Gear

Contents

The Gear component models an aircraft's landing gear, and above all the ground the aircraft stands on. Its own origin and $XY$ plane are the nominal ground plane, and its bogies, each a group of identical wheels, are placed on or above that plane (Figure 1). One Gear holds all of an aircraft's gear: nose, main, tail and outriggers share its ground plane.

The ground handling analyses need to know more than the gear's shape, so the Gear also holds how the gear behaves. Each bogie has a contact point and a pivot it rotates about, a steering limit, suspension travel that compresses and extends the strut, and tires built to Tire and Rim Association (TRA) dimensions [1, 2], whose static loaded, unloaded, grown and flat radii differ. The Gear holds the aircraft's center of gravity range too. From these, Auxiliary components hung from the Gear build off-nominal ground planes: the ground with chosen bogies in a chosen state, turned about their contact points or axles as the tail strike, tipback and roll analyses require (see Ground Analyses). Each bogie also has a retracted position, given either directly or by a retraction mechanism, so that the stowed gear can be checked against the aircraft.

Quick Start

1. Add a Gear. Set Model Units on the Design tab to the model's length unit. Place the Gear with the XForm tab so that its origin lies on the ground under the aircraft, in the attitude the aircraft stands in.
2. On the Bogie tab, Add a bogie for the nose gear and set its X Contact. Add another for the main gear, set its X Contact and Y Contact, and check Symmetrical. Set Across and Tandem for each.
3. On the Tire tab, choose each bogie and give its tire's Diameter and Width, in inches from the tire maker's data, and its rim.
4. On the Design tab, give the center of gravity and its range.
5. On the Retract tab, give each bogie's stowed position, or its retraction mechanism, and check it with Draw Gear Up and Down.

A new Gear has no bogies; until one is added it is only its ground plane.

The Gear's Frame and Ground Plane

Everything on the Gear's tabs is given in the Gear's own axes, which the XForm tab places in the model. With no rotation they are the model's axes: $X$ aft, $Y$ to the right, $Z$ up. The origin and the $XY$ plane are the nominal ground plane, the ground with the gear in its nominal state. To set the aircraft in a pitched attitude on the ground, pitch the Gear.

Figure 1. A Gear with a nose bogie and a symmetrical main bogie on its nominal ground plane, and the center of gravity with its range.

With Include Nominal Ground Plane checked, the Gear shows the ground plane as a square surface centered on its origin, extending Plane Size each way. Auto sets Plane Size to the length of the diagonal of the model's bounding box, so that the plane reaches past the whole aircraft. The plane is transparent to CFD meshing.

Model Units is the unit of the model's lengths. Tire sizes are given in inches, as tire makers give them, and converted to model units with it, so a Gear with the wrong Model Units has tires of the wrong size. It changes no other length, except that an Auxiliary clearance envelope read from a file is converted with it too.

Center of Gravity

The Center of Gravity group gives the nominal center of gravity and a range of it, a box from Min to Max in each direction, with Min $\le$ Nominal $\le$ Max. Local gives them in the Gear's axes, Global in the model's; the other set follows. The 3D view draws the nominal center of gravity as a grey point and the range as a grey box. The tipback and tipover analyses evaluate the nominal point and every corner of the box.

Bogies

A bogie is a group of identical wheels on one strut. The list on the Bogie tab shows each bogie's name, its configuration and its tire. The configuration is the FAA's designation [3]: the number of wheels in tandem, then the number across as S, D, T or Q for one to four (2D is two pairs of wheels, one behind the other). The tire is designated by diameter, width and rim diameter in inches, as 13.5x6-3.915. Choose a bogie in the list to edit it; the Tire and Retract tabs have their own bogie choice.

Across wheels are spaced side by side along $Y$ and Tandem wheels one behind the other along $X$, centered on the bogie's contact point (Figure 2). The distance between wheels across is set by one of:

Pitch between wheels in tandem has the same four modes, measured with the tire diameter. Whichever mode is chosen, the other values follow, so the tires can be spaced by the gap they need and the center distances read off.

Symmetrical adds a mirror image of the bogie in the Gear's $XZ$ plane, for main gear. Everything about the bogie, its stowed position included, is mirrored.

Figure 2. A symmetrical bogie, two across and two in tandem, from above. The contact point is the center of the bogie on the ground.

Contact Point and Suspension

X Contact, Y Contact and Z Above Ground place the bogie's contact point, the center of its footprint on the ground, in the Gear's axes. Z Above Ground is normally zero; a bogie that does not touch the ground in the nominal state, such as a tail bumper, is raised by it.

The bogie's pivot, the center of its axles, lies one static loaded radius (SLR, see Tires) above the contact point (Figure 3). The tires are drawn at their full, unloaded size about the pivot, so they reach below the ground by the tire's static deflection, as a loaded tire is flattened.

Figure 3. The bogie of Figure 2 from the side, with Compression Travel 0.4 and Extension Travel 0.2. The green line is the suspension travel the 3D view draws.

Suspension Travel gives the direction the pivot moves as the strut compresses, as a vector in the Gear's axes ($+Z$, straight up, by default), and how far it moves from the nominal state when fully compressed and fully extended. The ground analyses can use each bogie nominal, compressed or extended.

Bogie Rotation Limits are how far a bogie with wheels in tandem can pitch about its pivot, in degrees. The Plane 2pt Angle Contact analysis turns the bogies up to these limits.

Steering Angle is the most the bogie can steer, in degrees. The Ground Maneuverability analysis steers the bogie with the largest steering angle.

Tires

The Tire tab shapes each bogie's tires (Figure 4). Diameter and Width are the unloaded tire's outer diameter and section width. Each is given in inches (in) or in model units (Model); the other follows (Figure 5). Use inches for a tire from a maker's catalog: they do not scale when the model is scaled, while sizes in model units do.

Figure 4. The default tire, Full TRA, cut through its axle and seen from ahead, with the grown tire and the clearance envelope.

Figure 5. The Tire tab for the nose bogie of Figure 1, a 22 x 6.6 tire given in inches, in a model in feet: the model unit values beside them follow. The rim and shoulder are given as fractions of the tire, and the static loaded radius follows from the deflection.

The section height is $H = (D - D_\mathrm{rim})/2$. The static loaded radius is given directly, in inches or model units, or by the deflection (Deflection Frac H), the fraction of the tire's height above the wheel's flanges that the load flattens:

$$\mathrm{SLR} = \frac{D}{2} - \delta\,\frac{D - D_\mathrm{flange}}{2}$$

where $\delta$ is the deflection fraction (0.35 by default) and $D_\mathrm{flange}$ the diameter over the flanges.

The grown tire is the tire spinning at speed, grown by the TRA allowances: 4% in width, and in section height by the factor $1.115 - 0.075\,H/W$ [2]. The clearance envelope adds the TRA clearances for a bias-ply tire around the grown tire [2], in the TRA's envelope shape [1]; they grow with the tire's width and its speed rating. The Auxiliary Single Gear shows either one in place of the tire, to check that a wheel well clears them.

Tire Mode chooses the shape (Figure 6). The three TRA modes build the tire and wheel from all the dimensions above: Full TRA shows the wheel flanges, TRA w/ Faired Flange smooths the tire into the flanges, and TRA w/ Faired Wheel closes the wheel's face flat across the tire's width. The Balloon Tire modes are a tire of round section, of diameter equal to the width, for small aircraft: alone, on a wheel, or with a faired wheel. The balloon modes have no grown tire or clearance envelope of their own; the tire stands in for both.

Full TRA's wheel flanges follow the TRA's relations, which cover the sizes of tire the TRA standardizes. Where the flange meets the tire it leaves a narrow concave sliver, detail that external-flow CFD and many meshes are better without; the two faired TRA modes leave it out. The balloon modes suit bush planes, small UAS and radio-controlled models, whose tires are outside the TRA's range.

Figure 6. The default tire in each Tire Mode, cut through its axle, at one scale. The axle is dash-dotted.

Retraction

The Retract tab gives each bogie's retracted position, by one of two modes (Figure 7). Stowed Position places the retracted bogie directly. Gear Mechanism moves it there by a trunnion, a knee and a wrist, and so gives every position in between. Each mode's parameters are computed from the other's, so switching modes keeps the retracted position: in Stowed Position mode the mechanism is set to a single hinge that carries the tires to the stowed position, and in Gear Mechanism mode the stowed position is set to where the mechanism puts them.

Figure 7. The top of the Retract tab: the bogie, its Mode, here Gear Mechanism, and the tabs that hold each mode's controls.

The retracted position is a volume claim: it shows that the stowed gear fits inside the aircraft where it is meant to go, and clears the structure and the other systems there. Check it with the interference analyses, against the grown tire or the TRA clearance envelope of a Single Gear Auxiliary component.

Stowed Position

The Stow tab places the retracted bogie's pivot frame, the frame at the center of its axles with the Gear's axes, by a location and a rotation. Rel gives them in the frame chosen under Attach To Parent; Abs gives them in the Gear's axes.

Attach To Parent works like the XForm tab's Attach to Parent, except that the frame can be attached to any component in the model, not only the Gear's parent: choose a component, one of its surfaces, and how the frame is attached to it, by its component axes or a point given by UW, RST, LMN or $\eta$MN coordinates. A bogie stowed in a wheel well of a fuselage or a nacelle should be attached to it, so that it follows when that component moves or changes shape. A bogie attached to a Hinge rides on the Hinge's joint. With Contact, the default, the frame is the bogie's own pivot frame with the gear down, so Rel is the move from the down position.

Gear Mechanism

The mechanism swings the strut about a trunnion, folds it at a knee, and turns the bogie at its end, the wrist (Figure 10). The strut runs from the trunnion point to the pivot with the gear down. The controls are on the two Mechanism tabs (Figures 8-9).

Figure 8. The Mechanism (1/2) tab for the gear of Figure 10: the strut swings 90 degrees about a trunnion axis along $+Y$, through a trunnion point given in the Gear's axes (Abs).

Figure 9. The Mechanism (2/2) tab for the same gear: a straight strut with its knee halfway up, folding by $-100$ degrees, and no turn at the wrist.

The knee axis starts as the part of the trunnion axis square to the straight strut, turned by Azimuth Angle and Elevation Angle (Figure 11), and is fixed to the upper strut, so the Bend Angle turns it with the upper strut. With both angles at zero the strut folds in the plane it swings in. Before OpenVSP 3.56 the lower strut turned about the knee axis as it was before the Bend Angle turned it, not the axis drawn. A gear saved by an earlier version, with a Bend Angle other than 180 and a knee axis not along the trunnion axis, now retracts differently; set its Azimuth Angle and Elevation Angle again to keep the earlier motion. The Trunnion Point's Attach To Parent places it like the stowed position's, by translation only.

Figure 10. A gear retracting forward by a mechanism, from the side: down (grey), at Retract 0.5 (dashed) and up (blue). Retract Angle 90 about $+Y$ swings the strut forward; a Knee Rotation of $-100$ folds the lower strut back.

Figure 11. The knee axis of a straight strut, Azimuth Angle 30 and Elevation Angle 20, with the trunnion axis tilted out of the plane square to the strut. The part of the trunnion axis square to the strut (dashed) turns about the strut by Azimuth Angle (dotted), then tilts toward the trunnion by Elevation Angle, about the tilt axis square to the strut and the turned axis.

At an intermediate position $k$, every angle of the mechanism and the strut extension are taken at $k$ times their full value. A right-handed turn about the trunnion axis by Retract Angle comes first; the lower strut then turns about the knee, and the bogie about the wrist.

The 3D view draws the mechanism of each bogie: the trunnion axis dashed with an arrow for the positive sense, the strut from the trunnion through the knee to the pivot, and the knee axis, for each position drawn.

The mechanism is a simple model of the main strut, not a general linkage: it has the three joints above and no braces, side stays, drag struts, actuators, locks or doors, and its joints all move together, in proportion to Retract, with no sequencing. A gear whose motion it cannot follow is given by its Stowed Position instead. Use only the joints the gear needs; each one added is another part the real gear must carry.

Drawing the Gear Up and Down

Draw Gear on the Design tab chooses which positions the Gear shows: Down, Up, Up and Down, Intermediate, or All. The intermediate position, at each bogie's Retract value, exists only for bogies in Gear Mechanism mode. Each position drawn is a set of surfaces of the Gear, so it is also what CompGeom, interference checks and export see.

Ground Analyses

The Gear's nominal ground plane is the ground with every bogie in its nominal state. The ground handling analyses of the Geometry Analysis manager also need the ground in other states: under a flat tire or a compressed strut, or with the aircraft rotated onto its tail, its nose or a wing tip. An Auxiliary component hung from the Gear builds such an off-nominal ground plane. For each bogie it uses, it chooses one of the states the Gear defines, and the plane passes through those bogies' contact points in that state (see Ground Planes).

The 3pt Ground Plane rests on three bogies, for ground and ceiling clearance, weight distribution and ground maneuverability. The 2pt Ground Plane rests on two and turns about their front or rear axles, for the tail strike, tipback and tipover angles. The 1pt Ground Plane rests on one and rolls about its outer tire's contact point, for roll clearance. The analyses read the Gear's center of gravity range through these planes. The Plane Min/Max Distance analysis can also use the Gear's nominal ground plane directly.

Shared Tabs

The Gen, XForm, Mass and Sub tabs are common to every component; see Gen, XForm, Mass and Sub-Surfaces.

Scripting

The OpenVSP API manages bogies with these functions:

A bogie's parms are found with FindParm( bogie_id, name, group ). The groups are Bogie (layout, contact point, suspension, steering), Tire, Retract (mode, stowed position and mechanism), StowAttach and MechAttach. The modes are SpacingType and PitchType (BOGIE_SPACING_TYPE), TireMode (TIRE_MODES), the ...Mode parms of the tire dimensions (TIRE_DIM_MODES) and RetMode (GEAR_RETRACT_MODES). The Gear's own parms are in the groups Gear (m_ModelLenUnits, GearConfigMode) and GroundPlane (the plane and the center of gravity).

References

1. The Tire and Rim Association, Inc., "Engineering Design Information: Aircraft Tires and Rim Contours," Akron, OH, index of October 30, 2024. Page AC-12, Clearance Envelope Geometry Definition, Radial and Bias; page AC-30-B, Aircraft Rim Flange Height Formula.
2. The Tire and Rim Association, Inc., "2025 Aircraft Year Book," 2025. General Notes for Use of Aircraft Tire Data, note 13, Bias (Diagonal) Ply Aircraft Tire Standard Growth and Minimum Clearance Allowance, pp. 8-9.
3. Federal Aviation Administration, "Standard Naming Convention for Aircraft Landing Gear Configurations," Order 5300.7, U.S. Department of Transportation, 2005.