growingupisoptional

Another video on a windy day with a number of rough approaches.
Looks good.

http://www.youtube.com/watch?v=X8usNUJzJ1w

rcjets_63

My Feedback: (4)

As for the landing, and the several landing attempts, I think that repeated pilot error was a much bigger factor than the wind. On his first approach, he comes in too high and hot and runs out of runway. He aborts the landing way too late and nearly stalls the plane before the engine has a chance to hit full power. If he wasn't so high at the time (due to the magnitude of his screwed up first attempt) and the wind wasn't blowing so hard, my guess is that it would have hit the ground for sure. The other approaches weren't much better and for a while it looked like it was going into the pits or the fence.

However, I was impressed with the rotation part of the takeoff (at 1:20). This model seems to lift off the ground much gentler than I'd seen in other R/C Gripens. Maybe it's because he's only got a K-80 so he's not blasting off the ground like one does with a P-120.

killerG

My Feedback: (32)

I am thinking about getting the SM or JL Gripen. I have an older P-80 that only puts out 17lbs thrust. Would this be enough? What differences are there between the 2?
Thanks

joeflyer

My Feedback: (48)

The main difference between the two is weight. Skymaster has their ARF+ technology, the fiberglass is thinner and formers not as robust, making the airframe about 2 lbs. lighter. The Skymaster Gripen also has the main gear located about 1 inch forward of the scale location for smoother take offs. The fuel tanks are slightly smaller, about 72 oz. total capacity vs. Jet Legend at 78 oz.

On the other hand the Jet Legend Gripen is stronger, will take more punishment, and is a few hundred dollars cheaper. They also use aluminum rods for the canards, which don't flex like the Skymaster carbon fiber ones. I have owned both.

17 lbs. of thrust will fly the plane and is the minimum I'd recommend. The Skymaster one will accommodate a 14 lbs. turbine mounted in the tail without a tailpipe. That keeps the weight down, however from the videos I've seen flight performance is unimpressive. Personally I'd prefer at least 19 lbs. of thrust to fly it like a jet. I have a Jet Central Falcon in mine (23 lbs.) and it flys it well. I've seen them fly with a P-120 and a Jet Central Eagle and they're rockets.

If you're going to use your P-80 you'd be better off with the lighter Skymaster version. Do a search on Youtube and you'll find videos of Gripens flying with various turbines. There are also several links on Skymaster's website., You'll find a couple flying with a Kingtech K-80, which should perform similar to your P-80.

Joe

killerG

My Feedback: (32)

What weight does the SM come out dry, if you know?

SAAB 78

Joeflyer has made a very good summary.
To my opinion, the Jet Legend has an important avantage to my eyes, because you can buy the kit version, and this is the best way to have a light but strong jet when you are an experienced builder. ARF from China are most of the time heavy and not really made with the "state of the art" if you know what I mean...
So my JL with a G_Booster 80 (up to 20 lbs) fly really well and is very fast (see my video link herebefore on this topic). But in summer time I need 300 ft of perfect runway to take-off and more from a grass strip. However the GB80 (also called Evojet 90 in US) is a very good and light engine. My Gripen is 11Kg dry, that is light if you take into account the scale cockpit, missiles, external tanks, gun, LG doors, all lights with separate battery... You can save weight with Li-Ion batteries, Jetronic vent valves (30gr each!)...
The P80 is a good choice if you do some effort to save weight...
After you know the history... More power= engine heavier (often...)=more fuel= A/C heavy and fragile for landing...
The Gripen is a very good jet if you know how to land with the nose up.... in the contrary, be sure you can use a verrrrry long runway for the first flights! :-)

SAAB 78

All my missiles are now installed with fuel tank... hope it will take-off with my 20lbs engine!

rcjets_63

My Feedback: (4)

That looks friggin' awesome. Well done!!!!

Jim

SAAB 78

On the runway today

rcjets_63

My Feedback: (4)

Enough of the photos on the ground, let's see some photos of the Gripen in the air.

I've been pretty busy building a couple of jets for other folks and haven't had time to work on my Gripen. The next step was to lay up the fibreglass tanks but I know that I'll need to vacuum bag the tanks while the resin cures. Well, the $300-$500 price of a vacuum bag system was a bit of a demotivator, but I found a low cost solution at the supermarket.

Ziploc has a vacuum bag system that is perfect for vacuum bagging. The starter kit comes with the hand pump and 3 quart-sized bags for about $5. The quart size bags are likely too small but the gallon-sized bags (3 for about $4) are perfect for fuel tanks.

I did find that the bag leaked slightly overnight. My solution was to insert a thin bead of plumbers putty just below the ziploc seal. After vacuuming the bag, I put a bit of putty on the cutout for the pump. This worked great and the bag has kept a tight vacuum for a couple of days.

feketerc

Dear Visitors!

We are glad to see that our Gripen model video has been
appeared on the RC.Jets forum.

Here are the details of the model:
- JAS-39 Gripen B/D two seater.
- Scale: 1:5,5
- Body length: 2,69 m
- Wing span: 1,53 m
- Weight: max. 20 Kg

As this model has been made into the F4J scale jet class world
masters, this model has to perfectly match the original fighter. The
model had been built by this goal.

All the functions are working as the orginal functions on the real fighter.
So the flaps, canard, airbrakes, gears, cabin opening, full light system
with position stripes and so on...

We are over a 32 probe stars, and more needed, however the flight behaviour
is quite good!
Planned speed interval is: slow 50-60 Km/h, fast 250 Km/h.

The basic idea of building and creating this model is originated from Janos Horvath
whose company also built a really good turbine to this series. This it the HOFEM 180 N
turbine with 20-21 KN thrust on 118.000 rpm. Also the full landing gear system is made
in this factory.
The cockpit was built by Laszlo Lorincz who build other cockpit types for other planes too.
The body of the Gripen was planned and created by Jozsef Fekete in some thousands of
working hours around in 5 years.
Our helpers in constructing the model: Gyorgy Szabo, Tibor Kurilla, Lajos Molnar, Emil Arvai,
Laszlo Lorincz and many help comes from Sweden! Thanks for all of them.

The body is made of fully glass-carbon-cevlar + epoxi compozit.

Here you can see some photos about it.

More information is comming soon in this spring on the following website: http://jetmodell.uw.hu
(please note that this website will be updated very soon this year)

If you have questions, please contact us.

Janos Horvath - Jozsef Fekete - Miklos Alacs

rcjets_63

My Feedback: (4)

feketerc,

Thanks so much for posting the info and photos of the twin seater. It's awesome with all the scale details, particularly the nose gear door linkage; I don't doubt all the hard work that must have gone into the plane. Amazing!!!

As for the ongoing flight trim and canard discussion, the idea of having about 5-10 degrees of up-canard for landing is interesting. It should help to keep the nose up during approaches and slow the touchdown speed but it's also scale per the photo below.

Regards,

Jim

rcjets_63

My Feedback: (4)

Applied the first layer of cloth to the tanks tonight. I'm told this will be a one week adventure as there will be 7 layers (2 x 3/4 oz and 5 x 6 oz). Using yellow ripstop nylon from the local fabric store ($5.99 for 2 sq yards) as a peel ply (thanks B1 Bob) and 1 gallon sized Ziploc vacuum bags. We'll see how it turns out tomorrow morning.

Regards,

Jim

rcjets_63

My Feedback: (4)

Fuel tanks are coming along and I'm starting to get the hang of vacuum bagging weird shapes (the flats are easy but the 3-D corners are tough). In the mean time, I bought myself a little present, a JetLegend Gripen basic kit. The reasons I chose this kit are:

1) the flight control surfaces use hinges (not live hinged). The live hinging didn't last long on my (and other) Skymaster ARF+ Gripens.
2) the formers aren't yet glued in. I plan on doing several modifications and they will be easier to do without the formers in the way. Also, I can use the formers as templates, substitute the wood, increase the thickness, or laminate some CF to each side to increase the strength (particularly for the gear mounts).
3) The price of the Jet Legend basic kit is fantastic. $530 which includes fuel tanks and exhaust duct + $250 shipping. The Skymaster kit is $1295 + $250 shipping + $59 fuel tanks + $95 exhaust duct = $1699. I don't know about you, but I'll gladly glue in formers and hinges to save $919. That savings alone more than pays for the landing gear.
4) The canards in the JL kit have aluminum torque tubes. SM uses unidirectional CF rods which flex under torsion loads. In some cases (previously examined in this thread) the amount of flexure is alarming.
5) the JL kit includes a cockpit tub.
6) The JL kit has the landing gear in the scale location which allows use of bigger main fuel tanks. While the scale gear location makes rotation a big more difficult, I'll be adding fuel tanks behind the C.G. which should reduce this problem.

Many thanks to Mike and the Global Jet Club. Great service and communication. The kit should be here in four weeks.

Regards,

Jim

SAAB 78

Do not worry about the rotation with the JL Gripen! With a few canard in positive position it is ok.
With the kit, you will have the opportunity to customize your Gripen, and build it in the state of the art.
Pay less, get more!

IdefixRC

Hi Guys,

signing in.
Just got my JL ARF Gripen and will start building soon (just spend a few hours cleaning all parts - looks like mine came from the batch just before the annual spring cleaning in the factory. ......)
First impression: WOW is this airframe heavy........

(Jim, thanks for the copy of the manual).

I plan to install the following:

- JetCat P80SE
- JL JAS-39 retracts, oleos, wheels and brakes
- Spektrum AR9100 Receiver
- Servos:
- Elevons: Hitec 7954SH
- Canards: Hitec 7954SH
- Rudder: Hitec 5125MG (Slim Wing Digital)
- Steering: Hitec 645MG

The Spektrum AR9100 and the 7954 High Voltage Servos will be powered directly from 2x 2S LiPo Packs.
I will use a Spektrum VR5203 between the Receiver and the 2 standard power servos to step down the voltage to suitable levels (still need to check if the 2Amp the VR5203 is capable of will be enough).

I plan to first fly the model without the doors and add them only at a later stage.

Has anyone experience with the JL Wing mounting?
Looks like the JL Gripen does not have the carbon rod through the fuselage but only the 2 metal brackets front and aft.

regards,
Eric

jephry

Hey Eric,
Congrats on your purchase! Still waiting for my issue to be solved.. But if I can't wait, then I will built it soon after my repair on my F15.
Anyway hope to be able to learn from your built along the way.
When are you planning for the maiden?
I have also plan for the same servos to be installed.
Here is my setup:

- JetCat P80 V6 or Sim Jet 1700SP (20lbs Thrust)
- Futaba Fasst 7ch RX
- Elevons: Hitec 7954SH
- Canards: Hitec 7954SH
- Rudder: Hitec 5125MG
- Steering: Hitec 645MG

IdefixRC

slowly slowly...... I need to build it first :-)

Rliebl

Hi all Skymaster/ JL JAS 39 Gripen owners.
I just want to show you my crazy project to convert this Jet to EDF with an XPS Dynamax Fan System.
The Video is from the Maiden flight with some small problems.
I hope you enjoy the Movie.

http://www.youtube.com/watch?v=KrTgJuGU9sA

rcjets_63

My Feedback: (4)

Reinhard,

Congrats, on your electric conversion and maiden. You mentioned its a Skymaster/JL Gripen. Is it an old Skymaster version (same as current version of Jet Legend) with the original gear location or the new SM version with the gear moved forward. Nice rotation on takeoff.

Good decision aborting the landing on that first approach, but I was surprised that you raised the gear and did a wheels up landing. What happened?

Welcome to the Gripen club. Regards,

Jim

Rliebl

Hello Jim,

thank you,
it must be the new version, i get it last year in September.
Well at the first approach i had a slightly power reducing, so i tried to land, but unfortunately the front Gear didn´t came out, that´s why i did a belly landing.

SAAB 78

Well... it works! Congratulations!
Noise is quite realistic for an EDF model. Which landing gear it is? Spark? I have one and I have to admit that it is very reliable.
Do not hesitate to put other videos links on the forum!

baua

Hello Everyone!
I bought my JAS 39 Gripen a few days ago. Its the Paul Grays design, that Jim Fox sold, for what i Belive 10 years ago. I have a few questions regarding this model. Is it possible to convert it from EDF to take a turbine instead. I like the model very much. The meassures areLength 70" and wingspan 43,3". Maybe some of you have converted one of these models into Turbine. I am a member of a Swedish Forum, and I get different kind of answers from them. Also putting in apic of my MBS model of the mightyJ37 Viggen that I am building right now.

rcjets_63

My Feedback: (4)

Baua,

Welcome to the thread (and the Gripen club). Nice model you've got there. It looks like the plane could be converted to turbine (P-60 sized) if you can get enough fuel into it. You'll probably want about 68 oz or even a bit more. If you mount the turbine as far rearward as possible and mount the batteries, ECU, pump etc in the nose that should leave most of the center section available for fuel. You shouldn't have to bother with full ducting or a bypass and that will also make it easier to fit the tanks.

I'd say your biggest problem will be getting fuel tanks. You might check out the tanks that are made by Jet Tech, Dreamworks, or BVM for other models. Perhaps you can find something that will fit. Worse case scenario, you can mold your own like joeflyer and I did for our Gripens.

Regards,

Jim

Knife Liddle

Great video of the XPS powered Gripen!
I see it was flown on 12s 6000mah. Awesome!

Did you use the stock intake ducts, or did you make custom ducting? If you can please post some pictures of your installation. Thanks