By Dennis Weatherly
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Have you ever dreamed of seeing your model flash by on a low pass, wheels up? Thereís just something sleek about a model with retractable landing gear. Most of us never experiment with retracts, or if we do it is because some favorite scale project requires them.
Iíve often looked at my Cloud Dancer 40 and wondered what it would look like with the wheels up. I have also wondered about the change in flight performance that retracts might provide. Would the flight times be longer? Would the model seem to fly faster? Would the pleasant handling and stall characteristics change? Finally, would the added complexity make the model less of a "daily flyer"?
The first choice facing us is what style of retracts to use. There are two basic styles available: mechanical retracts that are powered by a separate servo, and pneumatic retracts that are typically powered by compressed air. There have also been self-contained, electric retracts available in the past (Giezendanner comes to mind) but I havenít seen this type for sale in the United States for some time.
Each of the two major types has benefits and limitations:
Light weight and simple
Push rod routing hassles
* No Nose Gear, so the push rods would be fairly simple.
* Light weight.
* No room for an air tank.
I selected a pair of Hobbico mechanical retracts and a Hobbico CS-63 retract servo. This servo has nearly 90 inch-ounces of torque and a low profile, so it could easily drive a pair of landing gear units and fit in the limited space I had in the Cloud Dancer.
Out With The Old
Now that I had the gear selected it was time to start the installation. The first order of business was to get the original fixed gear out of the wing. I spent quite a bit of time carefully measuring the fixed gear installation in the Cloud Dancer so I could position the retracts properly. These measurements brought a couple of issues to light:
* To retain the stock landing gear length, which I needed for the 12" prop I use, I would need to move the landing gear out about one inch. I couldnít leave them in the stock location, as the wheel wells would end up right where the wing hold down dowels are located.
* The Cloud Dancerís rib spacing required that I change from the stock 3 inch wheels to 2.5 inch wheels, to allow for the wheel well liners and a little misalignment after a "firm" landing.
I decided to cover the gear mount area with masking tape and carefully lay out the needed cut outs using a pencil. I removed the 1/16" balsa sheeting with a sharp Exacto knife and then turned my attention to getting the old gear mounts out of the wing. After much pondering and head scratching I decided to use a Dremel tool and their small circular saw blade to cut as far as I could along the gear blocks at each rib. I also used the saw to cut across each block on several places. Then I used a ľ" drill bit to drill holes the length of the gear block and carefully broke out the remaining pieces using needle nose pliers. This approach worked pretty well for the first gear block. Before I tackled the second gear block I had acquired a Great Planes Hinge Slotting Machine. This tool does an excellent job of making plunge cuts along and through the gear blocks. It worked much better than the Dremel approach.
Once I had the basic cutout made, I added a notch in the outermost rib to clear the retract unit. This was needed because I had to move the gear outboard. I already had 1/16" plywood reinforcements in place from the previous gear blocks. These reinforcements run from the leading edge to the spar, to tie everything together securely. I also added some ľ" triangle stock at these reinforcements to increase the gluing area for the new retract mounts. Notice in the picture that I still needed to remove the wooden block that held the old landing gearís torque arm. This was ground out using a high speed steel cutter in a Dremel tool.
In With The New
After some head scratching I decided to use 3/8" square maple for the retract mounts. I figured that if this was strong enough for engine mounts it would work great for retracts. In retrospect I wish I had used ľ" aircraft plywood instead. This would have left more of the plywood reinforcements intact and made for a stronger structure.
Notice in the picture on the right that I have covered the retract unit with waxed paper. This was to ensure that I didnít gum up the works with epoxy when I glued the mounts into place. I wanted the unit bolted to the mounts when I glued everything together to ensure proper alignment.
In the left and center pictures Iím doing a dry run of the retract installation. Be sure to cycle the gear up and down to look for binding or clearance problems. I ended up having to make my rib notch a touch deeper to clear the pivot block in the landing gear. Once everything is working smoothly, grab your favorite high strength adhesive and glue the mounts in place. The picture on the right shows the completed mount installation.
I was fortunate that the Cloud Dancer uses two wing hold down dowels in the leading edge. This left a perfect space for the retract servo. More head scratching ensued to try and figure out the best way to mount the servo and route the push rods. I ended up making a topless box that held the servo.
I made a cut out in the top leading edge top sheeting and glued the box down against the bottom leading edge sheeting. This gave a solid mount and provided just enough clearance for the DuBro #121 EZ Connectors to miss the battery pack in the fuselage. This arrangement also placed the push rods near the top of the wing, which allowed them to clear the wheels with a small bend in the middle. Everything is a close fit! Finally, I measured the length of stroke required on the landing gear and then chose holes in the servo output arm that would give the proper throw. This made final set up a snap and resulted in no binding at either the extended or retracted position. At this point, be sure your radio batteries are charged because I can guarantee youíll cycle the gear for 15 minutes just to watch it go up and down!
After everything was installed and working properly I set about "cleaning up" the installation. I fit 1/16" balsa on the exposed portions of the retract mounts to fair them into the wing sheeting.
I also formed wheel well liners from soft 1/16" balsa by soaking the wood in water, then wrapping it around a soda can and baking the whole assembly in the oven at 150 degrees until dry. Cutting and fitting these into the wing was a little tedious but the end result looks good, keeps unwanted airflow out of the wing and strengthens the wheel well cutout like a shear web strengthens a spar. They are also much easier to fit around the push rods after the push rods are in place. All finished up, the retracts added about six ounces to the airframe.
Was all the work worth the effort? The first time I flipped the gear switch on climb out was a real thrill. I followed this with the first low pass with the wheels up. Boy, did it look cool! Everything worked great until the first landing. That was when I discovered something I had missed. BE SURE to grind flats in the gear struts and TIGHTEN the setscrews that hold them in place. I hadnít, and the left strut spun 90 degrees when the Cloud Dancer touched down. It didnít flip over, but the CD sure came to a quick stop!
On the second flight I was surprised when I made a pretty smooth landing, then rolled into the short grass along the runway and ripped a retract unit out of the wing! Upon inspection I found that the maple mount had split along itís grain, which ran right through the semi-circular cutout needed to clear the spring in the gear strut. Perhaps the initial spinning strut incident had weakened the mount. This is another reason why I recommend using plywood rather than hardwood for the mounts.
To re-visit my initial questions:
* Would the flight times be longer? Yes, they appear to be longer but not by much. The Cloud Dancer is a pretty clean model, so the retracts might not have made a huge difference in drag. Plus, I tend to fly it faster and with more throttle now!
* Would the model seem to fly faster? Yes, it is faster. I have moved up 2 inches in prop pitch as a result of the cleaner airframe, with no detriment to loops and vertical lines.
* Would the pleasant handling and stall characteristics change? I havenít noticed any change in the way the plane handles, except that it required one turn of down trim. This may be due to the different airflow over the wing or just the higher airspeed.
* Finally, would the added complexity make the model less of a "daily flyer"? To be honest, yes. I am a bit paranoid about ripping out the gear mounts now (Iíve done it twice). I think that a stronger mount, using plywood rather than thicker maple, would help alleviate this concern. I will definitely build another sport model with retracts but they will be engineered in from the start.
This article reprinted courtesy of the Ezone.