Interesting Anti-Servo Tab Question

[bdflyer]

I was having an interesting discussion with another builder (an RV guy) on the differences between various controls approaches. We were mostly bantering about different failure modes and what happens if the failure were realized. So, we got to discuss the nature of the BD-4C anti-servo trim tab on the stabilator. Here, the failure mode was the jamming of the control cable back to the cockpit (hence, the inability to change the pitch angle of the trim tab). The hypothesis is that such a failure condition would be insignificant to safe flight, as the pitch angle of the stabilator can be adjusted to overcome the non-operable trim. Not being 100 percent certain on this, any thoughts from the group?

[crusty old aviator]

I wouldn't say it would be "insignificant," but you could still fly the plane until your arms became too fatigued to override the trim induced stick forces.

[SteveC]

Jamming of the cable will only affect the trim of the plane. While this could be an issue if it jammed while at an extreme setting, the stability (anti-servo) function will not be affected, as it is driven by movement of the stabilator. If the drive rod for the tab somehow failed, the stabilator will 'snatch' to full up or full down. Definitely a problem!

[Ron Blum]

If the tab "servoes" or "anti-servoes" is merely a function of control forces to the pilot. A tab that "servoes" helps the pilot; a tab that "anti-servoes" makes the forces higher (i.e. Helio Courier). Certificated airplanes have to have 2 independent ways to control pitch in case of a jam or disconnect of one system. Yes, you can fly the airplane home on trim alone ... it's good practice to try it under a controlled situation.

A jam is bad, and a runaway is worse (jam at the end of travel). If this happens, find the appropriate speed for the failed condition and use flaps (or don't use flaps accordingly). Flaps will pitch the nose down and require more nose up elevator (guessing it is not a canard :o)

Also an anti-servo tabbed surface is more effective, as a served surface has some portion (the tab) fighting what the control surface is trying to do. In other words, the tab goes the opposite direction of the control surface.

In your BD-4 case, fly the airplane home. The forces will be lighter, and the airplane will be draggier. I doubt the jack-knifed tail will stall, but that might depend on the angle it jambs.

[bdflyer]

Thanks to all! Great discussion!

[Byron J. Covey]

.....If the drive rod for the tab somehow failed, the stabilator will 'snatch' to full up or full down. Definitely a problem!

If that were true, I would ground the airplane until a re-designed stabilator could be installed.

For an in-flight jammed actuation system, unless something strange had just been done with the trim setting, there will be a trimmed configuration (flap setting, airspeed) that could be flown without excessive stick forces. [Edit:As mentioned above by Ron.]


BJC