S_Ellzey

For Valentine’s Day I received a most wonderful present from my wife, who flies jets also; a Jetcat P20. I spent some time trying to decide what to put it in. It had been a while since I designed something of my own, and hanging around Sam Snyder I felt the need to do in a balsa tree. So, I broke out one of my old designs and started to do some updates. The Crusader II flew for the first time almost 30 years ago, and over 20 years ago I started to work on a ducted fan that would have been the Crusader III. However I encountered a number of issues in the construction and ultimately abandoned it when it was about 70% complete. Since then I have drastically improved my design and construction skills, so I decided it was finally time for the Crusader III to come to life.

The construction all went well, everything fit together nicely, it had sufficient room inside for all the equipment (this is what killed the ducted fan attempt), and you could tell just by holding it that it was solid as a rock. However when it came time to finish it up and balance it something was not quite right. With the CG set where I wanted it the model would barely set on its nose gear. It was so bad that I had to move the CG forward close to an inch to get the model to taxi correctly. The model flew ok like this, but you could tell that it was nose heavy, which gave it something of a high landing speed.
After looking at the model a bit I finally figured out what went wrong. I knew where the CG should be, and placed the main wheels on the plans so that the model could not quite sit on its tail. I then drew a line straight up and told myself that is where the retracts go. Small problem; I installed the plate that the retracts attach to parallel to the wing skin rather than parallel to the center line, and since the retracts are on the forward part of the airfoil the struts point forward putting the wheels nearly an inch forward of where they should have been. I guess design and construction skills can always be improved more.

I thought of several approaches to correct the problem. Cutting up the wing and moving the gear just did not appeal to me. I could have shimmed the front of the retracts so as to level them, but that would leave the front of them setting pretty far out of the wing. What seemed to be the neatest solution was to replace the simple wire struts with a trailing arm strut that would allow the wheels to move back about an inch when on the ground, and would be perfectly straight when off the ground so that the wheels would go into the original wells.

I took a number of photos as I constructed these and would like to share them with you. This should give you an idea of what you can get done with a fairly small lathe and mill. The one shown is a Taig micro lathe. This unit has a fairly simple milling attachment that works pretty neat. I have had it about 25 years and it has been a part of pretty much every model project.

And now, on with the show.

S_Ellzey

After coming up with the design the blanks where cut for the upper strut and the trailing arm from 1” diameter 7075 aluminum rod. Each piece was put in the lathe and one end cleaned up which serves as a reference point as the part progresses.
While the parts can fit inside of a 1” cylinder, they cannot fit in the center of it. The clevises that the shock will mount to extend beyond 0.5” from the center line of the finished part. So it is necessary to hold the part off center in a four jaw chuck. A mark was made of about where the center needed to be, and the tail stock was used to help line it up.

S_Ellzey

Next, using a roughing bit, a section was turned down to the final diameter of the upper strut. Then the lathe bit was rotated to allow squaring up the end that will form the pivot joint. This is actually using the wrong edge of the bit, but it got through it without much complaining. Next the bit was turned to allow the edge to be turned that will form the lower surface of the upper shock attachment. Before removing the part from the lathe a starter bit was used to mark the center of the end of the strut for later reference. The roughing bit was used as the finishing bit since it left a nice radius in the corners that looked good.

S_Ellzey

continued

S_Ellzey

At this point it was necessary to flip the part around and work on the other end, but it needed to go back in so that the first section that was turned would still be in the center. Before being removed the part was marked relative to one of the jaws. It was then flipped over, trying to align the mark back up. Then a lathe bit was installed backwards in the tool holder, to allow a square edge going up against the part, and then the chuck was adjusted until the area that had already turned would lightly drag the back side of the bit all the way around. Once the part was realigned in the chuck the upper end of the strut could be turned to shape and then cut to the overall length. Before the part was removed it was drilled for the music wire pin that would hold it to the retract. An experienced machinist might wonder why I did not make this part in reverse order. I could have turned the upper end first and then flip the part over and used a 3 jaw chuck to hold the part centered while turning the middle and not have to go through the re-alignment process. The reason was that I was concerned that after drilling the pin hole that the end might be too flexible which might create problems turning the middle part.

S_Ellzey

At this point it was time to start the milling process. The part was clamped into the mill so that one side of the lobes could be machined off. The part did not have to be perfectly aligned. The first cut was the reference that the second cut would be made to. The flat that was created is placed against the jaw of the clamp so that the next cut would be perpendicular to the last. Each edge of the lobes where cut down to the height required. In this case only the part that would form the upper shock attachment needed to be left.

S_Ellzey

continued

S_Ellzey

The next part of the process was to cut the slot for the hinge. First the dimple that was added earlier was used to line up with the center of the part. Then with the end mill in place the part was pushed up against it. In order to set the depth of the cut a set of gauge blocks were used to set the position of the stop rod. If you look at the second photo you will see a shiny stack of disks towards the bottom of the photo that the stop rod is against. With the gauge blocks removed the mill will only cut to the desired depth.

S_Ellzey

continued

S_Ellzey

A lot of material does have to be removed to get to the final shape

S_Ellzey

The trailing arm was made in pretty much the same manner as the strut. The end was drilled and tapped for the set screw that would hold the axial while the part was still in the lathe. For tapping the lathe is turned by hand. Once the rest of the part was cut to shape I could finally see what two parts looked like together

S_Ellzey

Making the shocks was very straightforward. Since there is no protrusion beyond the diameter of the part they were made from 0.5” diameter stock. The most useful information here is on how to make a precise bore. A drill bit does not make a very precise deep hole, they tend to wander a bit, and at times leave what looks like a rifle bore. A two fluted end mill will make a very straight hole, however my set does not like to punch straight into a part. The center of the bit does not like to cut in this situation. So a small hole is drilled to the right depth, which is then followed by the end mill. Since the end of the bit is square it’s not going to follow the drills path in case it did wander around. The end mill needs to leave a slightly under sized hole so that you can finish up with a straight wall reamer. This will leave a clean, straight and precise bore. The reason for this care is because the lower part of the shock needs to move very smoothly in the upper part so that they will not try to grab each other.
The shocks have two springs in them. A small diameter one that runs the entire length that keeps the struts straight when there is no weight on them; they also carry almost all of the weight with the model setting on the ground. A second larger diameter spring goes around the first and does not get compressed until the struts are at their static position. This provides a stiffer shock for the last half of its travel to help prevent it from bottoming out.

S_Ellzey

Once the shocks where made it was time to drill everything for the pins that would hold it all together. When the parts were first made an undersized pilot hole was drilled in whatever part would be on the outside of the joint. These holes are always started with a starter bit which is very stiff so that the hole goes in the right place. The parts where clamped together and then drilled as a unit using the pilot hole to line up on. This way you know how the parts are going to fit together when finished. It also means that the each finished strut has matched parts, so it is necessary to keep the left and right sides together.

S_Ellzey

After the parts can be pinned together the last thing that needs to be done is to round off some corners so that everything can move. There are better tools to do this than those I have; a rotary table would be nice, but I don’t have one. So the part is marked with compass, clamped back in the mill and is slowly worked with the side of an end mill till it is close, and this is finished up with a file.

S_Ellzey

This process definitely created a large high strength dust bunny.

S_Ellzey

Needless to say there are many other small processes; fitting parts, smoothing edges and the like. But as you can see – just because it does not exist on the store shelf, does not mean that you can’t have it. In case anyone is interested, there is less than $20 worth of material involved here.
The final parts are working out quite well, I was able to move the CG back, and the model taxis very well, tracks well on takeoff and lands smoothly at what looks like a lower speed. If I set the model down I do have to reach under it and push the wheels back, but on landing the drag of the wheels pulls them back into place.

S_Ellzey

Overall I fell it turned out good.

Ram-bro

My Feedback: (101)

"Brilliant"

AndyAndrews

That's Really cool Steve. Thanks for sharing.

Andy

Dr Honda

My Feedback: (4)

Yep... nice work. It takes some skill to make good parts on manual machines.

drdoom

My Feedback: (9)

Gotta love the P20. And the wife who gives one... That is very sleek. Thanks for sharing.

DrScoles

My Feedback: (18)

Very cool! That nose gear is screaming for the same tx.

Canadian Man

My Feedback: (1)

Awesome!

Thanks[8D]

mr_matt

My Feedback: (10)

Steve that is a very nice design. One of the things I don't like about the current crop of trailing link designs is that over time, they will not go fully extended, the spring has so little of it force vector going to straight the strut. It looks like in your case the strut will always straighten out.

What kind of machine did you use? And what do you use for axle material?

H.Dale

My Feedback: (3)

Really neat Steve. I bet it is fast. You should try E-Flite electric retracts on it. They are simple and work flawlessly.

I've got a P-20 on an E-Turbinator and it's a lot of fun to fly. I carry my TX, fuel, propane and plane to the field and fly. I'm a little surprised that more people haven't tried one. They are perfect to carry out to the field during lunch or after work and get a couple of flights.

Hal