Laminar => turbulent transition criteria 2

[kolia]

Hello

Trying to define a few aerodynamic characteristics of a combat aircraft, I am looking for a criteria to calculate the position of the laminar/turbulent transition on a delta wing (typical combat aircraft).
I am not looking for something very sophisticated, but a simple relation suitable for preliminary design studies, based for instance on the profile length Reynolds number.
Does anyone know anything that could help me ?

Thank you for your suggestions !

Best regards
Nicolas

 

[borjame]

I don't know about the outside of the aircraft but in the engine we would approximate (for simple calcs)the wing as a cylinder in cross flow for the leading edge and then flow over curved plate for the rest of the wing. Your Re# is also going to be dependent on your flight point (altitude, hot day or cold day(at low altitudes), and speed). The basic Re# equation should suffice for the cylinder and flat plate, just the characteristic length variable would need to be adjusted for cylinder or plate. An intro aero book should cover this and I would suggest double checking me, I'm a simple mech eng grunt, not an aero guy.

 

[i278]

koila,

You may be in luck: for preliminary design efforts, there are couple ways you could approach this, depending on how optimistic you are...

The standard approach is to assume that transition occurs immediately--there is no laminar flow. This isn't as pessimistic as it may seem; all the real-life things that can cause early transition (dirt, bugs, dents, bumps, scratches, waviness) tend to congregrate at leading edges of aerodynamic surfaces (where the flow starts).

Assuming entirely turbulent flow can be overly conservative if you're looking at planes where extra attention has been given to the construction of smooth surfaces, long favourable pressure gradients, and long term cleaning of surfaces. Given that, I think it would be reasonable to assume that laminar flow could be maintained along favourable pressure gradients (basically, where the object cross-section is steadily increasing, like tapered noses and wing leading edges and such) until the flow gets to the first skin joint, or row of fasteners, or other surface discontinuity. IMHO, there are very few aircraft that this approach could reasonably apply to, though. Mostly one-off, composite aircraft.

It might interest you to know that NACA tests on actual production aircraft wings gave pretty much the same drag coefficient (~.008) regardless of airfoil section. These would all be rivetted metallic structures.

I hope that's helpful, it probably isn't what you were looking for, but may be what you need.

Regards