Effect of the surface roughness of a wing

[MattH150197]

Homework Statement

I am doing a report on the effect of the surface roughness of a wing on the wing's performance. This was done by testing a a wing design in a wind tunnel using a smooth, intermediate and then rough surface. The test involved varying the pitch angle and tunnel power setting (velocity) for each modification. The results that were recorded were the Lift and Drag for each angle and the power setting that was used for the recording.

The Attempt at a Solution

From these i have calculated velocity of wind tunnel then through this the Reynolds No., Cl and Cd and in my discussion have done graphs for Lift vs. pitch and Drag vs. pitch. I have discussed how the roughness affects the stall angle and min and max Cl and Cd values and also how the Reynolds number increases/ decreases the emphasis that roughness has on the performance. Also plotted L/D ratio graphs, is there any key features i have not discussed that you can notice? Appreciate any help you can offer!

 

[berkeman]

Sounds good to me so far, but I'm not an expert in the area. Can you post your results? You should delete any personal information, but it would help to see your report.

Also paging our resident Aero SA @boneh3ad for his thoughts.

 

[boneh3ad]

Whether or not you missed anything ultimately boils down to how detailed your report needs to be. Ultimately, all of the relevant effects can be capture by just saying $C_D$ and $C_L$ are affected, but the truly interesting stuff is in the details of how and why those effects occur.

 

[CWatters]

Not sure if of interest but...

When I flew gliders the club had an aerobatic glider that was sensitive to rain drops on the wing. It was said that this raised the stall speed too close to the max winching speed. For that reason the wing was always dried off carefully before winch launches if wet.

 

[MattH150197]

Whether or not you missed anything ultimately boils down to how detailed your report needs to be. Ultimately, all of the relevant effects can be capture by just saying $C_D$ and $C_L$ are affected, but the truly interesting stuff is in the details of how and why those effects occur.

Very good point and for example for the attached graph i have talked about how the rough surface doesn't produce lift as efficiently as the smoother conditions however the roughness seems to prolong the stall angle and how this may be due to roughness at the leading edge of the airfoil inducing it's own vortex and energising the layer of air above the upper surface and consequently the flow remains attached further down the wing than that of the smooth conditions. However the pressure distribution recordings would have really helped for this analysis but this wasn't recorded so its hard to discuss in detail.

 

[CWatters]

Very good point and for example for the attached graph i have talked about how the rough surface doesn't produce lift as efficiently as the smoother conditions however the roughness seems to prolong the stall angle and how this may be due to roughness at the leading edge of the airfoil inducing it's own vortex and energising the layer of air above the upper surface and consequently the flow remains attached further down the wing than that of the smooth conditions. However the pressure distribution recordings would have really helped for this analysis but this wasn't recorded so its hard to discuss in detail.

Aerodynamics isn't really my thing but I know some model aircraft in the 1970s used turbulators to try and improve performance. These were typically tapes cut with pinking shears (saw tooth) stuck to the top surface ahead of the spar. Some used a thin wire mounted ahead of the leading edge of the wing. Don't know if they are still used. I think aerofoil design has moved on a lot since then.

 

[MattH150197]

Aerodynamics isn't really my thing but I know some model aircraft in the 1970s used turbulators to try and improve performance. These were typically tapes cut with pinking shears (saw tooth) stuck to the top surface ahead of the spar. Some used a thin wire mounted ahead of the leading edge of the wing. Don't know if they are still used. I think aerofoil design has moved on a lot since then.

I actually used some of this for one of my modifications and saw pretty good results! I decided to compare my results to results obtained using xflr5 for the same wing to add something to my discussion. Thanks for your help guys!

 

[boneh3ad]

Overall this seems like a pretty solid experiment, though I'd be careful with the Reynolds number trend. There may be competing effects there that aren't fully captured here.

The bottom line here is that even small roughness can be important through its effects on the boundary layer. You've alluded to that previously, though I'd be very surprised if the roughness you've used is directly introducing any meaningful quantity of vorticity given how small it is. Larger roughness elements can do that, but something as small and distributed as sandpaper usually has a different effect. What you are actually (likely) doing with increased roughness is exciting unstable modes in the boundary layer, which alters the transition point. This is going to be by far the leading order effect on all of the trends you've noted so far.

That said, I still don't know much about the report. Is it schoolwork, and if so, what level of school? How detailed (scientifically) does it need to be?

 

[MattH150197]

Overall this seems like a pretty solid experiment, though I'd be careful with the Reynolds number trend. There may be competing effects there that aren't fully captured here.

The bottom line here is that even small roughness can be important through its effects on the boundary layer. You've alluded to that previously, though I'd be very surprised if the roughness you've used is directly introducing any meaningful quantity of vorticity given how small it is. Larger roughness elements can do that, but something as small and distributed as sandpaper usually has a different effect. What you are actually (likely) doing with increased roughness is exciting unstable modes in the boundary layer, which alters the transition point. This is going to be by far the leading order effect on all of the trends you've noted so far.

That said, I still don't know much about the report. Is it schoolwork, and if so, what level of school? How detailed (scientifically) does it need to be?

Thanks this is extremely useful!

 

[boneh3ad]

If you want something additional to try in your tunnel (assuming this is a 2D airfoil, i.e. not swept), then try putting a strip of sandpaper near the leading edge and then run the same test with just a 2D strip of tape with the same height as the sandpaper in the same location.