Patent Application
20070065289
This invention relates to a speed display system for displaying the maximum airspeed of a helicopter under current conditions and more particularly to a display and/or system for simultaneously displaying the actual speed of the helicopter and the maximum airspeed that if exceeded could result in retreating blade stall.
Helicopters have a unique position among aircraft in that they can stall by going too fast. For example, fixed wing aircraft may have structural limitations. However, on a helicopter, the advancing rotary blade has a higher air velocity then the retreating blade due to the summation of the velocity due to forward flight and the velocity due to the rotational speed of the blade. By contrast, the retreating blade has a lower air velocity due to the velocity of forward flight minus the velocity of the retreating blade. It is also well known to those of ordinary skill in the art that due to the flapping hinges or teeter mechanism, the helicopter's retreating blade will have a higher angle of attack then the advancing blade. Therefore, as the forward airspeed is increased, the velocity of airflow over the retreating blade will decrease eventually resulting in an angle of attack that exceeds the stall angle of attack. In this instance, the stall on the retreating blade will start at the tip and advance in an inward direction. The stall is then considered severe if the outer 25% of the blade is above the stall angle of attack. With a severe stall, the helicopter can become uncontrollable with a possibility of the loss of the aircraft and personnel.
In addition, to high speed flight, other conditions that can lead to retreating blade stall include low rotor speed, high gross weight, altitude, high load factor due to steady state high bank angle, abrupt control deflections and turbulent air with strong up drafts creating a momentary increase in the angle of attack.
In view of the above, helicopter manufacturers take such conditions into account in published tables in the Limitations section of the Flight Manual. This information is to advise pilots of the maximum airspeed to avoid a retreating blade stall. The problem with this approach is that the information in flight is dynamic. For example, the airspeed limit varies as the altitude of the helicopter changes and it changes with the temperature and gross weight. The problem is that a pilot cannot continuously be calculating a stall speed as the helicopter conditions change. Therefore, a pilot typically takes an overly cautious approach and uses a speed that is less than optimum.
It is now believed that there may be a large commercial market for a helicopter speed display and system for simultaneously displaying actual speed together with the dynamic maximum safe airspeed to avoid retreating blade stall in accordance with the present invention. It is believed that there will be a commercial market for such displays and such systems since they continuously indicate the actual speed and the maximum safe airspeed side-by-side or juxtapositioned in a manner to clearly indicate how close the aircraft is to its maximum safe speed. It is also believed that such displays and such systems can be manufactured at a competitive price, are durable, relatively easy to install and service, reliable and will enable a pilot to fly closer to a maximum speed safely.
In essence, the present invention contemplates a helicopter speed display wherein a first scale indicates the actual airspeed of a helicopter and a second scale indicates the maximum speed limit to avoid retreating blade stall adjacent to or juxtapositioned on the first scale. In a preferred embodiment of the invention, the display includes an arc or ring-shaped lighted segment which is analogous to the red line as indicated on the tachometer for land-based motor vehicles.
A further embodiment of the invention contemplates a helicopter high speed limit display system for indicating the actual airspeed of the aircraft and the speeds above which there is an imminent danger of a retreating blade stall juxtapositioned on the aircraft's actual airspeed. The system includes means for sensing the airspeed of a helicopter and for producing a signal indicative of the helicopter's actual airspeed. The system also includes means such as a barometric altimeter for sensing the altitude of the aircraft and for producing a signal indicative of the altitude of the aircraft. In addition to the above, the system includes means for indicating the gross weight of the helicopter as for example, a manual switch operated by the pilot to indicate whether the gross weight of the aircraft is above or below a preselected value such as 3,000 pounds. In this embodiment of the invention, a micro controller or computer includes two look-up tables, one for above 3,000 pounds and one for below 3,000 pounds and a display. The micro controller or computer processes the signals indicative of the altitude, temperature and weight and compares those values to the values in the look-up tables. The micro controller then produces an output signal to display the actual airspeed and the airspeed limit for avoiding retreating blade stall, so that, a pilot can fly at an airspeed which is close to the limit.
The invention will now be described in connection with the accompanying drawings wherein like reference numbers are used to designate like parts.
Basically, the helicopter airspeed display system in accordance with the present invention starts with a normal airspeed indication and adds a dynamic stall warning indication or warning to a pilot. The normal airspeed indicator is of conventional design and may take several forms as for example, a circular shape which is analogous to the face of a clock. It may be of a type having an internal pneumatic bellows, an electrically driven unit or other conventional mechanism and will not be described in detail.
In the present invention, the indication of a maximum airspeed for avoiding retreating blade stall under current conditions can be in the form of a second pointer, a lighted segment adjacent to the airspeed indicator or on a ring adjacent to the airspeed indicator. The dynamic stall warning indicator displays to the pilot the airspeed or airspeed band that should not be exceeded under current conditions. It is somewhat analogous to a red line on a tachometer for motor vehicles. However, unlike a tachometer on a motor vehicle, operation above the maximum speed can result in the loss of control of the helicopter and loss of life.
Referring now to
The pilot selects the gross weight of the aircraft with a toggle switch 28 to indicate whether the gross weight of the aircraft is above or below a preselected weight such as 3,000 pounds. This selection is then read by the micro controller 22. The micro controller 22 reads the inputs from the altitude encoder 24, temperature sensor 26 and switch 28 in a periodic manner in order to continuously update the pilot's display of the maximum airspeed limit.
The micro controller 22 accesses one of the two tables (see
In a preferred embodiment of the invention, the display board 34 as shown in
While the invention has been described in connection with its preferred embodiments, it should be recognized that changes and modifications may be made therein without departing from the scope of the appended claims.
