> The spoilers on an airplane wing will only increase drag if they're >> deployed >> beyond some critical threshold. Certainly if a spoiler goes to 30 degrees >> as a speedbrake or 60 degrees as a ground spoiler, there will be a lot of >> drag. But a big reason for using spoilers (rather than more aggressive >> aileron deflections) for roll control is because a few degrees of spoiler >> will reduce lift on that wing and induce a bank, without the additional >> drag >> you'd get from additional aileron deflection. >> > > We place pieces of tape on the upper surfaces of wings to trip the boundary > layers, and even those pieces of tape can raise the drag measurably. Tape! > A spoiler lifted even a minute amount in attached flow will cause the flow > downstream to detach, period. And we don't spend thousands of hours and > millions of dollars eliminating separated flow from aircraft for our health. > Every tiny little bit of separated flow causes huge amounts of drag. When > the B-47 was in use, workers had to wear special booties whenever they > walked on the upper surface of the wing. Scratches could lead to separated > regions which increased drag so much the airplane's mission could be put in > jeopardy. When the so-called "laminar flow" series (the NACA 23000 series) > of airfoils was introduced, engineers were disapointed with the drag > performance of the real wings built with those airfoil sections, compared to > those tested in the wind tunnel. They later found that dirt and dead bugs > (!) were causing the flow over the real wing to separate earlier than on the > pristine wings of the wind tunnel models, increasing the drag and > eliminating the benefits of the new design. Any other aerodynamicist will > agree with me, separation dominates. > > Right, spoilers are used for roll control. And if you fly an airplane with > spoilers you'll quickly see that the airplane doesn't require much rudder > input to coordinate the turn. In fact, many airplanes with spoilers require > *no* rudder input. That's because the spoiler is increasing drag on the > downward-rolling (lower lift) wing. If it reduced drag, you would get the > same adverse yaw that you get in an airplane with ailerons. Lack of adverse > yaw is one of the side benefits of spoilers. But you don't have to take if > from me. Discussing the control system for the Boeing 707, Anderson writes, > "Spoilers are essentially flat plates that deflect upward into the flow over > the top surface of the wing, 'spoiling' that flow and, hence, decreasing > lift and increasing drag." For other quick references, see also Etkin's and > McCormick's books. > > > > To look at it another way, if >> an airplane suffers a disconnection of a spoiler actuator, the spoiler >> won't >> just lie flat on the wing, it will "float" up a few degrees. That >> position >> is where drag is minimized. >> > > That will only happen if the volume below the spoiler is vented to the > lower surface or to the inside of the wing or if there is recirculating flow > over the spoiler. Otherwise, the spoiler *will* lay flat. > > > > For a well-documented automotive example of spoilers reducing drag, we >> don't >> need to look any further than my own '87-91 Porsche 928, which boasts a >> drag >> coefficient of 0.34 and a pretty obnoxious looking spoiler. Contrast that >> to the original 928 of 1978, which was aerodynamically identical aside >> from >> the lack of a spoiler, and demonstrated an unimpressive 0.41 drag >> coefficient. (Intermediate model years had a small lip spoiler, not >> unlike >> that on a Milano Verde, which reduced the drag coefficient to < .4, >> although >> I don't remember the exact value.) >> > > I can't say much about that without a picture and some analysis. Did you > make those measurements yourself are do they come from another source? > > I'm not an aerodynamicist, so all I really know about airflow over control surfaces comes from the data the aerodynamicists gave me when I was spending 40+ hours per week making decisions about spoiler vs. aileron responses to wheel inputs vs. trim inputs vs. speedbrake inputs vs. autopilot inputs. (And yes, many spoilers are subjected to positive pressure from a vented area underneath them, but I was lead to believe that even spoilers over unvented compartments could float up with a disconnected actuator, depending upon the airfoil design.)
The drag coefficients for the Porsche 928 with the different factory spoiler configurations are well documented (I didn't measure them myself). -Joe -- to be removed from alfa, see http://www.digest.net/bin/digest-subs.cgi or email "unsubscribe alfa" to [email protected]
