Temperature Effects

[feajob]

According to our customer specification, we are required to design the landing gears for a range of temperatures as follows:

-67F to +158F

From some curves in MMPDS, we can conclude that material allowables should be increased for cold temperature and decreased for hot temperature. So, conservatively we should consider only the impact of the elevated temperature. For our alloys this means that we should keep at least 5% positive M.S.

But, I think that it may be over-conservator to assume this impact in our stress (structural analysis).

Because, if Aircraft is landing in a extremely hot region (158 F) then it still takes time for landing gear (and other components of A/C) to reach to the ambient elevated temperature. Note that, Aircraft is coming from sky where temperature is low. Since, the most critical load conditions for landing gears are due to landing, consequently, it cannot reach to this elevated temperature.

I would like to know your experience with regard to temperature effects.

Thanks,
A.A.Y.

 

[btrueblood]

The really-really worst case is a forced return to field landing after takeoff, i.e. hot gear, full fuel load.

 

[rb1957]

which is why they have fuel dump systems, and allow a 6fps landing (instead of the 10fps typical design landing, which is almost a crash).

iguess a spec is a spec ... if that's the temperature range of the operation of the plane, then you have to design for it. mind you the temperature effect on strength is pretty small (isn't it?). and no doubt you're getting hit on 'cause you're over weight ! (well, not you personally, but your gear). this point could be discussed with the plane OEM, something like "your temperature envelop is driving X lbs into the gear ..." ... good luck !

i'd be more interested in temperature effects on the hydraulic system of the landing gear.

 

[WKTaylor]

feajob...

SAE AIR809 "Metal Dimensional Change with Temperature" is useful for predicting static and quasi-dynamic expansion and contraction values for various metallic materials.

Personal experience highlights extreme cold as the most serious environment. Nonmetallics (plastics elastomerss, etc) and fluids (oils, grease, etc) all behave badly (viscously, stiffly, stiction, etc) especially with real-world moisture contamination. Also the contraction of compressed gasses reduces the shock strut effectiveness, etc.

CAUTION: real world temps are usually worse than spec for long range high altitude flight. See +175F skin temps (dark/flat colors) in desert, and -75F in winter arctic flights. Sooo You may want to consider expanding spec boundaries.. at least for design sake.

Regards, Wil Taylor

 

[feajob]

Thank you for your responses.

btrueblood, there is a fuel dump system.

rb1957, you are right. Overall Impact is small (about 5%) for extremely hot temp.

wktaylor, thank you for your SAE reference. I am agree for operating purpose cold temperature is worst, but for structural static stress analysis strength allowable decrease for hot temperature. However, I think that we may experience more fatigue issues with extremely cold temperatures.

Thanks,
A.A.Y.

 

[btrueblood]

Fuel dump systems take time to operate, and there isn't always time in emergency situations to wait for it to do so...

but I'm also not a spec. writer.

 

[feajob]

Probably, a good compromise (more realistic and not too conservator approach) is to keep only for limit M.S (not Ult. M.S.) few percent M.S. for temperature effects.

As we know, for most aerospace alloys Ult. M.S. are more critical than limit M.S.

A.A.Y.