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New plane seems unbalanced

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Chris Lee12324/08/2015 19:59:46
60 forum posts
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I have recently converted my Hangar 9 Twist 40 to electric, I have it slightly nose heavy.

I have mounted the lipo on top of the fuselage just behind the motor, I don't want to have to keep taking the wings off to change the battery, I may cut a hatch in the top and have the pack inside but undecided.

When trying to fly straight and level it seems to be very unbalanced on the horizontal plane, if a little aileron is given to make it level it will tip to far. It seems a little extreme to me.

Any ideas? Usual suspects?

extra slim24/08/2015 20:22:46
448 forum posts
48 photos

Normally, with moving the CG forward the ailerons become more sensitive and the elevator less so, so that would be my guess, it's just like it has the rates turned up.surprise

Martin Harris24/08/2015 20:23:19
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Can you lower the battery to give some pendulum stability? You haven't made it obvious (to me, anyway!) if you flew it as an IC before conversion. If not, it doesn't look as though it's designed to be a stable model (mid-wing, no dihedral) as it's designed for 3D so perhaps it is just behaving as intended?

Biggles' Elder Brother - Moderator24/08/2015 21:11:18
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I think Martin may have it. If the Lipo is mounted well above the centre line you can create a lateral stability problem.

Imagine having a stick with a heavy weight at one end. If the weight is at the bottom and the stick is pivoted at the top, then when you disturb it, by moving it sideways, then it will naturally swing back to the centre - like a pendulum. The system is self stabilising.

Now reverse the situation, imagine the weight is at the top of the stick with the pivot at the bottom and the whole thing is perfectly balanced. But disturb that - even slightly - and it will fall. It won't restore itself to the centre. The system is not self stabilising. Worse, once it starts to move, its a devil to stop it!

A Lipo is a big heavy chunk to have up top! As soon as you input any roll the weight moves of the top and tends to accelerate the roll making the aileron feel very sensitive. (unlike a weight at the bottom which actually resists the roll). You then "correct" send the weight the other way and it accelerates off that way - result an aeroplane that feels like its lurghing from side to side while you desperately try to keep it in the one tiny precise position that its in balance!

BEB

Peter Jenkins24/08/2015 21:38:06
1252 forum posts
132 photos

I think the other issue is that you have introduced some side area ahead of the CG so your yaw stability will have been greatly reduced compared to when it was IC powered - that would seem consistent with your description of instability in roll since you will almost always get some yaw induced by using ailerons. You say you don't want to take the wings off to change the pack but in order to achieve that you have destabilised your airframe. You plan to cut a hatch in the top to put the LiPo inside and I think that, together with getting the CG in the correct place, will, I'm pretty certain, eliminate the problem. Generally, though, converting an IC to an electric is always going to be a compromise as weight will increase when compared to the fully fuelled IC weight. Also, unless you have taken steps to provide adequate battery cooling, you may suffer from overheating cells - not a good idea.

BTW I have never found that moving the CG forward makes the ailerons more sensitive.

Good luck.

Chris Lee12325/08/2015 09:24:11
60 forum posts
7 photos
Posted by Martin Harris on 24/08/2015 20:23:19:

Can you lower the battery to give some pendulum stability? You haven't made it obvious (to me, anyway!) if you flew it as an IC before conversion. If not, it doesn't look as though it's designed to be a stable model (mid-wing, no dihedral) as it's designed for 3D so perhaps it is just behaving as intended?

 

 

No i never flew it as a nitro, but a friend has the nitro version and its a lot more stable.

 

I will try to lower the battery, and put it inside to see if it has any affect.

Edited By Chris Lee 1 on 25/08/2015 09:25:03

Chris Lee12325/08/2015 09:26:38
60 forum posts
7 photos
Posted by Peter Jenkins on 24/08/2015 21:38:06:

I think the other issue is that you have introduced some side area ahead of the CG so your yaw stability will have been greatly reduced compared to when it was IC powered.

I don't under stand what you mean?

Neil R25/08/2015 10:12:45
22 forum posts

I always understood that the concept of pendulum stability was flawed - indeed it has often been the first thing I look for in any 'aero' book to judge whether I trust the author. IIRC Alistair Sutherland gets it right!

Pendulums rotate about a fixed pivot point. Aeroplanes don't have a 'fixed pivot point in the sky' - they rotate around their centre of gravity.

My own experience is that lateral (roll) stability is always best thought of in terms of how the the aircraft reacts to sideslip, and although I suspect there might be some secondary aerodynamic effect here of lowering the c.g, I'd be surprised if it's very noticeable.

Chris - Peter is talking about directional / yaw stability (the usual analogy is the weather vane) and suggesting that you may have added forward side area which is counteracting the stabilising effect of the fin & rudder. The two stabilities (directional and lateral/roll) are obviously linked by the resultant sideslip.

BEB - given your multi-rotor activities, I did read once (in a 'proper' academic paper) that moving their c.g. (batteries) upwards improves their stability! But I've long since lost the ability to follow the maths it was presenting.

Martin Harris25/08/2015 10:26:46
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I have to admit that I missed the point that the battery was sitting in the breeze and think Peter has hit on an excellent hypothesis.

However - and I don't claim an expert viewpoint - I would think that the "pendulum's" pivot point would relate to the aircraft's centres of gravity/pressure. It does seem that over the decades, aircraft designers have accepted the principle - most high wing aircraft having little or no dihedral and low wing (non-swept) ones having a significant amount...or is fuselage blanking in the sideslip the significant factor?

BEB?

 

 

Edited By Martin Harris on 25/08/2015 10:30:16

Dave Bran25/08/2015 11:30:48
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I have a Twist 40 (ASP 52 though I'm not averse to EP). The thought of an ugly great battery and wiring sticking up on top of the Fus though is enough........................I'm feeling ill now................... wink 2

(If it looks right, it probably is right Dept)

Dave Bran25/08/2015 11:43:32
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Nope, still feeling queasy, sure the plane would feel worse.................... wink 2

Mine is set up with max possible throws and a CG rearward by quite a margin on the stated one, and its NOT unduly nervous (for a 3D hooligan) or unstable.

You have something else affecting the plane IMO.......................

Dave Hopkin25/08/2015 11:45:49
3672 forum posts
294 photos

Apart from the aesthetics of having a lipo stuck on top of the nose behind the prop... I would have though it would create a fair amount of drag and as the wires on most Lipos come out of the side corner isnt it going to create unbalanced drag?

I would also have worries that a heavy arrival might dislodge the lipo and throw it directly into the prop - of course its probably well secured but..........

Bob Cotsford25/08/2015 15:18:10
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just to cover all the bases, and in view of this being a model that has not flown previously - I take it slop or sticking in the aileron linkages has been eliminated?

Chris Lee12325/08/2015 16:25:00
60 forum posts
7 photos
Posted by Bob Cotsford on 25/08/2015 15:18:10:

just to cover all the bases, and in view of this being a model that has not flown previously - I take it slop or sticking in the aileron linkages has been eliminated?

Yeah, no binding no slop.

Biggles' Elder Brother - Moderator25/08/2015 16:25:54
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Regarding the pendulum analogy. Yes its quite true that it is not perfect in this case because of the lack of a fixed pivot point - but the dynamics still hold true and its a good basic explanation.

Iamgine a model fitted with a 1m pole with a heavy weight at the end fixed vertically from the fuselage (!) If that pole was below the aircraft it would be very hard to roll! True the centre of the roll would now be below the aircraft (a sort of barrel roll!), but the additional moment of the weight resisting the roll would be enormous! Now imagine that the same pole was above the aircraft - your problem now would not be starting a roll - it would be stopping it! The model's vertical CoG would be above the fuselage and it may well have a tendancy to only be "happy" upside down!

BEB

Neil R25/08/2015 19:41:25
22 forum posts

Sorry BEB, it simply isn't true that the dynamics hold true. A vehicle which rotates around it's CG can't have an "additional moment" due to it's mass no matter whether it's 'normal' mass or on long sticks. The mass moment about the CG is (by definition) zero at ANY rotation angle.

The two effects that would be present in your examples are a) roll inertia will increase, which means that for a given aerodynamic rolling moment (for example from aileron deflection or rolling moment due to sideslip), roll acceleration (or deceleration) would be slower, and b) as you have indicated the changed rotation point would cause a centre of rotation further from the normal (fuselage) axis, which would introduce all sorts of second order aerodynamic effects, the generation of sidelsip angle being one such. I'm not saying that such aerodynamic effects wouldn't contribute to stability or instability, but this is not due to a mass moment.

Fortunately in the real world most CGs tend to live within the fuselage, and so such 'odd' aerodynamic effects are minimised.

I always found the concept of pendulum stability easiest to get sucked into for hang gliders. Yes it all looks like a pendulum, but the actual stabilising effects are aerodynamic not mass-moment related. As before, thinking about induced sidelsip and how it creates an aerodynamic rolling moment is usually the key to a better understanding.

Martin - yes it's fuselage blanking.

Chris - my vote also goes for the aero effect as first mentioned by Peter. Hopefully putting the battery inside should do the trick.

Peter Jenkins25/08/2015 20:58:41
1252 forum posts
132 photos
Posted by Chris Lee 1 on 25/08/2015 09:26:38:
Posted by Peter Jenkins on 24/08/2015 21:38:06:

I think the other issue is that you have introduced some side area ahead of the CG so your yaw stability will have been greatly reduced compared to when it was IC powered.

I don't under stand what you mean?

Chris, the fin and rudder provide weathercock stability to the aircraft. In other words they keep the aircraft pointing into the airflow like a wind vane or weathercock. The fin and rudder are well behind the CG but if you introduce extra side area (your lipo) in front the the CG then the weather cock effect is reduced and the aircraft's directional (or yaw) stability will be reduced. Having the LiPo close to the propeller will increase the airflow speed compared with the airflow speed further back at the fin so the side force from the LiPo will be further enhanced compared with the fin/rudder combination. Putting the LiPo inside the airframe will return to the designer's intended yaw stability and your aircraft will fly as it was intended to. Hope that helps with your understanding.

Chris Lee12326/08/2015 16:50:37
60 forum posts
7 photos
Posted by Peter Jenkins on 25/08/2015 20:58:41:
Posted by Chris Lee 1 on 25/08/2015 09:26:38:
Posted by Peter Jenkins on 24/08/2015 21:38:06:

I think the other issue is that you have introduced some side area ahead of the CG so your yaw stability will have been greatly reduced compared to when it was IC powered.

I don't under stand what you mean?

Chris, the fin and rudder provide weathercock stability to the aircraft. In other words they keep the aircraft pointing into the airflow like a wind vane or weathercock. The fin and rudder are well behind the CG but if you introduce extra side area (your lipo) in front the the CG then the weather cock effect is reduced and the aircraft's directional (or yaw) stability will be reduced. Having the LiPo close to the propeller will increase the airflow speed compared with the airflow speed further back at the fin so the side force from the LiPo will be further enhanced compared with the fin/rudder combination. Putting the LiPo inside the airframe will return to the designer's intended yaw stability and your aircraft will fly as it was intended to. Hope that helps with your understanding.

Brilliant thanks, hopefully I should have the lipo inside tonight and weather permitting give it ago!

Peter Jenkins04/09/2015 02:10:01
1252 forum posts
132 photos

Have you flown it yet Chris and what was the outcome?

Robert Armstrong 204/09/2015 09:49:58
234 forum posts
23 photos

In flight surely the airframe does have a 'pivot point'. It is 'suspended' by the lift provided by the wings (and a bit by fuz etc, but mainly wings). The centre of lift presumably lies in the wing. Therefore if the cog of the a/c is above where this centre of lift lies, it will reduce roll stability progressively, I would think. I thought this was why, for similar wings etc, high wing models are more stable than low wing.

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