Micro Sinbad

[Konrad]

Well, the effort to loose some tail weight was not only fun but productive. As shown the laminated outline with open structure is 38% lighter than the as cut sheet vertical fin.

I also maintain that the new structure is much stronger and more resistant to warpage. As a former material and manufacturing engineer it is incumbent that one use the advantages and limitation of the materials to the fullest. Key here is to use the wood grain to define the shape and resist warpage. The downside is that the OEM would have to provide basswood strips (actually 1/32 contest grade balsa would work and would be lighter still). The manual would have to describe the laminating process.

Ok, the 38% callout is a bit optimistic as both fins will need covering. This will lower the percentage spread. But the 0.45g loss would still be valid. This will translate into another 1.5 grams of saving from the loss of nose weight. For a total weight loss of close to 2 grams! This kind of weight improvement will be even greater with a restructured stab!

 

[Konrad]

Im a balsa noob so i like to skip thru it before a build and see whats coming up so I don't screw myself by gluing something early.

This is a concern particularly if deviating from the plans. In post #72 you can see I left off the top center sheeting to allow me to add the dihedral braces after I set the dihedral angle.

Here you can see I left off the rear magnet (on the fuselage), so as to still allow me to use the microwave oven to set my glue when setting up the tail. Also I left the front hold down pin unglued to allow ease of covering.

 

[Hank GB Z]

Well, the effort to loose some tail weight was not only fun but productive. As shown the laminated outline with open structure is 38% lighter than the as cut sheet vertical fin.
I also maintain that the new structure is much stronger and more resistant to warpage. As a former material and manufacturing engineer it is incumbent that one use the advantages and limitation of the materials to the fullest. Key here is to use the wood grain to define the shape and resist warpage. The down side is that the OEM would have to provide basswood strips (actually 1/32 contest grade balsa would work and would be lighter still). The manual would have to describe the laminating process.

Ok, the 38% call out is a bit optimistic as both fins will need covering. This will lower the percentage spread. But the 0.45g loss would still be valid. This will translate into another 1.5 grams of saving from the loss of nose weight. For a total weight loss of close to 2 grams! This kind of weight improvement will be even greater with a restructured stab!

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That’s cool Konrad. Save .48 grams on the tail saves 1.5 grams on the nose.

Hank

 

[Konrad]

More or less! Now that is assuming one needs nose weight. I make that assumption as the video linked earlier shows that the builder needed an oversized 500mAh cell. Even the development thread states that 8g of nose weight was need, about 1/5 the AUW!

Now a balance hit, is that I’ve moved 2 magnets from the front (canopy hold downs) to the rear, aft of the CofG, as wing hold downs.

Time will tell if the net change in weight really materializes. But I think the open tail structure looks better and is much stronger than the sheet tails.

 

[Konrad]

Here are some process hints I use with just about all my iron on coverings.

Most films have a grain. Some like LiteSpan have a very pronounced grain. I like to cut out my covering pieces so that the grain will span the ribs and formers so that grain helps control the sag between the ribs and formers. Usually this means running the grain the in the long direction of the part to be covered. Now some fuselage pieces might best have the grain oriented some other way to get the best effect around compound curves.

I like to use as low a heat setting as possible for all aspects of laying down the film. I like to think of film as a metal that is heat sensitive and we don't want to loose the temper.

A key point I rarely see mentioned is that the adhesive activates (becomes liquid) at a much lower temperature than that at which the film starts to shrink or stretch. This is a key concern as we don't want the adhesive to loose its bonding strength at the same time we are trying the shrink the film. This is key as to why I don't like heat guns to shrink my coverings. With a heat gun I can't control the application of heat with near the precision I can with an iron. Heat guns are great for stretching film, but I do avoid them when it comes to shrinking the film.

Here I'm showing how I place the iron to seal the perimeter and avoid placing any heat on the open surface of the part. I also show that when the glue is fluid you can use the drag of the iron to move the covering away from the center of the part, taking up some slack in the covering.

The last photo is showing how I avoid the perimeter of the part while shrinking the covering. You might have noticed that the iron has many odd shapes to the sides. This helps in selecting which side (feature) of the iron to use while shrinking the film and not loosen the adhesive.

 

[Konrad]

The Achille's heel of this Henry Struck design has for me been the stab mount. It basically just sits on a thinned spine stringer that has been thinned to half the cross section of the rest of the spine.

When I first saw that the micro SInbad had what looked like some added structure I was euphoric that this model might actually have a robust stab mount.
Alas to my thinking the stab mount is still the weak link in the design.

It looks like I'll need to rethink the whole rear end of this model. I like my hoop outlines and the open structure of the original design. But that stab's line contact glue area not so much. I'm thinking of lowering the stab to mount on the 2 3/32 stringers. This should still allow for the basic shape of the Sinbad. The only down side I can see is that the stab will be a bit lower in the weeds when landing. But really what is 12mm really going to harm.

BTW; Thanks to @poh for the uncut supply of orange Litespan

 

[Konrad]

New built up stab is much lighter, almost half the weight, at 2.54g than the sheeted stab 4.98g. And this is with the spruce elevator joiner and rear stab spar.

To show how I change the outline I've placed the two on top of each other. I did this because the length of the basswood strips I wanted use where a bit short.
I choose to use spruce because it is about 2.5 time more dense than balsa. I want the denser wood because I will have to change the configuration of the pass through window. The as cut tail parts has the pass through window in the rudder. I'll want to change this to using the fuselage as the pass through window.

P. S.
The right side of the sheet stab has been cut to form the mandrel, upon which the 3 pieces of 0.5mm basswood have been bent to form the outline of the stab.

 

[Konrad]

Just spent some time over at RCGoofs reading the development thread. If you knew how much I'm loath to spend any time on RCGroups you might understand how much I'm enjoying this build, to spend any time there.

I see that most of my concerns with the build have been voiced on the RCG thread. But as is typical of RCG I don't see many solutions offered. A positive note on that thread I did see a builder use the integral fuselage to fin "Fabric" fillet. This throws down the gauntlet in forcing me to actually do this on my build. I was planning on doing this, but now that I've seen it done I have no excuses for not trying. While my stab will be in a different place on the fuselage, I don't see that as being an issue.

Now nobody mentions the as cut stab mounting as being a concern. It looks like that thread died in early 2019. I'd like to learn in the subsequent years have there been any reports (feedback) as to the durability of that stab mount. @Wayne I recall you saying that there are a few local micro Sinbads flying around here. Have you had any feedback on the stab mount?

 

[Konrad]

Hum, I just looked at the video in the first RCGoofs posting and see that, that designer also didn't like the spine mounted stab. Now he has moved the stab all the way down to the crutch.

To my eye that looks like a very practical way to mount the stab. But for now I'm committed to using the mid turtle deck stringers.

Damn, I'm liking that crutch mount!

 

[Konrad]

Well here are some in process shots of the lowered stab and the in fuselage pass through window to allow the elevator joiner to, pass through.
Note this is different from the in rudder pass through window as found on the plans.

 

[Konrad]

Forgot to take photos of this. But the front of the fin needs some added support to keep it from retracting into the fuselage on a flip landing. I added a triangle piece of 3/32 balsa to the back of the rear former to support the front of the fin.

I've also added the dorsal fin radius. I've also added the lead on lead off radius to the sheeting that makes up the stab base/mount. I've also feather edged the 1/16 contest grade balsa to make a smooth transition for the fuse to fin covering transition.

I think this brings to an end the engineering changes I'll be making to the kit. The rest of the build will be basic craftsmanship and radio installation.

I'm so happy with how this kit is coming along. And that I was able to address the fatal flaw that had vexed me as a young 13 year old. Even as a kid I knew that stab mount was a bad idea. Seeing how well this mod has worked on this micro Sinbad I think I'll look over my 40" Sinbad kit to see what I need to do to bring this up to a "modern standard".

 

[Hawaii Sloper]

Forgot to take photos of this. But the front of the fin needs some added support to keep it from retracting into the fuselage on a flip landing. I added a triangle piece of 3/32 balsa to the back of the rear former to support the front of the fin.

I've also added the dorsal fin radius. I've also adder the lead on lead off radius to the sheeting that makes up the stab base/mount. I've also feather edged the 1/16 contest grade balsa to make a smooth transition for the fuse to fin covering transition.

I think this brings to an end the engineering changes I'll be making to the kit. The rest of the build will be basic craftsmanship and radio installation.

I'm so happy with how this kit is coming along. And that I was able to address the fatal flaw that had vexed me as a young 13 year old. Even as a kid I knew that stab mount was a bad idea. Seeing how well this mod has worked on this micro Sinbad I think I'll look over my 40" Sinbad kit to see what I need to do to bring this up to a "modern standard".

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@Konrad that wing covering is a piece of art work

 

[Hank GB Z]

Hey Konrad,


Great job on the fillet for the vertical stab and lowering the horizontal stab a bit. The Sinbad looks really good. Looking forward to seeing the fuse and tail feathers covered and have the whole thing come together.

Hank

 

[Hank GB Z]

Also I saw this the other day. This would make an awesome small RC glider.

 

[Konrad]

@Konrad that wing covering is a piece of art work

Thank you.

But these Hershey bar style wings really are easy. Key is to have a stable structure. This is a problem with micro models. This is why I changed the structure of the wing tips (laminated). As cut and designed the tip flexes and makes it almost impossible to get a good shrink. Also make sure you have smooth transition ramps like I used on the spar.

As @Hank GB Z is hinting, the fillet between the vertical fin and fuse is often very challenging. You can see how difficult this is in some of the manual photos and video. I've added some structure to the tail to stiffen the tail. I also added a larger dorsal fin fillet. I hope that when I get to covering the fuse that these benefits will show.

I may be wrong but I see the fin and fuse having to be covered as a single unit. This is an advance covering technique. And as designed I don't see adequate structure to do this. (See my process link at HIP on how I do this, I will try to take photos of how this is done on the micro Sinbad).

 

[Konrad]

Love the Mk1 eyeball and #11 cutting system along with the french curve!

Lots of good building techniques!

Also I saw this the other day. This would make an awesome small RC glider.

 

[Konrad]

This is a bit premature, but as the holiday break is coming up for many, I want to get the solution out for the few real issues I've had with the cut file to give folks a heads up. I was asked to keep track of these as some of my correction might make into future kits.

Most of these have been covered already.

First: make up a set of control horns that are close to twice a long as those supplied in the kit.
Second: I think the wing dihedral joint needs some braces.
Third: notch the wing tip to allow the spar to nest properly with the wing tip
Forth: add some balsa blocks to aid the covering to go over the spar at the tip.
Fifth: cut a new set of wing tips out of 1/32 balsa to make a laminated tip with the grain 45° to 90° to each other.
And last add wing tip brace aft of spar.

These are changes that I think are needed to get a strong wing and wing tip.

The wing hold down and tail changes don't quite rise to the must change level. But I strongly recommend getting some weight out of the tail and adding some strength to the stab mount.

P.S.
A must do is make sure the top of F4 allows the wing to seat on the top rails!

 

[Konrad]

Well, since I mentioned it and give credit to Dave Thornberg I thought I'd give the page numbers (pg98 and pg99). I like his call out of the transverse dowels as "archaic" and that serious Free-Flighter had given up on years ago (decades ago). Please think of ditching these on any of your microRC models, heck any model! They have a huge negative impact on glide performance!

Just finished re-reading Dave Thornberg Old Buzzard's Soaring Book for the umpteenth time! It is a real enjoyable read.

 

[u2builder]

I'm not building this model though I've been tempted. But I leaned a lot just reading the build thread, and I'm a fairly experienced builder. I love the many little details and especially the built up tail. I think one downside of the most the smaller - as well as some larger - laser cut glider kits nowadays are the sheet balsa tail surfaces with "lightening" holes. The lighten holes save negligible weight and weaken the already weak structure. Built up are stronger, look better and reduce weight in the tail and as much as three times more in the nose! Konrad presented a nice tutorial on building built up tails.

 

[Konrad]

Thank you.

That is my goal to try to get folks to go a bit beyond their comfort zone. I see that we have some novice builders here so I try to show the pit falls and how to avoid them.

I also try to show the OEM's and designers that we as modelers do take notice of the efforts and short cuts taken. A classic is the sheet tail feather. Yes, it can work but has a high cost. I think of it as the baseline by which the other options are measured.

Why should the designer go through the effort to make a built up surface if most folks don't understand the cost and benefit to the built up surface? I pointed the benefit of the laser as we see in the Steven's Aero products. Now as I don't have a laser I choose the classic laminated hoop. I used the laser cut sheet as a form to make the laminated hoops. This is easier for the home builder and is why I showed how I do it. After all, we build kits for the joy of building. There are enough ARF's out there, and good ones, that if the goal is to only fly and fly in a short amount of time from the time of purchase we don't need to build kits.

These micro kits are fun and easy (for the most part). The micro Sinbad is a step up in difficulty and reward.