Maximum Inertial Load

Question

Maximum Inertial Load

SAITAETGrad (Aeronautics)

(OP)

31 May 17 22:52

See image. The blue & orange curves represent + ultimate load factor curves for the shorter, lighter model of a type. The grey and yellow is for the longer, heavier model of the same type for which I just have some tabulated data. Both derivatives share common cg (x394). The shorter model distribution makes sense. The grey curve for the longer model makes sense too given the higher mass moment of inertia. But I don't understand the sudden change in the three last data points represented by the yellow curve. The tabulated maximum ultimate loads for the longer model are all given from the same yz axis. The slope change does occur just forward of the t-tail horizontal stabilizer.

I think I'm missing something fundamental here - is it obvious to others?

GregLocock (Automotive) · Answer 1 · 2017-06-01 01:04:22

Just a couple of WAsG, the maximum forces that can be generated aerodynamically by the stabilizer are not a function of the aircraft mass or length, or (b) tail strike

Cheers

Greg Locock

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rb1957 (Aerospace) · Answer 2 · 2017-06-01 12:00:43

you say you're comparing a shorted and longer plane, but the axis is in "fuselage station". won't it make more sense to plot in terms of normalised fuselage station (ie %age of length) ?

why does the load kick up towards the tail ? maybe ditching ? maybe dynamic landing ?

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SparWeb (Aerospace) · Answer 3 · 2017-06-01 19:03:58

...or "Mean Aerodynamic Chord" which is more commonly used, to amplify RB's comments.

My turn for a WAG: Fuselage elastic flex and recoil. I think an effect like that would be X^2 with length, but very small for most of the fuselage. You said this is a stretched fuselage, so maybe that effect begins to matter when it gets >40 feet from the spar.

STF

verymadmac (Mechanical) · Answer 4 · 2017-06-01 20:24:24

is the certification basis the same for both models.

It looks like 2 super imposed load cases. The gray line is 6.5G at the CG and yellow is 2.48 G so I would guess that the yellow is a pitch related load at maximum maneuvering load and the gray emergency landing maybe.

SAITAETGrad (Aeronautics) · Answer 5 · 2017-06-02 23:37:10

Certification Basis is the same. The steep curve intercepts the main gear at 2.96g. I would think RB is right...landing load. For interest:

rb1957 (Aerospace) · Answer 6 · 2017-06-05 12:23:59

those are pretty extreme g loads (>9) ... is this commercial, or military ?

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