US RE39295 E1
An ice detector for an aircraft comprises a strut and probe assembly, and it is positioned on the aircraft so that the pressure field around the ice detector causes a lower temperature region on the probe assembly compared to the aircraft. Ice will therefore form on the probe assembly before it forms on the aircraft to provide an early warning of icing conditions near freezing temperatures.
1. An ice detector for providing a signal indicating ice formation, said ice detector comprising a probe assembly protruding into an airstream and supported relative to a surface of a structure subject to icing, the airstream moving past said surface and said probe assembly, and said probe assembly including sections forming structural portions thatcomprising an airfoil cross sectional shape with a rounded section of the probe assembly facing in the direction of airflow and mounted on the structure in a position to provide an area of lower pressure toon a surface portion of the probe assembly than on the structure resulting in lower temperature on the surfacesportion of the airfoil shaped probe assembly than on the structure.
2. The ice detector of
3. The ice detector of claim 2 19, wherein said body member is positioned on an upstream side of a cylindrical probe section, said body member having an edge tapered from a surface on which the probe assembly is mounted toward an outer end of the cylindrical probe section.
4. The ice detector of claim 2 19, wherein probe assembly includes a cylindrical probe section and wherein said body member is positioned on a downstream side of the cylindrical probe section, and extends along a length of the cylindrical probe section a selected amount.
5. The ice detector of
6. The ice detector of
7. The ice detector of
8. The ice detector of
9. The ice detector of claim 8 1, wherein said airfoil cross sectional shape probe assembly is positioned on an aircraft comprising the structure, the airfoil cross-sectional shape probe assembly having an angle of attack that produces a lower pressure than an aircraft wing.
10. The ice detector of
11. The ice detector of
12. The probe assembly of
13. The probe assembly of
14. An ice detector for a structural airfoil comprising a probe assembly extending from an air vehicle and having a longitudinal length generally parallel to the longitudinal length of a structural airfoil of the air vehicle and said probe assembly having an airfoil-shaped cross section and being oriented so that the pressure field on the probe assembly airfoil shape provides a lower minimum pressure than a minimum pressure on a structural airfoil of the air vehicle at a desired angle of attack of the structural airfoil.
15. The ice detector of
16. The ice detector of
17. A method of providing advance warning of formation of ice on a structure comprising of an ice detector probe assembly, placing thean ice detector probe assembly having an airfoil shaped cross section, in a position in an airstream, configuring and positioning the ice detector probe assembly so the pressure field around the ice detector probe assembly causes a lower temperature at a location on the surface of the ice detector probe assembly than the temperature on the structure.
18. The method of
19. An ice detector for providing a signal indicating ice formation, said ice detector comprising a probe assembly protruding into an air stream and supported relative to a surface of a structure subject to icing, the air stream moving past said surface and said probe assembly, said probe assembly including a probe member, and a generally planar body member positioned adjacent to at least one of upstream and downstream sides of the probe assembly, whereby a lower area of pressure is created on a surface portion of the probe assembly and body member than on the structure, resulting in lower temperature on the surface portion of the probe assembly than on the structure.
20. An ice detector for providing a signal indicating ice formation on an aircraft wing subject to icing, said ice detector comprising a probe assembly protruding into an air stream from the aircraft and supported relative to the wing, said probe assembly including sections forming structural portions that provide an area of lower pressure on a surface portion of the probe assembly than on the wing when the angle of the attack of the air relative to the wing exceeds an angle of attack wherein the pressure coefficient on the wing is less than about −3.
The present invention relates to a configuration of an ice detector that detects ice at temperatures that are near freezing and which has a pressure field that reduces the pressure on surface regions so that such regions cool to a lower temperature as air flows past the detector to detect ice prior to formation on critical aircraft surfaces. The ice detector is used on air vehicles and provides a warning of actual ice accretion.
Existing magnetostrictive ice detectors perform well over the typical aircraft performance envelope. However, as more and more aircraft are designed with high performance wings situations may arise at temperatures near freezing where ice will form on a wing while the conventional ice detector provides no information indicating ice. The critical temperature is defined as the temperature above which no ice will form on a structure given the aircraft configuration and other atmospheric conditions. The critical temperature can be different for a typical airfoil configuration and for a conventional ice detector, at the same airspeed. The conventional ice detectors generally have a circular cross section probe.
A paper entitled “Equilibrium Temperature of an Unheated Icing Surface as a Function of Air Speed”, Messinger, B. L., J. Aeronaut. Sci., p. 29-42, Jan. 1953, provides insight into the thermodynamic balance at temperatures near the critical temperature for two dimensional cylinders. There comes a point in which the aerodynamic heating associated with direct impact cannot overcome the propensity of supercooled droplets (liquid water at temperatures below freezing) to change phase and remain on the structure as accreted ice. If the temperature is cold enough this will occur. In practice, the size of the ice detectors relative to the size of most wings can be selected so as to cause ice to accumulate on the detector faster than accretion on the wing, which is the intended result. This, however, did not take into account the fact that airflow over the lifting surface of the wing or airfoil can create localized areas of temperature colder than the ice detector. Hence ice accretion may occur on the wing at temperatures warmer than the conventional ice detector.
At high angles of attack, such as those present in takeoff and landing of an aircraft, the airflow around the leading edge of the wing accelerates around the top and creates a region of lower pressure or vacuum relative to ambient static pressure. This lower pressure in turn creates a temperature drop near the leading edge of the wing, and in the most extreme cases the area where the lower pressure occurs experiences ice accumulation. In other words, if supercooled droplets of water are present in the area of the wing where there is a lower pressure and a sufficient temperature drop occurs, ice will form.
The present invention relates to an ice detector strut and probe assembly that has a geometrical configuration that will alter the pressure distribution around the probe and reduce the temperature at some regions of the probe to a level less than the temperature on the critical surface of the aircraft that is to be protected from ice formation.
The geometrical configuration of the probe assembly can be an airfoil cross sectional shape, or can be a cylinder with a strut that alters the airflow to achieve the desired pressure distribution.
In one form, an airfoil cross section probe is oriented relative to a wing so that as the angle of attack of the wing increases, the angle of attack of the airfoil-shaped ice detector probe also changes and provides regions where a lower pressure occurs than at the associated wing surface. Using a probe with a shorter chord length, and having an appropriate airfoil shape relative to the shape of the wing, results in accretion of ice on the probe at temperatures above the critical temperature of the wing. Thus, ice accretes on the probe at temperatures warmer than that of the wing.
The airfoil-shaped probe is positioned so that the pressure field on the probe and adjacent to the probe is similar to, but creating lower pressure than, the wing airfoil at high operating angles of attack.
Additional forms of the invention show a cylindrical tube probe, that projects normal (or perpendicular) to the aircraft surface, and is arranged with a strut which modifies the flow past the probe in order to reduce the temperature on the probe. In other words, the strut geometry decreases the pressure and temperature at the probe surfaces to a level below that created by the wing or other structure with which the ice detector is used. In particular, the strut can incorporate bodies either fore or aft of the cylindrical probe with which to alter the pressure distribution around the probe.
Another form includes an axially extending rib on a lateral side of the probe. The rib will cause flow separation around the probe resulting in uneven or asymmetric pressure distribution with areas of the probe at a lower pressure than the aircraft skin and thus at a lower temperature.
Other methods can include strut and probe assemblies that have longitudinal axes that are not normal to the surface on which they are mounted, but inclined either forwardly or rearwardly so that the airflow past the probe is modified due to the probe inclination relative to the direction of airflow.
The flow can be guided and in all instances, the ice detector strut and probe assembly is formed to provide a pressure at a surface portion of the ice detector probe that is less than the pressure on the critical surface that is being protected by the ice detector. The reduction in pressure also causes a reduction in the temperature at the ice detector surface, thereby causing ice accretion at a warmer temperature than with conventional probes. The local pressure distribution on the ice detector probe is modified by the strut geometry, sweep of the probe, or by the formation of the airfoil shape cross section of the probe.
In performing analysis of the effectiveness of an airfoil cross section ice detector probe, LEWICEv 1.6 (or a comparable program), a Computational Fluid Dynamics (CFD) simulation of icing environments, can be used to parametrically determine the effect of variables such as liquid water content (LWC), Median Volume Diameter (MVD), ambient temperature, altitude, airspeed and probe and airfoil (wing) geometry through a series of analyses. The results of these analyses provide direction for certification authorities, including the U.S. Federal Aviation Administration, the Joint Airworthiness Authorities, and Transport Canada, who must certify aircraft as being airworthy. The analysis performed was to use a test structure comprising a typical airfoil cross section for a wing, and analyze it at a single angle of attack. For that angle of attack, the ambient temperature was dropped at a given or reference airspeed to determine if ice was forming on the test airfoil. Once ice began to form, the temperature at which the formation took place was deemed the “critical” temperature for that geometry, airspeed, angle of attack, altitude and LWC.
The plots include a plot 20 indicating ice will form below a particular temperature on this airfoil, and plotted along with it is a plot 22 of the average critical temperature. Plot 21 indicates temperatures above which ice will not form.
The bottom curve is the most negative localized pressure coefficient, indicating that the more negative the pressure coefficient, the more potential there is for cooling and thus for ice formation. The aerodynamic pressure coefficient Cp is defined by:
In a review of
The flow conditions have to be balanced or put into perspective by the flight envelope of the aircraft. It is unlikely that a commercial aircraft will fly at both high angles of attack and high speed. The situation that is shown in the plot of
By using an airfoil-shaped ice detector probe assembly, similar pressure field conditions can be provided at both the wing and the ice detector probe assembly with the airfoil-shaped probe oriented at an angle of attack to provide a lower pressure region than the airfoil of the wing or placed in a region of local AOA amplification.
After only 10 seconds, ice formed on the ice detector probe assembly under these conditions. In these same conditions, there was no ice formation on the conventional ice detector. If the airfoil-shaped probe assembly is placed and oriented correctly, it can be shown that the critical temperature of the ice detector is now above the critical temperature for a Whitcomb airfoil with a local AOA of 10°.
Because the NACA 0012 airfoil has a different pressure distribution than the Whitcomb airfoil, it was necessary to modify the orientation of the NACA 0012 airfoil-shaped probe assembly from that of the wing. The critical static temperature for the Whitcomb airfoil at an angle of attack of 10° is +0.68° C. The NACA 0012 airfoil ice detector probe assembly was oriented at 12° angle of attack to obtain a critical static temperature warmer than +0.68° C.
The other aerodynamic issue is one of how the probe assembly is located. The local angle of attack change is often not a one-to-one relationship with the aircraft angle of attack. For this reason it is important to understand the placement of the ice detector strut and probe assembly on the aircraft.
In other words, the airfoil-shaped probe assembly is mounted at a location so that at the normal desired takeoff and landing angle of attack, the airfoil-shaped ice detector probe assembly will be in a pressure field that is similar to that of the airfoil on the aircraft on which the ice detector is used.
The probe section 40 is cylindrical, as can be seen in
The leading edge of the afterbody or flat plate section, indicated at 54A is spaced from the trailing side of the probe section 40, as shown, by a distance equal to “d”. The distance “d” can be in the range of 0.025 inches, for example, and is enough so that it will not affect the vibration of the probe section 40, using the excitation and sensing circuitry.
In normal operation, the airflow around the conventionally shaped cylindrical cross section probe, from the direction indicated by arrow 55 in
In this form of the invention, an afterbody or flat plate section 72 is included in the strut assembly. This afterbody section 72 is a narrow, flat blade that is triangular in shape in top view and has an outer edge that tapers downwardly from the probe section 62 to the rear. The outer edge is rounded as well. The forward edge of afterbody 72 is spaced from the probe section 62 by a distance “d”, which again, is in the desired range, for example, 0.025 inches. The side of the afterbody section 72 adjacent the probe sections may be approximately the same height as the afterbody section 54 shown on
In this form of the invention a forebody or flat plate section 86 is included in the strut and probe assembly 81. The forebody section 86 is a narrow, flat blade that is triangular in shape in top view and has a leading edge that tapers upwardly from a leading end toward the probe section 82. The trailing edge of forebody 86 is concave to conform to the cylindrical shape of the probe section, and is spaced from the probe section 82 by a distance “d”, which again, is in the desired range, for example, 0.025 inches. The trailing edge of the forebody section 86, as shown, extends only about 60% to 80% of the axial length of the probe section, and tapers in thickness from the leading edge to the trailing edge, as shown in FIG. 14. The taper of the forebody from front to rear provides for a smooth acceleration of the airflow. The forebody 86 will divide the flow so that as air flows around the cylindrical probe section 82 in the direction indicated by arrow 89, it will create lower pressure on the leeward or downstream side areas indicated generally at 87, that is less than the pressure created on the aircraft skin 84. The addition of the forebody alters the pressure distribution around the probe and reduces the magnitude of the shed vortices and allows the flow to remain attached to the probe section 82 longer. Thus, areas of the probe section will be colder than what would exist without the influence of the forebody. In addition, the forebody acts like a shield to protect portions of the probe section and reduce the overall aerodynamic heating effects due to conduction. The circuitry 88 will provide an advance notice of icing conditions, before ice accretes on the aircraft.
The inclination of the strut and probe assembly in either a forward or rearward-swept configuration as shown in
The invention thus provides probe orientation and configuration to lower the pressure and temperature on portions of the probe surface to cause ice to accrete on the probe before the aircraft surface accretes ice.
Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.