AKB

My Feedback: (6)

So fellow Phantom owners, going to be getting ready to do the maiden on this bird and I would like to confirm the C of G settings with those that have good success with this aircraft. Mine is the Model C nose. The Skymaster manual says a range of 230-250 mm behind the leading edge of the wing. There have been some who have indicated on this forum, that they had experienced some elevator blanking issues, when full flaps were deployed. After reading this some again have remarked it was best to make sure that the aircraft was slowed before deploying gear and flaps. Also it was also suggested to make sure the control surface speed of the flaps to be throttled down to a slow speed as well. So it was also suggested by some to have the C of G moved back to 300 mm behind the leading edge. This is a further 2" back then the manual states. I guess I just want to make sure this is the setting that will control any elevator blanking when full flaps are deployed. I have already lost an aircraft before on total elevator loss when the nose was below the horizon and full flaps were used.

So to summarize 1) would the best setting for C of G be 300mm behind the leading edge of the wing 2) when activating full flaps make sure that airspeeds are at a slow safe maneuvering speed 3) and have the deployment speed of full flaps be quit slow? Again these safeguards in place so that the C of G is stable for the aircraft in all flight speeds and that the landings at full flap settings are predictable.

Thanks for your help.

Alan

dubd

Start at 300mm and adjust from there. She's a big *****cat at 300mm.

yeahbaby

My Feedback: (21)

+1 on CG.... P-kat......And always be at a reasonable speed when deploying flaps. She's a dream to fly and despite already owning another fabulous phantom, I wouldn't mind getting another one.

Have fun fun and good luck.

AKB

My Feedback: (6)

Thanks fellows for the help.

Alan

joeflyer

My Feedback: (48)

AKB,

You're looking at the wrong page in the manual. The recommended CG range for the Skymaster 1/7.5 version is 290-300 mm. The 230-250 mm. range you quoted is for the Xtreme 1/9.5 version. In both cases slightly behind their recommendation seems to work best.

f106jax

My Feedback: (16)

Alan,

I've found 305-310mm to be the sweet spot. 300mm is a perfect starting point. Wishing you the best with a great flying Phantom.

SStricker

I have been playing with the CG on my Skymaster F-4 and I have not yet found the aft limit. It just keeps getting better and better as the CG moves back. Right now it is balanced at the main spar with the front tank 7/8 full of fuel. I have moved it even further back for more testing at the next event.

When it was balanced at 300 mm with the front tank empty I found that the airplane felt heavy in general, the nose would fall through in the turns, it wouldn't hold knife edge with full rudder, the slow rolls were sloppy and the upline rolls were heavily top side biased indicating that the plane was carrying up elevator. Since shifting the CG back, it feels much lighter overall (even though it is 2 lbs heavier with ordinance and other add ons) it holds knife edge with 1/4 rudder and slow rolls like a dream.

I do however, generally land with the front tank full, I like to have ample fuel reserves at busy events. So I never really get to feel the CG with the front tank depleted. I hope this offers insight into your CG question.

ravill

My Feedback: (11)

This.

At this setting, that whole elevator blanking issue is gone and she's an absolute sweet heart to land. It's great flying a jet around knowing that your landing is going to be one of the least stressful things you'll do.

Svein Erik Riis

Anyone knows if this is ok for the FEJ 1/7 F-4 too?

ravill

My Feedback: (11)

If FEJ doesn't give you a CG (and they probably didn't even fly theirs , you are their guinea pig) this is likely a reasonable place to start.

JDjetjock

My Feedback: (2)

Hi Alan,

I'm patiently waiting for my SM F-4E...so thanks for asking about the best CofG. I'll probably start mine at the 305mm point.

Another two questions for the F-4 guys out there:

1. What is the approx neutral AOA point for the stab for trimmed level flight? This is not provided in the instructions.
2. Does the F-4 pitch up or down when deploying flaps? It is significant or easily trimable during test flights?

Much appreciated.

Jeff

AKB

My Feedback: (6)

Thanks again fellows for your replies. Hey Jeff have fun with your build.

Alan

yeahbaby

My Feedback: (21)

Unless someone posts before I do, I have a pix of where my neutral point was located with a CG at 300 mm

You will need a couple clicks of up trim when deploying flaps. If you deploy flaps at a higher than optimum speed the F4 will pitch forward and can be quite aggressive depending on how excessive the speed was.

Easy to trim inflight. I think my total up trim was around 1/8" from neutral flight to half to full flaps

kind of hard to see due to the glare but there are 3 small tick marks indicating from top down: clean wing, 1/2 flaps, full flaps. Mine weighed 40lbs dry with an AMT 450, Tam pipe and nozzles. great flying airplane.

JDjetjock

My Feedback: (2)

Thanks Buck. Just what I was looking for! I can make out the three tick marks...looks approx 2 inches below that panel line. I'll have a Mammoth in mine.

Cheers,

Jeff

yeahbaby

My Feedback: (21)

yup, roughly 2" below the panel line. One additional item to consider is to grind / file / sand etc. as much of the aft fuselage as feasible so that the elevator can get max up stabilizer deflection, w/o causing the servo to bind. On my first series of flights I maxed out my elevator throw on landing and had to compensate with additional power.

The F4 handled the additional landing speed with zero difficulty. The additional stabilizer throw enabled reasonable balance between power / pitch attitude.

Like Dantley said previously....she's a Pu55y Kat to land.

luv2flyrc

My Feedback: (6)

Thanks for posting that! I just set mine up a few nights ago and "eye balled" a neutral stab with the leading edge a good 1/2" higher than yours. I just made the adjustment to match yours, might have saved me a bit of a hairy maiden . It's deceiving because it sure looks like it has up elevator now.

Looks like the F4 has had a bit of a resurgence, a number of guys building one on here now! Hey Jeff, hope we can get together and fly them in Wingham

Mike

yeahbaby

My Feedback: (21)

Happy to help amigo. you will really enjoy watching it fly. I need another one Dammit!!!!

ChrisPetersen

My Feedback: (3)

This is a very timely thread for me too. I am getting ready to maiden mine finally and the info is much appreciated.

JDjetjock

My Feedback: (2)

Hi Mike,

If the jet is ready for Wingham, then we'll definitely fly together! Good luck with your maiden and keep me posted!

Jeff

SStricker

Here is my neutral point given these conditions. The CG is at the center of the main spar ~300mm with the front tank 7/8 full. The neutral trim point is somewhat speed dependent so the telemetry tells me that it's about 165 mph at maneuver entry. The F-4 likes speed. I have flown at reduced throttle but the plane seems to lock in at a higher speed. The neutral trim condition shown in the earlier post may reflect a more forward CG than mine and possibly neutral trim at a slower speed.

The first photo is neutral trim with flaps set at 0 deg. The second photo is neutral trim with flaps set at 45 deg. At 15 deg of flap there is very little trim change.

Craig B.

Hi Steve,


on my F4 I noted a tendency for the pitch trim to be affected by throttle settings. With full power the model would pitch up and at reduced power it would pitch down. The theory at the time was that varying exhaust gas velocity under the stab was affecting the pressure or lack thereof under the stab. At high throttle settings, the high exhaust gas velocity was reducing pressure under the stab generating a downwards force on the stab and this would pitch the nose up and vice versa.


I finished up putting in a throttle to down elevator mix which ironed it out. C of G at 300. Have you or anyone else noticed this?

JDjetjock

My Feedback: (2)

Thank you very much Steve. Seems that a good neutral point to start with for 1st flight with flaps up is at approx 2 inches down from that panel line. Expectations are minimal to nil pitch down with t/o flaps (at reduced speed) and approx 1/8" additional up trim with landing flaps (45 deg). This is valuable info to set up the plane for 1st flight and is one less unknown to worry about. And, as Buck says, ensure max stab travel is available, if needed.

Hope to see you guys at Kentucky jets.

Cheers,

SStricker

Craig B,
Yes, the Skymaster F-4 does pitch up with full throttle and even more so at high power low speed conditions. This is primarily caused by insufficient amount of down thrust in the Skymaster kit. Mine and most other SM F-4 owners have noticed this condition. I added some additional down thrust in mine but not quite enough. I worked with Mr Garcia as he was building his SM F-4 and he added more which seemed to correct the problem. As to the actual down thrust measurement I don't know either what it is stock or what mine or Mr Garcia's is now. What I will do on the next one is match the down thrust angle of the real F-4. I can see that it is quite significant from the drawings and from looking at in person.

yeahbaby

My Feedback: (21)

Are you guys experiencing pitch change with power using the stock pipe? I used tam's and didn't experience this phenomenon. I will caveat this by admitting any pitch change I may have experienced was barely perceptible with my flying skills.

The reason I ask about the stock pipe is because it has or had a slight upward vector at the end of the pipe, unlike tam's which exits straight.

cheers

SStricker

I have been using the Tam pipe. I have never flown the stock pipe so my personal observations are only relevant to the Tam pipe configuration.

The effect is most easily noticed, for example, at the top of a series of vertical rolls. The pitch up tendency becomes overwhelming as the airspeed approaches zero until down elevator no longer has any effect and the plane arches over toward inverted.