Aerial Topdressing Measurement Trials

New Zealand Journal of Agriculture, Volume 98, Issue 4, 15 April 1959, Page 369

Aerial Topdressing Measurement Trials

By

JEAN G. MILLER,

Biometrician, and

N. S. MOUNTIER,

Research

Officer, both of the Department of Agriculture, Wellington

AERIAL topdressing of hill country with fertiliser, particularly superphosphate, has been a regular practice in New Zealand since 1949. During the development stages and in the years since, the Department of Agriculture has conducted experiments to measure the spread of this fertiliser on the ground. The results of some of these trials have been published in detail (see "References"), but others have only been reported privately to those concerned with the particular trial. It is desirable to have all the results available together, and this article summarises all the main results of the trials conducted before 1957.

DIFFERENT types of aircraft have been used to drop superphosphate in many different forms. The spread and the accuracy with which material is dropped on to a given target area are greatly affected by both the weather at the time of the drop and the range and average size of the particles in the material. A precise comparison of the relative efficiency of the different aircraft would be fair only if measurements of spread were made when each was flown at its best operating height and speed and when using the material most suitable to it for any given conditions. Unfortunately it has not been possible to conduct enough trials to determine the best conditions for the different aircraft.

The performance of each aircraft has been assessed only under the conditions stated and for the specific types of material dropped, and results must not be considered to be generally applicable to all conditions. Methods of Measuring Spread Though the greatest advantage of using aircraft for dropping fertiliser is in the topdressing of hill country on which wheeled vehicles cannot be taken, most measurement trials are conducted on flat or nearly flat areas. In hill country it is too difficult to measure distances accurately and to find areas sufficiently uniform that local variations do not upset results, though in practice local irregularities

in the ground cause extra variability in the density of material from point to point. In trials, where only a very small fraction of the target area is included in the sample taken for measurement, it is obviously desirable to eliminate these effects, particularly where comparisons are being made. In each trial a target area was marked out and catchers placed in a regular pattern. Canvas catchers were used in the first experiment, then cardboard boxes, and finally kerosene tins. The tins appear to be the most satisfactory catchers. The pattern in which the . catchers were set out was also changed from experiment to experiment, but in most trials it was made up of rows at right angles to the line of flight of the aeroplane. The object was always to obtain a general pattern of fertiliser distribution, but in the later experiments attention was directed particularly to determining width of swath from one run of the plane. The fertiliser collected in each catcher was taken to the laboratory for accurate weighing and results were then converted to equivalent rate per acre. Traces of fertiliser often drift over quite a large area, but the rate of application over much of it is so small as to be worthless. Therefore it is convenient to set J cwt. per acre as the minimum effective rate. The area receiving J cwt. per acre or more and the width of spread, to i cwt. per acre limits are used as criteria in this report. In some trials sample catches were sieved to determine the distribution of particle size.

Trial with Grumman Avenger, 1949 This was the first large-scale measurement trial conducted by the Department of Agriculture. During 1949, the RNZAF had made a Grumman Avenger available for widespread experimental topdressing of properties in the Wairarapa and a technique had been evolved. In all operations ground staff and radio were used to guide the pilot accurately on to his lines of flight, which were spaced evenly at 2 chain intervals. This practice was followed in the experiment, which was conducted on the Ohakea aerodrome. The aim was to topdress an area 12 chains wide with six runs of the aircraft. Two hundred canvas catchers were spaced evenly over the target area. The length of each run during which the hopper was open was 40 chains. On one of the runways of the aerodrome which crossed the target area some more detailed measurements were obtained. The fertiliser dropped was double-screened hillside superphosphate with granules between 1 and J in. in diameter. The aircraft flew at 400 ft. and there was a light variable wind of about 7 m.p.h. Measurements showed that the fertiliser was deposited in bands with density of up to 6 cwt. per acre at the centres. These bands were, however, by no means evenly spaced (see Fig. 1), despite precision flying. This could have been caused by a variable sideways displacement due to changes in wind speed or direction. The shape of the distribution curve for a single run was estimated as far as possible from the runs which did not overlap. It was then calculated from the measurements taken on the cross runway that if the lines of flight were to be spaced at 1| chain intervals and the rate of discharge from the hopper correspondingly reduced, an even spread would be obtained with the density at all points being within the range of 2| to 4j cwt. per acre. However, it was shown in this trial and confirmed in later ones that it is impossible to lay down the bands with their centres at regular intervals; no matter how good is the flying this ideal spread is never likely to be achieved.

Factors Affecting Efficiency in Aerial Topdressing Though the series of trials reported in this article does not allow of precise evaluation of various factors governing the efficiency of aerial topdressing, the following conclusions can be fairly drawn: rn Unless special precautions are taken, a single application of fertiliser applied from an aeroplane is likely to be very unevenly distributed. This applies whether or not the material is granulated; in fact, powdered materials, particularly where use can be made of crosswinds, are probably more evenly distributed than are coarsely granulated materials. The pilot’s skill is probably the most important factor in securing evenness of spread. Where evenness of application is important several runs should be made over the area, a proportion of the total quantity to be applied being dropped on each run. nr] There is probably a serious loss of the “dust” fraction of materials dropped, especially with windy conditions and high flying. It is quite possible that much of this fine material is removed from the target area. Though the percentage lost in this manner is not known, there is sufficient evidence to show that it can be very substantial. With the larger types of aeroplane, or in any other circumstances where relatively high flying (by topdressing standards) is required, elimination of dusty material would seem to be essential. FTI Granular materials are probably dropped more precisely and must be used with applications from relatively high altitude. The most desirable type of granulation has not been established, but relatively fine granules appear to have certain advantages, particularly in securing more even application. Granular fertilisers which contain a high proportion of dust are, of course, just as likely to have this dust fraction blown from the dropping area as are powdered materials. A certain proportion of coarser granules may not be disadvantageous, especially with high flying and relatively fast aircraft. The narrowest width of spread will be achieved with coarse granules distributed from low-flying aircraft. Un The data do not permit evaluation of the different makes of aircraft, types of hopper, etc., for efficiency of spread. Speed of flying may be an important factor that requires further investigation. ryi Wind speed and direction in relation to aircraft height and direction of flying appear to be of overriding importance in determining the type and efficiency of spread. Granulation must help to overcome the wind effect to some degree, but granular fertilisers are still markedly affected by wind drift. The skilled pilot can do much to take advantage of wind in assisting distribution, but in all operations it is a serious limiting factor. It is probable that wind conditions are rarely suitable for the application of dusty fertilisers.

Trial with Light Aircraft (Tiger Moth), 1950

Because light aircraft operate without the assistance of ground control, the efficiency of fertiliser distribution from them must depend to a considerable degree on the skill of the pilot, In the detailed measurement trial conducted at Whatawhata, Waikato, in conjunction with James Aviation Ltd., an experienced pilot was operating under reasonably good practical conditions. The purpose of the trial was simply to find the type of distribution pattern that would be obtained with this aircraft spreading each of three fertilisers: ordinary superphosphate, serpentine superphosphate, and granulated superphosphate. ’ The area nn which the rateher<? (shallow cardboard boxes) were set out wal not steen thoXh rT was “Tea £“ s W whS e th d e O a7r a with foS? traverses dr ™ d °wa lety pelted fo? each of the three"fertilisers peatea for each of the three fertilisers, As the aircraft was operating under normal conditions of loading, accurate sampling of the materials to determine the size distribution of the particles was not possible. The wind at the time of the trial was light at ground level, but the pilot estimated it at

between 8 and 20 m.p.h. at the altitude of the aircraft, 70 ft., and blowing approximately at right angles to the lines of flight. The speed of the aircraft was 80 m.p.h. The distribution patterns for the three forms of phosphate are shown in Figs. 2,3, and 4. Distribution of Serpentine . ~ superphosphate Wind displaced the serpentine superphosphate (Fig. 2) to one side of the target area and a considerable amount was seen to fall outside the area, despite the efforts of the pilot to allow f° r wind drift. Some of the material fell i n Quite large lumps. The main area of heavy concentration of fertiliser ’ which can be seen in the diagram, is probabl y the running together of acre of the tar t area th On Sacresoi the target area the mean density was 1.3 cwt per acre. An , attempt f as made , to calculate for each run of the aeroplane the propertion of the fertiliser dropped which fell on the target area. By this method it was calculated that 62 per cent of the fertiliser dropped fell on the area receiving more than | cwt. per acre. Since calculations showed that the remainder of the target area could not

have received more than a further 8 per cent, about 30 per cent was not accounted for. Some of this loss was probably due to fertiliser bouncing out of the collecting boxes and some to fine dust drifting off the target area. Distribution of Ordinary Superphosphate The distribution pattern with ordinary superphosphate is shown in Fig. 3. Displacement of material to one side of the measuring area was again evident, but on this occasion the areas of heavy deposition were very small and it was not possible to identify the flight lanes. The mean density over the target area was 1.2 cwt. per acre. Even with an estimate of the probable pattern outside the target area, less than 50 per cent of the fertiliser dropped was accounted for. The accuracy of this figure is questionable, but it seems likely that more straight superphosphate than serpentine superphosphate became airborne and drifted right away from the collecting area. Distribution of Granulated Superphosphate Fig. 4 shows the distribution obtained with granulated superphosphate. This material, which was imported from

England, had particles of 9/64 to 12/64 in. diameter. It flowed much more rapidly than ordinary superphosphate or serpentine superphosphate, with the result that it all fell on the first two-thirds of the grid and the last third received nothing. (Not enough of this material was available for the pilot to make a trial run on which to gauge the correct hopper opening before dropping it on the experimental area proper.) With this reservation, the pattern in Fig. 4 shows that this material fell approximately where it was intended to fall. Though the wind was stronger at this time than when the other two phosphatic fertilisers were dropped, there was no marked displacement by wind. The two central runs overlapped and the

drop from the first run fell right on the boundary of the target area. The

granules could be seen very easily on the ground and they extended about 4 yds. outside the measuring area. On the 5 acres of target area a mean density of 1.3 cwt. per acre was recorded. Calculations from the pattern of a single run showed an application of only 57 per cent of the material dropped. Since the material is granular, with a negligible dust fraction, it would seem that the loss of over 40 per cent must be due to its bouncing out of the collecting boxes. If this were assumed, a correction could be made to the distribution diagram and the width of swath. However, there are two objections to such a procedure: 1. Later trials have thrown doubt on the validity of the bouncing explanation. 2. Only two-thirds of the length

of the target area was dressed and consequently this two-thirds was receiving fertiliser at lj times the intended rate.

Mean density (cwt./acre) Fertiliser On area dropped Area receiving receiving on target more than J cwt. On target more than of 5 acres per acre area J cwt. acres Serpentine superphosphate .. .. 2.8 1.3 2.2 Superphosphate .. .. .. ..3.7 1.2 .... 1.6 Granulated superphosphate .. ..3.4 1.3 .1-9

Trials with de Havilland Beaver, 1951 and 1953

These two trials were conducted in cooperation with Rural Aviation Ltd. at the Department of Agriculture’s Flock House Farm of Instruction, Bulls, on a flat target area. They aimed mainly at the comparison of different types of granulated superphosphate when dropped from this aircraft. In Trial 1 five materials were used and mechanical analyses of these were as follows:

vary almost as much along the direction of flight as across it. The average widths of the bands where the density as estimated from the amounts caught in the boxes was more than | cwt. per acre were: ft A. Aerial mixture No. 1 .. 72 B. Aerial mixture No. 2 .. 66 C. Aerial mixture No. 3 .. 54 D. Granulated superphosphate 66 E. “Super, compound” .. .. 90

Per cent which Per cent of material retained ■ passed a on *B/S sieve of mesh 60-mesh Material 8 16 30 60 sieve A. Aerial mixture No. 1 .... 2 51 31 13 3 B. Aerial mixture No. 2 .... 83 17 trace trace trace C. Aerial mixture No. 3 .. .. 100 nil nil nil . nil D. Granulated superphosphate .. 98 2 trace trace trace E. “Super, compound” .... 10 12 23 26 29

The aeroplane made three parallel runs over the area for each material, flying at 200 ft. and into a light variable wind of about 7 knots. Narrow cardboard boxes 9 in. deep were used as collectors and were set out in a regular grid over all the target area. All the granulated materials gave clearly defined band patterns of distribution, but sometimes it was apparent that two lines of flight had been very close together and the spreads had overlapped. The spread from the “super, compound” was much less regular and the density tended to

The greater the proportion of finer fractions in the fertiliser the greater was the width of spread. From one row where the boxes were at close (5 ft.) intervals detailed graphs of the cross sections of the distributions were drawn and are shown in Fig. 5. In these the spread of the different sized particles is shown. No marked consistent sideways drift was apparent, though in some runs of the finer materials, such as the aerial mixture No. 1 and “super, compound”, the smaller fractions were slightly displaced relative to the larger fractions. The variable nature of this drift is

probably a reflection of the changes in wind direction from moment to moment. It had been suspected from the previous trial with the Tiger Moth that a considerable amount of the finer fractions of powdery material such as superphosphate did not fall with the main bulk of fertiliser, but floated off. One of the advantages claimed for granulated materials was the elimination of this loss and the consequently more precise dropping of fertiliser on the target. Reliable estimates of the amount of each material which actually fell on the target area were attempted. These “recovery” figures, expressed as a percentage of the material released from the aircraft, were: Per cent A. Aerial mixture No. 1 .. 72 B. Aerial mixture No. 2 .. 68 C. Aerial mixture No. 3 .. 64 D. Granulated superphosphate 63 E. “Super, compound” .. 51 These figures are all surprisingly low. It seems unlikely that the large granules would have become airborne, but it is possible that they may have bounced out of even the relatively deep boxes. No completely satisfactory explanation can, however, be given. There was a slight trend with the granulated materials toward higher recovery of smaller particles, but this could be just a chance effect. The recovery of “super, compound” was fairly clearly lower than that of the granular materials. However, with the low recovery figures of the granulated materials unexplained it is difficult to assess the amount of “super, compound” that was actually lost off the target area by becoming airborne. In a subsidiary trial conducted later the same day the spreads were compared for single runs of the aircraft flying at 100, 200, and 400 ft. for each of two materials, “super, compound” and aerial mixture No. 1. Only one line across the target area was measured. The wind by this time was stronger than at the time of the main trial. “Super, compound” gave a peak density of about 2 cwt. per acre at 100 and 200 ft. The wind caused sideways displacement, particularly of the smaller particles, and the displacement was rather more marked at 200 ft. These effects can be seen in Fig. 6. At 400 ft. all the “super, compound” was blown beyond the target area. The drops of aerial mixture No. 1 also showed a sideways displacement. Here again the smaller particles were carried further than the large. The effect was somewhat greater at 200 ft. than at 100 ft., and still more marked at 400 ft.

In Trial 2 in 1953 five materials were again compared. These, chosen in light of the results of Trial 1, were: A, sieved serpentine superphosphate 16-60 mesh. B, a 50 : 50 mixture of sieved material and aerial mixture No. 2. C, aerial mixture No. 1. D, commercial serpentine superphosphate. E, “super, compound” of the same line used in the previous year.. The mechanical analysis of the materials was: Per cent ■which passed Per cent retained on 60-mesh *B/S sieve of mesh sieve Materia! 8 16 30 60 A . . 5 40 46 9 B . . 31 - 13 . 22 27 . 7 C .. 5 47 29 15 4 D . . 2 4 23 32 39 E .. 10 10 18 24 35 * British standard. Unfortunately conditions for this trial were very poor. The wind rose quickly from 10 knots at the beginning of the trial'to 18 knots, and to ensure that the fertiliser would land over the grid of boxes the pilot considered it

necessary to fly at not more than 100 ft., though he may have been lower. To try to overcome the loss due to bouncing of the granules, kerosene tins 13j in. high were used for collecting. The grid was designed to give a good picture of the crosswise distribution and recovery of the fertiliser. The aim of the drop was to have three clearly separated swaths for each material so that width of spread could be compared and this was achieved except for material A, where the last two runs overlapped and the first run was faulty owing to a leaking hopper. The grid had been designed with three rows of tins at 10 ft. intervals to show the crosswise distribution pattern and with a cluster of tins at each end to measure accurately the length of swath from each run. Because of the low height at which the pilot flew into the rather strong wind the width of spread of all materials was very small and the box spacing used did not show small differences in this width between different materials. In addition the rate of discharge of the materials varied considerably and the length of most swaths was not in accordance with expectations. Thus the information was not in general obtained for calculating the estimates of recovery as desired. Two runs of the materials

A and D appeared satisfactory for estimation and the results were: Per cent of material recovered A. Sieved serpentine superphosphate .. .. .. .. 73 D. Serpentine superphosphate .. 54 Again the recovery for the granular material A was lower than expected, but as there was great variation between the three crosslines of the grid, the estimate is probably very inaccurate. Particle distribution in the sieved superphosphate recovered was the same as that in the original material and observations during the trial indicated that there was little if any bouncing out of the kerosene tins. So

the loss of 27 per cent of this material is unexplained.

Trial with Bristol Freighter, 1954

A measurement trial was conducted during a demonstration being given by .a Bristol Freighter on a hill country station near Masterton. The load carried by this aircraft was large and was consequently spread over a large area. Since the ground was very broken, no attempt was made to measure the pattern of the complete drop. Instead, a single cross section was obtained from a grid of three close, parallel rows of kerosene tins running up and down a ridge. The grid was 20 chains long and the tins were 1 chain apart in each row. The material used was superphosphate on which a form of temporary granulation had been performed. This was done by adding water while the fertiliser was revolving in a concrete mixer. This gave a soft granule which tended to shatter on hitting the ground, so that an exact examination of the distribution of granule size across the line of flight was not possible. Some lumps of considerable size were formed and these dropped more or less directly under the aircraft. The mean height of flying was 400 ft. about 200 ft. above the highest point of the grid and 600 ft. above the lowest. The indicated air speed was 145 to 150 knots. The estimated wind speed for flights 1 and 2 was 18 to 20 knots at aircraft height and 15 to 25 knots at ground level; and for flight 3, 5 to 10 knots at ground level. The wind was nearly at right angles to the direction of flight. The distribution patterns are shown in Fig. 8 and the averages calculated from the three rows of the grid were:

The last column of the table has been calculated to give an estimate of the relative rate of discharge of material from the hopper at the time the aircraft was flying over the grid. It is clear that this rate was much heavier in flight 2 than in either flight lor 3. Only one line across the target

Mean rate of Estimated amount Total width Total width fertiliser applied dropped while between J cwt. between I cwt. Heaviest between J cwt. aircraft travels Flight per acre limits per acre limits deposition per acre limits I chain chains chains cwt./acre cwt./acre cwt. 1 *5 2% 1.4 0.9 0.8 2 81 5 2.7 1.2 1.1 3 3J 3 3.1 1.8 0.7

was measured. For best results the rate of drop achieved in flight 2 is nearer that desired, though it would appear that even wider hopper openings than were used in this flight would be needed to obtain a rate of 2 to 3 cwt. per acre. The width of spread achieved in flight 2 of this trial (8| chains receiving more than J cwt. per acre) is verymuch greater than that measured in any other trial. The distributions from this trial do not show the central area of very high concentration that is typical of the distributions measured from light aircraft. It appears that a heavy aircraft using granular fertiliser can make good use of wind drift. No attempt was made in this trial to meas-

ure any possible loss of material from the target area.

Trial with Dakota, 1955

In cooperation with Messrs James Aviation Ltd. measurements were made of the distribution from a modified DC3 (Dakota) on a flat property near Rukuhia Aerodrome. Again, with a large aircraft only a cross section of the distribution could be measured. The grid comprised two pairs of rows 6 chains apart with kerosene tins as the collecting receptacles. The tins were placed at J chain intervals within the rows. The material used was a roughly granulated superphosphate. Because the material was stored and loaded into the aircraft in bulk, it was difficult to obtain a sample that could be considered truly representative, but the mechanical analysis of the sample which was drawn was: Per cent Retained on: 8-mesh sieve . 34 16-mesh sieve 29 30-mesh sieve .... 11 60-mesh sieve .... 12 Passed 60-mesh sieve .. 13 Runs were made at 300, 500, and 700 ft. in conditions of no wind, and the patterns of spread for these are shown in Fig. 9. Though the general shape of the curves is similar in all, the amount collected from the run at 500 ft. (which was made first) was only about one-third of that from the other heights, though the weather remained constant. This suggests that the rate of delivery from the hopper varied considerably. The difference between flying at 300 and 700 ft. did not have any marked effect on the spread. For the runs at 300 and 700 ft. the peak rate of deposit was 9 cwt. per acre, and the average width of row which received 1 cwt. per acre or more was 120 ft. The average width of row receiving J cwt. per ? acre or more was 170 ft. and the average rate of deposition within this area was 3.5 cwt. per acre. ' The wind rose later and when it was medium to strong (9 to 10 knots in a direction at right angles to the flight path) two more runs were made, at 300 and 500 ft. respectively. These gave a very low rate of deposit over a very wide swath, but the peak density was less than 1 cwt. per acre and the total amount of material collected from each run was only a seventh of that collected from the runs in the first phase.. If the hopper was discharging at the same rate as at the end of the first phase, it seems that most of this material must have become airborne in the higher wind and drifted right off the target area. However, in view of the large change in deposit rate

which occurred during the first phase (between runs 1 and 2), it is not impossible that a further hopper change may have taken place before this second phase. Fig. 10 shows the graphical representations of these distributions. A justmeasurable amount of fertiliser was collected from a width of 270 ft. A further two runs were then made with a much increased hopper opening. The general shape of the distribution curve for these runs was very similar to that of the second-phase runs, with no

marked peak and a wide swath of roughly equal rates (see Fig. 11). For the drop from 500 ft. the wind was 14 knots across the line of flight. The peak density averaged over the four measuring rows was 1.1 cwt. per acre, and a rate of j cwt. per acre or more was deposited over a width of 180 ft. For the drop from 300 ft. the wind was 10 knots in the same direction. The peak density was 1.6 cwt. per acre and a rate of 1 cwt. per acre or more was deposited over a width of 200 ft.

The total amounts of fertiliser recovered from each of these drops were similar and were each roughly about a third of the totals recovered from the tw o satisfactory runs in phase one. The slightly greater flattening of the peak of the distribution curve for the drop from 500 ft. compared with that from 300 ft. and the conseq uen 11 y wider spread may have been due to the greater height of flying or to the stronger crosswind then blowing. Though the wind is given as 14 and 10 knots for the two runs

respectively, it was gusting up to 20 to 25 knots. Analysis of the results for the particle-size separations showed that most of the material which was fine enough to pass a 60-mesh sieve was lost off the target in all conditions of height and wind. It must be remembered, however, that the sample from the bulk with which comparisons are made may not have been very accurate. There also appeared to be some loss of material which passed a 30-mesh sieve. This was not noticeably greater at the higher wind speeds. These results indicate that though the total amount of fertiliser received on the target area varied very greatly for the different runs, the mechanical composition of the material remained more or less constant. Thus if the low rate of application under windy conditions resulted because most of the material dropped drifted away from the area, this loss occurred equally in particles of all sizes. It was observed that on motor cars parked to the leeward side of the target there was a distinct rattle, which suggests that not only the very fine dust was carried away. It was perhaps noteworthy that for the last run the

aircraft flew in a path which was 1 chain beyond the end of the grid, yet only an unmeasurable trace was collected in the last 14 tins of the grid. This means that there was a distance of 4| chains between the flight path of the aircraft and the nearest deposit of fertiliser. This was the minimum displacement suffered by even the largest particles under these conditions.

Trial with Beaver and Fletcher, 1955

When the trial with the Dakota was carried out both a de Havilland Beaver and a Fletcher FU 24 were nearby and the opportunity was taken to have them make some runs over the grid that had been set out for the Dakota. The spacing between tins (J chain) was rather wide for these smaller aircraft, because their width of spread is only about 1 chain. Comparisons of widths of spread for different heights of flying were consequently not very precise. Some of the runs of these aircraft were markedly skew to the direction of the grid and hence the diagrams show a spread somewhat wider than the true one. Corrections were made for this in the figures quoted in the table on page 384. As only a very light breeze was blowing (2 to 3 knots), the runs compare with the runs in the first phase of the Dakota trial. Beaver The Beaver made two runs at 100 ft. (the first missed one pair of the target rows of tins) and one run each at 200 and 300 ft. Details are shown in Fig. 12. . Approximately the same amount of material was collected from all rows of the second run at 100 ft. and of the run at 200 ft. Much less was collected from the run at 300 ft. and it is not clear from the graph whether this is due to a loss of fine material (row B suggests this), or to a lower, rate of discharge, possibly because the hopper was becoming empty (row A suggests this). There was no clearly defined difference in width of spread due to height of flying. The average width of spread to limits of j cwt. per acre was 67 ft. Fletcher The Fletcher made one run at each of the heights 100, 200, and 300 ft., and details of these are shown in Fig. 13. It can be seen that very little material was collected from the run at 300 ft. As there was very little wind, it must be presumed that the hopper had a low rate of discharge on this, the last run of the three. Again it is not clear

whether a consistently wider spread was obtained from 200 ft. than from 100 ft. The average width of spread to i cwt. per acre limits was 72 ft.

Conclusions from Trials

One of the most obvious conclusions from these trials is that no really satisfactory technique of measurement

has yet been achieved. No definite answers can be given on the effect of granulation on fertiliser distribution because of the failure to account for all material dropped, including the fractions of large particle size. A more satisfactory method of collecting fertiliser is badly needed and it is hoped to do some work on measurement technique in the near future.

The differences in pattern of spread between successive runs made by the same aircraft under apparently similar conditions are surprisingly large, and when the weather (especially wind) changes slightly, performances are obviously greatly affected. This fact and the general evidence of the trials show clearly that in practice distribution must be far from even. Where topdressing is to be repeated annually this may not be of much importance in that strips missed one year will probably get something the next. However, where a material is likely to be applied once only and where insecticides or trace elements are mixed with

fertilisers the need for even spread could be exceedingly important. It is not possible on the basis of these trials to make assessments of the relative efficiencies of different aircraft, or even in most cases of different forms of fertilisers, because each has been used under somewhat different conditions. This is unavoidable owing to wind variation. The most that can be said is that in all trials so far best results in width and density of spread came from the trial with the Bristol Freighter which used temporarily granulated superphosphate in a light crosswind.

Results in aerial topdressing trials are set out below, but the reservations already mentioned must be taken into account when comparisons are made. References Jean G. Miller, “Measurement of the Distribution of Granular Superphosphate from Aeroplanes”, “New Zealand Journal of Science and Technology”, vol. 33A, No. 2, pp. 17-27. P. B. Lynch, "Measuring Efficiency of Topdressing by Light Aircraft”, "The New Zealand Journal of Agriculture”, vol. 82, pp. 315-320. Jean G. Miller, “The Distribution of Fertiliser from a Beaver Aircraft”, “New Zealand Journal of Science and Technology”, vol. 38A, No. 4, pp. 383-396.

MEASUREMENT OF SPREAD ON GROUND IN AERIAL TOPDRESSING TRIALS Mean within Height of Peak Width to Width to i cwt. Aircraft Material Wind speed Wind direction flight density J cwt. 1 cwt. limits ft. cwt. cwt. Avenger .. .. Hillside super. .. .. 7 m.p.h. .. .. Variable .. .. 400 7.4 Not available owing to overlapping of runs Tiger Moth .. Serpentine super. . . .. • 8-20 m.p.h. .. Cross .. .. 70 9.2 No regular spread 2.2 Superphosphate . . .. 8-20 m.p.h. .. Cross .. .. 70 6.7 No regular spread 1.6 English granulated super. .. 8-20 m.p.h. .. Cross .. .. 70 5.5 No regular spread 1.9 Beaver .. .. Aerial No. 3 .. .. ;. 7-9 knots .. .. Against flight .. 200 5.1 54 ft. 59 ft. 2.8 Aerial No. 1 (fine) .. .. 7-9 knots .. . . Against flight . . 200 9.0 72 ft. 89 ft. 3.8 “Super, compound” .. . .. 7-9 knots ... .. Against flight .. 200 2.5 90 ft. (irregular) 105 ft. 1.7 Aerial No. 1.. .. .. 7-9 knots .. .. Against flight .. 100 4.5 80 ft. 100 ft. 2.4 Aerial No. 1 .. .. .. .. .. . . 200 2.4 75 ft. 80 ft. 1.4 Aerial No. 1 .. .. .. .. .. . . 400 2.2 115 ft. 135 ft. 1.2 Bristol .. .. Temp, granulated super. .. 18-20 knots .. Cross .. ... 400 1.4 5 ch. 11 ch. 0.9 Temp, granulated super. .. 18-20 knots .. Cross .. .. 400 2.7 8 ch. 92 ch. 1.2 Temp, granulated super. .. 5-10 knots .. .. Cross .. .. 400 3.1 32 ch. 5 ch. 1.8 Dakota ... .. Granulated super. . . .. Calm .. .. .. ■ ■ •• 300 and 700 9.0 170 ft. 190 ft. 3.5 Granulated super. .. .. 10 knots .. .. Cross .. .. 300 1.6 140 ft. 240 ft. 1.1 Granulated super. . . .. 14 knots .. .. Cross .. .. 500 1.1 180 ft. 285 ft. 0.8 Beaver .. .. Granulated super. .. .. Calm .. .. .. .. .. 200 6.2 67 ft. 94 ft. 2.8 Fletcher .. .. Granulated super .. .. Calm .. . . .. .. .. 200 6.2 72 ft. 107 ft. 2.6 384

♦ British standard.

* A further width of about 4 chains was dressed at about 0.3 cwt. per acre.