inflight strain analysis

[shearlag]

Dear all,
Iam working on in-flight strain analysis and for some reasons unable to match (not even reasonably) the in-flight strain recorded by the strain gauges. Assuming that the strain gauges are bonded properly and the inflight strain data are correct.
I have got few doubts like…
At maximum ‘g’ (normal acceleration ), maximum strain should be registered by all the strain gauges and conversely the maximum strain recorded throughout the sortie should be at maximum ‘g’ and in either case it is supposed to be the same strain gauge.

The strain gauges close to the cg of the aircraft should experience more strain…like the once put on the longerons and shear wall..

Irrespective of the maneuver, is it just ‘g’ which determines the load on the aircraft, or else pitch acceleration should be considered while evaluating strain on fuselage (as fuselage is most loaded in pull-out) & roll acceleration for wing

If any literature or papers is available kindly suggest me
Thanks in advance

with kindest regards,
shearlag

 

[carnage1]

max g for the overall aircraft will not be the highest strain on each piece the aircraft.
Take the assumptions you listed to there extreme.
is there more strain in the aircraft at 0 total g at mach 3 or is there more strain in the aircraft at .1 g near stall speed?

also 1 g acceleration straight down vs 1 g acceleration straight up, just do a simple free body diagram and you will see that your loading is totally different.

Luck is a difficult thing to verify and therefore should be tested often. - Me

 

[rb1957]

"At maximum 'g' (normal acceleration ), maximum strain should be registered by all the strain gauges ..."

i think this is a bit of a simplification, even assuming your gauges are on the wings. if it was true planes would be designed by one flight state, rather than thousands.

first, "g" is dependent on Weight ... the heavier the plane is the lower the g it produces (for a given control input or gust).

second, all sorts of things vary the airload distribution along (and across) the wing ... flaps, roll, L/E slats ...

to answer your final question, consider a roll ... the load on the wings is going to depend on Nz as much as it depends on Mx.

 

[shearlag]

sorry for late replying. Thanks carnage1 & rb1957 for your inputs.
i understand from your replies that ,three linear accelearations (nx,ny,nz & nz being predominant) & three angular accelearations (pitch acceleration, roll acceleration & yaw accelleration)are required to determine the load on the aircraft, depending on the maneuver. It is not just nz.
And as carnage1 says, load on aircraft straight up and down at same ‘g’are different . My understanding says 1g is achievable only at level flight. Am I right..? clarify plz…
And also, in one flight data ( pull out case) aircraft had done 1g to 3g in 1.7 sec . My question, does the aircraft structure respond equally fast & register the corresponding strain ? Can structure response time can be calculated..?
Thanks once again,
With kindest regards
Shearlag

 

[rb1957]

1g = straight and level flight
"does the aircraft structure respond equally fast & register the corresponding strain ?" ... no, it is more important about how long the 3g maneouvre was sustained. if you quickly pull and release the maneouvre, the structure doesn't have time to react the load. if you sustain the load for a period of time (see your 2nd question), you'll see the strain gauges achieve the 100% strains.
"Can structure response time can be calculated..?" ... not easily ! ... look at how long it takes to apply load on a full scale test (minutes !, hours ?). i guess you could plot s/gauge output and g against time and maybe you've got s/gauge output from a "steady-state" high g (test?) to show the structure's response rate. i guess you could (if you budget is big enough) model the plane with aero-elastics and/or do a GVT to quantify the structure's response rate. but i'm willing to bet you don't have either the information or the time to get it !!

is this "just" to correlate to internal loads predictions ?

 

[carnage1]

does the aircraft structure respond equally (perfectly) fast & register the corresponding strain ? No but I would assume it is lagging in the hundredths of seconds to seconds range.

Can structure response time can be calculated..?
I hope not because if it can someone will make you do it and that can't be fun (actually it can for simple structures but I don't know if you could get any useful data by the time you got done adding up the uncertainties in the calculation)

You might try analyzing some video of the type of plane you are dealing with doing some standard maneuvers and see if you can tell how long it takes for the plane to quit flexing after a maneuver.

Luck is a difficult thing to verify and therefore should be tested often. - Me

 

[GregLocock]

You will see lag between the reading in strain as the aircraft deflects. For a 747 I'd guess you might see a delay of the order of hundreds of milliseconds between different sensors located in different parts of the aircraft. The delay s strongly related to the natural frequencies of the structure.




Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[shearlag]

Thanks once again rb.., carnage & greg..
I do have GVT & MAST (Main Airframe Strength Test) results .As rb says, it almost took 45 min to load the structure and another 45 min to unload . Test was done with 10% increment in loading & it took 2-3 sec for the strain recorder to stabilize the readings on every 10% of loading. And as carnage writes ..yes, a lots of uncertainties involved in calculation of structure response time, t0 .

any further suggestions are most welcome

With kindest regards