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Old Sep 29, 2014, 09:26 PM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Scratch-built GP Viper 500 fuse for 426

I've learned about most of the modifications apparently required for the Great Planes Viper 500 fuselage for pylon racing. I'm going to get into Q-500 426 and I plan to build a couple Vipers this winter. I got my first one and after examining the fuse and contemplating the modifications, I decided it just might be better to design a new one with the fixes built in. I'm new to 426, and kind of new to pylon racing, so feel free to give me comments/suggestions along the way (even if it's too late in my build).

One of the main fixes I've learned about is changing the stab incidence -- apparently the Viper flies with lots of up trim requiring the trailing edge of the stab to be shimmed up about 1/16" or somewhere thereabouts (reducing stab incidence which would take out that up trim) . From what I can find, the incidences should be 0 - 0 - 0 (motor 0, wing 0, stab 0). But from examining the original design drawings the incidences are already 0 - 0 - 0. I fit mine together and everything measured out 0 - 0 - 0, or perhaps the stab was positive .5 to 1 degree depending on how hard I pressed down on the incidence meter. If, indeed the stab is positive, this correlates to up elevator trim needed. In any case, I'll not change the incidences on my fuse design and will just make sure the motor, wing and stab measure out at 0 - 0 - 0.

I'll also beef up the front end by incorporating 1/16" plywood on the outside in addition to the standard doublers. I'll reduce the hatch openings, add 3/32" balsa stringers in the corners on the bottom of the fuse from the landing gear plate aft and on the top of the fuse from the former at the wing trailing edge back. I'll add 1/16" ply triplers in the main landing gear plate area, doublers in the stab saddle area and a 1mm x 5mm x about 100mm carbon fiber strip below the stab doubler. I'm also modifying the former at the wing leading edge to accommodate the Tettra 6 oz. bubble-less fuel tank. The main fuselage sides will be 3/32" balsa, but I'll sand down to 1/16" or thereabouts. The blind nuts in the back of the firewall will be inset allowing for triangle stock all the way around the inside of the back of the firewall (instead of the tiny short pieces that fit between the gigantic blind nuts in the stock firewall).

I also got some Darrol Cady landing gear, wheels and axles.

Tim

Here's a few images of my first mock up so far:

Tim
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Old Yesterday, 09:04 AM
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Tim the plane is looking really good and your plans to beef it up in all the right places sounds good. But allow me to offer some suggestions for some things that I've learned over the years and from builders much better than me. The trick is to build it strong but keep it light. No one does this better than Chuck Bridge and I've studied a lot of his planes to see how he does it.

In stead of using triangle stock around the firewall I use chopped carbon mixed in epoxy and make thin but strong fillets in all of the corners in the nose. Not just the 4 around the firewall but also the 4 from the firewall back to the first former. I also use this trick and make fillets on the landing gear and wing hold down blocks. This might negate the need for a "tripler" on your landing gear, possibly saving weight. As for a hatch on the nose, there really is no need for this if you place the tank on the CG. The hoses are somewhat of a pain to run if you do this but not the end of the world and it really stiffens up the nose area.

The stringers along the bottom and top of the fuse are necessary and a great idea. I personally just run them from the TE former to the tail and use 1/4 triangle stock, and just run a bead of medium CA or thickened epoxy along the bottom corners of the fuse in the wing area. But that's a personal preference thing more than anything else. Oh and I also run a bead of medium CA in all the corners where the formers touch the sides of the fuse, again personal preference.

As for the blind nuts in the firewall I use the little Du-Bro kind, they don't require any cutting to fit in to the corners and I install them before making the fillets with epoxy and chopped carbon.

Now for the tail and setting the incidence. As you've found out trying to figure out the true incidence of the tail is somewhat a pain in the rear. I've never successfully gotten an incidence meter to work but then again I'm not all that smart. What has worked best for me is that I built a perfectly level table and I set the plane on there and make sure the firewall is perpendicular to the top of the table (making it 0) and then I set the wing and make sure it is setting at zero. Once I'm happy with all that I weigh it down with a 10lb dumbbell to make sure it doesn't move. Now it's time to get the tail ready. The fun part here is to make a line from LE to TE that is the same distance from the center of the wing, and this needs to be done on both tail halves. Once you have that set the tail in the fuse and measure up from the table top to where the line crosses the LE and TE. Once these 4 measurements are all the same tack the tail in with thin CA. This will hold it long enough for you to more securely attach the tail.

Once again I'd be willing to send over some pictures to help with this process.

Lonnie
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Old Yesterday, 04:21 PM
Tim Lampe; Hobbico R&D
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Hi Lonnie.

Thanks much for taking the time to share and write out your suggestions…

Yes, strong and light…that’s always the trick isn’t it? I have plenty of Dave Brown CF “tape” (looks more like strands of hair) I use all the time. But I’m contemplating using materials and methods that could be duplicated should a new fuse with these revisions ever go into production, so I may stick with tri-stock. Something to think about though for sure thank you.

Yes, I was already thinking of forgoing a front hatch. I imagine this will make the front end much more rigid.

And of course, medium CA fillets everywhere.

Yes, I’ve already designed the firewall for DuBro 6-32 blind nuts – they’re much smaller in diameter than the ones available from Great Planes and more suitable for this purpose where the quarters are close! If you look close in the second-to-last image in my opening post you’ll see the last 1/16” layer of the firewall is cut for the base of the blind nuts. Then, they won’t interfere with the triangle stock.

I’ll refer to your suggestions when setting the stab incidence.

Any images you feel like forwarding are much appreciated.

Thanks again.

Tim

EDIT: P.S. I didn't realize this is the Lonnie Finch I was talking to recently at the contest in Bloomington. Nice to meet you again!
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Old Yesterday, 08:02 PM
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Just a minor suggestion - your wing hold down blocks appear to be set down into the fuse a ways, leaving a gap between the wing and the block. These should really be flush to the bottom of the wing, without the gap - or filled in with something to eliminate this gap.

With the gap, as you tighten the wing bolts you are putting a LOT of strain on the joints holding those blocks in, as the bolts try to close up that gap. With the blocks flush to the bottom of the wing, it eliminates this strain on the mounting blocks, leaving only the strain of the lifting wing as the plane flies.

Also - if you're scratch building, it may be easier to leave this wing mounting blocks without holes for the bolts, then aligning your wing, drilling through the hard points in the wing into the blocks with the proper sized tap drill, then tapping the blocks, finally opening up the through holes in the wing with a close fitting drill size to pass the bolts without a lot of slop. Helps line up the wing, and it's a lot harder to try to match holes in a wing to mounting holes that are hidden beneath it in the mounting blocks.
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Old Yesterday, 09:41 PM
Tim Lampe; Hobbico R&D
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Thanks fisswater2.

I'm all over it...

I already considered switching from the original 10-24 metal bolts and blind nuts to 1/4-20 nylon bolts, but decided against it just to keep it simple and stay true to the original design where possible. One of the several advantages of going with nylon bolts is then, the wing bolt plates can be drilled and tapped instead of using blind nuts thus allowing the bolt plate (and nuts) to be at whatever angle is best -- not necessarily perpendicular to the bolts (making more room for the fuel tank as well).

After talking with Bernie Vanderleest over the phone this evening I decided I would go ahead and switch to nylon bolts and reconfigure the wing bolt plates. I'll lower the plates and make them parallel with the wing saddle in the location where the bolts will pass. In any regard, I was already going to fill the gap between the top of the wing bolt plates and the wing saddle with a block of balsa - this is the way the stock Viper 500 is already.

While I'm at it, I'm also going to stick with 10-24 bolts in the back of the wing, only switch those to nylon too - as well as the bolts that hold on the V-tail.

Tim
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Old Today, 08:52 AM
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Tim,

My only comment is regarding your switch installation. Based on the picture provided I can't see how you install the switch after the fuse is assembled.

Most of us use a slot method to install the switch using the on/off plate provided with the switch. The screws are left loose and the switch is slid under the tray and the plate on top of the tray and then secured with screws. Also, this may not matter to you and what you are doing but, JR, Spektrum and Futaba all have different mounting patterns and heights of the switches. Futaba is the smallest and shortest. If you set this airplane up for Futaba only, other users will need to modify the switch installation process to fit their needs.

Attached are a couple of JPEGs of the different trays I have done. The DARA EF-1 tray shows a hole (bigger than needed) for the switch to be installed into then slid into place.

Sorry, a couple last things:

Where are you putting your throttle servo?

Without any former in the middle of the fuselage, the sides of the fuselage are able to pinch under the wing. During the NATS inspection process several pilots removed this former on their Viper models and the result was airplanes that didn't meet the minimum width rule. we made them go back and put a spreader inside to meet the rule. They met the rule at the bottom of the fuselage where the sheeting was attached but not the top in the saddle area. This is more of an FYI than anything else.

DK
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Old Today, 09:04 AM
Tim Lampe; Hobbico R&D
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Champaign, IL
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Ha ha - thanks - good catch Dan on switch installation - would be difficult to install the switch after the servo mount plate is installed! (Though I suppose I could install the switch down through one of the servo openings before installing the servo.)

I'm not necessarily designing this for production, so at this point I'm just designing it for the equipment I will be using, but food for thought for future reference! I will slot the switch mount location though for switch installation/mounting after the servo tray is glued in.

Thanks for checking in Dan!

Tim
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Old Today, 09:23 AM
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Tim,

Yep it's me, glad to see you taking this on. I'd like to see these improvements incorporated into the Viper so a quality ARF would be available for those that are wanting to race but scared to get their hands dirty building.

I sent you an email with a couple pictures showing the tail installation.

Lonnie
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