By Frédérique Giroud and Dominique Legendre

Research and experiments on airdrops in France began with the first LABEX (LArgage Bombardier d’eau EXperimentation) experiment in 1995.

Following the acquisition of CL-415 waterbombers by the French government, it quickly became apparent that to optimize aerial firefighting, it was necessary to understand the dropping characteristics of the aircraft.

[This is the final article from a special edition of Wildfire Magazine on Aerial Firefighting around the World.]

Entente Valabre, the laboratory of the French Securité Civile, oversaw measuring the ground patterns of the French waterbomber fleet.

The aim of the LABEX research is to determine the ground patterns of various water-bombing aircraft and analyze the ground distribution of the released product by application rate class.

Each aircraft comes with a range of dropping programs. To optimize aircraft use in aerial forest fire fighting, it is essential to know the surface areas covered by the different applications – how much area is covered and by what depth of drop.

To complement these experiments at a real scale, with real aircraft, other experiments are carried out to understand and model aircraft drops are conducted. Analysis by Entente Valabre researchers of the airtanker ground patterns show that the product is not deposited uniformly, and is often deposited very irregularly, because of a complex fragmentation process of the liquid column released by the aircraft.

Although the release process seems simple, the fluid dynamics governing the transformation of a liquid block into a cloud of droplets are only partially understood. The droplets can be neither too fine to avoid dispersion and evaporation, nor too large to avoid harsh impacts on vegetation and the ground, resulting in projected objects. Large-scale fragmentation of a drop, which leads to the formation of liquid meteors that take longer to atomize into droplets, is a complex process and its physical origin is unexplained.

DETERMINATION OF GROUND PATTERNS

Ground patterns are evaluated on flat test areas, free of vegetation or covered only with herbaceous vegetation up to a height of 20 centimetres.

The test area dimensions are determined according to the aircraft to be tested and the areas is set up in a grid. The drop patterns are measured by distributing receptacles over the test area, comprising a fixed support and a collection cup designed to catch the liquid. After each test, the cups are hermetically sealed, collected and weighed. Each pattern is then plotted on a predefined scale, corresponding to classes of application rate in litres per square metre per day, or litre/metres².

DETERMINING RELEASE EFFICIENCY

The ground-pattern efficiency is primarily related to the application rates obtained, depending on the product released – water or retardant. In the Mediterranean region, under normal conditions of vegetation (10 metric tonnes per hectare), weather (wind less than 60 km/h) and terrain (not generating excessively difficult trajectories or significant turbulence), a rate of 0.8 lm2 is representative of an effective release using long-term retardant, while a rate of 1.6 l/m2 is representative of an effective release using water.

The various ground patterns parameters are:

• The length and depth for coverage rates above the representative application rates

• The area and product volume above representative application rates

• The homogeneity of the ground pattern

• The difference between the time the drop hits the ground and the time it totally covers the ground.

Ground pattern extension and concentration levels are not the only release characteristics that can determine the effectiveness of a drop. The speed of the released product can have a considerable influence on breaking fire-induced updrafts during direct attack, on upwind accuracy, or on safety on the ground. All these factors can be qualitatively related to the phases of ground-pattern formation. The three phases – retardant cloud tearing, atomization and dispersion – explain the various stages that take place between the initial break-up of the product cloud and the formation of droplets that have reached their final size. The rheology of the product, the height and speed of the aircraft, and the release system play an important role in these phenomena.

Observation of the drop is very important to assess its effectiveness, and valuable information can be gleaned from camera images. For this reason, a few subjective criteria are selected:

• Frontal view (cohesion, drift, separation phenomena, expansion, tearing)

• Lateral view (core erosion, deformation, disintegration, forward velocity)

• The status of the cloud when it hits the ground and the way it disperses while resting on the ground.

The drop patterns are measured by distributing receptacles over the test area, comprising a fixed support and a collection cup designed to catch the liquid. After each test, the cups are hermetically sealed, collected and weighed.

PRESENTATION OF RESULTS

The ground patterns are presented in the form of summary sheets in which the following are described:

• technical flight data (altitude, aircraft speed)

• technical loading data (weight, volume)

• meteorological conditions

• ground patterns characterization.

Following these analyses, recommendations are made regarding the use of different dropping programs for specific firefighting missions, for example initial attack or retardant barrier construction.

EXPERIMENTAL CAMPAIGNS IN FRANCE

Five experimental campaigns have been conducted in France:

LABEX 1996: Canadair CL-415, Hercules C130, Fokker 27, Tracker 2SF

LABEX 2009: Dash 8 Q400, Canadair CL-415, Tracker 2SF

LABEX 2013: Air Tractor AT-802

LABEX 2021: Dash 8 Q400 with retardant

LABEX 2024: Dash 8 Q400 with water

DROP MODELING

LABEX campaigns are time consuming and costly; therefore, it is essential to have a dropping model that can be used to vary, for example, the height of the drop, the wind direction and the characteristics of the product released – questions that water bomber pilots frequently ask to understand and optimize their practice.

With this in mind, a team at the Institut de Mecanique des Fluides de Toulouse is developing a dropping model that has already been tested against results obtained during various LABEX campaigns.

Initially, the approach was based on ground patterns from the US Department of Agriculture Forest Service’s open-access technical reports, which made it possible to study and model the main characteristics of a ground footprint, namely its length, width and density of deposited product.

The use of numerical simulation as a tool for predicting the drop cloud produced by an aircraft appears to be very attractive for gaining access to all the physical mechanisms involved. However, the computing resources available, even at the biggest French and European computing centers, do not currently allow researchers to describe the entire process from the aircraft to the ground.

The three phases – retardant cloud tearing, atomization and dispersion – explain the various stages that take place between the initial break-up of the product cloud and the formation of droplets that have reached their final size.

An approach developed by PhD candidate Corentin Calbrix in his thesis for Universite de Nimes was limited to describing the fragmentation of the released liquid in a zone close to the aircraft, i.e. 10 metres below the aircraft. Computational fluid dynamics studies of the CL-415 and Dash 8 drops were carried out, showing that fragmentation and dispersion in this zone prefigured ground deposition 50 metres below, which could be deduced by extrapolation of the drop cloud envelop. This approach can now be used to study drops by these aircraft under various operational conditions, particularly in the presence of crosswinds, to help interpret the differences in ground patterns observed during LABEX and provide useful information to optimize forest fire fighting.

Frontal and lateral observation of the release during a LABEX is an essential element in assessing the quality of a release. The eye can identify the quality of product fragmentation and dispersion and anticipate the characteristics of the ground pattern. Packages with a higher concentration of liquid are generally easy to observe and are likely to generate areas of higher concentration on the ground, making the ground pattern less uniform and of poorer quality.

The explanation for this fragmentation mechanism is not yet available: Is it a physical instability intrinsic to fragmentation at this scale, or the memory of a disturbance transmitted to the fluid by the release system? To answer this question, studies are underway at the Institut de Mecanique des Fluides de Toulouse combining wind tunnel experiments and numerical simulation.

A one-tenth scale dropping device has been installed in the 2.4 metre diameter wind tunnel, capable of delivering up to 25 m/s of relative wind. The fragmentation of the liquid column is filmed by a high-speed camera at 2,000 frames per second. The first results of the water release show a specific fragmentation mechanism that cannot be explained by literature results at smaller scales.

A computational fluid dynamics (CFD) code benchmark is underway to challenge various French and American research codes for the study of this instability; these codes must be able to reproduce wind tunnel experiments before the study of fragmentation at aircraft scale can begin.

Ultimately, understanding the origin of the instability and its modes will lead to proposals for better designs for release systems.

The aim of the LABEX research is to better understand and optimize aerial forest fire fighting. It is essential to know the different firefighting missions of aircraft to provide recommendations on the use of the different drop programs available on each aircraft. Climate change leads to changes in fire behavior, and the missions of firefighting aircraft must be adapted to these new phenomena. The tools and experiments described above will be used to improve existing dropping systems, to validate their operational use, and to assist pilots in the understanding of their practice.