Blackbird
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Older Cable-Planes And Control-Tabs

Sat Dec 22, 2007 5:11 am

I know on cable designs, the smaller earlier ones just had cables that directly connected to the control surface joint. Larger designs, and ones with greater control-power needed used control-tabs.

My question is, do the cables on control-tab airplanes connect only to the tab? Or do some connect to the control surface and some to the tabs?

Or does it vary from aircraft to aircraft?


Andrea Kent
 
rendezvous
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RE: Older Cable-Planes And Control-Tabs

Sat Dec 22, 2007 5:41 am

There are a couple of ways of doing it. There is a balance tab and a spring tab. Those are the two that spring to mind.

Balance tabs are small tabs at the rear of the control surface that are deflected by control inputs. This creates an aerodynamic force rear of the hinge line of the control surface, and therefor deflects it.

Balance tabs work fine, however are not very effective at low airspeeds. Controls become "sloppy" from what I've been told (I haven't actually flown a plane that uses balance tabs to move the control surfaces). To fix this issue, we move on to the spring tab:

If the balance tab is designed to be effective at low speeds, then you would find it overly sensitive at high speeds. For this reason, the spring tab was developed. It works on the same principle as the balance tab, however it is also attached to the control surface by a spring mechanism. You can go into the details about all the forces in different directions that this causes, however it serves the main purpose of dampening the control input at higher speed. At higher speed the spring stretches with the force and this effectively reduces the control surface deflection.

(it's been a while since I've studied/looked at this, so others can feel free to fix up mistakes!)
 
411A
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RE: Older Cable-Planes And Control-Tabs

Sun Dec 23, 2007 9:14 am



Quoting Rendezvous (Reply 1):
If the balance tab is designed to be effective at low speeds, then you would find it overly sensitive at high speeds. For this reason, the spring tab was developed. It works on the same principle as the balance tab, however it is also attached to the control surface by a spring mechanism. You can go into the details about all the forces in different directions that this causes, however it serves the main purpose of dampening the control input at higher speed. At higher speed the spring stretches with the force and this effectively reduces the control surface deflection.

Generally correct, and this was used to good effect on the DC-6B and Stratocruiser.
 
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Jetlagged
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RE: Older Cable-Planes And Control-Tabs

Sun Dec 23, 2007 3:49 pm

With balance tabs and spring tabs the control surface is moved directly by the pilot, the tabs move in the opposite direction to reduce control forces. I think the OP may have servo tabs in mind too. With this system, the control cables are only connected to the tab. However, once the servo tab reaches its endstop further cable movement will move the surface. In flight the servo tab won't normally reach its stop, as a small tab deflection will cause the surface to move in response. On ground, during full and free movement checks for example, the tabs will reach their stops and so the surfaces will move as well.
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Blackbird
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RE: Older Cable-Planes And Control-Tabs

Sun Dec 23, 2007 6:03 pm

I thought a servo tab, a control-tab, and a balance tab (not a balance panel) were all the same thing?


Andrea Kent
 
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Jetlagged
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RE: Older Cable-Planes And Control-Tabs

Sun Dec 23, 2007 9:35 pm



Quoting Blackbird (Reply 4):
I thought a servo tab, a control-tab, and a balance tab (not a balance panel) were all the same thing?

I suppose control tab is another name for servo tab. With a servo tab, the pilot has no direct control of the surface. He moves the tab, then the surface moves until its hinge moment balances the moment created by the tab. It's a position servo, hence the name. Balance tabs are different. They are usually geared to move in the opposite direction to the surface, which is moved directly by the pilot. The balance tab reduces the control hinge moment so reducing the force the pilot feels.
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
SFOMB67
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RE: Older Cable-Planes And Control-Tabs

Sun Dec 23, 2007 11:53 pm

When I worked on DC-8's, we had a different nominclature for the tabs, although the operation was pretty much per Jetlagged's discription. On the elevators, the inbd tab, actuated by the control column thru cables to a bellcrank in the tailcone which operated a pushrod thru the elevator structure to the inbd tab, was known as the control tab. A tab mounted just outboard of this tab, on the elevator trailing edge, was known as the gear tab. The gear tab was also called a boost tab. It was actuated by 2 pushrods connected to the stabilizer rear spar, which caused the tab to react in the opposite direction that the elevator was moved. Note that when the elevator was at neutral, both of these tabs were also neutral. During T/O when the pilot pulled back on the column (we will assume the elevators are now in a faired position caused by airflow), this caused the control tab to be actuated downward, pushing the elevator upward, and thusly pushing the tail of the acft down. The gear tab would also actuate in a downward direction as the elevator moved up, thus creating a boost on the elevator.
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