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Su-25 Frogfoot for slope

Looking at getting into PSS

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Robin Stevens16/02/2020 18:15:59
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22 forum posts
16 photos

Hi,

I've been thinking of getting into PSS for a while and could never find a model for a subject I was really interested in, so recently I've started to take the plunge and start to draw my own plan for a 1:8 Su-25 Frogfoot (about 1.8m span)

Su-25 in flight

The Frogfoot is a Russian ground attack aircraft of similar (but not quite the same) role to the A-10. I quite like the Russian designs and the Su-25 seems like a good plane to start with for PSS. The fuselage is almost rectangular all the way through it's length and doesn't really taper significantly so construction should be pretty simple.

Does anyone experienced with PSS have any recommendations on wing section to use? At the moment I'm thinking that the wing area should be sufficient with simply scaling down the design, no tweaking of the proportions.

As far as I'm aware this doesn't get modelled often, if there's anyone out there with another I'd be keen to hear from them.

Edited By Robin Stevens on 16/02/2020 18:17:23

Phil Cooke16/02/2020 18:58:30
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2647 forum posts
1851 photos

Hi Robin,

Great to see new interest in PSS flying - there's a lot of folk on here who I'm sure will offer advice and experience - what I would like to start by saying is you are clearly thinking along the right lines - the Su-25 is an absolute dead-ringer for a robust, scale soarer - it ticks a lot of boxes with an ample unswept wing and a relatively simple configuration!

I've seen a couple of examples on the slope over the years, Steve Griffiths drew and published a plan way back through Traplet then, now owed by Sarik and still available - only 42" span though - HERE

The full-size Frogfoot, like the A-10 was no speed machine so you dont need a slippery section - for a good all rounder I'd recommend something as old school as Eppler 205 for a model like this. You will be fine with a true scale wing in span and chord, its ideal - you'd get away with scale engine intakes and pods too IMO - others may prefer them slightly slimmed - but I'd go for it - again - this isnt built for speed.

Its a great subject - some superb schemes and loads of great details to consider modelling - great choice!!

Robin Stevens16/02/2020 19:44:41
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22 forum posts
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Hi Phil, thanks for the info. I had seen the Steve Griffiths plan on Sarik but without an image on their website I didn't pay much attention to it, looks a little on the small size for what I'm thinking but it's good to know what's out there.

As you say the Frogfoot isn't exactly a speed demon but it is pretty fast (around 850kph / 460kts). At the moment I'm thinking that the only concessions to scale I'll do is eliminating the weapon pylons to preserve the overall look of the plane, could always add them in later if needed. Nothing would be slimmed down for performance, the real thing can carry it's own weight in weapons and fuel so there should be plenty of overhead there.

In terms of construction I'm currently leaning towards a balsa stringer and former construction with a planked and glass skin. Currently 3d printing the CAD model (scaled down) I made yesterday so I have a good indication of whether I have the right shape.

Andy Meade16/02/2020 21:39:34
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2784 forum posts
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Nice subject, and a good size too! Iv'e been building it's fly-off-competition stable mate, the Ilyushin 102 for a couple of years on and off. All ready for glass now - she'll get there some day!

I think Phil is giving some great advice there, you don't want to fly her like a pylon racer, and she'll slow up nicely on the landing run with all the draggy bits. I'd be tempted to leave at least a bit of weapons rails in place, maybe with some magnets sunk in before glassing. That way you can add all the weaponry on for a big air day, or just for stlye points in the pits laugh

Good luck with the build and keep us all updated, please.

Martin Gay17/02/2020 12:58:16
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410 forum posts
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Robin,

I have a copy of the Steve Griffiths plan, and the build article to go with it.

I was also thinking along the lines of doing a larger version, so am very interested in watching your plan develop. If you need balsa/ply parts cutting I can do them on my CNC machine!

I tried to PM you but your profile is set to "Private".

Martin.

Edited By Martin Gay on 17/02/2020 12:58:54

Robin Stevens17/02/2020 22:10:14
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22 forum posts
16 photos

Will keep the pylons in mind, for the time being it looks like the fuselage should be draggy enough but like I said I can always change my mind later. The opportunity to have weapons all over it as variable drag/ballast is tempting though...

3d print of my CAD model is done (1:72 scale), I think I've got the shape and proportions right. The print is just as a test to make sure I'm happy with the shape before I start to take profiles from the model.

dsc_2509.jpg

Thanks for the offer Martin, there's someone in my club that's already offered the use of their CNC router but when I have the profiles and plan complete I can share the dxf files if 1.8m sounds about right for you.

As far as wing section goes I'd been recommended Eppler 374, which looking at airfoiltools looks like it's comparable to Eppler 205. However 205 has a flatter base which would be more convenient for building.

Had a look at different methods of wing detachment on the CAD model, I think having the port & stbd wings detach at the root is a better way to go than having a continuous detachable wing. The wings are very deep in the shoulder mount so there isn't much structure left if the entire wing pops off.

Andy Meade18/02/2020 08:55:34
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2784 forum posts
717 photos

The 374 is a good section too, for sure - very happy with it on my Sea Hawk. If it were me, I'd have the centre section of the wing built into the fuselage, and a two-piece plug-in wing. Leaves some scope for wing section tuning in the future, as well as a version with and without pylons / ordnance wink

Martin Gay18/02/2020 09:50:06
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410 forum posts
255 photos

Thank you, Robin. 1.8m sounds perfect.

E374 with 2 degrees of washout works very well on the swept wing Mass Build Sabre.

Andy Blackburn wrote a good article about the Jet Fighter as a PSS subject here:

**LINK**

Edited By Martin Gay on 18/02/2020 09:50:24

Edited By Martin Gay on 18/02/2020 09:51:39

Robin Stevens19/02/2020 19:45:28
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22 forum posts
16 photos

Going for E374 with the wing centre section built into the wing as advised. So far I'm around half way through planning out the fuselage profiles, hopefully will have something ready to be cut in a couple of weeks.

Thanks for the link Martin, interesting read with some things worth bearing in mind. Washout seems to be a key consideration, the Su-25 has a slight sweep to the wing so around 1 degree washout seems appropriate.

The plan is to have the fuselage being self-jigging where possible, the key advantage of CNC cutting to me is that accurate interlocking surfaces can easily be produced.

Martin Gay19/02/2020 19:56:15
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410 forum posts
255 photos

Robin,

Self jigging is definately the way to go.

Have you looked at the wing jigs we developed for the Sabre mass build?

They ensured correct sweep, dihedral and washout of the Sabre wings.

Lots of build blogs here:

https://www.modelflying.co.uk/forums/threads.asp?t=433

Robin Stevens19/02/2020 22:15:41
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22 forum posts
16 photos

The Sabre jigs are very impressive, definitely a technique I'm willing to copy.

The only thing I'm unsure about on the Frogfoot wing is whether to have the scale anhedral angle or to make it a flat wing, dihedral would look wrong so that's not an option

Harry Twist19/02/2020 22:39:21
357 forum posts
270 photos

Hi Robin, I would go scale anhedral ....or at least take a view... maybe somewhere midway between scale anhedral and flat? But not flat. The anhedral wing is such a characteristic of the Frogfoot it would be great to retain it.

Martin Gay19/02/2020 22:55:40
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410 forum posts
255 photos

The other way would to build the wing underside flat.

This would use the change in thickness from root to tip as the anhedral. It would look like a lot as we are used to seeing dihedral.

Andy Meade20/02/2020 08:38:26
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2784 forum posts
717 photos

The Antonov flies fine with anhedral - it's nothing to worry about, I'd stick it in, or go with section anhedral as per Martin's suggestion yes

Robin Stevens06/03/2020 18:51:30
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22 forum posts
16 photos

Just to prove I am still working on this I'll show my progress. Things have just been a little slow, real life always gets in the way.

Having a look at it I think Martin's suggestion of sectional anhedral is a good solution, but for the time being I'm concentrating on the fuselage. Most of the formers are about ready, I'm just planning out the stringers so there's a minimum level of fettling. The tail obviously needs to be planned out and the parts modelled.

frogfoot-profiles-1.jpg

Given that this is taking a bit longer than I'd hoped I'm thinking of doing a simple PSS model of the XP-79 so I have something to take to events and the like.

Martin Gay06/03/2020 22:47:38
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410 forum posts
255 photos

Next PSSA event is April 4/5 on the Great Orme.

I will have the buddy box with me so, weather permitting, you may get the chance to fly one or two PSS models.

It's also a great social event with lots of friendly banter as well.

Keep up the good work on the Frogfoot and don't worfy about how long it takes. I have a model that was three years in the making!

Paul Ashford11/08/2020 17:45:41
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24 forum posts
77 photos

This is a very semi scale model built over 30 years ago and was adapted from the Pavel Bosak plan for a 40 powered model that appeared in one of the mags at the time. It's not got the detail that most PSS models have these days but it does fly well. The wing is a thindown version as on the plan cut from foam and covered in balsa, blue foam was employed for the engine ducts. I have recently recovered the airframe in glass cloth and paint applied by airbrush. I have been pleased to get this old beast back in the air. I think the thing is you can achieve something that looks the part in the air, may not be a Concors winner but will give a good account of itself on the hill in a good blow. For info I have created an album with some more flying shots. Good luck with your own design and I will follow with interest.

Paul116107832_3082423055176943_3009439886216647583_o.jpg

Andy Meade11/08/2020 19:27:15
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2784 forum posts
717 photos

She looks super Paul, well done. I think a lot of the Bosak designs can be transferred to slope use quite well.

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