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Jun 19, 2019, 09:54 PM
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AntiArf's Avatar
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Build Log

Cessna CR2 racer 27.5" span W/retractable gear and flight video


Started working on a Cessna CR2 racer based on modified/enlarged 24" span Dave Rees plans from outerzone. The enlargement loses a bit of resolution, but par for the course for someone who generally builds from enlarged 3-views. The build plan is to also have retractable landing gear driven by a continuous rotation servo, all thread lead screw, and a few limit switches, resembling the full scale design in general layout.
https://outerzone.co.uk/images/_thumbs/plans/1258.jpg
The plan I printed is a stripped down version of the plan, which saves ink and removes all the unnecessary notes. The wing plan was used only for the outline, with a few more ribs added, as well as functional ailerons and a notched main spar. The fuselage back will also have added stringers, to more closely resemble the full scale.

Apparently the CR2 was initially silver and then painted red like the model below, which gives a choice of colors for the CR2. Already having 2 red racers, I may use the original silver scheme. As of now I have the cowling framed up and ready to skin with 1/32" sheet, as well as the short kit hand cut shown below. Acquired a few more Williams Brothers cylinders from Ebay to fabricate a 7 cylinder dummy engine using half cylinders. Cylinder halves with still look better than a paper dummy engine print, like my Herr Gee Bee R2 has.


Last edited by AntiArf; Aug 11, 2019 at 05:55 PM.
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Jun 20, 2019, 08:55 AM
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yomusj's Avatar
Looks like a lot of fun. Golden age racers are my favorite genre. I built this version of the 1922 Verville Sperry Racer last winter, from an Outerzone plan.
Jun 20, 2019, 03:48 PM
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portablevcb's Avatar
Cute little plane I had not heard of before. Looked it up and the CR3 is cute too.

The nice thing about these little planes is that 1/6 scale is usually less than 50" so fit in smaller spaces.

charlie
Jun 20, 2019, 05:38 PM
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Nice Sperry. Golden age racers have always been a favorite, with still at least a few like the Gilmore Red Lion on the list.
Charlie this one is exactly 1/8 scale at 27.5" span, which is small for a 1/8 scale model. Amazing to think that a 1/4 scale would be 55" span. Was looking at the normally oversized Williams Brothers 1/12 scale pilot, which would probably appear too small for the model. I have Sparky's light weight thin molded plastic shell Tuskeegee and jet pilot already already assembled and stashed. May use the Tuskeegee, who makes a reasonable vintage pilot also, like the one used in a Dayton Wright RB1 shown below.
Jun 20, 2019, 08:58 PM
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glewis's Avatar
Nice. And the perfect size too. We can stage a 1/8 scale air race. I'll fly the Laird Super Solution.
Glenn
Jun 26, 2019, 07:59 PM
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AntiArf's Avatar
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Cool I didn't know you had one of those Glenn. I remember when people used to build the Dumas kit, when building was more popular. Found your build: https://www.rcgroups.com/forums/show...Super-Solution
Built tail feathers using the laminate method, to create airfoiled surfaces for scale appearance. They're sizeable for a model this size, so 7 grams isn't terrible. Advantages of the laminate method are flat construction, strength, and the laminate layers providing a guide for symmetrical sculpting. The 1/32" balsa sheeted cowl has a removable battery door at the bottom, which is difficult to see in the photo , as it is taped in place. The battery will install in a box mounted on the firewall, that the motor will also mount on.

Had a gear driven all thread lead screw design fairly well thought out for the retractable landing gear, but am now seeing advantages to a nylon coated beading wire (cable) driven system, similar in operation to an outboard motorboat steering system.
Jun 30, 2019, 12:10 AM
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Working on the retractable landing gear. When modifying a Dynam servo for continuous rotation, don't solder the 3 pot leads together as some show on You Tube, as it doesn't work with this brand. Took a few minutes to solder them back on with good close up vision and lots of micro experience, but might not be so easy for some. The pot was centered to stop rotation, and then glued at a spot away from the wiper, to stop it from accidentally rotating as the now drilled out (was press fit) free spinning gear on the pot shaft could still easily turn the pot shaft. One good feature with the modified servo is that the speed can simply be controlled with transmitter travel percentage, versus requiring servo slow down ability. A cable winding drum (alum tubing) was pressed tightly over the center of a servo horn after cutting away the arms and sanding it into a small bushing, and then locked in place with thin CA. The cable drum on the winch servo is intentionally short, to reduce load on the servo output shaft. The cable end that holds the landing gear down will wind at the base of the drum, as landing will produce a load on the cable. A second washer will need to be placed mid drum, and then the second cable which raises the landing gear will wind in that area, followed by a cap washer at the end of the drum.

The black plastic part that the landing gear struts will mount on was fabricated from GWS spare parts, which seems to be both strong plastic and glues well with CA. The nylon coated silver beading wire has high pull strength for it's thin gauge, where a quality high test fishing line would also work. The wire ends are double looped through the aluminum tubing crimps shown in the first photo, to ensure they don't slip loose. The crimped loops on the ends of the two lengths of wire are inserted into the plastic landing gear base, where a screw passes through both of them. Easier said than done, but doable. The ends of the white plastic tubing cable guides shown in the second photo were heated and bent with a curved wire inserted into the ends. The top and bottom fuse keels were laminated with 1/32" ply on one side, as they required cutting through the balsa keels to embed the cable guides and wire slide rails. Testing the setup by pulling the cables to raise and lower the gear mounting base proved that the part needed bushings to slide smoothly on the wire rails. The bushings were made by drilling out aluminum tubing to fit the wire rails, and drilling the ends of the plastic parts open a bit to fit them into the part and glue in place.

The black plastic landing gear bases will need raised flanges glued to the sides, that will fit around the top fuselage keel when the landing gear is raised. The idea will be to have adjustable springs on the ends of the upper gear struts, that will rest against the flanges. Each landing gear assembly is comprised of 3 struts (see full scale photo in previous post). This will provide spring suspension and set the landing gear height, when deployed. I'm convinced Cessna did something similar, as spring suspension was mentioned. Other small details are lower strut stops that will be added to the bottom of the black plastic parts. You can't just have the landing gear hanging straight down when being raised, or the wheels won't fit well into the holes in the sides of the fuselage. Sketching a diagram of the gear layout proved that you need all the fuselage room and travel available for scale appearing gear strut length, when deployed. It also explained why the upper gear struts route downward toward the bottom of the fuselage, after being welded to the lower struts with a triangle bar arrangement, seen in full scale photos. When the gear is raised, the upper strut would clash with the upper fuselage inner surface, if it continued to route upward. The full scale photos don't provide much detail, but enough to decipher the basic principle of operation.
Jun 30, 2019, 02:14 PM
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turbonut's Avatar
Very cool...Love the creative part of the build...
Latest blog entry: In flight
Jul 03, 2019, 08:54 PM
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Thanks Turbo, this one's been getting interesting.

The retractable gear setup is now working with the limit switches functioning. Kinda like watching a miniature elevator in motion. There are fine adjustment screws that can be used to set the stop points, seen below mounted on the black plastic part that the landing gear struts will mount on. The limit switch tabs can also be bent, to get the adjustment in the ball park, before adjusting the fine adjustment screws. The landing gear is set to stop with a good millimeter of clearance between the gear mounting base and the fuselage keels, to ensure the limit switches trip, or the arrangement will continue moving and destroy itself. On that thought, I setup the master control switch mounted on the firewall, to work from a 3 position transmitter switch, so that the gear can be completely shut off in the center position, once the gear are lowered or raised. This required two master limit switches which are also adjustable, to get them to trip with relatively low servo motion, so that the winch servo speed can be controlled with total servo throw. A second option would be to use a mix and slave the winch servo from the master switch servo's channel, where the winch servo rate could be controlled from the slaved channel. The direction could even be controlled that way, if someone were to accidentally wire the master switch in reverse, although I made sure not to do that. I'm almost thinking everything up until now was the easy part, compared with fabricating the gear strut assemblies.

Thinking ahead I have a plan for a spring assist, that will spread the gear legs outward when deployed. Depending on the bottom of the fuselage wheel openings as fulcrum points to raise the gear struts and spread the wheels outward would require significant force, and would have to happen over a short distance of gear travel right at the very end. With the spring assist, the gear struts could be pushed inward as they are raised upward (retracting the gear) with much less force, so that the wheels will fit into the fuselage holes when retracted. Tabs running inward from the approximate fuselage center area could be fabricated to rub against the upper gear struts as they raise, slowly pushing them inward to the correct position for the wheels to fit properly in the fuselage holes, when fully retracted. I'm sure Cessna went to some effort to work these issues out on the full scale. The simple solution of depending on the bottom of the wheel openings as mechanical stops to raise the gear struts and spread the wheels outward has other issues, as precision stops on the bottom of the gear mounting block would need to be fabricated, to keep the gear legs from dangling straight down. The wheels would otherwise not fit properly into the fuselage holes. The mechanism does seem to have enough force to overcome the fulcrum problem mentioned without the spring assist, but I'm more concerned that something might break. Then again one could just let the wheels dangle down close together as they sit in the retracted position, and then land like a single wheel glider, hoping to maintain control until the wheels spread apart on their own. The phrase "this can't be good" comes to mind.
Jul 03, 2019, 09:20 PM
Big gov never Works
St. Martin's Avatar
You snuck this one in on me, Bill. I was bizzy on the Hunter, so not seeing what was new in the scale forum. As always, you amaze me. There was once a time in my life, when I did things like complex LG. I scratched an F-4F, but never finished it. I made up LG out of hardwood, using a sailboat winch servo and jack shaft, planning on using the wood parts for metal patterns. Never happened. The cost was too high. But, it was neat to watch it work. The gear was actually staggered in retraction. Probably due to my poor joint fitting.

Fuzz
Jul 04, 2019, 05:49 PM
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MrSmoothie's Avatar
Quote:
I built this version of the 1922 Verville Sperry Racer last winter, from an Outerzone
I always loved the Verville Sperry; beautiful job Yomusj -- did you have retracts, and how did it fly? I'll bet attaining CG on that one wasn't fun. I love all these racers, including the Cessna. What a cool project. Back when I was only building rubber scale models (over 30 years ago) they were my favorite subjects.

Good luck with this super interesting build, AntiArf!
Jul 04, 2019, 08:40 PM
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Thanks. I don't think the Verville had retracts, but there were racers from that era that did. Built the Dayton Wright RB1 and Bristol 72 with retractable gear. Nice thing about this subject and the Verville is that the rear fuselage isn't sheeted, making balance easier. Almost concerned about nose heaviness, but the tail and aileron torque rod servos can be mounted far back on the wing, as a wild card if needed.
Jul 05, 2019, 12:40 AM
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Fuzz I used to sail when we had a cottage on a lake and always wanted to try rc sailboating. Pretty much just made a sailboat winch here, and was thinking about that. I still have an old, beat up Dumas Sunfish that I thought about glassing over.

Assembled the gear struts. The drawing was used to bend the #2-56 all thread struts, holding tolerances within a mm or better which takes some time.

Pic 1. Tack glue the lower strut assemblies together with thick CA and activator. Once adjusted they were glued together firmly but keeping the glue from building up, as the wheel hub bushings have to fit over them and align with them, as well as leaving room for the upper strut to fit into the bushing. The clear nylon plastic bushing in the photo was used to slip over the threaded ends to make final adjustments to the bends where the clevises thread on, so as to not damage the threads. The clevises have a bit of slop when threaded on which is good, as it allows the final bends to be a hair off.

Pic 2. Glue the aluminum tubing wheel hub bushings onto the lower strut assemblies. Note that the upper struts have to slide in above the lower struts, so the bushing has to be glued against the bottom of the lower struts. The strut bend angles need to be precise, as the bushings glue on the struts and set the wheel camber. I was able to tweak these bends a bit with a pair of pliers, after the struts were thick CA glued together.

Pic 3. Slide the upper struts into the aluminum wheel hub bushings and glue in place. Tack gluing is a good thing, which allows some adjustment until satisfied with the positioning. Once positioned the bushings are flooded with thick CA and activator, locking the parts firmly together. There is about 3mm of upper strut sticking out of the wheel hub bushings, which will be used to thread on wheel retainer nuts. The nuts will be slightly off center on the wheels, oh well can't have it all. The hubs are a bit wider than the ideal (drawing) wheel position on either side, which will allow for some wheel adjustment using washers. Wire triangle shaped braces will be fabricated to connect the upper and lower struts, as on the full scale. Since the clevises are in that area, additional parts will be used to join the triangle brace wires to the clevises. The parts that thread on the top end of the upper struts will provide spring suspension and set the landing gear height when deployed, butting up against the black plastic landing gear mounting base. These parts were made with springs, blind nuts, aluminum tubing, and use lock nuts for after final adjustment is made.
Jul 05, 2019, 06:24 AM
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yomusj's Avatar
Quote:
Originally Posted by MrSmoothie
I always loved the Verville Sperry; beautiful job Yomusj -- did you have retracts, and how did it fly? I'll bet attaining CG on that one wasn't fun. I love all these racers, including the Cessna. What a cool project. Back when I was only building rubber scale models (over 30 years ago) they were my favorite subjects.

Good luck with this super interesting build, AntiArf!
I flew it without the LG, which was detachable for weight savings. I had roll axis stability problems during the initial flights, but think it may have been the brick. I plan to replace the receiver/servo combo eventually.

The Verville-Sperry R-3 Racer was a cantilever wing monoplane with a streamlined fuselage and the second aircraft with fully retractable landing gear, the first being the Dayton-Wright Racer.[1] In 1961, the R-3 racer was identified as one of the "Twelve Most Significant Aircraft of all Time" by Popular Mechanics magazine.[2] In 1924, an R-3 won the Pulitzer Trophy in Dayton, OH. (https://en.wikipedia.org/wiki/Verville-Sperry_R-3_Racer)
Last edited by yomusj; Jul 05, 2019 at 09:29 AM.
Jul 05, 2019, 10:55 AM
Big gov never Works
St. Martin's Avatar
Bill, that sailboat winch servo was about 4oz! After all, it was 1985 approx. IIRC it was a Kraft, which I used at the time.

Fuzz


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