OldAeroGuy wrote:morrisond wrote:OldAeroGuy wrote:
The tail loads during a stall are dynamic, ie the tail bounces around a lot in the wing wake and the concern is structural fatigue. Flight test airplanes do numerous stalls as part of a flight test program. Production airplanes will probably never encounter a stall in their service life so h. tail stall fatigue issues are a minor design condition.
If the keel beam has a strength issue, I doubt that it would be impacted by the h. tail fatigue condition.
What about when you are in a full stall (or spin) though and pointed at the ground and have to pull significant G's once you get the wing flying again to pull out of it?
You're being a bit dramatic. I was on the airplane or analyzed/reviewed data for 100's of stalls on the 737 Classic, 737NG, 747, 757 and 777 during configuration development and certification. Pulling more than 1.5 G's during recovery was a rare event with the norm being 1.05 - 1.20.
Spins on Part 25 airplanes during stall testing is very unlikely. About the most exciting thing that does happen during Part 25 stall tests are high bank angles due to asymmetric wing stalling. A sudden, uncommanded 60 deg bank can be pretty eye opening but they don't entail high recovery loads.
Remember at stall, the load factor is less than 1.0. During recovery, high load factors are not needed unless you're near the ground. The solution is to initiate all stall tests above 10,000 ft AGL.
Absent the tension in the keel beam due to pressurization, I would think the critical design condition for the keel beam is in compression as buckling happens is far lower than the allowable tension stress. In slender steel structures Fc is often around 18 ksi and Ft is at 36 ksi (using LFRD), pulling out of a dive should cause tension in the keel. I would think turbulence where the craft drops then hits the undisturbed air, similar to doing a belly flop into water would be a high compression load in the keel. Similarly landing hard where the gear is trying to stab thru the wing wells would have wicked compression in the keel.
I recall the 767's have had a number of hull buckling just ahead of the wing due to hard landings. Those are similar but the keel there is probably in tension, full compression at the crown due to the nose wheel hitting hard.