7.2 – Nose Gear Door Actuator Replacement

This entry is part 38 of 38 in the series 07 - Landing Gear

Early on in the build, I decided that I didn’t like the idea of closing the nose gear doors using hydraulics. There’s a lot that can go wrong there and it’s a more complicated approach.

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Hydraulic lines, cylinders, switches, etc. Yuch!

Fellow Builder Terry Miles had a nice solution that I liked.

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Another guy who had also come up with a mechanism was Ken Mishler. And he sold these.  So why reinvent the wheel?

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Unfortunately, Ken wasn’t making any at the time and didn’t know if he would. So I found a local machinist, drew up some plans and ended up with my version of the Terry Miles actuator.

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This worked very well. Went overcenter with the gear down and closed the doors when up. But it had three problems.

  1. It had a very narrow range of adjustment.  Basically, you could adjust the linkage. That’s it.
  2. Adjusted with the doors closed, the tire would rub on the door when spinning. This created a burning rubber smell in the cabin.
  3. With the gear up and doors closed, any increase in air pressure in the nose would cause the doors to open slightly. Two things caused this: a) air leaking from the nose oil cooler. and b) Air leaking into the nose through the gear doors.  Either of these would cause the “Gear Unsafe” warning light to come on.

There was a simple fix to these problems; Longer arms.  But my machinist was retired which meant that I didn’t have anyone to make me a new set.  I could have kludged on some extensions, but that’s not a good way to do things.

Then I found out that Ken was making his actuators again.  A bunch of people have installed these so I ordered one.

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Nicely built, the spring provides some give in the system so the doors stay closed even if the nose gear bounces a bit.

It also came with the rod ends to connect to the gear doors.

I used 1/4″ aluminum rod for the connecting rod between the upper and lower rod ends. First I had to determine the correct length of the base connecting rod.  So I installed the actuator in what I thought would be the best location. Then I measured the distance from the holes on the actuator to the holes on the gear door arms. Then I subtracted the length of the rod ends (to halfway the threaded part).  This gave me the length of the connecting rod.

I cut a pair that length (I think it was 3-1/2″) and started to drill the holes which would receive the rod ends. This brought me to my first, seemingly insurmountable problem. How do you drill a 5/32″ hole in the end of (what is effectively) an 8/32″ rod and be exactly in the center without wandering?  The obvious answer is a lathe. But I don’t have one nor do I have access to one.

So this is how I got around that minor barrier:

I chucked each piece of the 1/4″ rod into my drill.  Then I smoothed and flattened each end.

Next I got some 1/2″ (ID) tubing that I had laying around and cut off a piece about 3″ long.

Then I wrapped some tape around the 1/4″ rod near the end so that it just fit inside the 1/2″ tubing.

I then took a 7/16″ drill bit and clamped it in the bench vice and wrapped some masking tape around it until it just fit into the tubing.

Now I’m ready to go.  I chuck the 1/4″ rod into the drill, slide the 1/2″ tubing around it (the tubing extends past the end of the rod by about an inch), slide the end of the tubing over the 7/16″ drill bit and slowly start drilling. You just need to go a little because all you’re doing now is creating a pilot for drilling with the #21 bit.  By doing it this way, the pilot is almost exactly in the center.

Now slide the 1/2″ tubing off the rod, put a #21 bit in the vise, get the bit in the pilot that you just created and start drilling. Since you’re turning the stock instead of the bit, the hole will be perfectly centered in the rod.

Do that four more times and both end of the two connected rods are now drilled and ready for tapping the end with a 10-32 tap.

Here’s what it looked like after assembly:

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As soon as I raised the gear, I discovered the doors would close on the nose wheel. That’s because the linkage was too short. If I made them longer, the mechanism wouldn’t be overcenter when down… Unless I moved the actuator up.  So I did that (making new connecting rods wouldn’t be that much trouble). But once you move it up enough so that the mechanism is overcenter, the roller that rides on the gear leg becomes perpendicular to the leg. So now it becomes a stop. I asked Scott Swing and a couple other builders who used this mechanism how they got it overcenter and they all said they didn’t. No matter how much they tweaked it, they couldn’t get it to work properly when it was overcenter.

I decided to re-position the actuator so the doors would close properly and made stronger springs to hold the doors open. The concept of being overcenter is sound, but in reality, if the springs are not strong enough to overcome the force of the air on the doors, then they would never open far enough to go overcenter in the first place.

Next I discovered a problem that I ran into when adjusting my mechanism. I have to remove one of the rod ends to adjust the length of the linkage. This is a pain and you only can adjust it one turn at a time (which may be too much) and you can’t adjust it in position.

The next issue is that this mechanism is narrower than mine.  Because of that, the lower rod ends where beyond their allowable limits. The fix there was to rotate the rod end 90 degrees. But then the stud isn’t long enough to go through the gear door arm.

The solution to the adjustment problem is to use right hand threads at one end and left hand at the other.

Here’s what I did:

I ordered a pair of left hand threaded rod-ends for the bottom from McMaster-Carr.  I’ve never needed left hand taps before and they aren’t something you’ll find at Ace Hardware.  So I ordered those as well. Since I’ll be tapping blind holes, I ordered the set of left hand taps.  That way you’ll have the taper, plug and bottoming tap for just a couple dollars more.  I was going to order the left hand jam nuts but at McMaster you have to order 50 and it would cost about $10.  So I got six (two spares) from Spruce for about $3.

I made a new set of connecting rods with the top ends tapped for right hand threads and the bottom was tapped for left hand threads, I was almost ready to assemble everything. But there was one last step to be done. I flattened the rod in the middle to accept a 1/4″ wrench.

To attach the rod ends to the door arms, I drilled and tapped another piece of 1/4″ rod stock and cut them about 1/2″ long.  Then I drilled out the existing holes in the door arm to 1/4″, sanded the rod pieces, coated them with epoxy and inserted them in the door arm holes. Once the epoxy cured, I filed them flat.

Before screwing in the rod ends, I used some loctite to keep them from coming loose.

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I used a couple of thick washers to move the rod end forward. I was hoping that would be enough to get the mechanism overcenter. But it wasn’t.

Once I got everything assembled I didn’t like how close the actuator was to the hydraulic line coming out of the canard bulkhead.  So I ordered a close clearance 90 degree fitting and rerouted the hydraulic line.

Here’s the current setup:

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One the test flight, everything to worked fine. Except for the burning rubber smell. So I need to tweak the adjustment a bit more.

Change suggestion:

If you’re going with ANY mechanical door actuator, move the rear door arms forward about 3-4 inches.  They are where the manual has them to accommodate the hydraulic door actuator. But if you’re using a mechanical actuator, if they were just a bit forward then you could easily get the linkage overcenter.

 

 

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