Choose a location

In all cases the first action is to display on the screen the intended location of the excrescence at a convenient size, as follows:

• Choose the aircraft component tab (e.g. Wings).
• With the cursor outside of any sketch/graphic window, scroll down until the appropriate surface is shown (e.g. Port wing, lower surface).
• By default, the graphic will be in 'Drag/zoom sketch mode'. Put the cursor on the image. Hold down the left button to drag the sketch to the desired position (or use the arrow keys at the bottom of the image window). With the cursor on the image use scrolling to zoom in or out (or use the + and - keys at the bottom of the image window).

Drawing single line, polyline and polygon excrescences – steps, grooves or ridges

• From the drop-down list at the top of the image window select 'Add/edit multi-node excrescence'.

• Put the cursor on the image to locate the first node and left click.

  • For a 2-node, single-line excrescence, move the cursor to the desired location for the 2nd node and left click.
  • For a poly-line excrescence, hold down the Ctrl key while moving the cursor and using left click to define the node locations. Release the Ctrl key before locating the final node.
    If you forget to release the Ctrl button, left click again to get one extra node then right click on that node and select 'Delete node'.
  • For a polygon (closed) excrescence, hold down the Ctrl key while moving the cursor and using left click to define the node locations. Release the Ctrl key and hold down the Shift before locating the final node – this will add a line between the final node and the first node.
    If you forget to hold down Shift, you can still close the shape by holding down Ctrl and clicking on the last (or first) node. This will join the last node to the first.

  Notes:

  • Nodes can be repositioned at any time by dragging using the cursor; (hold down the left click button).
  • An additional node can be added between two nodes: place the cursor on the line joining the two nodes and left click.
  • A node can be deleted by placing the cursor over it, right clicking and selecting 'Delete node'.
  • The complete excrescence can be deleted by placing the cursor over any node, right clicking and selecting 'Delete shape'
  • The complete excrescence can be repositioned by placing the cursor over any node, holding down Shift and left click and dragging to the desired location.

• As the final node is placed the right-hand side of the screen will be populated.
• Text can be added under 'Title and Notes' as a reference for the excrescence.
• Under 'Nodes' the co-ordinates for each of the nodes is shown. Clicking on 'edit' allows the user to type in precise values (and the nodes will move on the image to the positions defined by the amended co-ordinates).
• Under the heading 'Edges' you will find, for each line, the co-ordinates of the mid-point, the local aerodynamic flow data at the mid-point, and an 'Edge details' form to be completed by the user.
• There is the option for the user to input their own local aerodynamic data, by selecting this under 'Flow Data Source'.
• For each edge select the 'Type' of excrescence from the drop-down list; i.e. step, rectangular ridge, square ridge or groove. Then enter the dimensions, edge treatment etc. as specified.

  Notes:

  • Heights are measured relative to the local skin surface: a positive value means the point projects the distance h from the surface into the airflow.
  • Height variation – The program allows the geometry at the ends of each line to be different (e.g. a misalignment or rigging variation). The average of the dimensions is used for the calculations provided that they are all of positive sign. In the case of a step the two heights entered may be of opposite sign, but the program will then require the step to be split into sections. It offers users the choice of doing this manually or allowing the program to split the step where the height is zero.
  • There is the option to copy the same edge features and dimensions to all of the lines – while retaining the ability to edit any individual item.

• Clicking on the 'Highlight' button emphasises on the sketch the line that is detailed in that section. (Clicking again removes the highlighting).
• Clicking on 'Calculate drag' will present the results for the particular edge as a local coefficient and an increment to aircraft drag. The drag increment at aircraft level is also shown in 'drag counts' (C_D × 10⁴). Clicking on the 'Calculate' button on the excrescence title line will calculate the drag for all edges that form that excrescence.
• To add another similar excrescence, right-click on the symbol ☰ on the excrescence title line and select 'Duplicate'. A second copy will appear on the graphic/sketch with the same geometric data. Drag this to the required location (using shift with left click), amend the dimensions (if required) and click on 'Calculate'.
• Left-clicking on the symbol ☰ on the excrescence 'Type' line provides links to the relevant ESDU methods and to diagrams for the definitions of the local excrescence geometry.

Local geometry for excrescences comprising forward-facing and rearward-facing steps.

When using the Tool to calculate the drag of steps, it is important to note that the software makes decisions about whether each particular edge is a forward or rearward facing step.

Polyline steps – In the case of a polyline shape (not a closed polygon) the software assumes that any step with positive height is forward facing and any step with negative height is rearward facing.

Polygon steps – If the shape is a polygon (a closed shape) the software will make assumptions about whether the steps are forward-facing or rearward-facing based on the complete shape.

Examples

• If a rectangle is drawn with all node heights positive this will be treated as a plate on the outside of the skin of the aircraft. Based on the direction of the freestream airflow, 'leading edges' of the shape will be determined to be forward facing and 'trailing edges' will be rearward facing.
• If a rectangle is drawn with all node heights negative this will be treated as a cavity. Based on the direction of the freestream airflow, 'leading edges' of the shape will be determined to be rearward facing and the 'trailing edges' will be forward facing.

The software states the assumption made for each edge and gives the user the ability to override the automated decision and make any edge forward or rearward facing. This is accessed by left-clicking on the symbol ☰ on the excrescence 'Step facing' line.

Another scenario to consider is where, after drawing a polygon, you want to look at the effect of removing one of the edges. This could be done by re-defining the shape, but the software enables this by means of the 'Omit this edge' button - the symbol ☰ at the start of the 'Edge' section and adjacent to the highlight button. The classification of the edges as forward or rearward facing steps and indeed, the calculations of drag for all the edges, are not changed by this. The 'Omit this edge' option simply removes the contribution of the selected edge from the total drag increment calculated.

For all calculations involving steps, it is strongly recommended that users check that the forward and rearward facing classifications are as intended.

Cavities

A cavity can be modelled by creating a multi-node, multi-line excrescence and setting all of the node heights to negative values. The program then assumes that the area enclosed by the polygon is recessed from the aircraft skin – i.e. a cavity.

However, for small circular cavities there is a quicker alternative method. For circular cavities of small diameter, the system offers a calculation based on one chosen aerodynamic data point. This is set up as follows:

• From the drop-down list at the top of the image window select 'Add/edit cavity'.
• Put the cursor on the image to locate the centre of the cavity and (left) click. The right-hand side of the screen will become populated.
• Text can be added under 'Title and Notes' as a reference for the excrescence.
• Under 'Location' the co-ordinates of the centre (the node) are shown. Clicking on 'edit' allows the user to type in precise values (and the node will move on the image to the position defined by the amended co-ordinates).
• The aerodynamic data extracted for the node position are shown under 'Local flow conditions'.
• Under 'Cavity details' select 'Circular Cavity' from the drop-down list.
• Enter cavity diameter and depth; (e.g. 100mm diameter, 3mm deep).
• Click on 'Calculate'.
• The results are displayed including drag based on cavity area and aircraft wing area – and as aircraft drag counts.
• If required use the cursor to drag the cavity node to a new location. Edit the diameter and depth as required and click on 'Calculate' to get a result for the amended location/geometry.

Moving, replicating and deleting excrescences

Once an excrescence has been created, it can be re-positioned by:

• holding down 'Shift';
• putting the cursor over any node;
• holding down the left button; and
• dragging.

An excrescence can be replicated by going to the symbol ☰ on the right-hand side at the start of the section for that excrescence and selecting 'Duplicate'.

To delete any excrescence either click on the symbol ☰ and select 'Delete' or 'right-click' on any node in the excrescence and select 'Delete'.

To delete all of the excrescences on the component (e.g. wing lower surface), click on 'Delete All' at the bottom of the sketch.

If you make a mistake:

Too few nodes added	Position the cursor on a line between any two nodes and left click to add a node. All nodes can be re-positioned by clicking and dragging (or entering co-ordinates in the right-hand window).
Too many nodes added	Place the cursor over the extra node, right click and select 'Delete node'.
To delete an excrescence	Either: click on any node, then the ☰ symbol and select 'Delete this excrescence' or 'right-click' on any node and select 'Delete shape'.
To delete all excrescences from a component	To delete all excrescences from an aircraft component (e.g. port wing upper surface) click on the 'Delete all' key just below the sketch.
To delete everything and start again	Click on 'Clear' at top right.