Darrell is not taking questions at this time.

Page 1: Questions 1-24, click on links or browse question page.
Q1 P-38 retract valve size? Q13 Top Flite 60 sized Corsair retract problem ?
Q2 Top Flite P-39 gear doors? Q14 Top Flite Spitfire gear fit ?
Q3 Spring Air for FW-190D? Q15 Pica Spitfire electrical or pneumatic retracts ?
Q4 Up down action changed to rotation action? Q16 Hawker Typhoon gear design questions ?
Q5 Palmer plans C-130 retracts? Q17 Mechanical retracts for giant Top Flite P-47 ?
Q6 Stronger retracts for Yellow P-38? Q18 Retracts for a Cactus Aviation Chipmunk ?
Q7 Century Jet retracts in a Pica P-51? Q19 Retracts for an 81" Mosquito ?
Q8 Top Flite P-51 gear door sequencing? Q20 Retracts for Top Flite P-47 ?
Q9 Piper Cub gear material? Q21 Air cylinder size ?
Q10 Adding drag link to Century Jet B-17 retracts? Q22 Retracts binding ?
Q11 Corsair tail wheel assembly ? Q23 Mechanical retracts for 1/7 scale P-47 ?
Q12 Basic retract operation and equipment ? Q24 Binding mechanical retract linkage ?

Question 1: "Am building a Ziroli P38. I have read in several web postings that the standard retract valve is not big enough for this application. Can you elaborate on this? Plans call for a medium size tank in each boom. My goal (intentions) are to put a receiver in each boom with separate controls for each side, this would translate into 2 airvalves, so each of the mains would have it's own valve and I would have to tee one or both valves to the nose gear. I have not even considered the gear doors yet. OH BOY what kinda trouble am I in now."

Darrell: " The needle orifices on the valves are not big enough to allow enough air to operate the cylinders correctly. Contact Sierra Precision at They have a valve with a larger orifice and spool that will handle most tri-gear systems. I don't recommend using separate valves for the mains. If you have any problems with one side not coming down, that will not be good. You would be better to belly the plane in than land on one main or a main and nose wheel. I would recommend an electronic sequence for the gear and doors if you are going to cycle them. You can use one valve for the gear and one for the doors. Let me know if you need any more help. Thanks, Darrell"

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Question 2: "Hi Darrell , I am building a Top Flite P-39 and I would like to make operational gear doors for front landing gear. I am using the Robart gear that was designed for the plane and if possible I would like to use the gear itself to activate the doors. Any help you could give me would be greatly appreciated."

Darrell: "Jim, One method to operate the doors on a nose or main gear is to install a small springboard in the gear well for the wheel to push against when it retracts. The doors are pulled closed when the wheels hit it. You have to use a stiff push rod from the doors to the springboard. A spring is used to push the springboard up to open the door and the wheel closes it."

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Question 3: "Hi Darrel, I am almost done building my FIRST war bird. It is a FW 190 D-9. I plan to use a pair of Spring Air retracts in it, and I have a couple of questions about them. As you likely know, the Spring Air units feature a spring inside the cylinders that cause the gear to deploy without air pressure( gear down ).The air is needed to retract the gear. My questions are: -Do I need to use some type of oil in my air system to keep the cylinders operating smoothly? If so, where do I add it? -I plan to use restrictors in the line to keep deployment speeds more scale. Should I use one for each leg, or just one before the " T " ? -How often should I change the air lines? When I bought these they were brand new, but sat for, I guess a couple of years, and the tubing just crumbled in my hands! -Any other tips for a first time pneumatic retract user? "

Darrell: "Ken, First, most modelers use either Marvels Magic Mystery Oil or Refrigerator Compressor Oil. Others use a Dow O-ring lubricant. Second, You can put a few drops into the end of one of the lines that goes into each end of each cylinder, and also some into the valve lines. The oil is used to keep the O-Rings from drying out. Most cylinders fail from being dry or from big temperature changes. Third, you will have to experiment with placement of the restrictors. Every system has it own quirks. Some need different restrictor sizes on each gear. Fourth, You should check the lines each season and replace any lines that seem to be to stiff or brittle. A few dollars on new air line is worth every penny you spend. Most Important: Be sure to cycle your gear several times before the first flight of the day. This will lubricate the cylinder walls and seat the o-rings. Also, don't paint your model with the gear exposed and get paint into the retract. I have seen several planes belly in to find that they filled the gear tracks with paint and balsa dust. Have fun, Darrell "

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Question 4: "Is it possible to customize normal "straight up/down" pneumatic gear ,to gear
that rotate to 90 degrees. How would one do this?"

Darrell: "Hello,Their are three ways that you can make a strut rotate as the gear leg retracts and extends, but all the methods use one
simple principle. They all need a "Lever" to force the gear to rotate as it moves. Some planes used gears, others used a "Heim Joint". It's best to look at a few full scale planes and see how they did it on the one you want to build. The other important factor, is that the strut has to be FREE moving in its trunion or support tube, so that the gear does not bind as it retracts. I know this is vague, but their are many ways depending on the gear design. If all else fails, call the manufacturer of the gear and ask if they can do the work. Thanks, Darrell"

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Question 5: "Darrel I am redoing the Palmer C-130 plans and will be enlarging it to 1/9th scale 176" ws. I want to have working retracts but the retracts on a C-130 raise straight up. Is there an easy way to do this with maybe modifying a set that is already around? I have not seen anyone selling a set of retracts for a C-130. Joe"

Darrell: "Joe, The C130 is a pretty simple gear design. As you said, they pull straight up into the fuse. Gene Barton make some retracts for a smaller model. If you can get me the drawings in Cad format, I can look at them to see what we can do. I would also need to know the estimated weight and tire size you would want to use. Good Luck, Darrell"

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Question 6: "I'm flying a Yellow P-38 and love it, but theretracts seem to be a weak link. The shafts snap off between the shear pins and the upper limit of the inner brass sleeve. Between me and my P-38 buddy Rich, we've snapped four of them, with very annoying, although not disastrous results. We fly off grass which is "kind of smooth." No problems on our field with other planes. Of course, none of them have the weight and the landing speed of the P-38! Each of the events has been associated with relatively minor sins: hitting a rough spot on takeoff, turning left too hard at the end of a landing rollout, etc. Interesting enough, none on actual landings, which have been smooth. My bird weighs 31#, which is well within specs, and it tracks straight on the ground without much/any steering input. Any words of wisdom? mark taylor "

Darrell: "Mark, There is always a weak link in every gear design. Some of them are the struts, some the trunions, and others the down locks or supports links. Some designs are great if you fly off of asphalt and are an excellent scale pilot. A gear manufacturer will always tell you that you need to land better. If you have a gear that keeps failing, you can only replace that part with a stronger one. But, you might find that you have a new weak part down the line. 31 lbs. On a tri-gear plane does not sound like a lot of weight, depending on your landing speed and if you tend to land on only one wheel, then that's a lot of weight. Thanks, Darrell"

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Question 7: " I've got a Pica 1/5th P-51 at 24 lbs that came with Century Jet retracts. I'd appreciate your opinion about these gear. Jerry Neuberger"

Darrell: "Jerry, You or someone with some retract experience will have to make the call on the quality of the CJM retracts. Try to cycle the gear and watch for any binding. CJM makes several different P-51 retracts. You can also put some loads on the gear when they are down and locked and see if they are going to hold up to the weight. "

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Question 8: "Dear Sir, I am about to embark on my first gear door sequencing system on a Top Flight Giant P-51. Do you know of any good articles or info that might help. I plan to use robart equipment but can find little on the parts needed much less how to do this all the way from the mechanics involved to radio programing. Regards, Don Beason"

Darrell: "Don, I do not really know of any articles that goes through the steps to set it up. You might be able to scan trough some old mags and see what you can uncover. I'm sure someone has done at least one article on this. Their are several ways you can make the inner gear doors cycle on a P-51. You have to decide if you are going to do it with air cylinders or mechanically with servo's. It also depends on how much money you want to spend. Electronic may be a few dollars cheaper in the long run. If you decide to go with air, contact UPC in Canada. They have all of the things needed. If you want to go with servo's, you can contact EMS/Jomar for an electronic sequencer and work them that way. It all depends on how you want to make them work. UPC's web site is EMS/Jomar's web site is Good Luck, Darrell "

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Question 9: "Hi Darrel, I need to make the main landing gear for my 105" Piper Cub. I would like to know what the best metal for a first time landingear builder to use is. I know they are not retracts but I'm sure you'll know. !!!Thanks!!!!"

Darrell: "Hello, You are going to need some welding experience if you are going to make nice gear for a Cub. Your best choice would be to use 4130 tubing. Most full scale aircraft from the 30's until today use this for their airframes. It is easy to find and work with. Select a tubing of 1/4" dia. and .049 wall. You could also use 4130 Solid bar at around 3/16" dia. Don't worry too much about weight. With a 105" WS, you won't even notice it. Cut and fit all of the parts and find a local welder that can connect it all together. If this seems like to much work, Robart makes several tube gear for Cubs at around $140 dollars. Good Luck, Darrell "

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Question 10: "Hello there Darrell. I had purchased a set of Century Jet B-17 retracts and immediately noticed that the mains were not designed the same way I have seen the Robart units done. The Century units lack the entire linkage that pulls the main strut assembly up into the engine nacelle as on the real aircraft. Yes these are only for visual purposes on the Robart units. So can you help me design something as a simple add on that recreates this drag linkage set up as the Robart units have? I thank you so very much for your time Darrell and have a Happy New Year! Best regards, Steven R Bolin"

Darrell: "Steve, You wil have to draw on paper or balsa the outline of the drag link from a picture or 3 view of what you want them to look like. Then when you get them to fold up the correct way, you need to make them out of a plastic sheet from a hobby shop or from scrap sheets of plastic from around the house. After they are painted it will be very hard to tell the difference from the real thing. Not knowing how the century jet gear work for the B-17, I would need to see a photo in order to make any other suggestions. Also try to use elastic or a small spring to make the down lock fold when the gear starts to move. Have fun and let me see what you come up with. Darrell"

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Question 11: "Darrell, I just acquired an F4U Corsair tailwheel assembly manufactured by Earl Aune. It's a beautiful piece of work---almost hate to "hide" it inside anything. I'm wondering about the actual integrity of the unit, because it's a VERY clever design, extremely realistic looking, but I know what it'll be subjected to, despite my best intentions (ie, not ALL of my landings will be
picture perfect ;-) I do have over 200 hours of light time on the Top Flite Giant Corsair, so I'm familiar with the breed, but this unit will be going into my Iron Bay F4U soon, and it will NOT be a hangar queen. Thanks for your thoughts!! Joe Hahn (aka F4you) "

Darrell: "Joe, Nice to meet you. I have not had the pleasure of using one of Earl's tail wheel retracts. I know they are made mostly of reinforced Plastic or G10 as some people call it. With most normal landings I am sure it will hold up fine. It's when you have a sideways landing or ground loop it on takeoff that it might not hold up. If you do damage the unit, it should be quite simple to repair. You might want to use a simple fixed tail wheel on the first few flight until you a comfortable with the new bird, then you can put the nice one in for the fly-in's. Let me know how it goes. Darrell "

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Question 12: "I'm about to build my first Warbird. Top Flite P-51D Mustang, .61-.90. I have successfully built and flown two trainers, and just finished a Piper Cub that flies great. But, I have no experience w/ retracts. I understand the basic principles of operation. Tower Hobbies lists a few Robart parts as accessories to the P-51. First, a specific set of retracts, then an air kit, then a pump, then robostruts... I am a little confused. Can you give me a list of the basic elements required for the operation of a retract system? What exactly do I need to make this work? Also, I'm not sure if you're familiar w/ this kit, but should the tailwheel retract as well as the main gear? Thanks! LIAM"

Darrell: "Liam, The retracts they list are the units that get mounted into the wing. The Robo Struts are if you want to replace the wire legs with something that looks like tube type struts instead of just wire. The air kit is the air tank, servo operated air valve, air line, and air line tees needed to make the retracts work. The pump is a over priced tire pump that you use to fill the tank with air for the system to work. Skip the pump and get an electric car pump for a few dollars more so you don't have to pump up your tank for 10 minutes. The air fill kit comes with instructions on how to hook up the entire system. The full scale P-51 had a retractable tail wheel, but for your fist retract plane, I would put in a fixed steerable tail wheel. You don't need the extra weight and nobody but you will ever notice. Good luck and let me know if you need any more info. Darrell"

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Question 13: "Is there a better more robust alternative rotating retracts for the corsair? I fly off a grass field and with only 5 flights, 4 have bent the levers. I may not be the best at landing but I have no problem landing the light wing loaded sport planes. Getting kind of gun shy at this point. Takeoffs are fine, flying is great, but landings are apprehensive. Edwin"

Darrell: "Edwin, I am not aware of any other retracts that will work in place of them. Rotating gear are the hardest to manufacture because of the rotating mechanics and the cost that most modelers are accustomed to paying for them. If enough people are willing to spring for a better set that will fit this size plane, I'm sure they could be designed. Top flight sells hundreds, if not thousands of this type of plane every year, but it would take thousands of dollars to design and manufacture a reliable set. Only a manufacturer with deep pockets is going to take on this type of endevor. The only real solution is to demand better gear, someone will take notice. I for one am listening. Just my opinion as always. Good Luck, Darrell"

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Question 14: "I am building a Top Flite Spitfire and I bought the Century Jet Retracts for it because I liked the operation of their rotaters for my P-40. Anyways I am having trouble fitting the strut and wheel into the wheel well. And Since I am tyring to go as scale as possible I want to put gear doors on and this problem is preventing me. If I were to cut the rest of the rib out after I sheet the wing the wheel would have to be less than an inch wide to fit in the well. I have yet to find one. Could you help?"

Darrell: "Ian, Check with William Bros., they make some thin wheels that might fit. What might be the problem is the axle may not be at the right angle to the strut. One end of the tire might be hitting the bottom of the well. You might be able to change the angle to make it fit. Some gear Manufacturers don't mount the axles at the correct angle. Most of the production kits have thin airfoils and don't leave much room for real struts and tires. Let me know how you make out. Darrell"

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Question 15: "Darrell, I´m building a 1/5 Pica spitfire.. I don´t want to use standard pneumatic retracts because I think they retract too fast... Do you have any experience with electrical driven retracts...? Regards, Rudy Ehren"

Darrell: "Rudy, You can make a set of pneumatic retracts work very slowly if you use a good valve. I do have experience with electric
retracts and they are more problems and weight than the advantage of a slow retraction. If any of the switches ever stop working, it will burn up the motor, battery or wiring. Contact Sierra at, they sell some very nice valves and retracts. Let me know if I can be of any further help. Darrell"

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Question 16: "Hi Darrell... I´m currently trying to design my own retracts to go into a 1:5 Hawker Typhoon from enlarged Taylor plans, and I have a few questions. Gear specs: Length from pivot to wheel axle: 10.1in Length from pivot to "groove" in side frame: 1/2in
Oleo leg dia: 3/4in
Wheel dia: 7in
Wheel weight: 12oz (each...)

1: With air retracts is it possible to get 15-20 cycles of the main gear? Me likes to take-off and land... And how much air would I have tocarry?
2: How do I determine how much power I need from the cylinder? Do I calculate moment arm based on lengths from pivot point? (If you know what I mean).
3: If I choose to use electric/servo power would a standard servo be enough power when used to drive a spindle type actuator? I guess that will depend on the ratio/thread of the spindle...or?
4: Should I use aluminium or steel for the retract "block" where the oleo attaches?

Hope that you can answer some of my questions?"

Darrell: "Giles, If you can supply me with a few scale photo's of the struts and dimentions from the retract you are going to use. I can make some recommendations as to what you can do for struts. I do not have the Topflight plans or photo's for the P-39. I'll help you out as much as I can. Darrell"

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Question 17: "Hi is it possible to put mechanical retracts in the top flight giant scale p-47 or would i have to use air retracts? thanks gene"

Darrell: "Gene, If you keep the plane light enough and only fly from a smooth blacktop or concrete runway, you might be able to get away with mechanical retracts. But with that size plane, you would be best advised to put a good set of Pneumatic Retracts with Oleo Struts and the right size tires. Mechanical gear usually have wire struts that has no forgiveness on Grass Runways. Good Luck, Darrell "

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Question 18: "Hi Darrell, I have a cactus aviation chipmunk with fixed gear. Is it possible to put retracts in it with compromising the wing tube and root strength? thx, George"

Darrell: "George, Most of the Cactus Aviation wings I have seen will have to have some type of modification done to them depending on what Brand of retract you are going to use. The Chipmunk should be fairly easy if you have built a plane or two and mounted retracts in them. If you are not sure how, try to get someone that can help you with the mounting. I have never put retracts in that exact model
but it should be easy. You may have to add a few ribs or rails depending on what is in their now. If you have a few photo's you could send, I could make a few simple suggestion. Let me know, Darrell "

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Question 19: "Darryl, Got any ideas for retracts that would work in the BT 81" Mosquito. I am not a total freak when it comes to scale and the plan gear looks just too flimsy for the work involved. I was thinking maybe a a set of good sturdy mechanicals with those "Fultz" double nose gear struts that could be dressd up to look pretty close. Any ideas? Glenn, Florida "

Darrell: "Glenn, Sierra Giant Scale has some mechanicals that might do the trick. Take a look and see what you think. The wire strut can be locked so it will not rotate. I have some air operated retracts that have the right retract angles and would not really weigh much more than mechanicals if you are interested. We have 95 degree and 105 degree retraction angles. I don't have 100 degree in the little gear. They would be perfect for a 81" plane. Let me know if pneumatic is an option.Thanks, Darrell "

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Question 20: "I am interested in building a TF giant P-47 but the retracts are holding me back. At my home field and at every fly-in that I attend, the warbirds guys spend most of their time there getting their pneumatic gear to work. Are these things that problematic or are most folks just not setting up and maintaining the retract system properly (I know what the manuf. would say)? Appears to me that there are just too many failure points in the pneumatic systems to be very reliable. It seems that with the torque and holding power of current digital servos that a large scale mechanical retract gear is possible. Is anyone pursuing the design of such? Thanks,
Chris Tomberlin"

Darrell: "Chris, Most of the air operated retracts on the market today are made by either Toy manufactures or by cottage industry modelers that are fed up with the low quality retracts that are available. The quality of gear goes hand in hand with the amount that the average modeler wants to pay for a set of gear. Modeler like everyone else has a budget that they try to stay within. Kits are on average 1/3 of the plane cost, motors are also about 1/3 or more of the cost. Then you have servo's, receiver, wheels, and the rest of the building materials. In my opinion, the retracts on a warbird is the single most important item that is needed. To skimp on the gear is the last thing to do. First, find a kit that has good quality gear like the PCM Models ME109 or the Vailly Aviation FW190, Tempest, or P-47. Then the rest is easy. You have every kind and size of motor available. Every servo you could want. Every Type of paint you could want. But the selection of gear is low because the Average Modeler does not want to spend that much on a set of gear. The reliability of pneumatic retracts is also affected by the amount of money spent on cheap retract valves and fittings.

Digital Servo's are an option, but a good set of retract will work every time if they are quality units made for the specific weight of the plane. Do some homework with the following manufacturers and I am sure you can find a set that will work.

Good Luck, Darrell"

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Question 21: "Hi Darrell, can I assume that all retract manufacturers make their own air cylinders? When I compare them to industrial air cylinder producers (Bimba, Clippard etc..) they are considerable smaller in overall length. For example, an air cylinder from Bimba with a 9/16" bore and 2" stroke will have an overall length of 4.25" while something similar from Robart will have an overall length of 2.5" Does Sierraprecision produce their own air cylinders and what is the recommended air cylinder bore for 1/4 scale models. Thanks, Norman"

Darrell: "Norman, Retract manufactures tend to make their own cylinders because they are trying to fit the smallest retract into the tightest area and most commercial cylinders are too long. They also don't want someone else's name on thier product if possible. Sierra Precision and Robart produce there own cylinders. Some of the others mfg. have them made by someone else and then put their name on them. The size of the cylinder depends on the lenght of the struts, the weight ofthe tires going to be used and the amount of mechanical advantage that the retract has on the trunion. We tend to go with a 7/8" to 1-1/8" cylinder to keep the systems pressure lower. Hope this helps. Darrell"

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Question 22: "Retracts for my topflite P40 gold edition, I only get 2 to 3 retracts per 110 lbs. of pressure, is this normal. I have changed tank, lines, valve and tee joints. I have noticed when right gear strut goes up and down,part of the strut rubs against the housing that is attached to the wing inside the wheel well.thanks for you help. JOHN"

Darrell: "John, My guess, not knowing what brand of retract you have, is that you may have the side plates binding and you need to much pressure to make them work on the first try. Try loosening up the mounting screws and see if that makes them work at lower pressure. If so, you may need to work on the rails to put them in better alignment so they don't bind up. This is a common problem with the stamped out side frames. Let me know if this helps. Darrell

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Question 23: "I know that on the 1/7 scale p-47 you can build it with the wheels that don't retract or with air retracts but can you use mechanical retracts. I fly off of smooth black top. thanks "

Darrell: "Why do you want to go with mechanical retracts. The air retracts that are available today are very reliable. I admit that some brands and better than other. Ask around and I am sure you will find a small set that will handle the 1/7 Scale P-47. There are a few mechanical units on the market but they don't have an oleo strut which is what is needed most. Even though you fly off of blacktop, someday you will want or have to land in the grass. Good Luck"

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Question 24: "Hi darrell I am buliding a Hangar nine P51 Mustang 60 size with mechanical retracts, when they are extended they are fine but when you close the retracts linkages are binding and wheels are not closing all the way, I am using a round servo arm do you think if I change the diameter of that servo arm meaning use bigger round would help or any other suggestions. Thanks Tony "

Darrell: "Tony, I am assuming you are using a 180 degree servo. The farther away you put the pushrod away from the center of the servo horn, the more travel you get. It sounds like you need a little larger horn as you said. Make sure you don't go to far and stall out the servo at the ends of the travel, it will put a drain on your receiver battery if it does not reach full travel. Hope this helps, Darrell"

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Notice: Questions or statements regarding product quality and/or usage are solely the opinion of the writer and not necessarily the opinion or recommendation of or owner/webmaster. By asking a question you are giving or owner/webmaster the right to post your question and name on this page. Not all questions are answered or posted. All questions and answers are copyright


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