How to Design a Aircraft Landing Gear System and How does it Work?

Question

I'm in Final Year of Aerospace Engineering Degree course, and as part of my Final year project I have been asked to design a Tricycle Landing Gear system for a Medium Size Private Jet aircraft.
So far I have got very limited resources and I havn't found good source to tell me how to design the layout including all the calculations for weight, roll, angle of turn and also which system to use for the retraction and extension of Landing Gear.

I Would very much appreciate it if anyone out there could give me some information as I need them ASAP.

Many Thanks for you kindness.

Reza

Answer 1

Look at books on rigid body dynamics and design of machinery. More specifically, look up four-bar linkages. These linkages are ideal for landing gear due to their mechanical advantage and the ability to 'lock' in a specific configuration.

The turn radius (which is what I believe you want) will depend upon the separation distance (D) between the rear two (non-swiveling) wheels of the 'tricycle' gear. The minimum possible turn radius (Rmin) will be half the separation distance (D).

Rmin = 0.5 *D

So to enable 'tight' turns you might want to put the rear wheels close together (small D). However, when you do that, the plane will be more likely to tip over during tight turns at high velocity. The designer's challenge is to balance a tight turn radius will adequate roll support.

To find the minimum separation between the two rear wheels consider the following force diagram. You want the plane to be able to execute a turn at radius R with forward velocity Vmax. The centripetal acceleration vector has magnitude (Vmax)^2/R and is oriented towards the hypothetical center of the circle that descibes the turn's radius. If you imagine looking at the rear of a plane turning right, the centripetal acceleration vector orginiates from the center of mass and points horizontally towards the right The acceleration of gravity pulls the plane downward towards the tarmac and has a magnitude of g=9.81 m/s^2. So the net acceleration on a turning airplane is a vector sum of these two accelerations. The 'sum' vector should point mostly downward. If you extend this vector, it should intersect the tarmac somewhere between the two rear wheels. If it doesn't, the plane will flip over during the turn.

You can use the above visualization to come up with a crieria for minimum distance between the rear wheels:

theta = arctan(g/(Vmax^2/R))

This equation states that the angle theta is the angle between the sum vector and the tarmac.

theta = arctan (L/ (.5*Dmin))

Theta is also the angle between the center of mass and the point where a rear wheel is touching the tarmac. The landing gear length (L) is the vertical distance between the tarmac and the plane's center of mass.

Equating the above two equations, we have

Dmin = 2*L * (Vmax^2)/(R*g)


Now as the designer you have to play around with D (separation distance between the two rear wheels) and L (landing gear length) to acheive an acceptable Rmin and Vmax.

Considering both turning radius and roll resistance, you want rear landing gear that are short (small L) and reasonably close together but not less than Dmin.

Answer 2

You start with the weight of the airplane and the maximum rate of descent at landing. Obviously, aircraft carrier landings (literally "crashing" on to the deck of a ship) are going to be much more demanding, then runway landings.

From this, you determine your structural ties to the airplane itself (what three point are best suited structurally to support the weight of the plane?). Rear gear location in terms of fore-aft placement with respect to the CG is also important, because you have to be able to rotate the aircraft during takeoff, without having it "rotate" on you by hitting a bump during taxi (this is why all planes started life as tail-draggers).

You use physics to determine the amount of energy the system must absorb due to repeated impacts (the gear obviously is not "rigid", but "springy").

Wheel diameter is determined by typical landing terrain and overall weight. Planes that are deigned to land "off-road" have bigger tires.

Lastly - you consider retractable or fixed and design the retracting mechanisms.

This is a very iterative (optimization) process.

Answer 3

That's a tough one. I'm an energy engineer and one customer, a company in Chicago, builds landing gear - they must have the info you are looking for. But - I can't remember their name. Maybe you can Google it. If I remember right, they go by a name that has to do with metal engineering.