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BVM F-4 CG

Old 12-27-2008, 01:43 AM
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motstud74
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Default BVM F-4 CG

I wanted to know where you guys have the CG set the best? The plans show 12-1/4” measured outboard along the leading edge. There is an addendum to change it to 12-3/4”. I remember speaking to Phil Nuza and he I thought told me to measure it in the middle of that box on the leading edge. If that the spot the CG is at 14" measured outboard along the leading edge. Any info will be greatly appreciated. Thank You.
Old 12-27-2008, 07:40 AM
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CraigG
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Default RE: BVM F-4 CG


ORIGINAL: motstud74

I wanted to know where you guys have the CG set the best? The plans show 12-1/4” measured outboard along the leading edge. There is an addendum to change it to 12-3/4”. I remember speaking to Phil Nuza and he I thought told me to measure it in the middle of that box on the leading edge. If that the spot the CG is at 14" measured outboard along the leading edge. Any info will be greatly appreciated. Thank You.
Motstud,

12-3/4" along the leading edge is the correct place for the CG and on my F-4 there is no box at that point. Not sure what Phil was talking about or if you misunderstood him but I would definitely start at the 12-3/4" point. Oddly, this is one airplane where a slight forward CG makes the airplane less stable.

Craig
Old 12-27-2008, 09:57 AM
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Tom Antlfinger
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Default RE: BVM F-4 CG

Hi Craig:

I always wondered why BV chose to use a distance along the leading edge on the F-4 and that little hole near the leading edge on the F-100D/F. With a swept wing, if you extend the C.G. point with a perpendicular line from the C.G at the root, out to the absolute front of the leading edge, you will get one measurement vs if you measure along that same perpendicular 1.5" more aft like the holes on the F-100. Why not do the measurement at the root like he does for most of his other models?

I notice on my Phantom that a perpendicular from the C.G. at the root measured about 1" back from the leading edge passes thru both the 12 3/4 point, as well as thru the center of the box at the front of the leading edge, if that makes any sense.

So when you do your C.G, how do you suspend the model.......right along the leading edge with a device like Tom Cook uses on the Firebird, or with your finger or some some sort of rocker device and inch or so back from the front of the leading edge?

Tom
Old 12-27-2008, 10:48 AM
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Default RE: BVM F-4 CG


ORIGINAL: Tom Antlfinger

Hi Craig:

I always wondered why BV chose to use a distance along the leading edge on the F-4 and that little hole near the leading edge on the F-100D/F. With a swept wing, if you extend the C.G. point with a perpendicular line from the C.G at the root, out to the absolute front of the leading edge, you will get one measurement vs if you measure along that same perpendicular 1.5" more aft like the holes on the F-100. Why not do the measurement at the root like he does for most of his other models?

I notice on my Phantom that a perpendicular from the C.G. at the root measured about 1" back from the leading edge passes thru both the 12 3/4 point, as well as thru the center of the box at the front of the leading edge, if that makes any sense.

So when you do your C.G, how do you suspend the model.......right along the leading edge with a device like Tom Cook uses on the Firebird, or with your finger or some some sort of rocker device and inch or so back from the front of the leading edge?

Tom
Hello Tom,

I agree that it would be much simpler for BV to just provide a distance back from the leading edge for the c.g. In reply to your second paragraph, no, it doesn't make sense! I just measured down the L.E. 12-3/4" (photo) and made a mark on both wings. I connected the marks with a string or straight edge and put a small piece of tape on the bottom on each side near the fus (two blue pieces of tape representing the 12-1/4" and 12-3/4" positions in photo). Not sure what "box" you guys are talking about or where the 1" behind the LE comes in to play.

I just use my flimsy wire Great Planes balancer to suspend the plane. It's too imprecise and awkward to try and balance it along the leading edge with your fingers. For my bigger planes I've got a contraption I made out of 2 X 4's to balance my planes but it takes at least 3 people. I think in the future I will use the JPO method of weighing each wheel and using their spreadsheet to calculate c.g., but of course you still need a longitudinal datum reference for the c.g.

Craig

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Old 12-27-2008, 03:43 PM
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yeahbaby
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Default RE: BVM F-4 CG

craig

what are those scoops on the bottom of the fuse? they are located on what looks like an access hatch.

that's a good looking wing
Old 12-27-2008, 05:18 PM
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Default RE: BVM F-4 CG

ORIGINAL: yeahbaby

craig

what are those scoops on the bottom of the fuse? they are located on what looks like an access hatch.

that's a good looking wing
Buck,

Those are air scoops called for in the plans/instructions (not scale). They provide extra cooling air flow into and around the exhaust tube.

Craig

PS: How's your new life?
Old 12-27-2008, 05:51 PM
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David Gladwin
 
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Default RE: BVM F-4 CG

Not sure why you chaps are bothered about this. I used to fly my F4 with the CG at 12 1/4 inches along the LE, flew fine, then BV sent me a note saying it flew better at 12 3/4 inches, moved mine back flew even better, think it improves landings too.

I just measured mine by standing at the front of the model and lifted it on two fingers pointing towards each other at the 12 3/4 point marked on pieces of masking tape. Simple quick, works ! Indicating the CG along the LE seems to me the simplest way of doing it and I now use exactly that technique on all swept wing jets, inc. 45 pounds of Hawk !

Regards,

David Gladwin.
Old 12-27-2008, 10:40 PM
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PHIL NUZA
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Default RE: BVM F-4 CG

I fly all of mine 14 1/8 on the leading edge puts it center in the box.
Old 12-27-2008, 11:04 PM
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DavidR
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Default RE: BVM F-4 CG

Me too on the center of the box, been that way for 8 years now.
Old 12-28-2008, 12:12 AM
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Default RE: BVM F-4 CG

LOL

ask me in 10 months. right now am as freaked out as i am excited.

thanks for the info. looks cool i may add it to mine just for schnitzingiggle

happy new year

buck


ORIGINAL: CraigG

ORIGINAL: yeahbaby

craig

what are those scoops on the bottom of the fuse? they are located on what looks like an access hatch.

that's a good looking wing
Buck,

Those are air scoops called for in the plans/instructions (not scale). They provide extra cooling air flow into and around the exhaust tube.

Craig

PS: How's your new life?
Old 12-28-2008, 02:08 AM
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Tom Antlfinger
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Default RE: BVM F-4 CG

Hi Craig:

Point I was trying to make was that if one is going to use the leading edge for C.G. on a swept wing, you have to balance preferably directly at the leading edge which can be mechanically difficult. As you move your finger aft of the apex of the leading edge, I think you can see in the photo that to maintain the same C.G. line, you have to move to a different location on the leading edge since the planform is not a rectangle, but 1/2 of a parallelogram.

Tom Cook's Firebird C.G. locater does a good job of placing the balance point right at the apex of the leading edge.

All of this is probably superfluous anyway since it seems that guys are flying anywhere from 12 1/2 out to 14 1/8 with good results as I have seen your excellent flights as well as David, Vern, and Phil's flights.

I usually go out and fly and see if it wants to tuck or snap and adjust the final C.G. accordingly like no doubt you do as well.
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Old 12-28-2008, 02:40 AM
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Default RE: BVM F-4 CG

Just pulled 6 ozs out of the nose on mine. I am at the back of the box and it flys nice. Took out some of the travel on the stab servo, about 20%. The airplane feels much lighter in pitch.
Planning on an all lipo battery fleet the F-4 is the last hold out. I dropped the 6 cell ECU nicad pack for a 3s Lipo pack. The next step is the 5 cell nicad rx pack but it probably will the cg too far aft.
Old 12-28-2008, 08:03 AM
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Default RE: BVM F-4 CG


ORIGINAL: Tom Antlfinger

Hi Craig:

Point I was trying to make was that if one is going to use the leading edge for C.G. on a swept wing, you have to balance preferably directly at the leading edge which can be mechanically difficult. As you move your finger aft of the apex of the leading edge, I think you can see in the photo that to maintain the same C.G. line, you have to move to a different location on the leading edge since the planform is not a rectangle, but 1/2 of a parallelogram.

Tom Cook's Firebird C.G. locater does a good job of placing the balance point right at the apex of the leading edge.

All of this is probably superfluous anyway since it seems that guys are flying anywhere from 12 1/2 out to 14 1/8 with good results as I have seen your excellent flights as well as David, Vern, and Phil's flights.

I usually go out and fly and see if it wants to tuck or snap and adjust the final C.G. accordingly like no doubt you do as well.
OK, that makes more sense. I guess the "box" everyone is referring to is the square panel just aft of the LE at about the 14" point (on top). If Phil and DavidR are using their fingers to balance at that point I suspect their actual cg is very close to mine. It's also apparent the cg is not critical within a range in that area.

I would still like to see a photo of David Gladwin bending over and balancing a 45 lb jet with his fingers.....from the front only please!

Craig
Old 12-28-2008, 11:28 AM
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Default RE: BVM F-4 CG

ORIGINAL: CraigG

I would still like to see a photo of David Gladwin bending over and balancing a 45 lb jet with his fingers.....from the front only please!

Craig
No problem for David "Edward Scissorhands" Gladwin

This is the balancer that one of our club members fabricated. The bases slide into one another so you can adjust it for various widths between the wings. You can get pretty close to the apex of the leading edge with it.
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Old 12-28-2008, 01:04 PM
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Default RE: BVM F-4 CG


ORIGINAL: Tom Antlfinger

This is the balancer that one of our club members fabricated. The bases slide into one another so you can adjust it for various widths between the wings. You can get pretty close to the apex of the leading edge with it.
Wow! No way now that I'm going to post a pic of my homemade 2 X 4 balancer.
Old 12-28-2008, 01:12 PM
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Default RE: BVM F-4 CG

Dude!!!![X(]

now that is one sweet cg machine

ORIGINAL: Tom Antlfinger

ORIGINAL: CraigG

I would still like to see a photo of David Gladwin bending over and balancing a 45 lb jet with his fingers.....from the front only please!

Craig
No problem for David "Edward Scissorhands" Gladwin

This is the balancer that one of our club members fabricated. The bases slide into one another so you can adjust it for various widths between the wings. You can get pretty close to the apex of the leading edge with it.
Old 12-28-2008, 04:43 PM
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Default RE: BVM F-4 CG

Now theres a challenge Tom, but it may not be a pretty sight. ! BUT gonna move my C of G back..... and the Phantom's !
DG.
Old 12-31-2008, 02:14 PM
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Default RE: BVM F-4 CG

Motstud,
A 45 lb. model is probably over the limit to balance using "model airplane techniques". Why not use the full size method of "weight and balance" used on all full size aircraft? After all where would you "pick up" a full size Phantom??? The method uses a simple equation which gives the distance from an imaginary "datum plane" to the CG location. Here it is:

SUM OF THE CLOCKWISE MOMENTS=SUM OF THE COUNTERCLOCKWISE MOMENTS.

All you need to do is pick a location for your "datum plane" (it can be anywhere, i usually use the nose of the airplane). Take accurate measurments from the datum plane to the nose gear and to each main gear. Record the weight on each gear separately and then calculate three moments: 1) datum to nose gear X weight on nose gear (in in.lbs), 2) datum to left main x weight on left main, 3) datum to right main x weight on right main.
The left side of the equation then becomes: Total weight of the airplane X distance from the datum to the C.G.
The right side of the equation then is: the sum of all three counterclockwise moments (left main moment + right main moment + nose gear moment. In in. lbs..
Solving the equation for C.G. you have:
C.G. = the sum of the three moments (in.lbs.) divided by the total weight of the airplane. The units of lbs. cancels out in the division and you are left with a distance in inches which is the C.G.'s location from the datum plane. Much more accurate than trying to "balance an airplane on your fingers or using a makeshift balance stand which at the weight of a Phantom or F-100 could poke holes in the wings.

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