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Split rudder solution and their effect on flutter

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Scheny

Well-Known Member
Joined
Feb 26, 2019
Messages
314
Location
Vienna, Austria
Hi,
as the Beast is designed to cruise at 220kt and topspeed should be above 260kt, I am going through great lengths to remove the likelyhood of flutter everywhere possible.

The problem we have to face is, that the engine produces around 150N of thrust at idle. The BD-5 is using a thrust attenuator that I do not like to build and as the Beast is way more efficient than the JSX-2, the idle thrust would almost be sufficient to keep it at level while idling (at pattern speed). This is also why airliners can sustain cruise speed on their descent while at idle...

Originally we evaluated this scissor type spoilers in the wing, but we already have part of the fuel tank there, so I would rather not have something electric being inside the fuel area (a wet wing construction). I like the idea from the Darkaero, where they use a split rudder as a drag device but I have serious doubts about whether it will work at high speeds. Basically (as far as I understood them) they have actuated both halves by a cable attached to each pedal. This means, you will only actuate one half rudder when flying a turn or you brake the same way as on ground by pressing both pedals, driving both halves apart.

My concerns are, that you now have a rather thin element where aero forces are acting upon and there is a spring acting against them only. So in my mind this looks like the invitation for flutter to me.

The option I came up with, is a mixer inside the tail and both halves being mounted on the same hinge line. An electric actuator presses the halves actively together and you can move them by rudder pedals as in any other plane. Then, the actuator could change the offset via the mixer and drive the halves from each other to braking position (+/-45° offset). This way, they are actively driven together when inactive and braking and steering motion is independent of each other (due to the mixer).

Another benefit would be, that you can control the airbrake with the thrust lever. I had a conversation with the engine manufacturer and they support a custom lever, where I could use a quadrant below idle for drag brake actuation. This has the additional benefit that you cannot by mistake activate drag brake while adding thrust and also complexity is reduced a lot.

Still, you have two aerodynamically unbalanced and rather thin halves. Who knows and can tell me whether this solution is viable or not?

Thanks in advance, Andreas
 
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