Balsa-Foamboard test build - FT Spitfire

[Rokcrln]

So are you mounting your servo's and receiver in the rear? What size battery are you running? On mine it is balanced with a 2200mha 3s and my servo's are right above the TE of the wing.

Printed out some new plans for an up coming build!

 

[Hai-Lee]

Unfortunately the answer to the servo positions question is definitely a YES. The idea was to build a standard plane and then the balsa reinforced and lightened version to see if there was any significant difference.

Anyway I spent the idle time adding some colour and the electronics access hatch to my sons plane as well as the aforementioned wheels. Here are a couple of poor pics.



Just a note! I used the balsa instead of paper/card for the fuselage decking so that the hatch would be able to handle being manipulated without easy damage.

I also found a distant source of the dreaded Balsa and 4 hours later I have it in my work area so the work continues.

With all that had been previously mentioned about reducing the size a tad to fit in the standard size wing hole in the fuselage I have really hacked into the poor old wing. So when I cut out the pieces I did not cut the holes for the spars or the servos as I may need to adjust them later in the build.

The spar heights which used to be 12mm are reduced to 10mm and the LE crush area of the fold would require a major adjustment of all of the peice sizes to maintain so I am going to short cut it and add another spar. More details and matching pics later.

Here is a pic of the wing FB cutouts so far with the paper stripped of course EXCEPT the areas which will become the internal surfaces of the wing, (for strength reasons at this stage).



More next post!

 

[Hai-Lee]

Just returned from my morning flying and the original design FT Spitfire, (my son'e), had 3 successful flights without incident and on this build Differential was added to the Ailerons. It rolls so sweet with the differential I will leave it. Will post a pic or two when they are made available to me. We have a field photographer, when he is not flying that is.

Have to deliver it so it does not suffer any Hanger Rash waiting in my workshop for collection.

May get some build time this afternoon for the Second, "Lightened", version this afternoon.

Fly high and land softly!

 

[Hai-Lee]

Got an order from the local model shop for 6 planes of one of my designs and so until the order is filled I will be time sharing this build with the shop order.

Anyway I planked the ailerons and determined how the wing is to go together at least for the first attempt. Here is the TEXT of what I have done so far.

1. Cut out 12.5mm LE strip to match wing underside piece
2. Cut 2 of 10mm FB Spars at least 540mm Long
3. Cut 2 of 6mm x 10mm Balsa LE Spars at least 360 long
4. Cut 2 20mm wide strips of 1mm Balsa longer than the LE spar and glue with one edge flush along a 10mm side of the spar
5. Chamfer the wing underside pieces and Top pieces from 3mm at the LE back a distance of 13mm.

Pics so far!






The wing upper and lower skin positioned behind the LE spar



More tomorrow, (Time budget willing).

 

[Userofmuchtape&glue]

Looks nice Hai-lee. do you think the time spent is worth it for the balsa?

 

[Hai-Lee]

As you already know the FB we use is significantly heavier than the FB FT used when designing their planes. Some of the FT designs are very difficult to get the CG correct without adding extra weight and so I was and am seeking a method of reducing weight without reducing design strength and so far this is working a treat even if it is a little tedious at times and a little fiddly.

One unexpected side bonus is that IF I build a warp into the structure I can remove it with a little hot water and a few clamps which simplifies the build detail a fair bit. Also I get to use Covering film and cover the exposed underside of the wing tips, (as you will see later).

So far this is all expected to be accomplished at a lower weight and greater strength than the original build. It is surprising how strong a foam balsa laminate really is and I may even play around in future with Balsa strips being used instead of CF reinforcing in places.

Whilst this procedure is not appealing to every forum user at least it might give them another material to consider when engineering their own pet projects in the future.

next time!

 

[Userofmuchtape&glue]

OKay, thanks.

Do you know HK sell 5mm foam thats really light for $2.50 per sheet with shipping?

 

[Rokcrln]

Great info even if I did get a touch lost with your wing pics but I'm sure it will make sense as you progress. Can you explain the differential you did on your son's ailerons that you liked so much? My regular FT Spit is lacking in the roll department. Another forum member has mentioned extending them 1"+ for great results. I'm almost to the point that my next FT size Spit will need to be created for fun flying as I work on the 6' Spit.

Thanks again for your time!

PS. Do you have links to any of these hobby shop builds you do as well?

 

[Hai-Lee]

Great info even if I did get a touch lost with your wing pics but I'm sure it will make sense as you progress. Can you explain the differential you did on your son's ailerons that you liked so much? My regular FT Spit is lacking in the roll department. Another forum member has mentioned extending them 1"+ for great results. I'm almost to the point that my next FT size Spit will need to be created for fun flying as I work on the 6' Spit.

Thanks again for your time!

PS. Do you have links to any of these hobby shop builds you do as well?

The wing build is simple so far. The paper was totally removed from the upper surface of the top wing pieces and the bottom of the wing bottom pieces leaving the paper on the other side in place, (for strength).

Then on the wing upper piece the areas not covered by the bottom wing skin were removed leaving the area inside the wing still covered with paper. When the paper is removed the Ailerons the upper wing surface and the lower wing piece all become separate from each other.

Because the LE of each wing is an area of great strength and extreme crush on the foam it was and is difficult to engineer it to be reduced by 1mm all round without resizing the upper and lower pieces. So I determined the area of the crush and removed it completely. It was replaced with a balsa spar of 5mm thickness with a 20mm wide strip of 1mm Balsa.

The upper and lower wing pieces needed to be tapered back to a total of 5mm thickness when glued together and onto the Balsa spar. When fitted together the balsa spar is to be tapered in line with the slope on the front of the wing upper panel and when properly tapered another 20mm strip of 1mm Balsa is to be glued to secure the LE spar to the Wing upper panel. Please note that before I attempt to join the wing panels together I will need to finish planking the wing under panel to increase its rigidity and stop it from curving or bulging due to pressure from the top wing panel passed through the spars.

The original wing spar itself was reduced in height by 2mm to 10mm and separated into 2 separate spars and they were also extended to the wing tip. They will be shaped later to allow the covering film to enclose the tip entirely. The reason I split the spar into 2 separate spars is that I want to support the centre fold section of the top wing panel equally to ensure a better control of the wing profile. The original design with its centrally mounted spar is quite strong but can cause a problem of 2 different wings profiles being built especially if the builder is inexperienced. Just a precaution on my part.

When it is time to "SEAL" the wing you will see that the TE will also need to the tapered a tad and yet again to allow for the planking without an increase in the wing external dimensions.

Please note that all of this fiddly detail is because I wanted to make the wing the same dimensions as the original design so that if the wing is damaged or the fuselage is damaged beyond repair I can just do a simple FB build as a quick replacement and the details of what i am cutting and resizing are irrelevant to your design as you will make such adjustments before you even make a single cut into the FB.

Also have covered the tail already and started the decking on the fuselage so it can be covered as well. Pics to follow in later post.

Have a gude dae!

 

[Hai-Lee]

OKay, thanks.

Do you know HK sell 5mm foam thats really light for $2.50 per sheet with shipping?

My supply situation has developed greatly where I can get virgin BOX quantities for 30% less than the discounted retail price already. My source is actually multiple sources and very reliable for supply even in great quantity.

Whilst I will admit that my FB will still be more expensive I cannot afford any damaged or warped sheets to be used in the planes I am trying to market.

Thank you for the info and I will definitely keep it in mind if saving a couple of dollars becomes an issue with my production for sale.

Fly High and Land Softly!

 

[Hai-Lee]

I took some time off today to push the wing construction a little further, up to the test fitting for the profile. I took a few more pics as well. First pics show the tail members covered in the silver film as well as some of the top deck work to date.



That was yesterday and today I firstly fitted the Balsa spar/Leading Edge to the other wing underside panel and the lower 20mm wide cap strip and then fitted the 5mm false ribs to the underside wing panels at 50mm intervals from the wing centre line leaving the first rib vacant because of the wing centre reinforcing, Last thing before wing fitment.



Then I joined the upper and lower wing panels at the Balsa leading edge and profiled the balsa spar to be flush with the wing upper wing panel profile. (Note that the wing panel had been tapered as mentioned previously).



Next step was to cover the wing upper leading edge with a 20mm wide cap strip.



When the glue was dry I sanded the LE to the correct profile and fitted the upper forward false wing ribs so that all was ready for a test fitting. The pics below show the test fitting into the STANDARD size wing mounting hole in the fuselage.



Next step which I will not supply a pic for is the fitting of the 75mm fuselage reinforcing skewer pieces in the bottom of the fuselage so that they are centrally located across the leading and trailing edge or the wing mounting hole to provide extra reinforcing when pulling high negative Gs.

More later! Getting close now! Fly high and land perfectly!

 

[Userofmuchtape&glue]

Wow I will never cease to be amazed at the creativity of some people!

The next step would be a full balsa version.

 

[Rokcrln]

That looks great! Thank you for the detailed wing construction break down, it all makes sense now. So are the external wing spars for strength, looks once covered or both? Also I was thinking about doing my next wing similar to your tail construction and cut multipal lightning holes in the wings foam and but then solid sheath with 1/16" balsa top and bottom. What are your thoughts on this?

Lastly is this wing going to be flat or will it have the 10* of dihedral?

 

[Hai-Lee]

The wing will have the standard amount of dihedral and as for the cutouts in the wing they could be done easily, (between the false ribs), but I would recommend leaving them to the last minute because if done sooner the foam may become too floppy and easily damaged.

As for an all Balsa build, NAH!, Got quite a few all Balsa models at the moment and they seem to always be in the repair line for minor damage including hangar rash.

As for your larger size scratch build I do not think my LE spar would scale very well but you could use a Round, (dowl) cross section of spruce or Oak and use a "Channel cut out of the foam around the LE, leaving the paper on the outer surface, and wrap the LE paper channel around the DOWL. The you could just plank the outside over the paper and foam.

I chose the LE spar for a number of reasons. The crush area of the original LE was really dense and heavy as well as very strong when glued. The weight of the LE spar as I built it is forward of the CG point and so actually beneficial to help overcome the balance issues I had previously with the original design version. Finally it was the first physical expression of a possible technique to repair FB models which have pole and tree strike damage to the wing leading edge. Sadly I have quite a few of those. :black_eyed: @$#%&%# POLES!!

I did a few "Minor" strength tests last night on the wing as it is, (only partially complete), has far less FLAPPING under load than the original and it withstood a moderate load over a few hours without developing a permanent distortion or warp which has been a problem with some of my planes in my hangar.

I am learning a lot with this endeavor so even if the result is a failure, (I doubt it), it has been really worth it.

More to come! The end is in sight!:applause:

 

[Grifflyer]

Wow nice job so far

 

[Hai-Lee]

So are the external wing spars for strength, looks once covered or both?

The external spars are the source of the majority if its strength and rigidity.

A brief explanation: The foam has three parts to consider when testing strength and rigidity, (there are a great deal more but this is a simple explanation of the balsa rib strength). The three points are the 2 outer surfaces and the centre.

In order for the foam to bend the inner surface of the bend will need to compress, the centre we will consider as stable and the bend line and the outer surface which must stretch for the curve to occur without permanent distortion or failure. A paper covering will not resist crush forces and does quickly fold becoming the axis of the bend/fold.

The Balsa has only one strength axis which is along the grain where it is very strong in resisting compression and expansion forces. With careful placement of the balsa so that the grain faces the bending forces the resistance of the surfaces of the foam to expansion and compression are greatly enhanced.

This means that when the bending forces are applied to the foam the inner surface of the bend which must compress has its compression resistance boosted by the grain of the balsa and the outer surface of the foam must stretch but its properties have been boosted by the balsa to resist stretching. So until the Balsa stretches or compresses against the grain the foam will want to maintain its shape or return to its original shape unless the forces cause a balsa failure.

The paper covered FB has resistance to the stretching forces, (unless wet), but has almost NO resistance to compression or crush forces. When a FB plane hits a pole the impact point collapses rapidly but as it is not a break the foam around the impact area, (to the sides) act in stretching mode and strongly resist the movement of the pole through the wing structure and so you end up with a large stretched and wrinkled impact zone.

With the wing and fuselage techniques I am using the actual item I am trying to bend has a Balsa/Foam/Air/Foam/Balsa cross section and so the two external surfaces are still balsa end grain and so I get the end grain effect to increase the strength.

The foam has a definite purpose in the build apart from the shape and that is to stop the balsa failing cross grain by keeping the balsa from flexing, bending and then failing. Remember the Balsa is only 1mm in thickness. In an all Balsa build the balsa used is relatively thick or the structure is weak. Gussets are used to stop the balsa distorting under lateral loads.

I could rave on further about structures and Balsa usage but that is not for this post. I hope this helps answer your questions!

Fly high and often!

 

[Hai-Lee]

Great info even if I did get a touch lost with your wing pics but I'm sure it will make sense as you progress. Can you explain the differential you did on your son's ailerons that you liked so much? My regular FT Spit is lacking in the roll department. Another forum member has mentioned extending them 1"+ for great results. I'm almost to the point that my next FT size Spit will need to be created for fun flying as I work on the 6' Spit.

Thanks again for your time!

PS. Do you have links to any of these hobby shop builds you do as well?

Sorry for the tardy response! Your question in relation to the differential is a good question so the spacial answer.

The differential as I built it in my sons Spitfire made the rolls almost as I have seen the real aircraft roll with no noticeable pitch or yaw effects even though the rolls were slow enough to mimic the real aircraft.

Differential is a simple concept and makes the plane just that little more realistic in the air. When an aileron or other surface is deflected upwards the pressure increases on the upper surface and decreases under the surface. The thing to consider is not really the lift or even the pressure changes but rather the DRAG caused by the surface deflection. Simply when raised the surface increases the drag on that side, (ailerons), by a small amount but the opposite aileron being lowered increases the drag on the other side at a far greater rate. This difference in drag amount can cause a yaw effect, (like a little added rudder), and this causes rolls which are sloppy and appear to be cylindrical rather than axial.

Another effect can be the nose high banked turns which make the plane look like it is very tail heavy when it definitely is not.

Adding differential is so very simple when done mechanically and on my sons Spitfire I just made sure that the servo output arms were leaning towards the LE of the wing when centred, (about 30 degrees of lean). I then fitted the pushrods etc as per normal and just flew it. When the servo output arm was at right angles to the wing, (my sons first and since written off Spitfire), the aileron rolls were very sloppy and the banked turns were terrible to watch compared to a real Spitfire.

Just make sure you have sufficient downwards deflection of the Ailerons as the upwards deflection does not matter too much. Try offsetting the servo output arms just a little towards the LE and see if it flies any better!

Later!

 

[Hai-Lee]

Today's work was badly effected by a migraine and a large amount of lethargy on my part but I just managed to bring the second wing up to the same build state as the one shown yesterday and test fit both wings.

After that I added a few more dummy ribs and worked out how to do the tips and the wing underside and Trailing Edge.

Anyway here is a pic of the wings test fitted into the fuselage.



More tomorrow if I get time.

Later!

 

[Userofmuchtape&glue]

That looks great! sorry about your head ache. they suck.

How strong do you think the fuse will be just behind the wing? looks kinda weak to me.

 

[Hai-Lee]

The tail can be held in the hands and over 2 kilos supported by the forward fuselage which does cause a little buckling but not breaking or permanent damage. I cannot envisage what speed and maneuver I could achieve that would generate such a force from the horizontal Stab and the elevator, (about 4 Gs which when added to the Main wing at the same loading would equate to over a 30 G total loading). The real test of its ultimate strength is the dirt dive which I will always do my best to avoid.

As is usual the only real test will be when it is in action but to quote the Military, "Our confidence is high".