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Old Jan 17, 2010, 10:30 AM
smithy
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Cornwall, UK
Joined Dec 2004
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Question
SU25 Frogfoot

Hi all, im close to sending off drawings iv'e done for a SU25 to be cut into parts. Its 66inch span 63inch long with a wing area of 485 sqin. Im hoping for an all up weight of about 80oz. Originally it was designed for 2 90mm fans but i think these would take me over my ideal weight limit. I am now thinking of adjusting the drawings to use 2 69mm fans instead, thinking i could save about 900g (fan units, motors and LiPo's). Do you think 2 69mm fans will be upto this size model? Another question, keeping to this scale and using 69mm fans would mean that my intake area would be more than the fan fsa, how would this affect performance?
Any help much appreciated.

Phil
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Old Jan 17, 2010, 06:31 PM
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hklagges's Avatar
Munich
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Quote:
Originally Posted by jumpy View Post
I am now thinking of adjusting the drawings to use 2 69mm fans instead, thinking i could save about 900g (fan units, motors and LiPo's). Do you think 2 69mm fans will be upto this size model?
Phil
Just to give you a data point for a 70mm power system:
- 2 Wemotec MiniFan Pro plus 2 HET2W25 plus 2 Roxxy ESCs plus long 4mm2 power cable all together weigh 530grams
- a 6s battery ok for that (eg Turnigy 30C 3.600mAh) weighs 580grams.
- they pull close 900 Watts each (40+ Amps each)
- thrust should be >= 2.4 kgs

Cheers,
Henrik
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Old Jan 17, 2010, 06:59 PM
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Phil............... nice plane to choose. I'll be following this one for sure. how about going the route I'm taking with the Jaguar and think about the WM400 MKII? great little fans, so I've read.

Rich
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Old Jan 17, 2010, 11:53 PM
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Well its very easy to achieve over 2 KW. in a 70mm sized fan. So say two 70mm fans at 2kw each for a total of 4kw combined. Is that enough? I would think so. I personally would stay with the twin 90's, even if you had to upscale your drawings to make work. 90's are a great cost effective sized fan vs. a 120mm size. Most who already fly 70mm fans have the lipos to run 90's and the size of the model is at a sweet spot. Bigger than a 70mm machine, but not quite as large as a 120+mm machine. Although your dimensions are around a 120mm sized jet. I think twin Tamjets 90mm 7 stator fans would be friggin killer in this jet. The frogfoot is russia's answer to our A-10. Very sweet and almost never modeled jet. I will for sure watch this thread. Great choice and I wish you the best of luck with the build!
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Old Jan 18, 2010, 04:32 AM
smithy
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Cornwall, UK
Joined Dec 2004
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Food for thaught indeed, thanks guys. The drawings are already designed for 2 Midi's and thay are a tight fit. Just thinking that if i redesign for 2 Mini's, which i already have btw, keeping it at the same size would mean that i would save weight, the tail pipe would be closer to scale and also the smaller ducting would mean i would have more room for retracts. The inlet area would be larger than fan fsa which i think would limit speed but im not looking for an all out speed 'demon'. O well, back to the drawing board

phil
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Old Jan 18, 2010, 04:44 AM
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sounds like a fun build! Frog is ne of my faves! and size wise sounds perfect for twin 70 or single 90. i biult a KP-44 version eons ago! thing to keep on the horizon is that the frogfoot can have a thrust issue like F-4's and Jags if you don't have enough length on tail pipe to hug the floor and even go up a tad on exit. Like F-4's if you have a hangin back fan or short tail pipe that aims down you'll have a plane that acts nose eavy on take off and tail heavy on throttle off.

Barry
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Old Jan 18, 2010, 04:49 AM
Will fly for food
Maryland
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I will keep an eye on this thread, no matter what size fans you figure on.

Would love to add a Frogfoot to the collection.

And the real thing was not a speed demon, high subsonic only.

I got to fly one in the Ukraine and was limited to 800 KPH.
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Old Jan 20, 2010, 03:23 AM
smithy
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Cornwall, UK
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Well here is were im at. Just finished redesigning for minifan setup. There is a lot more space now which allowed for much more straight through ducting, before there was quite a curve from inlet to fan inlet and also the exhaust was exiting at a slight downward and sideways angle. Now i have a 0degree outlet and straight inlet. Still got a few more bits to do but nearly ready to send of to be cut. Here's a screenshot of what i have so far.

Phil
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Old Jan 20, 2010, 03:39 AM
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Some time ago i've also drawn a Su-25, it was thought for 2 MiniFan but a bit smaller than yours.
Unfortunately i never finished this project.
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Old Jan 20, 2010, 06:17 AM
smithy
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Cornwall, UK
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Does anybody know how the mains gear retracts, ive been trawling through videos on youtube but cant quite make out how they retract. I know they retract forward but cant see if they also rotate through 90 degrees to lay flat in the wheel well similar to a Jaguar. My guess is they probably do to minimise space for ducting.

Phil
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Old Jan 20, 2010, 07:20 PM
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Frogfoot gear pics

hey Phil

I've looked at a bunch of pics on airliners.net for you and these are the best I can find. there are literally hundreds to look at on there if you are interested.......cockpit shots, paint schemes and all kinds of other things. here's a few of the gear and few cool schemes I found. there's deffinitely a twist exactly like the Jag as it retracts forward. you'll need a 90 degree rotating set, or make your own hinge point as I did with the Jag.
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Old Jan 20, 2010, 07:23 PM
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here's a few more

a few more gear pics and a really cool paint scheme.
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Old Jan 21, 2010, 01:13 AM
smithy
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Cornwall, UK
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Hey Rich, thanks for the pics, very helpful. I've also noticed that the main gear is quite short, if i keep it to scale i could have problems with ground interference taking off from grass, but on the plus side there's less room needed in the wheel well. Great colour schemes, i think my fav is the last one from the Czech airforce, no chance of loosing site of that in the air!

Phil
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Old Jan 21, 2010, 02:42 AM
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yeah, I agree totally. LOL. it's pretty loud and I kinda like the frog on the tail fin. you may run into FOD trouble, as you say. could you not put a screen somehwere in the intake, with a trap door system, so that if something hits the screen, the door opens and throws out the heavy object? they use FOD screens on turbines and it doesn't seem to affect performance too much. how well would they adapt to EDF use, I wonder? just a thought.

Rich
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Old Feb 14, 2010, 02:39 PM
smithy
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Cornwall, UK
Joined Dec 2004
524 Posts
Well i think i have finished the drawings, been going over them again and again to check for anything i have missed, can never be too carefull i suppose. First thing tomorrow sending the parts drawing off to be cut.
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Last edited by jumpy; Feb 16, 2010 at 04:26 AM.
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