HEAD UP DISPLAY MEANS FOR AIRCRAFT

Abstract: A head up display system for an aircraft including means for generating a display indicating the attitude of the aircraft, the display including a reference frame (2) which takes the form of a vertically-aligned cylindrical grid consisting of lines (3) in an axial direction which are representative of azimuth and circumferential lines (4) which are representative of altitude and viewed from a position (1) on the cylinder axis in a direction corresponding to the aircraft vector. A zero height datum on the reference frame may be provided, this conveniently taking the form of a shaded area (5) appearing to close in one end of the cylinder, whilst the other end (10) of the cylinder, representing the vertically upward direction, is apparently open. The display may also include circumferential markings (6, 7) on the cylinder indicative of a demand height determined by the aircraft's control settings, and of the height of the aircraft. The display may also possess a central area (11) within which the said reference frame does not extend hence enabling the central area to be available for additional symbology or an unobstructed forward view.

Description: HEAD UP DISPLAY MEANS FOR AIRCRAFT This invention relates to head up display means for aircraft, in which information about the orientation of the aircraft and the state of his equipment (including weapon systems) may be projected to the operator so as to be superimposed over his external view. For the past twenty years head up displays have become increasingly important in aircraft operations. This method of displaying information has a number of avantages over the head down display, the most important of these being that when the operator requires a view of the outside world as part of his operation he does not have to waste time changing his direction of gaze. Head up displays have their own special problems. First there are the engineering problems of creating clear accurate images with a light intensity that enables them to be seen against varying backgrounds. Then there are the human factors problems of interpreting the symbology, avoiding interference between different processing channels and selecting priorities among a large array of symbols. Added to these is the problem that the more symbology the sight glass carries the more obscuration there is of the view beyond. The conventional head up display used in fast-jet operation is the so-called"pitch ladder"display, consisting of a pattern of bars, each drawn essentially parallel to the actual horizon and forming a scale against which the position of an aircraft symbol indicates a measure of the attitude or climb/dive angle of the aircraft. Although especially in conditions of poor visibility or in acrobatic manoeuvres or high turbulence it is vitally important that when the operator returns his attention to the display after being required to look elsewhere the display will give him immediate, unambiguous information about the attitude of the aircraft, this type of display can give rise to ambiguities when, as is usual, only a small portion of the pattern is displayed within the field of view at any instant. This is illustrated by Figure 1, which illustrates schematically typical appearances of a pitch ladder display at three different positions of a roll manoeuvre in a plane slightly out of the vertical. The ladder display shows aircraft vector elevation as referred to the global horizon and therefore responds to aircraft pitch only under certain circumstances. When the aircraft's longitudinal axis is near the horizon and the wings are approximately level the ladder movement responds directly to aircraft pitch rate. For example if the ladder moves down 5 degrees the pilot knows that the aircraft's vector has pitched up 5 degrees. However, as the angle of bank increases the displayed pitch decreases as the cosine of the bank angle until at ninety degrees of bank the ladder does not respond to pitching movements at all but only vector elevation. In this situation it is the compass indication which responds to pitch changes. The compass is not designed for this purpose and its response is inadequate as a useful indicator for pitch control. It has the further disadvantage that it is working in the wrong plane. That is, as the nose pitches up in a tight turn the compass moves across the pilots field of view left to right or right to left. This response of the compass to pitch is a function of the sine of the angle of bank. Thus at forty five degrees of bank near the horizon the indication of pitch rate is shared between the movement of the pitch bars and compass rotation. Neither work in the plane of movement and their response characteristics are quite different. All the above applies only when the aircraft's longitudinal axis is near the horizon. At quite small elevation angles the natural horizon cannot be seen through the sight glass and what the pilot sees is the projection of a bar or bars orientated to the global horizon giving an indication of the aircraft's attitude in relation to the just-out-of-sight horizon. However, as the elevation increases this analogy of a relay of the nearest horizon acquires an unwanted and confusing extra dimension. At high angles of elevation the horizon is no longer anywhere near the bore sight of the aircraft but is all round the pilot, above his head, under his seat and out to each side. What a pilot sees of the actual horizon in these circumstances depends on which way he looks, There is certainly nothing to see of it dead ahead along the bore sight, which is where the instrument information must be presented. When the plane of pitching is truly vertical as in a perfect loop the pilot will see a relay of the horizon in his sight glass as though his wings were level and he can take this to imply that his plane of pitching is at 90 degrees to the horizon. At the zenith point, however, the term'nearest'horizon has no meaning as the angle is the same in all directions and the display is free to rotate through any angle which it does in a most distracting manner. This movement implies a spurious roll so the pilot has to learn to ignore it. Of course this rotation is only sustained if the pilot is executing an absolutely vertical climb. If he continues pulling round, as in a loop, the display rotates 180 degrees and then shows a relay of the approaching horizon but inverted. When, however, the pilot"loops"in a plane which is not exactly vertical, there is a marked difference in the way the manoeuvre is displayed. Figure 1 illustrates how the ladder type of display shows a false roll indication during looping manoeuvres in a plane slightly out of the vertical; since the ladder shows the angle between the longitudinal axis and the nearest horizon, it is seen that a spurious roll of 360 degrees during the manoeuvre is generated. Since the instrument shows a relay of the nearest horizon it has to rotate throughout the manoeuvre instead of snapping round only at the zenith and it thus shows a totally spurious rate of roll. A pilot who had not learned to use this display would automatically apply aileron to check this apparent roll and thus enter a spiral. This illustrates another disconcerting and potentially lethal characteristic of such displays. When the elevation angle is near to the horizon a rolling motion of the ladder is a direct indication of aircraft roll rate. When the elevation is 90 degrees up or down there is only a conditional relationship between aircraft roll and instrument rotation. Near the horizon this limitation is hardly noticed but at high angles of elevation it is the source of a seriously misleading display since not only can there be instrument roll when there is no aircraft roll but there can be the reverse where the instrument shows no roll but the aircraft is in a hard spiral. If for example a pilot, having momentarily blacked out, recovered consciousness in a steep descending spiral the ladder would show a steady dive at a steep nose down angle but with no roll as the aircraft maintains a constant relationship with the nearest horizon in its helix path. The only clue that this is a false picture comes from the compass which will be rotating rather than steady. This, however, is a poor clue because pilots are used to seeing the compass swing rapidly through 180 degrees at the zenith or nadir point when pulling through the vertical without any roll. If the pilot misses or does not understand this and does the instinctive thing which is to pull hard without any change in aileron he will merely tighten the spiral and incidentally fail to produce a change in the display. Failure to get rid of the roll in a diving spiral has always been acknowledged as a potential killer even when clear hood. Near the ground on instruments even a momentary confusion of this sort could easily prove fatal. Recent versions of the pitch ladder display, such as the display described in PCT application, publication no W092/01906, have introduced features such as variable ladder scales and distinctive climb/dive and zenith/nadir representations to clarify the presentation of information but these do not address the problem of spurious roll. They do not address either the problem of confusion of information by the overlaying of symbols one on another or whether this can be overcome by a re-distribution of symbology to different parts of the display area, particlarly towards the periphery of the display. This invention is directed to providing means for the rapid, intuitive recognition of the orientation of the aircraft as well as to the problem of overcrowding in a pilot's head up display reflector sight having particular regard to the demands of fighter and fighter bomber operations. Flight information is essential for keeping control of the aircraft. Weapons information is essential for the attack and in many cases a view of the outside world is also necessary both for weapons aiming and ground avoidance. These three are normally found contending for the pilot's attention in the central area of his vision. The weapons information symbols and flight information symbols vie for the same space and may interfere with each other during interpretation. Both tend to obscure the view of the external world. The invention provides a possible method of alleviating this difficulty by representing some of the essential flight information in a form which can be presented away from the crowded central area of the sight glass and towards the periphery of the pilot's visual field. Aircraft instrument presentations are based on the recognition of static patterns. Evidence is available to show that the visual system can respond to velocity input alone and that when it does so this constitutes an independent channel of information which does not conflict with foveal processing and that this response extends out to the limits of the periphery of vision. This display does not require foveal attention so the central area can be left clear. A cursory examination of the items which appear in existing displays shows us that these can be divided into two classes. First, those items which must be looked at directly to interpret their meaning, for example a digit group representing height or speed. Second, those items which can be monitored out of the corner of the eye, for example the vertical speed indicator or the attitude information conveyed by the pitch ladder. It is recognised that when aircraft flight information is presented in a form which requires direct foveation the pilot must divide his visual attention between the various items. The nearer these lie to the centre of the display the less time will be lost in transferring the gaze between them. This inevitably leads to overcrowding and obscuration in the central area. If, however, some of the essential information can be presented in a form which can be seen and used"out of the comer of the eye"then this could be pushed out into the periphery of the visual field. This invention consists of a head up display system for an aircraft including means for generating a display indicating the attitude of the aircraft, the display including a reference frame which takes the form of a vertically-aligned cylindrical grid consisting of lines in an axial direction which are representative of azimuth and circumferential lines which are representative of altitude and viewed from a position on the cylinder axis in a direction corresponding to the aircraft vector. Preferably, a zero height datum on the reference frame is provided, this convenientl y taking the form of a shaded area appearing to close in one end of the cylinder, whilst the other end of the cylinder, representing the vertically upward direction, is apparently open. The display may include means for particularly identifying a circumferential marking on the cylinder indicative of a demand height determined by the aircraft's control settings. The display may include means for particularly identifying a circumferential marking on the cylinder indicative of the altitude of the aircraft. The display may also possess a central area within which the said reference frame does not extend hence enabling the central area to be available for additional symbology or an unobstructed forward view. One immediate advantage of such a display is that it opens the possibility of using the very large, peripheral area of the retina which is totally unused with current narrow angle bore sight displays and thus alleviating the central overcrowding. Even a forty degree wide-angle display leaves eight percent of the retina unused so the potential savings in central clutter are extremely attractive. By way of example, one embodiment of the invention will now be described with reference to the drawings, of which Figure 2a is a schematic illustration of a cylindrical reference frame as generated in accordance with the invention but viewed externally, showing the aircraft position from which the display would be viewed, Figure 2b illustrates the extent of the reference frame which is displayed at any one time, Figure 2c is an illustration of a typical display generated at level flight some distance below the demand altitude, Figure 3a is an illustration of a typical display during a steep dive of about 60 degrees nose down with wings level, Figure 3b is an illustration of a typical display during a steep climb of about 60 degrees nose up with wings level, Figure 3c is an illustration of a typical display during a bank to port of about 30 degrees in a slight climb, and Figures 4a and 4b are illustrations of typical displays including peripheral heading information and a blanked-out central region. The current style of flight display, such as the pitch ladder described above, requires considerable foveal attention and cannot be simply relegated to a peripheral position on the retina Consequently it was necessary to design a new flight display using motion rather than foveally interpreted static pattern symbology. There are no known existing displays of this nature so the design is of necessity exploratory and it should be borne in mind that the display has been designed for use in an experimental task and ignores problems such as the display of absolute speed and heading peripherally. In order to accommodate hard manoeuvres near the vertical direction it is considered an essential first step to provide a indication of aircraft orientation (vector) that does not suffer from the limitations of the pitch ladder display. That is, the instrument's response to rates of aircraft pitch and roll must be consistent at all orientations. Referring to Figure 2a, means are provided for generating, on a screen forming the object of a head up display image, a representation of the view from a notional aircraft position 1 on the axis of a vertically-aligned cylindrical grid 2. The grid apparently being viewed by the aircraft pilot in the direction of the aircraft vector consists of a number of equally-spaced circumferential lines 3 corresponding to equal steps in altitude and a number of circumferentially-disposed lines 4 parallel to the cylinder axis which intersect the lines 3 and correspond to the azimuth of the aircraft vector. At a defined axial position of the cylinder there is provided an apparent opaque filled circle 5 blanking off the lower direction of the cylinder so that, as viewed from abo ve (including from the position 1) a distinctive filled-in circular indication is given of a zero height datum and the direction of the nadir (which corresponds to the centre of the circle 5). In the opposite (vertically upward) direction the cylinder is made to appear openended, the absence of an opaque screen avoiding any confusion in the pilot's mind between the opposite vertical directions. To provide additional information to the pilot, further, distinctive, circumferential lines are provided. In the present example, a single,"horizon"line 6 centred at the point 1 indicates the aircraft's current height and a doubled line 7 indicates the demand height as generated from the aircraft control data. These lines may be configured in a different. distinctive way and may, for example, be of different colours. In the case of Figure 2, the pilot is flying at a level below the demand height. As further assistance to the pilot, the cylinder grid lines are in pecked form above the line 6 and continuous below that line. The view of the cylinder as projected in the head up display is what would appear between the slant lines 8 and 9 indicated in Figure 2b. In the case illustrated, parts of both the filled circle 5 and the doubled demand height line 7 should be visible. The head up display image is accordingly in the form illustrated in Figure 2c. Figure 2c shows therefore how demand height is depicted by a double circumference drawn at a distance up or down dependant upon the error; when the aircraft is at the correct height the two circumferences bracket the single"horizon"line 6 (see Figure 4b). Like the filled height circle this double circle can be seen up or down the cylinder and allows rapid and confident acquisition from steep climbs or dives. The perspective angle of about 100 degs included angle gives a reasonably large movement of the"horizon"line 6 when flying level with minimum distortion of the upper and lower circles when at extreme angles of climb and descent. Unlike the ladder display, this display gives a continuous and natural representation of rates of pitch and roll for all angles of elevation. Aircraft elevation is not shown directly, but like the ladder display, it is implied from the elevation angle of the aircraft's vector. This is perfectly satisfactory for fixed wing aircraft at normal operating speed and angles of attack. Most ladder displays, but not all, have used the convention of pecked lines to imply down and solid lines up. In the case of the present invention this has been reversed on the grounds that it seems more appropriate to represent the ground with a solid line but it is not an important point since, as described, up and down are shown in a different way, as illustrated in Figure 3. When the aircraft is above some particular height the filled circle 5 is drawn at the lowest position and appears to the pilot, as shown in Figure 3a, as a distant solid circle much the same as the nadir circle in many ladder displays. This distinguishes up from down as the look-up view, as shown in Figure 3b, sees the open top 10 of the cylinder. Figure 3c shows a typical instrument display for banked flight near the horizontal. Figure 2c shows how, below the critical height, the filled circle 5 is drawn down the cylinder at a distance which depends on the aircraft's height. The effect is that near the ground the filled circle gets bigger and bigger obscuring the cylinder framework below it and giving a very dominating indication that the aircraft is approaching the ground. Since this display is intended to be peripheral the central 20 degrees is left free of symbology, in the form of a clear circle 11 as shown in Figure 4a. Aircraft"up"is shown by the segment of a circle lying on the upper edge of the cleared patch. It may be thought that a vertical dive does not appear very different from a vertical climb when the height is sufficient for the filled ground circle to be totally hidden"behind" the central cleared patch. In practice this is not important. If a pilot is unsure whether he is pointing straight up or straight down the normal recovery action is to pull down to the nearest horizon. As soon as this recovery is started the limit circle appears from behind the central clear circle and it is immediately obvious which way is up. At lower heights when the pull-out is critical the filled circle is too big to be hidden by the central clear patch so this momentary ambiguity does not exist. The pull-out is kept safe by keeping the filled circle out of, or just on the edge of, the central clearer patch. Very confident pull outs to low level from very steep dives can be made in this manner. Figures 4a and 4b show the method of displaying the demanded speed. When the speed is correct the collapsing circle lines up exactly with the fixed segment marking aircraft up. When the error is greater or less than some set limit the moving circle is stationary at the outer or inner limit positions shown in the diagram. Thus, in Figure 4a which shows a 45 degree bank to the right in level flight, no demand height is shown and the aircraft is indicated as well clear of the ground disc. In Figure 4b, in which the aircraft is in straight and level flight at the demand height, the speed is indicated as slow and the demand heading is some 20 degrees to the right. The pull-up disc has appeared at the top of the cleared central patch and the aircraft must be pitched up to keep it there. The small arrow at its side shows the direction to evade after weapon release at about 30 degrees nose up. The time-to-go arc shows about 20 seconds to run. Figures 4a and 4b therefore also show the method of depicting heading and demanded heading or track. Absolute heading is shown by a digital group. When banked this figure group does not roll with the aircraft but remains orientated to the cylinder and thus appears to be written on its surface. When the elevation angle is greater than 60 degrees this group is not drawn. Of course this figure group must be foveated for interpretation. The peripheral information is given by showing demanded heading with a heavy vertical line 12 drawn the full height of the cylinder at the appropriate location. When the aircraft is at a steep elevation angle this line can be seen whatever the difference between it and the current heading which makes acquiring a desired heading from a near vertical position quick and easy. When flying near the horizon it can only be seen within the 100 degrees of the perspective view. When the line is not in view an arrow, drawn around the heading digit group, indicates the shortest way to turn to acquire the demand heading.

Claims: CLAIMS 1. A head up display system for an aircraft including means for generating a display indicating the attitude of the aircraft, the display including a reference frame (2) which takes the form of a verticallyaligned cylindrical grid consisting of lines (3) in an axial direction which are representative of azimuth and circumferential lines (4) which are representative of altitude and viewed from a position (1) on the cylinder axis in a direction corresponding to the aircraft vector. 2. A head up display system according to Claim 1 in which a zero height datum on the reference frame is provided, this taking the form of a shaded area (5) appearing to close in one end of the cylinder, whilst the other end (10) of the cylinder, representing the vertically upward direction, is apparently open. 3. A head up display system according to either preceding claim including means for particularly identifying a circumferential marking (6) on the cylinder indicative of a demand height determined by the aircraft's control settings. 4. A head up display system according to any preceding claim including means for particularly identifying a circumferential marking (7) on the cylinder indicative of the altitude of the aircraft. 5. A head up display system according to any preceding claim including a central area (11) within which the said reference frame does not extend hence enabling the central area to be available for additional symbology or an unobstructed forward view. 6. A head up display system according to any preceding claim including information about the aircraft's flight parameters disposed in digital format in the peripheral area of the display. 7. A head up display system substantially as hereinbefore described with reference to Figures 2 to 4 of the drawings.

Inventors: DOYLE ANTHONY JOHN REDFERN, DOYLE, ANTHONY, JOHN, REDFERN
Applicants: DOYLE ANTHONY JOHN REDFERN, SECR DEFENCE, DOYLE, ANTHONY, JOHN, REDFERN, THE SECRETARY OF STATE FOR DEFENCE
Application year: 1995
Main classification: G01C23
IPC codes: G01C23, G02B27, G09G3