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May 07, 2008, 01:39 AM
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** PIRANHA! ** - 43" fff/monoblock/kfm4 aerobat

this plane was designed by tony65x55. it is a graceful flying 20oz class parkflyer capable of classic aerobatics, including inverted flight, loops, rolls... etc.

it is made primarily of fff (blucor, fan fold... whatever) & 2" foam (monoblock), with some 1/8" & 3/16" thick bamboo skewers. the monoblock construction makes it easy for neophytes to build a straight fuse. it also uses a 1/4" full span spruce wing spar and a 1/4" thick balsa wing joiner. wingtips & fuse reinforcements are made from 1/16" ply, and a 1/8" ply firewall is optional.

The base motor is a bp21, which is enough for most aerobatics. more power is required for unlimited vertical & more aggressive aerobatics. i have plans on swapping in my hacker a20-20l for fun, but that's sure to be too much.
[edit: The BP21 is OK if you already have one. If not, I recommend the AX2213n; the new kid on the block from HK.]

the unique feature of the piranha is it's thin symmetrical kfm4 airfoiled wing, which makes it essentially stall proof whether inverted or upright, and allows it to fly very stable in wind conditions that could ground similar planes using flat plate or standard airfoils.

imho, if you are comfortable flying a 3channel trainer and are looking for the next step up, the piranha is hard to beat. even if you're an expert pilot, the piranha offers a relaxing & nostalgic sport flight on those windy days.

here are some specs:
motor: Tower Pro 2408-21delta
prop: gws 8040hd
battery: HXT 3s1300 20C
esc: OEMRC Pulsar 18
servos: [email protected] HXT900
rx: Hitec 05S
AUW (w/batt): 23.1oz

the following post includes a thorough flight report. latter posts will include assembly advice that i thought may be helpful to have at the beginning of this thread. since the build was somewhat rushed, i didn't take as many pictures as i would've liked. some pics show my plane out of building sequence (ie finished pics shown before the finishing step). sorry 'bout that.

video of the piranha's maiden flight (34MB)

Pirahna Beta (7 min 15 sec) (7 min 15 sec)

it was a little windy during the maiden. i flew with the dual rates at 70%, with my cg roughly 1/2" forward from what the plans show.


[edit: The original Piranha Beta airframe finally bit the dust a few weekends ago... it's time to start working on another one, with added reinforcements and PB3 foam.

My latest power system recommendation: Turnigy 2836-1000, 3s1250 30C, 1060hd prop, & 30A esc]
Last edited by Truglodite; Feb 29, 2012 at 12:57 PM.
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May 07, 2008, 01:40 AM
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Flight Test Report (second flight)

before the flight, i tried taking off in 4" tall thick grass & it just wanted to plant the prop. i ended up having to take it to the baseball infield area of the park to perform a ROG takeoff. i think shorter landing gear would reduce the torque and make it more useable on taller grass (that or just bigger wheels to roll over it).

it easily survived a WOT vertical dive with zero flutter. i gave it a full elevator yank at the end of the dive too and noticed what was probably no more than 2" deflection at the wingtips. i decided to take a risk & try this just to prove the structure can handle it. must've been 45-50mph when i yanked on the elevator. of course there was no tip stalling.

i also played with full rates an all controls. we get 15' minimum loops and a roll rate of about 180deg per second. i found knife edge was limited. perhaps with more power it would be better; i was only able to hold sustained KE flight during the first half of the pack... the last half of the pack my KE's started decaying to vertical.

you probably already noticed in that video that my bp21 wasn't sustaining vertical flight. this time i was able to bring it in to somewhat of a hover about 20' high. when the plane started to to slide backwards (again, lack of power), it was eerily easy to control! in fact kept hanging on to the prop until i was about 10' high, added a tiny bit of down elevator, and she gradually transitioned to FF with absolutely no snaproll!!!

i also tried stalling it. i was effortlessly able to guide it through fig8's with full up elevator and about 1/3 throttle. it was more stable than anything else i've flown near stall... seemed i couldn't get the darn thing to snaproll no matter how hard i tried!

i also did several circuits inverted. it definately handles evenly with either side up. imho it's a great second plane for that reason... you can't stall it even when it's inverted and it tracks well even if you don't coorditate turns with the rudder.

i was flying a large portion at or near WOT, noticed the battery softening on me after around 9min 30sec on my timer, so i decided to bring her in. the approach was so easy it brought a smile to my face. just ease in up elevator until the elevator stick won't go any further, and use the throttle & rudder to hold course. i did have to add more throttle than i'm used to for the flair, but it landed very slow... rolling about 5' on the baseball infield.

Last edited by Truglodite; May 07, 2008 at 03:54 AM.
May 07, 2008, 01:40 AM
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step #1: cutting monoblock & gluing fuse skewers

after cutting the monoblock out on my scrollsaw, i cut recesses for the fise side skewers and used PU to glue them in. i also cut out recesses for the ply reinforcements, which get glued later on. note that the front skewer and ply reinforcements are actually sitting 1/4" too low in this picture.
Last edited by Truglodite; Jul 16, 2008 at 01:56 AM.
May 07, 2008, 01:51 AM
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step #2: glue wing sections, spars, and balsa joiner

i glued the 1/4" square spruce spars to front of each wing half first, then glued the 2 halves together with PU glue & strapping tape (the direction of the tape fibers is important... they should run along the span, not the chord)). i then used medium ca to glue the 1/4" balsa joiner to the front of the 1/4" spruce spars. i used ca to speed things up, but pu works if that's all you have

in the pic you'll notice there is an optional 5" long 1/8" thick bamboo skewer recessed into the trailing edge near the at the root. it's there to help prevent crushing the foam with the wing rubberbands. once dry, move on to the next step.
Last edited by Truglodite; May 17, 2008 at 04:27 AM.
May 07, 2008, 01:59 AM
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step #3: glue kfm panels, LE skewer, & center section to wing

first i cut 45's in the LE & joined the kfm4 panels along the LE using strapping tape (fibers running along the span). then i glued both kfm panels to one side of the wing so they line up with the wing tips and the line drawn on the plan. i let both sides dry under light clamping pressure (i used a dictionary & a 4 soda cans for clamping pressure).

then i cut an 8" long 3/16" thick bamboo skewer, applied glue where it rests inside the LE the left wing half, folded the left kfm panel over the skewer, & glued the panel down to the wing. i used masking tape to temporarily hold it while i finished gluing the right side panel. to line the skewer up with the LE on the right half i gently bent the skewer to line up with the tapered LE. once both halves were folded & glued with the skewer embedded in the LE, i applied heavy clamping pressure to achieve a straight & solid bond (over a case of soda), making sure the kf panels lined up with the grey line on the plans.

depending on how you clamped it, you might be able to glue the center section to the wing while you wait for it to dry. i used PU and a bit of strapping tape to hold the center section on, but the tape is unecessary & you can leave it off if you wish. keep in mind, the center section goes on the bottom of the wing.
Last edited by Truglodite; May 07, 2008 at 03:24 AM.
May 07, 2008, 02:01 AM
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step #4: glue fuse sides and ply reinforcements to monoblock

this is pretty simple, but you'll want your wing to be fully dry so you can use it get a good fit between the ply reinforcements and the LE skewer. i found it easier to fit the wing on and glue the ply to the monoblock first, then glue the fuse sides. if you glue the sides at the same time, you have to prevent the wing from sticking to the glue that squirts out from the fuse sides.

make sure the wing rests perpendicular to the sides of the monoblock when the LE skewer is fully engaged with the ply pieces. also try to get the LE skewer aligned with the rear of the wing (not too high or low). it's a good idea to err on the high side, because you can always cut & sand the slots to lower the wing LE later on. if the ply is too low... then you're in trouble.

after the ply peices are tacked up, you can glue the fuse sides to the monoblock.

DO NOT glue the side skewers to the fuse sides yet!!!
Last edited by Truglodite; May 07, 2008 at 03:31 AM.
May 07, 2008, 02:25 AM
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step #5: glue fuse decks, side skewers, firewall, and canopy

when gluing the top & bottom decks to the fuse, do your best to keep everything lined up & square, and don't forget to apply glue between the side skewers & the fuse sides. i simply cut a light v-notch where the skewers protrude through the top deck, and squeezed the sides together to make a tight fit.

i used 3/16" lite ply for a firewall. i tried clamping it so it included 2degrees right thrust and no up-down thrust. after testing, i think more right thrust was needed (will experiment with this), but zero up/down is good.

glue the canopy sides to the canopy monoblock & apply clamping pressure until completely dry.

DO NOT glue the canopy to the fuselage.
May 07, 2008, 02:26 AM
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step #6: glue elevator skewer, apply elevator hinges, and install stab/elev & fin to

i embedded an 1/8" thick bamboo skewer in the LE of the elevator using PU glue & masking tape. once dry, i removed the masking tape, primed the hinge line with UHU POR glue (no need to prime the papered surfaces), and applied my blenderm hinges. oh yeah, since the skewer is in the elevator i couldn't bevel it, so i had to cut a sharp angled bevel on the stab instead to allow full travel.

then i PU'd the stab square to the fuse sides, and let dry.

once dry i used tape to clamp the fin square to the stab.

[edit: note that my elevator seemed to flex quite a bit through the thin middle area even with the skewer installed. if i were allowed to use just one peice of of composite structurally on this plane, i'd use a piece of cf tubing to replace the elevator skewer.]
Last edited by Truglodite; May 07, 2008 at 03:45 AM.
May 07, 2008, 02:27 AM
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step #7: sand rudder hingeline in rear of fuse, install rudder, ailerons, and wingtip

since the rear of the fuse is 1/2" thick, and the fin/rudder are 1/4" thick, you need to sand a 45 degree bevel on the rear of the fuse to create a hinge line that lines up with edges of the fin. seems weird, but you'll understand what needs to be done when you get to this step.

once you bevel the rear of the fuse, install the rudder using strapping tape hinges. i didn't use blenderm on the rudder since it supports a tailwheel. of course, i primed the non-papered part of the hingeline with UHU to get a more permanent bond.

then i installed the ailerons using tape hinges. i primed with UHU & used blenderm on the ailerons.

once the ailerons were hinged, i glued the wingtips on with UHU POR (PU would also work here).
Last edited by Truglodite; Jun 26, 2008 at 03:33 AM.
May 07, 2008, 02:28 AM
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step #8: install servo horns, servos, and control rods

i used micro horns with 1/16"x3/4"x3/4" ply reinforcements. i used UHU to glue the ply to the foam, drilled a hole, then used PU to glue the horns in.

before gluing any servos in, fired up my electronics, centered the trims, and centered the servo arms (i also mark channel numbers on my wires at this point so i don't get monkey business later on ).

then i installed my aileron servos 1/3span out from the fuse, embedded in the wing from the top. they are held to the bottom kfm panel with UHU, and covered with strapping tape. i used UHU to glue my tail servos just in front of the stabilizer, but just behind the canopy makes it a bit easier to balance later on.

with the servos & horns installed & dry, i recentered my servo arms (i'm clumbsy) and made some cf control rods. the cf rods are light & very stiff for a more positive control response, but regular wire can be used if you wish (control rods are technically non-structural, so i took the liberty to use cf rods).
Last edited by Truglodite; May 07, 2008 at 03:46 AM.
May 07, 2008, 02:30 AM
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step #9: install canopy band skewers (or hinge), sand canopy & fuse

more than likely you'll be cutting fuse sides that look a bit different than mine. tony changed the sides so the canopy can be hinged if you prefer. since my sides were cut square near the canopy, i couldn't use a hinge. instead i used 2 peices of skewer & a rubberband to hold it on. the skewers need to positioned so the band doesn't slip off.

i sanded the canopy and the front underside of my fuse to a round profile using an oscillating belt sander, but sandpaper in the hand will work too. before continuing, i cut the fuse side skewers close to the fuse & sanded them flush. i used a sanding block to remove the little PU that squirted out of the fuse edges, and round the rest of the fuse corners.
Last edited by Truglodite; May 07, 2008 at 03:03 AM.
May 07, 2008, 02:35 AM
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step 10: ventilate & install hatches, mount motor, install landing gear

first cut whatever cooling holes you need to keep things cool, then install the hatches. i used plastic spoons for scoops. the exhaust for my esc hatch (top hatch) is on the side of the fuse. now install the hatches with tape hinges & a magnetic or velcro latch, whatever you prefer. since you probably won't need to get in to the esc hatch often, you can use tape instead of a magnet or velcro to hold it shut. i used magnets from my daughter's magnetix toys and 1/4" pieces of hacksaw blade to make latches for both the top and bottom hatches.

mount your motor. not much to say here. i made a 1/4" hole through the firewall to pass the wires to the esc.

i glued my main gear with a square of fff using pu. you might notice the little piece of wire i bent around the main gear near the fuse. it's there to prevent tearing the sides of the fuse out during "rougher than usual" landings.

my tail wheel is of the same i always use. it's only held on with a few pieces of strapping tape. if you bend it so the wheel is directly underneath the part that hooks under the rudder, it makes for a simple shock absorbing & robust tail wheel.
Last edited by Truglodite; May 07, 2008 at 03:06 AM.
May 07, 2008, 02:44 AM
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step 11: install electronics, wing band skewers, and battery mount

now for the finishing touches. besides the firewall hole, there's 2 more holes that need to be bored through the monoblock to pass wires through. one goes from the rear corner of the esc bay to the battery bay, and the other goes from the rear of the battery bay to the rear of the block into the tail area.

i located my rx in the rear of the battery bay, since it's a central location and didn't want it to get in the way while taking the wing on & off. i needed 2x 9" extensions to go from the ailerons to the canopy area, then 2 more 9" extensions to go from the canopy to the rx. i used 2x 12" extensions for the tail servos, and a 6" extension for the esc. using this setup, you can see i have very easy access to the aileron plugs when i need to R&R the wing. (see pic in step #4 above)

note that i had to splice in an extra 6" or so of 18ga wire to get my esc battery wire to the batter bay. it was easiest to solder the deans connector on after the battery wire & esc servo wire were already "fished" through the monoblock.

i installed the wing band skewers using a method i just came up with. using my combo square as a guide, i pierced about 1/2 way through each fuse side. this resulted in much less internal tearing that without the square. you might have noticed in an above picture i added some strapping tape on top of the kfm panel where the band goes. this is overkill, or possibly not necessary.

my battery mount is somewhat industrial by foamy standards, but i wanted to be sure the battery didn't fall out when i pull up hard during a recovery. i made it using 1/16" ply with velcro straps wrapped underneath. i added a strip of velcro on top to keep the pack from sliding back & forth, then used PU to glue the whole assembly in the battery bay.
Last edited by Truglodite; May 07, 2008 at 03:26 AM.
May 07, 2008, 03:10 AM
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sorry i've taken so long to post this. i started way earlier tonight & had some troubles with my isp along the way. i'm out of town until sunday, so i might not be able to respond until then. hopefully by then tony will already have the plans posted & some of you guys will already be finished building some more piranha's.

Last edited by Truglodite; May 07, 2008 at 03:29 AM.
May 07, 2008, 06:41 AM
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Thanks Kev

The design of the Piranha began when I started getting requests for a full aerobatic monoplane, capable of all standard aerobatic manoeuvres. I began by setting out my design criteria:
1. It had to be simple to make.
2. It had to be sturdy, but light.
3. It had to use inexpensive, common components so that those on a budget could afford to build and fly it.
4. It had to fly very well

With those design goals in mind, I began design of this aircraft in September/07. I used the tried and true monobloc method of construction. This system had served me well for several other aircraft and fulfills the first two design goals.

For those not familiar with the monobloc method of construction, it consists of a solid block of foam, precisely cut, that forms the forward portion of the fuselage. This block serves several functions. 1. It provides the forward part of the fuselage with a super rugged construction, easily capable of withstanding the normal bumps and bangs of RC flying. 2. It serves as an anchor point for all critical components. Motor mount, landing gear, radio gear, and battery are all housed within the monobloc. 3. It serves as the principal alignment point for all other parts, permitting the builder to easily ensure the aircraft will be built straight and true.

The design power system is the BP21 motor, also known as the TowerPro BM2408-21. This is a very inexpensive motor system, sold everywhere. For example, as of this writing, the motor, esc and prop are sold through Hobby City for a mere $14.49 USD. The Piranha also utilizes four inexpensive 9 gram servos, also commonly available at low prices. Hobby City currently lists those servos for a ultra-low $2.95 USD each.

With the equipment, construction methods and design goals determined, the next step was to establish the design parameters for the flying characteristics. For the aircraft to perform all necessary aerobatic manoeuvres a fully symmetrical airfoil was desirable. The standard fully symmetrical airfoil however, left much to be desired in regards to stall characteristics and speed envelope. Flat plate airfoils, while simple to build, have serious structural issues when built this large and aerodynamically, do not have good energy retention or lift.

I make no bones about being an unabashed fan of the Kline Fogleman (modified) airfoils. Many of my designs utilize the magnificent KFm3 airfoil and I considered this airfoil for use in the Piranha. After all, it does have an impressive track record. I felt, however, that the aerobatic design parameters required a different section to perform well. The KFm4 fulfilled these parameters nicely. This airfoil is a fully symmetrical KFm, featuring a step both top and bottom at the 50% chord mark. Additionally, the construction of the KFm4 permitted solid sparring with inexpensive spruce stick and balsa sheet.

My earlier testing of this airfoil section had revealed a wide speed envelope, mild stall and obviously, excellent inverted performance. Although this airfoil is not used as often as the more prevalent KFm2 and KFm3, I felt it had much to offer the Piranha.

As this aircraft was to be a aerobatic park flyer, I felt the aircraft needed lots of wing too. Total area is 450 square inches, plenty for light wing loading with minimal drag. Note: The CG marked on the plans is for aerobatic use. Move the CG .25” - .5" forward for your maiden flight.

Generous tail surfaces were deemed desirable to permit stability through all manoeuvres and large control surfaces allow very nimble performance.

The Piranha uses a simple wire undercarriage with a tailskid. While unnecessary, a tail wheel may be fitted.

The Piranha is not a beginner’s aircraft. Although construction is very simple and parts count is low, its flying characteristics require previous experience. It is a perfect choice to progress to after mastering the Blu-Baby series of trainers (both rudder and aileron) and will make an excellent third aircraft.

I would like to thank Kev (Truglodite) for the magnificent job he did in the development and Beta testing of the Piranha. He is a truly skilled builder and pilot and I could not have developed and posted this aircraft without his assistance.

I would also like to thank Dick Kline for his counsel and assistance in the development of the KFm4 airfoil section used in the Piranha.

The Piranha

Span: 44.5”
Weight: 20 oz RTF
Area: 450 sq in.
Airfoil: KFm4 fully symmetrical
Materials: ¼” Fan-Fold Foam or equivalent, 2” pink of blue foam, ¼” spruce stick, ¼” balsa sheet
Power System: BP21 or greater
Last edited by Tony65x55; May 07, 2008 at 08:49 AM.

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