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Look at this photo of some PreciseFlight© speed brakes: PreciseFlight Speedbrakes installed on a Cessna 180

On the website it is claimed that these little red plates can double drag and descent rates. I cannot believe that. Can this be true, and if yes, can you explain please how these small things can double the drag ?

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  • $\begingroup$ What about these speedbrakes, essentially big holes? Same manufacturer, same doubling of drag/descent rate. $\endgroup$
    – mins
    Nov 12, 2021 at 16:58
  • $\begingroup$ @mins surely this isn't serious ? $\endgroup$
    – TheEagle
    Nov 12, 2021 at 19:29
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    $\begingroup$ @Programmer It is serious. And don’t call him/her Shirley. $\endgroup$
    – StephenS
    Nov 13, 2021 at 17:14

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Essentially, they are selling a spoiler.

The way they can claim "doubling the drag" may be as follows:

The device spoils lift, thereby reducing lift cofficient. This results in the aircraft either having to fly faster to make the same lift, or at a higher AoA. Analysis of airfoil drag polars do show a sharp reduction of Lift/Drag ratio as AoA increases. Increasing speed will also increase drag exponentially.

The spoiler is not to be confused with flaps, which increase lift coefficient, allowing the aircraft to fly slower while maintaining lift (higher flap settings are generally used strictly to increase drag), making them useful for landing approaches.

While spoilers are valuable on extremely low rate of descent aircraft such as gliders, benefits of higher approach speeds are dubious for powered recreational aircraft. Some pilots may favor them over the old fashioned, but highly effective, forward slip.

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  • $\begingroup$ Oh yes, the reduction in lift seems plausible, thank you ! +1 $\endgroup$
    – TheEagle
    Nov 12, 2021 at 2:03
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    $\begingroup$ Also note that the Mooney's equivalent flat plate area is only 400 sq in, so the speed brakes are easily most of the way there on projected surface area alone. $\endgroup$
    – John K
    Nov 12, 2021 at 2:36
  • $\begingroup$ One can see how jet pilots might like them, adding drag rather than changing throttle. Need to know their effect on the stall speed of the aircraft. $\endgroup$ Nov 12, 2021 at 12:13
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    $\begingroup$ "The device spoils lift", if it spoiled lift it would be spoiler, not a speedbrake. If they stalled the wing on this small width, it would be a limited effect. I rather think the holes create large eddies, an efficient source of drag. $\endgroup$
    – mins
    Nov 12, 2021 at 17:10
  • $\begingroup$ @mins this device came out more than 20 years ago. Some pilots liked it because it helped stick fast landings. My preference is a tail speedbrake, like the F-86 Sabre has. $\endgroup$ Nov 12, 2021 at 18:26

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