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Wednesday, July 31, 2013

Landing brake - part 5

Electric actuator (14.2 hrs)

I mounted the thinner nuts from Aircraft Spruce on the speed-brake hinge, and that took care of the closing gap issue I was having, so I moved on to the next item on my list, the electric actuator.

Hinge center bolts using thinner profile nuts

The unit I purchased comes from ServoCity, the model number is HDA4-50

While I had a vague idea of how to mount it, I had no real plans, since this is not what the original design called for. Lucky for me, my good friend Mike came to the rescue with a 21 year old CSA newsletter (April 1992)! 

How could I have missed that!

In times like this, friends like Mike are better than money in the bank. 

I owe you again, BizMan!

1992 CSA article on electric landing brake

You might notice that these plans call for an actuator with 6” of travel, but I had already bought one with 4” of travel, so the measurements for how to position it (13” and 5.8”) were no longer applicable. I recalculated these distances, and came out with 12” and 4”. These would allow for 60˚ maximum opening, as well as full retraction. 

But first, I needed to build brackets on both ends of the actuator, and a plywood hard-point.

I modeled the top bracket after CS73 from the engine compartment, but I made it slightly longer because I wanted to add a way to adjust it up or down, by slotting the mounting holes.

Developing the bracket profile

Left and right brackets

After trimming

Test fit

The bottom mounts were not modified.

Bottom brackets marked and ready to be cut

Both brackets in place

Next, I went to work on the plywood hard-point.

0.25" plywood

I buried the nut-plates into recesses I routed into the back side of the plywood piece, then I floxed them in.

Routing the plywood for nut-plates

Recessed nut-plate detail

Actuator mounted on hard-point

Nut-plates floxed

In the meanwhile, I gently massaged the bracket placement in order to arrive at the prescribed 60˚ maximum opening.

Setting landing brake maximum opening limit

With all the geometry figured out, I drilled the bottom bracket holes into the speed-brake.

Bracket hinge bolt set 4" from landing brake hinge pin

The prescribed screws turned out to be too short, so I put in an order for longer ones with Aircraft Spruce.

Bottom bracket bolts are too short. Longer ones are on the way.

I then shifted my attention to the top bracket.

The top bracket hinge bolt is 12" from the landing brake hinge pin.

I mixed a new batch of flox, a bit on the dry side so that it would not run...

Flox applied to hard-point

... and attached the plywood to the front seat...

Hard-point attached to back side of the front seat. Bolts too long, shorter ones on order.

Actuator propped while curing

... finally, I cleaned the excess, radiused the edges, and let it dry overnight.

Excess flox cleaned up

The next morning I removed the actuator, and found that I had to do quite a bit of sanding to clean up stray flox, and smooth the radius up.

Hardened flox, clean up with sander needed.

Hard-point sanded and ready for fiberglassing

I ended up propping the fuselage in an unconventional way, so that I could work on the front seat while standing up. This was a huge improvement over working hunched over, or upside down.

I know it looks weird, but it worked!

Since I had to fiberglass over the top of the bolt holes, I filled them with Saran Wrap to keep the epoxy out of the holes.

Twisted Saran Wrap inserted in the hole

Excess Saran Wrap cut off to be reused in the other holes

Wrap will prevent epoxy from entering the hole

All holes plugged

I cut 5 staggered layers of BID...

5 layers of BID

... pre-pregged them...

BID getting pre-pregged

... and applied them to the plywood hard-point, finishing it with peel-ply...

Bid applied and peel-plied

With this layup curing overnight, I wanted to take a few minutes to talk about some of the features of this electric actuator...

Landing brake actuator introduction

Importance of adjustability

The next morning I drilled through the 5 layers of BID, exposing the Saran Wrap, and removed it with a scribe.

Saran Wrap being removed with a scribe

Then, I installed the actuator one last time.

Actuator in place, albeit with the wrong bolts

A view of the actuator from the back seat

As I expected, I ended up needing to adjust the length of the actuator ram again. One full turn clockwise did the trick.

Landing brake activation