SoarNeck
Dec 08, 2003, 07:30 PM
| spec
| @905222:The modified Senior Telemaster makes a great glider tug.
|> <b>Wingspan:</b> |< 95"
|> <b>Wing area:</b> |< 1330 sq. in.
|> <b>Weight:</b> |< 13.5 lbs (approx)
|> <b>Wing Loading:</b> |< 23 oz/sq.ft
|> <b>Motor:</b> |< Aveox 1817/4Y, ModelAirTech 1.6:1 belt drive
|> <b>ESC:</b> |< Aveox H160
|> <b>Radio:</b> |< Multiplex Profi 3030, IPD9 RX, MPX MCV2/Royal servos
|> <b>Available From:</b> |< <a href="http://hobby-lobby.com/">Hobby Lobby</a>
!Introduction
One of the fastest growing areas of model aviation at the moment is scale soaring, or more specifically, scale aerotowing. While I admittedly might be a little biased, I can think of few things more exciting than hooking a beautiful scale masterpiece up behind a powerful towplane, and heading out for a little afternoon soaring.
While traditionally sailplane tugs are large gas-powered monsters, this isn't an ideal system for everyone. Many clubs have problems with noise, as urban sprawl pushes more gas flying sites further out of major metropolitan areas. Some clubs, like <A href="http://www.soarcalgary.com/">mine</a>, have gone to a glider/electric-only system, and have started to look for alternatives to big, comparatively noisy gas engines.
One of the most popular unresolved questions on the RCGroups.com bulletin boards is the use of electric power in towing gliders - specifically, large gliders with wingspans greater than 4m. Print magazines have just started to run small articles on electric tugs, but that medium doesn't have the ability to go as in-depth as most people require.
Hopefully, this article will show you all that electric towing is a very viable option now, and help to guide you around some of the pitfalls that you might experience when trying towing for the first time. A large tug must be stoutly built, reliable, comfortable to fly, and generously powered. Each of these issues will be addressed separately below.
!Choosing a Power System
Unlike in the combustion realm, there are relatively few choices available for powering large electric models. Few speed controls can handle more than 30 cells in series, and fewer chargers can charge them in a reasonable time (I define "reasonable" to be within an hour - about the longest field charge that I will tolerate). To further limit the field, I personally feel that brushless motors are the best choice to power a large model, since they can deliver large amounts of power with minimal wastage and without the need for maintenance (and clouds of brush dust).
I was drawn to the Aveox line of motors after reading an article by Don Bailey, where he used an 1817-series motor to tow a Flair KA-8 with a large Piper cub. After running some numbers through MotoCalc, I decided that the 4 turn version of that motor (1817/4Y) would the best match for a dedicated tug, along with the Aveox H160 speed control (8-30 cells, 60A continuous, 100A peak). The motor is a sensored unit in its stock form, but it will also work nicely with any of the appropriate sensorless speed controls on the market.
Batteries also limit the system, in that they are a defined reservoir of electrical energy (excuse my blinding flash of the obvious). A powerful motor will require a large amount of energy to power it, and that power setting must be maintained for a reasonable length of time. A good tow will take between 1 and 2 minutes to gain reasonable height (1000-1500ft), and ideally it should be possible to get a couple of tows out of a given charge.
The 1817 series of motors is happiest when operating at around 30A continuous, so a quick ballpark calculation said that about 2 Ah of capacity would be needed to get two tows out of a given charge (30Ax 0.067 hours (ie. 4 mins) = 2Ah, or 2000mAh). This is well within the operating parameters of the newest sub-C sized Nicad and NiMh batteries, and I ultimately decided to use a 30 cell pack of GMVIS 2400 nicads. These cells are available from Icare Sailplanes for around $6-7 (Canadian) per cell, and have locally proven to be much more durable, and slightly more powerful, than the Sanyo RC2400 cell. A Schulze ISL330D will charge a30 cell pack of these cells at 2.6A peak (a little under an hour to charge), and a Schulze 630D will do them at 6.9A peak (a little under half an hour).
Sanyo 2600mah and GP 3000 and 3300 ni-mh cells would be another, slightly more expensive, option ($8-9 Cdn per cell from most sources), and might give 3 tows per charge. While Li-Poly batteries would enable you to get substantially more flight time before recharging, I shudder to think of what a pack capable of delivering 30A continuous at about 30V would cost. I would imagine that you could easily buy 3 or 4 packs of nicads for the same amount of money, without the need to transport and charge them in an ammunition blast box.
!Choosing an Airframe
The first decision that needed to be made was to select an appropriate tug airframe. Since the Hobby Lobby Senior Telemaster had been used quite successfully as a towplane in the past, and based on its popularity as a sport model, it was ultimately chosen as the subject of this conversion article. The Senior Telemaster is an ideal electric model, as it isn't nearly as overdesigned as most nitro/gas kits.
!Waiting for a Tow
As a long-time fan of scale models, Ryan van Beurden, generously offered both his piloting skills and the use of his <A href="http://www.roedelmodell.de/">Roedelmodel ASK-21</A> for the project. We had experimented with aerotowing in the past on a much smaller scale, but this was the first time that either of us had tried towing such a large model.
The ASK was a perfect choice here, as it was a well behaved, durable model that performed wonderfully over a wide range of conditions. A thread devoted to the sailplane can be found by <A href="http://www.rcgroups.com/forums/showthread.php?s=&threadid=127831">following this link.</A>
<img src="http://static.rcgroups.com/articles/liftzone/2003/nov/tug/camera.jpg"><A href="http://rcgroups.com/gallery/data/519/6409ASK_Loop.MPG">Video: ASK Test Flight</A>
@905223: Ryan van Beurden with his 4.2m Roedelmodel ASK-21
@905224: And people ask why we need to fly scale...soaring aesthetics don't get much better than this!
!Converting the Senior Telemaster
!!Fuselage
Since this article isn't intended to show a step-by-step build for the Senior Telemaster, but rather how to convert it for use as an electric tug, I'll rely more on photographs than on description for the following sections. If anything is unclear, please feel free to post to the linked comment thread at the end of the article, and I'll try to monitor that thread and respond as best I can.
I elected to start on the fuselage first when I built my Telemaster, mainly since that plan sheet came out of the box first! Speaking of which, I have to say that I found the plans that Hobby Lobby includes with this kit to be of fairly poor quality, both in clarity and description. The blue line machine that printed the sheets clearly needs to be cleaned up a bit, and even with 200+ models under my belt, I still had to do a bit of head scratching to figure out where all the various fuselage braces lined up. Nothing too major in the end, but something to keep in mind for those with less patience for ambiguity.
The first change that needs to be made is too set the firewall back from its stock location to one that suits your motor of choice. The ModelAirTech H1500 belt drive bolts nicely to a glow-style engine mount, and that was the method I ultimately chose to use. It also becomes very easy to adjust the thrust line of the motor later on, since you can add spacer washers under the four points of the motor mount.
@905225: Make sure to use plenty of bracing to keep the firewall in place.
@905226: You can see in this photo how far back the firewall needed to be moved. The original plan location is shown marked in pen.
My next small deviation from the plan was regarding the placement of the rudder and elevator servos, which I decided to move from under the wing to in the tail group. In order to make life easier, I ran the wiring for those servos early in the build process, leaving enough extra to make solder work easy. If you decide to do something similar, be sure to twist the servo wire at least once every 1" or so, and try not to install it under tension (to avoid wire fatigue when the fuselage twists).
Building the tug tow release came next in the build sequence. A release on the tug is important for safety, in case of a mechanical hangup or radio problem on the sailplane end of things. A small piece of 1/16" ply was recessed into the upper fuselage longerons in order to spread the load from the towhook properly, and was braced underneath with triangle stock. Next, a loop was formed out of heavy duty piano wire, which was securely anchored into the fuselage former just behind the wing. A piece of smooth 4-40 pushrod wire, sliding in a guide tube, is used to trap a loop built onto the end of the towline. When the wire is retracted back into the fuselage, the loop is released. I used a standard Futaba S148 servo for my release without a problem, though it wouldn't be wasted money if a stronger servo was chosen instead.
@905227: With the servos in the tail, the wiring runs to the receiver are rather long. They should be twisted to minimize radio problems.
@905228: A small piece of 1/16" ply provides a solid mount for the tug release.
@905229: A wide loop makes a crude but effective release bracket for the tug end of the system.
@905230: Here, you can see the linkage that drives the tow release.
!!The Tail Group
Relatively little has to be done to make the stab removable,which is especially valuable given how useful removable tail feathers are on a large model like a Telemaster. Before sheeting the stab center section, I cut four small pieces of 1/16" ply about 3/4" square, and drilled their center points to clear 4-40 bolts (I used metal, though plastic would be fine I'm sure). These are installed in the corners of the lower balsa stab center section sheeting, and must be securely bonded to the surrounding structure. Make sure to drill through the bottom balsa sheeting in order to transfer the location of the holes.
@905231: 4-40 bolts and blind nuts are used to make the tail group removable.
Next, the bolt hole locations from the stab needed to be transferred to the fuselage. First, I sheeted the stab platform with 1/16" ply to toughen the mount and take the load from the bolt heads. Next, I lined up the tail group and marked the location of the bolt holes onto the fuselage stab platform. I then drilled countersunk holes on the underside of the stab platform to clear the heads of 4-40 bolts, and hardened the surrounding balsa with thin c/a. Lastly, a hole was cut in the stabilizer platform to clear the 4-pin Dean's plug that I used to hook up the tail servos (which would be mounted in the stab itself).
@905232:Holes are drilled in the four corners of the stab platform to accept the tail bolts.
@905233:The stab platform here has been sheeted with plywood. A hole must be cut in the platform to clear the connector for the tail servos.
The mounting system is finished by either securing 4-40 blind nuts to the four pieces of ply in the stab itself, or by threading the pieces with a 4-40 tap (I used blind nuts for a more long-lasting system). Bolt the stab to the fuselage to make sure that the tail is still aligned properly.
Next, the servos are installed between the upper and lower sheeting in the stab center section. Since I used metal-geared Multiplex Royal Micro BB servos in this install, I had no problems sheeting over the finished installation. If a resin-geared servo is used instead, a hatch or removable cover would allow for gear changes in the event of damage. Since my mounting system replies on bonding the servo lugs to the face of the stabilizer rib for security, I used a doubler made of 1/32" ply on those ribs. Remember, there is very little vibration to deal with in an electric model, so hard-mounting the servos is perfectly acceptable.
The orientation of the two servos is dictated by the way that the control horns are installed on the rudder and elevator.
@905234: Here the elevator servo is being fitted into place...
@905235: ...and sheeted over. Be sure to use metal geared servos unless you make provisions for them to be removable.
@905236:Since the elevator servo horn projects below the stab, a small piece of balsa is used to provide an anchor point for the surrounding Ultracote. Be sure that the exit hole for the servo wires lines up with its counterpart in the fuselage!
@905237:The rudder servo being installed. The rudder servo horn extends through the upper center section sheeting.
Two small items finish off the list of modifications to the tail group. First, on the advice of others who had built Telemasters in the past, I added a fair amount of area to the rudder to boost its effectiveness. The new rudder is very effective, so I highly recommend this change.
Secondly, I added a small piece of vertical grain balsa between the stabilizer spars on either tip. This provides a secure, crush-proof structure to bolt the tail bracing wires to. Bracing wires leading from the tips of the stab to the top of the rudder are needed not for strength, but to allow the towline to slide from one side of the tail to the other in flight without removing the rudder in the process! Solid 2-56 wire could be used for these braces, but I elected to use braided cable from a Sullivan 4-40 pull-pull set instead (which is a little lighter).
@905238: In this photo, you can see the darker-grained wood that was added to boost the rudder area. The new rudder is very effective.
!!Building Carbon Landing Gear
With the immense torque of the Aveox 1817/4y motor, a large prop in the 18-20" range is needed to load the system properly. Given that the Telemaster isn't designed to take much more than a 12-14" prop, the stock aluminum landing gear bracket is pretty useless in this situation. That means that an interested modeler must either get some new taller gear custom formed, or build some themselves. While it should be reasonably easy to get a metal shop to custom form new aluminum gear (two of my LHS's offer this service), I decided as an experiment to try forming some composite gear at home.
The following pictures will detail my attempt to make active, sprung landing gear using small shock absorbers from an r/c car. The gear ended up being too flexible under the load of the finished model, and the Wilga-style scissor gear was abandoned, but the techniques would be suitable for lighter airplanes in the 6-8 pound range. I'd always had that idea floating around in my head, however, and had to give it a try.
To build this style of gear using my techniques, you'll need standard vacuum bagging equipment and some experience in laying up composite fabrics. My personal vacuum system uses an air-conditioner pump, and was assembled using techniques similar to these: <A href="http://www.badger.rchomepage.com/vacbag.html">Building a Vacuum Bagging System</A> (the fellow who put this page together is a fellow club member). Please use a respirator and skin protection when working with carbon fibre or epoxy.
To form the gear, I first cut a foam blank to the shape of the inside contour of the finished gear. I used pink Foamular insulating foam for the form, which required two layers of foam to be bonded together with a heavy coat of 3M77 spray adhesive. Protect the form with a release film of some description so the finished gear part won't stick to it. I used wax paper because it's cheap and readily available, but be warned that some people have had trouble with the wax dissolving in the epoxy and weakening the finished part. I use West System resin (105 resin 206 hardener) in most of my composite projects, and haven't had a problem with wax solubility yet, but your experiences may be different.
Next, make up two copies of outline of your landing gear in bagging mylar leaving the legs a little long, and wax these mylars with a couple of coats of mold release. The layup will be done from outside to inside on each of the mylars (ie outside layers are closest to the mylar), with the core layers added to only one side if the layup isn't perfectly symmetrical. The various layers of cloth are wetted out in a separate area to allow for excess resin to be drawn off, and are then arranged on the mylar. The mylars are then placed together with the fabric layers touching, and bent around the foam form. Don't worry about bits of cloth slipping out from between the mylars as you are forming it, since the finished gear will be cut to shape after it has cured. The whole mess is taped to the form, covered in a last layer of release film to protect the vacuum bag and breather cloth, and the vacuum pump is turned on .
The process isn't very difficult, but it is rather messy, so be prepared. Pull as much vacuum as your pump can take safely, and leave the gear to cure for 48 hours if possible. I didn't post-cure my landing gear, but it is an option.
@905239: Here is the Wilga-style landing gear bracket under vacuum.
The outside layers of cloth are the structural layers, so make sure that weave of the fabric is properly oriented. The majority of the strands should be running along the long dimension of the gear since that is the direction of loading, but some layers of bias cloth (with the weave at 45 degrees to the long axis of the gear) are useful to prevent the gear from twisting. Carbon fibre is a local choice for the load bearing outside layers since it is extremely strong for its weight...and good looking to boot. For a model this size and weight, three to four layers of 5 oz carbon cloth for each face are probably adequate (see the layup schedule at the end of this section). Unidirectional carbon will be stronger than plain-weave carbon if properly oriented, but it is usually more expensive, and isn't quite as good looking if used as an outside layer.
Building gear entirely of carbon is not required, however, since the inner layers of material don't see much load. Just like with a wing spar, the thicker the landing gear can be, the more load it can take, so the inner layers are really only used to build up thickness. Balsa might seem like a good choice, but people have had difficulty keeping the outer layers from delaminating in the past, so I would suggest using a number of layers of cheap fibreglass cloth instead (10-12 layers of 6 oz glass isn't overkill). The Wilga style bracket used only 8 layers of 6 oz glass as a core, and while it was strong, it was too flexible to be useful.
You can also use heavy kevlar cloth instead of glass which actually builds up thickness much quicker than fibreglass and is more durable, but kevlar is more expensive and is harder to come by. Cheap auto-repair fibreglass should be readily available at most automotive parts supply houses.
@905240:The swing arms for the Wilga gear can be bagged against a flat form.
@905241:A swing arm blank and a finished part for comparison
Once the landing gear has cured, remove the part from the vacuum bag. It should be fairly unkempt at this point, but it's very difficult to keep 15+ layers of cloth in alignment, so don't worry too much. Don't try to remove the mylars yet, but instead use them as a guide for trimming the flashing off your finished part. I used (and wore out) a brand-new bandsaw blade to cut the majority of the flashing off, and finished with a belt sander. USE A RESPIRATOR AND SKIN PROTECTION WHEN SANDING CARBON FIBRE! Your health will suffer if you don't, and carbon dust in your skin pores will have you scratching for days. After sanding to the edges of the mylar, peel the mylar back, and admire your fancy shiny carbon gear. Round the edges of the gear with sandpaper by hand to prevent nasty cuts to your fingers when handling the gear, which I guarantee will go septic and hurt for quite a while.
Next I will show why I advised that you leave the gear legs a little long. Certain innaccuracies can creep into the bagging process that might distort the gear a little bit, and a model that doesn't sit level on the ground doesn't look very good or track very straight on takeoff. Mount the gear to the fuselage of model, and block it level on your workbench. Next, scribe the center of the wheel axle at a precisely measured height on each leg. Drill the axle hole, round the end of the leg, and know that your model will now sit level.
@905242:Aligning the landing gear on the fuselage before drilling the retaining bolt holes.
@905243:The finished Wilga-style sprung gear on the Telemaster
Given the heavy weight of the Telemaster, even heavy-duty r/c touring car shocks weren't up to the challenge of supporting the model. On top of which, I didn't use enough core layers when making the first set of gear, and the bracket was too flexible side-to-side. For those that are up to a challenge,a heavier layup and monster truck shocks might work in this situation, but I ultimately decided to make a conventional bracket.
The flying pictures show a more conventional u-shaped gear bracket, which was formed with the same techniques and has proven to be extremely tough and durable. That bracket weighs a solid 9 ounces, but it's also about 40% larger than it ultimately needed to be. Properly sized gear should weigh less than 5 ounces. The layup schedule used (from outside to inside) two layers of non-bias 5.7 oz carbon cloth (plain weave), two layers of bias 5.7 oz carbon, two layers of 6 oz bias kevlar (plain weave) and a core of 4 layers of 6 oz glass (plain weave). The non-glass layers are repeated on the other side of the gear as well (ie. two layers of non-bias cloth on each side mean that four layers are used in the total layup, etc etc).
!To Be Continued...
Tune in in a few days for Part 2 of the Electric Aerotow Saga, which will conclude the assembly of the Senior Telemaster and present videos of successful aerotows.
| @905222:The modified Senior Telemaster makes a great glider tug.
|> <b>Wingspan:</b> |< 95"
|> <b>Wing area:</b> |< 1330 sq. in.
|> <b>Weight:</b> |< 13.5 lbs (approx)
|> <b>Wing Loading:</b> |< 23 oz/sq.ft
|> <b>Motor:</b> |< Aveox 1817/4Y, ModelAirTech 1.6:1 belt drive
|> <b>ESC:</b> |< Aveox H160
|> <b>Radio:</b> |< Multiplex Profi 3030, IPD9 RX, MPX MCV2/Royal servos
|> <b>Available From:</b> |< <a href="http://hobby-lobby.com/">Hobby Lobby</a>
!Introduction
One of the fastest growing areas of model aviation at the moment is scale soaring, or more specifically, scale aerotowing. While I admittedly might be a little biased, I can think of few things more exciting than hooking a beautiful scale masterpiece up behind a powerful towplane, and heading out for a little afternoon soaring.
While traditionally sailplane tugs are large gas-powered monsters, this isn't an ideal system for everyone. Many clubs have problems with noise, as urban sprawl pushes more gas flying sites further out of major metropolitan areas. Some clubs, like <A href="http://www.soarcalgary.com/">mine</a>, have gone to a glider/electric-only system, and have started to look for alternatives to big, comparatively noisy gas engines.
One of the most popular unresolved questions on the RCGroups.com bulletin boards is the use of electric power in towing gliders - specifically, large gliders with wingspans greater than 4m. Print magazines have just started to run small articles on electric tugs, but that medium doesn't have the ability to go as in-depth as most people require.
Hopefully, this article will show you all that electric towing is a very viable option now, and help to guide you around some of the pitfalls that you might experience when trying towing for the first time. A large tug must be stoutly built, reliable, comfortable to fly, and generously powered. Each of these issues will be addressed separately below.
!Choosing a Power System
Unlike in the combustion realm, there are relatively few choices available for powering large electric models. Few speed controls can handle more than 30 cells in series, and fewer chargers can charge them in a reasonable time (I define "reasonable" to be within an hour - about the longest field charge that I will tolerate). To further limit the field, I personally feel that brushless motors are the best choice to power a large model, since they can deliver large amounts of power with minimal wastage and without the need for maintenance (and clouds of brush dust).
I was drawn to the Aveox line of motors after reading an article by Don Bailey, where he used an 1817-series motor to tow a Flair KA-8 with a large Piper cub. After running some numbers through MotoCalc, I decided that the 4 turn version of that motor (1817/4Y) would the best match for a dedicated tug, along with the Aveox H160 speed control (8-30 cells, 60A continuous, 100A peak). The motor is a sensored unit in its stock form, but it will also work nicely with any of the appropriate sensorless speed controls on the market.
Batteries also limit the system, in that they are a defined reservoir of electrical energy (excuse my blinding flash of the obvious). A powerful motor will require a large amount of energy to power it, and that power setting must be maintained for a reasonable length of time. A good tow will take between 1 and 2 minutes to gain reasonable height (1000-1500ft), and ideally it should be possible to get a couple of tows out of a given charge.
The 1817 series of motors is happiest when operating at around 30A continuous, so a quick ballpark calculation said that about 2 Ah of capacity would be needed to get two tows out of a given charge (30Ax 0.067 hours (ie. 4 mins) = 2Ah, or 2000mAh). This is well within the operating parameters of the newest sub-C sized Nicad and NiMh batteries, and I ultimately decided to use a 30 cell pack of GMVIS 2400 nicads. These cells are available from Icare Sailplanes for around $6-7 (Canadian) per cell, and have locally proven to be much more durable, and slightly more powerful, than the Sanyo RC2400 cell. A Schulze ISL330D will charge a30 cell pack of these cells at 2.6A peak (a little under an hour to charge), and a Schulze 630D will do them at 6.9A peak (a little under half an hour).
Sanyo 2600mah and GP 3000 and 3300 ni-mh cells would be another, slightly more expensive, option ($8-9 Cdn per cell from most sources), and might give 3 tows per charge. While Li-Poly batteries would enable you to get substantially more flight time before recharging, I shudder to think of what a pack capable of delivering 30A continuous at about 30V would cost. I would imagine that you could easily buy 3 or 4 packs of nicads for the same amount of money, without the need to transport and charge them in an ammunition blast box.
!Choosing an Airframe
The first decision that needed to be made was to select an appropriate tug airframe. Since the Hobby Lobby Senior Telemaster had been used quite successfully as a towplane in the past, and based on its popularity as a sport model, it was ultimately chosen as the subject of this conversion article. The Senior Telemaster is an ideal electric model, as it isn't nearly as overdesigned as most nitro/gas kits.
!Waiting for a Tow
As a long-time fan of scale models, Ryan van Beurden, generously offered both his piloting skills and the use of his <A href="http://www.roedelmodell.de/">Roedelmodel ASK-21</A> for the project. We had experimented with aerotowing in the past on a much smaller scale, but this was the first time that either of us had tried towing such a large model.
The ASK was a perfect choice here, as it was a well behaved, durable model that performed wonderfully over a wide range of conditions. A thread devoted to the sailplane can be found by <A href="http://www.rcgroups.com/forums/showthread.php?s=&threadid=127831">following this link.</A>
<img src="http://static.rcgroups.com/articles/liftzone/2003/nov/tug/camera.jpg"><A href="http://rcgroups.com/gallery/data/519/6409ASK_Loop.MPG">Video: ASK Test Flight</A>
@905223: Ryan van Beurden with his 4.2m Roedelmodel ASK-21
@905224: And people ask why we need to fly scale...soaring aesthetics don't get much better than this!
!Converting the Senior Telemaster
!!Fuselage
Since this article isn't intended to show a step-by-step build for the Senior Telemaster, but rather how to convert it for use as an electric tug, I'll rely more on photographs than on description for the following sections. If anything is unclear, please feel free to post to the linked comment thread at the end of the article, and I'll try to monitor that thread and respond as best I can.
I elected to start on the fuselage first when I built my Telemaster, mainly since that plan sheet came out of the box first! Speaking of which, I have to say that I found the plans that Hobby Lobby includes with this kit to be of fairly poor quality, both in clarity and description. The blue line machine that printed the sheets clearly needs to be cleaned up a bit, and even with 200+ models under my belt, I still had to do a bit of head scratching to figure out where all the various fuselage braces lined up. Nothing too major in the end, but something to keep in mind for those with less patience for ambiguity.
The first change that needs to be made is too set the firewall back from its stock location to one that suits your motor of choice. The ModelAirTech H1500 belt drive bolts nicely to a glow-style engine mount, and that was the method I ultimately chose to use. It also becomes very easy to adjust the thrust line of the motor later on, since you can add spacer washers under the four points of the motor mount.
@905225: Make sure to use plenty of bracing to keep the firewall in place.
@905226: You can see in this photo how far back the firewall needed to be moved. The original plan location is shown marked in pen.
My next small deviation from the plan was regarding the placement of the rudder and elevator servos, which I decided to move from under the wing to in the tail group. In order to make life easier, I ran the wiring for those servos early in the build process, leaving enough extra to make solder work easy. If you decide to do something similar, be sure to twist the servo wire at least once every 1" or so, and try not to install it under tension (to avoid wire fatigue when the fuselage twists).
Building the tug tow release came next in the build sequence. A release on the tug is important for safety, in case of a mechanical hangup or radio problem on the sailplane end of things. A small piece of 1/16" ply was recessed into the upper fuselage longerons in order to spread the load from the towhook properly, and was braced underneath with triangle stock. Next, a loop was formed out of heavy duty piano wire, which was securely anchored into the fuselage former just behind the wing. A piece of smooth 4-40 pushrod wire, sliding in a guide tube, is used to trap a loop built onto the end of the towline. When the wire is retracted back into the fuselage, the loop is released. I used a standard Futaba S148 servo for my release without a problem, though it wouldn't be wasted money if a stronger servo was chosen instead.
@905227: With the servos in the tail, the wiring runs to the receiver are rather long. They should be twisted to minimize radio problems.
@905228: A small piece of 1/16" ply provides a solid mount for the tug release.
@905229: A wide loop makes a crude but effective release bracket for the tug end of the system.
@905230: Here, you can see the linkage that drives the tow release.
!!The Tail Group
Relatively little has to be done to make the stab removable,which is especially valuable given how useful removable tail feathers are on a large model like a Telemaster. Before sheeting the stab center section, I cut four small pieces of 1/16" ply about 3/4" square, and drilled their center points to clear 4-40 bolts (I used metal, though plastic would be fine I'm sure). These are installed in the corners of the lower balsa stab center section sheeting, and must be securely bonded to the surrounding structure. Make sure to drill through the bottom balsa sheeting in order to transfer the location of the holes.
@905231: 4-40 bolts and blind nuts are used to make the tail group removable.
Next, the bolt hole locations from the stab needed to be transferred to the fuselage. First, I sheeted the stab platform with 1/16" ply to toughen the mount and take the load from the bolt heads. Next, I lined up the tail group and marked the location of the bolt holes onto the fuselage stab platform. I then drilled countersunk holes on the underside of the stab platform to clear the heads of 4-40 bolts, and hardened the surrounding balsa with thin c/a. Lastly, a hole was cut in the stabilizer platform to clear the 4-pin Dean's plug that I used to hook up the tail servos (which would be mounted in the stab itself).
@905232:Holes are drilled in the four corners of the stab platform to accept the tail bolts.
@905233:The stab platform here has been sheeted with plywood. A hole must be cut in the platform to clear the connector for the tail servos.
The mounting system is finished by either securing 4-40 blind nuts to the four pieces of ply in the stab itself, or by threading the pieces with a 4-40 tap (I used blind nuts for a more long-lasting system). Bolt the stab to the fuselage to make sure that the tail is still aligned properly.
Next, the servos are installed between the upper and lower sheeting in the stab center section. Since I used metal-geared Multiplex Royal Micro BB servos in this install, I had no problems sheeting over the finished installation. If a resin-geared servo is used instead, a hatch or removable cover would allow for gear changes in the event of damage. Since my mounting system replies on bonding the servo lugs to the face of the stabilizer rib for security, I used a doubler made of 1/32" ply on those ribs. Remember, there is very little vibration to deal with in an electric model, so hard-mounting the servos is perfectly acceptable.
The orientation of the two servos is dictated by the way that the control horns are installed on the rudder and elevator.
@905234: Here the elevator servo is being fitted into place...
@905235: ...and sheeted over. Be sure to use metal geared servos unless you make provisions for them to be removable.
@905236:Since the elevator servo horn projects below the stab, a small piece of balsa is used to provide an anchor point for the surrounding Ultracote. Be sure that the exit hole for the servo wires lines up with its counterpart in the fuselage!
@905237:The rudder servo being installed. The rudder servo horn extends through the upper center section sheeting.
Two small items finish off the list of modifications to the tail group. First, on the advice of others who had built Telemasters in the past, I added a fair amount of area to the rudder to boost its effectiveness. The new rudder is very effective, so I highly recommend this change.
Secondly, I added a small piece of vertical grain balsa between the stabilizer spars on either tip. This provides a secure, crush-proof structure to bolt the tail bracing wires to. Bracing wires leading from the tips of the stab to the top of the rudder are needed not for strength, but to allow the towline to slide from one side of the tail to the other in flight without removing the rudder in the process! Solid 2-56 wire could be used for these braces, but I elected to use braided cable from a Sullivan 4-40 pull-pull set instead (which is a little lighter).
@905238: In this photo, you can see the darker-grained wood that was added to boost the rudder area. The new rudder is very effective.
!!Building Carbon Landing Gear
With the immense torque of the Aveox 1817/4y motor, a large prop in the 18-20" range is needed to load the system properly. Given that the Telemaster isn't designed to take much more than a 12-14" prop, the stock aluminum landing gear bracket is pretty useless in this situation. That means that an interested modeler must either get some new taller gear custom formed, or build some themselves. While it should be reasonably easy to get a metal shop to custom form new aluminum gear (two of my LHS's offer this service), I decided as an experiment to try forming some composite gear at home.
The following pictures will detail my attempt to make active, sprung landing gear using small shock absorbers from an r/c car. The gear ended up being too flexible under the load of the finished model, and the Wilga-style scissor gear was abandoned, but the techniques would be suitable for lighter airplanes in the 6-8 pound range. I'd always had that idea floating around in my head, however, and had to give it a try.
To build this style of gear using my techniques, you'll need standard vacuum bagging equipment and some experience in laying up composite fabrics. My personal vacuum system uses an air-conditioner pump, and was assembled using techniques similar to these: <A href="http://www.badger.rchomepage.com/vacbag.html">Building a Vacuum Bagging System</A> (the fellow who put this page together is a fellow club member). Please use a respirator and skin protection when working with carbon fibre or epoxy.
To form the gear, I first cut a foam blank to the shape of the inside contour of the finished gear. I used pink Foamular insulating foam for the form, which required two layers of foam to be bonded together with a heavy coat of 3M77 spray adhesive. Protect the form with a release film of some description so the finished gear part won't stick to it. I used wax paper because it's cheap and readily available, but be warned that some people have had trouble with the wax dissolving in the epoxy and weakening the finished part. I use West System resin (105 resin 206 hardener) in most of my composite projects, and haven't had a problem with wax solubility yet, but your experiences may be different.
Next, make up two copies of outline of your landing gear in bagging mylar leaving the legs a little long, and wax these mylars with a couple of coats of mold release. The layup will be done from outside to inside on each of the mylars (ie outside layers are closest to the mylar), with the core layers added to only one side if the layup isn't perfectly symmetrical. The various layers of cloth are wetted out in a separate area to allow for excess resin to be drawn off, and are then arranged on the mylar. The mylars are then placed together with the fabric layers touching, and bent around the foam form. Don't worry about bits of cloth slipping out from between the mylars as you are forming it, since the finished gear will be cut to shape after it has cured. The whole mess is taped to the form, covered in a last layer of release film to protect the vacuum bag and breather cloth, and the vacuum pump is turned on .
The process isn't very difficult, but it is rather messy, so be prepared. Pull as much vacuum as your pump can take safely, and leave the gear to cure for 48 hours if possible. I didn't post-cure my landing gear, but it is an option.
@905239: Here is the Wilga-style landing gear bracket under vacuum.
The outside layers of cloth are the structural layers, so make sure that weave of the fabric is properly oriented. The majority of the strands should be running along the long dimension of the gear since that is the direction of loading, but some layers of bias cloth (with the weave at 45 degrees to the long axis of the gear) are useful to prevent the gear from twisting. Carbon fibre is a local choice for the load bearing outside layers since it is extremely strong for its weight...and good looking to boot. For a model this size and weight, three to four layers of 5 oz carbon cloth for each face are probably adequate (see the layup schedule at the end of this section). Unidirectional carbon will be stronger than plain-weave carbon if properly oriented, but it is usually more expensive, and isn't quite as good looking if used as an outside layer.
Building gear entirely of carbon is not required, however, since the inner layers of material don't see much load. Just like with a wing spar, the thicker the landing gear can be, the more load it can take, so the inner layers are really only used to build up thickness. Balsa might seem like a good choice, but people have had difficulty keeping the outer layers from delaminating in the past, so I would suggest using a number of layers of cheap fibreglass cloth instead (10-12 layers of 6 oz glass isn't overkill). The Wilga style bracket used only 8 layers of 6 oz glass as a core, and while it was strong, it was too flexible to be useful.
You can also use heavy kevlar cloth instead of glass which actually builds up thickness much quicker than fibreglass and is more durable, but kevlar is more expensive and is harder to come by. Cheap auto-repair fibreglass should be readily available at most automotive parts supply houses.
@905240:The swing arms for the Wilga gear can be bagged against a flat form.
@905241:A swing arm blank and a finished part for comparison
Once the landing gear has cured, remove the part from the vacuum bag. It should be fairly unkempt at this point, but it's very difficult to keep 15+ layers of cloth in alignment, so don't worry too much. Don't try to remove the mylars yet, but instead use them as a guide for trimming the flashing off your finished part. I used (and wore out) a brand-new bandsaw blade to cut the majority of the flashing off, and finished with a belt sander. USE A RESPIRATOR AND SKIN PROTECTION WHEN SANDING CARBON FIBRE! Your health will suffer if you don't, and carbon dust in your skin pores will have you scratching for days. After sanding to the edges of the mylar, peel the mylar back, and admire your fancy shiny carbon gear. Round the edges of the gear with sandpaper by hand to prevent nasty cuts to your fingers when handling the gear, which I guarantee will go septic and hurt for quite a while.
Next I will show why I advised that you leave the gear legs a little long. Certain innaccuracies can creep into the bagging process that might distort the gear a little bit, and a model that doesn't sit level on the ground doesn't look very good or track very straight on takeoff. Mount the gear to the fuselage of model, and block it level on your workbench. Next, scribe the center of the wheel axle at a precisely measured height on each leg. Drill the axle hole, round the end of the leg, and know that your model will now sit level.
@905242:Aligning the landing gear on the fuselage before drilling the retaining bolt holes.
@905243:The finished Wilga-style sprung gear on the Telemaster
Given the heavy weight of the Telemaster, even heavy-duty r/c touring car shocks weren't up to the challenge of supporting the model. On top of which, I didn't use enough core layers when making the first set of gear, and the bracket was too flexible side-to-side. For those that are up to a challenge,a heavier layup and monster truck shocks might work in this situation, but I ultimately decided to make a conventional bracket.
The flying pictures show a more conventional u-shaped gear bracket, which was formed with the same techniques and has proven to be extremely tough and durable. That bracket weighs a solid 9 ounces, but it's also about 40% larger than it ultimately needed to be. Properly sized gear should weigh less than 5 ounces. The layup schedule used (from outside to inside) two layers of non-bias 5.7 oz carbon cloth (plain weave), two layers of bias 5.7 oz carbon, two layers of 6 oz bias kevlar (plain weave) and a core of 4 layers of 6 oz glass (plain weave). The non-glass layers are repeated on the other side of the gear as well (ie. two layers of non-bias cloth on each side mean that four layers are used in the total layup, etc etc).
!To Be Continued...
Tune in in a few days for Part 2 of the Electric Aerotow Saga, which will conclude the assembly of the Senior Telemaster and present videos of successful aerotows.