GB2312408A - Method and apparatus for calculating helicopter airspeed - Google Patents

Abstract

A method or apparatus for determining the airspeed of a helicopter, comprises establishing at least one cyclic pitch trim curve relating lateral and longitudinal cyclic blade pitch angle of the main rotor to airspeed, establishing the position of the centre of gravity of the helicopter relative to a datum position and using this to modify the trim curve to account for the offset, measuring the lateral and longitudinal cyclic pitch of the rotor blades and determining the airspeed from the modified trim curve using these angles. The pitch angles of the rotor blades may be determined by direct measurement at the blades or by measurement of the cyclic control stick position. The airspeed determined by the method may be displayed on an instrument showing both direction and magnitude. The method may be particularly useful at low speeds and near sea level. Compensation may be made for pitch and roll rates and accelerations.

Description

Description of Invention Title: METHOD AND APPARATUS FOR CALCULATING THE AIRSPEED OF A HELICOPTER This invention relates to a method and apparatus for calculating the airspeed of a helicopter, and particularly for calculating the magnitude and direction of the airspeed in low speed flight up to about 40 knots in all directions.

An accurate indication of the magnitude and direction of helicopter airspeed in low speed flight is highly desirable especially during certain operations such as rejected take-offs and when operating from ship decks and oil rig platforms.

Conventional fuselage mounted pitot-static airspeed measurement systems used on helicopters operate in an identical manner to fixed wing installations and suffer similar inaccuracies at low speed and during flight with high side slip angles. Furthermore, such devices are adversely affected especially at low airspeeds by the highly irregular airflow close to the helicopter fuselage due to the interaction of many influences including the effects of main rotor wake.

Other airspeed measurement systems particularly developed for helicopters such as the omnidirectional airspeed indicator of Pacer Systems Inc, or the Air Data System (ADS) of GEC may not be entirely satisfactory throughout all flight regimes.

As an alternative to such airspeed measurement devices there have been several prior proposals investigating the feasibility of using calculation procedures for determining the airspeed. Such systems, an example of which is disclosed in GB-A-2037688, are based on the fundamental idea that every flight state (defined by aircraft mass and inertias, linear and rotational attitudes. velocities and accelerations) corresponds to a unique combination of helicopter control parameters (main and tail rotor torques and control angles). This implies that if all control parameters and all flight state parameters except speed were continuously measured, then it should be possible (via look-up tables or calculation procedures) to determine accurately the true airspeed.

Such proposals, for example the aforementioned GB-A-2037688, cover the whole flight regime and require a large number of complex parameters to be continuously measured.

This results in an extremely complex and large system that is susceptible to considerable errors in the event of a fault in any of the individual measurements.

An objective of this invention therefore is to provide a method and apparatus for calculating the magnitude and direction of the airspeed of a helicopter that overcomes the aforesaid problems.

Accordingly in one aspect this invention provides a method for calculating the magnitude and direction of helicopter airspeed comprising the steps of establishing for the helicopter at least one cyclic pitch trim curve relating lateral and longitudinal cyclic blade pitch angle to airspeed, establishing an actual location of the centre of gravity of the helicopter relative a datum position and modifying the cyclic pitch trim curve depending on an offset of the actual centre of gravity position, measuring the angle of both lateral and longitudinal cyclic pitch of the rotor blades of a main sustaining rotor and applying the cyclic pitch angles to the cyclic pitch trim curve so as to determine the magnitude and direction of the helicopter airspeed.

In another aspect the invention provides apparatus for calculating the magnitude and direction of helicopter airspeed comprising measuring means for measuring the longitudinal and lateral cyclic pitch angles of the rotor blades of a main sustaining rotor and the location of the centre of gravity position relative a datum position, and computer means containing data in the form of a cyclic trim curve relating lateral and longitudinal cyclic blade angles to airspeed magnitude and direction whereby the magnitude and direction of the airspeed of the helicopter is calculated from said measured values of longitudinal and lateral cyclic pitch blade angles and said centre of gravity position.

The invention will now be described by way of example only and with reference to the accompanying drawings in which, Figures 1 to 6 are cyclic trim curves for a particular helicopter model in differing flight conditions, Figure 7 illustrates how the cyclic trim curves of Figures 1 to 6 are modified depending on flight conditions, Figure 8 is a graph plotting cyclic pitch against airspeed for the flight conditions of Figures 1 to 6, Figure 9 is a graph illustrating the variation of a cyclic vector angle against airspeed, Figure 10 illustrates the effect of adding a tail rotor on the cyclic trim curves, Figure 11 is a flow chart illustrating the procedure for calculating the magnitude and direction of airspeed, and Figures 12 and 13 are plots comparing a theoretical airspeed and an airspeed calculated according to the invention.

In considering problems associated with prior art airspeed calculating devices such as disclosed in GB-A-2037688 the inventors decided to concentrate only on the low airspeed flight regime, and to investigate the minimal system in terms of required measurements and calculations necessary to provide a sufficiently accurate calculation of the magnitude and direction of helicopter airspeed.

The horizontal airspeed and direction of flight of a helicopter are controlled by cyclic changes to the pitch angle of the blades of a main sustaining rotor as they rotate around an operational rotor disc. Such cyclic pitch changes combine longitudinal cyclic pitch changes and lateral cyclic pitch changes.

Initially, the inventors considered steady trimmed flight characteristics only. They recognised that,in the case of an isolated main sustaining rotor (i.e. no other helicopter components included) and with the helicopter centre of gravity on the sustaining rotor shaft axis (neutral), the vector of the rotor cyclic angles-to-trim at a particular airspeed varies with airspeed direction in a circular manner.

Figure 1 is a datum cyclic trim curve for such an isolated rotor with a neutral centre of gravity position, and plots longitudinal cyclic stick position against lateral cyclic stick position, expressed in terms of degrees of blade pitch angle, for "forwards", "backwards".

"sideways right" and "sideways left" flight. Figure 1 also shows the relationship between cyclic stick positions and airspeed, three representative airspeeds being shown by the concentric circles marked 10, 20 and 30 knots respectively.

This indicated to the inventors that a measurement of the cyclic stick position or the lateral and longitudinal cyclic blade pitch angles could, by applying the measurements to a cyclic trim curve, provide information in respect of both the magnitude and direction of the airspeed.

What was not clear was whether or not the information provided would be sufficiently accurate, and the inventors decided to investigate the effect of flight state changes and aircraft centre of gravity changes on the rotor cyclic pitch to trim requirements. Figures 2 to 6 inclusive along with Figure 1 (the datum) show the cyclic trim curves for the various configurations defined in Table 1 in which OGE and IGE mean, respectively, out of ground effect and in ground effect.

TABLE 1

Rotors Weight CG Flight State Figure 1 Main 12000 lb 0 on shaft level OGE (neutral) Figure 2 Main 12000 lb on shaft level IGE (neutral) rotor height 6.4 m Figure 3 Main 12000 Ib on shaft 600 ft/min (neutral) climb Figure 4 Main 9000 lb on shaft level OGE (neutral) Figure 5 Main 12000 lb 0.1 m aft of level OGE shaft axis Figure 6 Main + Tail 12000 lb neutral level OGE From a comparison of Figures 1 to 4 it will be seen that the only substantial differences from the cyclic trim curves of the datum case of Figure 1 occur in the curves for Figures 3 (600 ft/min climb) and Figure 4 (9000 Ib AUW), and then only at the higher airspeeds.

A comparison of Figures 1 and 5 shows that the effect of the movement aft of the centre of gravity has little effect on the form of the cyclic trim curve except that the centre is now offset predominantly in the longitudinal direction.

The effect of adding a tail rotor to the model is illustrated in the cyclic trim curve of Figure 6 which, when again compared to Figure 1, is seen now to have its centre offset predominantly in the lateral direction. The airspeed circles are no longer concentric once the tail rotor effect is added as the tail rotor force and hence lateral trim is a function of speed.

Various aspects of the cyclic blade angle requirements displayed in Figures 1 to 6 inclusive will now be discussed in more detail with reference to Figures 7 to 10 inclusive.

Figure 7 plots diagrammatically lateral cyclic pitch against longitudinal cyclic pitch, and illustrates the manner in which the centre of the cyclic trim curve is to be adjusted to take account of effects of the tail rotor (Figure 6) and the aft centre of gravity location (Figure 5). Thus starting from the datum position 21 for the isolated main rotor in hover with neutral c.g. position (Figure 1), the centre is offset predominantly laterally to position 21a due to the effect of the tail rotor in hover (a function of the all up weight) as illustrated in Figure 6.

Taking account now of the effect of the aft location of the centre of gravity illustrated in Figure 5, the centre of the cyclic trim curve is offset longitudinally to the position 2J b.

It is then necessary to introduce a correction of the tail rotor effect because of a change in that effect with airspeed. This is due to a non-linear relationship in the power requirement of the tail rotor with airspeed, which results in a correction parallel to the initial offse; between 21 and 21a but in an opposite sense, to establish for the particular coliflguration being considered, a corrected cyclic trim curve centre 21e for use in an algorithm for calculating the magnitude and direction of airspeed.

Figure 7 also indicates a forward beading flight line 22 from the adjusted centre position 21c and, as will be understood from the discussion of Figures 1 to 6, the radial location along the flight line 22 established by the measured lateral and longitudinal cyclic pitch angles will establish the airspeed of the helicopter.

With regard to the calculation of the heading of the helicopter, assume that the measured lateral and longitudinal cyclic pitch angles establish an actual heading such as illustrated at 23 in Figure 7. The angle from forward heading line 22 establishes the sideslip angle.

Figure 8 plots cyclic pitch angle (degrees) against airspeed (knots) for the various cases depicted in Table 1. With some minor deviation at the higher airspeeds particularly for cases 3 and 4, this graph illustrates a fairly linear relationship across the range of cases to demonstrate confidence In the cyclic trim curves as a method for calculating airspeed for a particular helicopter across a range of different operating conditions.

In Figure 9, airspeed (knots) is plotted against a heading deviation angle y, shown in Figure 7 as a deviation from the forward heading direction 22 for the cases detailed in Table 1.

The figure shows that the datum heading angle zH can be represented in a simple algorithm as a constant for all loading cases and throughout the speed range, and that this assumption would lead to errors in predicted sideslip angles of less than +20 . This illustrates further confidence in the method of the invention to calculate reasonably accurately within acceptable limits the heading (direction of airspeed) of the helicopter.

Figure 10 illustrates the effect of the tail rotor on the offset of the centre of the cyclic trim curves as noted previously in respect of Figure 6 and Figure 7. The effect is non-linear because the tail rotor needs to react the main rotor torque which changes non-linearly with airspeed.

The results hereinbefore defined convinced the inventors that the method of this invention clearly had the capability by way of a simple calculation procedure based on the measurement of longitudinal and lateral cyclic stick positions or direct rotor blade cyclic angle measurements and a knowledge of the helicopter centre of gravity position, to provide a reasonably accurate calculation of the magnitude and direction of a helicopter's airspeed.

The parameters can be used to make the calculations within the low speed regime by following the procedure outlined in the flowchart illustrated in Figure 11.

The calculated values of the magnitude and direction of airspeed can be displayed on any suitable indicating instrument in the cockpit. Such an instrument may for example comprise a variable length arrow device whose direction indicates the direction of the airspeed and whose length provides magnitude information. The instrument may also display both sets of calculations digitally.

The accuracy of the method of the invention is dependent on factors which may affect the non-linearity of the approximations used. These include fuselage effects, accuracy of c.g.

position and altitude.

It was thought that interactions between the main rotor wake and the fuselage/tailboom could cause the cyclic trim curves to become irregularly shaped with the magnitude of the velocity vector from the hover centre point changing with the heading angle. However, in the work conducted by the inventors this was not found to be substantial certainly for the particular helicopter model used.

As already noted, the centre of gravity position is important in determining the actual centre of the cyclic trim curve from which calculations are made, so that information on any significant changes in c.g. position is required during flight of the helicopter on which the invention is in use.

The cyclic trim positions defined in the cyclic trim curves are dependent on density altitude.

While this effect could be taken into account in the algorithm for undertaking the calculation procedures herein defined, the need for a low airspeed calculation at altitude is usually unnecessary, so that the algorithm would most likely be optimised for operation near sea level.

The calculation procedure hereinbefore defined is applicable only to steady trimmed flight conditions.

In any practical implementation of such a system it is preferable that non-steady flight conditions such as encountered in manoeuvring of the helicopter do not upset the calculations. Ideally, the calculations should remain as accurate through manoeuvres as in steady trimmed flight conditions, however, for practical purposes, it is probably sufficient that the airspeed magnitude and direction calculations move smoothly in the correct senses during the manoeuvre and without undue error.

To assess the dynamic characteristics of the low airspeed calculation algorithm, the helicopter model was set up in trim with the helicopter having an airspeed of 7 metres per second and with a sideslip angle of 135". A circular stirring of the cyclic control in a clockwise direction was then performed with the stick reaching its original trim position in 7 seconds.

Figure 12 plots longitudinal velocity (metres/second) against lateral velocity (metres/second) for this particular manoeuvre and compares a true output in broken line obtained from a state of the art computer model and an output calculated by an algorithm according to the method of this invention but without dynamic corrections in full line. Whilst the calculated output follows reasonably closely the actual output throughout the majority of the manoeuvre there are areas where considerable errors are evident.

From theoretical considerations, the inventors were able to define additional terms to be used in the trim algorithm to effectively modify the measured rotor cyclic displacements and compensate to a first order for the effects of such manoeuvres.

The additional terms investigated were those that compensate for pitch and roll rates and accelerations. The angular accelerations are brought about by rotor disc flapping which in turn requires rotor cyclic. The angular rates affect (through gyroscopic forces) the rotor cyclic pitch necessary to maintain the required rotor flapping during a particular manoeuvre.

The effect of the introduction of these terms into the calculation algorithm are illustrated in Figure 13 which is similar to Figure 12 and again compares a true output in broken line with a calculated output in full line. It will be seen that the calculated output is much more closely aligned with the true output throughout the entire manoeuvre.

This invention therefore provides a simple low airspeed magnitude and direction calculation method which in trimmed flight appears to be accurate to within about +4 knots magnitude and +8 degrees direction (heading). It has been shown also that errors in the calculations due to manoeuvre effects can be largely offset by introducing corrective terms in the algorithm in respect of measured pitch and roll rates and accelerations.

Whilst several embodiments have been described and illustrated it will be understood that many modifications may be made without departing from the scope of the invention.

Claims (10)

1. A method for calculating the magnitude and direction of helicopter airspeed comprising the steps of establishing for the helicopter at least one cyclic pitch trim curve relating lateral and longitudinal cyclic blade pitch angle to airspeed, establishing an actual location of the centre of gravity of the helicopter relative to a datum position and modifying the cyclic pitch trip curve depending on an offset of the actual centre of gravity position, determining the angle of both lateral and longitudinal cyclic pitch of the rotor blades of a main sustaining rotor and applying the cyclic pitch angles to the cyclic pitch trim curve so as to determine the magnitude and direction of the helicopter airspeed.

2. A method according to Claim 1 wherein the angle of the lateral and longitudinal cyclic pitch of the rotor blades of the main sustaining rotor is determined by measurement of longitudinal and lateral cyclic stick positions.

3. A method according to Claim 1 wherein the angle of both lateral and longitudinal cyclic pitch angles of the rotor blades of the main sustaining rotor are determined by direct rotor blade cyclic angle measurements.

4. A method according to any one of Claims 1 to 3 which includes displaying the values of the magnitude and direction of airspeed on an indicating instrument.

5. A method according to Claim 4 wherein the instrument comprises a variable length arrow device whose direction indicates the direction of the airspeed and whose length provides magnitude information.

6. A method according to any one of the preceding claims wherein the method is calibrated for operation near sea level.

7. A method according to any one of the preceding claims wherein in establishing for the helicopter at least one cyclic pitch trim curve, compensation is made for pitch and roll rates and accelerations.

8. A method for calculating the magnitude and direction of helicopter airspeeds substantially as hereinbefore described with reference to the accompanying drawings.

9. An apparatus for calculating the magnitude and direction of helicopter airspeed comprising means for determining the longitudinal and lateral cyclic pitch angles of the rotor blades of a main sustaining rotor and the location of the centre of gravity position relative to a datum position, processing means containing data in the form of a cyclic trim curve relating lateral and longitudinal cyclic blade angles to air speed magnitude and direction whereby the magnitude and direction of the airspeed of the helicopter is calculated from said determined values of longitudinal and lateral cyclic pitch blade angles and the centre of gravity position.

10. An apparatus for calculating the magnitude and direction of helicopter airspeed substantially as hereinbefore described with reference to and as shown in the accompanying drawings.