Elevator Weldment Interference with Vertical Stab
Posted: Thu May 07, 2020 4:23 pmSome builders may encounter interference between the elevator joiner weldment and the vertical tail's aft spar. This problem is not noticed when initially fitting the horizontal tail, but becomes apparent when the elevator and vertical stab are added in later steps. It shows itself as a rubbing or binding between the welded rib on the elevator joiner against the vertical stab, preventing full up-elevator travel.
1) What went wrong, and why?
The inability to achieve full up-travel is due to insufficient spacing between the elevator joiner weldment (aka "weldment") and the vertical stabilizer main spar (part # SNX-T09-01, aka "stab spar"). This is caused by two factors: 1) the entire horizontal stab could be located too far aft on the fuselage, and/or 2) The weldment count be riveted too close to the hingeline edge of the elevator root ribs. Both these factors will tend to decrease the gap, and without sufficient gap the weldment will bind on the stab spar.
2) What were the shortcomings, and why did they exist?
The elevator weldment is fit to the elevator root ribs using plans sheet SNX-T06, and the instructions for aligning the weldment are to align it with the top and forward edges. Builders can inadvertently position the weldment further aft than necessary. Furthermore, when the root ribs are fit to the elevator skins, the root ribs may not be tight against the hingeline edge of the skin. The rivets will hold the ribs firmly in position despite this, but the effect is to place the weldment further aft than ideal.
Additionally, the horizontal stabilizer is fitted to the fuselage longerons on plans sheet SNX-T01, which instructs the builder to align the horizontal stab a specific distance (see View A).
This should theoretically place the pre-drilled mounting holes in the forward horizontal stab spar nicely centered on the fuselage longerons. However, because the mounting holes are pr-drilled, it's not possible to adjust the fore-aft position of the stab further forward to help achieve more clearance for the weldment. If these holes were not pre-drilled, the entire stab could go further forward to create better clearance.
3) What actions were taken to fix the problem?
If a problem with binding is encountered, there are two possible fixes: grind the weldment to remove excess material, or relocate the weldment with regards to the elevator root ribs. If the stab has already been drilled to the fuselage longerons, it is not practical to move the stab further forward to eliminate binding the weldment.
Grinding the weldment is possible, within reason. The elevator drive horn is welded onto the joiner with a large weld fillet, and that fillet can be thinned down somewhat without compromising the strength of the part. Consult Sonex Aircraft tech support for specific details.
The entire weldment can be repositioned on the elevator root ribs such that it is further forward (towards the hingeline) This will require removing the old root ribs and replacing with new ribs. The weldment can be reused, and the new ribs can be match-drilled back to the skins using the existing skin holes.
4) What preventive actions should be taken into account for future efforts?
For scratch builders that do not have pre-located holes in the stab spar, it is recommended to not drill the mounting holes in SNX-T06-03 when fabricating this part. Instead, wait until the stab is fit to the fuselage longerons in the tail-rigging phase. Clamp the horizontal stab into place without drilling any holes and evaluate the fit of the elevator and vertical stab spar for binding. Should binding be present, the entire stab could be moved forward as needed to eliminate the binding.
For horizontal stabs with pre-drilled mounting holes, all users should attempt to locate the stab as far forward as possible, while still maintaining proper edge distance on the holes thru the fuselage longerons. There is typically enough edge distance to slide the stab forward or aft by up to an inch before edge distance is compromised, so understanding this allows the builder to place the stab as far forward as possible. This dynamic is not well documents in the plans.
5) What overall recommendations should be made for future projects?
In general, mounting holes and edge distance concerns can affect many places of the plans. If possible, wait to drill pilot holes if there is a possibility that doing so may compromise edge distance. Instead, leave pilot holes undrilled until the parts can be temporarily fitted and the edge space of all parts can be evaluated.
Key words:
elevator travel, elevator horn, control horn, horizontal stabilizer placement, elevator alignment, vertical stabilizer contact, up elevator travel, binding, rubbing
1) What went wrong, and why?
The inability to achieve full up-travel is due to insufficient spacing between the elevator joiner weldment (aka "weldment") and the vertical stabilizer main spar (part # SNX-T09-01, aka "stab spar"). This is caused by two factors: 1) the entire horizontal stab could be located too far aft on the fuselage, and/or 2) The weldment count be riveted too close to the hingeline edge of the elevator root ribs. Both these factors will tend to decrease the gap, and without sufficient gap the weldment will bind on the stab spar.
2) What were the shortcomings, and why did they exist?
The elevator weldment is fit to the elevator root ribs using plans sheet SNX-T06, and the instructions for aligning the weldment are to align it with the top and forward edges. Builders can inadvertently position the weldment further aft than necessary. Furthermore, when the root ribs are fit to the elevator skins, the root ribs may not be tight against the hingeline edge of the skin. The rivets will hold the ribs firmly in position despite this, but the effect is to place the weldment further aft than ideal.
Additionally, the horizontal stabilizer is fitted to the fuselage longerons on plans sheet SNX-T01, which instructs the builder to align the horizontal stab a specific distance (see View A).
This should theoretically place the pre-drilled mounting holes in the forward horizontal stab spar nicely centered on the fuselage longerons. However, because the mounting holes are pr-drilled, it's not possible to adjust the fore-aft position of the stab further forward to help achieve more clearance for the weldment. If these holes were not pre-drilled, the entire stab could go further forward to create better clearance.
3) What actions were taken to fix the problem?
If a problem with binding is encountered, there are two possible fixes: grind the weldment to remove excess material, or relocate the weldment with regards to the elevator root ribs. If the stab has already been drilled to the fuselage longerons, it is not practical to move the stab further forward to eliminate binding the weldment.
Grinding the weldment is possible, within reason. The elevator drive horn is welded onto the joiner with a large weld fillet, and that fillet can be thinned down somewhat without compromising the strength of the part. Consult Sonex Aircraft tech support for specific details.
The entire weldment can be repositioned on the elevator root ribs such that it is further forward (towards the hingeline) This will require removing the old root ribs and replacing with new ribs. The weldment can be reused, and the new ribs can be match-drilled back to the skins using the existing skin holes.
4) What preventive actions should be taken into account for future efforts?
For scratch builders that do not have pre-located holes in the stab spar, it is recommended to not drill the mounting holes in SNX-T06-03 when fabricating this part. Instead, wait until the stab is fit to the fuselage longerons in the tail-rigging phase. Clamp the horizontal stab into place without drilling any holes and evaluate the fit of the elevator and vertical stab spar for binding. Should binding be present, the entire stab could be moved forward as needed to eliminate the binding.
For horizontal stabs with pre-drilled mounting holes, all users should attempt to locate the stab as far forward as possible, while still maintaining proper edge distance on the holes thru the fuselage longerons. There is typically enough edge distance to slide the stab forward or aft by up to an inch before edge distance is compromised, so understanding this allows the builder to place the stab as far forward as possible. This dynamic is not well documents in the plans.
5) What overall recommendations should be made for future projects?
In general, mounting holes and edge distance concerns can affect many places of the plans. If possible, wait to drill pilot holes if there is a possibility that doing so may compromise edge distance. Instead, leave pilot holes undrilled until the parts can be temporarily fitted and the edge space of all parts can be evaluated.
Key words:
elevator travel, elevator horn, control horn, horizontal stabilizer placement, elevator alignment, vertical stabilizer contact, up elevator travel, binding, rubbing