Correct.
In a flat plate perpendicular to the flow, the B.L. is of course separated. Every time there's a B.L. separation, you can be sure that there's pressure drag as well. A B.L. separation completely changes the pressure distribution around the body.
No, the separation of the boundary layer is not necessary for pressure drag to appear.
Consider an airfoil in inviscid flow at zero AoA. As you said, in this case there's no either friction, nor pressure drag.
When you add the effects of viscosity and the boundary layer, not only you now have...
I also realized that the second answer to the OP in my first post was a mess, well I deserve it remains online as my personal wall of shame. :smile:
So I re-read the second question of the OP and now I think I understand what he was asking.
I think he's asking this: if we have a thin airfoil...
hi bone3ead,
yes, I realized the two cases are different. Let's consider the simpler example of the flat plate. In this case, the spanwise/chordwise gradients should be minimized.
I was wondering, for the experiment described above, which one between the three Re definitions, would give the...
So, suppose the following. Imagine an infinitely thin, 2D flat plate of finite chord, at a positive angle of attack. In potential flow, and adding the Kutta condition on the trailing edge, the result is positive lift and zero drag.
However: every infinitesimal force vector will be perpendicular...
Ok, meanwhile a simpler example came to my mind.
Consider a flat plate of infinite length and chord c at zero incidence. Incompressible flow and 100% turbulent B.L.
In this case, we know that, for example, the thickness of the B.L. at the trailing edge will be δ=f(Re).
Now we rotate the flat...
Suppose we have an infinite straight wing, using a given airfoil. Also, suppose for simplicity the B.L. is completely turbulent, and M<<1 (incompressible fluid).
As we know, the forces per unit length are: L=q⋅c⋅cl, D=q⋅c⋅cd, where cl and cd are the coefficients of the 2D airfoil for the given...
On an angled flat plate (AoA different than 0) in potential flow, the leading edge is a singularity point. This basically means that the velocity goes to infinity there, so you have infinity suction on an infinitely small area.
You can see the flat plate as a classic airfoil with a limit of...
Thank you all for your answers.
Of course the rigidity depends on the type of structures used. But, structure being the same, let's take a classic semi-monocoque design, with frames, stringers and stressed skin: since specific stiffness of wood is even better than aluminium, a wooden...
AFAIK, the wood used for aircraft structures should have a specific stiffness, that is specific Young's modulus and bending strength, somewhat higher than aluminium (see attached image).
If that is the case, why wood aircrafts are generally more subject to aeroelastic effects compared to...