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CG Jig Test Run

Old 04-20-2012, 10:23 AM
  #1  
floss
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Default CG Jig Test Run

I thought I would post this for comment,, a CG jig that I built off plans on the net.
If it is as simple and accurate as claimed then it should be worth its weight in gold.

I have put my scratchbuilt DIII in it seeing as this is flying on a guessed CG at the moment,, always suspected though that it is nose-heavy.

The jig says that the CG is 8-10mm rearwards of my guessed one so will re-balance and test fly hoping for a slower landing speed and nicer glide.

Steve
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Old 04-20-2012, 03:04 PM
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JetPlaneFlyer
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Steve,

No CG measurement method will tell you where the CG 'should be'. It will only measure where the CG 'is'. Flight testing is the best way to find out where the CG 'should be'.

If the measurement showed a more rearward CG position and you have added nose weight to move it forward then you will only end up making the nose heavy flying characteristics (if that's what it has) even worse.
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Old 04-21-2012, 01:19 AM
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floss
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Haven't added any weight,, understand fully that a more rearward CG needs weight reduction to balance - Cheers JPF
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Old 04-21-2012, 04:01 AM
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I under stand that this thing does not tell you where the planes balances but where the plane should blance. The blance point should be close to center of lift of the wing. Center of lift depends on several things. One is the airfoil shape. I see nothing on this device that checks the shape of the airfoil.
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Old 04-21-2012, 08:15 AM
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DG
I have had a few online chats with Pat Tritle and the guys on RC Groups about this and having been supplied all the factors that come to play in calculating CGs with measurements in biplanes never once did they mention wing profile? - thankfully because there is enough to factor in as it is without this

Wing chord, incidence, stagger and the difference in size and spacing between wings are leading factors with bipes.

I used to just place my CG 1/3 of the way back from the LE on my scratchbuilds but then I built a bipe with a significantly smaller lower wing and quite a nasty stagger between wings also and then it all went pear-shaped. Pat Tritle answered my plea for help in RC Groups a few years back but taking all the measurements to calculate correctly is time consuming.

This jig is said to place the CG with an accuracy of 1/16 inch, also reliable enough to be used on small models up to 1/4 scale.

It appears to be so simple but takes into account all of the parameters that I was given in RC groups. The fact that the slings suspend the aircraft and wrap the wings following their stagger and chord cover this well, then you simply rotate the dowel and the aircraft levels out and takes a level flying stance balancing under the plum-bob which denotes the centre of the sling set-up.
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Old 04-21-2012, 08:58 AM
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Floss.. Ill say again.. this device DOES NOT tell you where the CG 'should be' it's just a way of measuring where the CG 'is'.

Basically all that's happening is that the plane and the plumb-bob are both suspended from the same point so both their actual CG's end up directly below the suspension point. The plumb-bob therefore ends up pointing at where the planes CG physically is. Lines that go to the plane could wrap around the fuselage or hook over the prop, it would work just the same.


It's easy to prove this... just take the battery out of the plane's nose, and pop it in the cockpit. If the device really was telling you where the CG 'should be' then the indicated CG position would not move (where the CG should be can only change if you change the aerodynamics of the plane).. In reality of course the actual CG position has moved, the plane will swing forward and the plumb-bob will point at a position much further back, thereby proving that all it does is indicate where the CG 'is'.

You were right in that airfoil makes no difference to there the CG needs to be for stability, but the tail very much does. You cant miss out the tail in any meaningful CG calculation. I've got a CG calc in MS Excel that does biplanes. If you want it PM me (file too big to upload here). However if you are already flying the model just make adjustment from flight test results. if the model dives when slowed up and pitches strongly nose up at higher speed then that points very much toward CG too far forward. Use the dive test.
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Old 04-21-2012, 09:09 AM
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Understood JPF, cheers. I will contact you and take up your kind offer
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Old 04-21-2012, 10:59 AM
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degreen60
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Here is how I measure the wing cord on my scratch build multi wings planes so I can guess at a starting blance point.
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Old 04-23-2012, 09:49 PM
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7car7
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I like it. I think the concept is very cool. I agree, it is only showing where the CG *is*.

I might be tempted to try something like this, but the only issue I have is I'm concerned that the rope has too much drag on the wing surfaces and edges. (I am glad the OP stated that you twist the dowl - that would help).

So I'm wondering if one could just attach a couple of pulleys to a dowl, and run 2 big loops of string/cord thru the pulleys, and then the plane would not be fighting friction to let it settle at it's balance point?
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Old 04-23-2012, 11:57 PM
  #10  
Rolling Thunder
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Originally Posted by JetPlaneFlyer View Post
Floss.. Ill say again.. this device DOES NOT tell you where the CG 'should be' it's just a way of measuring where the CG 'is'.

Basically all that's happening is that the plane and the plumb-bob are both suspended from the same point so both their actual CG's end up directly below the suspension point. The plumb-bob therefore ends up pointing at where the planes CG physically is. Lines that go to the plane could wrap around the fuselage or hook over the prop, it would work just the same.


It's easy to prove this... just take the battery out of the plane's nose, and pop it in the cockpit. If the device really was telling you where the CG 'should be' then the indicated CG position would not move (where the CG should be can only change if you change the aerodynamics of the plane).. In reality of course the actual CG position has moved, the plane will swing forward and the plumb-bob will point at a position much further back, thereby proving that all it does is indicate where the CG 'is'.

You were right in that airfoil makes no difference to there the CG needs to be for stability, but the tail very much does. You cant miss out the tail in any meaningful CG calculation. I've got a CG calc in MS Excel that does biplanes. If you want it PM me (file too big to upload here). However if you are already flying the model just make adjustment from flight test results. if the model dives when slowed up and pitches strongly nose up at higher speed then that points very much toward CG too far forward. Use the dive test.
Dont know if this is correct but if you put the battery in the cockpit as stated then wouldnt the planes nose pitch up indicating the planes tail heavy.
The beauty of the jig is that 1st you must get the plane in a level attitude as shown in the pictures then the plumb bob shows the c.g.
.......I think
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Old 04-24-2012, 12:53 AM
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Originally Posted by Rolling Thunder View Post
Dont know if this is correct but if you put the battery in the cockpit as stated then wouldnt the planes nose pitch up indicating the planes tail heavy.
Yes, that's exactly what would happen it would swing forward on the trapeze with it's nose up. You could then re-level the plane but now the plumb bob would point at a position on the wing further back than before, which of course is correct because the CG has moved back.
This proves the point that I was trying to make in that this contraption only indicates where the CG physically 'is'.. not where it needs to be for proper flight.

Steve
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Old 04-24-2012, 05:07 AM
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So then the correct response would be that the jig shows where the C.G. physically is IN RELATION to the planes attitude in the jig.

So as long as youve got the plane hanging in an level flying attitude then the plumb bob will show the correct C.G.
....... I think
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Old 04-27-2012, 07:00 AM
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I love scratch-building but do admit my techo skills with where CG's should be and such are lacking. It was good of JPF and Degreen to comment here and I learnt from it, of course this tool shows where the CG is and not where it should be - cheers guys.

7car7 - I think the pulley idea may not work mate, the plane can't be free to find its own attitude, it has to be dialled in to a level flight attitude using the dowel that then relies on friction to not scroll back to its desired position.

This requirement was covered in the tools plan, it said if your dowel ends up too loose in the block then cut the bottom of the block through to the dowl hole and using a bolt make it into an adjustable clamp to once again tighten the dowels fit.

In the end it is you that dials in the correct flight attitude and this in a way pivots the aircraft forwards or backwards slightly too as well as nose up/down, the plane won't find this correct attitude itself if all was floating and free as with a pulley arrangement.

Hope this makes sense in words

Steve
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Old 04-27-2012, 07:09 AM
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Rolling Thunder

Agreed - a level flight attitude will always show exactly where the CG is no matter how crazily out of adjustment you have it - even if the battery was in the cockpit. So the tool is great at showing this.

The last line on the tools instructions says it all, from memory something like - "so you will always know where the CG is, finding where the CG should be is your problem"

...........I only read the last line after getting the advice from JPF
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Old 05-03-2012, 03:54 PM
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Just a quick question for my general education.
Am I correct in assuming that if the tail feathers are level when on a balance rig then adjustments can be made from that point, ie: nose down. It is sometimes hard to judge a plane because of the angles and shapes the fuse has.
I have this Maxford Alby finished. I had everything mounted. They called for 2.5 to 2.75 back from the LE. I put marks at 2-2.5-2.75 and set her at 2.5. She was tail heavy. At 2.75 she seemed balanced. I put a small bubble level on the tail and a second one on the nose (engine) to counter balance the first level. The bubble was dead center. I wanted a slight nose down so I have added a 1/2 oz of weights inside the batt compartment on the inside firewall.
Am I close enough or way off base.
Thanks
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Old 05-03-2012, 05:46 PM
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Sounds like you are pretty close. For first flights, I stay away from the rear balance point, as that will make controls more sensitive. You may find after several flights, that you would like more or less sensitivity, but that is personal.

The only way to really find out where the CG should be is by flying, not once but many flights. When you are comfortable with the trims, and the plane will fly hands off at a moderate throttle, then climb high, and do the dive test. That will show you if the CG is right.

My aerobatic friends test there planes in this way, on the first flight of the day, to make sure nothing has changed.
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Old 05-04-2012, 01:01 AM
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Wildflyer

Can you go over the dive test for us please, read about this years ago but have long forgotten - remember something about the plane diving then climbing etc.

Thanks.
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Old 05-04-2012, 06:21 AM
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Default CG Dive Test

OK this is how my flying buddy does it.

At a moderate throttle but not full wide open. Adjust the trims on your plane, until it is flying a nice straight and level flight, with hands off, across in front of you. Now put the plane into a 45degree dive (approximate angle ) then hands off, if the plane starts to curve upward the plane is nose heavy, if the plane starts to dive even steeper, then it is tail heavy. If it stays on the same downline then you are good.

Sort of the way this works is that if the plane is nose heavy, you are carrying at little up elevator trim, when the speed comes up in the dive, the up elevator trim is more effective at the higher speed, so it starts to climb. Of course the same idea hold for tail heavy, and the increase in angle.

The down force of a little up trimmed elevator, is what is carrying the nose heavy weight. Just like a pivoting lever.

It may take several flights to get this right, it took about 4 to get my F-16 EDF right. If you have a full symmetrical wing, then the right CG will mean it will take very little down to fly inverted.

I hope you can figure out what I mean.

Do the dive at a distance from you, it's hard to see the change in angle if you are too close.
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Old 05-04-2012, 06:37 AM
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Thanks Wf, just needed to hear it again and you've explained it well. I'll have a go.

Steve
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Old 05-04-2012, 05:20 PM
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I generally ignore the manufacturer's recommended CG, set it so it "looks" right when I balance the plane on two fingers and then fly it. I adjust from there until it is nailed. I view static CG settings as starting points. Frankly, they don't have to be that accurate. The accuracy comes from adjusting after flying. It wouldn't even occur to me to use fixture to set CG.
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Old 05-07-2012, 04:16 PM
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I like the idea of the Dive Test, and I'll try it.

But would it throw it off if a plane does not have the correct wing incidence? Or thrust angle?

Oh, and also, would it not do the same thing to just keep it going level and punch the throttle all the way?
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Old 05-07-2012, 07:16 PM
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Originally Posted by 7car7 View Post
I like the idea of the Dive Test, and I'll try it.

But would it throw it off if a plane does not have the correct wing incidence? Or thrust angle?

Oh, and also, would it not do the same thing to just keep it going level and punch the throttle all the way?
Incidence (within reason) wont matter because you trim that out with elevator.

The reason why you dive rather than open the throttle is to remove any effect from thrust angles.. So no, thrust angle shouldn't make much difference (within reason). Also no, opening the throttle is not the same because then you don't know if what the plane does is due to thrust or due to speed.

Steve

Last edited by JetPlaneFlyer; 05-07-2012 at 07:41 PM.
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