Tangential Tension

[Cube3]

I am trying to solve a problem that concerns model aircraft. I fly Vintage control line, for a good image that shows the object of this sport see:

http://controlline.org.uk/phpBB2/viewtopic.php?t=2787

I will try to describe as best I can what I do and what I want to achieve. The planes are flown by 2 lines that extend 52ft from a U shaped handle to the model, the 2 lines operate an elevator. The model is a very simple flying wing with the engine on bearers at the front, elevator at rear and a paper streamer attached. 2 planes fly in the circle at the same time and the object is to cut your opponents streamer. The whole semi-hemisphere is open flying space, therefore planes fly above your head against gravity and up wind on occasions. The faster and tighter turning model will have an advantage.
Sorry to waffle on, but I thought I should describe the background dynamics of the plane. My question is how best to achieve line tension and by that I mean the ability for the plane to keep the 2 lines taught throughout the combat bout in the most efficient manner to maintain maximum speed and plane stability.
Currently I use several ways for tension.
1. Line rake
2. Outboard wing tip weight
3. asymmetric wings (inboard wing larger then outer)
4. Engine bias (increase tangential thrust line)

See

http://www.cubethree.co.uk/aero/

Trying to apply some logic and an understanding of the aerodynamic affects of these methods, can someone help with a diagnosis and best practice.

Thanks in advance
Simon

 

[GregLocock]

Which line tends to go slack, or do both of them? In what manouevres? I imagine precession has some influence.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Cube3]

Both lines go slack at the same time, they are only 2" apart. This occurs when the model is up wind and above your head. The issue is that I can gain more than enough line tension from the methods described but in doing so I effects 2 things, speed and stability. I want to find the most efficient method or methods. As always in competion, I am looking for an advantage and based on science not tradition.
Simon

 

[rb1957]

part of the problem i can see is that the cables do two very different jobs ... one is "large-scale" (anchoring the model so it flies in a circle) and the other is micro-scale (controlling the movement of the model with very small inputs. i'm guessing one cable is fixed to the model, and the other is the control line to the elevator.

i keep thinking of a cable tensioner spring, so that you'd've a spring attached to your hand grip and the cable, with a loop of slack in the cable. this might be something to connect once the model has taken off.

 

[Cube3]

The lines are flexible, they are only 12 thou (0.3mm) diameter. they are only kept taught by the model trying to fly away from the centre.

 

[rb1957]

yeah, i've seen models flying before. the problem is that 'cause they're cables, they cannot react compression ... if you can pretension them, then the model has to react that tension inertially ... wouldn't the model bank a little more to compensate ?

 

[Cube3]

Quote "the problem is that 'cause they're cables, they cannot react compression"
I do not understand.

Quote "if you can pretension them, then the model has to react that tension inertially"

Not sure how I would pretention the lines, the only fixed point is the control handle I hold at the centre.

Quote "wouldn't the model bank a little more to compensate?" Whats making the plane bank?

 

[rb1957]

you can't push on a cable ...

under normal flight, the model's inertia (mostly centripedal acceleration) applies tension to the cables.

in abnormal flight, when the model is essentially flying toward the operator, the model is trying to apply a compression load. if the cables were rigid, they'd be able to react this. as cables, they can't, so they go slack.

if you pre-tension the cables, then they can react compression initially (up untill these loads equal the pre-tension).

let's go back a step ... am i right in thinking that there is one cable fixed to the model and one controlling the elevator ? your last comment suggests i'm not .. that you have both lines connected to a handle and you control the elevator by pivoting the handle (applying a control input to one line or the other). this means that one cable is controlling the elevator and controlling the flight-path of the model (since the act of pulling up on the elevator means that the other cable is slackened, no?).

it's these two different jobs you're asking the cables to do (control the elevator with small inputs and control the flight-path of the model) that is creating the difficulty. if you can separate these requirements, i think you can improve the model's response.

 

[Cube3]

The control system uses both wires, one wire pulls the elevator up the other wire pulls the elevator down. The system comprises of the 2 wires connected to a bellcrank and a rod from the bell crank is connected to an elevator horn and moves the elevator. This is sort of vaguely shown on

http://www.cubethree.co.uk/aero

I understand that a different control method may lead to gains in fine control but I am bound by the rules of this sport and cannot change this arrangement. I have so many questions about aerofoil shapes, elevator area versus elevator angle, model CofG's, best wing area, wing plan shapes etc. This sport has very traditional methods and I wish to quantify their merits.

 

[vortexman]

Cube3,

Increasing weight won't effectively increase control line tension unless the plane's velocity is sufficiently high, and has numerous disadvantages. In particular, increasing the weight of the plane will make it harder to do tight turns, and will increase the amount of 'induced drag' during those tight turns, which will slow the plane down.

I would be inclined to use the thrust angle and/or rudder to increase line tension. These measures would cost you a little bit of effective thrust and add a small amount of drag, but I think those penalties would be smaller than the penalties incurred by increased weight.

Good luck,

vortexman

 

[GregLocock]

I agree weight is the silliest method.

I don't see why changing the the inboard vs outboard wing size would help, directly.

So, that leaves you with thrust line or aerodynamic yaw.

I suspect a little of both would be more effective than just one or the other.

What I would do is think about building the body assymetrically so that it is a much more efficient airfoil in plan view, and mount the whole fin at an angle rather than using the rudder itself.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[davidjh]

You need to create a "force balance" - the old F=ma - analysis. Almost all aero performance and S&C text books start with this. There are probably several sources on the net where you can get some reasonable drag force and lift force values for your aerofoils and (importantly) the drag force of the lines themselves. Use a common scale, make a rig and measure the thrust of your engine. You can then vary the "flight condition" of your analysis, and see how the line tension value varies - just relative changes ought to be useful for you.

By-the-by I was into speed and rat racing "U" control some 4 decades ago, and your desription brought back many good memories.

 

[Cube3]

Thanks guys for your thoughts. Having thought about this in more detail over the last couple of days I agree with the thrust bias and yaw statements.
The wing tip weight is wrong on to points, it adds weight and does not work when flying directly above. Also that when turning 180 degrees the weight will tend to keep the wing to the outside of the manoeuvre radius, then gravity will overcome and the wing tip drop, this gives instability in flight. as you say just plain silly.
Asymmetric wings also introduce instability when turning from a clockwise to an anti-clockwise manoeuvre.
I can not introduce yaw by means of a rudder as these models do not have one!
I see that the line rake should be there only to compensate for the line drag, just to allow the model to fly straight as intended. No sure yet how to calculate that yet, but will look into it.
Engine bias is the way I am going to sort the line tension. I think I am going to try and make it adjustable so that I can tweak it up in high wind conditions. My only unconsidered force is the engine torque that is trying to tip the inboard wing down. Maybe a very small trailing edge trim to sort that one!
Would anyone like to suggest some information on aerofoils or should I start a new thread?
Simon

 

[rb1957]

if the plane is flying around a center, doesn't it naturally bank inbd wingtip down ?

but inclining the thrust line (and the torque line) outbd a few degs introduces a torque component that is nose down (or up, depending) ... i think it reduces the longitudinal component (which is causing the wing tip to drop).

thinking about this, is it better to have the most level wing possible ? then your engine torque can work with or against the natural bank of the model ... so that you could reduce the bank angle of the model if the engine (and prop) torque opposes the flight induced bank

 

[Dan320]

If the lies go slack when the a/c is overhead you could perhaps try to add lateral area to the fuselage and maybe a fin. This will (maybe) make the model want more to fly knife-edge. If the rules permit a deep fuselage and fin, that is. If the rules are liberal try something novel, like a verical small wing at the CG, lifting outwards. Something like a missile cruciform wing arrangement if you see what I mean...?

 

[saksmito]

As you say I think the solution could be to put a mechanism similar to the one in the bicicles to keep tension in the chain.
How much tension should you use? You can make triple integrals, but I think that the ultimate solution will be a manual adjustment according to your feeling.
On the other side, you could use a trigger in the handle to adjust tension manually during flight.
The problem I think is that this would increase weight, but shouldn´t be more than 10 grams if you use plastic materials for the pulleys and the supports.