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Skymaster 1:7.5 F-4 Phantom

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Skymaster 1:7.5 F-4 Phantom

Old 07-01-2017, 04:25 AM
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MaJ. Woody
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Default Skymaster 1:7.5 F-4 Phantom

Hi Guys.
I am close to pulling the trigger on this plane. I am planning to use a new Jet Central Rhino with 45 pounds of thrust for push. There are three things nagging at me though.
  1. Elevator Blanking: From everything I have read, the elevator blanking is eliminated by having a CG of at least 300mm. I recall a time on my FeiBao F-4 when I was base to final, Full flap and had no elevator. What a wild ride until I figured out what was going on
  2. Tailpipe: Right now the quote I am working up with BVM includes the Skymaster pipe for 180-200 size engines. I have read that the Tam pipe may be better and may eliminate pitch changes. Is this a must have item or just a consideration? Does the Skymaster pipe hold up well? I know the Skymaster pipe worked fine in my F-18 all those years.
  3. Heat: I worry about heat issues on the Stab. Understanding that the pipe angle is important to how much direct heat the stab gets.What is that angle supposed to be? seems like with the Skymaster pipe having the bend, it would have less of an angle and put more heat on the stab than the Tam. Is that correct? Are the engine bearers on an angle? Will heat in that area over time soften the glass or loosen the glue joints of the internal structure of the stab?
Looking forward to hearing from those with many flights on their Phantoms.
Thanks Guys!
Old 07-01-2017, 08:10 PM
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dubd
 
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Starting point for the CG is 300mm.

I've only used the Tam pipe and I dont have any issues with heat.
Old 07-02-2017, 01:02 AM
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MaJ. Woody
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Thanks for the reply.
Old 07-02-2017, 06:55 AM
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Craig B.
 
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I cooked my stab with the stock pipe and some engine issues. Bought the tam pipe. You will get bad throttle to pitch coupling with the stock pipe. Tam pipe is straight and generates more downthrust, less pitch coupling and directs exhaust away from stab. I would only use the tam pipe.
Old 07-02-2017, 04:02 PM
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MaJ. Woody
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Thanks Craig.
Old 07-02-2017, 06:31 PM
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JDjetjock
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For my F-4E, I found the CofG sweet spot to be 325mm, I have no tail blanking probs with flaps down. Inverted flight at that setting is nice too.

For the tailpipe, I have stuck with the stock tailpipe. Even though I have the larger tailpipe inlet size, it could be a bit larger. However, I'm not running into any problems yet with tail pipe or engine overheating. I'm running a Jet Central Mammoth SE in the Skymaster bypass. In my early flights, the plane was self-pitching up at low airspeed at high thrust settings. I dropped the aft tailpipe mount so the pipe is almost at its lowest point...this adds some downthrust and keeps the exhaust off the h-stabs. I'm also using an iGyro with the GPS2...this makes the plane fly beautifully and further damps out any adverse pitch behavior. I may try the TAM pipe later, but the stock pipe seems fine for now.

Heat on the H-stabs! I damaged the first of stabs on its first run-up. It seems the bays inside the stab are now sealed and will balloon if heated, damaging the stabs. Skymaster is now adding vent holes along the root of the stab...this works. I also added high temp foil on the bottom side and dropped the tailpipe lower in the aft mount (as mentioned above)...no issues now.

The F-4 is an awesome machine!

Cheers,
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Old 07-03-2017, 01:38 AM
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MaJ. Woody
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Thanks for all the info Jeff. One question. When you say dropped the tail pipe in the aft mount to you mean inside the tail cones? It seems you can change the angle by both blocking up the mounts inside the engine bay or removing material on the lower section of the tail cones.
Thanks!
Old 07-03-2017, 04:31 AM
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Chris Smith
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Originally Posted by JDjetjock View Post
For my F-4E, I found the CofG sweet spot to be 325mm, I have no tail blanking probs with flaps down. Inverted flight at that setting is nice too.

Cheers,
What configuration is the CG at 325? Fuel load, no fuel? Have you tested 325 with engine flame out?
Thanks
Old 07-03-2017, 04:47 AM
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RCFlyerDan
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Originally Posted by Chris Smith View Post
Have you tested 325 with engine flame out?
Thanks
Too funny! Only one way to test that too!! I have been wanting one of these for awhile, but don't want to go through the hassles and horror stories of waiting on it from the factory. I wished someone had them in stock already here in the states. I have always bought all of my jets already here to avoid some of the heck you guys have gone through to get them.
Old 07-03-2017, 06:18 AM
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tp777fo
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Hi all. I am building one from a white kit. have the doors on but need some insight on how the rams work. Could someone post some of pix of how you have your gear door air rams installed. Thanks!
Old 07-03-2017, 06:32 AM
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JDjetjock
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Originally Posted by MaJ. Woody View Post
Thanks for all the info Jeff. One question. When you say dropped the tail pipe in the aft mount to you mean inside the tail cones? It seems you can change the angle by both blocking up the mounts inside the engine bay or removing material on the lower section of the tail cones.
Thanks!
Yes, I removed material from the lower section of the tail cones. I can provide a pic if you would like. I recommend to maintain the downthrust angle of the engine mount...I measured it and if I remember, there is a built-in 5 deg down angle on the mount...you want to keep that or increase it more.

One thing with my engine and the stock tailpipe...it has a wicked sound! Everyone loves it.

Jeff
Old 07-03-2017, 06:48 AM
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JDjetjock
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Originally Posted by Chris Smith View Post
What configuration is the CG at 325? Fuel load, no fuel? Have you tested 325 with engine flame out?
Thanks
Hi Chris,

The config is ready for flight, tanks empty except the UAT/CAT tank full, and gear down, and no external stores. At idle it is well behaved, and landing approaches feel good with a little bit of power. I balance mine using scales under the gear and calculate the actual CG.
I did not have to add any extra weight and I have a full lighting kit and tail drogue system. Mind you I have a 5000mah ECU battery, two 3500mah Rx Lipos, and a 2-cell A123 battery (for the lighting) in the nose. The jet weighs 42.5 lbs without fuel (CAT tank full).

Hope this helps.

Jeff
Old 07-03-2017, 08:52 AM
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Chris Smith
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Originally Posted by JDjetjock View Post
Hi Chris,

The config is ready for flight, tanks empty except the UAT/CAT tank full, and gear down, and no external stores. At idle it is well behaved, and landing approaches feel good with a little bit of power. I balance mine using scales under the gear and calculate the actual CG.
I did not have to add any extra weight and I have a full lighting kit and tail drogue system. Mind you I have a 5000mah ECU battery, two 3500mah Rx Lipos, and a 2-cell A123 battery (for the lighting) in the nose. The jet weighs 42.5 lbs without fuel (CAT tank full).

Hope this helps.

Jeff
Jeff,
I also balance my jets with scales and actual CG calculation. Seems I've asked the right guy! Have you added the Takeoff fuel and calculated the T/O CG compared to the landing CG? Since we rarely land with only UAT fuel it should be different from the original CG.
It would be good to know what the CG shift is during flight.
Thanks!
By the way I am giving a how-to clinic on an easy way to find CG without CG machines or buying anything. the hands on how-to will be at Jets Over KY. Folks can look for the Nashville Jet Band tent for more info.
Old 07-03-2017, 09:58 AM
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ozief16
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Guys,

im getting close to a maiden with mine.

I'm running the three tanks in series from the front tank to one back then the other back tank. This should yield a pretty consistent landing cg (the back tanks sit VERY slightly forward of the CG). Jeff any idea what's the furthest aft cg you've actually flown with (ie the cg when you landed with the least fuel in the plane since any fuel burn results in an aft shift in cg)? Our strip isn't hugely long (750') so a good starting cg will be important. Leaning towards 310 with only uat fuel until I get more imperical data. If you've gotten yours down close to just uat fuel and 325 felt good that would be a great benchmark

I used the Tam pipe and nozzles, lights, and drag chute. I don't have a weight yet but it's starting to feel chubby!

thanks
Dave
Old 07-03-2017, 12:15 PM
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JDjetjock
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Guys,
I'm using the standard feed of the tanks (rear tanks in parallel feeding the front tank). After a 6.5 to 7 minute flight, the front tank is between 50% to 90% full. I find that flaring for landing feels right, but I would not want the CG any further back..it is touchy enough (running 50% expo). Attached are some pics to show the amount of elevator near touchdown. I will measure the CG for you guys later today with full tanks and with the front tank at 50%.

My next flight will be Wingham Jets in Canada later this month.

Cheers,
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Last edited by JDjetjock; 07-03-2017 at 12:19 PM.
Old 07-03-2017, 12:44 PM
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ozief16
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Awesome Jeff. That's great info. Thank you.

325 with a feed from the front tank to the backs might be too much then. I'm glad I asked!

Can you pls shoot a pic of the neutral posit of your elevator?

Dave
Old 07-03-2017, 03:06 PM
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ozief16
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Woody,

Of note, SM sent a pipe despite me asking them not to so I got to compare Tam and SM side by side...no comparison. I did wood ears to the inside of the engine mount rails to fully support the Tam pipe mounts

Also of note (remember I haven't flown it yet, so this is all conjecture), everyone that has seen them agrees the main gear mounts are insufficient. I made up 3mm Cf plates for the bottom of the mounting rails and tapped them for 8-32 (vs the wood screws that were stock). I then made maple blocks to tuck up tight under the bottom of the rails to tie the rails to the former just fwd/aft of the rail and the cf topskin. I will also add maple blocks on the front and back side of the said formers to tie those formers to the cf topskin of the wing outside of the gear mount area as well. I had a couple BV phantoms and always loved the strength of the gear (though I don't think I tested it).

With the nose nose gear I added a bunch of cf cloth in the nose for fear of the long moment arm of the nose slapping down if I pooch a landing

i used the 95mm air power ab rings

i was able to put all the 'important' stuff up on the main board over the intakes (uat, ecu, gear/door/gear blowdown/brake servos, central box for the Jeti (like a rx kind of), etc but titook somw planning
Old 07-03-2017, 04:25 PM
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JDjetjock
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Guys,

Here's the CG for my F-4 for different fuel states (Jet-A):
- full tanks (take-off): 302 mm (weight 53.3 lbs)
- front tank filled to the seam (approx 50% full), rear tanks empty - typical fuel quantity on landing: 309 mm (weight 45.5 lbs)
- zero fuel (except full CAT tank): 322 mm (weight 42.9 lbs)

So it appears the CG moves back approx 7 mm for a typical flight, and up to 20 mm if you have to stretch it out consuming most of the fuel. That last half of the front tank will effect the CG quite a bit.
Perhaps it may be wise to feed the front tank first and try to keep the CG near 305-310mm during the whole flight. That would mean balancing it near 305 mm with empty fuel.

What do you think?

I also beefed up the main gear mounts adding 1/4in ply...I'd have to show in a photo. I'll also send a pic of the H-stab neutral point for my CG setting.

Cheers,

Last edited by JDjetjock; 07-03-2017 at 04:28 PM.
Old 07-03-2017, 05:19 PM
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ozief16
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I think great data Jeff. Thanks. Much better than the wishy washy stuff I've heard before

BAHAHAHHAHAHA

i think ill go 310 with empty tanks and full uat so actual flight will be slightly forward of that to start

I'm debating making the Radom removable for batts but I need to cg first to figure out where to put batts first (mine is an E as well)


Dave
Old 07-03-2017, 05:34 PM
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JDjetjock
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Here's some other pics of the tailpipe, landing gear reinforcement and h-stab neutral posn. The hstab for trimmed flight at my CG is 46mm below that mold line. Note the small amounts of up-elevator mix for half and full flap.

Good luck!
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Old 07-03-2017, 06:15 PM
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Originally Posted by ozief16 View Post
I think great data Jeff. Thanks. Much better than the wishy washy stuff I've heard before

BAHAHAHHAHAHA

i think ill go 310 with empty tanks and full uat so actual flight will be slightly forward of that to start

I'm debating making the Radom removable for batts but I need to cg first to figure out where to put batts first (mine is an E as well)


Dave
you sunuva.....think tommyboy
Old 07-03-2017, 06:37 PM
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MaJ. Woody
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Thanks for all the great info Jeff!
Old 07-03-2017, 07:05 PM
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Geez buckaroo, get a life! You're on this thing too much! BAHAHAHHAHAHA

Jeff, thank you for all the great info.

Thanks to all

Dave
Old 07-04-2017, 01:13 AM
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Ha
Old 07-04-2017, 03:20 PM
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I am using 340 on CG with Tam pipe. Balance with empty tanks. Pipe has 5 degree down angle and sits low in the exit ducts at back. No trim changes during flight. It will fly on 310 but I would move back after first flights and I am sure you will like it much better further back

Non issues with bottom of stab

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