Piper PA-38 Tomahawk short kit build

Wingnutz · February 14, 2019, 04:42:15 PM

Although I left the GTA a couple of years ago, I didn't leave TEMAC completely and like a bad penny, still show up for a few events. I didn't stop building or flying models either and thought this build might interest a few TEMACers.
The Real Thing
In the 1970's and 80's Cessna was slowly developing a stranglehold on the two seat GA trainer market with its 150/152 model. In an effort to develop a competitor, Piper surveyed thousands of flight school CFIs. "Give us an aircraft that spins and handles like a heavier aircraft" was the reply. Armed with this advice, Piper engineers designed the Tomahawk...be careful what you ask for, because, spin it did and recovery demanded a technique more like a larger airplane, and unlike the gentler Cessnas. The aircraft earned a reputation and a nickname..."Traumahawk" for its stall/spin characteristics. The debate over whether poor design or poor pilot training is to blame is posted on many websites. Only 2400 ish we're produced but there are some fiercely loyal supporters who claim it's a better trainer than a 152. Certainly it has more room in the cockpit!
The Model
No Tomahawk BNFs, PNPs, ARFS or full kits out there, airplane with a reputation...so,...why did I choose to build the model? For better or worse, I recently became half owner of a full size 1982 Tomahawk II. I can't fly it until the weather improves and the other part owner and I pick it up from Miramichi NB.
In the meantime, I found a thread on RCGroups, ordered a laser cut short kit and started building while I'm on vacation here in Florida.
It's fairly small at 41" ws, but that's a good thing, as I'll have to get the partially completed model in the car for the trip home at the end of the month.
The build
No workshop and only my field box tools (I've done a bit of RC flying with the local club) have made it a challenge, but I've found a great LHS (Phil's) and scrounged andcut up a bit of drywall from the renovation in the unit above ours, to build on. The Mansell short kit from Laser Design Services in Texas is pretty complete and has gone together like a dream. In two days, I've built the work surfaces and framed up the fuse, the stabs, the elevators and the rudder.
Couple of interesting departures from the norm for a build this size.
Steerable Nose Gear
I was dreading this, having struggled with the nosegear on my Durafly Vampire while flying here, but the model and the LHS introduced me to DuBro's Steerable Nose Gear 1/2A size. Slick, simple solution..gets really easy when retracts are out of the picture!
Elevator Drive for T-tail
Mounting a servo on the fin or using pushrods are pretty much out of the question for this model, so the elevators are driven with a Sullivan cable which runs through a channel in the fin. Having built three t-tailed Tutors, this wasn't new to me, but has anyone come up with a good method to attach a control arm to the torque rod that connects the elevators? LMK!
I'll post as I build, hopefully finishing up with a photo of the model together with the full size.

piker · February 15, 2019, 01:14:36 PM

Hey Bill! I like that plane. It's cute.

Wingnutz · February 25, 2019, 05:18:11 PM

Yes Rob, it's cute. The model's nose gear will be on its own servo which will allow trimming the nose wheel separately from the rudder. I've only tried this on one other model, but it worked so well, I decided to try it here too. The idea is not mine so I've posted a link to where I found it.
https://www.youtube.com/watch?v=q-KWIzRyDq8
The servo is one salvaged from a "retired" model and I'll put a brand new one in when I get home from vacation. The servo mounting is designed to be removable.

Wingnutz · February 27, 2019, 01:49:58 PM

Decided to build this after we arrived here in Florida, so didn't pack building tools before I left home. Luxuries like scroll saw, band saw, drill press? Not here. My workbench is the glass topped table in our rented condo, the build boards are bits of scrounged dry wall and my cutting surface is a kitchen cutting board. Still, I wouldn't have missed it!
Yesterday was a rainy day, so framed up the "second" wing. Once it's finished, the wing(s) will be one piece, but I'm building it in two halves so they'll fit in our large cooler for the trip home in the car.
I decided to add flaps, so I had to decide how to re-frame the wing and mount the servos. I'm going to follow Rob Pike's advice from the water bomber build thread and glue the servos to the 1/8" ply hatches. Mounting wing servos has always been a fiddly, time-consuming task and this will simplify the process. Thanks Rob!
Even so, the servos have to be positioned carefully as the shallow depth and curve of the upper surface of the wing dictate where the servo has to sit. The wing stringers?, spars? ...anyway the long skinny bits that run from root to tip are 1/8x1/8" basswood and the plan calls for four full length and one broken to allow servo clearance. The main spar is 1/4x3/8" balsa.
The first picture shows the only saw and cutting surface I have...

Wingnutz · February 27, 2019, 02:12:02 PM

Here's a couple more shots of the real thing that I'm trying to model. Like most Tomahawks, C-FALH spent its early life as a basic trainer accumulating over 10,000 h so it's no youngster. It's been thoroughly overhauled recently and hopefully will serve us well for the next few years.
The third photo is a whimsical collage of Tomahawk renderings and apparently the age of pilotless GA aircraft has arrived! Only one appears to have a human pilot!

piker · February 27, 2019, 03:23:21 PM

Nice build, Bill.

It's sometimes refreshing and fun to get back to the basics and work with what you have at hand.

I used to do that at the cottage or even camping. I built most of my first Morphine racer, even sheeting the foam wings, at the campground while living in a tent for a week. It was a place I had lots of time to kill. However, I did plan the build so I had the supplies with me.

Wingnutz · February 28, 2019, 02:29:29 PM

Few tools, no workbench,...still, great fun! I'm not a sit by the pool or TV/YouTube type, so even on sunny days, there's often time to build. Ziploced all the little parts instead of leaving them all over the workbench like I usually do. Treated myself to some DuBro T-pins and if you build and don't have these, buy some...they're a joy to use!
All things come to an end and this morning, I had to start packing the framed up bits for the trip home. Thank goodness the model's small at 41" span and the bits are fitting in our large cooler.
The framed up pieces have been stored under the spare bed in our rented condo and I took photos as the bits re-emerged.
Nothing unusual, except the Tomahawk's T-tail presents a challenge driving the two separate elevators and I'm still hoping some knowledgeable TEMACer can offer suggestions to secure a control/servo horn to the torque rod which connects and drives the two elevators

Wingnutz · February 28, 2019, 02:41:58 PM

More framed up bits and packing the cooler for the trip home

Flat bits;wings, stabs, elevators and rudder are laying under a couple of rolled shirts and the vert stab has been removed so the fuse will fit...

piker · February 28, 2019, 04:36:20 PM

Quote from: Wingnutz on February 28, 2019, 02:29:29 PM
TEMACer can offer suggestions to secure a control/servo horn to the torque rod which connects and drives the two elevators

Cut a brass arm (length determined by pushrod location) with a large hole at one and to fit over the torque wire, and a small clevis hole at the other end. Slide it over the torque wire and solder in place. Easy (when you're home and you have tools), simple and effective. I see you already have the pushrod housing in place.

Note: Remove beer before travelling. It'll make a mess of those shirts.

Oh, and nice construction!!!

Robert

Frank v B · February 28, 2019, 08:23:09 PM

Bill,

I have a Sig metal horn and wire for elevators and ailerons in control line airplanes. I will dig it out and take a photo of it for you.

Safe home.

Frank

Found them. There were two in the original package. Feel free to make one follow you home.

Exactly the way Piker described.

Frank

piker · February 28, 2019, 11:36:49 PM

Even easier

Wingnutz · March 01, 2019, 12:42:07 AM

Rob, Frank,
Many thanks for your suggestions re the control horn for the elevator torque rod. The model is in the cooler (no beer) in the car so no more building for a few days. Frank, I'll take you up on the offer of one of those "control-horned thingies" and will be in touch about "making one follow me home"