Systems Jar exam yesterday
 

Anyone do yesterday´s Systems Jar paper find question two in the paper to be a bit strange. Have asked around but no one seems to be able to give a definite answer to it. Any ideas?

What causes torsion on a wing?

a) Positive sweep
b) Dihedral
c) Wing tip vortices
4) Propwash

Personally I found that paper a bit of a sod, and I went for option C, purley from the fact a vortex has a twisting motion.

There is hope, even though I found it a little touch and go :).

Good Luck with your results.

Keep the chin up guys - I was convinced that I had failed my systems exam but I passed. It is a bit of a lottery but remember, if you thought a question was a bit odd then someone will probably complain to the CAA and if it is dodgy then everyone will be awarded a mark (or two if the question was worth that etc!)

Hope all goes well!

Mister Geezer,

Thanks for the support, I took three exams yesterday, and to be quite honest I have no eye dear how it went! From experience of what is assessed, there appears to be so many questions designed to trip you up, I am not sure of how many RTFQ questions that caught me out!!:)

Anyway, like I said there is hope, take care.

I would say A is the right answer.

Sweep causes the wing tip to be out of plane with the wing root.

Therefor the Cp will be away from the root so there will be a moment caused by the force through the cp and distance between root and cp in the longitudinal. Which will give a torsional force. ie rotation in the pitch sense at the wing root.

MJ

I was there (wishing I wasn't)
I put 'c' my self,
If they had put 'wash in/out' I would of gone for that, not to sure about sweep Mad-Jock but you are frightening me with your case,
but even if your wrong (and you may be right) it sounds good bulls#it!;) but thinking about it you may well be right as air moving fast outwards from underneath will cause a bending moment :( (oh B@@@@llks) where did I put those CAA exam application forms?

I went for C too. Tried to reason it and that seemed about best.:confused:

Er....... (a) and (c) and possibly (d). I think we'll appeal this.

Can anyone else who sat this paper please confirm the exact wording?

Alex Whittingham
 

GO ALEX GO!
If you ask me, the wording in the first post is about right but answer 'a' might of been 'SWEEP BACK'
I'll speak to Geoff Bull at LGU tomorrow and see if they will second
your appeal.

But how dose this work? whats the drill?

My e.mail; [email protected]

I did last months Systems exams (LGU DL student) and thought that was a bitch of a paper too. (Got 79% so jobs a goodun!). There were 7 questions on gear/wheel/brakes and 5 on magnetos. What is that all about? So go get them. Agree with Alex it could be any of the three he mentions. But what do we know, we just spent hundreds of hours reading the books!

Alex. As far as i remeber the exact wording was as it appears at the top of this thread.

I think the answer to that question is A the positive sweep for the same reason mentioned by mad_jock.
Prop wash only re-energizes airflow over the wing thus creating more lift…
But could be wrong.

As someone once said to me " you could take the text books into the exam, and still not answer the question" What chance have we got ?

D.J.

I don't think you will have much chance of an appeal

Its basic engineering

Dihedral in static flight is a stability issue for dutch roll. It moves the CP in the vertical plane not the horizontal in relation to the wing root. And we need movement in the longitudinal to cause torsion.

Wing tip vortices are a function of drag and are external to the wing ie they are in the same plane as flight. So no moment so no torsion. (yes i know there will be a moment but not in the plane we are looking at it will be in the yaw axis but will be balanced)

Propwash as said before gives increased air flow over a portion of wing. But if the cp is is in the same plane as the wing root it will not give a torsional moment. Only an increased bending moment around the roll axis

A is the only answer

MJ

I must differ.

Although the twisting moment found on swept wings, when they flex in flight, is by far the most powerful, it is not the only torsional stress applied to a wing in flight.

If you have a difference in pressures, between the top of the aerofoil section and the bottom, and their mean points of action are not co-incidental, then you will have a twisting moment about the aerodynamic center. This is more marked with a cambered aerofoil with changing alpha.

If you have large wing tip vortices (low aspect ratio) then you will have more induced downwash at the tips, and consequently a reduced local alpha.

If you have reduced the alpha at one section (the tip) and increased the alpha at another (the root) to provide the total lift required, then there must be different pressure patterns over the tip section and the root section.

Different pressure patterns, different twisting moments, and therefore a torsion stress applied to the wing. Granted, it is not as marked or powerful, as the twisting moment found with a swept wing when it flexes in flight, but it exists.

Just ask the designers that use “wing warping” for lateral control!

Fair point.

Yet another great question in the JAR question bank.

I seem to remember something similar in the OAT feedback questions and Steve Chesher saying always go for the most marked effect.

MJ

I agree that this question is somewhat dubious (pretty dreadful actually), but I'm rather surpised that most contributors have ignored the torsional effects of dihedral. Dihedral causes the wing tips to be higher than the wing roots. So any drag acting at the tips tends to twist the wings leading edge up.

If we try hard enough we can find arguments that suggest that all four options are correct to some degree. But assuming all four factors apply to a given aircraft, then wing sweep is likely to be the most dominant in causing torsional loads on the wings. As stated in the previous post, if more than one option appears to be correct, students should pick that which they think is the most correct. In this case I'd go for sweep back.

Thank God! Nobody had mentioned the answer B! Thats what I put down. I dunno if it's right but I have my theories!

Well, I most certainly respect Mr Keith Williams opinion, as I know he knows his stuff, looks as if I got another question wrong!! Dam!!:eek:

Torsion
 

Mad_jock

Have to agreee with you there, although c would cause some torsional twisting due to pressure differential on the wing surfaces its not nearly as high as the sweep.

So in the eyes of the CAA, i think Alex you pointed this out to me once, whats the most appropriate.

If i remember correctly though this is most marked at lower speeds.?

If its wrong shoot me, its been 18 months since i did mine, if the plane aint stalling its still flying, besides im not getting much sweep on the PA44

:D