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While adding a slip to enhance my descent rate, I got to thinking that the airbrakes might be losing effectiveness as the slip twists them out of the relative wind. This thought arose because I did not achieve the sink rate I expected when entering a deep slip with airbrakes fully deployed.

With an eye toward formal analysis, what are slip drag and airbrake drag when controlled for beta? Does the airflow disturbance from a slip interact with that due to the airbrakes, and thus cause a change in overall drag/sink rate?

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  • $\begingroup$ You have one question in your title and one in the actual question, what's the actual question? $\endgroup$
    – GdD
    Jul 4 at 14:51
  • $\begingroup$ @GdD I don't quite agree. How do you see them as different? $\endgroup$
    – Kenn Sebesta
    Jul 4 at 14:52
  • $\begingroup$ Well I'm sure you realize from experience that there's no situation in which increasing the slip angle will actually decrease the sink rate, due to the possible effect you are mentioning. Precise in-flight explorations would be difficult due to inaccuracy of ASI during sideslip in most gliders. Are you hoping for an answer based on fluid dynamics modelling? $\endgroup$
    – quiet flyer
    Jul 4 at 14:58
  • $\begingroup$ @quietflyer I'm looking for a first-principles approach, although CFD will also be relevatory. A modern glider has a very slippery fuselage, and it's unclear to me if airbrakes work less well at the higher speeds I want for a slip. There's a lot of complex aerodynamics going on, so our intuitions are not trustworthy here. What I can say is that on a Stemme S6, slipping to land works well, airbrakes work well, but the two together don't give the cumulative effect I had expected. $\endgroup$
    – Kenn Sebesta
    Jul 4 at 15:01
  • $\begingroup$ One question is about sink rate, the other is about drag @KennSebesta. $\endgroup$
    – GdD
    Jul 4 at 15:05

1 Answer 1

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A simple evaluation of cosines will show little decrease in an airbraking surface, such as a flap, with an increase in sideslip angle.

However, after comparing the area of the entire side of the plane with that of the speedbrake one can see that total drag should be increasing.

This can be easily tested (under safe conditions) by forward slipping an aircraft with and without application of speed brakes. The drag generated by the side slip will most likely increase the rate of descent even if airbrake function is slightly degraded.

Care must be taken not to generate too much turbulence near control surfaces. Thus is why a forward slip with 30 degrees flaps is not recommended for the Cessna 172.

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  • $\begingroup$ I think there are several inherent assumptions you are making which are not supported by your argument as written: 1) that airbrakes only affect flow immediately downstream, and not spanwise flow; 2) that the effect from airbrakes stays uniform even at the higher speeds we fly when slipping; and 3) that the fuselage has any appreciable drag at high beta angles(a modern glider has an extremely small profile, and typically the vertical stab is not stalled in a slip). Any chance you could fill in any of those blanks? $\endgroup$
    – Kenn Sebesta
    Jul 5 at 5:05
  • $\begingroup$ @KennSebesta the question is about the effect of slipping on airbrake performance, not the effect of the airbrake on airflow. As for 3, turning an aircraft sideways, aka uncoordinated, creates large amounts of drag. It might help to tell us what type of aircraft is in question here. $\endgroup$
    – Robert DiGiovanni
    Jul 5 at 10:58
  • $\begingroup$ My question seeks to explore the quantitative effect, not the qualitative one. In that sense, it's not helpful to limit to one airplane. However, what got me thinking about this experience slipping a Stemme S6 vs a Citabria 7ECA, and a Grob 103 vs a Schweizer 2-33. 21st century aerodynamics vs 1950s flat-steel design! $\endgroup$
    – Kenn Sebesta
    Jul 5 at 12:26
  • $\begingroup$ @KennSebesta fair enough. One for the wind tunnels, or in the air actually flying it. But your question does raise a very interesting issue on drag coefficients of fuselages at various angles of attack (and empennage too). Quantitative data would help. If I find any I will post. $\endgroup$
    – Robert DiGiovanni
    Jul 5 at 13:13
  • $\begingroup$ Food for thought-- Bailey-Moyes Dragonfly -- en.wikipedia.org/wiki/Bailey-Moyes_Dragonfly -- no fuselage to speak of at all, yet sideslipping is an effective way to descend steeper. Why? Well, you are still dragging that big vertical fin sideways through the air. Plus the rudder is deflected. And the dihedral wing makes lots of roll torque in a sideslip, so the ailerons are deflected as well. But add a flat-sided covering to the open fuselage framework, and the drag at any given sideslip angle would surely be much higher still. $\endgroup$
    – quiet flyer
    Jul 5 at 16:59

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