Kyosho F-4 mods - RC Groups
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Sep 05, 2008, 02:51 PM
cray2602's Avatar

Kyosho F-4 mods

Thought i would start a mod thread for this little bird. Hopefully everybody else who has modded their K F-4 will post thier own mods for us all to glean from.

My setup is as follows. -

575grams AUW on 4s setup with a AA battery under the tail to get CG right.

LSJS 4500kv motor (had to grind the motor standoff adapter down about 1/16" in diameter to fit into the stock fan shroud)

1550 20C 4s 180 grams Rhino battery (from Hobby City)

CC Phoenix 35amp ESC

stock fan

4s LSJS setup = 28amps
stock 3s setup = 17amps

To get the CG back where it belongs with a heavy 4s battery i had to make a battery holder box and locate the battery just in front of the servos and down inside the intake channel.

Since i have no way to adjust the throws via the radio at max throw on the servos and elevons i did not have enough elevon throw for it to fly well with my 4s setup. My solution was to add 1/2" long extensions to the elevons. This made a huge difference in handling of the plane, complete night and day difference. It flies excellent with the extensions added, without them the plane would not flare enough on landing and it would get stuck in the middle of a rolling inverted maneuver and become rather sluggish even with CG set exactly spot on.

I also tried the stock motor on 4s battery. It was a screamer and made more power on the bench with a few less amps than the LSJS did. But it burned up within 4 flights. Tim Johnson claims 40+ flights on his stock motor with 4s battery, so it might be a hit or miss with that. But its worth a try, if the stocker doesnt burn up its a very cheap and easy upgrade to just add a 4s battery and ESC. I was pushing mine hard when it went too. A heat sink might help out a bit.
Last edited by cray2602; Sep 14, 2008 at 12:33 AM.
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Sep 05, 2008, 03:38 PM
Member 120mph Club
ATIS's Avatar

Nice work. It appears the speed demon has a hold of you. Any videos?
Sep 05, 2008, 03:41 PM
cray2602's Avatar
No videos yet. I usualy am flying alone at home and my 4 year old son isnt too good with a camera. Im not the best at hand launching so i figure if im building any more hand launch EDF's they better have enough power to climb straight up from a stand still.
Sep 14, 2008, 12:31 AM
cray2602's Avatar
I took the stock fan out today and dropped in a Hyperflow fan to see if it fit and it fit perfect. Any fan with an outside diameter of 56mm and maybe 57mm should be a simple drop in installation in this F-4. A few more mm in FSA should equate to a modest power increase. I dont personaly recomend the Hyperflow fan, i just wanted to see what size fan would fit. The quality on my Hyperflow fan is terrible, the fan itself was so far out of concentric there was no way to get it in balance.
Sep 22, 2008, 11:28 PM
cray2602's Avatar
Made some more changes to my F-4. I swapped the stock fan for a 55mm inner diameter fan. I cut a GWS 6 blade rotor down to fit in the 55mm fan shroud. I got the thrust up to 490grams with that setup on the 4s. Thats roughly the same thrust that i got from the stock motor on 4s. I then calculated the correct exhaust exit nozle diameter for this 55mm fan and made a couple nozzles as the stock exhaust diameter for this plane is grossly over sized even for a 55mm fan. I got an exta 140grams of thrust from the nozzles alone. Static thrust is now 630grams on first run up with fresh charged battery.

Here is a pic of the exhaust nozzles. They arent pretty, it was just a quick build up to see what the results would be.
Last edited by cray2602; Sep 24, 2008 at 11:24 AM.
Sep 22, 2008, 11:34 PM
cray2602's Avatar
My final setup (i hope) is as follows.

LSJS 4500kv motor
1550 4s battery
55mm fan with GWS 6 blade rotor
CC Phoenix 35 ESC

AUW = 575 grams
630 grams thrust

Stock thrust was 300 grams.
Sep 23, 2008, 12:47 AM
MICKEL's Avatar
Man! You're definitely going straight up vertical! I bet that plane hops right out of your hand.

Way cool bro.
Sep 24, 2008, 11:30 AM
cray2602's Avatar
I got to fly it with the exhaust nozzles on last evening. Huge improvement in power. I had to dial down my reflex trim to 10-15% for hand launch, it never even flattened out on takeoff just went out and straight up. Half throttle keeps it cruising nice and smooth around the corners with no drop in speed. What a blast.
Oct 29, 2008, 10:09 AM
cray2602's Avatar
Heres a short vid of my modified F-4. An easy straight and level toss gets her nose up and climbing right away. I am getting about 1 minute of full WOT but half throttle gives a nice 4 minutes of flight time and it still runs way faster than it needs to at half throttle. A 25C battery i think would improve flight time a little since the 20C just isnt holding voltage once the capacity drops a little.
Oct 30, 2008, 04:35 AM
MICKEL's Avatar
NICE!!!! That thing hauls balls!!!! Nice work Cray! Check out Team Orion Avionics packs. I even put the 3s910 in my Mini LST 2 and it kicked @$$!!!! No puffing like some of the other 1300+ mah packs I've tried. Let me add that a 4wd 1/18 scale take off draws some serious amps! Kokam cells are the ticket. Damn you're fast!!!!
Oct 30, 2008, 09:48 AM
cray2602's Avatar
I will look into those lipos. The problem is i need a 4s 1500-1550 mah pack and that seems to be a rather odd size or something. Also id rather not spend $100+ on a battery pack when i can get a Chino pack for 1/3rd that price. And a few of my Chino cheapo packs are outperforming some of the US name brand lipos. But ill definately check those out.
Nov 03, 2008, 08:17 PM
cray2602's Avatar
I checked my thrust with a 1300 3s battery and got 420 grams thrust at 21amps. Thats only 4 amps higher than the stock setup and 120 grams more thrust. With an AUW of 480 grams with that battery thats about .87:1 thrust to weight. Stock setup gave me around .66:1 . That should give me a solid 4.5~5 minute flight on 3s at about 80% throttle with a good amount of speed and performance.

For a dirt cheap power upgrade in this plane you could simply put the stock motor in a 55mm fan like an Alpha 5 blade and make the exhaust nozzles. Depending on the fan i think it would be close to my setup on 3s and maybe even a bit better.
Nov 17, 2008, 06:50 PM
Registered User
cray, would you mind posting dimensions/instructions for building your nozzle?
Nov 18, 2008, 11:34 PM
cray2602's Avatar
My nozzles measure 1.25" in diameter. But thats for a 55mm fan not the stock fan. Ill have to crunch the numbers a couple more times and make sure the numbers are right.
To make the nozzles. What i did was take a really small plastic cup and cut the bottom out. Then cut the top side off to leave the nozzle at the length i wanted. What you are left with is a slice of the cup, then cut the section length wise so you can roll it up like a tube. I put some rubber glue around the outside of the nozzle where it will contact the factory exhaust ports. Roll it up untill its small enough to fit inside and then push the end of the nozzle up into the f-4 exhaust ports. Let it unroll untill the outside of it grips the exhaust ports. Then what i did was roll up the smaller end untill it was the diameter i wanted and put a small piece of tape on the inside of the nozzle to keep it from unravelling. Make sure the nozzles are pointing straight and even with each other or you will end up with the thrust pushing the nose up or down.

Its rather crude and probably could be done a little better if i wanted to do it right. But its effective enough for me and was fast and easy to build.

If you try it on the stock setup post the results. Id be interested to know if it makes any difference on the stock power setup. Some of my jets benefit from smaller nozzles and some dont.
Last edited by cray2602; Nov 22, 2008 at 10:54 AM.
Nov 19, 2008, 08:51 PM
Registered User
I am going to try it on the stock setup. Just waiting on a battery.

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