Airplane Construction

[AirKadet]

I heard that there are a couple of retract options: gas and mechanical. What are the advantages and disadvantages of each type? Is one more reliable than the other?

[jesseandaprildyer]

I have had ok luck with the mechanical retracts but the thing that i noticed was that with the Low profile retracts from JR i think they are the JRPS791's, that they are so much torque that the servo arm will bend and break. It happened to me this past weekend. Theree is no guarentee that all systems are flawless, you just have to make sure you get the throws as close as possible.

With the air retracts the good thing about them if i uderstand it correctly is once it gets to a certain spot it is going to stop. Unlike the mechanicals it will only push so much and then stop./ With the high torque servos they will keep pushing until either the gears strip out or it breaks or stretches out the ball link connector.

Another disadvantage is that if you play with the retracts alot in the flight or you forgot to pump them up before take-off you may lose the use of your gears.

Hope this helps. I see you posted this a while back but i figure my 2 cenets may be good enough.

Jesse

[AirKadet]

Hi Jesse,

Thanks for the post. I asked the question after seeing another P-51 land on one wheel this spring after the compressed air-driven gear didn't come down. I was wondering why would anyone risk such an expensive plane with gear that were so dependent on having air in the tank.

By the way, great flight with the P-51 last weekend! You have a great-looking/flying plane. All of the pre-flight radio and prop problems must have been really frustrating though! ;)

I have a couple of pictures of your flight that I showed to your wife at the field. Hopefully, I'll get time to post them on the web this week.

Welcome to the club!

Gilbert

[Darren1492]

Hey Gilbert, Pneumatics are more Scale like, and ALOT more expensive. Robart makes a very sturdy and reasonably priced mechanical retract. If setup properly using endpoint adjustments they'll work perfectly every time. You can call me if you have anymore questions. Hope this helps?:D

[Eddie Teague]

I don't like either of yalls (Jesse & Darren) post on retracts!!! Most problems with any retracts, mechanical or pneumatics, can be contributed to :---SH$T happens--builder era or OOOOOPs bad landing:rolleyes:

[Darren1492]

Eddie, who pulled your string??? I've told you about interrupting when grown ups are talking!!!:mad:

[Eddie Teague]

I pulled my own string:p

[wingman371]

quit pulling on that thing eddie. Retracs fail because they fail not always operator error, if you want retracs that will come down if the air pressure fails get spring down. Nort very scale like but 90 percent of the time they will flop down.

[Eddie Teague]

nobody squeezed your head--- Wingman!!!!

[Chris Smith]

Gilbert,

Retracts are like any other aspect of modelling. Once you've seen them perform properly, and have someone show you how to set them up, they will be as reliable as any other part of your airplane. There is no mystery to them, but like an engine, it helps to have someone who nows how to work them, help you.

Which kind you choose depends on the airplane usually. All of my warbirds from .40 size up have retracts. Most are air systems and are very reliable even on our field. I have at least 3 aircraft that weigh between 14-17 pounds dry and loaded with fuel, weigh between 19-25 lbs. All are equipped with (Pnuematic) 3-gear air-up/air-down retract systems. These aircraft land at 40-70 miles per hour. The retracts are extremely reliable. They have to be in order to withstand the landing loads, and you can image the cost and damage if they fail.

The key to a reliable air retract system is the airline tube installation and its connections. All connections are safety wired for example. Other key items are the proper air-tank size and the use of 100 psi air charge. Forget hand pumps. Air loss or inadequate pressure account for most gear troubles. In addition, all of my airplanes with air retracts have an on-board air gauge so I always know if there is a slow leak, or how much air is used per flight. And it's great for minor troubleshooting. After retract installation I charge the air-tank to 100psi and let it sit over-nite. If it loses air I find the slow leak and fix it. Just because you can't hear it leaking doesn't mean its not.

Air retracts use air. There is no other stable or safer gas. The Europeans are starting to use hydraulic systems on larger jets and warbirds, but they are very expensive and require more maintenance. Air systems are the preferred choice for larger and heavier airplanes. For choosing between mechanical or air retracts, 60 size and larger airplanes would be a good rule of thumb for choosing air, although you will find many ARF .60 size warbirds work ok with mechanicals when set up right. Trouble is on rougher fields, the locking mechanisms are much more prone to excessive play and vibration which can cause un-wanted ground handling characteristics, and possible jamming in the wheel well after take-off. Mechanicals are also not designed to be fitted with robo-struts or equivilent, which really help in rougher grass or higher landing loads.

Even so, I have one .40 size warbird with mechanical servo actuated retracts, and they are also reliable. No failure in 5+years. But, the airplane weighs under 6 pounds, and has no nose retract. 3-gear mechanicals (includes nose gear), are twice as involved to set up with servo
actuation and linkage. So for my smaller 3-gear airplanes like my .40 size Hangar 9 T-34, I use an air up/spring down set of retracts.

The real potential problem with a servo actuated retract is if the servo stalls under load due to linkage or a jam. This will likely result in excessively high amp draw from the battery. You can loose an airplane to this in a matter of seconds depending on the battery charge at the time. Even with a fresh charge a stalled retract servo will draw so much amperage, the airplane won't last a whole flight. So mechanical retracts need to be set-up properly, and if possible use their own battery in addition to the receiver battery.

There is more but it would take a long post to discuss it all.

For a smaller 2-gear airplane by all means try a mechanical servo actuated system. For larger aircraft use air systems and some simple installation methods.

Don't hesitate to try them out. By the way, if you ever build a twin engine aircraft retracts are a must. On most twins, the reduction in drag by retracting the gear will determine if you can save the model if an engine fails. Over-powering twins can make things worse. Your best friend in the case of an engine failure is minimize drag.

When our field is open again come find me and I'll answer any questions. If you want I'll also bring out some of my larger airplanes so you can see how the retracts are installed.

Chris

[AirKadet]

Chris,
Thanks for the informative and highly detailed response. I reallly appreciate it! Now, I have to improve my flying skills so that I feel comfortable buying a warbird that needs retracts.

Gilbert

[Chris Smith]

You'll have that warbird soon enough. Keep flying.