability Information: from 'More Helicopter Aerodynamics', p 2:

"Sikorsky Advancing Blade Concept (ABC) test bed aircraft, which when hovering close to the ground is subjected to random side forces. This craft has a nicely rounded fuselage bottom that presents no logical point where the downwash can separate. This wandwering seperation point produces erratic side forces the pilo9t has trouble anticipating."

By self; Might the landing gear and the doors or the now exposed receptacle areas breakup the aerodynamic stability?

Intermeshing:

"The investigation indicates that for all types of helicopters, except for the synchropter type just mentioned, lateral stability may be expected, contrary to the longitudinal stability which is a serious problem for most helicopters." ~ [Source ~ SCR p.56] This seem to contradict the information above, from that on page 28 of the same document.

Potential Solutions:

Perhaps a low center of gravity and high lateral drag will reduce this. A high vertical empennage should also help.

DESIGN: UniCopter - Rotor - Disk - Pre-cone

Interleaving:

Side by Side - Focke Fa-61:

The rotors, which were mounted on outriggers, were inclined slightly inward to provide lateral stability.

For all large transport craft: Stability ~ Roll / Sideslip in Ground effect. The following may be of concern to the Interleaving configuration. From posting on PPRuNe;

The same craft but different conclusions.

By Dave Jackson

Focke-Achgelis Fa223

Sud Est Aviation (Aerospatial) SE-3000

Hendrick Focke, the inventor of the FA 223, said in the Jan/1947 issue of the 'American Helicopter' magazine;

"The Fa223 is statically and dynamically completely stable around all axes other than the longitudinal one. At traveling speed of 140 to 150 km/h all controls can be released, because longitudinal instability disappears at about 120 km/h. Then this aircraft behaves just like a normal airplane and is automatically stable."

___________________

Jean Boulet, the test pilot of the SE3000, said some time later;

"Despite the first successes of the Focke aircraft, the two lateral rotors formula is not satisfactory. It shows a good longitudinal stability, because the empenage is well out of the rotor's stream, but in return the formula has defects concerning the lateral stability:

- Dynamic instability of roll while hovering in ground effect; the ground effect increases on the side toward which the aircraft is leans, tends to push the aircraft to the other side.

- Static instability of roll in translation at low speed; when a turn is engaged, at a speed lower than the climbing speed, the increase in the relative speed of the external hub produces an increase in lift which leads to increase in initial tilt.

It will be necessary to wait for the era of automatic pilots which will artificially fix these defects, to see reappear the lateral rotor's formula with the Bell XV 15."

Translated by Claude Dawson from the French publication 'History of the Helicopter'by Jean Boulet.

_________________________________________

By Matthew Parsons

Jean Boulet's words are less convincing at first glance. In a ground effect hover, a disturbance that causes an uncommanded roll will result in placing one rotor closer to the ground. If that does in fact increase that rotor's lift due to added ground effect, then that rotor will create a restoring moment. That makes the roll statically stable. How that same effect could lead to dynamic instability is not obvious to me.

His second point sounds more like a spiral stability concern. I'm not convinced that the effect described creates an undesirable flying condition.

Both his observations have to do with low speed roll channel. I'm curious if it's a control power issue. It seems to me that it would be very easy to create very strong lateral control power with this configuration.

_____________________________________

By Dave Jackson	Quote: - Dynamic instability of roll while hovering in ground effect; the ground effect increases on the side toward which the aircraft is leans, tends to push the aircraft to the other side.
	Mathew, Your questioning of Jean Boulet's statement above to 'dynamic' stability sounds valid. However, his statement does suggest the possibility of 'static' instability on rotorcraft with long fuselages. This instability may be an insignificant problem for current helicopters, but on future transport helicopters the ratio of payload to empty weight will increases. This will result in the ratio of the fuselage's side X-sectional area to the rotor thrust ratio increasing also. In turn, this means that the lateral area of the fuselage will present a larger area for the downwash in ground effect to push against. For an example, consider the situation of a helicopter that is transitioning to Starboard. When out of ground effect, the cyclic will be to the right and the rotor disk(s) will be lower on the Starboard side. As the craft descends it will come into ground effect. There will be a higher 'pressure' under the Starboard half of the disk area then there is under the Port half. This will require the pilot to apply an ever increasing cyclic to the right, which might, or might not, provide static stability. However, at a low enough elevation, the thrust against the fuselage may apply a stronger force to Port than the lateral component of the disk thrust is applying to Starboard. The craft then starts transitioning to Port. What is the pilot to do, since more right cyclic will only exasperate the situation? The following sketch is borrowed from Sikorsky's presentation to the Jan/06 AHS VLADC. It is displayed solely for the purpose of advancing rotorcraft technology.

For more see; B280.html - Low_Speed

For more see; http://www.pprune.org/forums/showthread.php?t=219968

Pitch-Roll Coupling:

See: Couplings

Pitch- Sideslip Coupling:

See: Couplings

Dynamic Stability:

Hovering:

xxx

Forward Flight:

Short Period Mode: An oscillation of the pitch of the craft at mainly constant speed.

Long Period Mode ~ Phugoid: An oscillation of the pitch and mainly constant angle of attack. The intermeshing should hold heading because it is symmetrical.

Dutch Roll: A combination of yaw and roll in an oscillating pattern. To much dihedral effect can produce an unstable Dutch roll. Dutch Roll, Design Compromises by R. Prouty in Rotor & Wing

Spiral Dive: To much directional stability can produce an unstable spiral dive.

"Dynamic Lateral Stability: is indicated if a left rolling moment is produced by a side slip to the right and if the damping in roll, the damping in yaw and the directional stability are positive. For centrally located rotor axes these conditions are fulfilled. In a synchropter rotor, however, the static directional stability is negative if the right rotor turns clockwise seen from above. The investigation indicates that for all types of helicopters, except for the synchropter type just mentioned, lateral stability may be expected, contrary to the longitudinal stability which is a serious problem for most helicopter types." ~ [Source ~ SCR p.56]

Autorotation:

My thoughts;~ If the power is lost then the torque induced downward pitching moment on the craft is lost. However the decent and the horizontal stabilizer should reestablish the nose down force. It should be noted that the high rigidness of the rotors might mean that the HS is not very large.

Concern re intermeshing configuration; OTHER: Aerodynamics ~ General - Autorotation ~ Synchropter Directional Stability Should this be in OTHER: Dynamics - Control?

The Six Equations of Motion:		Symbol Definitions
	The Longitudinal Equations:
	Longitudinal force (forward)	X_P + X_S + X_H + X_V + X_F + X_E = GW sin(θ) + (GW /g) * (x^{..} - y^{.} r + ż^{.} * q) \|\| x is acceleration; y & z are velocity
	Vertical Force (down)	Z_P + Z_S + Z_H + Z_V + Z_F + Z_E = -GW cos(θ) + (GW /g) * (z^{..} - x^{.} q + y^{.} * p) \|\| z is acceleration; x & y are velocity
	Pitching Moment (nose up)	(M_P - X_P * (z_P + z_CG) + Z_P * (x_P + x_CG)) + (M_S - X_S * (z_S + z_CG) + Z_S * (x_S + x_CG)) + Z_E * (x_E + x_CG) - X_H ( z_H + z_CG) + Z_H (x_H + x_CG) - X_V * (z_V + z_CG) + M_F + Z_F * (x_F + x_CG) - X_F * (z_F + z_CG) - X_E * (z_E + z_CG) = I_yy * q^{.} - pr (I_zz - I_xx)* \|\| q is velocity
	The Lateral Equations:
	Lateral Force (right)	Y_P + Y_S+ Y_V + Y_F + Y_E = -GW sin(Φ) ) + (GW /g) * (y^{..} - x^{.} r + ż^{.} * p) \|\| y is acceleration; x & z are velocity
	Rolling Moment (down to right)	(R_P + Y_P (z_P + z_CG) + Z_P * (y_M + z_CG)) + (R_S + Y_S * (z_S + z_CG) + Z_S * (-y_M + z_CG)) + Y_V * (z_V + z_CG) + Y_F * (z_F + z_CG) + R_F = I_xx * p^{.} - qr * (I_yy - I_zz)* \|\| p is velocity
	Yawing Moment (nose to right)	(N_P - Y_P (x_P + x_CG)) - (N_S + Y_S * (x_S + x_CG)) - Y_V * (x_V + x_CG) + N_F - Y_F * (x_F + x_CG) - Y_E * (x_E + x_CG) = I_zz * ŕ^{.} - pq * (I_xx - I_yy)* \|\| r is velocity

From a rigorous standpoint, the SynchroLite should be augmented with 3 more equations representing the coning, longitudinal and lateral flapping of the rotor, which is not attached very rigidly to the airframe.

Stability Derivatives:

FAA Regulations:

PART 29--AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY ROTORCRAFT

	• 29.171	Stability: general.
	• 29.173	Static longitudinal stability.
	• 29.175	Demonstration of static longitudinal stability.
	• 29.177	Static directional stability.
	• 29.181	Dynamic stability: Category A rotorcraft.

Possibly Relevant Pages:

	DESIGN: SynchroLite ~ Rotor - Disk - Centers, Radii & Moments	0729
	DESIGN: SynchroLite ~ Rotor - Blade - Composite - VR-7b - Centers, Radii & Moments	0826

Outside Information:

Excellent overview; 'Rotorcraft Stability and Control: Past, Present and Future' in January 2003 issue of Journal of the America Helicopter Society.

Methods for obtaining desired helicopter stability characteristics and procedures for stability predictions. http://naca.larc.nasa.gov/reports/1958/naca-report-1350/ Have hard copy.

Stability Augmentation System:

Information; [Source ~ HT, section 15-6]

Last Revised: December 6, 2007