Landing gear questions

[9GClub]

Hi guys,

I'm new on flightinfo.com and I thought I'd get down to business by asking some pesky questions right off the bat......

1.) Why are the four-wheel main bogies (just the wheel trucks-- I'm not talking about the vertical struts) on different aircraft cocked at different angles when they're extended? For example, they're cocked slightly "nose-up" on the B757 and nose-down on the -67. Also, the outboard (wing) trucks on the -47 are cocked up around 70 degrees to the horizontal, whereas the trucks on the L-1011 appear to be perpendicular (0 degrees) to the strut. I'd be surprised if it was arbitrary, especially considering the complexities of modern gear design (for example, the B773 main trucks lock perpendicular to the struts on rotation in order to kick the fulcrum back a few feet for tail-strike avoidance).

2.) On the MD-80 class of jets, I've noticed a small "platform" attached to the nosegear...... it's fairly prominent and can been seen clearly right behind the nose tires in most photos with the gear down. It's not a "mudflap" or anything goofy like that is it? Convenient stepstool for prospective stowaways maybe..... I'm out of ideas......

3.) Is there a "standard" tyre pressure (notice how I'm deftly switching back and forth between the American and English spellings of "tire" in order to appease any British brethren) for commercial jets? If not, how about a tire pressure "window"..... I'm sure it's modified according to aircraft type, load, surface conditions, etc., but are there any generalizations that can be made? For example, I read somewhere that the USN uses 200 psi for land ops and 300 psi for carrier ops with its Hornets.

Thanks in advance for any insights you can provide. I look forward to learning from y'all.

 

[avbug]

If you look at any landing gear containing a shock strut or oleo strut, you'll note that it fully extends (so far as the scissor linkage will allow). A multiple wheel gear bogie is no different; the tilting action is by design of the strut, and occurs when the strut is at full extention. This design also tends to smooth the touchdown.

Many aircraft utilize gear switch logic that requires the mains to go to full extention and the gear to tilt to tell the aircraft computers that the airplane is in the air. Essentially an advanced squat switch arrangement. As an example, on the B747, upon touchdown, the logic circuitry Arms/disarms the landing gear lever latch, activates anti-skid touch down protection, activates automatic ground spoilers on landing, activates/deactivates takeoff warning circuitry, activates/deactivates ground safety relay, etc.

On some aircraft, such as the 777, the gear must tilt to fit in the gear wells. In the case of the 777, the main landing gear is tilted down (forward) five degrees for retraction and extention. In the extended position, it's tilted up (front end up) 13 degrees. The five degree tilt is to accomodate the gear well. The reverse tilt when fully extended serves gear logic (as any squat switch arrangement), but also helps smooth the touchdown, and assists in providing torque to drop the nose of the aircraft after touchdown.

The 767 works the other way, having a forward tilt (nose end down) to landing, and tends to land more firmly as a result. In the case of the 767, the tendency is to pitch forward suddenly on landing; the forward tilted gear helps arrest this tendency, whereas a rear tilted gear (front end up) would tend to exascerbate the problem (make it worse).

The A330 Airbus tilts back (nose end up) for similiar reasons to that of the 777. On the A346, the center landing gear trails (tips back) for retraction and extention to clear the gear doors, and then tilts forward when extended.

Above all other reasons, the primary purpose in 'tilting' the gear bogie is to fit it in the gear well and conserve space.

In some designs, the tilt sequence also assists in the way the aircraft comes off the ground, and provides overrotation protection (preventing tail strikes).

Some aircraft, such as the MD-11, DC10, and Airbus A300, don't utilize gear tilt. On the A300, the gear handle can't be moved to retract the gear unless it's level. On the A310, the same applies, except that the gear handle will not move unless the gear is tilted forward (to facilitate fitting in the gear bay and to accomodate the retraction sequence).

There exists no standard or uniform method of determining aircraft tire inflation; the specific needs of a particular make and model determine what the tire pressure should be; this is specified both by the aircraft manufacturer, and the component (wheel and tire) manufacturer(s).

When a wheel assembly is serviced, it is done based on several factors, chief among them being temperature, and weather the aircraft weight is on the wheel. Overinflation can cause premature wear to a tire, whereas underinflation results in high tire carcas temperatures and the potential for a thermal failure. Specifics as to tire inflation for any particular make, model, and serial number, are found in the approved manufacturer service and maintenance publications for that aircraft.

 

[9GClub]

Avbug,

Dang, that was extensive (and more complicated than I anticipated). Thanks for taking the time to help me out, I appreciate it!

 

[sweptback]

Supposedly the reason for the weird tilt on the 767 gear was that they uncovered a design flaw as they were putting the first one together. The only way they could get the gear to fit in the bay was to tilt it the opposite direction of practically every other gear out there.

 

[DC9stick]

The "platform" on the MD-80 series is just what you suspected, a mud flap, actually called a spray deflector it deflectes water away from the fuselage and engines thereby preventing ingestion.
The DC-9 series could use the spray deflector or chined tires, the MD-80's require the spray deflector.

 

[Donsa320]

2.) On the MD-80 class of jets, I've noticed a small "platform" attached to the nosegear...... it's fairly prominent and can been seen clearly right behind the nose tires in most photos with the gear down. It's not a "mudflap" or anything goofy like that is it? Convenient stepstool for prospective stowaways maybe..... I'm out of ideas......
It is in fact a spray deflector to keep water spray and snow under the wings and out of the engines. It works fairly well except in deep snow in a strong crosswind take-off. Then the spray plume tends to swamp the upwind engine. Compressor stalls result. If the spray deflector is not used then the nosewheel tires will have "chines" on their sidewalls to obtain the deflection action.

 

[bafanguy]

Supposedly the reason for the weird tilt on the 767 gear was that they uncovered a design flaw as they were putting the first one together. The only way they could get the gear to fit in the bay was to tilt it the opposite direction of practically every other gear out there.

Adjusting MLG truck position to fit the assembly into the wheelwell is nothing new. The CV-880 did that eons ago. In fact, there was a truck-position cylinder on each MLG assembly to insure movement to a position that would allow it to fit in the wells. The FE checked this on walkaround for proper servicing. In the cockpit near the gear position lights was a truck position light indicating when the trucks were properly positioned after liftoff. If I remember correctly, if you didn't get proper indication, you didn't retract the gear.

 

[EagleRJ]

Re: tire pressure

Tire pressure varies widely by aircraft type, but it generally increases from GA to commuter to large jet to military. It usually is higher the greater the load on the tire and the greater the speed the aircraft lands at. The highest I've seen was on the tire of an F-89 Scorpion in a museum. If I remember correctly, it was 400 PSI!
Mechanics are usually responsible for checking the pressure, although some aircraft have the capability of sensing a low tire.
Fun facts- aircraft tires are filled with nitrogen to avoid corrosion in the wheel and stabilize temperature-related pressure changes. Also, aircraft tires are always made from natural rubber. Synthetics can't take the heat.

Adjusting MLG truck position to fit the assembly into the wheelwell is nothing new. The CV-880 did that eons ago. In fact, there was a truck-position cylinder on each MLG assembly to insure movement to a position that would allow it to fit in the wells. The FE checked this on walkaround for proper servicing. In the cockpit near the gear position lights was a truck position light indicating when the trucks were properly positioned after liftoff. If I remember correctly, if you didn't get proper indication, you didn't retract the gear.

The all-time winner for a strange gear retraction has to be the XB-70 Valkryie. It had two main four-wheel bogies that would twist 90 degrees, the bogies would fold flat against the struts, and the whole thing would fold aft into the fuselage. Must have worked great for parallel-parking the jet on the ramp!