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The Sig LT-25

A conversion article on the laser-cut trainer by Bernard Cawley, Jr.

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Specifications:
  • Plane: Sig LT-25, kit number RC-74
  • Type: High wing cabin trainer/sport/utility plane.
  • No. of channels: 4 (ailerons, elevator, throttle, rudder)
  • Wingspan: 63 inches (160 cm)
  • Wing area: 724 square inches (48 square decimeters)
  • Airfoil: Clark Y
  • Length: 51 1/4 inches (130 cm)
  • Propshaft to ground: 8.25 inches (21 cm) in flying attitude
  • Weight: 4 lbs. 9 ounces (2.08 kg) to 4 lbs 15 ounces (2.24 kg)as built
  • Wing loading: 14.5 to 15.7 ounces/square foot (43 to 47 g/square decimeter)
  • Propeller: Master Airscrew 12X8 wood Electric
  • Battery: 10 cells from 1400 to 2000 mAh
  • Speed controls: Aveox L30, MEC MX-80

Power systems tested:

  • Aveox 1114/4Y on Modelair-Tech H500 belt drive at 3.6:1
  • MEC Turbo 10GT on MEC SuperBox at 5:1
  • Kyosho Magnetic Mayhem Reverse on MEC SuperBox at 3.333:1

Available from:

Sig Manufacturing Co., Inc.
401 So. Front Street
Montezuma, IA 50171-0520 USA

 

Introduction

Are you looking for an easy-building glow kit that is also an easy conversion to E-power? How about a trainer that is bigger and flies with more authority than the usual "oh-five" powered ship – one that would mix well with the .25 or .40 powered trainers at the flying field? Or, how about a versatile, reasonably aerobatic plane that can do utility duty such as aerial photography or power system testing? Or, perhaps, do you want to see if all this ballyhoo about laser cut kits is for real? Well, if you said yes to any one of these, the Sig Kadet LT-25 may be just what you’re looking for.

The Sig Kadet family, especially the Senior and Seniorita, have been favorites among E-flyers for quite awhile. More recently, the LT-40, which is simpler to build and is a partially laser-cut kit, has been a successful conversion, even though it has the now-traditional poplar ply (so-called "light ply") trainer fuselage. Introduced early this year, the LT-25 is the newest member of this family.

The first time I saw it was at the Sig booth at the Northwest Model Expo in Puyallup. Several things about it caught my eye. For one, it is a taildragger – so there’s no nosewheel to discard or main gear to relocate. It also has ailerons, like the LT-40 (and unlike the Senior/Seniorita). The tailwheel is operated by a nice pull-pull system, which is included in the kit. I thought that was a nice touch. After pondering for awhile as I always do, I decided to buy one.

 

Kit Contents

The next step, of course, was to open the box and take a look. Out came the now-customary illustrated manual (this one is 40 pages long), and two folded sheets of full sized plans. I also took a peek at the laser cutting I had been reading about.   I was NOT disappointed.  In fact, I was amazed – even the 5/16 inch thick stabilizer and fin parts were cleanly cut - so were the lightening holes in the tapered elevator and rudder parts. The wood itself was all good quality stuff, as I have come to expect from Sig.

Poring over the plans and looking closer at the kit I soon figured out that converting this airplane to electric power was going to be almost too easy. Harold Hester seems to have designed it as an Electric from the outset, for he’s done almost all the things we would normally do to convert a plane like this. It has no bass or spruce beams in the wing structure. It has no plywood fuselage sides or doublers – instead the structure is virtually all 1/8 and 3/32 inch balsa, with only a plywood firewall, landing gear mount plate and wing dihedral joiner. Really, all there is to do to convert it is to figure out how to mount the motor, and how to mount and access the flight battery. A little further on I’ll detail how I went about doing this, and I’ll make a couple of suggestions of other ways to do it. But first, let’s look at the numbers and see what sort of power systems we might use.

 

Power Choices – Lots of ‘Em

Based on the kit box information of 724 square inches of wing area and a flying weight of 4 to 4.25 lbs., we can use some of the common rules-of-thumb to get an idea of the power requirements. For instance, the old standby rule Bob Kopski first published in the April 1984 issue of Model Aviation (yes, it has been that long!) suggests that for good takeoffs one needs 30-50W of input power per pound of airplane weight (or 65-110W per kilogram). A related rule proposed by Keith Shaw in the July 1987 issue of Model Builder says one needs 40-60W per pound (90-130W/kg) for sport aerobatics such as loops, rolls, Cuban eights and so forth.. The two of these have come to be combined to give us the 50W/lb (110W/kg) figure we often use today. So, if we figure an electric version of the LT-25 will weigh around 5 pounds, that gives 250W needed. Setting a maximum current draw at full throttle to 30A (4 minutes of full power on RC2000’s), which is well within the capability of a good rare-earth magnet brushed or brushless motor, we need at least 8.333 – say 9 – cells.

Another way to look at it is known as Orme’s law (since Matt Orme of Aveox proposed it on the EFLIGHT mailing list and in the August/September 1998 issue of Sailplane and Electric Modeler). He suggests one cell for every 50 square inches of wing area for a high wing trainer type airplane, such as this one. This gives a little higher cell count – 14.5 cells. At 30A this is 435W input – or 87W/lb. That much power would give decidedly un-trainerlike performance, or much longer flights if propped/geared for a lower peak current or flown with a disciplined hand on the throttle. So, depending on the approach, anywhere from 9-15 cells are suggested. This makes an Astro 05G powered conversion sound doable at the low end. Something like an Astro 15G, an MEC Turbo 10 or a small brushless would be more in order, however, or perhaps even an Astro 035 or 05 with Astro’s Superbox. But, any motor than can handle 25-30A on 10-14 cells will do just fine. A particularly inexpensive choice I tried is the Kyosho Magnetic Mayhem Reverse "monster truck" motor. 

My experience with other models (and the fact that I had an unused 10 cell 1400 SCR pack and two motors to match) led me to make my LT-25 a 10 cell model. With ElectriCalc I was able to take this basic assumption of 10 cells and the choice to start with the Master Airscrew 12X8 wood Electric prop to find a gear ratio for my Aveox 1114/4Y and my Model Electronics Turbo 10GT – 3.6:1 in the first case, 5:1 in the second. In fact, E-Calc predicted that the performance with either of these systems would be quite lively, with climb rates in excess of 1200 feet per minute. So my task was to build the airplane, while finding a home for a 10 cell battery and figuring out a way to mount either or both of these motors with their appropriate reduction drives.

 

Building It – Almost Too Easy

With no structural changes to make, I got right into building the airplane. Immediately I began to see that all the good words I’d heard about the laser cut parts were true. I’ve never before built a kit in which every pre-cut part fit exactly right. I have now.

One small problem: the manual has you build the wings first, and at step three I ran into a little snag. It directs you to cut the lower spar to length on the plan. I did that for the left wing, and found that the photocopy of the plan I’d made to cut apart apparently had shrunk a bit. When I dry-assembled the pre-slotted shear web and ribs to the spar I wound up with the tip rib about 3/16 inch past the end of the spar cap. So, I’d recommend NOT cutting the lower spars off until the tip rib is fitted into place. The only other "problem" I found was simply that the aileron throws listed on the back of the manual are incorrect – the linkage provides for some differential, and the throws should be 1/2 inch up, 3/8 inch down, rather than 3/8 inch both ways.

There are a couple of odd sequencing choices in the manual. For example, you’re told how to install the pull-pull cables for the tailwheel, which assumes the tailwheel assembly is installed, and several pages later it describes how to assemble the tailwheel. And of course, you can simply skip all the fuel tank installation stuff (and just leave out the balsa tank compartment floor).

Could it be built lighter if some changes were made? Well, yes – but with all that wing area and the virtually all-balsa construction it certainly isn’t worth the trouble to substitute parts made of smaller wood sizes or even re-trace parts onto lighter stock the of the same thicknesses. If the parts didn’t fit so well perhaps there’d be more incentive to do some of this, but it’s too easy to just glue the parts together as supplied. By the way, some have speculated about leaving out the diagonal ribs that fit between the front and rear spars, but I don’t know why one would do that. They fit perfectly and add a tremendous amount of torsional strength to the wing, which, when completely framed-up, can’t be twisted without breaking something. Besides, they look cool under a translucent covering.

In the end I remade exactly one part: I used the supplied light ply "tank hatch" as a pattern and cut a balsa one from one of the fuselage doubler sheets (of which there is a lot of material left over). I think I did that just for the principal of the thing, rather than because it was necessary.

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The battery is too heavy to be placed all the way up front where a gas tank would normally go, so I decided to use this space for the ESC (in the picture, MEC MX-80) and the receiver battery.

I covered my LT-25 with my favorite iron-on covering, Coverite’s Micafilm. This lightweight but tough stuff, when applied properly, will not sag in the sun and lasts for years and years. For the base color I used the unpigmented (so called "clear") version, that weighs about 40% of what MonoKote does. For an accent color I used translucent orange, which is a little heavier. This covering choice probably saved me 1.5 ounces or so over Monokote or a bit more over UltraCote/Oracover. This was the third plane I’d done using Solarfilm’s Balsaloc as the adhesive for Micafilm rather than Coverite’s Balsarite. I see little difference in performance, and Balsaloc is a lot friendlier to work with, being water based rather than thinned with toluene.

By adding some blue Coverite Graphics stripes to the white and orange, I was able to make a color scheme that was sort of an inverse of the white with blue trim and orange stripes color scheme shown on the kit box, and still use the supplied colorful graphics for the vertical tail. Completing the look of the model were the "window" and "windshield" stickers supplied, a matching blue AMA number for the wing and the letter "E" to put on the vertical fin (so it says Kadet LT-25E) from Vinyl Graphics by Greg.

For wheels I used a three inch pair of the newest version of the DuBro lightweight wheels, which are now made more like Sullivan SkyLites, having a harder outer layer, and a 1 1/2 inch Dave Brown LectraLite for a tailwheel. I also have a pair of #9G Trexler wheels I plan to use when flying off of a rough meadow with no prepared runway that is near my house.

The landing gear, by the way, is quite tall with at least 8 inches from the prop shaft to the ground with the plane in flying attitude. The gear is also pretty stiff, so there is no problem swinging a 12 or 13 inch prop, maybe even a 14 incher without any concern for prop strikes, even on a less-than-graceful touch-and-go. I speak from experience here.

I also modified the tail section to make it removable, anticipating a trip to the American Southwest this past summer with the plane – but that is material for another article…

All up weight, with the 10 cell 1400SCR pack installed, was 4 lbs. 9.2 ounces before the first flight.

 

Motor Mounting – Several Options

At the top of the initial fuselage construction page the manual says "NOTE: You will need to have the engine that you will be using before starting fuselage construction". It then describes how to lay out the correct locations for the supplied glass-filled nylon engine mounts on the firewall. This is good advice. Regardless of the motor/gearbox you might select, the firewall is probably going to be the most heavily modified part in the kit. As a minimum it will need a moderate-to-large hole through which to pass motor wiring (and perhaps part of the motor) and some additional cooling holes to allow air to the speed control and/or the battery. In my case, I started out mounting the Aveox 1114/4Y on a Modelair-Tech H-500 belt drive, using the kit’s engine mounts.

Putting a reduction gear on the little Aveox is a bit of a challenge, as it is barely an inch in diameter and has mounting holes with an odd spacing which don’t match any commercially available gearboxes or belt drives. A little fussing (and a custom made spacer that Tom Hunt provided) let me mate the motor to the H-500. This turned out to be a fortuitous choice, as the glass-filled nylon engine mounts supplied in the kit fit up with the top of the belt drive perfectly. To mount the 1114-4Y/H-500 combination all I had to do was shorten the supplied mount beams about 3/8 of an inch, then bolt them to the firewall with the tops of the mounts flush with the top of the firewall. The spacing between them shown on the plan works just fine. A pair of holes in each mounting beam lead to holes I drilled and tapped for 4-40 screws in the top of the H-500’s housing. Two 4-40 cap screws attach each engine mount beam to the H-500, hanging the assembly neatly in the nose of the airplane, with the prop shaft just a little bit higher than that of the OS .25 FP shown on the plans. A hole in the firewall to allow that long, skinny motor to pass through was needed near the bottom. This hole would later be used to route wiring when I tried the Turbo 10GT and then a Magnetic Mayhem Reverse as alternatives to the Aveox, as you can see in some of the photos.

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Photos of my 1114-4Y/H-500 combo installed in the LT-25.  The ModelAir-Tech belt drive fits perfectly between the supplied motor mounts for the traditional glow motor. 

 

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My Model Electronics Corporation Turbo 10 GT motor in an MEC Superbox.   I mounted this motor using a Stitzer Aero-Vee mount.
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The "Magnetic Mayhem" motor mounted to the nose, also with a Stitzer mount.  This is an inexpensive ferrite motor that can be found in most R/C car shops.  I used an MEC Superbox (3.3:1 ratio).

For mounting the Turbo 10GT and the Magnetic Mayhem, I am using one of my favorite motor mounts – a Stitzer AeroVee MM-1. This is a sheet metal implementation of the Kopski vee-block and inner-pushrod-tube-strap method. You just bolt it to the firewall and secure the motor in the vee-shaped cradle with two lengths of plastic pushrod stock and/or two wire ties. This puts the whole motor and gearbox ahead of the firewall, though. This, in turn, puts the prop a moderate distance past the nose of the fuselage unless the gearbox has a really short shaft (I took about 3/8 of an inch off the MEC SuperBox shaft for my Magnetic Mayhem installation). This mount is also ideal for an Astro 035SG or 05SG or even a MaxCim 1315 or Aveox 1406 or 1409 series motor as well as the ferrites I’m using. In fact, for the truly power-hungry, the fuselage is wide enough for the similar but larger MM-2, which accommodates motors such as the Astro 25 and 40.

 

Battery Mounting – Time to Get Creative

I puzzled over how to carry the motor battery for some time, and came up with a solution that works, but would work better if the battery were a bit further forward in the airplane. I also managed to implement the solution using nothing but what came in the kit except for some 1/64 ply. I wanted the battery mount to make it easy to remove and replace packs for cooling and recharging without removing the wing. I also would like the battery to be able to leave the airplane in a hurry without tearing the rest of the airplane up (been there, done that, got the garbage bag).

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A picture of the battery hatch.
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A shot showing an open hatch to demonstrate the battery access.
 

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The battery pack installed in the airplane.

Herm Perez, a modeler known to be a lover of Kadets, advocates loading the battery through a windshield hatch.  Early in the construction I took my 10 cell flat pack and laid it down on the plan in various orientations, trying to figure out how to make all that work. It was apparent that if I put the battery straight in through the windshield and carried it horizontally it would be pretty close to the aileron servo and linkage. So, I tried angling it downward into the fuselage. This led it to interfere with where the elevator and rudder servos and the tiller assembly for the tailwheel are designed to go. I finally decided to move the servos and tiller aft as far as they would go in the compartment (using more scrap from the doubler sheets to provide a set of rails for the servo mounts to sit on as they would have if I’d installed them where they were supposed to go). This left lots of room for the battery to angle down from a windshield hatch.

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I moved the rudder and elevator servos backwards to accomodate the battery pack.   This is one of the few changes needed to convert this kit.

 

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Don't forget to provide some cooling air for your batteries.  I left a portion of this lightening hole uncovered so the air that enters the front of the plane (through holes in the firewall) can pass over the batteries and leave via the underside of the fuselage.

The actual tray was made from 1/8 inch balsa – another part from one of the doubler sheets, laminated with 1/64 ply on both sides, and stiffened with a scrap of rear spar material on the bottom. It is supported on both ends by scraps of aileron stock, which set the angle of the tray in the fuselage. The aft tray mount sits where the servos would go on the edge of the doubler. The forward end is just forward of F-2 (the bulkhead at the forward end of the cabin). The tray is attached to the aileron stock shelves with 4-40 nylon screws to make it easy to remove. I also had to enlarge the opening in F-2 to allow the width of the pack to slide through easily. A patch of Velcro at the forward end retains the pack there, and a loop made from a Velcro tie-strap holds it at the back. A scrap of EPP foam is glued to the very back of the tray to prevent the battery from sliding back onto the servos.

The windshield hatch is simply a portion of the part that normally goes there, hinged by the press-on "decal" windshield and held closed at the bottom by a flat nylon landing gear strap from one of my "bits" drawers. All this was done after the airplane was otherwise finished and covered.

As I mentioned, this arrangement puts the battery just a bit too far aft for the airplane to balance at the recommended location, even though the CG of the pack is forward of the balance point a bit. This almost forced me to add lead to the nose (sacrilege to any self-respecting electric flyer), even with the speed control mounted as far forward as possible right behind the firewall on the left side. Since I was using the BEC in the Aveox L30 controller, there was no receiver battery to mount "as far forward as possible", so I wound up using a 2 1/2 inch diameter prop spinner I had left over from another project instead of the 2 inch one I’d planned to use. That just brought the plane into balance. I suppose another solution would be to use a heavy prop like a Graupner fixed-blade unit or an APC rather than the Master Airscrew wood Electric prop, which is very light.

I had to use the same spinner on the MEC Turbo 10GT installation. It turns out that the GT, plus an MEC Superbox, the MM-1, an MEC MX-80 speed control AND a 270 mAh receiver battery I put in in anticipation of using a non-BEC controller later was about the same weight as the Aveox 1114/4Y, the L30 control, the H-500 and the kit’s engine mounts. The Magnetic Mayhem is a little bit heavier, so I was finally able to use the smaller spinner with it.

What I really need to do is tear out the battery tray and relocate it lower and further forward, so that I can install and remove the battery through the gas tank hatch instead of the windshield. Those of you using 12 or 14 cells in two shorter packs can probably more easily locate them far enough forward to balance.

 

Flying

Those of you getting this far (or skipping to this point) probably want to know about the plane’s performance, right? Well, OK.

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A shot of Howard Battan's LT-25 flying overhead with the
sun shining through the translucent white Micafilm.

With the Aveox 1114/4Y/3.6:1 H-500 power system, the maximum current draw is about 37A and input power in the flat of the discharge curve is about 345W according to my WhattMeter. Takeoffs in calm air are in 20-30 feet. With a breeze, they’re almost instantaneous. Climbout is very near the E-Calc predicted 45 degrees. The airplane rolls smoothly to the right with very little elevator correction needed when inverted. Inverted flight at full power, too, needs very little forward stick. It will barely do 1/2 of an outside loop from inverted flight At slow speeds, especially, the rudder is the more powerful lateral control, and it can be flown on the rudder and elevator quite nicely.

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The LT-25 on final.  The plane is very gentle at low speeds due to the 15 oz/sq. ft. wing loading and Clark Y airfoil.  This photo shows my plane, prop stopped, about to land at the Celebration of Silent Flight. Digital photo by Scott Kindt.

When slowed down, the wing loading of under 15 ounces per square foot on that true Clark Y wing makes for a gentle floater. In fact, it just doesn’t want to come down if the prop is braked to a stop. Several of my friends have suggested setting my low throttle trim before takeoff to a low "idle" and this has proven to be just the ticket to get a positive descent rate on final approach. Landed this way it will do a very graceful three point at a very low speed, and with throttle fed back in is off again in no time.

Stalls are straight ahead and with little altitude loss. Overall its manners are trustworthy and predictable – just what you would want in a trainer or a "just go flying" plane. It is threatening to displace the Goldberg Mirage 550 as my favorite plane of that sort. I think if it fit in my little car a bit easier there would be no question about it.

According to my flight log, on that pack of 10 1400 SCRs I mentioned, flights have ranged from 4 1/2 to 10 1/2 minutes, with most flights in the 5 1/2 to 6 1/2 minute range. Our illustrious editor, when he flew it at Chilliwack last spring, had a 7 minute 20 second flight on it. Remember, this is on "only" 1400 mAh cells. I later got a pack of RC2000s, which are in an MEC PowerTube. Flights on that pack average 7 1/2 to 8 minutes long.

Hanging a 7 ounce 35mm camera on the side for aerial picture taking seems to go completely unnoticed by the LT-25, at least in all upright attitudes. The trim doesn’t change and the flight times are not noticeably shorter. But this, too, is the subject of another article…

Performance on the Turbo 10GT/5:1 MEC SuperBox is a just a little bit lower. I’ve only three flights on that configuration so it’s hard to say much more. I have a note in my log on shooting touch and goes for 7 1/2 minutes on the 1400 pack and still having more than enough to taxi back to the pits at the Boeing Hawks’ field.

I have eight flights, so far, on the Magnetic Mayhem Reverse/3.33:1 MEC SuperBox. With this combination, the maximum current draw is about 28 Amps (about 270W in the flat part of the discharge curve). There is a second LT-25 flying in my area with a Magnetic Mayhem Reverse running the Master Airscrew 3.5:1 gearbox, on 10 cells, also using the Master Airscrew 12X8 wood E-prop.

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This is a shot of the motor installation on Howard Battan's Lt-25.  He is using a Magnetic Mayhem ferrite motor on 10 cells and a Master Airscrew 3.5:1 gearbox.  This is a very inexpensive combination which flies the plane nicely.  Howard made his mount using a ply tobe with wood blocks which attach to the kit engine mounts.

Again, as predicted by E-Calc, this is not as sprightly a performer, but it still gets off the runway in less than 100 feet and still loops easily from level flight. It will also take off readily out of thick wet grass. Even on this relatively low power, it feels quite solid, with about a 30 degree climb-out. This would be an ideal combination for flight training, I think, and one that is very inexpensive to set up. Of course, at this power level, outside loops are out of the question, but it still has enough power for sustained inverted flight, and I got 9 1/2 minutes of shooting touch and goes on the 1400 pack on a recent Sunday evening.

 

Recommendation

The Bottom Line – It’s a Keeper!!

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Two Kadets from the Pacific Northwest.  The blue one belongs to Howard Battan.

The Sig Kadet LT-25 is a beautifully made kit that is essentially Electric-ready out of the box. It builds with no fuss – the parts all fit. It is a forgiving, gentle flyer, just as a good trainer should be, but can be surprisingly aerobatic. With a 250W power system it would make a great trainer and touch-and-go (circuit and bump) airplane, maybe even a good night flyer with Tim Cone’s Night Ops or a set of RAM Skylights. With 350W or more it is a sporty performer and has much to teach me about the proper coordination of all four controls. I like it, a lot. If this is the sort of plane you’re looking for, you will, too.

 

Credits

Thanks to Howard Battan for providing digital photos of his plane for this article.

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