Aerodymamics...

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posted on Jul, 8 2006 @ 01:09 PM
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Just a quick question. I saw a document about aerodymanics today. In the program I got the informmation that there are two kind of wings, negative-v (W/v not sure) wich means that the wings are bent down. Planes like fighters have these kind of wings because they are far more agile. However not so stabile, that problem is solved by computer systems. Then there are the positive-v(w) wich means that the wings are bent upwards. Passangerplanes use these kinds of wings for the stability. My question is: What makes the negative-v(w) so unstabile, and what makes positive-v(w) so stabile. if I understood the program correctly, it has something to do with the weight being shifted from the center to the tips...??

Example of negative and positive v.



[edit on 8-7-2006 by Figher Master FIN]




posted on Jul, 8 2006 @ 01:16 PM
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You can do a simple intuitive experiment to allow you too see why. Take a length of metal coathanger and bend it into a V or chevron shape. Hold it by its ends and see which way it "wants" to rotate to. The center of gravity will try to rotate lower than the ends.

The positive V is like a marble sitting inside a bowl, but the Negative V is like trying to balance a marble on the same bowl inverted or its convex surface (just an analogy).



posted on Jul, 8 2006 @ 01:45 PM
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Your V is known as dihedral - pic #1 of the airliner - and anhedral, pic #2 of the Harrier.

Dihedral is used as a stabilizing device when the CG is above the wing.
The fuselage will remain level due to the dihedral effect until a control force is input.

Same thing with anhedral for the plane in the pic.
The CG is below the wing and the fuselage remains level due to weight until a control force is input.

Some aircraft, like the F4 Phantom have a fairly flat wing with dihedral out toward the wingtips.
The horizontal stabilizer has anhedral.

Anhedral on the stabilizer because in almost all cases the stabilizer imparts a downward force and not a lifting force like most think.
Lose the horizontal stabilizer and you get a nose down pitch.

The fairly extreme anhedral on the F4 may have to do with getting the horizontal stabilizer into "clean air" much like the T tail does.

Aircraft with no dihedral such as the WW1 Spad were a bitch to fly due to the lack of leveling effect from either dihedral or low CG.

Aileron input was a continual requirement for the pilot.
It did make for a very maneuverable aircraft, but the normal biplane fighters of the era had dihedral and were about as maneuverable.



posted on Jul, 9 2006 @ 04:10 AM
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originally posted by ben91069
You can do a simple intuitive experiment to allow you too see why. Take a length of metal coathanger and bend it into a V or chevron shape. Hold it by its ends and see which way it "wants" to rotate to. The center of gravity will try to rotate lower than the ends.

The positive V is like a marble sitting inside a bowl, but the Negative V is like trying to balance a marble on the same bowl inverted or its convex surface (just an analogy).


I understand your explanation, and it is a good analogy, however it doesn't explain why it happends. Why does the CG want to be lower than the wingtips.



Originally posted by Desert Dawg
Your V is known as dihedral - pic #1 of the airliner - and anhedral, pic #2 of the Harrier.

Dihedral is used as a stabilizing device when the CG is above the wing.
The fuselage will remain level due to the dihedral effect until a control force is input.

Same thing with anhedral for the plane in the pic.
The CG is below the wing and the fuselage remains level due to weight until a control force is input.

Some aircraft, like the F4 Phantom have a fairly flat wing with dihedral out toward the wingtips.
The horizontal stabilizer has anhedral.

Anhedral on the stabilizer because in almost all cases the stabilizer imparts a downward force and not a lifting force like most think.
Lose the horizontal stabilizer and you get a nose down pitch.

The fairly extreme anhedral on the F4 may have to do with getting the horizontal stabilizer into "clean air" much like the T tail does.

Aircraft with no dihedral such as the WW1 Spad were a bitch to fly due to the lack of leveling effect from either dihedral or low CG.

Aileron input was a continual requirement for the pilot.
It did make for a very maneuverable aircraft, but the normal biplane fighters of the era had dihedral and were about as maneuverable.


In the 747 we are talking about a "dihedral"?? And you are saying that the CG is above the wing. Is it becasue the 747 has two floors and the upper floor is above the wing wich makes the CG to high up. And by using "dihedral" the center of gravity will change to go under the wings, wich is more stabile??

However I don't really understand the "anhedral". You said that the CG is below the wing. Shouldn't the CG be above the wing to make the plane unstabile, and the unstability would make the plane more agile??



posted on Jul, 9 2006 @ 04:50 AM
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The Phantoms wing design is due to the fact that McDonnell made a mistake when designing it, without getting technical as that isn't my style. The original design for the F-4 had the same flat wing and tail as their previous fighter, the single engine F-3 Demon. Angling the tail downwards was simple but angling the wing upwards would have required a major structural redesign at a fairly late stage in the design process so they hit upon the expedient of angling the outer wings upwards at an exaggerated angle which would give the overall net angle required without altering the wing root at all. The net dihedral of the F-4 is the angle you get if you draw a straight line from the root to the wingtip in head on view.

In contrast to this the original designs from which the TSR 2 was evolved, the English Electric P.17 and the Supermarine 571, already incorporated anhedral in their shoulder mounted wings but BAC chose to change this to a flat wing with exaggerated anhedral on the outer panels because it gave better stability and a rock steady ride at very low levels at supersonic speed. As can be seen here, in head on view, the wingtips of both the P.17 and TSR.2 are in about the same position in relation to the fuselage despite the change, this illustration also applies to the F-4 example I gave, but the other way up







[edit on 9-7-2006 by waynos]

[edit on 9/7/2006 by Mirthful Me]



posted on Jul, 9 2006 @ 05:31 AM
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I just came o think about an other question. Cargo planes like the C-5 and the C-17 have top mounted wings, this clearly puts the center of gravity (CG) below the wings wich makes it more stabile. Why does the 747 have low moounted wings insted. It would be much more stabile to have top-mounted wings right?

Looking at pictures of the C-5 you can defenetly see that it has negative-v or dihedral as you said Desert Dawg, however it has the wings top mounted. So does this give the plane both manoverability and stability?



[edit on 9-7-2006 by Figher Master FIN]



posted on Jul, 9 2006 @ 07:43 AM
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They use anhedral wings in combination with swept wings to bring back maneuverability in the lateral direction.

Maneuverability = destabilized.

When you have a swept wing it causes more lateral stability, so if you go into a roll the dipping wing will have more lift than the trailing wing (higher wing) and that causes a rolling moment that tries to correct the plane back to level (i.e. less maneuverability). This is caused by an upwash on the dropping wing and a downwash on the rising wing (due to the sideslip component of the velocity vector).

If you combine a dihedral wing configuration with this it gets even more stable because you are increasing the upwash on the dropping wing and increasing the downwash (decreasing the lift) on the rising wing.

So the anhedral diminishes the lateral stability of the swept wing configuration by causing a compensating downwash on the dropping wing.

I've made a little cartoon that I hope illustrates this well enough. (pardon my rudimentary sketching)




posted on Jul, 9 2006 @ 10:02 AM
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Can I express me with a simple smily.--->


I do not understanad this sentence "This is caused by an upwash on the dropping wing and a downwash on the rising wing (due to the sideslip component of the velocity vector)."

And you mentioned that if you combine dihedral wings with an anhedral swept wing th plane will be even more stabile, but how can you use both anhedral and dihedral wings at the same time. Is that what the C-5 is all about?

Mod Edit: Removed Quote Of Entire Previous Post.

[edit on 9/7/2006 by Mirthful Me]



posted on Jul, 9 2006 @ 10:12 AM
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Okay, as you start into a roll (banking turn) you get slip (sideways movement) which causes two things:

1. The dropping wing will experienced a faster airflow than the lifting wing, so you get increased lift on the lower wing relative to the upper wing. That causes the plane to want to level out.

2. The slip introduces a sideways velocity vector onto the wing. For the rising wing this sideways air movement will wash across the top of the wing causing a downward force vector (working against lift, therefore decreasing apparent lift on that wing). Again, these differences in lift on the two wings result in a rolling moment that wants to make the plane level out - which means it is less maneuverable (takes more control force to keep it banked).

So if your wings are angled up (as in my picture) the sideways moving air actually starts impinging on the bottom of the lower wing instead of just moving across it (even more lift on that wing) and starts impinging on the top surface of the rising wing more (even less lift on that wing) so the rolling moment to level out is even greater - even more control force to keep banking.

BUT, with an anhedral wing, the sideways air movement impinges on the TOP surface of the lower wing (a downward force on that wing) decreasing the rolling moment that wants to level the plane, so it takes less control force to stay in a bank.

No, I wasn't talking about both (you can't have both). I was just trying to show you how a dihedral angle makes a plane more stable in a banked turn, and a anhedral makes it less stable. For a fighter with swept wings you want to have anhedral so that you can stay maneuverable.

[edit on 7-9-2006 by Valhall]



posted on Jul, 9 2006 @ 10:22 AM
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Think of it this way Fin. The wing box (where the wing joins the fuselage) determines your center of gravity. By having the wing box LOW, your fuselage is going to want to roll upright constantly, and stay level, because all the weight is low. (747 and other commercial planes) By having the wing box HIGH, the plane is going to want to keep rolling when you get in a turn because the weight is so high. (C-5, C-17 etc) By having the plane want to roll all the time, you gain manuverability. The more unstable a plane is, the more manuverable it is. With a C-5 or C-17, you have so much weight in just the basic airframe, that you have to have a high wing box to make them more able to turn and just fly.

(For those that have studied this, yes I'm aware this is a wickedly simplified explanation, just trying to explain it without getting super technical)



posted on Jul, 9 2006 @ 10:23 AM
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Originally posted by Valhall
Okay, as you start into a roll (banking turn) you get slip (sideways movement) which causes two things:

1. The dropping wing will experienced a faster airflow than the lifting wing, so you get increased lift on the lower wing relative to the upper wing. That causes the plane to want to level out.

2. The slip introduces a sideways velocity vector onto the wing. For the rising wing this sideways air movement will wash across the top of the wing causing a downward force vector (working against lift, therefore decreasing apparent lift on that wing). Again, these differences in lift on the two wings result in a rolling moment that wants to make the plane level out - which means it is less maneuverable (takes more control force to keep it banked).

So if your wings are angled up (as in my picture) the sideways moving air actually starts impinging on the bottom of the lower wing instead of just moving across it (even more lift on that wing) and starts impinging on the top surface of the rising wing more (even less lift on that wing) so the rolling moment to level out is even greater - even more control force to keep banking.

BUT, with an anhedral wing, the sideways air movement impinges on the TOP surface of the lower wing (a downward force on that wing) decreasing the rolling moment that wants to level the plane, so it takes less control force to stay in a bank.

No, I wasn't talking about both (you can't have both). I was just trying to show you how a dihedral angle makes a plane more stable in a banked turn, and a anhedral makes it less stable. For a fighter with swept wings you want to have anhedral so that you can stay maneuverable.

[edit on 7-9-2006 by Valhall]


Can I express myself again with a simple smily.--->


Indeed this is pretty comlicated, but I think I do understand the basics now, I will still read this through a couple of times and return with questions if those are in place.

EDIT: For zap

Indeed zap, keep it simple due I'am not at that level yet.

[edit on 9-7-2006 by Figher Master FIN]



posted on Jul, 9 2006 @ 10:29 AM
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I think talking about manouverability is slightly misleading though (but I'm no expert myself). It is not just 'stability' in general that is affected by anhedral/dihedral, but specifically lateral stability. Being laterally unstable does not impart highter manmouverability, it merely makes the plane want to roll over onto its back while flying in a straight line, therefore shoulder mounted anhedral wings tend to dictate a larger than usual fin and rudder in ordeer to counteract this tendency. Its true that the C-5 etc are easier to turn if they are easier to bank but this is not the same thing as being 'manouverable' FIN's question in this regard higher up the thread conjures up wierd images in my mind of C-5 pilots trying to do Red Arrows impressions


For example of the shoulder wing = bigger fin look at pictures of the C-17, or even compare side views of the BAe 146 with the Embraer 190 (same size of aircraft) and look at the difference in fin area.

[edit on 9-7-2006 by waynos]



posted on Jul, 9 2006 @ 10:31 AM
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Originally posted by waynos
I think talking about manouverability is slightly misleading though (but I'm no expert myself). It is not just 'stability' in general that is affected by anhedral/dihedral, but specifically lateral stability. Being laterally unstable does not impart highter manmouverability, it merely makes the plane want to roll over onto its back while flying in a straight line, therefore shoulder mounted anhedral wings tend to dictate a larger than usual fin and rudder in ordeer to counteract this tendency. Its true that the C-5 etc are easier to turn if they are easier to bank but this is not the same thing as being 'manouverable' FIN's question in this regard higher up the thread conjures up wierd images in my mind of C-5 pilots trying to do Red Arrows impressions


Of the thread for a while, the Red-Arrows were truly outstanding. And I do see that they are famous for a reason.



posted on Jul, 9 2006 @ 10:33 AM
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You're right waynos, but I've talked to Fin in chat before, so I have an idea of where he's at as far as studying aerodynamics, and didn't want to bring in like 12 different concepts at once.


Although have you ever seen a pair of C-5s do an overhead approach?
That's something I'd love to see again. It's.....uhm......amazing I think would be a good word to see something that HUGE do an overhead break.



posted on Jul, 9 2006 @ 10:34 AM
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I saw them last Sunday, most display teams do the headlong split where they appear to just miss each other in crossing but I've only seen the Arrows do it with multiple aircraft while manoevering and looping! Amazing.

*cough* back on topic, I'll be back shortly with a couple of pictures to illustrate that last point I made above.



posted on Jul, 9 2006 @ 10:41 AM
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Lateral instability doesn't equate to wanting to flip on your back in level flight (not unless you've got one wing gone
). It equates to whether the plane wants to return to equilibrium when taken FROM level flight. The more the plane wants to return to flying level when taken from level, the less maneuverable the plane is due to lateral stability.

Your example of the humongoid tail fins is a prime example of what we're talking about here. If you need a barn door to get enough control force to keep a bank - you're dealing with a less than "highly maneuverable" design.



posted on Jul, 9 2006 @ 10:42 AM
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Here we go, here are two similar sized 80-90 seat regional aircraft. As can be seen the E190 has a low set dihedral wing which makes it more laterally stable whereas the BAe 146 has a shoulder mounted anhedral wing, the result being that the 146 needs almost double the fin area to counteract the reduced lateral stability.







Lateral instability doesn't equate to wanting to flip on your back in level flight


Maybe that wasn't the right way to put it, especially as it is a gross over-exaggeration of what I mean't. Don't worry if you are due to fly on a 146, it doesn't want to roll over all the time
I was just trying to get across that the reduced stability is in the lateral axis alone, not generally.



[edit on 9-7-2006 by waynos]



posted on Jul, 9 2006 @ 10:56 AM
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Here is another way of imparting lateral stability on a civil aircraft. The inboard section of the wing has a very pronounced dihedral , which provides the ehnanced lateral stability, and the outer section then levels out somewhat. The reason for this is that excess dihedral can actually reduce useful lift if extended right across the span and this layout gives a useful compromise. It is effectively the reverse of the solution used on the TSR-2 and F-4 that I mentioned earlier.

This example is a Citation X but the very same method is used all the way up to the 777 and A380.




posted on Jul, 9 2006 @ 11:56 AM
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Waynos: The reason for the extra camber at the wing root is that lift created there induces much less wingtip vortex drag. What you see is not really dihedral, more an increase in wing camber and twist.






OK, onto the wider topic. 3 kinds of lateral stability issue:

- roll
- slow spiral
- dutch roll

We'll leave dutch roll out as its too complicated for the moment.


Simple roll and roll stability has already been dealt with very well by Valhall.
One little factor that was missed, straightforward enough, if a wing has dihedral 3 degrees, if the aircraft is banked at 3 degrees, the lower wing is generating its lift straight up, while the higher wing is generating its lift at 6 degrees to the vertical - this difference in components will tend to rise the lower wing - meaning the roll movement will oscillate [well, it'll actually converge with damping by wing/tailplanes].


However, when in side-slip, there is also the effect of the vertical fin to be considered, it will tend to turn the aircraft into the sideslip, and if the fin is big enough [in comparison to the wings and wing di/an hedral], will induce a slow spiral [typical time period being around 100 seconds]. Some aircraft are stable, some unstable.



FIN, try here:

selair.selkirk.bc.ca...

[edit on 9-7-2006 by kilcoo316]



posted on Jul, 9 2006 @ 12:05 PM
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Zap since I'am 15 years old I can only count 1+1.

This brings up an other question, So If I udnerstand you zap you say that wings that are mounted up high make the plane more manouverable, (If we make this very simple) How come no fighters use top-mounted wings. (Of course there are exeptions like Saab 1050E)



These are my pictures dedicated to Waynos. (To make this expirience ultiamte you need to listen to Elvis Presly, I can't help falling in love with you
)











[edit on 9-7-2006 by Figher Master FIN]

[edit on 9-7-2006 by Figher Master FIN]





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