Dihedral

[Ministry_Cork]

All,
A question that has come up in preperation for the ppl exams:

Wing Dihedral
A: Increases from Root to Tip
B: Decreases from Root to Tip
C: Gives Lateral Stability
D: Gives Directional Stability

My understanding is that dihedral gives lateral stability and therfor C should be correct however the correct answer has been given as A. I cannot find any reference to wing dihedral increasing from root to tip as i would assume (probably incorrectly) that the wing angle remains almost contstant from root to tip with referene to the horizontal on light aircraft? Looking at the A380 however it looks like dihedral decreases from root to tip?

Does anyone have a clear definition of Dihedral with specific reference to the anlges?


Thanks in advanace.
M

[Re-Heat]

Wikipedia has a very detailed entry.

http://en.wikipedia.org/wiki/Dihedral_(aircraft)

[Ministry_Cork]

Thanks Re-Heat,
As detailed as it is however it cannot answer the question.

[olliew]

The definition of dihedral as according to L.J. Clancys Aerodynamics is: 'The angle between the plane of each wing and the horizontal'. I would consider your question source as answer C is correct.

[Lightning Mate]

...."in preperatipon for the ppl exams"....

good grief.....

[Re-Heat]

Well, it does answer it indirectly if you read through it - you're not going to find the answer printed directly on a website anywhere!

Think about how it resolves minor upset flight and the mechanics of it doing so.

[GOLF_BRAVO_ZULU]

How about keeping it simple as per AP3456?

Vol 1, Pt 1, Sect 1, Ch 14:


73. The factors affecting static lateral stability are:
a. Wing contributions due to:
(1) Dihedral.
(2) Sweepback

The answer option A would give a dihedral angle that increased towards the tip; so the wing would curve up. Option B would be a curve in the opposite direction. You don't see many aeroplanes with either characteristic.

The question was probably written by a Quantity Surveyor with English as a second language.

[Ministry_Cork]

Thanks re-heat, but i find the following site addresses the questions in my initial post more accurately. i.e it discussus the angle of dihedral.


Airfield Models - Calculating the Dihedral of a Wing (http://www.airfieldmodels.com/information_source/math_and_science_of_model_aircraft/formulas/dihedral.htm)

[Ministry_Cork]

Thanks GBZ, thats exactly what i was thinking, and why i mentioned the A380, if you look at it head on it appears that the angle of the wing with reference to the horizontal reduces from root to tip.?

[Ministry_Cork]

Maybe a little too complex for ppl level Lightning - I suppose its really in prep for the ATPLs but need to get these out of the way first :-)

[ClippedCub]

Wings deform under one g load once airborne. On the ground, the A380 appears to sag, but aerodynamically, once airborne, under one g, the wing bows upward. That's what is felt by the airplane in operation, producing 'C'.

[Ministry_Cork]

Thansk CC, it seems that wing Dihedral is an angle raising the centerline of the wing tip above the centerline of the wing root. It can also be expressed as a measurement of length. given this definition it seems that any curve of the wing is irrelevant as the measurement is from root to tip and alonh the horizontal - a diagram for which would produce a triangle. then wingspan * sin of the angle gives the valiue for dihedral. If i come across the question in the exams i think i will go with - "Gives lateral stability"


Thanks for your input.

[wheelbrace]

I would take this question at face value - it is asking what wing dihedral is, not its effect. The wikipedia reference answers the former and (maybe unhelpfully?) expands on it to explain the latter. Look up anhedral for the reverse.

Just a thought. As for ATPL preparation, you will need a first class degree(with Hons.) in second guessing and bull**** before even contemplating opening that shiny package from BGS :)

[Ministry_Cork]

Wheelbrace - looking formward to that shiny package and yes, it will indeed be from BGS but not for 12 months or so. are you saying you would go with A or C then?

[Lightning Mate]

Ministry Cork,

Which book is giving you that answer?
I teach this stuff to ATPL and university degree level, and some of the books out there do give incorrect answers.

Dihedral is a contributing factor in static and dynamic lateral stability.

LM

[Ministry_Cork]

Lightning Mate,
It was given to me by my instructor in a sample paper so i do not know the source- i havent looked through any of the books for that specific question yet. not to drift off the subject but tere were a few ambiguous questipons in there e.g what advantge does an alternator offer over a generator? two of the possible answers were a: its lighter b: it produces power at low rpm. I answered b but was told that in fact a was correct.

We were told that there will be a few questions in the real exam like this where they could be as GBZ said earlier "probably written by a Quantity Surveyor with English as a second language." While im confident that i understand the subject i am inclined to agree and move on. but having said that i think i will make good use of the note paper in the exam if i come across such questions!

M

[Re-Heat]

It is possible for it to do either A or B depending upon design, so they are clearly not the answer. The answer is C.

[ClippedCub]

B: Decreases from Root to Tip

Is termed Anhedral.

[Re-Heat]

No, ClippedClub - it is polyhedral (though likely to be anhedral at the root, I admit...), as is A. See a 777 wing in flight for example.

[ClippedCub]

though likely to be anhedral

Based on high wing transports.:ok:

[mushingstall]

Ministry Cork,

Perhaps the question is referring to the distance between the wing and the horizontal. Assuming a constant dihedral angle would still result in the distance between the plane of the wing and the horizontal plane increasing from root to tip which would give A as the correct answer.

A rather ambigious question. I went for C initally too.

[GOLF_BRAVO_ZULU]

Oh dear. Dihedral; tips are higher than the root. Anhedral; tips are lower than the root. Polyhedral; differing degrees of dihedral or anhedral; or both as in a gull wing or cranked wing. The options A and B in the exam question imply polyhedral, as it is written.

In most simple wings, the dihedral is constant. It does not increase towards the tip. If it did it would be a positive polyhedral.

[Ministry_Cork]

GBZ - Do you agree with the following to calculate the value of dihedral for a given wing?

Wingspan = 60" (note that we will be using half the wingspan)
Dihedral angle = 5°
Dihedral measurement = Unknown
1) find the Sine of the angle:
Ø = 5°
Sine 5° = 0.087156
2) Plug in the answer above to find the opposite side of the triangle.
0.087156 = Dihedral ÷ 30"
Dihedral = 30" x 0.087156
Dihedral = 2.6" (under each tip)

[GOLF_BRAVO_ZULU]

According to my trusty Casio fx-20, yes; except the dihedral is expressed as 5 deg. The wing tip datum would, indeed be 2.615" above root datum, though. That's got to be a model!

I don't recall seeing dihedral expressed as a gradient or tip height above root; hence my earlier reference to Quantity Surveyors. I may have had a sheltered life, though.

[Lightning Mate]

Good grief...
Good grief...
Good grief...

[Airbusfreak]

the funniest thing here is that question which i recognise from my ppl days is in (or at least was in) the Irish ppl question bank which is jaa and can also come up in uk etc.. its just one of those ones where you learn the answer because somebody wrote a ****e question...

[Halfbaked_Boy]

The answer is 'C'.

[AlphaMale]

Wing Dihedral
A: Increases from Root to Tip
B: Decreases from Root to Tip
C: Gives Lateral Stability
D: Gives Directional Stability

I think A & B are there as to confuse you with washout. i.e. the angle decrease from root to tip in order to make the root of the wing stall before the tip an subsequently the ailerons aft of the wing tip leading edge.

So after disregarding A & B you’ll be left with C & D, basics of the Tec notes will tell you that the fin gives you directional stability (around the normal axis – movement is given by the rudder in a yaw motion).

So you’re only left with C and ‘Lateral Stability’ is gained by having dihedral as seen on most low wing aircraft, or adhedral as seen on some high wing aircraft like a BAe146 or a Harrier Jet. Lateral stability is to stop the aircraft from sideslip.

By having dihedral (as mentioned in the answers) the lower wing in a sideslip will experience a greater angle of attack than the raised wing thus giving greater lift and regaining control of the aircraft so to speak.

… I think that makes sense?

[AVIATOR1982]

I may be wrong here but doesn't the aircraft pitch about the lateral axis, roll about the longitudinal axis and yaw about the normal/vertical axis, dihedral therefore increases longitudinal stability not lateral stability? Therefore C can't be correct,Its been a while since i took the aero exam so like I said I may be wrong.

[AVIATOR1982]

And thinking about it all the question wants to know is that you know the difference between Anhedral and Dihedral, its one of those questions thats not worded the best. Dihedral means the tip is higher than the root, answer is A, C and D are there to trick.

[Halfbaked_Boy]

AVIATOR1982,

Bit of confusion here I think buddy!

Have a good think back to when you studied PoF, and hopefully you'll recall that -

In terminology used when discussing stability, you are referencing to the movement of an axis, not movement around an axis.

Hence lateral stability is the stability of the aeroplane in roll, i.e. when rolling the lateral axis is moving, whereas the longitudinal axis remains stationary. (Well ok, it's rolling too, bit think of what you can actually see moving.)

Lateral stability - Stability in roll.

Longitudinal stability - Stability in pitch.

I used to find it helpful to imagine the longitudinal axis like a boat. Smooth water and it's stable. Rough sea and it's unstable (longitudinally).

Hope this helps :)

[AVIATOR1982]

You are quite correct, having just looked up my notes which i should have done before hand:ugh:
Answer is still A though!!

[AlphaMale]

I'm still sticking with 'C' but due to the question not being very detailed it could very well 'A' also. As you said the wing tip is higher than the root. :ok:

I've been in a similar situation when doing PoF exam and it asked where the oil pressure sensor is situated in a dry sump engine. Knowing that you want to measure the pressure before it enters the engine it would need to be between the pump and engine. But answers were 'Outer side of the pump' or 'In-side of the pump'? Do you take it the oil pump is pumping the oil out to the engine or in to the engine?

I put the answer as In-side thinking outside would be the oil tank and filters etc. I had it wrong :ugh: The pump sucks the oil from the inside the tank and pushes it outside to the engine :(

[Halfbaked_Boy]

I'm still sticking with 'C' on this one, reason being -

The way the question is phrased (if indeed the OP has copied it word for word), if you choose answer 'A', what you're saying effectively is that dihedral increases from root to tip. Not the height of the wing above the horizontal plane, but the dihedral. So you'd end up with a wing shaped like a banana, which in essence isn't completely incorrect. However, as somebody previously mentioned, the A380, and I'm sure other aeroplanes, displays polyhedral characteristics. On the ground, fully fuelled, I'm sure there's a measureable degree of anhedral, but once the wings are load bearing, this is transformed into dihedral.

So, in essence, if you take a very broad view of the wording in the question, A and B could both be considered correct, but are opposite answers and can therefore be disregarded.

Personally I think the question should be, very simply, 'C'! Certain words or phrases in aviation jump out hand in hand to me, and 'dihedral' and 'static lateral stability' are two of them! :ok:

[debiassi]

Dihedral is the V shape formed when looking at the wings from the front so the dihedral would increase from the wing root to the tip. The opposite in aircraft wing is Anhedral and the Anhedral would decrease from wing root to wingtip.

[Pugilistic Animus]

I believe Mad(flt)Scientist expanded considerably on this subject a while ago,...but the short answer is 'helps the plane recover from side slip'=="lateral stabilty",..but in terms of pure geometry it's A V-shaped the legs on the V go-up

is this a trick question:confused:

[Halfbaked_Boy]

I can see your point, but am misunderstanding something here -

If the answer is in fact 'A', I still think it's very badly worded - the dihedral does not increase from root to tip, it is the distance of the wing segment from the horizontal plane of the aircraft that increases with increasing distance from the root (in the case of dihedral).

Dihedral (or anhedral), is simply an expression for the angle formed between the wing and the lateral axis.

Or can somebody correct me? :confused:

[Pugilistic Animus]

http://www.pprune.org/tech-log/307358-use-anahedral-lateral-stability.html?highlight=anhedral

actually this guy does it better than me:ok: but I mean 'lateral stabilty' more in ol' colloquial pilot horse hooey whereas MFS is talking like an experienced engineer:}

except further discussion of roll helix angle and acceptable/ unacceptable roll stabilty is in order:ouch::8

PA