13 votes
Accepted

Is airflow ever laminar in the boundary layer, near the wing leading edge?

In still air every boundary layer starts laminar. How soon it transitions to a turbulent boundary layer depends on: the local Reynolds number, the pressure gradient, wing sweep and disturbances like ...
Peter Kämpf's user avatar
10 votes
Accepted

What is the relation between an airplane's altitude and the drag it is experiencing?

Yes, it does - skin friction increases with an increasing altitude. The mechanism is linked more to temperature than to density, but the reasoning in your question is correct. There might be a few ...
Peter Kämpf's user avatar
9 votes

Is airflow ever laminar in the boundary layer, near the wing leading edge?

Reynolds number is basically the ratio between the viscous and inertial forces acting on the system. At low Reynolds numbers, the viscous force is dominant compared to the inertial force and it is the ...
aeroalias's user avatar
  • 100k
8 votes
Accepted

Does zero-lift angle of attack depend on Reynolds number?

The aerodynamic coefficients $c_l,c_d,c_m$ are in general functions of the angle of attack $\alpha$, Reynolds $Re$ and Mach number $Ma$. For your experiments, since you are operating in a very low $...
ares's user avatar
  • 614
6 votes
Accepted

How do i find a supercritical airfoil for my preliminary design?

Answer to the implicit question: Yes, the more recent airliner airfoils are indeed secret. Look for reports like this to see what is out there. Answer to question 1: Try the linked NASA Technical ...
Peter Kämpf's user avatar
6 votes
Accepted

Why does a turbulent boundary layer cause more friction drag than a laminar boundary layer?

No, a higher Reynolds number only signifies a lower ratio of viscous to inertial forces. Since you increase the Reynolds number by either increasing speed or length, both will drive up the inertial ...
Peter Kämpf's user avatar
5 votes
Accepted

How does drag change with Reynolds number?

Recall the definition of the Reynolds number: Where ρ is the density, v is the velocity, μ is the viscosity of air and l is the characteristic length. The characteristic length is typically ...
LaVolpe's user avatar
  • 1,010
5 votes

Why XFOIL predicts a lower drag coefficient at higher Re?

Aerodynamic friction is caused by viscosity. The Reynolds number tells you how big viscosity is in relation to inertial forces. A bigger Reynolds number signifies lower viscosity. This means a higher ...
Peter Kämpf's user avatar
4 votes
Accepted

Why don't we simulate similarity parameters simultaneously in the same wind tunnel?

It would be great if we could match all parameters at once. But that would require a tunnel large enough to accommodate the full size airplane and powerful enough to match the actual flight speed. The ...
Peter Kämpf's user avatar
3 votes
Accepted

Leading edge or trailing edge stall?

The Benedek 10355 clearly stalls with separation starting from the trailing edge. There is some discussion whether a laminar-to-turbulent transition due to rising pressure will always involve a ...
Peter Kämpf's user avatar
3 votes
Accepted

Is it possible that as the Reynolds number increases, a laminar bubble increases in extension?

The laminar "bubble" on the bottom is caused by the wavy contour. There is a pressure rise at about 25% on the lower side which causes the boundary layer to thicken, but the subsequent flat ...
Peter Kämpf's user avatar
3 votes

Does Lift/Drag generally improve at higher Reynolds numbers?

Yes, the boundary layer that surrounds the airfoil is known to decrease its thickness as the Reynolds number increases; its relative thickness to the characteristic length (the cord for example) ...
abcd's user avatar
  • 257
3 votes

would a dimpled heat shield reduce heat transfer during reentry?

Near as I know, the key to designing good re-entry vehicles or shielding is large flat plates, primarily because the design ensures the energy transfer from kinetic and potential energy of the craft ...
Romeo_4808N's user avatar
  • 73.5k
2 votes
Accepted

would a dimpled heat shield reduce heat transfer during reentry?

Golfballs are dimpled because the resulting turbulent boundary layer can follow the contour of the ball much better. Subsonic turbulent boundary layers have more drag than laminar ones, but fast ...
Koyovis's user avatar
  • 61.6k
2 votes

Does Lift/Drag generally improve at higher Reynolds numbers?

An intuitive way to look at it, is that an increase in Reynolds number can be interpreted as a decrease in viscosity taking all other components constant. $\mathrm{Re} = \frac{\rho u L}{\mu} = \frac{...
hrobeers's user avatar
  • 156
2 votes

Why does a turbulent boundary layer cause more friction drag than a laminar boundary layer?

Good question. The thing is in the velocity profiles for laminar and turbulent boundary layers. Lets look at the picture below. The profiles are little different. The turbulent profile is "fatter&...
Darjan's user avatar
  • 1,145
2 votes

Is the Reynolds number the same for both the airfoil and wing? If not, why/how does it change?

From the equation that defines it, you can see that its value depends on some properties of the fluid (air), the speed that an object moves through the fluid, and the length of the object. Nothing ...
Camille Goudeseune's user avatar
2 votes

What chord should I use to calculate Reynolds number?

Depends on what you are trying to accomplish. If you are comparing different aircraft, it probably doesn't matter as long as you always do it the same way. If you are designing a wing, the exposed ...
Pilothead's user avatar
  • 20k
2 votes

What chord should I use to calculate Reynolds number?

Reynolds number is a variable parameter. There are multiple definition and you can use one of the two MAC, the important thing is that you specify the reference of your calculation. Usually between ...
albiremo's user avatar
  • 491
2 votes

Why don't we simulate similarity parameters simultaneously in the same wind tunnel?

It is extremely expensive to build a wind tunnel that can be both pressurized and handle high-velocity flow. Such large-section wind tunnels do exist, but to maintain high pressure during a test run ...
niels nielsen's user avatar
1 vote

Wind turbine airfoil on low Reynolds number aircraft

Do not take the origin of an airfoil as a criterium for its selection. Judge it purely on its merits for the intended purpose. If a wind turbine airfoil gives you better L/D than any other, use it. ...
Peter Kämpf's user avatar
1 vote

At which point in scale reduction do slats and flaps become unnecessary?

Putting slats and flaps on a 10 cm model is extreme, but would add a touch of realism for the serious hobbyist. At which point in scale do slats and flaps become unnecessary? We turn to the lift ...
Robert DiGiovanni's user avatar
1 vote

Turbulent flows over 6 series airfoils

Flow at the forward stagnation point of an airfoil is laminar, because the local velocity will be zero. The boundary layer originating form the stagnation point will usually not remain laminar for ...
AEhere supports Monica's user avatar
1 vote

Why does a turbulent boundary layer cause more friction drag than a laminar boundary layer?

The basic reason is that the energy to create the turbulence comes in the first place from the forward motion of the aircraft or air vehicle.
J. Southworth's user avatar

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