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12:17 AM
@DanHulme gravity acts at the CG and exerts no pitch torque
 
 
7 hours later…
7:15 AM
@quietflyer the same is true of airships: it's actually the lift that's causing torque about the CG
 
 
7 hours later…
2:08 PM
@DanHulme I have to disagree with your statement there Dan. Buoyancy acts vertically upward while aerodynamic lift does not. If you adopt a method of analysis that focus on the vertical component of the lift vector, you make the error of overlooking the horizontal component of the lift vector and the roll torque that would be generated by that component if the wing is not located right at the CG.
Also your way of thinking overlooks the fact that if we make the vertical fin too large as we design the plane, we may reduce sideslip to the point where dihedral or high wing placement can't generate enough roll torque towards wings-level, and the aircraft will become spirally unstable. Actually many full-scale planes are indeed spirally unstable but enlarging the wing would typically make them more so.
make that enlarging the "fin" not wing. I can provides many outside links to this effect if it is of interest. Btw here is a related ASE question -- aviation.stackexchange.com/…
 
@TannerSwett If you were to reach up there with your giant hand and roll the airship 90° clockwise, it would now be out of balance because the gondola is on the left (looking from the tail) and there's nothing on the right to counter balance it. Gravity alone would be enough to cause it to roll back to the bottom, would it not?
 
perhaps the critical part is that buoyancy is always in the opposite direction of gravity, which makes things stable as soon as the center of mass is below the center of volume (for a fully submerged object like an airship in air).
aerodynamic lift depends on the orientation of the object, the airspeed, etc. Which means that as soon as it rotates the lift direction and torque changes as well
 
2:36 PM
@quietflyer did you mean to post a link there?
@FreeMan gravity is enough because it's providing both the lift and the weight in the airship case
 
@DanHulme Yes-- Btw here is a related ASE question -- aviation.stackexchange.com/questions/56507/…
Here's another answer to a related question aviation.stackexchange.com/questions/53437/…
 
3:08 PM
Here come some outside links re these topics--
1) web.archive.org/web/20071217231328/http://www.djaerotech.com/… - pay particular attention to the sentence that begins "The effect of the curvature of the flow acting on the fin and rudder is to yaw the model towards the outside of the turn. " By the way I disagree with the author's inclusion of "centrifugal force". There is a valid way to make essentially the same argument without invoking "centrifugal force".
2) web.archive.org/web/20080101024627/http://www.djaerotech.com/… - note the sentence "However, if you have negative spiral stability, reducing the tail area can improve it."
3)
https://www.av8n.com/how/htm/roll.html#sec-dihedral-no-slip
That's all for now
ok a few more
5) "Circling the Holighaus way" by Richard Johnson-- note reference to roll torque caused by interaction between sideslip and dihedral.
5) "Circling the Holighaus way" by Richard Johnson-- note reference to roll torque caused by interaction between sideslip and dihedral. wisoar.org/Documents/…
6) Links to articles by Blaine Beron-Rawdon-- how to optimize dihedral geometry for radio-controlled sailplanes-- these articles are ALL about the interaction between sideslip and dihedral -- rcgroups.com/forums/showpost.php?p=40143536&postcount=26
Ok gotta run now
 
3:57 PM
@DanHulme That's my thinking. I think @TannerSwett is over thinking it.
A simple experiment: take a rubber ball, glue/tape/whatever/(don't staple) a small weight to it as a gondola, float it in the tub. No matter what orientation you have the "airship" in the tub, the gondola weight will fall to the bottom as the ball floats in the water.
unless you manage to perfectly balance it on top and there's no other movement in the water
 
4:32 PM
I voted to approve this B-17 tag wiki edit but it does contain an inaccuracy. The B-17 was the backbone of the USAAF's air campaigns, but the rest of the Allies hardly used any. Canada had six for Transatlantic mail delivery; the UK had 84 but used them mostly for coastal patrol; the other ~12600 of them were used by the US.
It's a shame there's no "improve" option for tag wiki edits.
 
a new tag was created for the B-17, yet the tag has a typo, it reads b17 not b-17 -- tried to create (to replace) a b-17 tag, but the system won't accept it due to the similarity, what to do now?
 
post on meta
probably you need actual SE staff to edit the tag name
 
5:13 PM
Other option might be to remove b17, let it die, and then add b-17
 
@Machavity I'm sure you are right re the B-29 backing up; I deleted my answer and added a comment under John K's.
 
 
1 hour later…
6:49 PM
@FreeMan "Gravity alone would be enough to cause it to roll back to the bottom, would it not?" - No, it would not. Spacecraft are affected by gravity alone, and that wouldn't happen to a spacecraft. It only rolls back to the bottom because the ship is surrounded by air and affected by aerodynamic forces.
(Are they still called "aerodynamic forces" in still air? Hmmmm.)
"I think @TannerSwett is over thinking it." - I don't deny that. :D But I also think that if thinking leads you to the wrong conclusion, the solution is even more thinking, not less thinking.
 
but the force of (the Earth's) gravity on a spacecraft is significantly smaller than it is on a craft within the Earth's atmosphere, no?
#IANAP #IANAP
I am not a pilot and I am not a physicist
 
Not really... but maybe. If I remember right, the International Space Station experiences something like 90% of the gravity that we earth-dwellers experience. Or maybe 99%.
In any case, if an object is affected only by gravity and not by any other forces, then the object will not gain or lose any angular momentum. If it's not rotating, it won't begin to rotate.
(Unless different parts of the object experience different amounts of acceleration due to gravity. But this effect is insignificant for objects near Earth.)
Now, granted, I'm assuming that you're looking at the object's rotation about its own center of mass. It's actually valid to look at the rotation about any point, but you'll ultimately end up with the same answer.
 

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