Tuesday, May 30, 2017

Installing a “Bender Baffle” Piggybacked With Regulator Relocation Mod - Part Six

This post will cover the majority of the fabrication of the control hardware that will make the baffle plate assembly inside the air duct move by pulling a knob from inside the cockpit. To adjust the baffle position, a Bowden style cable was chosen. Decided to use an A-740 control cable that has built in friction detents … this is the same control cable that was installed for the parking for the parking brake control.
The A-740 control cable that will be installed in the cockpit to control the “Bender baffle”. The control has a 3" range of motion.

Quite a bit of consideration was given to where the control for the baffle should be located within the cockpit, taking into consideration ease of installation and ease of use. Almost immediately dismissed the idea of mounting the baffle control knob in the center console area along with the other controls because of the way the control cable would need to be run on the forward side of the firewall. Not say’ in it can’t or shouldn’t be done, but it would require multiple bends of the cable and figured it easier if a straighter approach was incorporated.

As previously mentioned, when deciding on a mounting location for the baffle control ease of installation and operation were high on the list but I also desired the control to be out of the way. The location finally decided upon to mount the baffle control knob is to the far left and under the instrument panel base. This location creates a couple of challenges but overall is straight forward. The plan is to tuck the control as far left as practical and still be able to get fingers around the knob but also forward enough so the entire control knob is under the instrument panel when the baffle is fully open (which will be the norm 95% of the time). This will eliminate the possibility of snagging it with a shoe or knee during cockpit ingress and egress. Taking a photo while holding the control in position from outside the cockpit proved very difficult, so I have doctored an older photo and added a red arrow in the photo to show the approximate mounting location for the knob.
The red arrow in this photo points to the general location the control knob for the “Bender baffle” will be located. The plan is to tuck the knob a little forward under the instrument panel so it is completely out of the way when not in use.

The above mounting location presents a couple of small challenges that fellow builders considering this location need to be cognizant of when designing the mounting bracket for the control knob. The first issue is not to drill into the edge of the  flange on the F-1202H-L canopy rib (although if one does decide to do so, it would be OK as long as the hole ends up in the center of the flange … I just opted to avoid it altogether). The second is not to drill into the AV-50000A Control Module. Even with the mentioned concerns, there is still plenty of room to rivet a bracket onto the bottom of the instrument panel base and avoid conflicts with the surrounding items. To avoid the flange of the F-1202H canopy rib, decided to make a mounting bracket with an offset which will allow the control knob to be mounted further to the left yet keeps the rivets used to mount the bracket clear of the flange of the flange on the F-1204H-L canopy rib. Below is a photo of the unfinished bracket that shows the offset which will allow the control knob to be further to the left.
Proof of concept bracket for the “Bender baffle” control cable made from .063" aluminum sheet. The trial fit looks promising although do plan on remove some more material from the bracket to trim it down a little more.

Prior to working on the control arm for the baffle, figured it best to establish hard point stops for the baffle so the overall rotational angles can be determined. This was done by making marks denoting the desired range of motion and using some tracing paper to make a template for cutting a piece of aluminum. Decided to make both stops from one piece of aluminum which complicated the fabrication a little. First an Avery hand seaming tool was used to make a measured bend in a scrap piece of aluminum which I discovered required 3/32" of material to make the bend. Knowing this, I made two marks on the aluminum one where I wanted the stop to be placed and another 3/32" back from the desired line which is where the seeming tool jaws were placed. The fit of the resulting bracket was spot on if I must say so myself …love it when a plan comes together the first try.
The hard stops for the “Bender baffle” are made from one piece of aluminum and meet the baffle plate perfectly square at both ends. Decided to go this route as opposed to making two stops and then needing to drill 4 holes. This bracket uses the rivet holes already drilled in the upper doubler plate. The stop is wider than it needs to be so will likely trim some more material away.
Baffle plate in the full open position on the stop.
Baffle plate in the full closed position on the stop.

Now onto making the baffle assembly swing. For the control arm, decided to use a piece of angle which I drilled a 1/2" hole into so it could be slid over the baffle’s shaft. I did play around with trying to twist a piece aluminum bar stock but think it may have been too thick because I was getting stress marks so decided to scrap that idea. Eventually, two screws will secure the control arm onto the baffle shaft but for the mockup the holes were left undrilled and the control arm material was left longer than necessary.
Drilling the 1/2" hole in angle stock that will become the control arm so it can be slipped over the baffle shaft.

Quite a while ago I did the math and figured I would need approximately a 2" control arm attached to the baffle’s pivot shaft to make a 90 degree swing using a 3" pull. However, had to use that data as a reference point because as it turns out, the baffle actually swings a little beyond 90 degrees. So I mocked up a control arm and placed a cotton swab stick at the 2" point from centerline of the baffle shaft and measured how much pull was necessary to go stop to stop. This was done using a piece of waxed cord slipped over the swab stick and using a ruler to measure the pull distance. To do that, the cord was run through the cowl hinge eyelets so the measuring point would not move. The 2" point was close … but finally decided upon using 1 7/8" which seems to nicely move the baffle from stop to stop using slightly under a 3" pull.
The mockup to determine the necessary length for the control arm. Looking closely at the photo, one can see the waxed cord attached to a cotton swab stick (hard to see but it is under the right clamp) placed 2" from center of the baffle shaft. After a little testing, decided to move the stick to the 1 7/8" point which allowed for a full range of baffle motion from stop to stop with a 3" pull on the cord.

At this point, the cowl was placed in position to verify the clearances between the baffle control arm and the Rotax 912’s valve cover, spark plug wire and the three hoses in the vicinity. Looking at the following three photos below one can see the clearances are acceptable … plus the control arm will be shortened a little more because for this trial a fair amount of excess material was left beyond where the hole will be drilled for the control cable attachment point.
Squaring up the baffle control arm and the cotton swab stick for a range of motion test.
Baffle control arm position with the baffle in the fully open position.
Baffle control arm position with the baffle in the fully closed position.

Satisfied the control arm created no interference with nearby objects throughout its full range of motion, the next step was to fabricate a mounting pad that will become the control cable’s mounting point. Once made, the mounting pad was temporarily taped in position to check the overall fit. The “B” nut for the cable end was placed on top of the pad just to verify there were still no clearance issues, even though it is actually higher than what the final position will be when the “B” nut is placed in the mounting hole that will be drilled in the pad.
The mounting pad for the control cable temporarily taped onto the control arm for a test fit. The “B” nut for the end of the control cable was just set on the pad upside down to verify clearances. Later a 3/16" hole will be drilled into the pad at a point 1 7/8" from the center of the baffle shaft so the “B” nut can be inserted into the hole and secured with a cotter pin.

At this point the mounting pad was riveted onto the control arm. Of note: The pad for the “B” nut mounting is still larger than it needs to be. This was done on purpose because the cotter pin or quick release clip used to secure the shaft of the “B” nut will also need to have enough clearance so it does not hit the body of the control arm as it swings through its range of motion.

Return from the future: The mounting pad on the control arm shown below was  miss-drilled  and needed to be remade ... in doing so, the design was changed to a more simplistic version. The new design and reason for the miss-drilled hole are covered in the post  Part 7.
Apologize for the poor lighting it was almost dark when this photo was taken ... would have used the flash, but the camera battery was almost dead. Completed control arm with the mounting pad riveted in place. The mounting pad will be trimmed down more based on the final location of the mounting hole and cotter pin clearances.

The next work session will involve mounting the control knob bracket and securing the Bowden cable.