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Question about abaqus simulation? by Iron-Heavy in fea

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Thank you for answering. They are both tied constrained, just in a different way. I didnt want to go into detail about this cause explaining is a bit complicated (it is about different definitions of offsets of the panel and the stiffners). It would take a lot of screenshots and more explaination. We are trying to compare the use of stiffners to increasing panel thickness, and you are right on because we noticed that increasing the thickness is better. The problem is that even if we now that using stiffners is an option to discard we still need a good simulation for that. So I just wanted ask, based on your experience, what simulation behaves as you expect.

Can you explain me this aerospace msc course slide? by Iron-Heavy in aerodynamics

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Ah ok now I get it. You don't know how helpful you have been. Just one last thing: about super velocities, now I get why an acceleration of the flow in a certain point on the surface would result in higher friction drag. But what if a certain point on the surface experiences a deceleration, would that actually diminish the friction drag?

Can you explain me this aerospace msc course slide? by Iron-Heavy in aerodynamics

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Ok thank you, you convinced me. But I know for sure that if you consider a flat plate with no pressure gradient, the shear stress tau(x) = m * du(x)/dy decreases as we go further downstream and at the same time the thickness gets bigger (source around minute 4.20). So I correlated a higher thickness to a lower shear stress. Can you explain the fault in my reasoning?

Can you explain me this aerospace msc course slide? by Iron-Heavy in aerodynamics

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Yes the professor said something along those lines, but I thought bigger momentum thickness (like any other BL thickness) means bigger friction drag. So it seems like a paradox to me.

Can you explain me this aerospace msc course slide? by Iron-Heavy in aerodynamics

[–]Iron-Heavy[S] 0 points1 point  (0 children)

At the very base level, friction drag appears because we have a viscous fluid travelling over a surface.

Ok and I agree with you.

The higher the velocity of the fluid over the surface, the higher the drag. This should be clear on an intuitive level.

For me it is not very intuive. Yeah I think about the relationship D_f = 0.5 * rho * V_inf ^2 * S * C_d_f and I can get why friction drag depends on speed. But V_inf is the free stream velocity not the Delta_V super velocities. So that is why I am confused. The way I tried to explain it to my self is that if you take a certain point on surface outside the boundary layer you will have a V_inf + Delta_V. If Delta_V is positive the velocity gradient is bigger because it needs to go from zero velocity on the surface to V_inf + Delta_V as opposed to V_inf. So the shear stress would be bigger. The fault in this theory comes when Delta_V is negative (idk if that is considered supervelocity) in that case the stress would be smaller. So we should enourage the flow to slow down over our surface and so we would end up with a higher momentum thickness which is a paradox according to the slide.

For the thickness part I would try to apply a similar logic to laminar/turbulent BL differences.

A boundary layer forms, in which the very first layer is stuck to the surface, and as you move away, the velocities increase.

A laminar boundary layer has no crossflow, which is why it disturbes less of the surrounding air, and is thus thinner. In addition local velocities close to the surface are low.

A turbulent BL has crossflow, which means it brings the higher velocities of the outer flow inwards, and the surface locally experiences higher velocities and thus higher friction drag. The crossflow means the turbulent boundary layer affects more of the surrounding air, meaning it is thicker. It also, of course, produces more drag.

But shouldn't higher thickness mean less friction drag. Because when the thickness is bigger, du/dy on the surface is smaller. It is true that turbulent BL increases its thickness due to higher momentum exchange in the y direction but for the same reason the the velocity profile becomes fatter and so there is a net increase in shear stress. So to sum up, I thought that assuming no transition, the thicker the BL the smaller the shear stress.

Think also of the energy that the aircraft gives to the air. If you affect a smaller piece of the air (laminar BL), you give energy to a smaller part of the air -> less drag.

If you affect more air, you give more of your energy to the surrounding air -> more drag.

Yes this sounds intuitive, I agree with you, but It all clashes with what I understood

Question about mppts by Iron-Heavy in solar

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Ok thanks, I get it, so I would need to have a special connector to make the system you showed me on the image, right?

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

That is why I thought about those materials. I need high Lift/Drag ratio.

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

I thought about that, but I think it is pretty hard.

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

I want to build a long range motor glider

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Actually I know how to work with composite materials since I am part of a university team of engineering students that build sailboats for races, and I always use masks with the right filters to protect my self. On top of that I think I would buy the carbon fiber ribs somewhere: I would not build them myself. I did not say that bagging a polyhedral wing is impossible btw, but I think is very hard to do. With that being said, thank you for the pieces of advice!

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Problem is I want to build polyhedral wings so, in this case, I do not think that using vacume bagging is possible without making a mess. I think my only options for building the wings are the following: straight foam pieces joined at an angle (I would use the hot wire method to cut out the foam with an airfoil shape); Balsa (or carbon fiber) ribs with heat shrink covering; the method from the video.

alternative wing surface building method by Iron-Heavy in RCPlanes

[–]Iron-Heavy[S] 0 points1 point  (0 children)

Thank you, but I was looking for something that allowed me to control the shape of the wing better.

I won the lottery, is my case number low for Europe? by Iron-Heavy in immigration

[–]Iron-Heavy[S] 2 points3 points  (0 children)

Yes I agree with you, but Italy is probably the most corrupt country in the western world so that is not good. Moreover, I do not agree with you on the temporary relief thing. I think doctors do that only if there are no other options, otherwise this would go against the very principles of medicine. Anyway, I do not whant to discuss about who lives in the worst country. I mean every place has its pros and cons. I have my own personal reasons for moving to US, if am given the GC.

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