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Old Oct 25, 2014, 12:13 AM
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United States, CT, Oxford
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Build Log
Another JMP F-4 Phantom conversion twin Jet Fan 120s

Picked up a shiny new Tom Cook F-4 Phantom today. Ok, it's not shiny, and it's not exactly new.
Plane has been on Ebay for some time, when I first saw it, I thought it was one of the ugliest Cook F-4's I had seen. We worked out a good price, and I spent some quality time on the Jersey Turnpike today!
I assumed it was well used (read proven) and my plan was to strip it, go thru make sure what was there was good, update it, new paint, and a pair of Jet Fan 120s

As I start pulling it apart, I notice no slimmy mess inside. Pull the fans and they look brand new. Look in the exhausts, doesn't look like they have ever been run. Fuel tank is bone dry and perfectly clean

My first thought is cool, never flown! My second thought was crap, never flewn.... why not Have a little extra work ahead to go thru it
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Old Oct 25, 2014, 12:21 AM
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Started pulling fans and ply ducts out. Everything is covered in overspray, and years of dust. But nothing looks like it was ever used
Too bad no one fly's Turbax's with 7.5s anymore!
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Old Oct 25, 2014, 12:53 AM
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Got the old fans and exhaust ducts out. They only weighed 4.8 lbs, plus 32 oz of fuel totals 6.8 lbs, which is less than I was hoping. Was hoping the electric would be closer in weight, just being unrealistic. Batteries just weight too much

Speaking of batteries, look how nice one set of batteries fits where the main fuel tank used to be

I think a pair of Jet Fan 90s with CC120HV ESCs, wire, and two 5300 12S packs is going to weigh roughly 10lbs, and the Jetfan 120 just shy of 12 lbs.

Its a big plane so I'm not really sweating 25-27#. As long I can get at least one to one, I'm happy. As shown it weighs 13#, which seems pretty light for its size. Figure it's going to gain a couple pounds during the rework, plus power system

I have a pair of Jetfan 90s for another project and sat one in there. When you think how little thrust the Turbax's made with the 7.5, or the later Dynamax's with 65 or 77s, it is pretty tempting to try out the 90s with the bellmouth

Plane looks like it has a lot of drag, and I just don't know how the 90s would fly it. I'm thinking it really won't notice the two extra pounds with the bigger fans, but should really notice the 8ish lbs of extra thrust
What do you guys think?
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Old Oct 25, 2014, 07:42 AM
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Congratulations on the purchase and good luck. I think that I would have stayed with twin 90 fans, overall probably less weight, less cost and excellent performance. Not talking from experience yet though.
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Old Oct 25, 2014, 08:29 AM
deltas are cool
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Tehachapi ,CA.
Joined Apr 2006
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Man! What a blast from the past.....not sure which way I'd go...but like.the.idea.of.2x 120 JF.that should be@ 15-16lbs.each.....the old turbax had @ 4-5 lbs static.you might look at the 120's on 10s each
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Old Oct 25, 2014, 08:35 AM
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Thanks

That's a good idea, save some battery weight. Has anyone posted any data running the 120s on 10S yet?
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Old Oct 25, 2014, 08:48 AM
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Quote:
Originally Posted by N99JH View Post
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Congratulations on the purchase and good luck. I think that I would have stayed with twin 90 fans, overall probably less weight, less cost and excellent performance. Not talking from experience yet though.

Thanks
I hear you on the lighter weight and cost
The thing that struck me was the 90 some odd amps for the 90mm on 12S making 10-11 lbs of thrust, and what Rainer posted about the 120s making 14-15 lbs on the same 90 amps. Realize the lower Eflux, but I find that pretty interesting
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Old Oct 25, 2014, 09:04 AM
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Congratulations on the acquisition. Back in the mid 1990's I had a Cook F-4. Really nice stable flier. As was customary at the time I had two Dynamax fans with OS 91's. Plane weighed in at 33 lbs. wet and flew well at that weight.

I recently bought one of Tams center bodies and put an HET 800-685 motor in it. I installed it in my Falcon 120 and was really impressed with the performance. With my rather crude measurement system (digital fish scale hooked to the back of plane) I was getting 16+ lbs. of thrust on 12S. If you are asking for input, I would like to suggest that you consider two Dynamax EDF units (perhaps using Tam's center body and a 685KV motor or similar).

As mentioned earlier, the plane files well but my only problem was trying to land with a lot of flap. The plane was not very responsive to elevator input and I believe that part of it was that with a lot of flap (in the order of 50 deg.) the elevator was getting blanked.

Also, when I first flew mine, it was VERY tail heavy. As I recall, the CG on the plans was incorrect. I believe that it was 1"+ too far aft. I might suggest that you calculate the CG.

Ed
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Old Oct 25, 2014, 11:49 AM
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Thanks A10
This one was beyond my limits ($) back then, but they left a lasting impression. The fact it never flew, and some of the things I have found so far has me a little worried about what I can't see.
First things first, need to finish tearing it down, and strip the paint

Yes, I am asking for opinions. Seeing the JetFan's performance is what got me heading back down the Jet path after so many years away. I like the light weight, performance numbers, and low cost.
Will have to do some reading, but I'm imagining a converted Dynamax will be heavy compared to the Jetfan 120. Higher static thrust, lower Eflux, more amps??
Need to look at the E turbax as well I guess, being it already has the fans. My first instinct is the new fan designs are more efficient and lighter.

Have been reading thru old threads I could find, and the aft CG seems to be a issue in every thread. Only thing I can think of is they were flying fairly slow back in the 80s, so maybe it needed to be balanced closer to neutral to allow it to rotate and flare?? Not sure, but from what I have been able to find so far, it seems 3/4-1.5" fwd of plan CG is what seems to be working.

If you have any links to JMP threads, please feel free to post links. I thought I would have found many more conversion threads, but I have only found a couple
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Old Oct 25, 2014, 12:57 PM
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Green Valley, AZ
Joined Aug 2004
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Inlets

Remember to Wrap the inlets, they need to be beefed up, remember that this airplanes was designed / made for .65 engines. When we put O.S. 91's the inlets collapsed on mine.

Ralph
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Old Oct 25, 2014, 03:47 PM
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Österreich, Stmk., Graz-Umgebung
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JF90 vs. JF120

Nice projekt

If itīs possible, use the biggest fan you can get in a plane!!!

I have a "Tetris MiG29/35" with 2x JetFan90 on 1x 12s 9.000mah. It flys really nice and powerfull, but i miss some extra power

The only thing to change the setup to 2x Jetfan120 is - the inletarea should be a little bit bigger.
I can use the same ESC (YEP120A), the same BEC and the same batterys...

So: If itīs possible, use the biggest fan you can get in a plane!!!

Regards, George
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Old Oct 25, 2014, 06:41 PM
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another cool project!!

biggest fan you have the FSA for
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Old Oct 25, 2014, 10:59 PM
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ragtop:
I would like to share my experiences with my plane if you don't mind.

My plane was originally built by a BVM rep in Austin, TX. by the name of Rick Schafer. The plane was designed for Dynamax fans but Rick converted it to Violett fans at the owners request. The plane eventually ended up in the hands of Charlie Trivitts from Delaware. Charlie was flying it at the 1992 Jets Over Deland when he had two engine failures shortly after takeoff and ended up putting the plane down in the trees. The plane was badly damaged and the fuse was broken off from the wing leading edge forward. I managed to buy the plane from Charlie and had it shipped to me in Denver in 1992. I called Tom Cook and he laid up a fuse front section for me. I grafted it onto the remaining fuse. While I was at it I changed the intakes back to the original so they would fit a Dynamax (one of the things that Charlie told me was that the plane did not have enough power to fly with authority with the smaller BVM fans) so I went back to Dynamax fans.

When I got the plane it was done up in the old Playboy Bunny F-4 scheme except that everything that was black on the full scale was now a pearlescent white and all that was white (emblems, etc. ) was now black. I went back to the original Playboy Black Bunny scheme. I first flew the plane at the Jefco modelers field in Denver on two OS 77's and the CG per the plans. See the photo below of the takeoff. We managed to get the plane back on the ground safely with only some minor damage. I repaired the damage and decided I needed a longer runway so I went to a full scale airport in Colorado Springs, CO. for the second flight. By this time I had upgraded from the OS 77's to OS 91's but the CG was still per the plans. The photos show the results. Same nose up pitch but this time I was not able to land the plane safely. I relaxed the elevator to try to get the plane to climb a little but every time I relaxed the elevator, the plane sunk so I was stuck. As the plane made a left hand turn towards the full scale parking area I decided to pull the throttles back and belly landed the plane. Unfortunately, when the plane landed it landed on the top of a dirt mound and broke the front of the fuse off AGAIN. I repaired it and finally ended up recalculating the CG with the help of the late Ivan Munninghoff. Once we moved the CG forward plane flew vey well.

One thing I will offer is that I think you should install 127mm fans as I think 90's or even 115's are too small. I say this because while you can get the efflux and the power, you can not get the same amount of volume from the smaller fans. This point was made for me when I flew my 15.5 lb. BVM Maverick on a Jet Fan 90. While the fan flew the plane well, I had to keep the throttle at about 75% or more to keep the plane "on the step". Once I went back to the 115mm BVM fan the plane flew much better and I fly most of the flights at about 50-60% power. The 90mm fan flew the plane (15.5 bs. on 10S) at 125 mph. while the BVM fan (on 12S) flew the same plane (now 18.5 lbs . with the bigger fan and batteries) at 160+ mph.

Others may offer different suggestions and a lot are based on first hand experience but so are my observations and mine are based on flying this plane with Dynamax fans and SEEING it fly with the 115mm BVM fans and there is an enormous difference. If I had the plane, I would put Dynamax fans in it and have fun. Incidentally, I recently converted my 120" span 53 lb. A-10 from Dynamax glow to Dynamax EDF (double the thrust from glow to EDF) and the difference is huge.

I have attached some photos of my JMP F-4 circa 1992 thru 1995.

Just my 2cents.

Ed
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Old Oct 25, 2014, 11:17 PM
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Thanks Ed

I think the 90s are out

Have read a lot on these planes and the aft CG issues. Once you rebalanced it, and had it dialed in, where was the CG?
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Old Oct 25, 2014, 11:33 PM
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Started stripping it tonight. Read that Citrastrip works pretty good on glass, so gave it a shot. Unfortunately, not so good on this one. Given the vintage, I'm thinking its Super Poxy, or Hobby Poxy paint. The black came right off, but the green and tan are holding strong.

Going to have to do it the the hard way, and theres a lot of paint on this thing. Looks like old Feather fill primer, then a Beige, then a tan, then the green camo. Little too much for the shop, need to move this exercise out to the garage

The striper did make the paint sand a little easier. Going to try roughing it up and let the striper fill in all the scratches, and sit for a while. Striper sat for 30-45 min tonight, and the paint just laughed at it
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