Chapter: 15. Defining and Tuning Planes

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Chapter: 15. Defining and Tuning Planes

15.01 Defining a Plane 'from Scratch'

This section is an example of how to define a new plane 'from scratch'. You can follow it through without prior knowledge of Piano. If during the tutorial this text gets covered up, just click on a visible part of it or move other windows aside.

Assuming you have just launched Piano and have not yet loaded any aircraft or assigned any values, you are ready to start. Otherwise, go first to the 'Param' menu and select the 'Reset...' item to make sure you start with a 'clean sheet'.

For this example, we'll stick with old-fashioned imperial units: Choose 'Select Units...' under the 'Misc' menu to make sure that this is the case for all of them.

Normally, the most convenient way to define a new plane is through a 'palette'. Go to the 'Param' menu, 'Palette...' sub-menu, and use 'Load Palette...' to find and load the one called 'NEW PLANE (vitals+)'. This shows some basic parameters (accessed by a single click). Those shown in red constitute a 'vital' minimum set. However, now that you know this, you can close the palette (click its top left corner), because you won't need it!

For the purposes of this demo, it is easier to assign values to parameters directly within the User's Guide, as you read it. Do this simply by clicking on the blue parameter names. For example, click on this: sweep-deg , type 25, then hit OK or <Return>. Just move the parameter's dialog if it hides this text. We'll follow the order in which parameters appear in the palette, although any other order would be fine too.

We are going to generate a simple 100-seater aircraft model, using round figures for the maximum takeoff mass, wing area, and other characteristics. Input these numbers:

number-of-pax 100

mto-mass 100e3 pounds

wing-area 1000 sq.feet

Initially we define the wing as a simple trapezoid. In the following, you'll notice that the aspect-ratio's dialog includes a tick-box labelled 'use internal calculation'. You must untick this before you can input a value. (To understand more about the different types of parameters, read Chapter#08section02 later on). The thickness/chord ratio (t/c) is for the moment assumed constant from root to tip:

aspect-ratio 9

sweep-deg 25 degrees

t/c-root 0.15

taper 0.25

We supply approximate values for the design [max.operating] Mach and a typical cruise altitude. These are needed for calibrations and stressing, not for performance calculations (details are in Chapter#04section04 ):

design-cruise-mach 0.82

design-cruise-altitude 39000 feet

The fuselage is divided into a front part, a mid-part of constant cross-section which contains the cabin, and a rear part. External width should suit a five-abreast configuration, and external depth is for now assumed to be the same:

fuse-width 11 feet

seats-abreast 5

front-fuse-length 15 feet

mid-fuse-length 50 feet

rear-fuse-length 30 feet

Typical engine thrust for a twin 100-seater would be between 15k and 18k lbf. The nacelle's dimensions are initially based on a given width:

reference-thrust-per-engine 17000 pounds-force

nac-width 6 feet

Although all the 'red' parameters are now ok, we have yet to specify the number and location of the engines. We assume that engines are mounted on the wing at 30% of the exposed span (by implication, two), and none are mounted on the fuselage:

nacs-mounted-on-wing 0.3

nacs-mounted-on-fuse Leave this unchanged (blank).

The basic specification is now complete. Of course, we may have chosen some conflicting or inappropriate values, as you will see right away when we examine the aircraft:

The 'redesign' procedure that calculates all relevant geometry, mass and balance characteristics will execute automatically as soon as you ask for any kind of output (see Chapter#08section07 ): Go under the 'Report' menu and select the '3-View' item. You should now get a blocking message, saying that the cabin is not long enough to hold 100 passengers, 5-abreast. There are many things we could do about that; for the moment let us say that 30% of the rear fuselage length is also available for accommodation:

cabin-in-rear-fuse-fraction 0.3

(You would normally look for this parameter in another palette, such as 'Fuselage geometry', or under the 'Group' menu, or via 'Params With...' , 'Param' menu).

If you now select '3-View' once again, you will finally be able to see the first picture of a very simple 100-seater. It should be clear that Piano is at this time making several default assumptions regarding configuration and other characteristics. For example, it has picked a low-wing configuration, a conventional tail, certain high-lift devices, and is also using default shapes for the front and rear fuselage and for the nacelles. You will be able to change any of these (and many more) easily when you are slightly more familiar with Piano's features. For now, we'll just refine the wing a bit:

Add a trailing-edge break to the wing planform at 25% of the exposed span, and specify that the t/c ratio at that point is 80% of the root value, and at the wingtip it is 70%:

planform-break-fraction 0.25

t/c-tip/root 0.7

t/c-break/root 0.8

Select '3-View' again to see the changes. The plane is automatically redesigned. You can already obtain much useful and detailed information from under the 'Report' menu, for example select the 'Geometry' report or the 'Mass' breakdown.

One thing you cannot yet obtain is a 'Range' report. You get a blocking message instead, because you have not yet supplied any engine data (ratings, fuel flows, etc). To do so, go to the 'Eng' menu, select 'Load Engine', and load the engine called:

"Medium-BPR Fan '90s"

This is a typical powerplant for the 100-seater class. Its characteristics are 'rubberisable' and will be scaled automatically to the thrust we have already specified, namely 17,000 lbf. (Don't worry about any messages shown when the data are loaded; details about engines can be found in Chapter 7).

We must also specify the way in which the range is to be calculated: Go to the 'Report' menu and select 'Range Modes...'. Click on 'Flight Levels', delete any existing text and specify the single number 390 (equivalent to 39,000 feet). Also click the 'Specify' option inside the 'Cruise Mach' section and type 0.8 (that value is probably already there), then click "OK".

You can now obtain a 'Range' report, or the 'Field Lengths' performance from under the 'Report' menu.

The only thing that remains is to save the aircraft. Go under the 'Plane' menu and select 'Store Plane...'. Save it as "100 seater" or something similar. (Later on, you may prefer to create your own folder[s] within "planes" to keep various aircraft separate).

15.02 Matching a Set of Requirements

The "100 seater" we have just defined is a simple point-design. Its performance is a direct 'fall-out' depending on the current weights, geometry, aerodynamics, etc. You can adjust the design to match a set of requirements through parametric studies or optimisation (see Chapter 14). Before trying this 'for real' you should become reasonably familiar with Piano's parameters and be able to create more detailed models of planes.

This section shows you how easy it is to set up and run a multi-variable optimisation for initial sizing purposes. However, optimisation does not constitute a panacea. It involves certain pitfalls (again, see Chapter 14), and is not a substitute for experience, or for careful, consistent design definition.

We are going to try and determine the 'best' combination of max.takeoff mass, wing-area, thrust, aspect-ratio and sweepback that will yield a nominally 'optimum' design. Let us assume that our target aircraft must achieve a design range of 1,700 nautical miles and be able to use 5,000-ft. long runways.

Make sure you are still using the "100 seater" we just defined. (If not, use the 'Reset...' item as before and load the plane you saved, via 'Load Plane...' ).

Go under the 'Study' menu and select 'Optimization'. You are now in the main optimisation dialog. Click the 'Set Variables...' button. The first four parameters you see (the ones that are ticked) will constitute the optimisation variables. You can leave all the numbers as they are and just click the OK button.

Next click the 'Set Constrains...' button. Leave the tick-boxes as they are. Type 1700 nautical miles for the range, 5000 feet for the takeoff field length and also 5000 for the landing field length. You can skip everything else. (There is another constraint ticked which ensures that the fuel capacity will be sufficient, but this won't be relevant in this exercise).

You also use the optimisation dialog to decide what objective to minimise. Leave the default selection at 'Compound function'. This is just an arbitrary objective function that looks for an optimum somewhere between a 'minimum-mass' and 'minimum-fuel' design. You can leave all the remaining numbers as they are, except for the 'Number of restarts', which you might as well set to just 2 or 3 to reduce processing time for this example.

You can now initiate the optimisation by clicking the 'Start' button. Click 'Continue' in the dialog that appears. The whole process should take a fairly short time in any modern Mac. As we just specified, it will (re)start itself a few times to improve the solution, although you can interrupt it at any time through a button on the tracing window that is displayed.

At the end of the final pass, use the '3-View', 'Range' and 'Field Lengths' items under the 'Report' menu to examine your final design and to confirm that it satisfies your range and takeoff distance constraints.

If you have been using the numbers given in this tutorial, you probably ended up with roughly the following 'optimal' values:

mto-mass 95600 pounds

reference-thrust-per-engine 17300 pounds-force

wing-area 953 sq.feet

aspect-ratio 9.6

Having obtained an initial 'taste' for defining aircraft from scratch and sizing them to meet a specific set of requirements, you may now prefer to move to other chapters and learn more details about Piano's various parameters and features.

15.03 'Tuning' Planes and the Database

Piano has a database of more than 250 existing or projected planes. You can modify any of these and/or create your own models as required. Levels of accuracy vary depending on the available data. Each plane has some calibration comments associated with it (see 'Notes..' under the 'Param' menu). The ones with extensive notes generally indicate a high degree of confidence (e.g. F100/F70, 767, DC-9 plus derivatives). Others contain few notes and tend to be based on marketing brochures or information from Jane's, AWST, or Flight Intl. (e.g. Yakovlev projects). A great variety of private and non-proprietary commercial sources have been used; no guarantees are given about the accuracy of any information in the database, nor is the endorsement of any manufacturer implied.

When modelling a plane, different users will naturally focus on different aspects of the design. The following are just a few general indications of areas where attention should be paid.

Geometry is a basic consideration. Gross errors often arise in the definition of the wing area, which varies between manufacturers, or even within the same organisation. Piano uses a trapezoidal wing-area plus an optional planform break, but also allows the most commonly used alternative values to be input. Details are given in Chapter#03section02 through to Chapter#03section05 . Areas quoted without explanation in brochures or in the press need to be treated with caution!

Arbitrary shapes can be defined for the front and rear fuselage and for the nacelles, but existing shapes often provide a close match and can be used instead (adjusted to the current plane's dimensions, see Chapter#03section07 ). Wetted area estimates often remain satisfactory in such cases, even though the appearance changes significantly.

When you are modelling a 'frozen' design, you may prefer to override the tail sizing and balancing routines, fixing the stabiliser and fin areas and the wing location as described in Chapter#08section12 and Chapter#08section13 . Geometric fuel capacity can be adjusted as explained in Chapter#03section16 (this capacity will change if you modify the wing).

Piano automatically generates a detailed mass breakdown for any plane. Although there are factors and other parameters that let you tailor all component contributions individually, such an exercise is normally not necessary. The only relevant values that need to be exact when modelling a known design are the MTOW, OEW, MLW and MZFW. An adjustment procedure is summarised in Chapter#04section14 .

Comprehensive knowledge of actual aerodynamic lift/drag polars for existing planes is rare. If you possess such data, you can use them directly to override or complement Piano's calculations as explained in Chapter#06section01 . Otherwise you will need to use judgement or experience gained with similar designs to alter various zero-lift, induced or compressibility-related drag contributions. If your information about the performance of a particular plane is limited to simple quotes of total range or specific-air-range (SAR) values, you have to make a further judgement in balancing the effects of engine SFC and airframe aerodynamics at a representative point condition (see Chapter#09section18 ).

Piano includes more than 25 engine models. These are based on actual engine characteristics (so-called 'decks') or competitor simulations. Each model can be scaled to any thrust and its ratings and fuel flows (or SFC) adjusted individually. You can select a representative configuration by examining the engine choices made for similar aircraft in the database, or you can supply your own engine models as explained in Chapter 7.

Ultimately, the strength of any analysis using Piano (or any other tool) depends on the experience of the user and the quality of the input. For these, there can be no substitute.

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