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Jul 14, 2005, 11:41 PM
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DCobra's Avatar

F-20 Tigershark parkjet (Jetset44 T-38 Mod)


For those of you that haven't followed the F-20 redesign of Jetset44's T-38 plans in the T-38 parkjet thread, there's a new parkjet tearing up the soccerfield skies.

After about 3 weeks of research, playing with CAD, drawing, re-drawing, and building.... I maidened the F-20 today, with great success

Here's how it went...

I put the finishing touches on and set up the radio before leaving the house. I was a bit rushed to maiden, so some things are jury rigged a bit. The cockpit is held with tape, and 4 quarters are taped to the nose to ensure good CG balance. Tailcone held with tape as well. A final weigh-in is done, to see what the difference of the newly installed centerline tank and the 4 quarters of ballast had added. With my stomach in knots over the prospect of throwing 3 weeks of work into the wind, I leave the house.

I take my F-15 parkjet along as a security blanket. After all, it's the only plane I've owned longer than 3 months that wasn't beat to a pulp or in pieces. I figure some quality time with the Strike Eagle will get the knots out of my stomach, and loosen up my fingers.

I remember to bring a camera, after all, this may be the last time I see this sweet plane in one piece. I pose the plane in the grass, and as I lean it against the car to get a shot of the fuel tank, the camera battery dies. Oh well, it held up all through the build taking pictures.

I get everything sorted out, and get the F-15 in the air. After a minute or two of flying, I find myself bored with the familiarity and reliability of my trusty F-15. Besides, it's starting to get dark, so I land uneventfully as usual.

Now my stomach starts to knot up again. I go through the familiar routine of range checking, and visually checking that nothing was damaged/ changed while carrying the plane to the field.

Now it's do or die time. I grab the centerline tank, and cock the plane back over my shoulder in my throwing hand. I slowly ease the throttle up near halfway until I like the sound the Himax, gearbox, and prop are making.
I'm almost tempted to squeeze my eyes shut, after all... my last two maiden flights went down in flames....and the planes sit repaired....collecting dust as reminders of those failures... with problems waiting to be sorted out when I come back to them.
I think to myself, "I'm gonna toss this one so hard it can't help but climb out."

And the thought is very reassuring with the solid feel of the centerline tank in my hand.

I throw!

And she actually FLIES!

But she's sinking... gone 20 feet out, but losing the 6 feet of altitude quickly!
I stab for full throttle, and the Himax roars to life with a high pitched whine!
I gently pull back on the elevator...don't want to overdo it and stall!

She's clawing for the sky now...picking up speed and altitude, but wanting to roll to the right, and wanting to pitch forward if I ease off the elevator.

I fumble for my trim tabs, and hear the reassuring beeps of my Eclipse 7. stick.... straighter now but not where it needs to be. I turn left, and make my downwind run compensating to keep her straight and level.
I make another left turn to head back upwind. Let's punch it and get some altitude to get this baby trimmed.

Ok a couple clicks here, and a couple clicks there. Ah now thats much better.

And she's FLYING!

Lets do a couple more racetracks around the field. Wow...she likes the speed but she doesn't like to glide much.

Ok...I seem to have a grasp on her tendencies...lets try a few axial rolls and see what all the T-38 and F-5 guys are bragging about. Ah nice.....much faster than my F-15 and doesn't want to drop the nose! Lets try it on aileron high rates...ooooooh ahhhhh.

Ok it's starting to get dark, and this baby moves too fast....I don't want to see her go down over mis-orientation.

Let's line up for final....ok she's lined up and starting to sink...lets drop throttle and glide her in a bit....WHOA 6 feet off the deck, and way short of the runway! Hit the afterburners...climb, climb, climb!!!!

She comes in waaaaay HOT. I don't think the same throttle management for landing the F15 is going to work here.

Ok....I could drop the flaps, but that always makes my ailerons mushy on the F-15.
Let's come in with no flaps... better to check the flaps out next time at 100 feet.

Ok, were lined up again....gonna ease off the throttle, and wait until the last second to cut it and flare. Ok...ok...coming in...looks good...OH NO OVERSHOOT! And right toward that huge puddle!
Ah touchdown....and cleared the puddle. ONE PIECE! WOOHOO!

I hope you guys enjoyed being right there with me.

More pics and video coming soon!
I apologize for the poor picture camera phone and bad lighting. Much better pics coming as soon as I can upload them.

Link to parts templates:

Much thanks to RCParkflyer for providing hosting of the files on his

Last edited by DCobra; Oct 10, 2005 at 09:42 AM.
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Jul 15, 2005, 12:15 AM
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DCobra's Avatar
Here are the current specs, and this might change as I rework some things I don't currently like.

Airframe constucted from FFF, CF rods, aluminum tubes (elevator rod sleeves), Gorilla Glue, UHU Creativ, and Scotch Satin tape. Blank CD used for aileron control horns, scrap plastic used for elevator control horn. Styro coffee cup used for exhaust can.
Centerline tank constructed from craft styrofoam, FFF, lite ply, popsicle sticks, balsa, paper towel tube, and mounted with 1/4 x 2-1/2 inch nylon bolts with 1/4 nylon nuts.
Action Jack...your model rocket tank would have been much better. This was thrown together quick and dirty to get her in the air. Centerline tank weight...2 oz

Electronics used:
(3) GWS Naro Standard servos
Hitec Electron 6 Rx
Castle Phoenix 25 ESC
Himax 2015-4100 with GWS gearbox, 5.33 ratio and GWS 9x7 prop
3S Apogee 1570 mah pack
Approximately 24 inches of 18 gauge wire and Dean's connectors for battery/ ESC leads. All leads twisted and wrapped in foil.

Some flight observations:
It may be a tad too nose heavy. I'm going to try taking away the 4 ballast quarters one at a time and see how it flies. I tend to prefer nose heavy flight, so we'll see where it ends up, but I'd like to shed some weight so I can add some paint.

Current weight is +/- 21 oz. Not sure of exact weight because I was in a hurry, and it's hard to weigh with the tank mounted.

I still have alot of sanding to do, but I also want to add some reinforcement tape to the wing edges, and possibly a coat of polycrilic to stiffen the wings and vertical. I'll also be using polycrilic to smooth some of the sanded areas.

Centerline tank will be redone, hopefully lighter. I'll be adding missile rails, and possibly missiles made from straws, balsa, etc.

Hopefully, I'll lose some weight before she's completely done, if not I'll probably re-gear the Himax with B gears and APC 9x6 prop for more thrust/ speed.

Jul 15, 2005, 01:13 AM
CAP 232-aholic
DiabloKid's Avatar
nice work there Paul! Sounds like you had fun

any chance of getting plans on this one?

Jul 15, 2005, 08:03 AM
Living a Dream
Hans-Joachim's Avatar
Congratulation to your F-20 Maiden!!

Thats a very nice flight report.
Ive enjoyed being right there with you!!

Keep on the good work, Paul.


PS: Im waiting for more pics and a video!
Jul 15, 2005, 08:12 AM
EB-66C Team Member
J Morgan's Avatar
Great job Paul! You described the exact feelings I ALWAYS have on a maiden.

Latest blog entry: Ziroli A-1H at 100"ws
Jul 15, 2005, 11:34 AM
Registered User
Action Jack's Avatar
Congratulations, Paul! Stories with happy endings are always good.

Not sure how much weight you will save by using rocket parts to build your centreline tank. Those plastic nose cones are heavy. They look nice though.
Oh vanity always comes with such a heavy cost.

Now get some paint on that bird!

Jul 15, 2005, 02:18 PM
Park Jet Guru
jetset44's Avatar
Now this has got to be one of the most exciting maiden flight stories I've ever read! I feel like I was standing right beside you the whole time. CONGRATS Paul!

Jul 15, 2005, 10:05 PM
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DCobra's Avatar
DiabloKid - Thank you! Chance for plans? Plans are in the works! I tried to keep plenty of documentation while I was building so that I could put together some plans and possibly an assembly guide if anyone was interested in building this. Of course, I got carried away with building a few times and forgot to take a picture of a step, or forgot to trace a particular part on paper to scan in later. I'll be recreating everything, modifying what I think could have been done better differently, and filling in any gaps over the next few weeks until I feel I have a set of part templates and an assembly drawing worthy of posting. I'd estimate as soon as a week, and as long as 3 until I'm ready to share with the community. Thanks again!

Hans-Joachim - Danke! I was glad to be able to share the experience, and wish you guys could have been there! A few months ago, I met another flyer at the park I fly at and was hoping he'd be available to take inflight pictures or video. Unfortunately he wasn't available, and having a fellow Ezoner there to hold the camera would have been great! Of course I would have loved to have an Ezoner there to high-five, but my little boy made up for it with his excitement

J Morgan - Thank you! I'm glad to hear you still get the butterflies on your maidens. I don't think I'd want to be in the hobby anymore if I didn't get excited over it!

Action Jack - Special thanks for being the inspiration behind the centerline tank. It was worth it's weight in gold when I came in hot, and I'll definitely be improving it with the rocket nose cones, even if it is just an aesthetic improvement. The tank really made it possible to put the stabs and wing in very close to scale positions without risking damage everytime it lands. Paint is coming soon...I'm checking out some airbrush options because I have a special scheme planned for this one. Thanks again, I owe you one!

Steve - The most thanks is due to you, mostly for inspiring me, but also for allowing and even encouraging the modification of your T-38 design into the F-20. It definitely made it easier than a ground up build would have been, particularly with the sparse amount of F-20 info, pics, and 3 views on the net. In case you were wondering, the forward fuse and the stabs were unchanged, everything else got a tweak to some degree, but tweaking from a baseline product made things ALOT easier. If you had told me last July that I'd have flown (2) F-18's, an F-15, and designed an F-20 successfully in a years time, I would have suggested rehab.
Thanks again for not only inspiring me, but for bringing my building skills up to a level where I can start taking on the challenge of design as well.
[EDIT] Oh yeah Steve, before I forget because I have before...the foam bending technique I used was largely inspired by your technique on the F-14 build. I just took it to an extreme, so really the credit for heat gun and depron/ FFF is due to you.

Last edited by DCobra; Jul 15, 2005 at 10:32 PM.
Jul 16, 2005, 09:13 AM
Giddy up!
Sal C's Avatar
Nice work DC! This one definately looks like a candidate for edf, I see Minifan 480 written all over it...
Jul 16, 2005, 11:49 AM
Living a Dream
Hans-Joachim's Avatar
Here is a willing camera man, but 6928 km away

What about the motor/prop offset to wing and elevator in thrustline?
Do you have to trim the elevator in normal (park like) flight?

Jul 16, 2005, 12:54 PM
Registered User
congrats paul!
that looks absolutely awesome! can't wait to see the plans and your input on how you did the foam bending. i am working on a similar foam bending for my next project... any help/insight would be invaluable.
i know what you mean about butterflies in the stomach... esp when designing your own plane from the ground up
what paint scheme are you planning on?
Jul 16, 2005, 01:37 PM
CAP 232-aholic
DiabloKid's Avatar

the wingtips need missiles!!

Jul 16, 2005, 04:15 PM
Live to fly, fly to live!
DCobra's Avatar
Sal C - Thanks! As soon as I saw how large the intakes were, I was thinking the same thing. I took it to the flying field to show off before it was flyable, and my friend there suggested a fan as well. I've got no fan experience though. I'd definitely like to try it on this plane, but would build a new one so I could eliminate some weight and possibly use retracts. Maybe when the plans go up someone can lead the way on a fan conversion

Hans-Joachim - I think I got the motor/prop offset very close to perfect on this one. I put the motor mount stick a bit lower than center because I didn't want it too high above the wings. Something I read in DAF's posts about his 42" WS F-4 led me to this decision. I also believe I had thrustline issues on the previous two planes I built, 3DFoamy's X-29 and Stefan Lamers F-16. In both cases I believe the thrustline was off due to my fault, not the design. Both planes would handlaunch then roll left and go into the ground...I think it was a combination of torque roll and downwward thrust pushing the tail up and the nose down. Both of those planes are going to come off the shelf soon and see if I was right. Thanks for offering to film

Beanie - Thanks! I'll be putting some pics up very soon of the foam bending, but here's a quick and dirty description. I've been using a 3/4 inch dowel, wide masking tape, and a heat gun. The tape isn't needed if using FFF, just leave the protective film on the outside of the bend. What I do is just work it slowly with the dowel, and when I get it to the shape I'm looking for I tape the foam edges together in a way that it will hold the shape. Then I use the heatgun on it on both sides. I just try not to get close enough to do any melting, but if you do melt it slightly on the surface it's no big deal. I leave the tape for a few minutes while it cools, then remove the tape and the foam holds shape very well. Another tool that would be helpful is wider length of PVC pipe, I'm going to try that soon....the 3/4 dowel is a bit narrow.
Paint scheme...I was going to keep this hush-hush but here's what I decided on in the pictures. The spotted "Tigershark" camo looks to be a winner.

DiabloKid - Oh yeah...they look pretty naked with no rails on 'em at least! If I put rails, then I might as well put armament out there as well!


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