FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1946
1946 - 1735.PDF
SEPTEMBER 5TH, 1946 FLIGHT 247 Pressure-Pattern Flying A Useful Method of Checking Navigation from Barometric Pressure Data PRESSURE-PATTERN flying is a broad term embrac-ing all aspects of navigation which are dependentupon pressure-height data ; it is not another inde- pendent method of navigation. The Americans have called the principle Aerologation, derived presumably from Aerology, their name for the science known to us as meteorology. The Empire Air Navigation School have considered the principle worthy of experiment and have already made long-distance test flights, the results of which proved that the method has economic advantages of possible value to civil airlines. It must be appreciated that the idea is young, and at the moment the principle has been applied in only two directions: single-heading flight and the other, a -methdti of checking drift whilst flying blind over the sea, which for the purposes of this article will be called pressure drift-finding. Before describing either method some basic principles and limitations must be considered. In latitudes greater than 25 deg from the equator the geostrophic wind- formula may be applied. This is a working approximation connecting wind velocity with the gradients of atmospheric pressure. The wind tends to blow along the isobars, strongly where the isobars are close, and weakly where they are far apart. Also, Buys Ballot's Law states that, in the northern hemisphere, if you stand with your back to the wind, pressure is lower on your left hand than on your right, and in the southern hemisphere the opposite is true. Single-heading Flight "Whereas a Great-Circle route is the shortest distance between any two places, it is not necessarily the quickest route to fly, bearing in mind the influence of the wind. If it were possible to fly around a pressure system, using a tail wind, naturally the distance would be covered in a shorter time, but as long distances have to be flown, pass- ins, perhaps, through several weather systems, this is KH7OFT. HORIZONTAL SEPARATION : Both fly-ing on a single heading but in opposite directions, two aircraft influenced by thewind make good on the tracks B and C on either side of the Great Circle A. seldom possible. It is possible, however, to calculate a mean wind over the whole route and by its application steer a constant course which will subject the aircraft to more favourable winds. Thus it follows that although a greater ground distance may have to be flown, the ground speed will be faster and the air distance less owing to the avoidance of unfavourable winds. The time-saving possible in this method has been proved by several long- distance test flights by the E.A.N.S. aircraft Aries. The pre-flight calculations are dependent upon accurate pressure forecasting, or predetermination of the height of pressure surfaces. The height of the pressure surface at the time of depar- ture at the planned height of flight is required, and the same for the destination, but at the time when the aircraft may be expected to be half-way to the destination. The forecast pressure estimated at the time of arrival at destination would be unsatisfactory owing to variations which might take place during the period of the flight. The two leadings will produce a pressure gradient or mean slope of the iso- baric surface to be flown, and hence the mean drift angle ; this will give the single heading. The formula used to obtain this result K (D2—Di).is as follows: Zn = • TAS Where Zn=cross-wind effect in nautical miles, K is depen- dent upon latitude (~ 7—yi—r~ )• D2—Di are the ^ \Sin mean latitude/ height readings in feet. The second formula is Sin X = Zntotal distance where X = mean drift angle. Pressure Drift-finding IO3ZOFT. IO28OFT. IO24OFT. IO3OOFT. KD35OFT. 3OMINS. too NAUTICAL MILES SEA .LEVEL , DRIFT DRIFT FINDING : Illustrating the use made by a navigatorof information from pressure data. The diagram X shows drif. between single readings and Y the drift resulting fromseveral readings in the height diagram, all at latitude 5o°N. In the ordinary course of flying, a constant check is kept by visual methods upon the amount of drift. This is impossible, of course, when flying blind, or above ten- tenths cloud, and may be an embarrassment if continued for a long period when other aids to navigation are not available. With the assistance of a radio or radar altimeter, the actual height of an aircraft above the sea may be obtained, subject naturally to instrument errors. Further, by using an altimeter as an aneriod barometer, it is possible to fly at a given pressure level. If a constant level is flown, successive readings of height will represent the variation, higher or lower, of the pressure level. It follows naturally that from the difference in the two readings a height is found, which represents the difference in surface level of the given pressure, over a known dis- tance, the air miles travelled by the aircraft. Since the geostrophic wind varies with the latitude, the sin of the latter is again included in the calculation, the formula . J K (D2-D1)for which is Vn (wind component) = -r-.——--— 5 K(Air dist. flown) 21.47having the same value as before -—=—-. beam wind component in knots, which is laid off at right angles to the course, at the distance flown during the period of the two readings. The direction of drift caused by the beam wind component is dependent upon whether the pressure is rising or falling. If the pressure is rising, and the direction of flight is towards a " High," in the northern hemisphere the wind will be from the starboard side, giving The result is a
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
