Aerodynamic derailment
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@Polygeekery said in Novel Bioweapon Conspiracy Thread:
@Bulb said in Novel Bioweapon Conspiracy Thread:
@Polygeekery That's keel effect. Calling it pendulum effect is a misnomer, and it has nothing to do with whether the plane is high or low wing anyway. It produces restoring force because the vertical stabilizer extends upwards from the fuselage independent of where the wings are.
You seem to be very knowledgeable on the subject so I am not trying to doubt you, but in the linked article it says that "pendulum effect" is another name for it.
In aeronautics, the keel effect (also known as the pendulum effect or pendulum stability)
I always forget some call it that way, because it's such a misnomer. It's almost exactly unlike pendulum.
In addition:
Keel effect is also called "Pendulum Effect" because a lower center of gravity increases the effect of sideways forces (above the center of gravity) in producing a rolling moment. This is because the moment arm is longer, not because of gravitational forces. A low center of gravity is like a pendulum (which has a very low center of gravity).
As you have likely noticed over the years I do not hold Wikipedia to be an infallible source so could you ELI5 how this is incorrect? In your other replies you used lots of words that I understand, but in the order you used them my eyes and mind tend to lose focus and I just stop reading.
What they say is technically correct, but very misleading
- Pendulum: the anchor point is fixed and opposed any force applied to it. Gravity pulls on the centre of mass, and since it can't get away from the anchor, it gets pulled towards right below it (except it overshoots and keeps swinging, because it's a second-order feedback). From any initial condition.
- Keel effect: Because the sideways aerodynamic force caused by the plane moving sideways—relative to the body axis—acts above centre of mass, the plane rolls away from the side-slip. Gravity and orientation play no role, only the relative motion through air matters. If the plane is banked 45° in a turn, but kept coordinated with a rudder (the nose is aligned with the motion), there will be no keel effect.
Dihedral has the same effect. In side-slip the wing into the slip sees the air coming slightly from below, increasing the angle of attack and therefore lift, while the wing away sees the air coming slightly from above, reducing the angle of attack and lift.
The keel effect is usually quite small, which is why most aircraft also have dihedral (even a C172 has a slight dihedral). Only high swept wing aircraft tend to have anhedral, because wing sweep itself has same effect as dihedral (the wing toward the slip has effectively increased span) and so does the flow around the fuselage. Together these effects are called “lateral stability”.
The same principle as keel effect is weathervaning. It makes the airplane align itself with the motion, turning into the direction of the slip. This is “directional stability”.
Since both effects are second order feedback (acceleration is proportional to displacement), oscillation called dutch roll results, which manifests as the aircraft wagging the tail. That's why planes are designed with only moderate lateral stability; otherwise the dutch roll would make very unpleasant ride. Transport aircraft additionally have “yaw damper” to counter the dutch roll as it still gets quite noticeable with the long fuselage.
Now when the plane starts turning into the side-slip due to weathervaning, the outer wing moves a bit faster while the inner wing a bit slower. This increases lift on the outside of the turn, banking the plane more into the turn, opposing lateral stability. It grows faster as the curve tightens, eventually outgrowing lateral stability and resulting in a spiral divergence (often called “graveyard spiral”, because it is often how aircraft crash if an untrained pilot enters clouds).
Between the oscillating stability at low bank angles, and spiral divergence at high bank angles, is a narrow stability region. A long tail keeps the plane in slight skid in the turn (the tail is aligned properly, but the air flow is curved, so the nose isn't) so at some point the lateral stability and tendency for spiral divergence cancel out, producing stable shallow turn. But only gliders tend to have long enough tails to reliably stay in such turn with hands off controls.
@Polygeekery said in Novel Bioweapon Conspiracy Thread:
@Bulb said in Novel Bioweapon Conspiracy Thread:
No aircraft will right itself from inversion.
Perhaps I phrased that wrong in attempting to use the extreme to illustrate a point?
I don't think there is a point here.
@Bulb said in Novel Bioweapon Conspiracy Thread:
Cessna trainer (e.g. 172) is not significantly more roll-stable then comparable low-wing trainers. It cannot be, otherwise it would constantly wag the tail in a dutch roll oscillation, which would be extremely uncomfortable.
This seems entirely counterintuitive both based on just observation in the difference in CG and my experience flying with friends. Let's say you are in a bank and you go to neutral stick, you are saying that a plane similar to a 172 would not flatten out on its own more quickly than something like a Mooney 201?
If you are in a bank and go to neutral stick, a Cessna 172 will, with neutral stick above certain bank angle, go into a spiral just like any other plane. Depending on the plane that angle might be a bit stepper or shallower, but not significantly. It's probably comparable with low wing trainers like Piper Cherokee. A Mooney will probably show more tendency for spiralling, but that's because it is a higher performance aircraft designed to be more manoeuvrable at the expense of stability, not because it is low wing.
Also note that the keel effect is lower in a high wing like Cessna 172, because the high wing moves centre of mass up, but the wing is not generating any sideways force, the fuselage and the tail do.
What about resistance to cross wind at lower altitudes?
There is no such thing as cross-wind while flying. When flying, the aircraft moves relative to the air mass and what the ground is doing somewhere down below is completely irrelevant. Cross-wind only matters when landing and a bit on take-off.
When approaching to land with cross wind, the ground moves somewhat sideways under the airplane, but because the wheels can't turn that way, the pilot, just before touching down, aligns the the plane with the ground by banking it into the wind, but using opposite rudder to prevent it from turning. That's active control; stability has little to do with it.
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re: re: Aerodynamic derailment
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@boomzilla said in Aerodynamic derailment:
re: re: Aerodynamic derailment
(The Plane Saga)
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@Luhmann said in Aerodynamic derailment:
@boomzilla said in Aerodynamic derailment:
RE: re: re: Aerodynamic derailment(The Plane Saga)
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@boomzilla Needs more emojis.
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@Bulb said in Aerodynamic derailment:
There is no such thing as cross-wind while flying. When flying, the aircraft moves relative to the air mass and what the ground is doing somewhere down below is completely irrelevant. Cross-wind only matters when landing and a bit on take-off.
what the ground is doing below has a lot of impact on your course to point B. If my destination is at 360 degrees (exactly north) and there's winds aloft from 270 at 15 knots, my nose is not going to be pointing at 360 if I want to stay on course. I still have to account for the wind to crab and keep a ground course of 360 while my airplane is on a heading of, <360. I mean, yes, a large important part of flying is landing and taking off safely, but another important part is getting where you intend to go without finding yourself several miles east of where you wanted to go because you didn't take into account a steady wind blowing you off course.
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@The_Quiet_One Yes, for navigation it matters. But we are talking about aerodynamics and for that it does not.
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This post is deleted!
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@Bulb said in Aerodynamic derailment:
increasing the angle of attack and therefore lift
Assuming the AOA doesn't increase to the point of a stall, in which case the lift would dramatically decrease.
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@boomzilla
that big RE in front can't be very aerodynamic
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@Luhmann kind of like how derailments aren't very conducive to operating trains, eh?
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@boomzilla
Can't derail if it isn't on rails!
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"Serving coffee on aircraft causes turbulence."
@Bulb said in Aerodynamic derailment:
What they say is technically correct, but very misleading
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@Bulb said in Aerodynamic derailment:
What they say is technically correct, but very misleading
First time on the Internet?
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@izzion said in Aerodynamic derailment:
@Bulb said in Aerodynamic derailment:
What they say is technically correct, but very misleading
First time on the Internet?
That's not the internet. That's Wikipedia referencing something that was probably written on paper 50 years ago by someone who should have known better. Because biologists naming things using free associations that make absolutely no sense to anybody else is just par for the course, but most physicists recognize the value of terminology that is neither ambiguous nor misleading.
