Center of Gravity Machine for a 3D plane

[David22]

I bought the Hangar 9 Twist 60 and installed this in it.

http://www.vimeo.com/2766872

[ndb8fxe]

very innovative. Have you tried flying yet? I must say it's a great idea.

[David22]

No unfortunatly the plane is new and there is snow on the ground.
Still have to maiden it. Still have to start it.:)

Going to make one for a large glider.
It will be 1/4 inch carbon rod and somewhere around 28 to 36 inches or so long.
But exactly the same thing but way lighter.
It will also be used as a spar for the tail.:eek:

David22

[JetPlaneFlyer]

What are you trying to achieve by moving the CG in flight:confused:

[David22]

Exactly what I have been thinking for about six months now and still ask today.

What can be achieved by moving the cg durring flight with a 3d plane.


My next question is what can be achieved by moving the cg durring flight with a sailplane?

I figure it will make for some pretty cool 3d tricks and as for the sailplane it will let me fly longer.

It is just for fun, for competitions it is not allowed. As a sport flyer my rules are basically non existent.

I know some expert about planes could answer the questions better than me.

BTW the extra weight is not really an issue for me. The 4-5 oz in total weight of the cg machine pod has been compensated for with a larger lighter motor and light
lipo battery.

David22

[JetPlaneFlyer]

Hmmm...It may be fun experimenting but for most aircraft there is an optimum CG position and once you have found it then there is no point in further adjustment, so for most models i cant see what benifit it could deliver. Certainly in the case of the glider then performance in terms of sink rate would be damaged due to increased weight of the CG adjusting mechanism.

3D may be different, making the model go deliberately unstable by moving the CG aft could produce some rather interesting uncontrolled aerobatics :eek: .. Just make sure you can get the CG forward again before the model meets the ground ;)

The extra weight would surely be a problem on a 3D model when it came to hovering type 'tricks'?... Don’t they need to be as light as possible? Would moving the battery foward and aft rather than adding a big hunk of lead not be a better way to do it (though maybe harder to implement)

Steve

[David22]

The idea is to put as little weight as possable right on the recommended cg and send it as far back as possable. For larger nitro planes 4 oz does not make much a difference as for large e sailplanes 1-3 oz won't make a difference either. But in the tail it will.:)

[JetPlaneFlyer]

But in the tail it will.:)

yeah, it will make it crash :rolleyes:

[Dad_Roman]

Would moving the battery foward and aft......
You mean like this??? :D


I have found that a maxed out 3D setup generally makes a plane a handful to land (for obvious reasons) but being able to have a radical 3D CG in flight then a docile CG on landing is a true pleasure.

I love my 4Star ;)

[kcaldwel]

George Moffat used a mercury pump system in the Nimbus 4 full scale sailplane to shift the CG during flight. He did it to avoid the trim drag of a deflected elevator. The % gain was very small, and I don't believe it has ever been repeated. He had mercury tanks in the tail and near the CG I believe.

Other than trim drag, what advantage do you see for a sailplane?

Kevin

[David22]

Ok one question for you guys.

Why is it not permitted in contests?
Why was it outlawed?
why is it against the rules?

That is why i did it!

David22

[David22]

Dad

You are the one who started me on this.
I asked has anyone done this before and you showed me your pics.
That was months ago, however if it were not for you, my inspiration would be way way less.
BTW I just found out yesterday:eek: it was against the rules when my hobby shop guy said so! That is why i jumped right onto the glider version right away!

Thanks Dad

David22

If I crash on my maiden you guys are going to really laugh lol

Twisting away
You mean like this??? :D


I have found that a maxed out 3D setup generally makes a plane a handful to land (for obvious reasons) but being able to have a radical 3D CG in flight then a docile CG on landing is a true pleasure.

I love my 4Star ;)

[Brandano]

The idea that by having the CG far back the plane will hang better by the prop is false. It won't work, and I could go to great length discussing why it won't, but unfortunately the illusion is really strong and it's difficult to get people to let go of an illusion. In any case, it could perhaps work with an articulated rotor, but on a rigid rotor it won't make any difference.

[David22]

You are probably right! Brandano
My hobby guy said the same thing about the `hang better by the prop.``3d effect and said the opposite is true to what i have done.

He tried to explain to me that if I had the weight the other way towards the motor it would be easier to hover with a 3d plane.


But all the 3D learning sites say for better 3D, move the cg back?

[JetPlaneFlyer]

Ok one question for you guys.

Why is it not permitted in contests?
Why was it outlawed?
why is it against the rules?

That is why i did it!

David22

How do you know it is outlawed in all contests?.. I'm not aware of any such rule in, for instance, glider contests (though i've not searched through the rule book I have to admit)
If the rule is there maybe it's there because it would make the model potentially un-stable and un-controllable and so a hazard to those on the ground?

In a glider any incredibly tiny advantage that could be gained by reducing 'trim drag' would be more than outweighed by the added weight, small though it may be.

Steve

[David22]

I was hopeing to hear from someone why they are not allowed as well.

It never crossed my mind it was because of safety, I thought it was because of an unfair advantage.

I will look into it and find out why cg machines are a no no in a plane/glider for competition.

David22

[David22]

For high speed records with an f5b or very fast plane they won't alow me to put an onboard fpv system on it.
"No communication with the plane"

[Brandano]

You are probably right! Brandano
My hobby guy said the same thing about the `hang better by the prop.``3d effect and said the opposite is true to what i have done.

He tried to explain to me that if I had the weight the other way towards the motor it would be easier to hover with a 3d plane.


But all the 3D learning sites say for better 3D, move the cg back?
Well, it's indifferent if the CG close, far away or even in front of the prop. When the plane tilts the prop tilts with it, the vector of thrust is always in the same position relative to the CG. What really matters is how far away the CG is from the tail feathers, the longer the moment arm and the more effective these will be to keep the plane under control. moving the CG backward makes it easier to transition to the hover, since the plane won't tend so much to drop its nose after the stall, but won't really make much of a difference in the hover proper.

[kcaldwel]

Actually, the glider rules specifically allow altering the CG in flight:

5.Q.2.1.2 Organisation of a round

h. In flight, lifting area, angle of attack and centre of gravity may be altered by remote control.

There does not seem to be a specific rule allowing or outlawing CG shift in the aerobatic rules.

http://www.fai.org/aeromodelling/documents/sc4

Kevin

[David22]

I looked everywhere for that regulation
I am sure it is correct. I phoned the guy who told me that it was banned and he said just for a particular contest.


Oh well, for what it costs i will make one up for a glider.

If all fails it will allow me to find the "G" spot on my gliders. lol
ok bad joke

It won't be a waste of time.

David22

but thanks all
will keep you posted even if I crash.

[Brandano]

Well, if it isn't explicitly forbidden you could shift the center of pressure instead, by adding retractable canards in front of the wing

[David22]

In the case of modeling science do up have any links for retractable canards?

David22

[Brandano]

Well, the one full scale example for retractable canards as a way to shift the center of pressure as a trimming measure is on the F14 A, in the form of the fixed pitch "glove vanes". These were removed in the later versions, though. another plane that uses variable geometry canards, again as a way to trim for cruise is the Beech Starship.

[David22]

Obviously you know way more about this than I do but,
Glove vanes for a glider?
I thought they are for jets with wings that can be pulled back?

I thought instead of useing my flaps or elevator to create lift on a glider shifting the cg back would be better for me. less drag right.

For my 3d plane it is more of a joke, something fun. For my sailplane I would be much more acurate with the weight, and how much lead in the tail is enough.

The glove vanes must slow me down, kind on like flaps or elevator trim in a trermal?

So that is why my simple system, my dream, would be best. Once i get used to a really far back cg, I can remove the cg machine and install it into another plane.

No?
David22

[Brandano]

The scope of the glove vanes on the F14 was to trim the plane and unload the tail surfaces, which in turn results in less induced drag at the cost of higher surface dragTo get the same effect on a glider probably it would be better to change the sweep of the main wing, or to have the entire wing mounted in such a way that it could be moved forward and back. I think that it would really be way too complex for any benefit to offset the disadvantages. I remember vaguely a full size experimental flying wing glider that used variable sweep for trim, though.

[David22]

Your glider explaination is over my abilities to fabricate that kind of glove vane set up. It is too complicatted for me let alone understand.

I understand what you meant about the transition for a 3d plane to go into a hover, but what is going to happen once the cg is sent back and I have become hovering?

What should I expect, an easier time entering a hover, but a harder time hovering ?

I thought once I am hovering it would be a joke to stay like that. But no.:(

David22

[JetPlaneFlyer]

David,
I know nothing about 3D, so i'll not comment on it.. However for the glider moving the CG back wont "create more lift" all it will do is decrease longitudinal stability or even , if the CG goes back far enough, make the model unstable.
Flaps increase the camber of the wing giving it a higher Cl max (i.e. it can produce more lift before it stalls)... Moving the CG can never achieve the same thing.

I'd not get too excited about retractable canards and the like.. As far as I know they are mainly used on supersonic/transonic aircraft to compensate for the movement in aerodynamic centre that occurs as the speed of sound is reached... Hardly an issue for a RC glider ;) . Strangely enough this same issue is where adjustable CG is sometimes used in full size aircraft, the Concord being an example.

Steve

[David22]

I am still confused, I thought shifting cg in a glider or 3d plane was not allowed for a reason, but now "in writing" it says it is allowed?

It must mention it is allowed for a reason?
I do not understand, sorry?

[kcaldwel]

As I posted, CG shifting does not appear to be disallowed by the FAI, or the AMA, for either gliders or aerobatic aircraft. Your information it was illegal was wrong.

AMA rules:

http://www.modelaircraft.org/events/compreg.aspx

The only possible advantages I can see are reducing the very small trim drag to fly at different speeds for a sailplane, or making a 3D model more stable for landing.

If you investigate the pitch stability of aircraft, you will see they become unstable once the CG is moved back past the aircraft neutral point. It is unlikely that the aircraft can be flown without electronic stability augmentation with the CG behind the neutral point.

The only reason it is likely mentioned that it *is* allowed in the FAI sailplane rules, is because it was tried, and brought up discussion at a contest. Since the advantage is likely to be infitismal, there is no reason to disallow it.

Kevin

[David22]

I understand.