Building the Top Flite Giant Scale P-40 – P4 Cowl Mounting

After we built the first P-40 (this one being round two) and got a few flights on it we began to immediately see problems with the cowl mountings.  It seemed impossible to keep the screws from backing out and even around the ones that didn’t the vibration immediately started chewing up the mounting holes.  No amount of reinforcing of the fiberglass helped and while bonded washers have proven to be effective in this role they would have completely trashed the scale lines of this bird.  So when cowl number two arrived I got creative and decided we needed a better mounting system that didn’t result in having to put a bunch of (I don’t remember the exact number  but it seems like 20) holes in the cowl.  I came up with a system of mounting using blocks epoxied inside the cowl with 2 guide pins and two bolts.  It seemed to be working quite well… right up until an unfortunate lack of airspeed, altitude and ideas destroyed more than just the cowl.  So fast forward to today and I had to recreate the mounting method.

This time I thought I’d document a bit in case someone else wants to try it.  It worked well but it’s a bit devilish to get the alignment right.  It’s amazing how tightly it holds though with only two pins and two bolts holding it and zero holes in the cowl to mar the great looks.  The basic idea is to use the two bottom mounting blocks on the firewall for alignment pins, then using the the blocks on the bottom front of the motor box to accept bolts that are reachable via the (already needed) exhaust cutout.

Here is one of the firewall mounting blocks after drilling and the addition of an “alignment pin”.

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Then I marked and drilled a block with a matching hole.  Where I was careful not to drill all the way through the firewall, this block can be drilled all the way through.  I matched it up to be flush with the outside edge of the block attached to the firewall and also beveled the hole so that the locator pin could be inserted easier.  Here it is after the initial fit.

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The tricky part is to then put the cowl on and get it aligned as perfectly as possible.  Using the spinner back plate helps.  Once it’s right, mark the inside of the cowl and get prepared to apply some epoxy.

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Importantly at this point, where the block is going to be glued to the inside of the cowl, you need to clean the cowl inside surface with something that will cut through the mold release agent (think wax) that was applied when the cowl was made.  You can use some acetone (caustic stuff so be careful and make sure you have good ventilation) and/or sand the surface before applying epoxy.  Standard clamps can hold these blocks in place until they are set.  If you do this in the general area before you mark the block locations it will make you life easier in the long run.  Ask me how I figured that one out!

Similarly, the forward mounting blocks must be carefully aligned, marked and glued in place.  These I pre-drilled to snugly pass an 8-32 bolt and drilled and tapped the existing bottom motor box blocks to match.  Thin CA (very thin preferably) will harden those threads nicely, though you may have to re-run the tap 15 minutes later to make sure the CA didn’t block the threads.  Here is one method of holding the forward blocks in place while the glue dries… This method used masking tape and a “cut to length” dowel.  If you look closely you can see the holes to pass the bolts through have already been pre-drilled.

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In order to be reachable from outside the cowl the bolts are inserted from the bottom.  Happily this results in a very firm mount.  I used allen bolts which require a fairly long ball driver to access in this position.  Once you have these all in place you may (if your like me) find the blocks don’t perfectly match the surfaces to which they are attaching and one or more may need to be redone.  Sometimes a little sawing or sanding to make the surfaces match will suffice.  Otherwise a hammer will remove the block and a layer of the fiberglass with it (if you epoxied it well).

I’ll edit this post in the next few days to add a picture showing the mounting bolts in place.  I’ve done this twice now and it probably took me 3-4 hours to get it right, even the second time.  It’s time consuming and painstaking work but it leaves no marks and the cowl seems rock solid when it’s done.


Xion Wing Lock – Pt. 2

Last weekend at the field during the IMAC contest I lost one of the cams and the washer on the port wing of my Slick.  No big deal really… I have clips on my anti rotation pins on each wing so I needed a click of aileron trim and all was fine.  After the flight when I went looking for the reason I spotted the wing gap and on investigation found that the cam and washer were gone and the stud had rotated out a bit.  I didn’t have my spare so that wing went back to the nylon bolt for the rest of the day.

So what happened?  While it is impossible to tell for sure, I believe the problem was rooted in not seating the wing down snug against the body.  So when I locked the cam down it pulled the wing in a bit but not enough to put pressure against the cam and therefore with some vibration the cam escaped.  Maybe the stud rotated enough to further loosen as well.  Possibly contributing to this is that I have not glued the washers down inside the body either… perhaps if I had the slack in the system would not have been enough to allow the cam to escape??

The only other issue I can see is that perhaps the stud managed to pull out of the blind nut embedded in the wing root.  I am really not fond of blind nuts in any case.  I find most of them to be ill fitting/loose.  When used in conjunction with a standard bolt there just seems to be a lot of play and I have seen a bolt manage to walk itself out a thread or two before without rotating at all because of vibration and slop.

I have continued to use the system through another 6 flights or so and so far have not had a sign of any movement.  I may add some plumbers tape to the studs to cut down on “rattle” between the studs and the blind nuts before my next trip to the field.  Otherwise I will keep with it and see how it goes.

(Don’t recommend these anymore… see part 3)

New Gadget – Xion Wing Lock System

While walking the aisles at the Weak Signals show in Toledo I ran across the Xion Wing Lock System.  These little gadgets are an interesting alternative to 1/4-20 or 6mm wing bolts that screw into the wing root to hold the wings in place on your favorite aerobatic aircraft.  After watching the demo a couple times and recalling the times I’ve found a wing bolt rolling around in the bottom of the plane or struggled to get a socket wrench or screwdriver onto the wing bolts… I thought these might be an interesting alternative.  So wallet a bit lighter, I pocketed two sets of these and moved on with high hopes.  Here’s a stock image of the Wing Lock system from the Redwing RC website.

As you can see, the wing lock system consists of a stud that threads into the wing root, a rubber washer that installs onto the stud inside your aircraft body and the lever/cam lock that slots into the stud and, when rotated 90 degrees to flat against the body of the aircraft, tightens the connection.  Here is an example installed in my aircraft.


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To get to this point you have to drill out the wing bolt holes a bit and screw the studs into the wings.  (The enlarged hole is only 1/8″ or less larger so going back to 1/4″ bolts is not a problem if you later decide these are not for you.)

Now I will be the first to admit that I’m a bit of a gadget fan when it comes to RC.  I like trying out the new doodads and these are no exception.  I must also say that my current system of nylon bolts backed up by clips in the anti-rotation pins works excellently… So why the new system?

In addition to the allure of getting to play with a new gadget, I have always had trouble reaching the wing bolts inside my airframe.  The bolts are close up behind the main wing tube and with my large hands it requires a screwdriver to get the bolts inserted and tightened.  I was hoping that these wing locks would resolve that issue.  They looked very easy to connect at the show…

So are these the ultimate RC gadget?  Will they keep your wings firmly in place? Are they the solution to my wing bolt woes?  Here’s my experience so far.

I flew 3 times today after installing the Wing Lock system.  The wings stayed firmly in place and the wing locks didn’t seem to have budged… Not the slightest movement or loosening was apparent.  There’s seems to be a lot of questions out there on the web about whether these things will hold.  I’d have to say I’m no longer concerned about that.  Based on my experience, I’d have to say that they do work as advertised in this regard.  In addition the craftsmanship seems to be excellent and they are very light, which is always a plus.

The major selling point on these seems to be that they make wing assembly quicker and easier.  In my particular application it’s a bit difficult to reach the assembly point inside the aircraft body and at first blush this system didn’t make that any easier.  My fingers still don’t reach there easily and I can drop these into the bowels of the airframe just as I did the wing bolts!  I must say they are significantly faster (no 15 turns of a screwdriver in a tight area) so in that regard the system works as claimed.  I do appreciate this because it can be a back breaker at times to reach the wing bolts.  Using the wing locks doesn’t change how hard it is to reach this part of the airplane but I can’t blame the Xion folks for how my airframe is laid out and they do go together faster than my old wing bolts so less time spent hunched over is an improvement.

Not much can be done about where I have to reach to attach my wings and where things land when you drop them inside this airframe…. so I’m going to try tethering the cam locks to help limit the number of times I have to fish out the parts!  Maybe by doing that and with a bit more practice installing the cam levers I can maximize the benefit of the system.

Overall I like the Xion system and they certainly make assembly of my aircraft a bit quicker.  It’s always difficult to justify spending $15-20 when you have something that works already in place but I don’t regret spending the money for the Wing Locks.  I think I’ll keep using them and will update with any new revelations or changes I make.  So far I’m still thinking these are pretty cool.

(Don’t recommend these anymore… see Part 3)

P-51 Red Tail – Two Starboard Ailerons??

I’ve been spending a bit more time working on the replacement wing for my Giant Scale P-51 Red Tail Mustang.  After cutting out the covering appropriately and recovering the tips in red and adding the yellow stripes I started working on linkages.  When I went to attach the control horn to the port side aileron I discovered that there was no hardwood mounting plate on the bottom of the aileron to mount to.  Of course I did so by drilling a hole and then realizing the drill went in far to easily!  Stopping to consider I realized there was an appropriate right and left aileron and I assumed I had picked up the wrong one…  (please imagine appropriate cursing as I realized I had already applied the small red and larger yellow material to the tip of what I imagined was the “wrong” aileron, not to mention the new hole I had just drilled)

I then went to retrieve the other aileron and quickly determined that it was an exact replica of the one I had already worked on…. not good.  As you may have figured out for yourself by now, they should NOT be duplicates.  In particular the port side aileron should have a hardwood plate in the appropriate spot to mount the control horn (close to the inboard end of the surface on the bottom).  The starboard side aileron would have a similar plate but because these surfaces are tapered the two are mirror images, not duplicates.  What happened, it appears is that I have two ailerons that would both work on the starboard side but not the port side.  Here is a picture of the two ailerons side by side (you can see the half-circle hardwood plate through the material if you look closely).

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I double checked the flaps and they are fine but the ailerons I have are not.  Either the plate was simply installed on the wrong side or I have two starboard ailerons.

Not caring for the option of calling the hobby shop and/or Top Flite with the inevitable delays that would ensue before they came up with a replacement part, I decided I needed to correct this issue.  Don’t get me wrong; they are all good folks and I feel confident they would do all they could to help resolve the issue but what are the chances they have an extra port side aileron laying around?  It took months to get the replacement wing the first time.  The balsa is certainly not going to be sufficient to hold the control horn screws securely for this big heavy bird so some surgery was in order.  It was time for the “modeling” part of the hobby!

I located a small piece of 3/16″ aircraft ply from another project and cut a nice square out.  I noticed my old wing used square insert and the new one uses the half circles so I figure either works as well and I can cut squares easier than circles!  I then cut away the covering material around the area I had to work on and used the ply square as a template to mark the surface.  Using an X-acto knife I cut around the block and then carved away balsa until I could fit the ply block in place.

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Once satisfied with the fit I mixed up some 15 minute epoxy and coated every surface that would have contact with the new block with epoxy.  Once in place I wiped the excess epoxy off with alcohol and set the piece aside to dry.  With some minimal sanding and a bit of patchwork covering I should be able to get back to making some progress tomorrow!  Here’s the repair awaiting some recovering work.

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One lesson to take from all this is to keep the ailerons,flaps and whatever other parts of your ARF came partly assembled or packed together… together.  If the ailerons had been correct I still might have picked up the wrong one and run into this same issue.  Marking them might be a good idea if you are going to be disassembling and reassembling things.

I wonder if somewhere out there is someone with two starboard side ailerons on their Giant Scale P-51??


P-51 Red Tail – Main Gear wheel pant attachment. A better way?

After my off field landing I decided to assemble a replacement wing.  I’ve been working the replacement wing a little at a time.  The latest project was to attach the wheel pants/doors to the Robart retracts (electric conversion).  The manual directs the builder to glue these wheel pants on with Epoxy but that seemed like it could be a bit weak.  Maybe I’m just overly cautious, but I thought something a bit more secure might be in order.  I couldn’t help but think if it didn’t pop loose between the attachment blocks and the wheel pants it would surely vibrate off on the other end between the blocks and the struts themselves.  Here is a view of my “improved attachment method”.


I sanded the back of the fiberglass door and the strut then used epoxy as directed but added some wire to hold it tight against the strut.  In order to keep the wire from causing a gap I had to cut a couple of grooves across the top of the blocks as well.  Here’s a view of both blocks in place.


I used one continuous wrap of wire rather than two separate wraps, thus the crossover you can see on the lower end (right).  That seemed like it would, at a minimum, keep the blocks attached to the struts but what about the wheel pant attachment to the blocks?  On that end I decided that a button head screw would add at least a bit of extra hold while also holding things in alignment while the glue hardened.  Below you can see the view with the gear up.  You can clearly see the two button head screws (#2 x 3/8″ I believe).


Hopefully between all of these modifications the wheel pants will stay put.  Lots more work to do but with the gear in and pants in place it feels like I might be closer to finished than just getting started.

Cable Ties, spiral wrap and RC aircraft wiring

I’ve always been a proponent of wiring being tightly tied down on an RC airplane.  You certainly don’t want the wires to flop around and catch on servo arms, linkages and the like.  This would likely result in the wires pulling loose, abrading against the internal structure of the plane or just breaking due to repeated bending.  Worst of all if the wire starts to break inside the insulation where you can’t see it, the connection is likely intermittent and therefore difficult to troubleshoot.

Usually folks control this by tying the wires to the internal structure with cable/wire ties.  These are usually made from flexible nylon with locking teeth built in that make them hold nice and tight and just about impossible to loosen.  That’s all well and good but take a look at what happens when you pull these up, perhaps a bit to snug, against your servo wires.


As you can see the insulation on the red and black wire (for instance) has been pinched and cut through down to the conductor.  It’s not impossible that somewhere down the road these two wires would touch or simply be cut and as a result a short or open circuit would occur with unfortunate results.  Since this is the power lead on an ignition for a small gas engine… at best an open circuit occurs and the engine quits unexpectedly.  At worst… well the flaming parts of airplane falling out of the sky probably won’t win you any “Club Member of the Year” awards.  (Hey, I did say “worst”)

So my suggestion is this:  Continue to use wire ties as needed but only do so with proper protection around the wires.  For instance, wrap the wires first in a spiral wire wrap like the material provided by the ignition manufacturer.  Then use your cable tie and tighten it down just enough to get a good grip on the wrap material.  Something like this:


This wrap is also useful to protect the braided ground wire on the outside of the spark plug lead from getting abraded by the edge of a fiberglass cowl.  You only want to protect the necessary part of that lead however as this nylon material is typically not made to withstand the temperatures of your exhaust or cooling fins on a gas engine.  Here is an example:


Of course, this is just one option.  I’m also a big fan of Velcro but that’s for another post.

Building the Top Flite Giant Scale P-40 – P3 Canopy Mounting

Today we tackled the mounting of the canopy on Kelly’s P-40.  Of course the manual directs the canopy to be glued down but we agreed that doing so just creates a problem.  With a detailed cockpit and a pilot that can turn his head, it becomes almost a certainty that the canopy will have to be removed once or twice so gluing it down is just not an option.  Magnets didn’t seem to be a good option for this large a canopy on a big plane like this so screws seemed to be the best option.

The issue with using screws is there is not a lot of structure to screw to, so a bit of reinforcing is in order.  We tried to use less screws this time as we thought it was a bit overdone on the last model.  First we placed the canopy on the model and determined where to drill holes.  After  that, we taped down the canopy and drilled pilot holes before running screws down to see where the reinforcement is needed.  Here it is at that point.


I have marked the screw locations with a green arrow in this shot and of course there are two more on the far side that cannot be seen from this angle.  As you can see, the front two are at least mostly embedded in some stringers:


Just to be sure these bit well, I dripped some thin CA on the holes (after removing the screws) then used some thick CA to add a small square of “popsicle stick’ to the stringer to give the screw a bit extra to bite into.

Along the sides of the canopy we were out in the middle of balsa sheet so a similar process was used.  Note that before running the screws back in you should allow for the glue to dry completely and then re drill them with a bit that is appropriately sized to open up just a pilot hole.  Otherwise you run the risk of splitting the new reinforcing wood which defeats the purpose of adding it in the first place.  Here is a view along the side after the reinforcement was finished.


I am hopeful that this will be enough to give us a good firm attachment and if they start to loosen up, at least there will be enough wood to drill out for inserts.  I also think we will go to a true button head screw on this go round and touch them with some appropriate green paint to make them blend in better.

Once this is complete, we can work on the cockpit interior at our leisure.  Once the interior is complete we can then grind down any screws that might protrude to far into the cockpit.

Next is likely to be a bit of cockpit interior or perhaps servo control linkage installation.


Building the Top Flite Giant Scale P-40 – P2 MPI Switch

This post is a continuation on the topic of the Top Flite P-40 ARF.  This time I wanted to just quickly show what we have done with the MPI switch that Kelly used in his first P-40.  We decided to install this same switch (which is undamaged) into the new bird but we were never happy with the location so we decided on a new position.

The MPI switch that Kelly purchased is the unit with dual HD switches, charge jacks and an integrated fuel dot.  It is a fairly massive piece of hardware and comes anodized in one of several colors.  Here’s a picture of the one we are working with:


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You may notice that this one has an added hole right above the word Miracle… we added that in order to better stabilize the switch plate as we had it mounted in an area that was very thin balsa and curved at that.  This was never a satisfactory configuration because of continued issues with it occasionally coming lose and the fact that it looked out of place on the side of a big war bird.  I have some other issues with the unit as well.  One that always confused me was why you would create a switch panel that has the switch “on” position being down when referenced to the lettering on the front panel??  The second is that the unit has a very complex shaped hole to mount it and MPI does not provide… not even for download… a template to cut the mounting hole for it.  It is a non-trivial task to cut a proper opening and the lip available around it that allows for a margin of error is extremely small.  This thing requires a bit of modeling skill just to get it mounted and that seems unnecessary to me.  To bad as the MPI switch products seem to be otherwise quite high quality.

Here’s what I consider to be the necessary tool to make yourself a template for this beast:

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This is a contour gauge, commonly used for transferring complex curves and the like in woodworking.  By pressing this against the side of the switch, the basic pattern can be transferred to card stock.  Do it again from the other side and make a few measurements to decide how far apart the two sides are and a basic template can be drawn and cut out such as the one below:


This isn’t perfect but after transferring to the selected area of the airplane and a little work with a Rotozip bit results in this rough opening.  We had tested the fit and pre-drilled the mounting screw holes by the time of this pic:


After that it’s just bit of clean up and the switch drops in to look like this (note this is flipped upside down from the last pic):


When we are ready we will put the screws in place and all is well.  This area is under the planes “chin” at the back of the cowl where it will be easy to reach in and feel for the switches.  We were careful to arrange the panel so that forward on the switches represents “ON” and rearward is off.  Likewise we will make note of the charge jack polarity before the final buttoning up happens.

This location seems like an improvement over the side of the body just below and forward of the cockpit area that we used on the last plane.  That area is just light balsa sheeting while this one is ply and therefore much more sturdy.  The new location is also well hidden but still reasonably accessible.  Having placed fuel dots on the bottom of airplanes before I can attest that this mounting location will require much more care when fueling to avoid spillage but it is the best compromise between accessibility and functionality that we found available on this particular airplane.

Next up we will do some work mounting the canopy such that removal is possible when needed but it remains secure.

Next time we’ll do a bit of canopy work.

Building the Top Flite Giant Scale P-40 – P1 RotoFlow

Last winter my friend Kelly and I put together his Giant Scale P-40 ARF.  It flew and looked great.  Here’s a stock photo from Top Flite



Unfortunately, in late summer the airplane made its last flight.  An engine out landing combined with some unfortunate tail winds combined to result in a badly damaged airplane.  The decision was made to replace the plane rather than attempt a rebuild

While I don’t plan on documenting the whole build, I thought it might be worth posting a bit on where we decided to make a few different choices of equipment or modifications from the reccomended.

One thing we decided on was the use of a JL Products 24 ounce RotoFlow gas tank.  Of course, the plane is not designed for this tank shape so some  modifications were in order.

First, in order to gain enough clearance we had to trim out the former at the back of the motorbox.  I should have taken a before and after picture but all I got was an after.  The part that was removed was an inverted “T” shape that I’ve outlined in this picture.


Secondly, the tank neck and overflow won’t quite fit in the forward bulkhead just behind the firewall, so a “V” cut was made as seen here:

2013-12-15 15.04.03 And finally, since the tank is a bit longer than the supplied unit, a bit of a trim is necessary on the radio tray that fits directly aft of the tank.  Here is the piece that must be cut out.

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Once that is done, the tank fits nicely and with a minimum of padding will be a nice snug fit.  This modification was done on on the first plane and now on the second.  Here is the current plane during the tank fitting today.

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Next time, I’ll go over the switch installation change we decided on for the new airplane.

Conversion of Robart retracts from Air to Electric for the Top Flite P51

After the “off-airport” landing of my Mustang a while back, I decided that it was time to get the retracts working.  I had them in the plane (locked down) for several flights and they did not appreciably change the flying characteristics of the plane and seemed at least as capable as the wire gear in absorbing light to moderately bumpy landings.  I hope I’m beyond hard landings unless a mechanical issue arises so hoping I can avoid any truly hard landings in the future.

Of course, these retracts were bought in 2004 and were built to be run by air but I had pretty much made the decision when I assembled the bird that they were going to have to be converted to electric use.  The area where the air tank would normally go was filled with padding and batteries and pretty well buried anyway so the conversion was a given if retracts were ever to be functional.  The retracts got a little minor repair and have been removed from the wing anyway since a new wing is on order!  The current wing is fixable but a fair amount of crushed wood and cracks in the main spar convinced me it would be easier and maybe smarter to just get a new one.

After taking a little time to replenish the piggy bank, I ordered the complete main and tail wheel conversion kits from Robart via my favorite local hobby shop.  A week or two later I had them in my hands and I sat down at the bench to get started.  This will give you an idea what the kit looks like for the mains.




Here is what the starboard main looks like before the conversion and then after….



I found the instructions to be above average but the one part that was not spelled out clearly is that there is a difference in the two replacement trunnions.  At least I didn’t notice it being mentioned anywhere.  There is a right and a left and if you use the wrong one you end up with what is pictured above!  Yep, I had a 50/50 chance and I chose unwisely.  Oh, the retract works great… if you want the wheel to thrust upward through the wing instead of dropping down below it!!  Hah!

So I got to take it all apart again and do it a second time.  I was getting pretty good at it by then and hadn’t yet put the thread lock on it since I’ve done a bit of modeling before and pretty much knew that I’d mess it up somehow.  It took another 20 minutes or so but then all seemed good and worked pretty well… mostly.

The next problem I had was that occasionally one of the retracts would slow down to about half speed or just outright stop completely.  Since this system was designed to stop at the end of travel solely by sensing the increase in current draw caused by the increase in physical resistance, I concluded that the controller has no way to know the difference between end of travel and any other major increase in mechanical resistance.  I tried a little lubrication (WD-40) and that certainly helped but one retract would still stop in mid stroke on occasion.  After loosening the screws up a bit and exercising things I re-tightened and apparently the better alignment that resulted has resolved the issue.  I still have not applied thread lock as I’m saving that for the final install once I insure everything is well when they are mounted in the new wing.  Unfortunately, Top-Flite has not been as quick to send out a new wing as Robart was at sending the conversion kits so this last test fitting will have to wait for a while yet.

The tail wheel kit is much simpler.  Here is the kit as it came out of the package.


After the mains, the tail gear presented no challenge at all.  The only gotcha, which IS clearly spelled out is the possibility to set the controller to the wrong “type” for this retract with the possibility of burning out the motor.  Read carefully and follow instructions and you should not have problems.  I didn’t!

One final warning is that the actuator wires that enter near the clevis pin actually interfere with the insertion of the pin.  You can see the issue here.


A bit of judicious wiggling and a bit of push on the wires to help them clear the pin is all that is necessary to avoid the wires and since they are protected with a sleeve of some sort it does not appear that rubbing will be a problem down the road.

That’s about all there is to say beyond what is clearly documented in the instructions that come in the kit(s).  For about $300 you get what seems to be a bit simpler system which should result in some improved reliability.  I have heard that some are unhappy with the ~10 second time for the gear to fully retract or extend but I like the smooth action better than the usual slap and clunk actions I have seen from many air systems.  This time can be somewhat decreased or increased by feeding the system higher or lower voltage.

I plan to feed mine direct from the flight battery/receiver voltage as the system uses little current and I am carrying 2 x 2300mah A123s so battery capacity is not a big issue for me.  I supply the ignition from these same batteries using an Ultra IBEC and even so I’ve been flying 6 or more flights between charges with no issue.  At worst I expect to charge every 4 or 5 flights with the new added load.  I’ll keep an eye on battery capacity and usage for a bit to see how that goes.