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Two basic types of horizontal stabilizer systems

by Warren Hubbard
Tags: aerodynamics, basic, horizontal, stabilizer, systems, types
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Warren Hubbard
#1
Feb4-14, 06:47 AM
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There are two basic types of horizontal stabilizer systems on most general aviation aircraft. Horizontal stabilizer with elevator, and or horizontal stabilator. If the horizontal stabilator system that is on most piper aircraft and some high performance fighter aircraft, then why are so many tailwheel and other aircraft designed with the horizontal stabs and elevator. Stabilator systems are supposed to be more effective and generally better by design. I am seeking opinions for an experimental aircraft that I am working on. It will be a tailwheel type aircraft.
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Travis_King
#2
Feb4-14, 09:24 AM
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I imagine it has a great deal to do with cost. Elevators do the job perfectly adequately for the flight conditions of most aircraft (which basically just try to maintain stable flight paths, rather than maneuver), so it's not worth the added cost to design them with stabilators. Elevators are also much simpler to service, so there's reduced cost and reduced risk there.
AlephZero
#3
Feb4-14, 01:48 PM
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Another issue is that stabilators tend to have no force feedback to the pilot controls (the stabillator pivots about its aerodynamic center of pressure), so either you need another component like an "anti-servo tab" to provide some feedback force to the pilot (which is a requirement for aircraft certification), and/or a computer system to maintain the aircraft stability in hands-off flight.

Elevators are intrinsically stable, as well as being simpler and probably cheaper.

Warren Hubbard
#4
Feb4-14, 02:40 PM
P: 2
Two basic types of horizontal stabilizer systems

With that said, would I be correct in assuming the anti servo tab would be a spring tab? And if that is correct, it would take someone smarter than myself to figure the spring force to provide that correct feed back pressure. Not only the design is costly, but the upkeep to insure proper function is provided could be a potential problem. The fact that piper had these on their general aviation trainer's is a surprise. In my flight instructing days I just moved them up and down on preflight to make sure they were moving in the right direction, I didn't take into account the engineering that went along with them. Thanks guys for the input.
AlephZero
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Feb4-14, 04:05 PM
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I don't know the specifics of the Cessna design, but don't confuse servo tabs and anti-servo tabs.

A servo tab reduces the pilot control force by providing "power assistance" to move the main tab. In the simplest version, the plot controls only move the servo tab at the rear of the main tab, and the aerodynamic forces on the servo tab then move the main control surface. There is a damper to stop the system oscillating, but otherwise the main control surface is free to move where it wants to.

That simple idea has the problem that the amount of servo effect varies with the air speed, and the mechanical spring of the "spring tab" reduces that variation.

An anti-servo tab works in the opposite way to increase the control force. I would imagine there is a mechanical linkage between the tab and the servo, so the servo is forced to turn through a bigger angle than the main tab in the same direction, not the opposite direction like a servo tab.

Stabiliators are lighter and provide more control force for a given size compared with an elevator - unless you move them too far and they stall, when the control force suddenly disappears!!!


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