2004 R/C Modeler
Vol. 41 - No. 4 - Pg. 10
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TIGER MOTH 120
TIGER MOTH 120 ARF
Mfg. Sug. Retail Price
Total Wing Area
Total Stab Area
Mfg. Rec. Engine
Rec. Fuel Tank Size
Rec. No. of Channels
Rec. Control Functions
Basic Materials Used In Construction
Building Instructions on Plan Sheets
Fuel Tank Used
Weight, Ready to Fly
WE LIKED THE:
WE DIDN'T LIKE THE:
Pacific Aeromodel Manufacturing Company's huge Tiger Moth 120 is a thoughtfully designed and artfully produced ARF airplane with "good scale fidelity." With a wingspan of 78", wing area of 1841 square inches, a flying weight of 10-11 pounds and requiring five servos with four channels, it boasts smooth, stable, scale-like flight to delight the relative newcomer as well as the "old pro." But the fun doesn't stop there. It is also capable of flying some of the classic maneuvers as well. And get this, based on a telephone call to PAM this morning we were told that the list price for this beauty is only $399.99. Now that's hard to believe. If I were building this IMAA-legal model from a kit I have no doubt it would end up costing me more than that to get all of what they put in the box! Incidentally, in addition to the red/silver color scheme of the review model, the 120 is also available in a olive/yellow military color scheme, or an all yellow trainer color scheme. If those colors are not enough to choose from, the TM is also available as an Almost Ready to Cover (ARC).
How's that for a product review start? I hope it piqued your interest because this one sure caught my attention. The TM 120's airframe is constructed of balsa and hardwood covered with a durable heat-shrink Ultracote-type film. It only takes a quick pass with an iron or heat gun working at about the same temperature as MonoKote to remove the minimal but inevitable wrinkles. In checking out the airframe construction I found it to be flawless. All of the pieces of our review kit fit accurately and there was no need to "fix" anything left undone.
In this review I'm not going to get into the details of building the model. Instead, I'm going to comment on the engine installation details, alert you to a couple very easily solved potential problems and, of course, comment about our test flight.
Tiger Moth Overview:
As we cautiously removed the packaged pieces from the huge 7-1/2" x 22-3/4" x 52" graphic-covered box, it was clear that Pacific Aeromodels has done all of the building-related grunt work for you. And, they've done it right. They've built the airframe. They've covered it and applied the graphics. They've built the struts. They've prepared and installed the pull/pull control wires. The wing flying wires, which are structural not decorative, are also finished. They are neatly packaged and require only a clevis adjustment to snug them up when attached. The main landing gear with its support struts is also prepared and even chrome-plated! Pacific Aeromodels has even applied the instrument panel decal in the aft cockpit and the cockpit combing. In other words, they've done just about all they could do short of installing your R/C system and engine.
Having prepared product reports on many ARF's since they first appeared on the scene some years ago, many of which were also well done, I found the TM 120 full of nice surprises. For example, most ARF's I've reviewed have you mount the servos onto a ply plate, factory-installed inside the fuselage. As a general practice I always add ply doublers to areas where the servo screws will be installed to get a little more "meat" for the screws. To my surprise, those doublers were already installed. Many modelers file a flat in the landing gear axle for the wheel collar set screws. Surprise, the flats were there. For pattern and larger planes we usually pin the rudder, elevator, and aileron hinges. Surprise, the outboard hinges on these surfaces were toothpick-pinned and the film-covered balsa fairings were factory applied to the main gear strut. We also like the knurled edge jam nuts on the control wires so that after minor adjustments are made, a wrench isn't needed to lock the jam nuts in place. (A drop of blue Loctite to all the clevises and their jam nuts helps assure they won't work loose.)
The wing struts are made of aluminum with film-covered balsa glued to the sides to give them shape. Bolts on each end of each strut mount them to the wings via aluminum brackets machined from a block of aluminum, and aluminum brackets are used to attach the flying wires at their ends. "Fish strings" are found in the bottom wing panels to help pull the servo wires through the wing to their exit holes in the wing panel root. The intake air holes in the front and side vent of the flawless gelcoat-finished cowl were factory cut. Panel lines were also factory carved in the cowl. The floors of the cockpits are removable, held in place with one screw each. That's handy because it provides additional access to the servo compartment, both from the cockpit and through the bottom wing saddle area. And there are lots and lots of other such nice surprises seldom found on an ARF model. It's clear that someone who flies R/C was intimately involved in the production of this one!
The only thing we don't like about the model is the hard foam wheels they included, though that was easily remedied. The factory wheels probably work okay on grass fields but all of the club and public fields at which we fly hereabouts are blacktop. So, we switched to rubber wheels to get more shock absorption. We do know of guys flying the TM 120 off of blacktop with the foam wheels, however, and they report having no problems other than those wheels do seem to wear relatively easily, as you would expect.
Assembling The Model:
After the servos and linkages are installed, actual assembly of the plane begins by bolting on the removable tail feathers. Two long bolts pass up through the bottom of the fuselage, through the stab and into blind nuts factory-installed into the bottom block of the vertical fin. Unfortunately, one of the blind nuts inside the bottom of the fin came loose as we attempted to install the hold-down bolts. We removed the covering from one side of the fin, reinstalled the blind nut, gluing it in place this time, and recovered with silver MonoKote - which matched perfectly. When we contacted Pacific Aeromodels to inform them of the problem, they commented that they were not aware of it perhaps because most (all?) people who have built the plane glue the stab and fin permanently in place. Makes sense to me. For product reviews we usually try to follow the assembly instructions more or less to the letter unless something in them seems to be wrong, so ours is bolted on. However, we do have plans to get out the adhesive after completing the test flights.
The top wing panels slide onto two stainless rods to attach to the center section which, in turn, is rigidly held to the fuselage with three struts on each side. The bottom wing panels slide together on a stainless rod and bolt to the bottom of the fuselage. Being able to take the wings apart makes for convenient transport if your space is limited. If you have the transport room you may want to glue the bottom wing panels together.
However, with all that said I did run into some challenging problems here and there. Relatively minor though they were, in our view they do not detract at all from the quality of this airplane. The first problem I confronted was as I prepared to install the engine. The info about the plane indicates that a .74-.90 2-stroke or .90-1.20 4-stroke engine is recommended. I chose to go the 4-stroke route. Using a 2-piece engine mount provided (firewall holes are factory-drilled and blind nuts installed) the engine is bolted upside down on the top side of the mount. When I received the kit for review I anticipated using a new O.S. 120 Surpass pumper that has been waiting for a ride. But, to my disappointment, that didn't work out.
Located at just about the center of the engine mount flanges, the O.S. 120 has a triangular-shaped brace extending from the case to the top surface of the flanges. Since I was unwilling to grind off that piece (the O.S. factory must have had a reason for putting it there) or to cut a slot into the engine mount arm (and weaken the engine mount to accommodate the brace), I decided to get a Saito 120 4-stroke as shown in the manual. I did check out several other engine mounts to see if I could mount the engine in the conventional way, but the location of the prop shaft so near the top of the firewall creates a problem. To get the proper prop shaft location, the top holes of the other engine mounts would end up above the top of the firewall! In addition to possibly reworking one of those engine mounts, the factory-installed blind nuts would have had to be removed and new ones installed, which meant that the factory-installed fuel tank would have to be removed, and ... though there are undoubtedly other solutions, getting a great performing Saito 120 seemed the most sensible and expeditious way to go.
While on the subject of mounting the engine, I would also like to alert you to something which should have been addressed in the sometimes too brief assembly instructions. (They also forget to mention some of the neat things they've already done for you.) As mentioned earlier, a 2-piece engine mount is provided. The kit also includes some rubbery thick urethane washers that are used behind the engine mount and serve a dual purpose. They are intended to minimize engine vibration and build in right thrust. Two of the washers are almost twice the thickness of the other two. With the two thicker ones mounted under the left side and thinner ones under the right side of the engine mount, you have right thrust. But that's true only if you go about it correctly.
When I prepared to drill the holes in the engine mount for the engine, the two engine mount arms were bolted to the firewall with washers in place. Because the engine mount is in two pieces, rather than right thrust, what I had was one engine mount arm sticking farther forward than the other! Oops! That won't work.
After giving the situation some thought and getting some helpful advice from other pilots at our club field, I arrived at a solution. Temporarily install the engine mount onto the firewall without the washers. Mark and drill the holes for the engine. Then bolt the engine to the right and left engine mount rails. That locks the engine mount rails together. Remove the engine/engine mount assembly from the firewall and then reinstall it using the washers provided as outlined in the instructions. Voila! Right thrust.
When the engine mount bolts are tightened to the firewall be sure that the back of the left and right sides of the engine mount are parallel to the firewall from top to bottom. If you were to tighten the top or bottom bolts more than the other, you could also put in some unwanted down thrust on one side or the other. I used two scraps of balsa sanded to the proper thickness for each side of the engine mount as a guide to make sure each engine mount side was parallel to the firewall. Finally, to add a little more "meat" in support of the engine mount I filled the gap behind the engine mount between the plastic washers with RTV, aka silicon seal. Not only does that provide some additional support for the engine mount but it also prevents that area from becoming a sump for engine oil, etc. Also, be sure to use a threadlocking compound on these bolts or they will loosen up very quickly.
I am sure that everyone who flies a 4-stroke engine knows that if the engine is to be mounted inverted the chances are it will idle slower, better, and more reliably if an on-board glow driver is installed. A RAM Glow Driver was installed on the review model. This is a nice unit that comes with the battery all wired and has an LED indicator light. The battery was mounted inside the top of the roomy fuel tank compartment in a block of foam rubber above the receiver and its battery pack. If you are flying with a late model Futaba or Hitec PCM however, you will have to send to RAM for an "amplifier" if you want to use a "Y" to connect it to the throttle channel. When we connected the unit to a switch-operated channel, such as the retract channel, it worked fine without the amplifier. We do prefer having it connected to the throttle, however, and are presently waiting for the amplifier to arrive.
One of the nice things about the Tiger Moth is that it is not necessary to make a cut-out for the engine. The entire engine is inside the cowl. When I prepared to mount the cowl I found no mention of the recommended location of the cowl hold-down screws. Screws through balsa on the top of the fuselage obviously wouldn't work. So, despite the fact that it was factory-installed, I removed the fuel tank to get inside. Removing the fuel tank also allowed the installation of foam rubber around the sides of the ply fuel tank supports. Surprise, doublers for the cowl hold-down screws are factory-installed. To mark the location of the cowl mount screw doublers I pushed a large straight pin up through the factory-drilled pilot hole in the doublers to puncture the covering material and mark their location. At the same time, I also discovered similar doublers for the windscreen mounting screws, and marked their location in a similar manner.
I used the hardwood beams inside the nose area to which the cabane supports mount as a great location for additional cowl mount screws. To provide some support for the bottom of the cowl so it wasn't just hanging out, a balsa block was added onto the bottom front of the fuselage in line with the firewall. Due to the design of the cowl, there is more than enough air exit space along the sides, so cooling should be no problem.
One thing that disappointed me was the need to cut a rather large hole into the left side of the cowl for the Saito 120's rather large, new cone-shaped muffler. Not nice. And then I wondered how the muffler is to be installed after the cowl is on. Before getting out the Dremel, I thought it would be wise to check with our local hobby shop owner, Jay Replogle. As usual, he had a practical solution. Jay recommended some after-market fittings that enabled me to run an extension tube down the back of the engine to position the factory muffler at the bottom rear of the cowl. With that mod it was necessary only to relieve an area on the bottom of the cowl for the muffler.
The muffler was mounted to a half-round cut-out in the bottom cowl mounting block with a wire tie. To make a soft mount for the muffler, the cut-out in the block was lined with RTV. That mod works great because in addition to looking much better the exhaust blows out of the bottom, which also keep the plane cleaner. The only other holes I had to make in the cowl were for the fueler valve and needle valve access.
If you would like to make the same modification, the parts we used are; MACS #1674 90-degree Motor Adapter, MACS #9411 flexible tubing, #1654 straight Motor Adapter onto which the Saito muffler is screwed. The nice thing about this set-up is that the flexible tubing actually screws into the 90-degree fittings that screw into the engine and muffler. It's a nice clean set-up that's very easy to install.
Control And Flying Wires:
Since I had a metal elevator clevis pull off a pushrod some years ago, I have viewed metal clevises with a high degree of suspect. My anxiety was further supported when, in finalizing the plane for the first flight and checking the clevis jam nuts, we found that the up-elevator clevis jam nut for the right elevator half wouldn't tighten. Our first thought was that the threads on the jam nut were bad. After checking further we found that despite earlier checks of the clevises, the jam nut was simply pushing the clevis off of the threaded fitting! One good tug and the clevis came off despite our test tugs earlier. Check each of those metal clevises thoroughly! Then check them a second time, especially on the control wires. And after the flight trim is adjusted, a drop of CA or thread locker would be a good idea. It's always much better to discover problems like that on the ground rather than in the air.
Once we got the wings and their struts bolted on and flying wires
snapped snugly in place, it seemed a good idea to mark them to make
assembly at the field go more quickly. So each of the struts was
marked: L/Wing-F, L/Wing-R, etc. When we removed the flying wires we
attached a little tab to the attach clevis on each set; L/Wing-top,
L/Wing-bottom, etc. It does take some time to put the plane together,
but it's worth it. And remember, the flying wires are not decoration;
they are required. Pacific Aeromodels' Tiger Moth 120 looks really
great all ready to go.
As you would expect, setting the plane up at the field drew a group of on-lookers and well-wishers. When I commented that it was an ARF they were very surprised. When I asked what they thought the kit cost, their guess was $550-$600. When I said the list price was $399.99, they didn't believe me.
Because I was armed with my Nikon to shoot the in-flight pictures, I asked test pilot Jay Replogle to work the transmitter for the Tiger Moth's first flight. With the Futaba 8-ch. transmitter programmed, the control surfaces and C.G. set up as recommended in the instructions and about 40 minutes of break-in time on the engine, a range check was completed and the tank filled. With the Saito running smoothly, it was ready to go.
I released the plane and we taxied out to the runway. When it was our turn, Jay rolled the Moth onto the runway, paused a bit, then slowly opened the throttle and the plane started moving forward. With that huge rudder the plane easily held the centerline as it picked up speed. Shortly, the tail lifted, then the main gear wheels were spinning free. It really looked great as it lifted off - smooth and majestic is the best way I can describe it.
The Moth climbed out easily, made a gentle turn and in little time was up to our test altitude. Our first test was to throttle down almost to an idle to check out its slow flight characteristics. It just floated up there smooth and slow with no hint of a tip stall. Slow down too much and the Moth simply drops its nose and continues flying.
Back to mid-throttle and Jay first checked its response to aileron controls. Having built in differential as I normally do with this type of plane, the response was great. Rudder and elevator controls were equally responsive. Then it was time to open up the engine for a wing-over, split "S" loop, and so on. The really nice thing about this Tiger Moth is that it not only performs all of the more sedate scale maneuvers with ease, but you can also crank it up and wring it out for a few tricks if you want.
Jay was having as good a time flying the Moth as I was with the camera, and we both lost track of the time. The engine quit. Jay yelled out, "Deadstick," made a gentle turn and floated easily to the apron where it touched down and rolled to a stop almost in front of us. I breathed a deep sigh of relief after a very successful first flight. And thanks to Jay's piloting skills, I was sure I had some fine in-flight photos.