Ch 21 Strakes – Fuel/Baggage

Date:  05-15-2018

Number of Hours:  6

Manual Reference:

Here we go again. This is my fourth strake skin and I’m starting to get the hang of it. I guess I’m a slow learner. Left bottom this time.

Here I’m using the right skin as a template to get me started on fitting the foam to the curved fuselage side. It definitely helped. Interestingly the top skin curves are not the same as the bottom skin curves. So if you are building one of these planes take note. Not all skins are equal.

Here are three skins together. My workshop is so small this is the best way to store them while I work on the last one.

Again I had to tweak the ribs and baffles a little. Final tweak will be before I glass the skin on permanently. I think the bottom’s have been easier because the curve is less in the airfoil profile.

Here’s my skin, a good enough fit now for glassing the inside.

The usual marking for later peel ply.

Fourth strake skin glassed and peel plied with 4″ pieces.

OK so whats this? Its a Garmin GMU 11 Magnetometer and it needs to live in relative magnetic isolation in the plane. This is the electronic version of a compass. A helpful email back from Garmin suggests this is the place.  Except its too long to fit, especially with a cable attached.

Plan B is here. It is close to those AN6 wing bolts but maybe far enough away? I could hack a cavity that gets it in the first picture position, that would be Plan C. Its just more work. Consider me undecided.

 

Date:  05-14-2018

Number of Hours:  6

Manual Reference:  21-3

More fitting of the bottom skin today.

I again needed to slightly adjust the ribs and baffles and then a lot of careful fitting at the spar.

For some reason this is fitting a lot better than the tops have.

As before I’ve marked out where the ribs go so I can see where to put the peel ply later.

After micro and then one ply of BID. Pretty much the same as doing the top skins.

One difference is that I have a ramp fitting here for the fuel to flow out. Its quite even, not how it looks in the photo.

This is the outlet I’m working around. The bottom of this is just slightly higher than the bottom of the skin with the graduation. This means this is a very thin part of the tank and I may add a couple of plies of glass on the outside later to reinforce this area. It is going to be close to where my armpit cooling ducts will live so it may be well protected anyway.

Date:  05-12-2018

Number of Hours:  4

Manual Reference:  21-3

The fuselage needs to be on its back like a turtle for a while. With a distinct lack of military precision I assembled a team of six. Now one got stuck in traffic on the way so our team of five got it done. My three giant windows come out easily with a new glass sucker I’ve acquired. We just carried the plane outside, rotated the spar on the ground which was padded with a few rubber mats and carried it inside again.

I found it pretty easy but one said it was quite a workout and another discovered he was not as strong or agile as he thought. I guess I’m not pulling my weight. Apart from having kittens every time we got close to damaging anything, like the car parked really close to us, and of course, the front strake outboard rib, it was all good.

With the strake ribs still exposed I spent a couple of hours just trimming the hat tops. Partly because I like neat work but really to remove the frayed edges and get a little sandpaper on there. It will get more prep work later before the top goes back on.

I am very mindful of not leaving little bits of half attached fluff for when the strakes are closed. These would eventually break off, float around for a while and then get fuel soaked and sink. More time later they find their way to fuel filters and start blocking them.

Experience of others shows that this takes around 50 hours and a few people have had engine failures on this issue. I believe I’m 100% (hahah if thats possible)  protected because I have the 15″X15″ giant Holley Hyramat fuel filter in my common sump. However, thats no reason to be complacent. That filter is not cleanable and really expensive so another good reason not to fill it with ‘fod’ prematurely!

As usual I spent a while doing the rough cut from one big sheet for the two strake bottoms. This is my last big sheet of foam the right thickness and it would be a big delay if I had to get more. Like months…

Here’s the first rough cut fit. The side edge has a gentle curve that needs a careful amount of sanding to get right. I’d forgotten already how much time these guys take. While the way forward is close to worked out, its just a lot of time and effort now.

Meanwhile I have a long list taped to my wall of small jobs for execution while she is in ‘cat belly scratch’ position. This was the landing gear donuts. When I floxed that gap from above I couldn’t get to the underside without the flox oozing out. This little mod just prevents the gear tabs from moving on the through tube. I like to think it also reinforces the tabs themselves with extra thickness.

Yes now I’m looking at this I’m seeing the sharpish corner of the gear and the SS brake line (in a plastic tube) along with the pee tube that could be better supported and protected from chafing. More jobs for the list while inverted.

Date:  05-09-2018

Number of Hours:  3

Manual Reference:  no ref

Today was popping off the top strake skins and a quick initial trim.

While they still look a little rough I have a perfect match to the tops now.

I still need to route out the middle section so I can get a glass to glass with the tops and with the sides like a flox corner.

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It does feel like some progress.

This shot is before trimming. Just the tapes ribs and baffles. I need to allow for the mouse holes of course and tidy all that up later.

 

Starting to look more like an airplane.

 

 

Date:  05-08-2018

Number of Hours:  10

Manual Reference:  no ref

Right top hats were like the left. I did all the prep in one day and the the layup of the tapes in 6 hours the next day.

I got everything sitting down where it should. Easier with LESS cuts as it turns out.

This time I did the tape layups in two sections. A-C and then later D-F. On the previous strake things were starting to go off when doing it all at once. I just measured what was needed and labeled a pic of the strakes for reference. Then as you see I had the 3″ wide tapes also labeled before I cut them out with scissors.

No need for too many pics, it was like the other side. Lots of tapes after floxing the joins.

In case this process makes no sense I will attempt to explain with a little test piece.

The job is to make a hat top on our curved piece that fits the rib perfectly. The underside of the top is glassed, the rib is the finished piece installed in the plane. As we are popping the top off later we need some release tape on the top. That’s the grey stuff.

Then we want the top and the rib to match perfectly. This is the trick. I just put a LOT of flox in the 90 degree join area. This takes up the slack space and gives the glass a nice filleted curve to lay into.

Next up a couple of plies of BID to join the top and rib side.

After cure the top comes off, no problem. The grey tape does its job as a release agent.

Can you see how bumpy the bottom part of the top is. Doesn’t matter because the flox takes up the space. Its hard to see but assume you see a perfect match.

Later you can route into the join down to the glass. Then its ready to be floxed on. If you route into the rib a little you are making a sort of flox corner. Glass to glass everywhere rib sides to top connected with the flox. You also get the rib well sealed and of course the main point is that when you glass/flox the top on it isn’t going to leak because you are glassing it over a wide area – the ‘hat tops’. This cheesy example is just on one side of course. Most of the ribs look like they have a T section on top.

I hope this and tomorrows pics of the hat tops (9th May 18) are some help.

 

 

 

Date:  05-06-2018

Number of Hours:  8

Manual Reference:  no ref

This is an idea that many people add to strake construction. We want to try and eliminate leaks with a really wide gluing surface for the top skin. It also allows a type of ledge for the top to sit on that is sealed all the way around. My method is nearest to one in part two of a ‘Cozy Girrrls’ video where the technique is well explained.

First up was a really good clean up of the rib and baffle surfaces with 40 grit sandpaper. I later gave it a wipe with acetone on a paper towel as initial prep and clean.

I marked out underneath where the ribs were and then ensured I had plenty of release duct tape in place. Yeah, I have enough.

Next was to get the top where it needs to sit. I’m not 100% happy with this but it is quite fixable with a tweak later to get the top perfectly flat and smooth.  Its pretty close sitting like this with a little hot glue and some weights while I get those top hats in place.

Then some planning. I needed 24 two ply BID tapes and that needed a bit of a plan. I have strake segments A to F with 4 tapes in each. All of them different sizes to the .1 of an inch. It worked out! I planned my tapes and cut to plan and then mounted them to plan in order. You can see them in the pic above all with labels and set into 6 groups.

Quite some hours of taping followed. I used a LOT of flox in the joins before putting the tapes down. Much of it will be ground out later.

It took so long that by the time I was getting to the last couple of groups the epoxy was getting really sticky. That just made it easier working partly upside down.

I had cut a lot of peel ply but I think things were starting to go off and I feel only a little of these tapes will need anything more bonded to them. If so, I’ll sand. I really don’t want to risk any extra air bubbles trying to use peel ply on very sticky tapes. Best to let them cure. I have the heater on as the temperature is getting down a little and I like at least 20C for all layups if not 25-26C. Being fuel tank related its worth a bit of expense running my gas heater overnight.

Date:  05-05-2018

Number of Hours:  12

Manual Reference:  21-3

Both top skins need to be fitted to the ribs and strakes. I started on the left side.

Yes slits with a razor blade down to the glass, 1/4″ apart.

Well that didn’t work so I made more slits in two more directions. I certainly had a nice floppy piece of foam by then.

I soon saw that my floppy top piece was not going to sit happily and I worked on reshaping BL23 yet again as well as the baffles in-between. After quite a LOT of work it sort of sat OK. I could at least see that it was possible. The small cuts made the top flexible as I wanted but it tended to droop in some places as well.

Eventually I had the top sitting close to what it should and a good result seems possible. Those hundreds of cuts took many hours. I may have to slightly fill in a couple of places to get the smooth airfoil that is required. It has been a significant challenge to blend these two distant airfoil shapes so far.

Here’s the view from underneath and it looks like I am going to win, even though its been the hard way.

With the right side I just did cuts in one direction 0.5″ apart. I took a tracing of the rib and baffle changes on the left side and transferred them to this right hand side strake. Wow, it was quick and easy to do. I must have learned something in doing the left side! Much to my surprise the right side skin sat happily with a few weight on the ribs and baffles. Less cuts worked a LOT better than more cuts. If I had had spare foam I would have redone the left side like this, BUT I don’t. So the left skin will need some careful treatment (micro in the cuts later) so it is strong. It was some relief to have one side a touch easier.

The left side will be like herding cats when it comes time to set it in position for the hat tops. I can’t wait! (sarcasm here)

Meanwhile as I mention cats, there was some work to do on the mouse holes. These allow the fuel to flow between the ‘obstructions’. The ones on the top are less important than the bottom. Having the top ones should allow fuel to equalise more quickly when I’m getting near full tanks.

Here I’ve make more holes in the plans .8″ radius. You might be able to see I’ve also grounded out the centers so I can get a glass to glass micro fill to seal these guys up.

There were 14 new mouse holes to fill with micro here before I can get on with the hat tops. Slow progress but progress all the same.

 

 

 

 

 

 

 

Date:  05-01-2018

Number of Hours:  6

Manual Reference:  21-3

The top right inside strake skin was not that different to the left. No surprise. Just careful fitting.

I decided to cut part of the leading edge rib template off so I could ‘flow’ the top down without trimming it to the exact size. Just in case I needed it, I made a template pic of each rib. I did the same thing before cutting on the other side too. I think if the leading edge itself is preserved it will be academic to make that last little curve in foam later.

More cutting and fitting these big pieces of foam.

Here’s the inside glassed and good for the next step.

I quite like the look of having something more like an aeroplane or airplane if you are from the USA. You can see the baffles (and rib BL23) on the left are ready for a proper trim.

Date:  04-27-2018

Number of Hours:  5

Manual Reference:  21-3

Today’s was some time fitting the left top skin to its exact position. Just tweaking yesterdays fit.

The top fit is working out. I’m just slightly under where the wing will meet the spar. So perhaps some fairing filler here. It shouldn’t need much.

The corner fit is good. It is possible that I’d be better off from a drag perspective with the spar being the point of maximum thickness. However I would have lost some fuel volume and I believe some of the extra lift characteristics. I suspect it would be in the noise and hardly measurable.

I have beveled both these edges for a good fit.

I’ve now trimmed the front area and left a good inch or so that will be trimmed once the top skin is ready for floxing on. Meanwhile I can see how much the foam needs to bend, to make a fit.

The next step is to glass the underside of the top skin. I’m using the Cozy method here (rather than the LE plans) where the skin is glassed then later the foam is cut in strips to make it all curve easily. The foam ply keeps the strips from falling off. Once this is done I’ll again depart from the plans methods of both planes.

You can see I’ve previously marked where the ribs and baffles are. I’ve microed the skin and now one ply of BID. The idea here was to just peel ply wide strips at these points rather than one really big big sheet of peel ply that might end in grief. I’ll watch for bubbles as it cures. Its looking good at this stage.

Date:  04-26-2018

Number of Hours:  5

Manual Reference:  21-3

The top left strake skin has begun. I spent some time reviewing my notes and a couple of online videos building the confidence to move ahead. I know my strakes have a difficult transition from one air foil to another so a bit of a plan is in order.

I first laid out some plastic to make extra sure that I was correctly cutting out from my foam sheet.

I only have two of these pieces of foam, two skins each. With the plastic confirming my measurements I cut out the first skin, way oversized of course.

Fitting to the curved side took a little time. I also had to bevel the edges inside to allow for the curve on the fuselage wall.

With the skin sitting on top now I could see where the baffles needed trimming. I also cut off some of the carefully preserved rib tips. I have traced then out on paper before the cuts you see above. The skin needs to sit flat so these might as well go now.

More work and I can see a little heat is going to be needed to make a bend… however I’m going to keep trimming until I’m close to a good fit first. Its a start today. I have four of these skins to do and I think once I have this one right the next three will be much easier.

Date:  04-25-2018

Number of Hours:  7

Manual Reference:  21-3

I’ve been preparing the leading edge strips for some days now. 1.9″ wide and length as required. One ply of BID each side and the fuel safe epoxy.

As you can see above, there are three strips per side. This is similar to the plans construction except I’m not doing the fairing block or the kink in the strakes. I’m also making this LE nose area a little smaller than plans to allow more fuel.

After a lot of fiddling around I fitted the six pieces dry. As usual its not plans positions either. It makes more sense to me to have these stripe parallel to the leading edge of the strakes. I think the plans mean you to do that but on the numbers given I can’t see how.

I worked on 0.7″ gap measure with those rules. That gave me a ‘fitting point’. I then floxed in a piece, clamped it in place. After that I hot glued sticks to the outside leading edge aluminium straight edge which is also hot glued in place.  Then I retrieved my rules and clamps when it set. A guy only has so many six inch rulers.

You can see the result here. Those leading edge pieces will probably get a lot of careful trimming upper and lower to fit the tops and bottoms. I think the edge angles will need adjusting. All good.

Here’s another shot showing them reasonably straight. It doesn’t matter as its all internal, but I like to go for a straight line when I can.

Another view as I did the other side. More sticks and clamps. That piece was made from two pieces of foam I laminated together due to a lack of 12mm foam. Its not some fancy structural reason.

Again reasonably straight. I can remove those forward dowels once this cures, I have a box like structure in place.

The basic strake shape with ribs and baffles has arrived.

Date:  04-24-2018

Number of Hours:  3

Manual Reference:  no ref

After the baffles had cured overnight they needed a rough trim.

I measured things out slightly oversized and hit it with the vibration cutter. More exact fitting will come later. Baffle fit is however not critical because to captops will take care of it.

They look a bit better now.

Meanwhile I was still making layups for the leading edge strips. Once this piece is cured I’ll have enough material in total to proceed.