Structural questions

[Shah]

Any structural engineers around. Currently taking some local Engineering papers and there's some questions which I can't really get the answers.

1) What is the maximum countersinking depth to material thickness ratio?
a) 1/3
b) 2/3
3) 3/4

2) A cantilever beam
a) no bending stress at the wing tip
b) moderate bending stress at wing tip
c) greatest bending stress at the wing tip

3) What is the main advantage of machine structures over riveted structures? a) saving in weight
b) reduce in notching effect
c) reduce in corrosion at joint

4) Bending stress of the beam
a) parallel to the axis of the beam
b) perpendicular to the axis of the beam
c) longitudinal to the axis of the beam

5) Alternate repair method to prevent knife edge to occur
a) dimpling
b) bonding


Any ideas guys?

[Genghis the Engineer]

For (1) I should look in the "aviation mechanics handbook", or some similar databook.

(2) Think about it this way. Start your bending moment diagram at the tip. Moving a nothingth of an inch towards the root, there's no force applying, over no distance, so there's no bending moment.

(3) Looking at what this means. Is it possible to machine (say) 1/16" thick wing skins? - of course not - so that's (a) out. Does machined mean that there's no sharp edges in the structure? - nope, so that's (b) out. Does a machined structure have a joint between two flat plates that can trap moisture? - nope, so the answer must be (c)

(4) If the stress can bend a beam (rather than compress, extent or twist it) surely there's only one direction it can be acting. If this isn't clear, take a plastic ruler, hold it down on one end against the top of the desk, and with your other hand hand it. Which way are you applying the stress?

(5) I think there are a few words missing here, I'm not sure I can see what the question is.

G

[Shah]

For (1) I should look in the "aviation mechanics handbook", or some similar databook.

(2) Think about it this way. Start your bending moment diagram at the tip. Moving a nothingth of an inch towards the root, there's no force applying, over no distance, so there's no bending moment.

(3) Looking at what this means. Is it possible to machine (say) 1/16" thick wing skins? - of course not - so that's (a) out. Does machined mean that there's no sharp edges in the structure? - nope, so that's (b) out. Does a machined structure have a joint between two flat plates that can trap moisture? - nope, so the answer must be (c)

(4) If the stress can bend a beam (rather than compress, extent or twist it) surely there's only one direction it can be acting. If this isn't clear, take a plastic ruler, hold it down on one end against the top of the desk, and with your other hand hand it. Which way are you applying the stress?

(5) I think there are a few words missing here, I'm not sure I can see what the question is.

G

for question nr 1 - i did look up in some mechs books, FAA books but don't seem to get a definite ans.
for nr 4 - i think it is perpendicular to the axis of the beam?
nr 3 - During Airbus course, the instructor once said that most of airbus skin structure is machine primarily to save weight so which is the best ans?
nr 5 - that's just all of it. nothing missing..

shah

[Grunf]

Shah:

1) t=1.5*td where t is the material thickness and td is max c/s depth. therefore your answer is 2/3

2) I would go with Genghis on that one.

3) I would say weight reduction, from design point view. This is a general idea when they teach you or better re-teach you when you start working for one of big manufacturers. Machined is always better then a lot of sheet metal attached with a lot of fasteners. Too heavy.

Also, not to forget, machined structure makes stress engineers much happier:)

4) Same as Genghis although I would like you to say that force (or moment) is what acts on a beam, not stress.

5) Bonding is your answer to eliminating knife edge

I hope it helps.

Cheers

[Shah]

thanks guys!

[North Stand Tier3]

For (5),its dimpling.When a sheet of material is too thin for countersinking,knife edging will occur and the hole may also open up if its way too thin.A countersunk hole should not affect the drilled hole size.The hole in this case would be dimpled using a special tool but its not a very common practice nowadays on larger a/c.Last dimpling I did was on an ATP eons ago!When you find the possibility of this happening on larger a/c skin repairs (when you have to open holes up to the next size),most manufacturers call for protruding head fasteners to be fitted as long as the repair isn't in a critical airflow area.Hope this helps!

[Grunf]

North Stand (and Shah):

Several repair methodologies - SRMs - and Liaison and Stress tools suggest bonding as repair for knife edge.

If there is a need for a public reference I would like to point to Michael (Chang Yang) Niu's book:

Airframe Stress analysis and Sizing (ISBN 962-7128-07-4), p. 277 Figure 9.1.5 (c).

This specific repair is coming from MD and Lockheed background. Boeing used to do the same.
Nowadays you do go with protruding fasteners, one oversize.

Dimpling can be used a manufacturing methodology in order to prevent knife edge from happening.

Shah:

I gave you the reference so you can use it for your test. Book is available in any bigger university library.

Regards,