Gordon Mc

My Feedback: (11)

Back to the tanks. Sideshow Bob told me he had tried making his own tanks a while ago, for his Skymaster F15, and he'd given up after a while. One of the things he seemed to struggle with a bit was getting the material to stay where he put it when wrapping it around a compound curve etc.

I figured the easy solution to that was vacuum bagging, but unfortunately I never did finish building my home-made vacuum bagging system (can you tell that I enjoy building stuff ? ...and that I never have enough time? ) , so I figured maybe this was a perfect excuse to buy some more tools (as if us guys ever need a real excuse !). So, I called up ACP (Aerospace composites - http://www.acp-composites.com/about.htm ) in Livermore and told the guy I was interested in buying one of his bagging systems. He asked me what I was going to do with it, and after spending about 15 minutes with me on the phone he said "You know, I could sell you a bagging system if you want, and make a nice little profit on it, but you really don't need that", and he proceeded to explain to me what method I should use that would save me a bunch of money. It's not often that you get a company that will do that for you, so take this as a ringing endorsement of ACP.

Anyway, the first thing I now needed to do, was get rid of any unnecessary concave curves, so I reworked the "bubble" on the tank plugs, as shown in the next pics:

Gordon Mc

My Feedback: (11)

I applied a coat of spackle to fill in any holed in the foam, then sanded it all smooth. Then I applied a layer of 3/4 oz glass cloth all over, and immediately (while everything was still wet) then applied the kevlar/carbon cloth (From Art's Hobbies) on top of that. Next I applied strips of 2" peel-ply, and then the magic ... industrial grade cling-film - the stuff that is used to wrap carbdoard boxes etc for added strength. I picked up this 6" wide stuff at Office Depot - enough to do a hundred tanks, for about $2. I simply stretched the film as I was wrapping it around the glassed tank, which applied pretty even pressure all round and kept the cloth firmly in place (even over the tight corners) until everything hardened up.

The other 2 pics here show the larger of the 2 tanks just after the cling film & peel-ply have been removed.

Gordon Mc

My Feedback: (11)

Then it was time to split the tanks in order to remove the foam. A small circular saw blade made the cuts through the kevlar etc, then a mini-saw was used to cut the foam. Although the foam can be disolved chemically, apparently that can be very messy, so I used Dremel sanding drums etc to remove most of the material, then only used thinners to disolve the very last little bits. A couple of slightly rough spots remained, but a light dusting with some 220 grit sandpaper took care of that.

Gordon Mc

My Feedback: (11)

Tank hardware was added before joining the tank halves again, while we have easy access to both sides of the fittings for ensuring good glue application.

I chose to use the BVM tank hardware - a brass nipple for the overflow, that has nuts on both the outside & inside of the tank, and the main stopper being an aluminium screw-in part with an o-ring for a leak-free fit (the aluminium part screws into a brass fitting that we glue into the tank).

Then, for joining the tank halves again, I thought about making a lip on the inside of one half to ensure that I didn't get any glue "drips" on the inside of the tank creating a rough surface that could be abrasive to the clunk line, but eventually chose instead to tack-glue the halves together with PIC Plasti-Stic, then smear aeropoxy along the join line to fill in the gaps without the glue being able to "drip" inside, then I applied a Kevlar bandage around the outside to provide the strength to keep the parts together.

BTW, for those not familiar with this stuff, the white strip in the second pic is the peel-ply, whcih is simply wrapped around the 1" kevlar bandage.

Total time spent for building both tanks - probably about 3 hours of actual work, spread over 3 days to let everything cure.

WhoDaMan

My Feedback: (164)

Here she is gordon,,,, you were right


http://www.nylon.net/sabrina/oddthings.htm

Gordon Mc

My Feedback: (11)

Access to the wing-bolts etc is made through the cockpit area, so anything I do in the cockpit has to be easily removable. With that in mind I'm not going to bother doing a highly detailed and accurate cockpit tub, because access is more problematic than I care to deal with.. I am using a full-body pilot though, so I needed to create a basic removable tub to support his seat etc.

I added a fixed semi-bulkhead at the point where the fuelage slopes down for the rear end of the cockpit. Then I needed to figure out how deep the tub needed to be to get the pilot at the right height, so I just threw together a little bundle of sticks that could be dropped into the cockpit area with the pilot sitting inside, then I kept adding balsa spacers under the plot's butt until I got him where I wanted him, and used that information to determine the tub dimensions.

Then I took a pic of the ejetcion seat from a Revell kit, and blew it up to the correct size and used that as a rough template to knock together a quick & dirty ejection seat that would position the pilot where I wanted him and still give me the optimum amount of space under him to fit other gear.

Please ignore the horrendous black paint on the seat - that's just a temporary sealant and the final color will be much more appropriate once I add the necessary detail work etc.

Gordon Mc

My Feedback: (11)

LOL ... had to be something like that !

Jim Patrick

So Gordon,
What kind of baffles did you put in your home made tanks??

mickreeves

To GORDON Mc
Quote "Earlier, it was suggested that a dual wall pipe would weigh twice what the single-wall one that came with the kit does,"
Can you tell me what thickness steel sheet was used for your twin walled duct. We found that the thinner {and lighter] material did not have enough stiffness to resist the suck-in effect due to the low pressure inside the duct. Occasionally ducts have been known to suddenly collapse inwards. I would also like to what the twin duct cost.
Mick

mickreeves

Tanks.
I try to make things easy and economic for my customers. The soda bottle tank works very well. Why do you want to make things difficult for yourself
Mick

seanreit

My Feedback: (60)

Gordon, take another look at your fuel line above, I think you need to switch that out to Tygon or other. Looks like you got glow lines on there. I recongnize I'm saying the obvious, but just wanted to make sure you didn't get too far on that. You probably are just doing a leak check with alcohol ?

seanreit

My Feedback: (60)

Mick, here in the states we are "encouraged" to go with Kevlar Tanks whenever possible. Although here is a testament to their strength, last Friday I totaled my Bobcat at FIJR. The tanks came out completely unharmed, airplane was in 500 pieces.

There are so many Turbine Flights over here and that many more crashes. Anything to keep a fire from occurring is "encouraged". Not mandatory, just not a bad idea. Probably has nothing to do with the success you guys are having over there.

Sean

Gordon Mc

My Feedback: (11)

For the smaller (rearward) tank, I didn't bother with a baffle - it's only marginally larger than the centre tank on my Bobcat, which works fine without a baffle. For the wider (forward) tank, I made up a couple of pieces of suitably shaped G10 with small holes cut-out on the outer surfaces, so that fuel can move around freely, but not too quickly - basically a variation of the technique that BV uses in the two wing mounted tanks that go in the Bobcat.

Gordon Mc

My Feedback: (11)

Hi Mick,

Sorry - I don't know off-hand what the thickness is, but I can try to measure it if you like, and/or ask Tam what material he used. I won't pretend to know much about pipe design or construction, but perhaps when you go to two layers each one can be thinner than if only a single one is used ? Dunno. If you'd like to know more, and get it from the horse's mouth rather than get my uneducated guesses on the subject, I can drop you Tam's email address and / or number so that you can chat with him about it.

As for cost - IIRC, I paid Tam $140. That's certainly more than the $35 that UPS quoted me for shipping my old pipe back to you to have it shortened, but the difference is less than the cost of one servo, so I figured it was no big deal.

Note for anyone else reading this thread and thinking about building the kit - the switch to the dual wall pipe is simply my personal preference, not something that has to be done. I'm not the only person to make that choice, as one of the magazine reviewers also did so, but there are also others who are perfectly happy with the single-wall pipe. If I didn't have a world reknowned pipe specialist living in the same town as me, I may well have stayed with the single-wall pipe too, as the hassle of getting a different pipe made up could possibly have negated the small benefit I see in the dual wall system.


I explained that a little bit in my post last night, but didn't go into great detail - so lemme see if I can do a better job this time around. I know that soda bottle tanks are very popular in many countries (just as plasma bags are / were at some point), but over here our regulatory authority is a tad more anal about trying to manage fire risk than you seem to face over there (maybe it just rains so much in Britain that you don't have to worry about burning up thousands of acres if a model catches fire when it goes in ? ).

In addition to any pressure we get from the AMA about fire risk, some of the counties, flying clubs, etc are also so afraid of a potential fire that they also impose restrictions of their own, and in some clubs all it will take is one 5 acre fire, and turbines will be outlawed there for good. So, self-interest dictates that we do what we can to reduce the liklihood of us getting into such a situation. Whether that be by turbine pilots having a self-imposed "no fly duration" at the height of the California fire season, or by voluntarily adopting better fire prevention techniques such as kevlar tanks or engine-to-tank crash separation techniques as attempted in the BVM Bobcat, or pilots clubbing together to buy a quad-bike armed with multiple fire extinguishers to have at hand whenever they fly, etc., etc., ... anything at all that we do to help reduce fire risk helps to ensure that we may actually have a field to fly at next year - so given the $50k or so that I have "invested" in my jets, a little extra time or money for a kevlar tank is basically a no brainer in my opinion.

There has been some talk about possibly outlawing soda-pop tanks, just as plasma-bag tanks were banned, but for now it's 'voluntary' for us to do whatever we can to reduce the risk of fire in a crash. My personal opinion is that in a crash soda-pop bottles will burst much easier than sturdy kevlar tanks do, and I've also had a chemist tell me that the material used for many soda bottles may actually become more brittle over time especially with exposure to kerosene & UV etc.

Dunno whether any of those are proven facts to anyone's satisfaction, but the strength issue alone seems reasonable enough for me to want to do my bit to reduce the liklihood of us losing a field due to a fire if I'm unlucky enough to stuff the model at some point. This is really no different than how some people continue to use soda-bottle tanks in their Roos etc., while others choose to replace the tanks with Kevlar units from e.g. gary Meuller in order to do their bit to mitigate fire risks.

Also, as I mentioned last night, I figured that it might be nice to have some baffled tanks to avoid having quite a few pounds of weight being able to move back & forth at will. (several pounds being able to shift fore & aft by a foot or so doesn't inspire me as being immensely good for the CG). Again, just a personal preference, plus I'd been interested in having a go at making my own tanks for a while, and this just ended up being the perfect opportunity.

While I'm mentioning CG ... for anyone else building this kit, be advised that the CG shown on the plans differs from the suggested CG that is now posted on Mick's website. The plans show a CG that is a fair bit aft of the website recommendation, so please take note.

----

BTW Mick - at one point you indicated that you'd be able to give me the info about where the nose-gear should be modified to add bolts (or whatever) for steering, since my early nose-gear mistakenly had no steering mechansim on it. Can you share that info here ? The NG steering is one of the next things that I have to figure out how to engineer, so any tips you are willing to share would be welcome.

Cheers,
Gordon

Gordon Mc

My Feedback: (11)

Cheers Sean ... as far as I'm concerned, don't ever be afraid of stating the obvious, as (a) it may sometimes not be as obvious as you think, and (b) even geniuses (or is that genii ?) like me [X(] screw up once or twice a week.

In this case, the silicon fuel tube was initially just used to perform pressurized under-water leak tests, after which I simply looped the silicon line from the inlet to the outlet in order to keep all dirt out of the tanks while construction of the aircraft continues. Once I start the actual plumbing, that line will come off and be replaced by safety-wired tygon line.

Later,
Gordon

Gordon Mc

My Feedback: (11)

Now, I did take a bunch of photos while doing the following steps, in order to share the technique with you guys, but now I can't find the darn things, so we'll have to just improvise with descriptions and a few pics of the completed item...

Next up ... the tires. I think I mentioned previously that I was considering doing new tires, for two reasons:
[ul][*] One of the surfaces I fly form is pretty rough, and while the fairly soft foam tires provided with Mick's gear is probably fine for some surfaces, it would probably only last a few brake applications on our runway surfaces before that foam got chewed to pieces.[*] I found that in order to have the rear edge of the tire NOT be higher than the surrounding wing sheeting, I had to rotate the gear strut to lie the wheel slightly angled ; unfortunately, doing so caused an inordinate amount of toe-out, which is pretty undesirable. I tried finding a way of recessing the gear further into the wing to let the whole wheel sit higher up (rather than just the rearward portion), but couldn't come up with enough difference - at that point I realized that the cross-section of Mick's tires is pretty round, "ballooning" the tires beyond the sides of the wheel hubs, and I realized that if I simply made new tires that didn't balloon, I'd get enough of a difference in the height of the tire at the rear of the wing, to get it to sit fully enclosed without any toe-out. The pics I found of full-scale Hunter gear (one enclosed below) suggested a flatter tire profile anyway, so this seemed like a reasonable change to make.
[/ul]

Gordon Mc

My Feedback: (11)

With that in mind, I set of to get new tires. While I was hunting for a supplier, I saw a thread by "dbarrym" about how he home made some tires from layers of tough rubber that his company uses to make medical braces etc. (Barry - please correct me if I've got the use wrong). I enquired about getting some of this material, and Barry very graciously sent me enough to make 3 tires. (I needed 2, but wanted enough material to allow me to screw up once without ending up screwed).

The material he sent me is two layers of hard 1/2" rubberized compound, glued together with some super-secret shoe-glue You can see a pic of it shown below (and the original MR tire next to it). I covered a face of the material with wide masking tape to allow me to mark it, then drew out the circle that forms outline of the outside of the tire (this is where I had a load of pics that would have saved me many words...) I used a regular scroll-saw to roughly cut the rubber to the circular outline, then drilled a hole through the center and used that to install a mandrel that I swiped from a large (6" ) sanding disk (the kind that you chuck in your 1/2" drill).

I then shoved the mandrel in the chuck on my lathe, and proceeded to do the finish cutting on the tire outer surface, clean up the 2 faces, and finally put a bit of a radius on the corners.

Then I cut out the center of the tire out on the scroll saw again, to match the inner hub diameter, then used some dremel milling bits to cut the necessary recesses for the hub edges to sit around the inner part of the tire.

At this point I had a hole in the rubber that was of course smaller (by about 3/16" in radius, or 3/8" overall, at a guess) than the widest part of the hub. Since it's a one-piece hub, that meant I needed to stretch the rubber slowly over the hub to get it in place. Since I was familiar with the tools that BVM sells for replacing tires on his hubs, I decided to create a similar tool out of a plastic spinner, some plywood, and some brass tube. The brass tube was selected to match the diameter of the axle hole in the hub, and was aeropoxied into the spinner (ensuring perpedicular alignment), then I added a series of plywood disks until I could flair the spinner rear end out to match the outer diameter of the hub. The brass tube was then inserted into the hub hole, the spinner was lubed up with plenty of 409, and then the new tire was slowly worked down over the spinner until it plopped into place on the hub. Once the 409 evaported, PIC Plasti-stic was then used to adhere the tire to the hub.

The end result was a tire that is hopefully harder wearing than the original, and which also no longer has the tire "toe-out or stick out" problem that I encountered earlier.

Kevin Greene

My Feedback: (85)

What size is the tire OD??? Can you post a close up pic of Mick's wheels??

Kevin

Gordon Mc

My Feedback: (11)

Nose gear doors.

There are 2 NG doors - one opens rearward behind the strut, the other one opens forward (the NG itself retracts forward).

When I initially cut the gear doors out based on the scribed outlines on the fuselage bottom, I encountered a slight problem - when the gear retracted, the tire hit the fuselage forward of the door cut-out, so the gear would not fully retract. I reworked that by lengthening the door and splicng the newly cut-off piece back onto the gear door, and thought I'd solved that problem. WRONG !!! When I extended the door forward, I allowed enough for the tire to clear as it retracted, but I stupidly had neglected to consider the issue of how the door would be closed... as well as a hinge, obviously I needed something to allow an air cylinder to push the door open and pull it closed, but when I installed an Usher offset door hinge with it's associated horn, the nosegear tire now hit the horn ! DUH.

Once again, I enlarged the gear door forward, until I eventually had the necessary clearance. Of course, now the door looks like crap, so it will need some tidy-up. I added a 1/64" plywood inner lip for the door to sit on when it closes, and used a simple Du-bro hinge for the rear gear door. (That one will be opened & closed via a pushrod connecting it to the nose gear strut). I also had to go back and rework the clearance that I'd left on the rear door, as I'd originally kept it tight enough that the door would open 90 degrees ... then realised afterwards that much more clearance is needed since the door (per the full-scale photo) actually opens WAY beyond 90 degrees (if it didn't, then the NG tire would hit the door when the strut is compressed, which is how I first noticed my mistake)

Gordon Mc

My Feedback: (11)

OD is 110mm or 4 3/8".

Not sure what you want a pic of - the tire, the hub, or wot. If you want to see the detail work on the hubs, I'm afraid you'll have to wait a bit... the gear door portion that is attached to the struts actually covers the outer face of the hub, and until I disassemble that, you won't see much of the hubs.

BTW, I should have made it clearer, but in the prior pics, the tire that you see actually mounted on the hubs is the replacement ; the one that is just a tire with no hub is Mick's original tire.

Kevin Greene

My Feedback: (85)

Thanks Gordon----The reason for my inquiry is that I was curious if the MiG wheels/brakes from BVM would work. The are very thin but only 4 inches in diameter.

Kevin

Gordon Mc

My Feedback: (11)

The BVM parts might work, though there would clearly have to be work done to adapt them to Mick's struts, and I think there's a good chance that there's not enough material in the brake hubs and/or struts to handle that without substantially weakening the axle retention. (Mick uses a bolt - 1/4x20 ? - as the axle, and the strut is tapped to receive it).

If anyone wants to go down the BVM parts route, it would probably make more sense to have completely new struts made too.

Gordon Mc

My Feedback: (11)

I spent a while trying to figure out the best way to lay out certain components in the nose area ... as well as 2 or 3 batteries, I need to fit in an air cylinder for the forward nose gear, and the servo for the NG steering, and obviously I wanted to try to maximize use of the area while making sure that none of the components gets in each other's way (don't want a battery lead interfering with hte steering mechanism, or the steering servo arm interfering with the door cyliner etc.). I also wanted to make absolutely certain that removing any of the components for maintenance was as painless as possible. (None of this nonsense about spending 5 minutes getting just one screw out, as I've had to deal with on some friends aircraft.)

What I eventually ended up deciding was that I wanted the servo upside down - to keep the pull-pull lines as short and as close to horizontal as possible, and to allow me to have both the steering cables and the cylinder pushrod protected by a "tunnel" that would keep these items well separated from batteries, leads, etc. I also wanted everything retained by a simple mechanism involving rearward facing screws. Here's what I ended up with:

Gordon Mc

My Feedback: (11)

Since my nosegear was missing its steering mechanism, and I hadn't got the info from Mick about his mechanism and its placement, I played around with a few prospective geometries, then just went for it.

I elected to use a couple of the plastic parts that are typically used for controlling aileron torque rods on small models, and attach those to my selected point on the sides of the nose gear strut via 4-40 set screws. Given the size of the gap between the NG rails, even when I opened the forward sectoin up a bit I was still going to be a bit tight on space, so I simply resolved that by reducing the width of the chosen plastic parts as shown. The NG was then drilled and tapped, and some temporary kevlar thread used to check whether guide channels would be needed. Once I put the "real" pull-pull lines in, they will be kept out of the way during retraction by means of simply knotting a rubber band around the middle of each line, and stretching the bands to tension the lines ; the other ends of the bands will simply be looped around one of the white platsic clips shown, which will simply be CA'd to the inside of the upper fuz sides.

iiievolution

Hi I thought you might like to see a picture of my MRM Gloster Javelin. I am in Cyprus. It flies a treat.