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CG located on fuselage?

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CG located on fuselage?

Old 02-23-2017, 11:13 AM
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oly 9
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Default CG located on fuselage?

Hi gentlemen, I found a CG calc tool that used the dimensions of the wing, tail, and distance between the wing leading edge and the elev leading edge. I have used this before with some success. For grins and giggles I played with changing the distance between the two leading edges. The recommended CG kept moving rearward. I went as far as to extend the distance enough where the airplane would probably not fly but I was trying to see where the CG would end .Well I got one spec that put the CG in the middle of the fuselage between the wing and the tail. I am trying to understand the physic behind this. I assume that the center of lift would always be somewhere on the wing. Would this work with the cg that far off the wing. If anyone can explain why the tail distance from the wing affects cg. I understand that the longer the arm the smoother the reaction etc. but how does this affect cg? anyone help/And thanks.
Old 02-24-2017, 01:55 AM
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Sounds like your calculator has an error. Balance can be behind the wing, but the planform would be for a tandem wing model, balance could be forward of the wing, but it would be a canard layout. Models with a conventional planform can have the balance around 100% chord, but these would be specific models, free flight power comes to mind, and very much 'single speed' trims with long moments.
Evan.
Old 02-25-2017, 07:05 AM
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ahicks
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Something lost on some people is that tail should carry some of the plane's weight. All else being equal, I can see where a longer tail moment might move the CG back. Taken to extreme, it would be impossible to keep "all else being equal" due to the weight of the structure required to get the tail back that far and remain structurally sound.
Old 02-25-2017, 07:59 PM
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There's no error. As you make the fuselage longer the overall stability neutral point moves back. Make the fuselage excessively long enough and the NP ends up behind the wing. And the CG set for some degree of stability follows the neutral point.

Back in the 1950's there was a movement among free flight power flyers to make the fuselage longer and longer and the stabilizers larger and larger. This led to models such as the Civvy Boy where the CG was right on the trailing edge of the wing. And one particular extra long .049 competition power model had the CG indicated at 110% back from the leading edge. That's right, it was 10% behind the trailing edge.

What happens is that as the CG moves to 30% and further back is that the design starts to behave more like a tandem wing airplane. That is the stabilizer lifts upward same as the wing. The further back the more the stabilizer lifts.

What the CG calculator does not tell you is that for a smaller size stabilizer you can over load the lifting area and it will have too high a "wing loading" and not fly well at all. The stab ends up "stalling" when loaded too strongly unless it's a larger size stabilizer like those old free flight models had with their 25 and 30% size stabs. Bad and confusing things occur at about that time.
Old 02-26-2017, 11:44 AM
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oly 9
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Thanks I think I got it now
Old 03-06-2017, 04:12 PM
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Bozarth
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Originally Posted by ahicks
Something lost on some people is that tail should carry some of the plane's weight. ....
Does this apply with a conventional design with positive stability?

Kurt
Old 03-08-2017, 11:13 AM
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rt3232
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would depend on weather the stab had an airfoil or incidence both
Old 03-08-2017, 11:39 AM
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Top_Gunn
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Originally Posted by Bozarth
Does this apply with a conventional design with positive stability?

Kurt

If the center of gravity is forward of the center of pressure, as it is on most conventional designs, the horizontal tail surfaces keep the tail from rising.
Old 03-09-2017, 01:41 PM
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Originally Posted by Bozarth
Does this apply with a conventional design with positive stability?

Kurt
The lift from a wing through the normal range of flight is focused on the 25% chord point. Then if there is some camber then you also see some pitching moment. But if we look at symmetrical airfoils for simplicity the lift occurs at the 25% point with no pitching moment.

So if the CG of the model is set to some point behind this 25% point as is very often the case then the horizontal tail will be lifting upward by some amount. It is purely related to the location of the CG with respect to the wing. The rest such as airfoils used on the stabilizer and angles between the wing and stabilizer only act to trim the model for a given stability margin and flight speed. This can alter slightly when we use camber in the wing. At some flight trims a CG at something like 30% with a strongly cambered airfoil might be lifting upwards. At some other speed it might be creating no lift and at yet a third speed might be lifting downwards. It depends on how strong the pitching moment is and the flight speed at any given time.

But even with a strong pitching moment as we enlarge the stabilizer or lengthen the tail moment and use that to allow us to shift the CG rearwards the stabilizer will shift to lifting upwards all the time and more and more strongly the more the CG shifts to the rear.

This is one reason why old time free flight models so often use lifting airfoils on the stabilizers. The use of an airfoil does not automatically make the tail a lifting tail. Instead it is related to the strongly rearward CG locations such models typically used. Anything from 35 to 60% for a location of the CG was not at all unusual depending on individual designs. One of my own models is an electric powered Henry Struck Record Hound. And at 50% CG it is actually still more nose heavy than is optimal for the glide. I'm waiting on a brushless motor to arrive and then I can install it along with the swap to light Lipo packs and finally get the CG back where it should be. I've only avoided ballasting it further back up to now because it was already quite heavy thanks to the old school geared brush motor it has lived with through the late 80's and all of the 90's.
Old 03-09-2017, 02:25 PM
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All Day Dan
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Mr Mathews, Am I correct to assume that when you say "chord" you mean the Mean Aerodynamic Chord? Dan.
Old 03-09-2017, 04:22 PM
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Yes, that would be the understanding on an overall wing. But when I was describing the lift at the 25% chord of any given airfoil it was more a 2D airfoil thing. But of course when that translates to a real wing it would be related to the MAC.
Old 10-20-2017, 08:59 AM
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jeffEE
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SO... would this airframe have its CG somewhere between the wings? I ask because I am in the first steps of altering a Balsa USA kit into the Fokker V8. But the CG position has put a stop to the bash for now.
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Old 10-20-2017, 02:03 PM
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Rodney
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Originally Posted by ahicks
Something lost on some people is that tail should carry some of the plane's weight. All else being equal, I can see where a longer tail moment might move the CG back. Taken to extreme, it would be impossible to keep "all else being equal" due to the weight of the structure required to get the tail back that far and remain structurally sound.
Not true, most of our models (and real planes) are set up where the stab is usually lifting DOWN for stable flight.
Old 10-21-2017, 07:17 PM
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JohnBuckner
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Originally Posted by jeffEE
SO... would this airframe have its CG somewhere between the wings? I ask because I am in the first steps of altering a Balsa USA kit into the Fokker V8. But the CG position has put a stop to the bash for now.

jeffEE What a wonderful challenge and subject for a kit bash, I have always loved the more unusual subjects and have great respect for the wonderful art of the kit bash simple or major so please do not give up for now. Yes I am certain that the CG for that experimental Fokker with the triplane bay all the way forward and the biplane bay well aft of the pilot would would indeed have the CG somewhere between the two. Even with the horizontal tail surface and apparently an elevator bringing up the rear this airplane is far closer to a tandem wing airplane than a conventional multi plane aircraft.

Rather than shelve the project do give some thought to the idea of using simple stick chuck gliders to experiment with CG locations. Even though it uses triplane front bays and the biplane aft bay, since neither uses any stagger then it would still be useful to substitute monoplane surfaces for the stick to work out useful CG's. I would just try to acheve the same relative positions of the three surface as well as planforms, areas and relative positioning of the three surfaces.

I wish you well and please do not give up

John
Old 10-22-2017, 10:59 AM
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speedracerntrixie
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Originally Posted by Rodney
Not true, most of our models (and real planes) are set up where the stab is usually lifting DOWN for stable flight.

You are absolutely correct that most guys do set up their models this way. However many guys are starting to realize that this setup has many more disadvantages then it does advantages especially when one started getting into advanced flying and aerobatics.
Old 11-06-2017, 05:41 PM
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Jeffee, if this reply is not too late for you I would suggest that with a complex multi surface design of that sort that you make up an all balsa or all thin foam test glider and play around with the CG location. Ideally the rear set of lifting surfaces would be adjustable in angle compared to the forward surface set to play with CG vs decalage angles (as in front set to rear set in this case, not the classic biplane upper to lower wing angles) to find the critical rearward point where the CG and related trim angle between front and rear surfaces still provides a proper recover from a slight stall. But just barely. That would be your most rearward CG location for the proper model.

With something of the sort you cannot rely on the usual CG calculations. They just do not provide for all the interactions of a total of 6 flying surfaces like a test glider would do.

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