FT Storch Light depron conversion

One year ago I've bougth FT Simple Storch SpeedBuild Kit. The assembly was a breeze, and I had tons of fun flying it ever since. You can learn more about this plane and how I've managed to turn it into slow flying, docile trainer plane from this article. I've enjoyed all the qualities of the airplane, along with all the advantages of the material it was made of. But the biggest strength of this material - covering with very light kind of paper - turned out to be it's worst enemy. The paper covering started to delaminate and warp the plane just because of literally absorbing moisture from the air. After that, the plane started to loose it's rigidity, some cracks and wrinkles started to appear in the most stressed areas. At the beginning of this summer, the plane meet it's end: under the heavy sun, the first glue joints started to come loose. Even though I loved the plane so much, it was the time to find a replacement.

The story of my new plane begins way before that, when I started my RC adventure with scratch built FT Baby Blender. Since I live in Europe, I never had access to DTFB; so I started by experimenting with whatever I could find. Since then I've built five of them, constantly refining the construction and technology; looking for the best solutions in both ease of build and overall durability. The next step was the arrival of FT Sorch kit - I've traced all the parts on a piece of stiff and thick cardboard/foam sandwitch board before the assembly. At this point, the obvious choice for DTFB Storch replacement was to build the very same plane using much more durable, and more commonly available material: depron.

As always in case of "material conversion" projects, I started by figuring out how to use and/or modify original plans (in this case: traced out parts) to cut out parts that could be assembled easily/at all. Since depron is not covered with anything (and I decided to leave it this way), all the "paper tabs" were useless. To keep the fuselage strong enough, I used only one thickness of the foam: 6mm. Not being limited by the pakage size, I decided to cut out fuselage sides as two parts only, from the nose to the tail, without the cut in the middle.

I actually never removed the power pod from my previous plane, so this time I decided to build the firewall in permanently. I like the overall arrangement of the electronics though, so I traced out the shape of the original power pod on the plans. That allowed me to cut out two "nose doublers" that will hold the firewall in the correct position.

After that, I cut out three rectangles for the windshield, nose cover and the electronics tray from depron, and the firewall itself from the 3mm plywood; and started to glue them to one of the fuselage sides. Since the firewall was going to be glued in permanently, I had to drill the hole to pass the motor wires trough before installation.

It was the latest stage I could easily glue in plywood patches to reinforce locations for landing gear and rubber band mounting rods. It would be slightly better to do it one step before, but I simply forget to do that. All the forementioned locations are the same as on the original plans except that rather being located inside the thickness of upper/lower fuselage skin, I decided to move them towards the middle of the fuselage - to a point 10mm from the edge of the fuselage sides. Then I glued in 30x30mm patches of 1,5mm plywood, with the appropriate hole in the middle. The upper forward patch have two holes 10mm apart - this one will hold both the front  rubber band mounting rod and the central (main) landing gear strut. After glueing them in, the fuselage side looks like this:

You can easily notice that there is one additional patch added at the bottom of the fuselage, 120mm behind the rear one for the main landing gear. This one is meant to accept additional LG strut for the "optional summer landing gear". If you do not intend to take off and land on the water, you may disregard it; otherwise I strongly recommend to put it in place.

The next step was to join the other fuselage side:

To complete this part of the fuselage construction, I drilled out all 8 LG holes (4 on each side) to 3mm diameter, and glued in brass tubes, 3mm in outer diameter with the 2mm hole inside.

Then I started to work on the wings. First, I had to choose the airfoil. Luckily for me, I already did that when redesigning my Baby Blenders; and both planes have exactly the same wing chord (except control surfaces). Since my BB profile worked exceptionally well in the past, I used it again without any modifications. It is rather thick, providing tons of lift and gentle stall characteristics; but does not like to fly inverted and noticeably limits the max speed - something I can live with in case of basic trainer plane. The profile is in fact "hand drawn", with the following dimensions:

- length: 150mm;

- thickness: 20mm;

- largest thickness location: 40mm back from leading edge.

Those dimensions refer to the wing rib template, not the actual wing. The final product (assembled wing) will be slightly larger because of the thickness of the skin and the additional material at the leading and trailing edges. Since the Storch (just like Baby Blender) have constand wing chord, having one rib template is enough to make all of them:

In the type of construction I decided to use, you need a lot of wing ribs to support the skin. 20 of them were 3mm thick and 4 were 6mm.

I cut the wing skin exactly to the original plans, without control surfaces. In the sake of ease of assembly, I decided to use four panels instead of two. To achieve that, I cut the full size wing panel roughly in half of it's span; I also decided to leave the wing tips square at this point (to be modified later) and to decrease the wingspan by 50mm - solely to make the whole plane fit inside my car.

The spar I used for this plane is one piece balsa, 1m long, 6mm thick and of the height equal to the widest part of the chosen profile (20mm). As you may notice, 1m spar is not long enough to cover 1,45m wing; but since the tips are not under the heavy load, I decided to use balsa for the central part and fill the missing ends with 6mm depron. Before proceeding to assembly stage, I cut all the ribs into two pieces at their widest location, and made holes in the aft parts to pass the cables trough.

I started the assembly by glueing the spar to the both of the central wing panels, 45mm back from the leading edge mark. Then I glued in the first batch of the ribs: four 6mm ribs in two pairs at the location where fuselage sides and the rubber bands will put the highest load on the wing skin, and then six 3mm ribs spaced evenly (sor t of) on both sides. Then, I marked flap servo locations and cut the holes in both the skin and the ribs in such a way the servo mounted iside will be glued by the biggest surface possible.

Then I glued in the outer panels, depron spar extensions and other ribs (in this order). As you may notice on the picture below, I did not put all the aft parts of the ribs in place - only the ones that will hold the aileron servos, two on each panel joining line and one more at the each wing tip. Then I cut aileron servo holes the same way I did for the flaps. At this point I also glued in the nose stiffener. I used dedicated balsa profile, but any round wooden (or carbon, if you like) rod of approx. 5mm diameter will do the trick. This stiffener is primarily to make the wing panels bending easier; not to provide any additional strength. That's why it needs to be the same for the entire wingspan (as opposed to the main spar).

With the last part of the internal structure in place, I started to mount electronics. First: servos. I centered and installed servo arms on all four of them and glued them in, and - to make those locations stiffer - glued small rectangles of 6mm depron around them. Here's how the wing looks like at this stage:

It does look a little messy, but it is about to get worse. I decided to mount the LEDs inside the wing to make it visible during the evening hours and allow me to fly troughout the winter. I will not describe the exact mounting locations and techniques by words; I'd better let the pictures do that for me.

With all the stuff in place, it was time to "close the wing". I started from the central panels. To bend the 3mm depron over 5mm rod without having it crack, I do not use any fancy method like covering it with tape or heating up. My best friend here is the nose stiffener. All I need to do is to press the depron sheet against the stiffener (from outside) with one hand while supporting the stiffener from inside with the other hand. I do it a little bit at a time, moving along the whole lenght of the panel I'm going to bend. The key to success is to squeeze the sheet against the rod; the bend will form "by itself". Do not try to bend the foam to early because it will definitely crack.

I repeat the squeezing process until the foam holds the bend just shy of 90 degrees at rest. At this time I reach for another magic trick, this time related to the glue I'm using: UHU Por. This type of glue can be used in two ways. One of them is to cover both joined surface with very thin layer of glue, wait approx 1 to 2 minutes and join them using light pressure. This way the glued parts will join instantly without any play. This method is perfect for covering wings since the wing panel does not want to stay on the curved ribs by itself. What I do is to put a moderate volume of glue on all ribs, the spar and the trailing egde, then press the covering panel agains them and separate immediatelly. That leaves all joining surfaces covered with thin layer of the glue in proper places. After a minute of waiting I join them again - this time permanently. I use additional weights to keep everything secured for another hour or so; this is not required (theoretically), but better safe than sorry.

After closing both central panels I've checked the electrical connections - this is the last stage where any corrections or repairs can be done without cutting trough the wing. Fortunatelly, everything was still in perfect condition.

Closing the outer panels was just repeating the process again. It didn't take long before the wing was completely closed and ready to accept control surfaces:

The control surfaces are made from 6mm depron; with the flap having exactly the same dimensions as on the original plans and the ailerons having reduced length to acommodate shorter wingspan. Those surfaces are mounted in typical way with packing tape hinge on the top, so I will not describe it in greater detail. The last thing added to the wing is the load spreading structure to protect the wing from being crushed by the rubber. There are tons of ways how to do that; but I decided to create a "ladder" of thin plywood stripes. The exact number of stripes is not important; but at least one of them (the widest one in my case) needs to sit exactly on the spar, and all other stripes needs to be positioned so the rubber bands never touch the depron. The final product looks like this:

Two more plywood stripes - longer and wider than those on the top - are added at the bottom of the wing, front to back, to protect both the fuselage and the wing from damaging each other.

The final touch is to shape the wing tips. I started with the shape from the original plans; but it did not work very well due to altered wing profile. I decided to use the wing rib profile (including a the fact of having 6mm spar somewhere in the middle) to shape the wing tip.

As you can see on the picture below (the wing shown upside down), using the wing rib profile for shapening the wing tip have a very nice - and very desired in this case - effect: when looking on the wing tip forward to back, the entire wingtip edge looks like two straight lines that seem to lay on the flat surface.  This way I can cover the wing tip with one flat piece of depron without dealing with any bizzare curves.

With the wing completed, it's time to finish the fuselage. I started with the top, covering it with two pieces of 6mm depron: one rectangular, reaching from the windshield back to the point where the fuselage "bends", and the other one triangular from the bend location all the way to the tail. Then I cut out the slots for horizontal and vertical stabilizers. Those slots are exactly the same as on the original plans except their width - I had to make them wider to 6mm. All the tail feathers were cut out from 6mm depron using the original plans, with the pie-shaped hole in the rudder slightly enlarged to make room for thicker elevator. I did add the reinforcing strip of plywood to the elevator to keep it from bending; and also added the lights to the rudder:

After having all this in place, I added more lights inside the fuselage. Those are mounted to the middle of the upper surface; I'm sorry I didn't take any pictures, but you can easily deduce their location by the light pattern visible trough the foam:

Now it was time to mount the motor to the firewall, sneak the ESC inside the nose, install the tail servos and get all the cables where they should be. Tail servos installation is - yet again - very typical and their location is exactly as on the original plans. After all the electronics were inside (including reciever) I glued the last parts of fuselage skin. Three pieces of depron were used: one triangular at the rear, and two rectangular ones covering the LG mounting locations. The front piece does not cover all the fuselage - it streaches between main landing gear legs only, leaving the front and the middle of the plane open. The front opening is for the battery, the middle one makes electronics manipulation easier and created great spot for "cargo bay" (to be installed in the future).

And that's basically it. The landing gear is made in the same fasion as the original one except the fact that it is not glued in; rather than that, LG legs are just put inside the brass tubes. Tle front and back legs are hold in place using rubber bands; the central one holds itself in place without any additional devices.

The controls setup does not differ at all from my previous Storch; please check my previous article to find more details.

Here's how the beauty looked like when it was completely assembled and ready to fly for the very first time:

It was finally the time for the maiden flight. If you were patient (or determined) enough to get here, here's your reward:

(if for some reason you can't watch the video here, please follow this link directly to YouTube)

The plane turned out to fly very similar to the original one; especially with the flaps deployed. With flaps retracted it does not accelerate as well (due to thick wing profile); yet still it can fly fast and perform some basic aerobatics. It can negotiate moderate wind gusts, but definitely excels in no wind conditions; and the night flying was a blast. Here's the video of some evening flights performed several days after the maiden:

(again, if the video is not visible, please use this direct link)

When writing this article, I already have several hours of flying this plane on my account. At this point I can confirm that the plane meets all the expectations:

- it can fly as slow and as stable as the original SpeedBuild kit, except for no "self-leveling" tendency due to no dihedral on the wing. It can also fly fast (relatively; it's still high wing trainer) even without using a lot of power.

- stall characteristics are MUCH more gentle. The plane stalls at approximately the same speed/attitude, but does that "in slow motion" and the dynamics of this event depends greatly on the plane's speed and elevator deflection (and thus can be controlled). That allows controlled sub-stall maneuvers (e.g. very tight turns) without a risk of unexpected, immediate spin, and also very gentle and stable "tree points landings" (with proper flare and full wing stall just before touchdown).

At this point I can say the goal was meet in more than 100%; and that I will love the plane just like (if not even more) the original one.