I'm kinda sorry that I brought this up. You've already decided to use blade type airbrakes and two of the designers that we know of who used the split flap type of airbrake abandoned it in favor schempp-Hirth blades. A third example would be the Me-163 but both the type of wing and the operating requirements are different so the fact that it worked for that airplane isn't relevant. But since I opened this can of worms I should explain myself. I mentioned the similarity to a stepped airfoil yesterday. The attachment and
this web page should explane what I meant by that. The vortex that forms behind a step fills in the airfoil shape and is able to expand or contract with changes in speed and attitude. Somehow this makes for a very stable wing (although I don't see how a wing that relies on separation could be very efficient but the wind tunnel testing shows that some step configurations can improve L/D in a
very narrow range). For the most part a step acts like a poorly designed drag brake or spoiler with a lot of lag. The best steps act like this poor spoiler with a permanent lag period. I know that Orion understands what spoiler lag is but I wouldn't be surprised if Topaze has heard that myth that I've heard from glider guiders that lag is from the spoiler being buried in the boundary layer and therefor can't be effective until it lifts up above the BL. What actually happens is that the BL separates from the free edge of the flap and becomes a free shear layer. As free shear layers are wont to do it curves toward the slower side and rolls up into a startup vortex. After a set period which can be predicted with the Strouhal number the vortex bursts and the spoiler becomes effective. There's
a beautiful example of this startup vortex at 1:22 of this 3:04 minute video on Youtube. The hollow behind a step provides a haven where the vortex can settle and persist. I know it works this way for low AR and, I believe, sweep but I'm still somewhat doubtful about high AR. The graphs on my web page are from an article in Journal of Aerospace Engineering, vol. 11, January 98 "Aerodynamic Performance of an Airfoil with Step-Induced Vortex for Lift Augmentation" By Fathi Finaish and Stephen Witherspoon. The link to ordering info is dead. I don't know where to order back issues now and I can't find the copy I got through the library periodicals service. Here's the abstract I scrounged from
the wayback machine
Quote:
AERODYNAMIC PERFORMANCE OF AN AIRFOIL WITH STEP-INDUCED VORTEX FOR LIFT AUGMENTATION
ABSTRACT: Physical and numerical experiments on flow developments around an NACA-0012 airfoil were conducted to explore the possibility of enhancing the airfoil's aerodynamic performance by vortex lift augmentation. The paper focuses on the effects of the separated flow and subsequent vortex formation, generated by backward-facing steps on pressure distributions and corresponding flow occurrences around the airfoil. Various step configurations are examined to determine their effect on lift and on lift-to-drag ratios. A discussion of the effects of main geometrical parameters of upper and lower surface steps on the airfoil performance, based on computational and physical flow visualization experiments, are presented. The results suggest that incorporation of backward-facing steps on the lower surface that are located at the midchord and extend back to the trailing edge with 50% depth of the airfoil chord may lead to considerable enhancements in lift coefficients and lift-to-drag ratios. The data produced may serve as a reference for future studies on the possible use of separated vortex structures in enhancing the aerodynamic or hydrodynamic performance of vehicles and structures.
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A few days ago Alan Shaw sent an article to the Atlantica mailing list from the January 14. 2008 issue of Aviation Week & Space Technology. Somebody has been granted a patent for a center body airbrake for BWB aircraft. It's basically the flap I described with some holes to prevent the startup vortex from forming. Apparently the inventors were surprised that it enhanced lift, they were just looking for an airbrake
