Re: Elevator Trim Springy Thingy Really Needed?


David J. Gall
 

Not stupid at all. That’s almost exactly what John Roncz did in designing the Long-EZ replacement canard airfoil, except he gave it a little “kick” up right at the trailing edge that helps “push” the trailing edge of the elevator “down into the airstream” so to speak. That’s all the Roncz canard needs — no spring, no sparrow strainer — just that little “reflex” (and proper mass balance weights) and the airplane can fly at low, low cruise speed with the elevator control and trim system completely disconnected (not that you’d do that on purpose, of course!).

That’s the gold standard that we should strive toward while acknowledging that the existing system “works” but (for the GU canard) is deficient and has potentially fatal failure mode(s). The LS-1 canard also has a potentially fatal failure mode if the sparrow strainers break.

David J. Gall

On Jul 29, 2022, at 12:08 PM, Sandaircraft <Snoboarders@...> wrote:

Just stupid non engineer thinking, but on the ls if the issue is from the under camber, if elevators are built without it and built symmetrical wouldnt that elevate the problem or does that change the airfoil too much?
On Jul 29, 2022, at 10:57 AM, Rick Hole via groups.io <r.hole@...> wrote:

The elevator spring and sparrow strainer work together to establish a default for elevator deflection which is somewhere within the range of sustainable flight should either fail or elevator linkage fail. The builder should consider failure modes in understanding what each does.

Relying on spring pressure alone, and the spring system fails can, and has, caused catastrophic failure of the aircraft. Sparrow strainers could potentially become attached. Elevator linkages have broken. In all cases we want the elevator to stay in a flyable state. In this event altitude can be controlled with the throttle if necessary to make an emergency landing.
Velocities have the same basic issue. I have seen builder omit the sparrow strainer or even (!) install it upside down and flown successfully that way and never seen a failure mode arise.
But it only takes once...

As for depending on autopilot to hold the elevator, remember they are not designed to continuously provide pressure, only to maintain altitude given a properly trimmed condition. I have seen this principle ignored and pilots go to unusual attitude when the autopilot is disengaged either manually or unexpectedly. It is at best embarrassing in front of passengers and at worse...

Rick Hole









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