Question from a float pilot.

This forum has been developed to discuss helicopter related topics.

Moderators: ahramin, sky's the limit, sepia, Sulako, North Shore

Post Reply
Northern Skies
Rank 8
Rank 8
Posts: 769
Joined: Wed May 26, 2004 2:00 pm

Question from a float pilot.

#1 Post by Northern Skies » Fri Dec 18, 2009 10:55 pm

This summer I had the pleasure of operating alongside a Kaman K-max. It was fun to follow it and watch how a synchropter works. But there was one thing I couldn't figure out, and forgot to ask the pilot:

How does a synchropter control yaw?
---------- ADS -----------

Rank 3
Rank 3
Posts: 195
Joined: Tue Sep 26, 2006 7:59 pm
Location: Rotting at an IA base near you

Re: Question from a float pilot.

#2 Post by kingeddie » Sat Dec 19, 2009 10:07 am

Merry Xmas

When there is a pedal input one rotor has its pitch reduced and the other has its pitch increased.

This makes the torque reactions unequal giving a net torque in one direction.

The best info is in the second article ..

So godamn complicated no wonder KMAN is the only one making it , HHAHAHAHAHA

Main article: Contra-rotating propellers
While several nations experimented with contra-rotating propellers in aircraft, only the United Kingdom and Soviet Union produced them in large numbers. The U.S. worked with several prototypes, including the tail-sitting Convair XFY and Lockheed XFV "Pogo" VTOL fighters, but jet engine technology was advancing rapidly and the designs were deemed unnecessary. Kaman Aircraft designed the H-43 Huskie and K-Max light utility helicopter with intermeshing contra-rotating blades.

Tandem-rotor designs such as the Boeing Vertol CH-46 Sea Knight and CH-47 Chinook use a counter-rotating arrangement to offset torque; the rotors do not share a common coaxial hub.

A new usage of contra-rotating propulsion in aircraft can be found in the F-35B variant of the new F-35 Lightning II strike fighter, which uses a lift fan with contra-rotating blades.

Contra-rotating propellers, also referred to as coaxial contra-rotating propellers, apply the maximum power of usually a single piston or turboprop engine to drive two propellers in opposite rotation. Contra-rotating propellers are common in some marine transmission systems, in particular for medium to large size planing leisure crafts. Two propellers are arranged one behind the other, and power is transferred from the engine via a planetary gear or spur gear transmission. Contra-rotating propellers should not be confused with counter-rotating propellers, a term which describes multi-engined aircraft with the airscrew on one engine turning clockwise and the other counter-clockwise.

When airspeed is low the mass of the air flowing through the propeller disk (thrust) causes a significant amount of tangential or rotational air flow to be created by the spinning blades. The energy of this tangential air flow is wasted in a single propeller design. To use this wasted effort the placement of a second propeller behind the first takes advantage of the disturbed airflow. The tangential air flow also causes handling problems at low speed as the air strikes the rudder/fin, causing the aircraft to swerve left or right, depending of the direction of propeller rotation.

If it is well designed, a contra-rotating propeller will have no rotational air flow, pushing a maximum amount of air uniformly through the propeller disk, resulting in high performance and low induced energy loss. It also serves to counter the asymmetrical torque effect of a conventional propeller (see P-factor). Some contra-rotating systems were designed to be used at take off for maximum power and efficiency, and allowing one of the propellers to be disabled during cruise to extend flight time.

Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers[1]. However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 db, and tangentially 10db[1]. Most of this extra noise can be found in the higher frequencies. These substantial noise problems will limit commercial applications unless solutions can be found. One possibility is to enclose the contra-rotating propellers in a shroud. It is also helpful if the two propellers have a different number of blades (e.g. four blades on the forward propeller and five on the aft).

The efficiency of a contra-rotating prop is somewhat offset by its mechanical complexity. Nonetheless, coaxial contra-rotating propellers and rotors are moderately common in military aircraft and nava
l applications, such as torpedoes, where the added maintenance is not as much of a concern to government budgets


Up to now, the intermesher rotors reacted in much the same manner as any helicopter. However, in the directional system the similarity ends. Due to the couter-rotating rotors, as explained in previous paragraphs, rotor torque is nullified. It is important to keep in mind that this is true only if both rotor receive equal pitch changes. So far in our discussion, we know how to fly vertically and in any given direction. To make turns, the rudder pedals are linked mechanically to the collective and fore-aft cyclic systems. Turning the intermesher is brought about by intentionally changing the torque relationship between the rotors with the application of the pedals.

When the pilot applies pedal, both differential collective and differential cyclic are applied to the rotors. For example,when right pedal is applied, the left rotor increases in pitch and the right rotor decreases in pitch (see fig. 5). This action is known as differential collective and causes the left rotor to produce more torque reaction and the right rotor less, thus turning the helicopter to the right.

Conversely, the pitch and torque reaction of the right rotor increases and that of the left decreases when left pedal is applied causing the helicopter to turn and roll to the left. It must be remembered that these effects occur while the helicopter is in powered flight.

As differential collective is induced in the rotors,another action as differential cyclic, takes place simultaneously. Application of right rudder pedal not only causes the left rotor to increase in pitch and torque reaction, but also tilt forward,(see fig.6). the right rotor decreases in pitch and tilts aft. This action tilts the left rotor forwards and the right rotor aft , causing them to „ push and pull „ in the turn. Thus an aerodynamic force is induced wich assists the helicopter in the turn to the right. The action of differential cyclic may be compared to the use of oars in turning a row boat. When in a right turn, the oarsman facing the stern uses the oar in his right hand to pull the boat around in the direction of the turn and either pushes with his left oar or allows it to drag in the water to assist in turning. Application of left rudder pedal in the helicopter causes the right rotor to „ push „ and the left rotor to „ pull or drag „ and the left rotor to „ pull or drag „ turning the aircraft to the left.


As described in previous paraghraphs during power-on flights ( engine driving the rotors ), the differential torque reaction helps turn the helicopter. We have learned that in the intermesher configuration,in powered flight,the greatest torque reaction is produced by the HIGH lift rotor. During autorotation the rotors are driven by an external force, the flow of air up through the rotors produced by the helicopter`s rate of descent. Again, due to the characteristics of the intermesher configuration, the rotor having the HIGHER pitch now provides the greatest reaction due to rransmission friction and causes the fuselage to turn in the same direction as the fuselage. Consequently, application of right pedal would apply more pitch on the left rotor which would cause the helicopter to yaw to the left-an undesirable situation! The solution is in the incorporation into the controls a mechanism known as the „ reverser „ . The purpose of this devise is to maintain a consistent relationsship betwen the application of pedal and direction of turn. This is accomplished by reversing the differential collective in the rotors while in autorotation thus by reversing the differential collective in the rotors while in autorotation thus making the inside rotor in a turn the high pitch rotor and the rotor ( see fig. 7 ). The helicopter then turns in the desired direction.


To accomplish the above, the reversing mechanism is installed in the control module between the pedals and collective system. Ist only purpose is to reverse the differential collective to the rotors when in descending flight 0 – 10 percent collective position and autorotation. The reverser mechanism is a self – contained unit consisting of three control connections – input from the rudder pedals output to the collective systems,and control input from the collective lever.

The reverser never needs adjustment other than initial rigging.It is designed to mechanically and automatically revers the differential collective input to the rotor from the pedals as required between power – on flight and descending – autorotation. The reverser has two main control positions, „normal“ and „reverse“. During normal power-on flight, the reverser is in the normal position. With application of right pedal, the left rotor increases pitch and the right rotor decreases pitch.This causes the helicopter to turn to the right. On entering descents while maintaining the same amount of right pedal, the pilot lowers his collective lever to the full „ down „ position,and the signal rod from the collective lever automatically and mechanically causes an overcentering lever in the reverser to shift. This moves the output side of the reverser in the opposite direction from that applied by the right pedal. This action reverses the pitch between the rotors, decreasing pitch in the left rotor, and increasing pitch in the right rotor. The direction of turn is now in the same direction as pedal applied and,so far,the reverser has accomplished what it was designed to do. How ever, as in most reversing mechanisms, there is a transition area which, in this case, occurs in a neutral area, which the reverser must pass through in order to reverse the control direction.

The neutral area exists at approximately the 25 – 30 percent collective position. As the pilot lowers the collective lever from a normal powewr– on flight condition to the full „ down „ position for descents,the reverser shifts from normal to reverse. During this time, at some intermediate collective lever position, the reverser passes through this neutral area. There are times in flight when the pilot may fly at this intermediate collective stick position (partial power flight during a descent). If a pilot were to apply rudder pedal with the reverser in this area, there would be no differential collective output and all the pilot would be relving on for a turning force would be differential cyclic. Consequently, the directional control power would be less due to the lack of differential collective.

To augment this neutral area characteristic, a control linkage has been added to the differential cyclic system to augment the differential cyclic control. This is called the differential cyclic shifter.


The primary purpose of the DCS linkage is to increase the output of the already present differential cyclic control ( more fore / aft tilting of the rotors ) so there will be adequate directional control whenever the reverser is in, or near, the neutral area.

Tthe result of all this mechanical mixing is that the pilot just pushes the pedal in desired direction to get normal aircraft response. Larger pedal inputs may be required, with the collective in the neutral area, to get the same aircraft response. More pedal response can be achieved by moving the collective control up or down out of the neutral area.

The pedals are also connected to the rudder which moves in direct proportion to pedal input. The rudder reduces pilot workload by increasing directional stability in forward flight.
---------- ADS -----------

User avatar
sky's the limit
Rank Moderator
Rank Moderator
Posts: 4614
Joined: Sat Jan 22, 2005 11:38 am
Location: Now where's the starter button on this thing???

Re: Question from a float pilot.

#3 Post by sky's the limit » Sat Dec 19, 2009 11:09 am

Thanks for posting that Eddie,

I was going to try to explain it, but that's MUCH better than what I could have pulled off....

---------- ADS -----------

Post Reply

Return to “Helicopter Forum”