Official Freewing JAS 39 Gripen 80mm EDF Jet

[janmb]

Wow! You’d think some people are getting paid by the word

Or maybe they just have a burning commitment to the subject matter and like to share their competence? In this case, for once, with a positive result as well.

[Pogo]

My Gripen is on it's way (sort of) but between the U.S. postal service and Canadian customs I expect several weeks wait, sadly. (Just couldn't bring myself to do Fedex.) Flying here is pretty much done for the season anyway...In the meantime I have some aero questions:
I can't get my head around how having a gyro on the canards (in rate mode) will do anything but stabilize the jet in turbulent conditions (assuming the elevons are also stabilized). I've used EagleTree stabs in my jets for years, usually only on the roll axis and always in "rate" mode only and plan to do the same with the Gripen, the canards in pitch only. I suppose "heading hold" mode might get the canards acting more like the full size in high alpha flight but don't know (?).
Secondly, seeing as how the full size Gripen has 2 control surfaces per wing, would there be any advantage to doing so with the model? Could the inner surfaces be used as flaps I wonder? - video seems to show them up in crow on landing however. It could add some interesting possibilities..
Lastly, for those of you with the model, is there any chance of adding speedbrakes?- is there physical room for small servos to be buried just forward of the panels, maybe down low near the wing root?
Tom

[F106A]

I’m going to do a repaint.
Do you remove the markings and decals.
Seems that trying to remove them will cause a lot of damage that will have to be repaired later.
Thanks,
Jon

[Pogo]

I’m going to do a repaint.
Do you remove the markings and decals.
Seems that trying to remove them will cause a lot of damage that will have to be repaired later.
Thanks,
Jon

Jon, I've repainted a couple of FW jets and done it right over the existing paint and markings (as have many others on here). The fact that they're waterslide decals and therefore thin render them invisible once painted over. Latex house paint, Krylon enamels and Testors and Tamiya paints all work well with no compatibility issues.
Tom

[Airguardian]

Do you remove the markings and decals.

I heard some did it with mineral spirits?
Not sure about this, do search for confirmation!

Or maybe they just have a burning commitment to the subject matter and like to share their competence?

"Burning commitment"... like how that sounds I think it fits!

In the meantime I have some aero questions:
I can't get my head around how having a gyro on the canards (in rate mode) will do anything but stabilize the jet in turbulent conditions (assuming the elevons are also stabilized).

If you *only* drop the gyro in, then it will do precisely that, as you suggest, for the most part.
The trick is moving CG backwards in addition to using the gyro. No need to get into tail-heavy realm for it to work the magic, but you want it pretty balanced ideally.

It also depends on particular gyro control algorithm. Supposedly, many gyros should have as 'setpoint' the angular rate at which the plane turns in a particular axis (in practice I think it's a bit more complicated) so the gyro would interpret pilot input as a certain amount of º/s in turn rate. When you comand the jet to 'pitch up', the whole aerodynamics of the jet and the elevons on the tail overpowering canards will set the plane to overshoot that setpoint, so the gyro will 'reduce' the angle of attack on the canards to match the commanded turn rate. This yields a seemingly pitched down canards.

Besides, with a balanced CG you can add a certain amount of 'pitch-down' offset built in, so the canards will tend to balance about a more 'pitch-down' position that benefits the handling of such an aircraft, by virtue of redirecting airflow in an energized fashion over the wing.

You can also turn off the pitch to canard mix so the canards will fight your inputs (without overcoming them) and exagerate this tendency, but I would not use this as the default flight mode, but rather program it to be switchable.

I suppose "heading hold" mode might get the canards acting more like the full size in high alpha flight but don't know (?).

I'd stay away from heading hold on aircraft. Can be used but it is tricky and may not produce the expected results. Direct rate stabilization is better suited to fixed wing aircraft, IMO.

Secondly, seeing as how the full size Gripen has 2 control surfaces per wing, would there be any advantage to doing so with the model? Could the inner surfaces be used as flaps I wonder? - video seems to show them up in crow on landing however. It could add some interesting possibilities..

Sure, would require some modding... but you'd have extra control authority and could do flaps (with canard compensation) as the fullscale jet.

Lastly, for those of you with the model, is there any chance of adding speedbrakes?- is there physical room for small servos to be buried just forward of the panels, maybe down low near the wing root?

I think so, but again, lotsa work to do! :)

[Alpha]

Speed brakes would be something I'd enjoy seeing someone attempt. There isn't a lot of space in there, but with a conformal door it *could* be done. If it would help, I could share some images from the CAD of what it looks like under those areas.

[xviper]

I think full (high rates) down elevator and down canards would make for very effective speed brakes after touch down.

[TangoVector]

I think full (high rates) down elevator and down canards would make for very effective speed brakes after touch down.

Or just prior to touchdown for maximum effect, at least once

[MountainRC]

Here she is all done!!! Came home to her on the deck and had to get right at it putting her together everything went together with out problems only thing is when pulling rudder wire through to front go under the Ruder mounting bracket the direction show both FYI everyone and only thing I had to do was align the canards. Safe flying everyone shes a real nice bird tomorrow’s the maiden...

[TangoVector]

Hi guys, this is a hobby and it’s for fun. If you don’t find in depth background information about the full scale Gripen interesting or fun, just use the scroll-function on your device and keep enjoying this forum.

If you’re reading this, then hopefully you might learn and/or appreciate something from it, so here goes…
I found this dedicated site about “A journey of change in the aircraft industry” that is a description by Saab how they have been working the last 15 years. In their own words: “A Journey of Change in the Aircraft Industry focuses on military aviation and the Gripen fighter aircraft system, as well as operations at the Saab Aeronautics business area.”
And: “With A Journey of Change in the Aircraft Industry, Saab wishes to inspire others to initiate and conduct change management, regardless of organisational type.
The target groups for the publication are customers, governments, armed forces, politicians, government agencies, industries, partners, suppliers, research institutes and academia.”

The website is not with lots of formulas, but quite in depth anyway.

If this feels exciting, head over to https://saabaircraftindustry.com and knock yourself out.
If the part of aerodynamics is more of an interest, the “good stuff” I’ve found so far are at https://saabaircraftindustry.com/en/.../#avsnitt-4267

If following links is too much of a chore and you just want the meat of it, here are some cut&paste from the site that I with my perspective as a hobby model pilot find interesting (maybe biased by working full time as an offshore helicopter pilot and previous helicopter aerodynamics instructor, anyhoo)…

Everything about the Gripens design is to make it light and agile. By using a canard they could have positive force from canard, wing and trailing edge elevons compared to the wing on a normal delta having to produce enough lift to also handle the negative force created by the elevon. This lead to that a smaller and thereby lighter wing could be designed, still giving the desired amount of lift. By actively using the canard, the servo-speed and size of servos could be lowered and thereby made lighter.

About how the Gripen is controlled in pitch:
“The canard is controllable and quickens the aircraft response speed to the desired level. The trailing edge control surface provides the main aerodynamic moment for the aircraft’s rotation in pitch and roll whereas the leading edge flap ensures that the airflow over the wing remains optimal at all times. Differential trailing edge control surface command results in a roll.”
With authors note:
“The Gripen is controlled using both the canards and the trailing edge control surfaces in order to reduce the speed at which the control surfaces need to move when carrying out different manoeuvres. The canards speed up the aircraft’s response in a pitching command from the control stick. At subsonic speeds, the canards and trailing edge control surfaces are used more or less to the same extent; at supersonic speeds, the trailing edge control surfaces are used much more.”
Want to know more and why? Follow the links above!

Something else that made me go “Wow, cooool” is the fact that the statically unstable Gripen actually is stable when the AoA exceeds 55 degrees! So when flight-testing and to see how the FBW-system was able to handle a spin, the aircraft was flown to an AoA exceeding 55deg where the system was bypassed and the pilots inputs went straight to the control-surfaces since the aircraft is stable in that flight-regime. After putting the aircraft into a spin, the FBW system was reconnected and it returned the aircraft to the normal flight envelope. There is a nice graph of the pitch moment as a function of AoA that I didn’t understand, mostly because I haven’t seen one before and don’t really have an interest of seeing one again, but it did look cool.

There are lots of interesting stuff about the development of the Gripen, so follow the link if you want to. There are two versions of the site, one in English and the other in Swedish if preferred. I’m happy with what I’ve learnt and if I have misunderstood anything or everything, I want to continue my life in ignorant bliss.

Cheers!
/Tobias

[Airguardian]

I think full (high rates) down elevator and down canards would make for very effective speed brakes after touch down.

Yes... but I'd be wary of tip-over considerations after having a tip-over myself on the third flight... which got reduced to just a take-off attempt.

And BTW, one of the struts went flying, the grub screws don't come 'loctited' from factory, be careful with that.
Another strut (or the same?) got bent in the process. There's a point of the strut that has four drilled holes, considerably reducing strut cross section so they bend rather easily apparently at that 'weakened' section. After being bent, the gear door would not close. Dismounted everything, straightened the strut with some 'percussive persuassion' and now it's all good again.

Here go some pics to show what I'm talking about:

Bent strut at section with four perforations.



All good again after 'persuading' the strut with the right tools

[Airguardian]

If the part of aerodynamics is more of an interest, the “good stuff” I’ve found so far are at https://saabaircraftindustry.com/en/.../#avsnitt-4267[/COLOR][/FONT][/COLOR][/FONT]

Good find, thanks for sharing!

[Warbirdfan29]

The first upgrade part...

Hello Radar-Guy, is it possible to get the file? Kind Regards warbidfan29

[Warbirdfan29]

Hello James,

this is for you. The Gripen was painted in the "old Saab Viggen" colors.

[Warbirdfan29]

I like "splinter camo" paint schemes. I´ve repaintet my Freewing 70mm F-16 C Falcon V2 into the unique BDU Green Splinter Aggressor colors. Nice!!!

[Mtpisgah99]

OHH BOY. i fly flybarless heli and setting it up the heli and GYRO is pain (swash plate and tail). however, heli's R&D is all in there but costly ($) electronics. this GRIPEN jet is going to be trial and error for me. but, if James can fly this gripen may be, may be i can fly it too. only if i stop thinking too much and just follow him, oh well !!!

[Pogo]

Speed brakes would be something I'd enjoy seeing someone attempt. There isn't a lot of space in there, but with a conformal door it *could* be done. If it would help, I could share some images from the CAD of what it looks like under those areas.

Yes please Alpha, should be helpful. I've just been looking at linear actuators that might be an option...
Tom

[Pogo]

"If you *only* drop the gyro in, then it will do precisely that, as you suggest, for the most part.
The trick is moving CG backwards in addition to using the gyro. No need to get into tail-heavy realm for it to work the magic, but you want it pretty balanced ideally.

It also depends on particular gyro control algorithm. Supposedly, many gyros should have as 'setpoint' the angular rate at which the plane turns in a particular axis (in practice I think it's a bit more complicated) so the gyro would interpret pilot input as a certain amount of º/s in turn rate. When you comand the jet to 'pitch up', the whole aerodynamics of the jet and the elevons on the tail overpowering canards will set the plane to overshoot that setpoint, so the gyro will 'reduce' the angle of attack on the canards to match the commanded turn rate. This yields a seemingly pitched down canards.

Besides, with a balanced CG you can add a certain amount of 'pitch-down' offset built in, so the canards will tend to balance about a more 'pitch-down' position that benefits the handling of such an aircraft, by virtue of redirecting airflow in an energized fashion over the wing.

You can also turn off the pitch to canard mix so the canards will fight your inputs (without overcoming them) and exagerate this tendency, but I would not use this as the default flight mode, but rather program it to be switchable."

Thank you J-Sandro- that all makes sense. I think you have experience with the Eagle Tree Guardian?




"I'd stay away from heading hold on aircraft. Can be used but it is tricky and may not produce the expected results. Direct rate stabilization is better suited to fixed wing aircraft, IMO."

Yes, wasn't really suggesting it's use- just noodling. I've tried "heading hold" in the past with mixed results.



"Sure, would require some modding... but you'd have extra control authority and could do flaps (with canard compensation) as the fullscale jet."

Wouldn't be so difficult and seeing as this jet seems to need weight in the ass end the 2 extra servos aren't really a penalty.




"I think so, but again, lotsa work to do! :)"

Indeed! We shall see. Thanks Sandro.
Tom

[kallend]

Hello Radar-Guy, is it possible to get the file? Kind Regards warbidfan29

+1 on that. Those are very nice.

[Radar-Guy]

Hello Radar-Guy, is it possible to get the file? Kind Regards warbidfan29

Great paint jobs! Here you go, I've added a hole to the center section to clear the axle and make it an easy fit to the wheel. Please note, that it's based on pics I found showing the Czech C model.