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Dave Platt T-28B Trojan 1/7 Scale 65-inch

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Dave Platt T-28B Trojan 1/7 Scale 65-inch

Old 06-02-2012, 03:15 PM
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ChinookRC
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Default Dave Platt T-28B Trojan 1/7 Scale 65-inch

After completing a Top Flite Corsair and a TF Bearcat this past winter and realizing that it would be some time before I had enough experience to feel confident enough to fly them I started looking around for another warbird kit that would have flight characteristics more suited to a novice flyer. I wanted a 60-size plane that I could convert to electric and use my existing AXI 4130 motors, CC 100A ESCs and 6S Lipos. To further cut down on costs I wanted something that wouldn't look strange without retracts.

The T-28 seemed like a good candidate and when a Dave Platt kit went up for sale locally, I grabbed it for $200. There's no date on the box or plans but looking at the bio on Dave's website I'm assuming that the kit dates between 1972 when he started the company, to 1975 when he sold it to the no longer extant PICO as there is no reference to PICO anywhere on the packaging.

Iwish I could say I've aged as well as this similarly dated kit! The contents appear to have remained in the box all this time and the wood is in perfect shape with no warping and only one cracked plank. The 3-piece cowl looks good and the trike LG isn't rusted. The canopy has been warped somewhat along one bottom edge but I expect that it won't be noticeable once it's glued on. The water slide decal sheets need to be flattened out but are otherwise ok and the plans are in perfect condition. The only complaint I have is that the main plan is split into two sheets and I haven't figured out how to join the scanned PDFs so I can print them out as one sheet at work. For a new builder like myself the instructions are minimal, even more sparse than the 1979 TFBearcat I recently built.

I don't expect there are a lot of these 40 year old kits out there waiting to be built, and I haven't been able to find a build thread anywhere so it looks like this will be the first and probably last 65" Dave Platt T-28 build thread. But I thought the same when building the '79 Bearcat and was surprised when quite a few people came out the woodwork with kits just finished or in progress and posted lots of help and encouragement on the build thread. I hope the same happens with this DP T-28!

It's summer with vacations, long weekends, flying time and laziness in general so the build will be leisurely. The plan is to have it in the air this season, which means before the end of October or five months.

Here are some photos of the kit. The scans I have of the plans are too large to post so just send me a message if you'd like me to email them to you.
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Old 06-02-2012, 03:25 PM
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LargeScale88
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Dave Platt airplanes are great. I have heard building them is a little different, but very scale and very nice looking when completed.

Can't wait to see it come alive!

Jason
Old 06-02-2012, 09:35 PM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I'm not sure about the other T-28's designed by Platt but this one is far from scale, eschewing the stubby nose and short tail for what I can only guess is a more forgiving long tail and a slightly longer nose, perhaps for balance. If anything, it looks more like a pattern ship than a T-28 but it's certainly recognizable as a T-28. And that's fine with me - I'm more interested in easy flying characteristics than in scale accuracy, especially since it seems I tend to lose patience with the finishing work at the end of a build.

The build starts a lot different that I'm used to, starting in the middle of the wing with the center W1 rib and working out on the left side to W11. No spending a week or two putting together fin, rudder, stab and elevators wondering if it'll ever start taking shape. After one evening half the wing has been put together. I thought I'd have a few weeks to shop around for alternative servos to the cheap and reliable Futaba S3004's I've been using but with this sequence I'm going to be shopping for S3004's next week.
    [*]epoxied rib doublers as needed[*]CA'd the bottom spars to the dihedral joiner. Not too happy that this key part is balsa.[*]CA'd the fore and aft halves of the center W1s to the dihedral joiner (7.5 degrees)[*]pinned down the left spar with the right half up in the air at a 15 degree angle[*]the rib notches fit nice and tight around the spar for the dry fit[/list]
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Old 06-02-2012, 10:03 PM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I finally got a chance to build washout into a wing. There's no mention of washout in the Platt documentation, and I can't find any recommendations on whether this particular model needs it or not, but I couldn't resist the opportunity to go ahead and build it in. Following the excellent article on washout attached (from AMAIthink, but I can't find a link) I brushed up on my trig and calculated the TElift needed at the tip rib W11 to induce 2 degrees of washout (~5mm). The wing root has a positive angle of incidence of 4.5 degrees and the choice of 2 degrees was arbitrary.

The plans show a 1/4"x1/4" jig running below the aft portion of the ribs so I added a small 5mm block at W11, with 3 more blocks of descending sizes under W10, W9, and W8 to give a gradual twist out toward the tip. It looks like it should work pretty well.

On the Corsair and Bearcat I didn't follow plans for bell cranks on the flaps and ailerons and used four co-located servos instead, but on this model I'll stick with the plan. So I glued in the bell crank platforms and servo rails, CA'd the ribs to the bottom spar, CA'd the top spar in place and added epoxy fillets. I considered adding spar webbing but decided to continue the weight saving philosophy and to follow the plan. Expected weight is only 7.5lb so it should be plenty strong as is.

The low weight and the low height of the prop hub compared to a tail dragger also got me looking at the recommended prop size - 11x7, 11x8, 12x6 - and the realization that an AXI 4130 and 16x8x3 just won't work. So I'll have to look into more modest motor options. I'm starting to feel confident that this plane is a good choice for my flying skills.

Checked the hardware bits and they're in great condition. I need to research the best hardware to use for a slop-free bell crank setup though.
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Old 06-04-2012, 08:10 PM
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ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I sent in an order for some of the Dave Platt how-to videos and when Dave himself replied, I asked him about the date of the kit (he said 1973, before it went to Pica who 'cheapened' the quality) and about the longer than scale tail (no comment from Dave).

The left half of the wing structure is complete after trimming off the excess spar, LE and TE length. The LE is straight while the TEhas a noticeable curve up toward the tip because of the washout shims.

Iused the same washout jig w/shims on the right half. LE and TE to be glued in place tomorrow.
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Old 06-04-2012, 10:25 PM
  #6  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch


ORIGINAL: ChinookRC

.......... I need to research the best hardware to use for a slop-free bell crank setup though.

Use ball links. Either the helicopter ones or the DuBro hardware (they have a pretty extensive selection including metric). Just about anything else will develop slop. Also, pay attention to the pivot points on the bellcranks. Its usually just a plastic sleeve with a washer on the top. The fit is not very precise so they can rock a bit introducing slop. Especially over time. Since this style of aileron actuation is almost never used anymore I'm not sure if there is anything better out there so you might want to look around.

On some helicopters, they use plastic bellcranks but there is a flanged ball bearing on both sides with a brass or aluminum sleeve to take up the space between the two. You can then run your bolt tight and not have to worry about slop.

On another note; I've seen a couple of these fly many, many moons ago. They didn't look like they needed any washout. Very majestic and solid flyers. However, I didn't build the planes so who knows but adding washout was just not common back then.

Mike
Old 06-04-2012, 10:31 PM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch


ORIGINAL: ChinookRC

......... about the longer than scale tail (no comment from Dave).
I'm not sure how much research you've done but the three view you show here looks like the A model or maybe even a prototype. The B that your kit is, had a more squared off cowl and I think a bigger engine. I think there were several other changes as well. Its late and I don't have my T-28 stuff in front of me right now but your kit is probably more scale than first glance.

Mike
Old 06-05-2012, 09:28 PM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Mike.. would you recommend ball links over the e/z connectors? I suppose the latter with the set screw has a greater risk of something going wrong. The stock and Dubro bell cranks both have brass sleeves but I'll look around to see what the heli guys have - bearings sound like a good idea.

There's an ongoing thread covering the DP 1/5 scale kit where I saw specs for the real thing showing 3 degrees of washout. We'll see how it turns out on this model.

Thanks for pointing out the potential that I'm using an incorrect 3-view... if you have something more definitive for the T-28B I'd appreciate a copy. I'll take some time to trace the outlines in Autocad and overlay the results. The attachment here is a snap of the 3-view included on the plans, I'm guessing it should be correct.

+++

Finished the right half of the wing and glued up the wing planks.
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Old 06-06-2012, 04:52 AM
  #9  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I'm going to assume he meant these kind of ball links: [link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD912&P=ML[/link]
Old 06-07-2012, 04:20 AM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Thanks - I'll drop by the LHS this week and pick up the ball links (I see now that they'll offer more degrees of freedom than the e/z connectors) and some servos so Ican start sheeting the wings.

I superimposed traces of the model side view and the 3-view that is included on the plans and the model is definitely quite a bit longer in the tail than the 3-view. The nose is also more narrow to give an overall more streamlined shape. And this is fine with me - I'm not looking for scale accuracy so much as a nice flyer that I can graduate to from my Nexstar trainer.
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Old 06-07-2012, 08:42 PM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

The prop size limitation of 12 inches is giving me some grief selecting an electric motor. Using the WebOCalc online calculator it appears that I need something 600-700Kv, 40 to 50A on 5S or 6S.

Ihave a 670Kv Turnigy G46 sitting around that gives me the attached specs, which are pretty much as good as anything else I'm finding on HK. The similarly sized 700 Kv rated Turnigy L5055C-700 gives slightly better power output, but I'm not sure it would be worth buying another motor.

The 800Kv Rimfire 46 might be the best choice but it's $100.

Have no idea at this point whether a 5S or 6S will provide better balance. I'm pretty sure the battery mount will extend through the firewall into the motor box to bring it forward enough.

Appreciate any ideas.
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Old 06-08-2012, 10:47 AM
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optech
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch


ORIGINAL: smithcreek

I'm going to assume he meant these kind of ball links: [link]http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXD912&P=ML[/link]
Yes these are the Heim style I was talking about but the ball and socket type work well too if they'll fit. I definitely recommend these style over e/z connectors. In my opinion E/Z connectors are no more precise than z-bends. They depend on the slip fit of the connector and the servo arm which wears quickly and develops slop.

Mike
Old 06-08-2012, 10:53 AM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

ORIGINAL: ChinookRC


I superimposed traces of the model side view and the 3-view that is included on the plans and the model is definitely quite a bit longer in the tail than the 3-view. The nose is also more narrow to give an overall more streamlined shape. And this is fine with me - I'm not looking for scale accuracy so much as a nice flyer that I can graduate to from my Nexstar trainer.
I didn't realize the plans included a three view of a B model. As a Dave Platt kit I'm a little surprised it doesn't match the models plans better as evidenced by your tracings. As I go back and look I take it the included three-view was used for competition documentation which is even more surprising. In anycase, the built planes look nice and sure do fly good,

That Rimfire does look like a good motor. I like Rimfires and have several but if your gonna spend that kind of money on another motor, go look at Scorpion motors. Those are very nicely built, and efficient for a reasonable price.

Oh.... and for this size plane I would go 6 cells. Your amps would be lower and you'll probably need the nose weight without have to get too radical with the mounting.

Mike
Old 06-08-2012, 12:51 PM
  #14  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Here is one in Navy grey. I think the high visibility scheme , white with day-glow red, tends to amplify the poor proportions of this kit. The grey scheme tends to move the eye in such a matter as to minimize the poor proportions.
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Old 06-08-2012, 11:24 PM
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ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Appreciate all the advice guys - learn something new everyday.

I'll go with the swivel ball link as recommended. I hate messing around with the ball and socket type and yes, the pin is often loose inside the servo arm hole. Hopefully will find the links and servos in stock at one of the shops nearby tomorrow.

I'm looking at a 790Kv Scorpion S-4020-8. Running it through the calculator at 6S on a 12x6 gives it unlimited vertical, hover capability, and pulls 90A for almost 2000W in. I get the feeling that this could cause the fastest crash of a T-28 ever. Kind of steep at $160 but haven't looked around. I suppose I should start tame and work up.

After building two boring blue warbirds I'm looking forward to choosing a color scheme. The white and red is too common at my field (foamies), the grey is nice but maybe a bit too monotone like the blue I've been doing. Lots of time to think about this.

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Old 06-10-2012, 07:22 AM
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ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

In the end I didn't go with the swivel ball links because
  1. cost - at $4 a pair it would have been better to buy two more S3004 servos to co-locate in the wing
  2. space - binding would have been a problem because the linkages are all located close to ribs and the holes the rods pass through would have had to be enlarged considerably to accomodate the relatively hefty ball links passing through.
So as I stick-built the rods I ended up with quite a hodge-podge of z-bends, l-bends, nylon retainers, metal clevises, copper wire wrapped joints, threaded and unthreaded 2-56 rods. The pins all fit very snugly though and the only slop I can detect is in the flap linkage where the geometry is causing some flexing of the rod at the ends of throws. I expect the needed flap throw will be much less than the max so this flexing shouldn't be an operating issue. The aileron links are rock solid and the rod/arm geometry is straight with no unusual stress on the servo.

All said and done though, I'll be avoiding bell crank mechanisms in the future! This was good for experience, but it's pretty obvious that putting individual servos in place with short straight rods to the horns is faster and much easier to adjust and repair. I'll come to regret this, but at least I've done it once.

Time to move on and sheet the bottom.
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Old 06-10-2012, 10:31 AM
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Hey Texas.. I ran into one of your T-28 pics by chance in another thread. Great covering job! Can definitely see the pattern ship influence from this angle.

I wonder how it would fly if I took an inch or two out of the tail between the canopy and the fin extension whatever that piece is called...
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Old 06-10-2012, 05:21 PM
  #18  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

The T-28 is a nice flying plane. Have you considered the Eflite Power 25 motor? 870KV, and it spins a 12x8 in my T-34 on 3s lipos.  The P-51 TFGE Im building started out with the bellcranks and pushrods, but after I got the wing assembled and looked at where the servos goes, realized I couldnt get the link installed. I opened the wing where the bellcrank is and ripped it all out and stuck rails in for servos instead.  I opened that wing up so much to do mods it looks like I had already flown and crashed it.

Looking forward to updates on this build, its one plane I would like to build if I can find the right kit.
Old 06-10-2012, 08:41 PM
  #19  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Looks like the Power 25 is rated for 3-5.5lb planes and this will come in at 7.5lb, probably higher with the 5S or 6S Lipo. The 12-inch prop limitation means I need a big motor with high Kv and there aren't that many out there. The Power 60 is 400 Kv. This wasn't an issue I was aware of initially so this has been a good hands on intro to sizing electrics.

Another lesson today was to not use pins with little round heads - they leave little round dents and the heads pull off! Sheeted the bottom of the wing and epoxied the fixed LG key blocks. Made a 4-foot aluminum sanding bar to help make straight 90deg sheet edges, something I've been having problems with.

I've had the decals under one of my building boards for a week and they're still warped but otherwise in good shape. I really like the detailed placement guides Dave provides for all those little "hoist" etc decals. I've been guessing where to put them until now.
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Old 06-17-2012, 07:02 PM
  #20  
ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I got a little frustrated this week sheeting the wing and haven't been posting, but in the meantime I did manage to finish the wing:
    [*]Drilled out the fixed LG key block and cut slots in the bottom of the wing. One of the wire LG needs to be adjusted to fit the angle.[*]For some reason it didn't occur to me to use the water spray bottle sitting on my table until I got to the last wing panel, and then the edges finally clamped down tight to the LE and TE. The other top panel doesn't look very good and I was po'd. It's structurally sound though and some balsa filler covered up the thin gap at the LE.[*]There isn't a section view of the wing where the servos are installed so I couldn't plan ahead for having my servo arms sticking up above the top sheet. No way to tell at this time if it's going to be a problem but I went ahead and cut out access holes. These will be inside the fuse and should serve for maintenance.[*]Building in washout meants that the TEs had an upward curve which complicated building the flap/aileron pieces. I decided to pin the F/A LE to the wing TE, following the curve, and then I glued the lower F/A skin in place while still pinned to the wing. The ribs were CAd in place also. The result was pretty good - the F/A piece held the same curve as the wing TE.[/list]
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Old 06-17-2012, 07:22 PM
  #21  
ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

    [*]The flaps and ailerons were cut into their respective pieces and I marked off the hinge locations on the wing TE with push pins, then pressed the F/As against the pin heads to mark the positions. Must be a better way to do this.[*]Cut slits for the CA hinges. The flap hinges are at the bottom of the TE, which was different from the Corsair and Bearcat.[*]Realized I'd forgotten to install the horn support ply inside the F/A structures so I put in small balsa blocks and screwed in the stock horns temporarily for fitting the control rods. Seemed to work ok. I dripped some thin CA in the screw holes to harden them and will epoxy them and the horns after covering.[*]I wasn't too happy with my choice of threaded control rods exiting the wing skin (except one, which needed a z-bend and a bend to make it over the cross-wise rod at the bell crank). The metal Sullivan clevises were impossible to screw on so I switched to nylon which were much easier.[*]Took a long time to get the surfaces lined up with balanced throws on each side. The CA hinges won't be glued until after covering so it's hard to tell right now how much accurate it is.[/list]Filled in the most obvious dings and dents and have set the wing aside for now. The tip blocks will be added just before covering to avoid hanger rash.
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Old 06-18-2012, 06:59 AM
  #22  
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Looking good!
Old 06-22-2012, 09:22 PM
  #23  
ChinookRC
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I put the wing away and glued the fuse formers, stringers and longerons together in a couple hours this evening. There's a 2deg downward thrust angle built into the firewall.

I received the DP videos I ordered and have been watching the Building and Fiberglassing series.. really informative. I'm kind of mystified at how he uses thin Zap CA for almost everything it seems. He puts pieces together, dabs a bit on and that's it. He does mention quite often that he's just tacking the pieces and will go back later to do a proper gluing job, but he doesn't show how he does that! Thin CA again?

I tried this with thin Mercury CA and the pieces didn't hold so I went back to applying mediun CA to one of the pieces and joining and holding them in place for 30sec to a minute. Makes me wonder if I should pick up some thin Zap, or just continue doing what I'm doing.
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Old 06-23-2012, 10:45 AM
  #24  
tailskid
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

Too bad that darn thing is a one piece wing......especially the "1/5" version!
Old 06-23-2012, 11:36 AM
  #25  
airraptor
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Default RE: Dave Platt T-28B Trojan 1/7 Scale 65-inch

I have this kit NIB and i plan to build some day. Did you end up shortening the tail section? I wonder if you could just move all the formers in about a inch or more progressively to make it look better?

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