A Sport-scale PT-19 for 20 Cell Systems [Archive]

Bill Bowne

Apr 01, 1997, 01:00 AM

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<td><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c12.jpg" width="323" height="207" </td>
<td>MODEL DATA SHEET: <ul>
<li>Wing area: 750 sq. in. </li>
<li>Span: 72 in.</li>
<li>Airfoil: Clark YH </li>
<li>Weight: 120 oz. </li>
<li>Wing Loading: 23 oz/ft. sq. </li>
<li>Motor used: Astro Flight Cobalt 40 Geared </li>
<li>Number of cells: 20 </li>
<li>Propellor: 13 X 10 </li>
<li>Watts/pound: 53</li>
</ul>
</td>
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<h2 align="left">INTRODUCTION</h2>

How's this for why I designed and built the PT-19: My wife told me to!

You see, she wants to fly it in AMA Fun Scale competition. I figured if I built it for
her, then I might be able to get to fly it, too. Maybe. If she lets me! 

Anyway, we both have soft spots for the colorful "Between the Wars" military
planes. Somehow, the bright blues and yellows, plus the pre-WWII roundels make those war
planes seem quaint and almost harmless. I guess the difference between those planes and
modern jets is like the difference between a Shetland sheepdog and a pit bull: both have
teeth, but a sheltie is a lot less threatening than a pit bull. It's also a heckuva lot
prettier. 

Like most Electric modelers, we started with glow models, then moved up to electrics.
Our first electrics were ferrite "05" models, then we gradually transitioned to
Cobalt motors and began flying larger and larger power systems. 

So far, we're happiest with Astro Flight's geared 40 motors. They work like four-stroke
60's, turning bigger, more efficient props than direct drive motors. Those bigger props
lend themselves to larger models with lighter wing loadings. Bigger models can absorb
heavier (less perfect!) building, and the planes are much better in windy weather. 

I've built one direct drive 40 pattern model and two geared 40 models (a Waco YKS and
the PT-19). The geared models definitely fly better! They aren't quite as fast as the
direct drive models, but they are much more maneuverable, and they feel much more `solid'
in the air. To sweeten the deal, the gear drives need less amperage to fly the planes,
while still flying them well. I usually get a full 5 minutes of aerobatic flight with my
Waco, but I rarely reached 3 minutes with my direct drive 40 pattern model. 

For the PT-19, I finally figured out how to avoid the one thing I really dislike about
larger electric packs: Having to take the wing off to change them. It isn't a real bother
with powered gliders and models without ailerons, but one of these days I'm sure I'll
forget to plug the ailerons back in and I won't notice before take off. So, I put a simple
quick release in the nose hatch and rigged a battery pack quick change system to ease
swapping packs. The hatch latch details are on the plans, but it was easy to rig from some
bent scrap push rod wire, the spring from a long dead ball point pen, and some scrap wood.


It makes more sense to take packs out to cool and recharge between flights. It also
really helps to have more than one flight pack. That's why all my 40 powered models are
built around 20 cell packs, using Sanyo SCR cells. I have three of them, and I use a <a href="#Battery Cooling Boxes">battery cooling box</a> between flights.

A gentle reminder to you beginners: My design is a simple model of a full-scale
trainer, but it isn't intended to be an R/C trainer! The full-scale PT-19 was built to
train full-scale pilots, not modelers. Please, build and learn on a true electric or even
glow trainer before tackling a model like this. 

(My fellow electric purists are probably staring and muttering at the last paragraph.
If you see one, please kick him/her and tell him to move on to this paragraph.) Why do I
even mention Glow power? Because it works and beginners can get a good start on it. Yes,
I'd rather see beginners start the Electric way, but the most important thing is that they
START and LEARN successfully. There are a lot of glow fliers out there who can help future
electric fliers get started. You can convert glow fliers to Electrics, but it's a lot
harder to convert ex-modelers who gave up the whole hobby in disgust. Besides, glow fliers
help keep the bugs away (or at least their models do!). 

 

<h2>CONSTRUCTION</h2>

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<td><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19fuse.gif" width="350" height="228"></td>
<td><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19wing.gif" width="375" height="140"></td>
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<td colspan="2">High-quality, full-sized plans are available for
this and other models from the E Zone's plans service.</td>
</tr>
</table>

<h3>General hints: </h3>

Make a kit of all the major parts before starting construction, use only thick and thin
cyanoacrylates (CA's), and cover the model with light weight iron-ons. 

Keep trying to make the model as light as possible. The completed model (with motor
& battery) should weigh less than twice the weight of the motor and batteries. If it
weighs more, then it probably won't fly. With a 3 lb Astro Flight geared 40 power system,
then, your PT-19 shouldn't weigh over 9 lbs. 

<h3>A. WING. </h3>

If the wings aren't right, then the rest of the airplane is useless. So, I do them
first. I used a wing jig. Since this is an almost flat-bottomed airfoil, you can build it
on a board. I've had an Adjusto-Jig for over 10 years and I use it for all the built up
wings I build. It takes a few minutes to set it up and cut the jig holes, but it saves
much more time and effort. With the jig, I can build the wing in one piece, build in the
washout, and do almost all the construction before removing the wing from the jig.
Finally, I KNOW the wing will be straight when I'm done. Which ever way you choose,
remember to build in 3 degrees of dihedral PER PANEL (that means elevate the tip ribs
2" EACH). Want to guess who misread his own plans and only built in 1 degree per
panel? That extra fiberglass tape in the center section cost me at least 6 ounces of
wasted weight! 

(1) Make the alignment holes in the balsa ribs and slide them into place on the jig
rods. Glue the plywood rib doublers in place. 

(2) Glue in the spruce spars, balsa leading and trailing edges, and plywood dihedral
brace. PLEASE, use AIRCRAFT, not "light" ply for the dihedral brace. 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c2.jpg" width="323" height="226" 

(3) Glue in the landing gear blocks and verticals. Trim the bottom leading edge
sheeting to fit around the gear blocks and glue it into place. Add the top leading edge
sheeting, the top and bottom trailing edge sheeting, and the bottom center section
sheeting. Wait on the top center section sheeting until the flap servo mount and aileron
servo cord ducting is completed. 

(4) Remove the wing from the jig. Cut, fit, and glue in place the 1/16 balsa shear
webbing. Add the aileron servo mounts, the flap servo mount, and the aileron servo cord
ducts. Now, sheet the top center section. 

(5) Build the ailerons and flaps over the plans. Make sure you use real aircraft ply
for the control horn braces, not light ply. Light ply won't hold wood screws for more than
a few flights, not even if you harden it with CA. 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c1.jpg" width="323" height="179" 

(6) Fit the ailerons and flaps into place, then temporarily tape them on. Fit and glue
on the trailing edge center section. Glue the wing tip blocks together over the plans,
then glue them to R13. Add the braces, and scrap LE fairing blocks. 

(7) Take the wing, go outside, and sand, sand, sand. You warm climate people can have
some fun by providing your climatologically challenged neighbors with a simulated
snowstorm! Or, if you have neighbors you don't like, sand upwind of them and leave a trail
of sawdust to draw termites to their house! 

(8) Set up the aileron servo mounts. I've used bellcranks, but their slop always makes
me cranky (Sorry!). Now, I use a single micro servo for each aileron, although a standard
servo works as well. It's just be a bit heavier. I used a single standard (S-148) servo
for the flaps. Using servos like this lets me put short, straight pushrods between the
control surfaces and the servos. Fluttering is for butterflies, not model airplanes! 

(9) Sand flat the wing leading edge inside the fuselage, glue on the 1/16" plywood
bolt plate, then fiberglass the wing center section. Fit the landing gear, then put the
wing aside until the fuselage is ready for it. 

<h3>B. TAIL SURFACES. </h3>

Build these on a flat surface. I basically build them all the same way: 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c3.jpg" width="323" height="219" 

(1) Pin down the perimeter parts and wick thin CA into the joints, then glue in the
internal parts. 

(2) After the glue cures, take them off the board and sand them to shape. Taper the
rudder trailing edge to about 3/32" and the elevator tailing edge to about 1/8".
Round the horizontal and vertical stab leading edges, but sand the elevator and rudder
leading edges to "V" cross-sections (except at the rudder balance area - just
round the front of that and leave the bottom flat.). 

(3) Glue a 1/2" wide strip of fiberglass tape around the leading edge where the
rudder horn will go. For insurance, CA two small (about 1" x 1/2") fiberglass
patches to secure the elevator joiner to the elevators (I also used a "Y" shaped
push rod to two elevator horns. (Belt and suspenders? The elevator is under a lot of
stress, especially the spruce joiner. I don't like running out of elevator when I'm
pulling out of a low altitude loop.) 

 

<h3>C. FUSELAGE. </h3>

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c5.jpg" width="323" height="182" 

(1) Start by making up the bulkhead sub-assemblies. Please, use good aircraft ply for
the bottom of F2. Most airplanes fly better WITH wings than without! 

(2) Mark the top spruce longerons where they have to be cut at F2, saw them about
half-way through, then pin them down. Glue the 1/16" balsa sides and the horizontal
stab mounts to the longerons. Remember to build a left and a right side! 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c4.jpg" width="323" height="138" 

(3) Add the wing saddle doubler, bottom rear spruce longeron, and 1/8 X 1/4 nose bottom
brace. Use F2 and F5 for alignment. 

(4) Add the 1/4 sq. balsa diagonals and verticals, using the bulkheads to align them
(but don't glue the bulkheads in place!). Do the same with the 1/4 X 3/16 verticals and
diagonals between F1 and F5. Finally, add the 1/16 balsa tail filler and the ply gussets. 

(5) Use a triangle to glue F2 and F5 to one side, then glue the other side to this sub
assembly. Over a straight centerline, pull the nose sides together and glue in F1 and the
ply nose ring. 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c11.jpg" width="323" height="195" 

(6) Sand each aft fuselage sides to 1/16" thick at the tail. Simulate the tail
post with a piece of 1/4 X 1/2 balsa, then glue another piece just in front of it. Let the
glue cure, then remove the "tail post". Now, glue in the F6 and F7 bulkheads and
all cross-members aft of F5, then add the bottom balsa 1/4 sq. longeron. 

(7) Glue in the wing bolt plate, then mount the wing to the fuselage. Be sure the wing
is square with the fuselage! If you don't plan to use flaps, I suggest you re-engineer the
bolt plate rearward and cut out a few grams. 

(8) Add the balsa triangle braces behind F1 and around the wing bolt plate. 

(9) Add bulkheads F3 and F4, but DON'T add F5d. 

(10) Glue in the center 3/16 sq. balsa longeron between F1 and F5, then sheet the top
of the nose and the cockpit area with flexible 1/16 balsa. I did the nose top first, then
the rest. To make up each section of sheeting, first join together enough 1/16 sheet to go
from side to side in one piece. Use some scrap paper to approximate how large the panels
should be, then cut them about 1/16" oversized. Wet one side of the panel with warm
water and glue its center to the 3/16 sq. central longeron. Bend the sides of the panel
down and glue them in place with thick CA, then run thin CA along the bulkhead/sheeting
seam. Tape down the top sheeting and let it dry, then repeat for the center top sheeting. 

(11) Trim the sheeting flush with the front of the nose ring and with the rear of F5,
then glue F5d to the back of F5. Sand F5d flush with the top of the 1/16 sheeting
overlapping F5, then ensure the 3/16 sq. central longeron segment running from F5d to F7
is level with the top of the sheeting at F5. Next, glue in the rest of the top longerons. 

(12) Fit the motor mount and motor into place and then build up the nose from scrap
blocks. 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c6.jpg" width="323" height="207" 

(13) Build up the bottom nose hatch. Start by placing the front and rear plywood
bulkheads in place, then drill through the front one for the two 1/8" dowels. Next,
glue the 1/4 sq. hatch sides to the plywood bulkheads, then glue the SOFT 3/16" balsa
sheet crosswise over the 1/4 sq. Remove the hatch and glue in the front filler block, then
finish drilling for the 1/8" dowels and glue them in place. Put the hatch back under
the nose and temporarily fix it in place by running a small wood screw through the bottom
of F2 and into the rear plywood bulkhead. 

(14) Take the fuselage outside and sand (Don't draw the termites from your neighbor's
house!). Use the plywood bulkheads as guides to round off the bottom. Sand the front of
the nose cowl to shape and hollow out the nose blocks. (Incidentally, I had to disassemble
the motor from the gearbox to remove it, but since it only requires the removal of two
screws, it's easy to do). 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c8.jpg" width="323" height="143" 

(15) Glue in the cockpit top doublers. Use a pattern to draw the cockpit openings on
the fuselage top to guide you. Make the cutouts a bit smaller than the final openings, so
you have room to sand and trim. Once the openings satisfy you, turn the fuselage over and
glue in the cockpit floor rears and the pylon mount. Assemble and test fit the turnover
pylon, but don't glue it in place until the fuselage is covered. 

(16) Align and glue on the horizontal stab. Remember, you have to have the elevator
hinged and attached before gluing on the vertical stab! 

(17) Install the pushrods & bracing, then add the side stringers (use hard balsa!)
and sand the fuselage to final shape. 

(18) Make and install the hatch latch. Don't worry about using the exact same dimension
parts, just use what you have on hand. I used the spring loaded hatch latch on one side,
then I backed it up with a Goldberg nylon hatch catch on the OTHER side. Why? Because the
spring loaded hatch could fail in flight and I could lose the hatch. Why not use just the
Goldberg catches? Because they don't leave any leeway to easily grasp and pull off the
hatch. Hence, the TWO latches. 

(19) Between the motor weight perched out on that long nose and all the battery weight,
you'll be fighting to keep the model from being nose heavy, not tail heavy. So, go right
ahead and install the rudder and elevator servos into place. PLEASE, DON'T glue your servo
mounts only to the 1/16 balsa sides. They just aren't strong enough to handle that kind of
strain. Instead, glue them to the spruce longerons and add side braces (as the plans
show), then complete the pushrods. 

<h3>D. COVERING. </h3>

I used Coverite's 21st Century film for the wings, tail surfaces, and roundels, while
Black Baron film provided the blue fuselage. Yes, I did all the markings with film,
including the rudder stripes. The black wing walks are black Super Monokote, sanded to a
flat finish with 400 grit sandpaper BEFORE being ironed on. 

Finally, I drew the panel lines with a Sharpie marking pen. If you aren't careful, you
can rub it off (but not quickly!). No problem, 'cause it's easy to reapply. 

You stubborn older modelers (My wife frequently says that should include me, no matter
how frequently and loudly I deny it!) will cover the PT-19 however you want to. Once the
wing is covered, I recommend you consider covering the horizontal stab FIRST, then the
(unattached) vertical stab. Next, hinge the elevator, THEN glue the vertical stab into
place. After that, cover the fuselage, then the rudder. Finally, glue the rudder into
place. You always want to work DOWN in size, covering large parts before covering small
ones. You can always cut small sheets from large, but not the reverse! 

I find it hard to get many coverings to stick to fiberglass center section tape,
whether it be glued on with epoxy or CA. To make the coverings stick, slap a coat of
Balsarite over the fiberglass. Of course, if you plan to cover the model with Micafilm,
you'll Balsarite the whole plane, but that's another story.) 

<h3>E. POWER.</h3>

20 Sanyo SCRs motivate the geared 40, swinging a Zinger 13 X 10 prop. This combination
works well, but I do want to experiment with a 13 X 9 (once I can find one!). An Astro
Flight 205 speed control directs the electron flow. There are several other good ones on
the market, but whatever you use, USE A FUSE!!! You may think you can cut the throttle
before digging in the prop during a ground loop or before crash impact, but what about if
the radio quits? Fuses are lots cheaper than speed controls! 

Mount the speed control on the deck in the upper nose. It'll stay out of the way when
you slide motor packs in and out. In the case of an 'aircraft mishap' (Air Force lingo for
'crash'), it also lessens the likelihood the speed control gets caught in the middle when
the pack and motor play hammer and anvil. 

<img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c7.jpg" width="323" height="182" 
The battery pack, secured to its tray

As for the battery pack, the 20 cell pack is THE major mass component of the model. So,
it has to be securely tied down. Problem is, if it can't also easily escape the model in a
crash, it'll make like an angry rhino in a Volkswagen. So, the battery sled and rails will
guide the pack forward in a crash, but keep it from floating around during aerobatics. I
know that from experience! 

<table border="0" cellpadding="5" width="100%">
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<td><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c10.jpg" width="323" height="218" </td>
<td><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c9.jpg" width="323" height="203" </td>
</tr>
<tr>
<td>The bottom of the fuselage, showing the battery rails</td>
<td>The bottom of the fuselage with the pack in place.</td>
</tr>
</table>

 

<h3>F. FINAL ASSEMBLY.</h3>

Now, for the finishing touches. Make the windscreens from .015" butyrate plastic.
I slit the covering and glued them down with RC 56. After there were in place, I carefully
ironed on small strips of blue covering film to simulate the windscreen framing and
bottom. As long as you keep the heat down, you can iron to the butyrate without deforming
it. 

Paint the plastic nerd pilots and glue them into place, then paint and install the
crash pylon. Hinge the ailerons and flaps (You already did the elevators and the rudder,
right?.) Reinstall the landing gear (Robart strut shells nicely simulate oleo gears).
Position the motor battery to balance the model and glue the rails into place. 

I connected the rudder and tail wheel steering arm using a strap cut from a coffee can
lid. I also put a piece of fuel tubing (an artifact from my less mature glow power days)
over the steering arm. It spares the rudder servo any nasty jolts caused by rough fields. 

<h3>G. CONTROL SURFACE THROWS. </h3>

Elevator and Ailerons: +/- 3/8" 
Rudder: +/- 1/2" 
Flaps: -1/2" (for initial trials)

 

<h2>FLYING</h2>

The winds were right down the runway, and blowing at less than 15 mph when we took the
PT-19 out for its maiden flight. Taxying was easy on our grass strip, and the tail came up
before the model had moved more than 3 feet. 

Climb out wasn't ballistic, but it was very comfortable. One swing around the field
brought us up to a "two-mistakes high" altitude, and I started experimenting. 

The model isn't treacherous, but it also isn't a plane you can ignore while flying. It
is big, and a bit clumsy. Straight and level stalls are slow, with little tendency to drop
a wing, while high speed stalls will trigger a over the top snap and spin IF PRESSED.
Spins are fairly tight, but releasing the sticks and pushing slightly down brings the
model right out. Stall turns are really pretty, especially against a clear blue sky. 

I didn't use the flaps until the second flight. Dropping them caused little ballooning,
and they worked well in delaying the stall. Where I screwed up was in leaving them down
whilst landing in that head wind. I forgot to keep in more throttle when landing, and the
wind gusted just as I was about to touch down. The model slowed and lifted, and I
mistakenly concentrated on keeping the wings level with aileron (I should have added power
and kept the wings level with RUDDER, just like I tell everyone else to do. Sigh!). The
left wing dropped, the tip dug in, and the model cartwheeled. That's how I know the
battery tray system works! The battery pack slid out without causing any significant
damage. I had the model repaired and ready to fly within an hour. 

Now, I just hope my wife doesn't order me to build a B-36.....

<address>
Bill Bowne 
<a href="http://rcgroups.com/shared/nospam.php?u=CaptainBB&d=aol.com">CaptainBB(at)aol.com</a> 
307 Colorado Trail 
Browns Mills, NJ 08015
</address>

<table border="0" cellpadding="5" width="100%">
<tr>
<td width="50%"><img src="http://static.rcgroups.com/articles/ezonemag/plans/pt19bill/pt19_c13.jpg" width="323" height="179"> 
Author Bill Bowne with his PT-19. Fetch, Bill!</td>
<td width="50%">Plans Available from the<a href="/articles/ezonemag/plans/pt19bill/../../../pages/orderpln.htm"> E Zone's
On-line Plans Service.</a>For more information, send an SASE to:
<address>
The E Zone 
501 Goodwin Dr 
Richardson, TX 75081
</address>
</td>
</tr>
</table>

 

<hr>

<h2><a name="Battery Cooling Boxes">Battery Cooling Boxes </a></h2>

When Uncle Sam sent us on a three year "vacation" in Germany, we saw how some
German Electric fliers did things. Some methods were better than ours, some weren't, and
some were just different. 

The battery cooling box is one idea we adopted. Several German fliers used a variety of
containers and cooling fans to blow air over flight packs before recharging them. 

The most common method used lengths of PVC pipe with electronics cooling fans installed
in one end. Cutting a large section of the pipe allowed easy access to the man-made breeze
within. Fan power usually came from a direct connection to the car battery. 

I liked the PVC pipe method, but I didn't want to be restricted to the relatively small
diameter pipe available in my area. So, I bought a square Rubbermaid snap lid storage
container (which also cost less than two feet of large PVC pipe in our local hardware
store). I used an old soldering pencil tip to melt a round hole in one end and a series of
slits in the other. 

For the cooling fan, I swiped an idea from Bob Afflerback, flying buddy and friend. He
pointed out that the heating element in a hand-held AC hair dryer did the AC to DC
conversion and that minus the heating element, it would run very well on a six cell pack.
It just wouldn't get hot. I persuaded Mickey that she could get more flights if she let me
cannibalize one of her old hair dryers, so she let me have an old gun-type dryer she
didn't like. 

I pulled out all the heating wiring, cut off the cord and installed a pair of Sermos
connectors, and rewired around the switch. ONCE YOU DO THIS, DO NOT PLUG THE
BLOWER INTO 110 AC POWER! 

The round nozzle of the blower fits into the round hole in the Rubbermaid box. For
power, I always have on hand at least one old 6 or 7 cell pack (that protects the car
battery, and lets me run down partially discharged packs). I just snap off the lid, insert
the pack, snap the lid back on, push the nozzle into the hole, and then plug in the
battery. The batteries are cool in minutes. As an added bonus, I can plug the blower
nozzle into a convenient port on the model to cool the batteries in place, should I feel
to lazy to bother removing them. It just takes a little longer. It also works well to get
campfires started!