Bill's Skywriter...err Pitts Python? Build...

[Wingnutz]

Decided I would post the tail and fuse parts of my build as I'm not following the plan here. I am not an experienced builder and this may turn out to be a lesson in HNTDIY (How Not To Do It Yourself!) Add the fact that once again my life is in a state of turmoil, with happy and sad stuff going on and the outcome is anything but assured...
The plan tail feathers seem overbuilt and overweight and are at least partly responsible for the model's tendency to be tail heavy...ergo my tail feathers were built to plan outline but not with plan materials. Borrowing from Polaris building and indoor building, I built the tail feathers out of depron covered with Econokote...gives a light build...flying will tell if they're strong enough...there is a carbon strip embedded in the h. stab to stiffen it.
Sorry if the thumbnail presents upside down...it pops right side up when expanded.

[Wingnutz]

Fuselage
I like the look of some of the big American radial powered biplanes...Super Stearmans and Wacos coming to mind...I wasn't as enthusiastic about the nose of the Skywriter...so...time to start figuring out if I could put a radial cowling on my Skywriter...sounded...challenging and intimidating at the same time
Started looking at radial engine noses and settled on the cowling of a P-47 because, unlike most radial engine cowlings, the P-47's is oval, not round. This shape would allow me to employ Frank's Norseman battery mounting system and get the battery under the motor...almost as far forward as the back of the prop, again battling tail heaviness without adding ballast.
I bought a Parkzone P-47 cowling which looked great but didn't appear to want to match up with the Skywriter fuse. Ordered two more P-47 cowlings from Park Flyer plastics and one of them looked doable.
Head scratching ensued over how to retain the character of the Skywriter fuse from the cockpit area back with a P-47 cowling up front...hmmm
Stole fuse construction ideas from Rob Pike's build but re- designed the nose so that top and bottom run parallel and the sides taper in to accomodate the P-47 cowling...
Used my magnetic building table to try to keep things square and plumb...

[sihinch]

Looks great Bill.

We're going to have 6 or 7 different Skywriters! How exciting.

[Wingnutz]

While I was surprised, honoured(?) and amused to be awarded the Crash of the Year trophy last night, I'm not anxious to repeat, so hopefully this all comes together well.
Unlike some of the other builds, I have not lengthened the nose.
The white firewall is actually a sheet of depron which I'm using to create a template before I cut a ply firewall. Firewall and motor are stuck in place with two-sided tape, so I can see where everything's likely to end up. I've angled the firewall to create down and right thrust but still have to locate the motor accurately to compensate.
The battery is a 3300 4S which will sit on a piece of ply between F1 and F2 and be anchored with a battery velcro strap which can be strapped/unstrapped through the cowling. It's Frank's idea from the Norseman build.

I think the thrust line will be approx 3/4" lower than the original Skywriter's...less downthrust? Suggestions, opinions appreciated...before I start gluing

[piker]

That looks great!  Good work.

About down thrust.  Hmmm.... I'm wondering if we even need any.  I put in the 2 degrees that the plans called for but I'm now second guessing that... now that you ask.

My understanding of down or up thrust is that it's used to counteract the pitching effect caused by the separation between the thrust line of the motor and the drag line of the aircraft.  With a high wing aircraft the line of average drag is above the thrust line, so application of power with tend to swing the nose up.  With a low wing aircraft we provide UP thrust for the same reason but in the opposite direction.  For an aircraft where the wing and tail are inline with the motor (or at least very close) there's no significant pitching effect with application of power, so no need for up or down thrust.  In the case of the biplane, with one wing above the thrust line and the other below, I would assume the average drag line is closer to the thrust line causing very little pitch effect.  I bet we don't even need down thrust on the Skywriter. 

However, if you're going lower with the motor, and therefore further below the average drag line, you may be introducing the need for a little down thrust.  Still, based on the above pile of bull... I certainly wouldn't add more than the recommended 2 degrees.   

What do YOU think?

[Papa]

Go without for a start and if you need it just use washers to achieve it.


Jack.

[Wingnutz]

Thanks guys. I like the washer solution. Rob, I think you're right about compensating for a lowered thrustline with downthrust...how much? I dunno...
Added some gussets (I think they're called scab gussets) to the rear part of the fuse. Hopefully the gain in stiffness and strength outweighs the gain in weight! More gussets to go but they can be done after the firewall and front formers go on.

[Papa]

Bill, IMHO that will be a weak fuselage. While it will save on weight it won't stand up to a robust landing. I think you are over compensating for weight. To strengthen the fuselage you should add trusses to distribute the stress loads. If you do a single truss top left to bottom right corners on each side you will get an "X" shape when viewed through the side. This will add great strength for only a slight gain in weight.


Jack.

[Wingnutz]

I will add some trusses...good idea... Thanks

[Wingnutz]

Added diagonal bracing to create Warren bracing style trusses out of each fuse side. Scab gussets helped to provide gluing surface area. Will add similar to top and bottom later.

[Michael]

Nice!

[Papa]

Hi Bill, it has always been my understanding that the strongest truss structure has the opposite side trusses go to opposite corners. This includes sides and top. You have both sides going to the same corners. I have seen it done both ways but I believe the opposite structure divides and distributes the stress more efficiently. Jack.

[Wingnutz]

Hi Jack,
Your suggestion and the photo make sense. Followed the bracing from the Norseman fuselage and what I've seen in other models in deciding to do the two fuse sides the same. Will alternate the top and bottom bracing.

[Wingnutz]

In the midst of Christmas chaos, found a little time to work on my Skywriter. Built a 3/32 ply plate between F3 and F4 to stiffen the fuse, mount the cabane struts and the tail feather servos. Figured out a way to make the cabanes removable so they can be out of the way for covering. Struts are not bent yet as I won't be sure where to bend them until the fuse is completely framed.
Plates for mounting the lower wing, to support the battery and to mount the L.G. are next and then the nose job can proceed. Took a look at what I've done on the fuse so far and realized I haven't used anything from the short kit or even cut as per the plan.
Maybe that's why I'm so slow...every step on the fuse has involved lots of head scratching!
Top photo is  from the top, bottom from the bottom.

[Wingnutz]

Plugged away at my Skywriter fuse over the holidays. Lots of head scratching as I try to solve a custom designed fuselage...but some slow progress. Wanted yo solve the problems of wire routing and power train component location before I started sheeting.
Motor, battery and ESC are located and pretty much mounted...photos below.
One problem has me still scratching my head. Battery is plugged and unplugged through the prop arc. This system requires a way to plug in with no chance of the motor starting...or removing the prop every time and re-installing after the battery is plugged...a nuisance I' m living with on my Norseman.
Suggestions?