FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1931
1931 - 0778.PDF
SUPPLEMENT TO FLIGHT 54 THE AIRCRAFT ENGINEER JULY 24, 1931 connecting the altimeter reading, the temperature and the equivalent standard height. Then at each altimeter height the equivalent standard height may be read off against the intersection of temperature with altimeter height. The standard heights corresponding to altimeter read- ings may then be entered in the table. It is preferable, however, to obtain the standard height with a greater degree of accuracy, and the following equation connect- ing altimeter height, temperature and standard height may be used for all levels below the lower level of the isothermal atmosphere, viz., below 35,332 ft., or 10,769 m.: — H, = (1.238 Ha + 120 to - 1,800). Hs = Standard height in feet. Ha = Altimeter height in feet (locked altimeter). to — Observed temperature in degrees C. (The plus sign becomes minus, of course, if t0 is nega- tive.) Having filled in the equivalent standard heights in the table, the climb in standard atmosphere is known from the time column, but the commencement of this climb is very rarely from zero standard feet. This could only be so if the barometer reading at the aerodrome level was 760 mm. and the ground temperatures + 15 deg. C. For instance, the commencement of the climb might have taken place when the locked altimeter indicated 200 ft. If the temperature had been 4- 16£ deg. the start would have been made from a height of + 427.6 ft. in standard atmosphere. If the temperature had been 12| deg. instead, the start would have been made from a height of - 52.4 ft. in standard atmosphere. RATE OF CLIMB. The average rate of climb at various altitudes is very easily calculated from the time climbs to standard heights, but on plotting the results it will usually be found that the points are very scattered, no matter how carefully the previous work has been carried out. It is difficult then to determine at sight a line which represents the rate of climb—and, therefore, the ceiling —accurately. A few trial rate of climb curves will probably be necessary before one is found which, on working back, gives the time to climb figures which agree with the time curve. In the case of a normally aspirated engine, the rate of climb curve is usually a straight line, that is to Bay, the reduction of rate from ground level to absolute ceiling is regular. Such an engine, however, may possess features which permit partial maintenance of ground power for the first few thousands of feet, and in such cases it is desirable to check back, after having deter- mined a rate of climb curve which gives the correct time to a height near the ceiling, to verify that it also gives the correct times at heights nearer the ground. In the case of a supercharged engine, the rate of climb is usually maintained, or even increased, from the ground up to a certain altitude, after which it falls in the normal manner. In some cases, the rate of climb is not maintained, but the fall in rate is not so marked for the first few thousands of feet. The shape of the curve below the altitude where the fall-off commences to be normal depends on the type of supercharger, etc., and is difficult to determine accur- ately when the plotted points are widely scattered. The time to the altitude where the fall-off commences to be THE nwe -4Ece««Agy TO CUIHB FWOM reno STAMDMP HE MgKIMT AT WHICH . ri-lOMT tCTUAoLY COMMENCED From this example it is clear that the curve obtained by plotting climb times against the figures for standard height must be moved across the paper as shown in Fig. 3, so that the curve intersects zero height, which is the same as adding (or subtracting, as the case may be) the time necessary to climb through the number of stan- dard feet separating zero standard height from the point where the climb actually commenced. It is more convenient, of course, to state the climb times finally as times to regular intervals of standard height. These may then be read off the curve and entered in the appropriate column in the table. At the foot of this column are spaces in which to enter service and absolute ceilings. These values are obtainable by plotting the " rate of climb " curve. normal can be found fairly accurately, and as the interval times to altitudes below this level are not 01 very great interest, the precise shape of the curve below this" altitude, that is, whether it is a straight line or not, is relatively unimportant. For this reason, it is advisable to commence by assum- ing that the rate of climb curve is composed of two straight lines, the one commencing at the ground and going up to what may be called the " supercharge limit," and the other commencing at the supercharge limit and going up to ceiling. The height of the supercharge limit can be fixed tor the preliminary curve by inspection of the engine power curve if available; failing this, the r.p.m. climbing will give a fair indication. A line must then be drawn through the plotted points 726/
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
