Sethhunter

While I've been enjoying flying the Pfalz DXV and Halbi DIII, I've been thinking about the next project and finally started my third in a series of Pfalz aircraft. The DrI! For awhile I thought about refitting my DIIIa with a third wing, as the Eversbusch brothers tried in 1917, but since that experiment didn't pan out very well, I chose the more successful, purpose-built DrI. While not especially fast, the Pfalz DrI, with a counter-rotating 160 Hp Siemens & Halske SH.III engine, had the fastest climb of any German aircraft; 11.5 minutes to 5,000m! I drew the plans to 1/3 scale, then decided to build a 1/4 scale prototype first, to prove out the design, and hopefully have a light-weight fun-size airplane to chase and be chased. My weight goal is around 15 lbs, Fuji 43 powered. This airplane will have less detail than others I've built, with the goal of flying in the Spring. The 1/3 scale, if I get there, will be more scale. Mainly the short cuts I'm taking to save time, is to build 1-piece wings. This means considerable simplification of the fairings around the lower wing. Due to the short tail moment, I increased the stab area about 10%. And the controls will not have cables. Otherwise the outlines are scale, which makes for a pretty unusual airplane; The fuselage is round and stocky and the middle wing is very high aspect ratio (16:1). I suppose it qualifies as a sesquiplane triplane!

I have the wings and fuselage framed - pictures will follow.

Lucky Dog

My Feedback: (1)

Interesting subject! Looking forward to following your build Seth. Subscribed.

ARUP

Nice!

abufletcher

I just knew you'd come up with something special! [8D]

foodstick

WOW, a 1/4 plane as a prep for a 1/3 ? You do have drive !

I would never get the second one done.

It will be fun to watch this come along.

It is a interesting plane after studying the picture closer.

destrat

Sethhunter:

Excellent choice my friend, and staying in the Pfalz Pfamily to boot. I know that you'll perform you magic as you have on the past two subjects, can't wait to see you get started! I will send you a PM, it seems as though I do not have a current e-mail address for you.

Dan

bogbeagle

Did the type suffer any problems with the torsional stiffness of the lower wing?

I ask because I see that it has just the one interplane strut and the wing's structure is clearly very light. It puts me in mind of the Nieuport 11, some of which did experience wing failures, owing to a lack of torsional stiffness.

geezeraviation

My Feedback: (3)

Hi Seth, I was keeping an eye out for your next project. I am not surprised you chose something unusual.
I'll just hook up here and sit back.
Doc

Sethhunter

bogbeagle, that's a great question. I don't think the airplane had sufficient deployment to find out if wing stiffness would be a problem. It's not mentioned in Pfalz Aircraft of WWI (Herris). Herris did say the main problem was the power plant, which apparently was not very reliable. Only 10 were delivered to the front. He notes it climbed over 3 times faster than an Albatros DV, but was slower than allied biplanes and less maneuverable than the Fokker Dr.I. I'm not sure how a counter-rotating radial works - sounds like maybe a planetary gearbox where the ring gear is connected the block, the sun connected to the prop, and the carrier connected to the firewall - so the block rotates one direction and the prop rotates the other. Just a guess - could account for the reliability problems.

I started the build with the wings (I usually build them last, and it drags out the end so I thought I'd get those done first). I also short-cut the plans step, mostly just laid out the major components and dove in. The bottom wing will be attached to the fuselage with traditional R/C-type bolts and dowels. The middle wing will pass through an opening in the fuselage, and be bolted inside the fuselage. The interplane struts will then drop through slots in the middle wing and bolt to the middle and bottom wings. The top wing will have buried, horizontal slots that receive L-bends in the tops of the cabane struts; so the wing drops down on the "L's", then slides back .5". Little "keepers" get screwed over the recessed slots, trapping the cabanes in place. I don't know if that makes sense... we'll find out as it comes together.

Oh - because the middle wing has brackets to bolt to the interplane struts, those brackets must pass through the fuse too. So I'll build the fairings onto the wing, making the opening in the fuselage much larger, allowing the brackets to pass through. That's the theory, anyway!

So I started with the bottom wing. Since there really wasn't room for secondary spars, I laminated CF to the insides of the trailing edge sheeting, and put shear webs between the top and bottoms.

I also cut spacers and glued them to the LE to improve the glue joint to the ribs. The main spars have CF reinforcement. Remember - I want this thing to be a dog-fighter!

I made a few simple jigs to cut the webs and spacers to length. The brass tube sets the height of the spacer, and the gap between spacers. The spacers also form a rabbet for the top and bottom sheeting edges to sit in. That is, the sheet butts against the LE, down on the spacer and ribs.

Lastly, to make sure the MODEL doesn't have stiffness problems with the bottom wing, I made sure the strut anchor ribs are tightly connected to the TE and front spars. I put two holes in the pad to secure an external bracket, which will have two bolts through the strut, to handle bending moments. Buried under the pad are two nyloc nuts.

geezeraviation

My Feedback: (3)

The Semens Halske contra rotating engine was also used in the SSW DIII and DIV and that aircraft was also a fast climber and bears resemblence to a beer keg with wings. The Pfalz is also sort of stubby and round. There is a support for the front of the engine, between it and the prop that was a rigid part of the engine mounting thus all the "spokes" in the front of the cowl. Because of thier late entry into the conflict and due to lots of teething problems with the S-Halskey engine they were mainly assigned to home guard units for defense against night bomber raids utilizing thier rapid climb abilities
With the carbon fiber and sheer webs the wing should be quite strong enough. Is there a way you could use a sort of hinge for your L strut brackets? They could fold down for removal and require a smaller hole. Just thinking
Doc

JohnF

Hey Seth, great project you have picked. Look forward to seeing it at the field.

Sethhunter

I like the idea of hinging the brackets, but in this case I don't think I'll have to. The fairing radius will be a bit larger than the height of the bracket, and since the fairing will be part of the wing, rather than the fuselage, the opening in the fuselage will be big enough.

So the top wing is constructed similar to the bottom, except there's depth in the section to have secondary spars. Here too the spars are reinforced with .031 thick CF. The CF is 48". I put a 1/32 balsa strip on the outboard ends of the spars, to support the sheeting past the CF. All the 48" spars and sheeting are scarf joined towards the outboard ends of the 84" wings. With flat 1 piece wings, there are no joints in the center.

The wing tips are made from a lamination of balsa top and bottom with 1/64 ply core. I used the wing itself as a mold (bean-bag dive weights work great!).

The "Y" shaped interplane struts anchor in two locations, so the anchor ribs are beefy plywood - cut to form hard-point pads for the external brackets to mount to. Since I installed these after the wing was built, I cut them in two, with a tapered joint between the main spars, so I could insert them into the wing and glue them back together in position. This anchors also transmit the side load of the flying wires to the spars.

Sethhunter

The cabane anchors are similar, except rather than using a bracket, I created a block with a groove, and Fiberglass plate bolted over half the groove (and a thinner FG backing plate), creating a buried hole. So the horizontal L bend on the top of the cabane (kind of like a pin) goes in the slot, and the wing gets pushed back to slide the pin into the hole. A plate gets screwed over the slot behind the strut to hold it in. (That's the theory, I hope it works!) The hole in the sheeting near the mounting block is an exit point for the aileron cables, to run down the cabane struts. I'll put some cosmetic covers over the exposed FG plates.

For the sake of construction speed, I'll be mounting servos the traditional way (well, not really traditional, lets call it the X-gen way); hanging in the breeze. It hurts but will give me more incentive to build a more detailed bird.

It just strikes me how small the full scale was. I mean a .5 meter chord on the middle wing? Wow.

Sethhunter

Happy New Year! It's been a good time for building. Here's a few pictures of the fuselage coming together.
I built the round fuselage in two halves starting with a ply box, then balsa stringers, and finally diagonal balsa sheeting.
One of the more tricky bits is the joint between the fuselage and the bottom wing. Only the front third intersects the fuselage, so the box
offers only a partial saddle for the wing. We'll see in a later post how I extended support for the aft section of the wing, after the halves were joined.
The fuselage will have 3 hatches; one on top between the cabane struts (access fuel tank, receivers, batteries), on the bottom behind the firewall (access ignition and EIS battery, servos), and where the bottom wing comes off (access bolts for middle wing). I like diagonal strips for sheeting because it requires less fitting, and the arched grain seems to provide more strength. On the downside, when used to make halves, each half tends to spring-back a little when removed from the board (in my case, about 1/4" each). It's pretty easy to pull them back together when the halves are joined. The top and bottom longerons are 3/8 sq balsa on each half, or 3/8 x 3/4 when the halves are glued together.

The first picture shows the "half" firewalls being glued to the plywood sides with big hardwood gussets. After the glue cured, I drilled and doweled both the sides and firewall to the gussets. I used a tenoning jig on a tablesaw to hold a hardwood board at 45 degrees, then cross-cut the 45, flipped and cut again, to make an accurate triangular gusset where the grain runs parallel to the long side for max strength.

Sethhunter

Before joining the halves, I made a lamination of 3 layers of 1/16 ply with a wire for the tail skid embedded, and mounted to one half. The same plate will support the stabilizer. Then I glued the whole business together with a second plywood "full" firewall, to tie together the "half" firewalls that were butt joined together.

destrat

Seth:

Boy I sure wished I knew your secret for designing and building so fast! The speed you go you'll get every one built that you have on your "bucket list".
Everything looks just great. keep up the good work!

Dan

Sethhunter

Hi Dan - yes this is going together pretty quick, but to be completely honest, I had about a week head start before I started posting. With these pictures, I'm all caught-up. My day job starts tomorrow, so progress is bound to slow down. These pictures show the deck framing the top hatch, which provides a bay for the fuel tank, and holes to strap down receivers, etc. This deck also provides more torsional stiffness behind the firewall. The picture of the bottom shows the sub-bulkhead to receive the dowels on the lower wing LE, and a picture of the cantilever saddles glued to a bulkhead, reaching back to provide foundation for the fairing, and notches for a plate to receive the hold-down bolts. The wing fit nicely on the saddles, has a traditional groove for the rear LG wire.

The two hardwood blocks are glued and pinned to the bulkhead, and stick down into a recess in the top of the bottom wing. There, they butt against the LG block, so hard landing loads are transmitted to the bulkhead, not just the wing bolts.

From here, fun-with-fairings begins with 1/64 ply bottom sheet on the saddle, and "ribs" made of strips of ply between the bottom sheet and the fuselage. These strip-ribs are easier to put in place than cut balsa ribs. Lastly, strips of balsa and lots of thin CA make quick work of the fairings, and some aggressive sanding finishes them off for now.

When the hatches and middle wing saddle are done, I'll go over the fuselage for final filling and sanding. I'm thinking of putting .5 oz FG cloth down with highly thinned epoxy, then painting. It might be a little heavier than putting solartex on the fuselage. I'm also thinking this shape would be a challenge to get an iron-on fabric to fit right without lots of seams.

Sethhunter

A few more.... these fiberglass plates will bolt to bulkheads to hold strut wires in place. That is, the LG and cabane struts will have 1/8 music wire cores. Those wires will extend into the fuselage and have an L bend on the ends. These plates have slotted plywood liners that will hold the wires in place. And it makes them easy to remove. Seeing this model in 3D really drives home how stocky the airplane was. Next step - cut some holes in the fuselage for the middle wing. If you look back at the plywood fuselage sides, there's a big cut-out for the wing. And there's tiny 1/2 circle cutouts in front and back of the wing cutouts. The straight bottoms of those semi-circles are datums indicating the height and angle to align the middle wing. So I can suspend the wing in the cutouts, on a fixture that references those semi-circles, and build up the fairings . Since the fairings will be attached to the wing, one side will be just a little bigger than the other, so the wing fits tightly and slips out easily in one direction. That's the idea anyway. We'll see! Lastly - a picture of all three wings together shows just how narrow that middle wing is.

David Bathe

Dude... Nice job!

abufletcher

I'm not sure if that makes me feel better or worse! You got all of this done in one week???? [X(]

geezeraviation

My Feedback: (3)

More time gluing and less time posting, my, look what we can do.
Seth it looke great, nothing looks heavy (can you see weight?) what is your hoped for completed weight.
Doc

Sethhunter

Well, two weeks. My family hardly saw me at all during the Holiday! So far the three wings and fuselage weigh 5 lbs. I'm hoping all up weight will be 15 lbs, carried by 1,670 sq-in for a wing loading of about 20 oz/sq-ft, and a power loading of about a .25 hp/lb. The original had such a big prop, the front landing gear was pretty long - the hub towered over the mechanic's head. At this weight, power and angle-at-rest, I'm a little worried about it trying to float off the ground. Should take some care to get the tail up and flying first! [sm=what_smile.gif]

abufletcher

Ain't that the truth! (Says the guy with over 12,000 posts. [:'(] )

Sethhunter

Thanks David - Something important about the spiral sheeting method I didn't mention: Since each half springs back (twists) a little as the stressed strips try to straighten-out, you need to layout the halves with the spiral pattern mirrored (like "book-matched" veneer, say down-and-back on both sides). Then both tend to pull each other back together when you glue the halves together and line up straight. If you laid them out in the same spiral direction (like down-and-back on one side, and down-and-forward on the other), the spring-back on one half would follow the spring back on the other - gluing together with a back permanent twist.

If you're really picky, its good to try and build each half the same - same angle, similar density strips, same widths, at least in roughly the same locations, etc. It's not all that sensitive, but little things help. I also used 1/8 balsa spacers (glued to the board and longerons) to lift the top/bottom longerons and bulkheads above the board, so strips and bulkheads don't accidentally get glued to the board, and the longerons overhang the edge of the board, so the strip lengths don't need to be trimmed carefully while gluing. To separate the shell, just cut the spacers, then trim the ends of the strips. A few of the strips need to be spiled (tapered ends) to lay flat. That slows down the layup, but it shouldn't need too many.

geezeraviation

My Feedback: (3)

Seth
I had to go back to posts 14 and 15 to look at the sides before being joined, and didnt find what I hoped to find and that is a partial sheeting pic. Did you start in the center of each side and go towards the front and back alternately or did you start at the front/back and go full length from there? Just curious.
Doc