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Why did the Bede Bd5 have strakes on the horizontal tail?

Is it a sexy way of adding horizontal tail volume or reducing the span of the horizontal tail by increasing it's chord?

Fred
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  • Strakes on the tail are usually there to account for some unwanted tendency, like uncommanded Dutch Rolls or similar. I am looking for a clear picture of strakes and are not seeing any. Do you have a reference? https://www.airspacemag.com/history-of-flight/thrill-flying-worlds-smallest-jet-180952134/ – CrossRoads Jul 08 '19 at 17:32

1 Answers1

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They work like large vortex generators to delay flow separation on the underside when the stabilator is at high (leading edge down) angles at low speed. IIRC, the BD-5 had stabilator stalling issues during development. That's one way of doing it. Another way is to put VGs along the underside leading edge.

The Cessna Cardinal had a similar problem when it first entered service and Cessna's fix was to put LE slots in (a famous Service Bulletin known as the Cardinal Rule, with a comprehensive fleet mod campaign for the initial 500 or so aircraft), more or less for the same effect.

enter image description here

John K
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  • And I thought the Caravan was the only mess that required VGs on a tail surface! (https://aviation.stackexchange.com/a/50000/23223). – AEhere supports Monica Jul 08 '19 at 18:03
  • Why would the tail stall? Too much aspect ratio? Horiz. tail looks pretty slender. It looks to me there was not enough chord so the tail was flying outside of it's Re range. Is that correct? Or was it downwash from the wing, or other? – Fred Jul 08 '19 at 18:48
  • @Fred those sound like separate questions, you should ask them in the proper format. – AEhere supports Monica Jul 08 '19 at 20:20
  • A stabilator tends to get a little closer to stalling AOA when fully aircraft nose up than a fixed stab/elevator. Extend flap to increase the strength and angle of the downwash, add a little sideslip so that air is flowing across and down at the root, and you can exceed stall AOA at the root. The Thorp T-18 had a stabilator stalling issue with full flap and side slip and would pitch over in a very annoying way, at the worst time, so Thorp told builders to limit flap to the second last notch. I have seen T-18s with the same sort of strakes added by owners but couldn't find a pic on line. – John K Jul 08 '19 at 21:17