PA-28-140 Cherokee trim tab adjustment
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PA-28-140 Cherokee trim tab adjustment
I am seeking an explanation of the operation of the Cherokee trim tab after alteration to the length of the rod connecting the trim barrel to the trim tab.
The Cherokee owner complained that he was using all of the nose up trim when landing with forward c of g and full flaps and asked if an adjustment could be made so that there was more nose up trim available. The mechanic made some checks of the rigging and then adjusted the rod connecting the trim barrel to the trim tab so that the rod was shorter which made the trim tab move down slightly.
A test flight proved that the situation was now worse, all of the nose up trim was used earlier in the approach and there was more back pressure required on final. The mechanic couldn't see how that would be possible so he shortened the rod further, moving the tab further down. The test flight showed that the situation was worse again so the owner asked that the rod be returned to the original position, i.e lengthened so that the trim tab moved up. Test flight showed that it was operating as it did originally, still ran out of trim but much better than after the two remedial adjustments.
It seemed, to all involved, that moving the tab down should reduce the requirement for nose up trim but the test flight showed the opposite. The aircraft is fitted with A.M.R&D. wingtips and stabilator tips and vortex generators on the wings.
The Cherokee owner complained that he was using all of the nose up trim when landing with forward c of g and full flaps and asked if an adjustment could be made so that there was more nose up trim available. The mechanic made some checks of the rigging and then adjusted the rod connecting the trim barrel to the trim tab so that the rod was shorter which made the trim tab move down slightly.
A test flight proved that the situation was now worse, all of the nose up trim was used earlier in the approach and there was more back pressure required on final. The mechanic couldn't see how that would be possible so he shortened the rod further, moving the tab further down. The test flight showed that the situation was worse again so the owner asked that the rod be returned to the original position, i.e lengthened so that the trim tab moved up. Test flight showed that it was operating as it did originally, still ran out of trim but much better than after the two remedial adjustments.
It seemed, to all involved, that moving the tab down should reduce the requirement for nose up trim but the test flight showed the opposite. The aircraft is fitted with A.M.R&D. wingtips and stabilator tips and vortex generators on the wings.
Last edited by uncle8; 13th May 2012 at 10:17.
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Hi uncle8,
I think you'll find it is an "anti servo trim tab" - which works in the opposite sense to a normal trim tab.
Servo tab - Wikipedia, the free encyclopedia
I think you'll find it is an "anti servo trim tab" - which works in the opposite sense to a normal trim tab.
Servo tab - Wikipedia, the free encyclopedia
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Thanks rudderrudderrat, I understand that. The anti servo tab is used as a trim tab too.
When you wind the trim tab wheel for nose up, the trim/anti servo tab moves down. That's why we thought that adjusting the rod to move the tab down would provide more nose up trim. We were wrong and I'd like to know why.
When you wind the trim tab wheel for nose up, the trim/anti servo tab moves down. That's why we thought that adjusting the rod to move the tab down would provide more nose up trim. We were wrong and I'd like to know why.
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Many years ago I worked with a Cherokee six and indeed it run out of nose up trim at forward c.g loading. The reverse effect experienced could possibly be explained by partial stalling of the stabilator as Piper call it due to the increased AoA produced by the tab adjustment.
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ONE: don't go against the aircraft manufactuer's reccomendations.
two...having flown the gammut of cherokee types I remind you that the last inch of yoke travel (nose up) requires a slight upward as well as rearward force. Also it has been noted many times that proper lubrication of the yoke mechanism should be accomplished.
three...stabilator stall also happens in the Cessna 177...they even have a slot to reduce this.
FOUR>..two ways to handle this situation in forward C/G (but within limits)...
try landing with one notch LESS flap if runway length is not a problem.
Move the CG aft slightly by putting something in the luggage area and properly securing it...something you might find useful like survival supplies.
two...having flown the gammut of cherokee types I remind you that the last inch of yoke travel (nose up) requires a slight upward as well as rearward force. Also it has been noted many times that proper lubrication of the yoke mechanism should be accomplished.
three...stabilator stall also happens in the Cessna 177...they even have a slot to reduce this.
FOUR>..two ways to handle this situation in forward C/G (but within limits)...
try landing with one notch LESS flap if runway length is not a problem.
Move the CG aft slightly by putting something in the luggage area and properly securing it...something you might find useful like survival supplies.
