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Old Mar 13, 2015, 07:28 PM
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Question
When is a plane in perfect balance?

When is a plane is perfect balance? Is it when the horizontal stab is level?
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Old Mar 13, 2015, 09:51 PM
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When the wing sits level. The tail should be slightly negative at that point.

Actually when it's that close it's close enough either way. There's a range of acceptable positioning where at the forward extent the model will behave like an overly stable trainer. As you shift it to the rear over about a 8 to 12% chord range it becomes less and less pitch stable in that it takes longer and more altitude to recover hands off from a dive. When it's at neutral stability it won't recover at all with no input. This rearward extreme is where a lot of fun fly models and serious sport fliers set their balance point locations.

When you get down to the ragged edge you work with in flight testing for feel and results. We only balance on our fingers or other points to get to where it's safe enough to fly the first flights.
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Old Mar 14, 2015, 08:39 AM
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When the plane flys reasonably level, and you roll inverted -- and it then needs no (or very little elevator) for level flight. That's my personal criteria.
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Old Mar 14, 2015, 08:55 AM
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When is a plane in perfect balance?

But, it can also depend on the plane and the flier.

From a CG point of view, when a plane flies how I want it to, that to me is near enough.

If you gave the exact same model to two different fliers, and told them the CG was out and they needed to correct it. You could easily end up with two different CG locations.

It comes down to personal 'feel'.

The CG dimension specified on a plan or in the instructions, should be a safe starting point, and is where it should be for the first flight.

It's then up to the flier to decide if they want to move it.
Where the elevator is in relation to the horizontal stab, is not really to do with 'balance', but is generally a good place to start.
Though for a first flight many might recommend a couple of clicks of up elevator before launch.
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Old Mar 14, 2015, 09:19 AM
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"When is a plane is perfect balance? Is it when the horizontal stab is level?"

An airplane is no more than an arrow. It will fly in the direction of the tail feathers. If those tail feathers are other that in the direction of desired flight, you will have to force the aircraft to fly in the attitude you want it to, by some method, or other.

Les
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Old Mar 14, 2015, 10:57 AM
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Quote:
Originally Posted by Marion View Post
When the plane flys reasonably level, and you roll inverted -- and it then needs no (or very little elevator) for level flight. That's my personal criteria.
This is the sort of balance point for a serious aerobatic model being flown by an advanced pilot.

A trainer or advanced trainer would be set so it needs some amount more down to hold it inverted so that it's more positively stable when upright.

Of course a newer pilot is likely not doing that much inverted flying anyway so this technique isn't likely one they would think of to use. But it does illustrate how the model behaves as we move the CG back towards the neutral stability point.
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Old Mar 14, 2015, 10:59 AM
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If talking about static measurement, then I just make sure the fuse centerline is slightly nose down. Yes, it may be wrong compared to the aerodynamic center, but, it has worked for me.

When flying I verify the CG with a dive test.
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Old Mar 14, 2015, 12:42 PM
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Agreed. The CG is correct when the plane flies as you like it to fly.

NOTE: the CG marked on the plans/in the manual is no more than a hint. Usually just a reasonably safe starting point.

Steve
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Old Mar 14, 2015, 01:22 PM
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assuming that all things are strait and square!

then put the plane in a 45 degree climb, the roll inverted, and the plane should continue on the same line!

that is the simple way too tell, there is books written on how too properly balance an airplane, which usually needs too be done, on a very calm day!

the plane also needs too be balanced laterally, (side too side)!
I turn the plane upside down, rest it on the rudder, then hold on too the nose cone, with the engine free wheeling! and make sure the plane doesn't fall too the same side every time I let go of the wings!

once you get it statically balanced, then you perform the flying part of it! ( refer too first option) 45 degree inverted roll!

unless your flying in competition, that will get you into a happy place with your balance!
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Old Mar 14, 2015, 04:07 PM
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Quote:
then put the plane in a 45 degree climb, the roll inverted, and the plane should continue on the same line!
Here again this is a technique for setting the balance and associated elevator trim right on or darn close to the neutral stability point. For a pattern or serious fun flying aerobatic model it's perfect.

But it isn't right and it won't work for trimming a trainer or advanced trainer where we WANT some positive stability for the new or lower time pilot.
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Old Mar 14, 2015, 08:11 PM
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For non-aerobatic stuff:

Trim the plane for straight and level 'hands off' flight. Without touching the throttle use the elevator to put the plane in a 45deg dive (doesn't have to be exact, just enough to get the plane to speed up). Then let go the elevator stick. If the plane rises out of the dive it is forward balance. If it continues on the down line it is neutral. If it tucks and dives steeper the CG is to the rear of neutral.

FWIW, if the CG is too far back you'll know it without this test

The idea is you can do slight adjustments to get a more neutral controls....if you want. I've rarely felt the need to get close to neutral. I have had a few planes that I have moved the CG aft of the plans, usually due to things like swept wings or highly staggered biplanes.

charlie
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Old Mar 14, 2015, 09:46 PM
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Question: wouldn't the angle between the main wing and the horizontal stabilizer (Decalage?) make a difference in this formula?
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Old Mar 15, 2015, 02:35 AM
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I'm always mystified when it's claimed that an aeroplane can fly "exactly the same", whether it's erect or inverted, whilst maintaining the same trim settings.

It doesn't quite make sense, in my head.

Will someone explain to me, how that works?
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Old Mar 15, 2015, 03:47 AM
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Why not ? If it has zero camber airfoils with the thrust line, wing and tail all on the same datum line. How's it gonna know which way up you think it is ?

OTOH most aircraft can't do that because they don't meet most of those conditions.

Steve
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Old Mar 15, 2015, 06:39 AM
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I don't get it either... gravity is a flight trim factor. With a perfectly symmetrical plane, upright flight needs a slight bit of up trim to counter gravity. When inverted, it would need a slight bit of down trim to factor for gravity.

In my head, I'm seeing these two trims adding to each other for a variation that does not provide the same trim for upright and inverted.

The only way that I can manage to get my head around equality of upright/inverted flight, is for trim adjustment between the two. This could be automatic however... such as a sensor like is used in an IPhone that trims out the variation.
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