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 A Squared 1st Aug 2016 08:57

Originally Posted by Mach Stall (Post 9451185)

The question is how much hotter.

Not that hard to calculate. Lets say we have an airplane landing at 250,000 Kg in an unchecked 600 ft/minute impact. That's a pretty good thump. In fact it's the FAR 25 design limits for hard landing. The kinetic energy would be 250,000 kg X ( 3.05 m/s )**2 = 2,325,625 joules or 2,326 Kilojoules. Lets say all kinetic energy was converted with perfect 100% efficiency into heat in the landing gear struts. And lets say that the collective main landing gear struts is 200 Kg of steel. Obviously the landing gear struts of a 250,000 kg airplane is going to be a *lot* more than 200 kg, but for illustrative purposes it's useful to use a ridiculously small mass. Steel has a specific heat of 0.49 kilojoules/kg degree C so, that 2326 kjoules will heat our 200 kg of landing gear about 23 degrees celcius. Not quite enough to turn steel blue. What sort of impact would it take to turn steel blue? My information is that steel turns blue at about 300 decrees C, Using the same airplane mass, and same landing gear mass, and working backwards, we find that a 250,000 kg airplane would have to touch down at a vertical speed of 2125 ft/min in order to raise the temperature of 200 kg of steel 300 degrees C. Remember we're assuming that the entire kinetic energy will be transformed perfectly efficiently into heat energy and concentrated exclusively into 200 kg of steel, which is absurd on several levels. Bottom line is there is just no way a hard landing could generate enough temperature rise in the metal portions of the landing gear to turn steel blue. Someone was having pulse1 on.

 riff_raff 2nd Aug 2016 06:38

A Squared-

Good post. The only thing I would add is that the heat transfer mechanism to the strut is via compression work applied to the hydraulic fluid mass within the cylinder. Obviously, the hydraulic fluid mass absorbing the energy would get hotter than the strut body it is transferring heat to.

 twochai 3rd Aug 2016 14:14

I recall being told by a landing gear designer that the critical L/G loads were those induced by wheel spin-up on touchdown!

 Volume 4th Aug 2016 12:18

which of course is only relevant if you have more than one axle per leg or more than two main landing gear legs. The Vulcan has 4 tires on 2 axle per leg.
Even the 4 tires on one axle design of the Trident prevents most of this type of load.
This is how it looks if you turn an A380 around tight corners...

 Mach Stall 4th Aug 2016 18:11

The sole concern with A Squared's calculation is that it assumes thermal equilibrium is reached for the assumed gear strut mass (i.e., plenty of time for the thermal energy to diffuse through that mass) -- whereas in reality, we're talking about on the order of a half-second for strut compression. During this short time, the fluid friction would transfer heat to the comparatively tiny local mass of steel surfaces adjacent to the heat source. Bottom line, the uppermost piston and cylinder steel surfaces would see a drastically higher peak temperature rise than would the bulk mass of the strut.

As I said in my previous post (having run the same sort of back-of-the-envelope calc), I thought the temperature rise of the whole strut would be under 50 deg C, and I remain in agreement with A Squared on that front. And since pulse1 described the entire lower leg being blue, I still think there was probably some other cause at play in that large-scale thermal damage.

But I'm leaving the door just a little cracked in my mind that during a violently hard landing, some small, localized surface (e.g., upper piston) could possibly get hot enough to blue steel (over 275 deg C). I just can't quite rule it out intuitively without seeing actual data or a dynamic thermal analysis.

You generated an interesting sub-discussion, pulse1.

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