Yellow F-4 turbine rebuild - RC Groups
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Apr 29, 2011, 12:12 AM
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Build Log

Yellow F-4 turbine rebuild

I thought I'd start this build log so that anyone can follow the (re)building and turbine conversion of a Yellow Aircraft F-4 that I picked up second hand.

This is by no means the only build thread online for this kit, but the others are a little bit bit patchy in terms of describing some of the build decisions that will need to be made, so I want to put up a pretty thorough record of mine.

It's one of the old Yellow kits - the instructions have no colour photos, and I suspect that makes it a pre-1992 (or so?) model that's been around the traps for a fair time. From what I gather it's had at least two former lives - one as a 91 powered DF and then more recently as an EDF conversion.

It's generally in OK condition and is a good starting point for my third foray into a jet construction. Some shipping bumps, tears and bruises are evident, but generally it's OK. The wings have been glassed to the fuse.

I've been told the earlier versions of the kit aren't up to the turbine challenge, being too weak, especially at the wing fuse junction. However, I did a test by suspending the frame by the outermost ends of the wingtips and loading in 11Kg of metal objects into the belly of the fuse. There were no worrying signs of cracking or undue bowing, so I reckon it'll be OK.

Other issues I've read about include:
-weakness and flimsiness aound the u/c bays resulting in cracked wings;
-being too heavy with a turbine and the heavy fuel load onboard for adequate flight performance;
-major thrust loss (30%!) from using a bifurcated tube;
-difficulty with finding room to carry sufficient fuel and complex plumbing;
-difficulty with building a rigid rudder control system; and
-difficulty rotating at take-off because of the rearward location of the u/c.

Ok, so I'll try to tackle each of these in the following posts, as I get to them.

The pics show what it looked like just before I purchased the airframe.
Last edited by GRVW; Apr 30, 2011 at 10:19 PM.
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May 02, 2011, 03:07 AM
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Just thought I'd put up a link to a good F-4 build thread that helped me sort out some thinking so far:,35377.0.html by mpx. A cracker, except that the website crashed some years ago and the embedded photos are lost. (I obtained some of the photos from mpx directly which I might re-post here later, when I get up to the relevant construction bit.)
May 13, 2011, 12:59 AM
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One issue that has been tackled by builders in different ways is the location of the fuel tanks. Some people have made custom saddle tanks that fit around the engine bay, but generally this requires removal of the stubs of the middle wing spars which intrude into the engine bay, and I'm a bit reluctant to do that for strength's sake. Jet-Tech LLC make some nice saddle tanks for this kit if you go down that path, see . Other people have plumbed in a series of four Sullivan/Dubro tanks in the engine bay/saddle tank area but I think the fewer the number of tanks the more reliable the whole show is.
A copy of a June-July 2010 RC Jet International has a useful article by Ralph Esposito on building his Yellow F-4 turbine, and in it he said he used a standard 50cc Dubro tank in the rear part of the canopy opening. This makes it forward of the engine bay and ahead of the CG. So I emailed Frank, asking if there was an issue with changing CG as the tank drains in flight, and he said it wasn't really noticeable. That setup is a much much simpler and less expensive option which I'm going to run with. Attached are Frank's photos of a similar tank installation in his L-39 (not the same, just similar).
May 13, 2011, 01:09 AM
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To make space for the 50 cc Dubro tank at the back of the canopy opening, I needed to cut out the inside walls of the ducting. The photo shows one half of the inside ducting of my F-4 already cut away, with the other half soon to be removed. The outside wall of the ducting remains intact and may hopefully allow some smoother airflow into the engine area (I've seen some installations where the ducting has been entirely removed in order to fit everything in.)
May 18, 2011, 07:16 AM
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well done !

Hi Gowrie
Great to see this build thread,good on you for having go at this type for conversion to turbine,I'm sure many will get a lot of help from this build and give some inspiration to tackle similar projects.I will follow this with keen interest,keep it up mate.

ps sent you a PM cheers Rich
Jul 24, 2011, 10:04 PM
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OK, very slow progress but going to pick up some speed now, hopefully.
A few people have reported punching the landing gear through the top of the wing after numerous or less-than-ideal landings, so I have laid carbon fibre reinforcing around and into the retract wells. See images. I think that will double the spread of forces from any rough-ish landing. It feels rigid and strong, so hopefully that'll be enough. I could have added more but I'm conscious about adding extra weight on this airframe. It looks untidy but it'll be fine once I square it up a bit, and fill and paint it.
Jul 24, 2011, 10:26 PM
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Made the elevator pushrod using 4-40 threaded rod, jacketed by carbon fibre tube which is secured by a washer and nut tightened each end. This method produces an intrinsically very rigid and reliable pushrod, just so long as you cut the carbon tube squarely. Also shown in the pickky (but not very clearly) are the chunky alunminium horns I made for the elevators - these now fit nicely into the pivot blocks and make this whole business end rigid. I brazed together the two short threaded rod stems onto the main rod end. I'm no expert brazer, but I can't see this joint letting go any time soon.
Jul 25, 2011, 12:08 AM
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I was having dificulty finding a stainless steel bifurcated tailpipe for this kit. Tamjets make one which everybody I've spoken to says is a work of art, construction-wise. However, some people have also reported unexpectedly large thrust losses with this pipe. Tam himself said the losses he's experienced with it are very minimal (1/4lb), given that all bifurcated pipes are lossy. Some others reckon they lost around 3lbs, which would turn the exercise into a very marginal one for those with 12lb turbines. Many variables are at play here, including the distance between the pipe's throat and the end of the turbine, so it could be worth building a thrust test rig and doing some tuning of the distance. I'd order the Tam pipe, but the factor most at play for me is cost. To buy the Tam pipe and ship it to Australia is almost as expensive as what I paid for all the rest of the kit + retracts.
After I put a wanted ad online at rctrader a local jet builder in Sydney replied and has a pipe that, with a bit of shortening, will probably work fine. I'ts a very reasonable price, so I'll go with that.
Aug 04, 2011, 06:54 AM
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Here is the pipe I bought second hand from a very helpful guy in Sydney. It's from a larger Rafale, and looks like it has a much larger diameter compared to the Tams pipe for the F4 - it might make a starting point to compare the effect of pipe size on thrust loss. Anyone want to chime in here?
The blue tape shows where I'll need to trim it down to fit the fuse. How the trimming is to be done I haven't yet worked out. But, I've been busily adapting an old (and far too large) spot welder to help out with this task.
Already I can see, however, that it's going to be difficult to get it installed through the engine bay.
Aug 07, 2011, 02:39 PM
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therealhiphop's Avatar
Subscribing - I have a NIB Y/A F-4 kit that I plan to convert to electric. I am interested in watching your build.
Aug 13, 2011, 01:12 AM
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Ok, did a bit of work over the last couple of days to trim the pipe. I used a Dremel with a cut-off wheel to slice through the stainless tubes, then used a large and not very reliable spot welder to join some new spacers used to separate the outer from the inner tubes. Sounds easier than it was.
I also fashioned a bellmouth out of some steel tube I had, then spot welded this to the inner tube mouth - though I'm not sure it was worth the trouble, given that this pipe is pretty huge compared to the turbine.
Today I whipped up a stand for the bifurcated pipe which joins onto the test bed for the turbine - that way I can test the whole shebang. I need to take the turbine to full power, but all seems promising - see the vid.

Jet joe with bifurcated pipe (2 min 20 sec)
Aug 13, 2011, 03:10 AM
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Gave this setup full throttle in a test out at the field this afternoon. All good. I can't really tell what the thrust loss might be, but I reckon there is some. What I can definitely say is that it is fearsomely noisy! The bifurcated pipe somehow makes it really, really loud.
Jul 02, 2012, 11:08 PM
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Ok, it's been almost a year since my last post. In that time I have built, flown and crashed a Foxcub trainer using this turbine engine. The idea was to get used to landing on a fairly short (450 foot) strip using a Foxcub trainer rather than the F-4. Except, the trainer was much faster than I'd hoped and much harder to land - and in the end after only one or two semi-enjoyable flights, something happened mid-air and I had a flame-out and lost radio contact. I still have absolutely no idea what caused it though I suspect the receiver. Anyhow, the turbine escaped major damage. $100 worth of parts and it is back to its old self.
I've ordered another Foxcub but it hasn't arrived yet (I'm actually worried my supplier has disappeared into the ether), so I'll proceed with the F-4 in the meantime.
Just an update on my post #7 where I made a custom Y-pushrod: in the end this had to be ditched for two separate arrowshaft pushrods because the angled elevator pivot blocks wanted to pull the Y-junction apart as the pushrod went through its travel.
This photo shows some slightly beefed up trunion blocks I made for the main retracts. A lot of work so I'm hoping they're worth it. The pikkie shows the new (wider) one installed compared to the original.
Jul 02, 2012, 11:29 PM
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The installation of the tailpipe took some fiddling and coaxing. It is much bigger than the Tams pipe and took a lot of jiggling to get it in the fuselage and sitting properly. The photo shows the pipe in place and the engine installed as far forward as I can get it and still have room for the fuel tank in front.

I was helped by my friend Bill who owns a good laser level. We set the aircraft on a large flat table and the laser beams made it clear what adjustments to the pipe angle needed to be made to get the correct relationship to the wing. It also helped to find exactly where horizontal is with respect to the elevator halves.

I needed to set the pipe and engine as low as possible in order to get the thrust line fairly parallel with the fuselage.This required building two aluminium engine mounts which are bent down to get the centreline of the engine low enough down.
Jul 02, 2012, 11:36 PM
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I've been keen to prevent the tail getting too hot. I covered the bottom of the elevators and tail with two layers of adhesive foil. The pipe protrudes past the nozzles about 10mm - not such a good look, but I was worried the nozzles might melt or get burned. There is about a 1.5 mm spacer between the nozzles and the outer wall of the thrust tube. Hopefully some cooling air will pass through.

I started up the turbine in the aircraft last week. Nothing seemed to get too hot, so that is pleasing.

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