SU-25/39 Frogfoot/Hot Wire| Maiden video added - Page 3 - RC Groups
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Oct 10, 2012, 03:24 AM
jumpy's Avatar
Hey D, she's looking good.

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Oct 10, 2012, 08:19 AM
You are a "go" for reentry
Maxthrottle's Avatar
Wow that's just a few more hours? She's definitly looking like an SU should.
Your brothers gonna start asking for one you know
Should have cut two of everything.
Oct 10, 2012, 08:19 AM
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Waltjg's Avatar
D, looks really great so far! And your wing looks super too, much more detail than the program that I use. You mentioned space for retracts, that is always the problem that I run into wether they Re in the wing or the fuse, they require a lot of room!!! W
Oct 10, 2012, 10:19 AM
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D_FAST's Avatar

Yup,i have a total of 12 hrs. cutting and mounting templates and bow time.

Well my brother did a lot of the bow time, as i was busy cutting out stations and getting the foam blocks ready. so technically its 24 man hours.

@Walt, I did some measurements last night, looks like some out there on the market will work. its getting the twist and turn option thats going to be tricky.
When i cad'd this up i was using a retract with a 21mm depth, most are all 24-26mm.

The cavity measures 24x66x120mm DxWxL

The only thing i would change on my design is swooping the ducts a little higher, and just tad bit more back over the landing gear cavities.
I also need to thicken up a few stations, I also wish i cut a brace out of every 3rd station to help with some structural support as its completely hollow thru the spine of the fuse. I have no plans on glassing the inside.some of the turtleneck is already smooshy Got to get glass on it fast!

After sitting, staring at what has evolved from some flat foam,to what we have now, and butting brains with my bro, i'm changing up the wing design.

Going to cad up a simple tube in tube spar,with locating pins and few lite ply mounting tabs.

Wieght so far,4.6oz

I can already see my hopes of a 14oz empty airframe weight is going to be more like 16oz
Last edited by D_FAST; Oct 10, 2012 at 12:46 PM.
Oct 10, 2012, 01:13 PM
Erik with a K
tvcasualty's Avatar
Nice work D_FAST!

I thought you might find this interesting:
Oct 10, 2012, 04:22 PM
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unclecrash's Avatar
Looking good D. I was thinking the same thing with the bracing. I should have left a vertical section on all the pieces that get narrow about midway back on the yf23. They would have stood up better and matched up alot better. But its just a big learning curve going from balsa to foam It's not too big a deal with the green ,blue, and pink foam it is kind of rigid I bet the white being a bit softer and lighter it probably moves around a bit more.
Last edited by unclecrash; Oct 10, 2012 at 04:30 PM.
Oct 11, 2012, 12:51 AM
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D_FAST's Avatar
Got a bit more done.

Some things i got to get cad'd before continueing the glueing.

Need to solidify my wing design, get some lite ply formers going for the elevator &rudder support and there control connections, draw up my thrust tubes, and get some motors and esc's ordered.
Oct 11, 2012, 03:21 AM
You are a "go" for reentry
Maxthrottle's Avatar
Man I have no idea why none of the manufacturers haven't gotten around to doing these after doing so many repeats of the L-39.

I know you're looking to get this one out the door before the paint dries but to our previous conversation, whats your take on doing this in a single bifurcated 90mm sitting further up to make room for the gear. You almost have room for some of the standard 2.5 and 3.5kg type Etracts.

Its one thing to look at CAD renderings but having the hard body in front of you whats your take?

Thanks D-Fast.
Oct 11, 2012, 12:06 PM
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D_FAST's Avatar
as you can see Max, things got pretty thin, quick on me trying to keep things scale. Single 67mm model has not been fine tuned like enlarging the exhaust nacelles.

I could however scale this model up add in 90mm unit and generate the ducting.
What your fan unit? I have a changesun 90m 12 blade already modeled to scale if that"ll work?

Never mind all that ^^^^^^^^^^^^^^^^^^^^^^^^^ 53-54mm exhaust on a bifurcated exhaust
Remember though i scaled this back as far as i could go and enlarge the nacelles slightly off scale to make everything work.
Last edited by D_FAST; Oct 11, 2012 at 12:39 PM.
Oct 11, 2012, 01:07 PM
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Maxthrottle's Avatar
I had in mind the CS90/12 as it does well bifurcated.

Because the fsa of 2x64mm ~/= 1x90mm fsa the scale shouldn't need to change by any large amount.
But the conversation in Jumpy's thread being Bifurcated would be hinded on a 25/39 because the nacelles being set further apart.

Since you have model in hand in your estimate is it doable moving it up and slightly back without creating agreesive angles to the bifurcated duct.

With your current 64mm scale what did you make the inlet area or fsa% and the exhaust fsa%.
I think I missed why the scale would have to be larger to handle the 90mm.

In any case that twin 64mm is the perfect size so I would be more inclined to fudge the nozzles than make it bigger.

Small scale question; are you going to add a small OLS in the nose? Small sheet of clear plastic with a keychain cam behind it? It'd be the best view in the house
Last edited by Maxthrottle; Oct 11, 2012 at 03:07 PM.
Oct 11, 2012, 01:45 PM
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D_FAST's Avatar
I don't think your getting a 90mm in here. I tried several curve algorithms, these are the best i got.
Besides I dont think you would ever get it to balance without major amounts of dead weight.

Now a 70mm would drop right in here, plenty of space for etracts and no acute angles in the ducting.
Oct 11, 2012, 03:44 PM
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Maxthrottle's Avatar
OK. Ya you guys are right, that's a little too much angle at the face of the fan.
Even a Bi 70 may still have the same concerns if you're shooting for at least 100mm of straight duct fore and aft of the fan. A single 6S 70 may still struggle for a model this size especially if the duct has significant bends.

So I guess twins are a must but tracking the duct up and over the gear bay a little more. To do that it may be better to do plug in wings.
Actually thats pretty quick work on the rendering. Thanks.

My guess is if 64s the center belly can be the stow for knee strut/wheels with the rotating retracts mounted angled in toward the center. This way you would only need clearance to mount the retract between the floor and the duct. The wheels would have room beside the duct in the center stowed at a slight angle.
Oct 11, 2012, 04:46 PM
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Maxthrottle's Avatar
have you thought of liquid sheeting instead of glass all around?

Liquid Sheeting a round fuselage (23 min 39 sec)
Oct 15, 2012, 07:55 AM
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D_FAST's Avatar
Some more progress this past sunday.

Weight: 10oz
Does not include wings but has the edf shrouds and 2 elevator servos installed

2x motors = 3.5oz
2x Esc = 1.6oz
3x Servos = 1.05oz
1x receiver = 0.5oz
1x 2700mah 65c 4s = 11.7oz
Misc wires, soldier, leads,etc... 3oz
Filler,glass,paint 4-5 oz?

Total: 36oz

if all holds true im gonna be one happy camper considering my 2x50mm build was up at 39oz after paint
Last edited by D_FAST; Oct 15, 2012 at 08:17 AM.
Oct 15, 2012, 05:29 PM
Registered User
LuvEvolution7's Avatar
wow, how did I miss this one!? LOL.

wish I could learn some CAD skills. I really need to get out of pencil and paper drawings. I'm working on a 1/4 scale Piaggio Avanti II and it would be awesome to do it on the PC, rather than on paper.


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