FR2582614A1 - ORDER OF AIRPLANE CONTRAROTATIVE PROPELLERS - Google Patents

Abstract

CONTROL OF CONTRAROTTATIVE AIRPLANE PROPELLERS INCLUDING: A. Means 62, 100, 106 to control the speed of the forward propeller; B. OF MEANS 64, 102, 108 TO CONTROL THE SPEED OF THE REAR PROPELLER; C. MEANS 124 TO CONTROL THE PHASE ANGLE BETWEEN THE TWO PROPELLERS.

Description

The present invention relates to a control of

counter-rotating airplane lines.

  FIG. 1 represents an airplane 3 having mo-

  gas turbine torers 6 mounted at the rear. The motors 5 each drive a front propeller 9F and a rear propeller

  9A which rotate around an axis in opposite directions.

  Figure 2 shows in more detail

  the engine propeller system in Figure 1. To the left

  che there is shown a gas turbine engine 15 of the F404 type manufactured by the holder of the present application. In

  the scope of the present invention, we can consider the mo-

  gas turbine generator 15 as a gas generator which

  generate a gaseous stream of high energy 33 and provides the

  gas stream 33 at a propulsion stage 36.

  The propulsion stage 36 extracts the energy from the

  rant gas 33 directly by means of sets of vanes

  low speed counter-rotating turbine. (This is a different

  compared to the traditional approach of using a high speed turbine whose speed is reduced on the

  path to a propeller by a reduction gearbox

  tion.) A first set of blades 39 extracts the pro-

  coming from the gas stream 33 and driving the propeller in rotation

  before 9F. A second set of blades 42 drives in rata-

  tion the rear propeller 9A but in a direction opposite to that of the front propeller 9F. Bearings 47 support the blade assemblies and the propellers and allow this counter-rotation. A step variation mechanism 52 to modify

  the pitch of the propellers 9A and 9F is shown schematically

  S matic. It is desirable to be able to control the mechanic

  pitch variation me 52 so the propeller pitch

  either one that is suitable under the operating conditions

engine.

  The object of the present invention is to provide a

  control of counter-propeller pitch variation mechanism

  rotary. The present invention further aims to provide a command to both synchronize and set

  in phase the propellers of a contra-rotating propeller system.

  According to an embodiment of the present invention

  the steps, speed and phase angles of the he-

counter-rotating aircraft files.

  The following description refers to the figures

  attached which respectively represent:

  - Figure 1, an airplane with propellers

trarotatives;

  - Figure 2, a detailed view of the counter-propellers

  rotary of Figure 1; - Figure 3, an embodiment of the present invention;

  - Figure 4, a block diagram of the operation-

  part of a computer program presented to the

end of description;

  - Figure 5, a second embodiment of the present invention;

  - Figures 4A and SA, a block diagram of the

  posants which calculates the signal Y2D of figures 4 and 5;

  - Figures 6A to D, a sequence of cal-

  block diagram ass which explains a simplification

  in Figure 4 resulting in Figure 5.

  - 3- Figure 3 shows an embodiment of the present invention. 50F and 50A speed detectors

  (also shown in Figure 2) measure the speeds

  pectives of the front 9F and rear 9A propellers. These speed sensors provide digital signals on the lines

  52 and 54 which are subtracted in summers 56 and 58.

  Such a speed measurement is conventional in the art

gas turbine engines.

  A common speed request signal for propellers 9A and 9F is present on line 60 and is indicated by XN48D and XN49D. (The symbol "XN48D" etc. is used to

  in order to be correlated with the Source Code of a pro-

  computer gram which will be studied next. The "D" in the symbol means "request".) The speed request signal on line 60 is the result of a request for the flight of the airplane and is generated either by the pilot of the airplane or by automatic control. This signal indicates the speeds at which the propellers 9A and 9F must rotate. In the case of a signal creation by automatic control, the speed request signal is programmed as a

  - function of pressures P2 (total pressure at the inlet of the mo-

  tor) and P46 (pressure turbine outlet pressure

  intermediate) of the gas turbine engine 15 of FIG. 2.

  The intermediate pressure turbine is not shown but is located immediately upstream of the arrow 33. This

  programming is well known in the art.

  The speed request signal on line 60 is added in summers 56 and 58. These summers provide as a result error signals for speed, speed, U1 and U2. A speed error signal indicates the difference between the requested speed (for example XN48D) and the measured speed (for example XN48F) of a propeller (here 9F). Speed error signals U1 and U2 are sent

  respectively to numerical controls 62 and 64.

  The digital controls 62 and 64 operate as proportional / integral controls and calculate the respective speed error signals Ul and U2 for

  thus providing calculated demand signals Y1D and Y2D.

  Numerical controls 62 and 64 are shown as separate blocks for ease of explanation. However, in one embodiment, they are in the form of a single digital computer program which calculates the speed error signals U1 and U2. We can

  use commands 62 and 64 to reduce interactions

  between the two blower control loops 130

  and 134 (described below). The source code for such a pro-

  gram is given at the end of the description.

  Y1D and Y2D are presented as position requests to summers 70 and 72 in which digital signals ACTiF and ACT2F are subtracted. ACTiF and ACT2F

  are position signals which indicate the positions

  real lines of the linear hydraulic pistons 81 and 84 of FIG. 2 and which are supplied by sensors 74 and 77. Since the geometry of the transmission rods between the pistons and the propeller blades is known in advance, the

  no propeller can be derived from ACT1F and ACTZF.

  Thus, ACT1F and ACT2F directly indicate the position of the

  pistons (pistons 81 and 84) and indirectly the pitch of the

  running. ACTIVE and ACT2F are direct position signals-

  indirectly measured and step signals indirectly measured.

  The summers 70 AND 72 produce at their outputs position error (or step) signals YE1 and YEZ. These position error signals are sent to compensation blocks 89 and 92. The compensation blocks 89 and 92

  provide analog signals XMA1 and XMA2 (by oppo-

  sition to the digital signals called YE1 and YE2) which are sent to amplifiers 96 and 98 which control the constant torque motor servo valves (TMSV) 100 and 102 which in turn control the cylinders 106 and 108 which, in

  in this case take the form of pistons 81 and 84 as shown

  Figure 2. The use of an amplifier 96 to control a TMSV 100 and a cylinder 106 in Figure 2 to modify the pitch of a 9F aircraft propeller is considered to be part of human knowledge. of the design technique of control systems.

  The outputs of the operating blocks 106 and 108 represent

  present the actual spatial positions of the pistons 81 and 84 of Figure 2, which adjust the pitch of the propellers as discussed above. These outputs are not signals. The

  blocks 74 and 77 provide the signals indicating the positions

  tions. The block 15 called "engine" provides the air flow 33

  of Figure 2 so as to drive the propellers 9F and 9A.

  Between cylinders 106 and 108 there are rod blocks-

  mechanical line 106A and 108A. The presence of these blocks emphasizes the fact that a linear displacement of the pistons

  81 and 84 is transformed into a modification of propeller pitch.

  Thus, the engine block 15 acts on the propellers having pitches

  which are determined by the cylinders 106 and 108. The not deter-

  mine the speed of the propeller for an output power of

  aircraft engine and operating conditions

  nees. The invention thus described can be redefined as follows. The speed request signals XN48D and XN49D on line 60 together with the measured speed signals XN48F and XN49F on lines 52 and 54 provide the speed error signals U1 and U2 which are processed by digital controls 62 and 64 to get Y1D and Y2D. Y1D and Y2D are then modified by signals from

  ACTIVE and ACT2F step reaction in order to provide the

  step error (YE1 and YE2). These pitch error signals force the cylinders 106 and 108 to decrease the pitch error (i.e. to decrease the

  difference between the requested step and the measured step)

  following the steps of the propeller blades. These modifications of the pitch of the blades act on the load of the sets of turbine blades 39 and 42 of FIG. 2, thus changing the speed of the modified propeller, since the energy in the air flow

  33 will remain, in general, constant.

  The invention thus described comprises a control system with two loops for each propeller, both being closed loops. One of them is a pitch feedback loop indicated by arrows 120 and 124 and the second is a speed feedback loop indicated by arrows 130 and 134. These loops have the function of

  maintain propeller speeds at the single speed requested

  designed on line 60. That is to say that they synchronize the propellers. The present invention does not only seek to

  synchronize the propellers 9A and 9F but also to synchro-

  nize in phase so as to regulate noise and vibrations

  tions. The phase synchronization is defined in connection with FIG. 1. The phase angle is the angle PH between the blade number 1F of the front propeller 9F and the blade number lA of the rear propeller A, but measured at a predetermined time,

  as for example when the blade 1F is in the posi-

  "noon". (It is of course necessary to define the phase angle PH at a determined time or with respect to

  any other reference, because the angle PH constantly changes

  due to counter rotation.) Phase synchronization is the process of adjusting the PH phase angle. The present invention adjusts the PH phase angle as follows. The blocks 120A and 122 of FIG. 3 provide a phase signal PH indicative of the phase angle PH of FIG. 1. One way of obtaining such a phase signal is to start the propellers with a known phase angle and then count the subsequent number of revolutions of each blade using the sensors 50F and 50A in figure 2. It

  There are other methods to ensure the phase of the he-

-7-

lines 9A and 9F.

  Regardless of the method used, the digital PH phase signal indicative of the PH angle in Figure 1

  is subtracted in the summer 130 from a phase signal of-

  mandé PHAD. PHAD is supplied by the pilot or by an automatic control. The summator 130 then provides a signal

  U6 phase error which is sent to a sync command

  phase 124 which also receives a speed signal

  indicating the engine speed on line 125. The com-

  phase synchronization command 124, in response to

  trées which have just been described, provides a phase control signal Y3. The phase control signal Y3 is

  added in summator 72 for so, effectively

  ve, modify the YE2 step error signal. This forces the jack 108 to try to decrease the pitch error signal

  YE2 by adjusting the pitch of the rear propeller 9A.

  For example, in the case where both the error and speed signals U1 and U2 indicate that the propellers are synchronized at the requested speed, the pilot of the aircraft may wish to modify the phase angle PH. If it wishes to increase the phase angle, it will create a PHAD phase request signal which will produce a U6 error signal. The error signal U6 will effectively increase the pitch error signal YE2, forcing the pitch of the rear propeller 9A

  in Figures 1 and 2 to be increased. This increase ra-

  will slow down the rear propeller, thus tending to decrease the error

to not.

  Various important aspects of this in-

  vention will now be studied. First, we have

  previously wrote two closed feedback loops for each

  one of the propellers. In addition, a third reaction loop, a phase reaction loop comprising the blocks 120A,

  122 and 124 and summers 72 and 130 is added to the heli-

  this rear 9A. Thus the control diagram includes two feedback loops for the forward propeller, but three loops - 8 -

  reaction for the rear propeller.

  Second, the speed feedback loops

  are not open during phase synchronization.

  That is, for example, although the control signal

  phase signal YE2 is sent to the summator 72 during a phase modification, neither the pitch error signals YlD and Y2D sent to the summers 70 and 72, nor the measured step signals ACTIVE and ACTZF are disconnected from the summers 70 and 72. That is, the speed loops, the loops

  step and the phase loop remain closed and function-

at any time.

  Thirdly, the phase adjustment is carried out by modifying the pitch of the rear propeller. Tests and models have shown that the changes in pitch of the rear propeller 9A disturb the front propeller 9F much less than the changes in pitch of the front propeller 9F disturb the rear propeller 9A: the disturbance detected at before is

  lower than the disturbance detected at the rear. Therefore

  quent, the present invention aims to obtain a synch-

  ronization in phase by operating the rear propeller

earlier than the forward propeller.

  Fourth, from the fact that the turbine sets

  ne 39 and 42 are fluidly coupled, their speeds re-

  latives may differ. So synchronization in phase

described above is possible.

  We will now comment on the software code that will

  to follow. Lines 1 and 2 of the code refer to the operation

  The summers 56 and 58 of FIG. 3. Lines 3-6 refer to the digital representations of proportional / integral control (p / i) indicated by blocks 62 and 64. For continuous analog control, lines 3- 6 can be described as shown in Figure 4. The command

  p / i (i.e. blocks 62 and 64 in Figure 3) is ap-

  peeled "proportional" because of the term "a" in block 170 of Figure 4 ("a" in this analog case would have a value of 0.00105). The output (for example Y2B on line 9 of the code and shown in Figure 4) is proportional to the input (by

  example U2 on lines 4 and 6 of the code and shown fi-

  gure 4) a proportionality factor "a".

  The command is called "integral" because of the term b / s of block 176 in which "b" would have the value of

  0.003 in this analog case. 1 / s refers to an integral

  tion in the time domain. The constants a and b of blocks 170 and 176 are represented in the numerical case (i.e. non-analog digital) by the numbers

  0.001065 and 0.00003 in lines 4 and 6 of the code.

  For block 64 of Figure 3, lines 4 and 6

  represent the state transition ABCD or the form of equa-

  differential control command transformed Z compara-

ble 0.001065 + 0t00003 Z-1

1- Z-1

  for a sample period of T = 0.01 seconds. The cons-

  aunts a and b of blocks 170 and 176 are not identical to those of the Z transform due to approximations in the relation in which Z 1 + sT z 1 - sT

  In other words, the values of "a" and "b" differ

  slightly depending on whether one operates in the field

  analog or digital.

  The terms XllB and X21B in lines 7 and 9 are basic values that are programmed as a function of the engine operating point. G48 and G49 on lines and 6 are loop gain multipliers programmed as a function of the operating point. In the

- 10 -

  lines 3 and 4, X1 and X2 are the past values of the states

  and U1 and U2 are the past values of speed errors.

  In lines 3 and 4, XNEWi and XNEW2 are the pre-

  of states resulting from digital rectangular integration. The variable Xl in line 5 is the current value of the state and is the same as the variable XNEWl in line 3. In the same way for XZ line 6 compared to

  XNEW2 line 4. U1 and UZ on lines S and 6 are the

  their current speed errors. This distinction in the nomenclature is made by the particular software used in the design. The equations in the lines

  3-6 represent classic differential equations.

  Lines 11 and 12 refer to operation

summers 70 and 72.

  The functions of lines 13-16 have not been studied.

  gods before. These lines create what can be called

  "hat functions". The calculated results of "functions-

  headings "are the variables GF1 and GF2 in lines 18, 24, 26, 29 and 30. The operation" SIGN "in lines 13 and 14 refers to the function SIGN: for example" SIGN (l, 0, YEl) "has a value of +1 when YE1 is positive and of -1 when YE1 is negative. The operation of the hat functions is to give the variables GF1 and GF2 larger values when the error signals Y1 and Y2 are small, making XMA1 and XMA2 on lines 29 and 30

  taller. This allows the non-functional band to be exceeded.

  servo valves with constant torque motor (TMSV). It has been noted that the non-operating band of the TMSV servo valves results in an error in the angle of

  not real. The "hat" function reduces this error.

  Lines 29 and 30 of the code refer to the compensation blocks 89 and 92 in FIG. 3. GF1 and GF2 are the multipliers of the "hat" function, YE1 and YEZ are

  position errors (i.e. positions of the pis-

  tones 81 and 84 in Figure 2) and G is a constant of

- 11 -

  conversion. (the units of X4A1 and XMA2 must be in mil-

  ampere, while those of YE1 and YE2 are not).

  Line 32 of the code refers to the summator 130 of

  Figure 3. Lines 32-37 limit U6 to values

  taken between -45 and +45 degrees. One function of this limitation is to treat all propeller blades in the same way. So, for example, if the phase angle PH increases

  from 44 to 46 degrees, lines 31-39 of the code will have for e-

  to redefine the phase angle from 44 to 1 degree. We measure

  so now the phase between a pair of different blades

  your. This 45 degree limit assumes 8 blades per propeller (360/8 = 45). If a different number of blades is used, the

  limitation range will of course be different.

  Lines 46-48 refer to a second set

  ble of differential equations which represent a command

  proportional / integral which is practically identical to the proportional / integral command used in 64, block 124 figure 3. The principles described above in relation to figure 4 apply to lines 46-48 in the same way. Line 47 refers to the signal Y3 which is applied to the summator 72 of Figure 3. G1 in line 47 is a

  loop gain multiplier programmed to be functional

tion of the operating point.

  Line 2 of the code contains the variable "SETTING" which is sent to the adder 58 figure 3. This variable is controlled by the pilot and allows him to choose different

speeds for the two propellers.

  -12 - Source Code I 1 Ul = XN48D-XN48F

2 U2 = (XN49D + SETUP) -XN49F

3 XNEWl = Xl + G48 * U1

4 XNEWZ = X2 + G49 * U2Z

Y1 = 0.00014 * X1 + 0.00427 * U1 * G48

6 Y2 = 0.00003 * X2 + 0.001065 * U2 * G49

7 Y1D = Yl1 + XllB

8 C REFLECTS INITIAL CONDITIONS

9 Y2D = Y2 + X21B

C REFLECTS INITIAL CONDITIONS

11 YE1 = Y1D-ACT1F

12 YE2 = (Y2D + Y3) -ACT2F

13 Ml = SIGN (1,0, YE1) * (1,0-HT1) / ELIM1

14 M2 = SIGN (1,0, YE2) * (1,0-HT2) / ELIM2

T1 = Ml * YE1 + HT1

16 T2 = M2 * YE2 + HT2

17 IF (ABS (YE1) .LT.ELIM1) THEN

18 GFl = T1

19 OTHERWISE

GF1 = 1.0

21 END IF

22 C

23 IF (ABS (YE2). LT.ELIM2) THEN

24 GF2 = T2

OTHER

26 GF2 = 1.0

27 END IF

28 C

29 XMAl = G * YE1 * GF1

XMA1 = G * YE2 * GF2

31 IF (L4.GT.0) THEN

32 U6 = PHAD-PH

- 13 -

33 IF (U6.LT.-22,5) THEN

* 34 U6 = U6 + 45.

IF (U6.GT.22,5) THEN

36 U6 = U6 + 45

37 END IF

38 OTHERWISE

39 U6 = 0

END IF

41 IF (SP.LT. 1) THEN

42 XX3 = 0

43 OTHERWISE

44 XX3 = X3

END IF

46 XNEW3 = XX3 + U6 * G1

47 Y3 = 0.00003 * X3 + 0.001065 * U6 * G1

48 C

49 RETURN

VS

51 C CALCULATION ON ACCEPTED VALUES

52 70 RETURN

53 C TERMINAL CALCULATIONS

54 80 RETURN

END

Source Code II

4 XNEW2 = X2 + (G49 * U2 + Y3)

  6 Y2 = 0.00003 * X2 + 0.001065 * (U2 * G49 + Y3)

12 YE2 = Y2D-ACT2F

- 14 -

  We have described an order to order a set of

  ble of propellers driven by counter-rotating turbines. The

  control has two loops for each propeller: one

  speed control key and a pitch control loop. In addition, the command has a third loop for

  one of the propellers which is a phase control loop.

  All these loops are preferably closed to all mo-

  and the phase control loop works pre-

  ference by adjusting the pitch (therefore the speed) of

the rear propeller.

  The previous discussion focused on the realization

  tion of speed error signals U1 and U2 by the summa-

  56 and 58. It will be noted that these speed error signals are practically similar to thrust error signals, the thrust of the propellers being a function of the speed of the propeller. Therefore, a thrust sensor can be substituted for the speed detectors 50A and 50F and the signal on line 60 can be treated as a thrust request signal rather than a speed demand signal. The thrust measurements can be carried out by numerous methods known in the art. For example, thrust is a function of the total pressure drop

  (i.e. pressure ratio) through the sets

  blades of counter-rotating turbine blades 39 and 42 Figure 1.

  You can also measure the thrust using extenso-

  meters attached to the trees or cylinders that support the he-

  lines 9A and 9F. The elongation of the tree is a measure of the

  thrust. In addition, the thrust can be obtained in-

  direct. A set of counter-rotating vanes 9A and 9F can-

  They can either be made to full size or to scale and operate in a load cell. The thrust for different operating conditions, including different propeller and blade pitch speeds is recorded on a program. Then, in real flight, the

- 15 -

  conditions such as the propeller speed and the blade pitch are measured and / or derives the thrust from the program based on these measurements. Consequently, the speed loops 130 and 134 can be considered as push loops, with the push which becomes the requested parameter on line 60 and also which is the parameter on

which the loops close.

  A second embodiment is based on the

  the numerical control 64 of FIG. 3 is similar

  the synchronization control in phase 124. The situation

  milarity is represented by the similarity of form of the li-

  Genes 4 and 6 of the Source Code (for numerical control 64)

  compared with lines 46 and 47 (for the command of syn-

  chronization in phase 124). Recognition of this simi-

  the size eliminates the phase synchronization command 124 as shown in Figure 5. Figure 5 is implemented by replacing lines 4, 6 and 12 with lines

  Source Code II, and eliminating the li-

genes 41-48 included.

  A justification for this replacement in the figure

  re 5 is represented by the sequence of FIGS. 6A-D. The fi-

  gure 6A is a diagram which is similar to that of the fi-

  gure 4 but to which an input 200 which represents the signal Y3 of FIG. 3 has been added. Y3 is the output of the synchronization command in phase 124. FIG. 6B is obtained by decreasing the closed loop containing the blocks 202 and 204 of FIG. 6A at a simplified "offset", that is to say at the block

  206 Figure 6B. Figure 6C is obtained from the fig-

  re 6B by breaking line 208 in Figure 6B as shown

  felt by lightening bolts 210. FIG. 6C thus becomes FIG. 6B, but with an open entry 212 and an open loop 214. FIG. 6D, the block 216 of FIG. 6C has been separated into blocks 218 and 220 with the block 218 being meaningless

  inverse so that s / (s + a) is the inverse of (s + a) / s.

  The function f (s) of block 220 of FIG. 6D represents

2582 614

- 16 -

  shows a part of the phase synchronization command 124 in FIG. 3. If block 220 has the form (s + a) / s, as indicated by arrow 22, and if the terms "a" of blocks 218 and 220 are identical, then block 218 cancels block 220. This cancellation has a significant result: the integrators present in blocks 218 and 220 are eliminated. This is important because an integrator in a

  any control system must be carefully designed

  is lying. The integrator is always "in operation", that is to say that it always integrates its input, always producing an output signal (except of course for the case where the input has been zero for a fairly long time

  with respect to the time constant of the integrator).

  The Source Code, when modified by the Source Code

  It implements the concepts represented in FIG.

  integration takes place for each of the propellers, i.e. on lines 3-9 of the Source Code. In addition, even with a

  Source code similar to that described at the beginning, the time constants of the synchronization command in phase 124 and the

  numerical control 64 are the same. That is, the coefficients 0, 00003 and 0.001065 on lines 6 and 47 are the same. This makes it possible to eliminate the block 124 of FIG. 3 and to introduce U6 directly into the block 64 with the appropriate program gain G1 as indicated by the line in broken lines 250 of FIG. 3. Consequently, the processing of Y2D of Figure 4 by the components of Figure 4A is modified to become the treatment of Y2D in

  Figure 5 by the components of Figure SA.

- 17 -

Claims (6)

  1. Control for airplane front and rear propellers which rotate around a common axis, characterized in that it comprises: a) means (62, 100, 106) for controlling the speed of the front propeller; b) means (64, 102, 108) for controlling the speed of the rear propeller; c) means (124) for controlling the angle of phase between the two propellers.   2. Control for airplane front and rear propellers which rotate around a common axis, characterized in that they comprise: a) means (62, 100, 106) for modifying the pitch of the front propeller in response at the speed of the front propeller, and b) means (64, 102, 108, 124) for modifying the pitch of the rear propeller in response to both the speed of the front propeller and to   the phase angle between the two propellers.   3. Control for airplane front and rear propellers which rotate around a common axis, characterized in that it comprises: a) means for modifying the pitch of the front propeller in response to the thrust of the propeller before and,   b) means for modifying the rear propeller   re in response to both the push of the he-   rear line and at the phase angle between the two propellers.   4. Control for airplane front and rear propellers which rotate around a common axis, characterized in that it comprises: a) means for modifying the pitch of the front propeller in response to the speed of the propeller - 18 -   forward and forward propeller pitch, and b) means for changing the pitch of the rear propeller in response to both the speed of the rear propeller, the pitch of the rear propeller and the angle phase between the two propellers.   5. Order for a pair of co-propellers   contra-rotating axles characterized in that it comprises:   - means (62, 100, 106) for controlling the vi-   tesse of a first propeller by adjusting the pitch of the first propeller;   - means (64, 102, 108) for controlling the vi-   size of the second propeller by adjusting the pitch of the second propeller; - means (124) for measuring the relative phase angle between the first and the second propellers; and   - means for adjusting the phase angle in re-   sliding the pitch of one of the propellers, thus modifying the vi-   size of this propeller 6. Control for airplane front and rear propellers which rotate in opposite directions around a common axis, characterized in that it comprises: a) a forward speed detector (SOF) to provide a forward speed signal indicative of the speed of the forward propeller; b) a rear speed detector (50A) for   provide a reverse speed signal   tif the speed of the rear propeller; c) a forward pitch detector (74) for providing a forward pitch signal indicative of the pitch of the forward propeller; d) a rear pitch detector (77) for providing a rear pitch signal indicative of the pitch of the rear propeller;   e) a phase detector (120A, 122) for   provide a phase signal indicative of the angle - 19 -   phase between the two propellers; f) a front pitch cylinder (106) for modifying the pitch of the front propeller; g) a rear pitch cylinder (108) for modifying the pitch of the rear propeller;   h) means of comparison (56, 58) of the   forward and reverse speed signals with a   speed required to provide   forward and reverse speed error signals; i) means (62, 64) for processing the forward and reverse speed error signals and for providing processed forward and reverse speed error signals; j) means (70, 72) for comparing the forward and reverse speed signals processed with the respective forward and reverse step signals and for providing the forward and reverse step error signals; k) means (106, 108) for modifying the respective pitches of the front and rear propellers by   response to step error signals respectively   tifs of j); 1) means for modifying the processed rear speed error signal; m) means (130) for comparing the phase signal with a requested phase signal and for providing a phase error signal; and n) means (124) for modifying the processed reverse speed error signal of i) in response to the response to the phase error signal.