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oipigface

Graecalis build log

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Gromit

Jon,  Firstly i must say i love your new Avatar :D . 

I shall be following your build thread with great interest :thumbsup:

 

  Stu.

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oipigface

I bought a new toy that has helped enormously getting the Graecalis straight and true. See how in Part 3:

Graecalis Build 3.pdf

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Bobbyr

Nice 1 John ,

A lazy Level !!!!

Nice Job mate , you need to stick all the PDFs into a book , your build post are Excellent and very informative . :)

Bob

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giammarco

fantastic...... 

 

Very good Works

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Pete Burgess

Hi John, a few details about your "new toy" would be very useful, I might get one.

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oipigface

Hi John, a few details about your "new toy" would be very useful, I might get one.

Hello Pete,

I went for this one from my local DIY shop in Axminster: http://www.axminster.co.uk/bosch-quigo-2-cross-line-laser.

There are others. They all seem to be about the same price, and have similar capabilities. That is, they project laser light along two planes at right angles to each other. It is not necessary to set them up exactly level, because they self-level within + or -4 degrees of true horizontal. I think they all have a universal bracket as well, so they can be attached to anything less than about 40mm thick.

Cheers

John

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ercolizzato

Ciao John,

Congratulation for your Graecalis, you are doing a good work.

I like your new laser tool...our fuse seems to be symmetric....

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Woodstock

Ha ha, laser.  I just walk to the back of the workshop and eyeball it!

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Woodstock

I bought a new toy that has helped enormously getting the Graecalis straight and true. See how in Part 3:

When I saw the single 10 mm servo there, my first reaction was that that seemed really small for an elevator of this size (I have full-size servo of 10 or 14 KG or so for elevator).  Then I researched the specs of the MKS, and they claim torque of up to 8.2 Kg.  Really :blink: ?!  Do people find in practice that that is a realistic claim?  I find it REALLY hard to believe :unsure: ..

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oipigface

Ha ha, laser.  I just walk to the back of the workshop and eyeball it!

Your workshop must be bigger than mine. I've barely got room to get in mine when the Graecalis fuselage is in there too.

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oipigface

When I saw the single 10 mm servo there, my first reaction was that that seemed really small for an elevator of this size (I have full-size servo of 10 or 14 KG or so for elevator).  Then I researched the specs of the MKS, and they claim torque of up to 8.2 Kg.  Really :blink: ?!  Do people find in practice that that is a realistic claim?  I find it REALLY hard to believe :unsure: ..

It's actually a 12mm servo, and is supposedly more powerful than whatever it is that Stefano recommends. I am using a similarly rated full- size Alturn USA servo for the rudder. Now you've raised the question, I realise that I have no idea how much it takes to turn a 9kg model!

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oipigface

Now you've raised the question, I realise that I have no idea how much it takes to turn a 9kg model!

Now I do. At http://www.mnbigbirds.com/Servo%20Torque%20Caculator.htm, Chuck Gadd gives the straight dope, together with a calculator. The weight of the model is irrelevant, although I imagine that moving the CG back a mm would increase the torque required in a heavy model by more than in a light one. I did consider mass balancing the rudder and elevator, but to get the CG of both of them onto their hinge lines would add about half a kilo to the weight of the model (unless they were suspended on arms forward of the surface) so I decided not to.

Here's the formula and the small print. (C2 and V2 mean C and V squared, respectively. IPad won't do superscripts properly, and my PC is old, knackered and will be replaced in the next few days.)

The maximum torque requirement does not always occur at full deflection. This calculator determines the torque at every control position, from 1 degree to the max. deflection specified.

The result is the max torque found and the position of the control surface when the max torque was reached.

The formula used to calculate the torque is as follows :

Torque (oz-in) = 8.5E-6 * ( C2 V2 L sin(S1) tan(S1) / tan(S2)]

Where:

C = Control surface chord in cm

L = Control surface length in cm

V = Speed in MPH

S1 = Max control surface deflection in degrees

S2 = Max servo deflection in degrees

Reducing the servo deflection from the default 60 degrees is similar to using ATV / Dual Rates to reduce the control throws. If you vary the servo deflection from the normal 60 degrees, you will see that using "Dual rates / ATV" to set the proper control surface deflection greatly increases the load on the servo.

Note the following assumptions:

The angle of incidence of the wing, stab, or fuse is zero (relative to the airflow).

Angular velocity and acceleration of the aircraft is zero.

Air flow may be modeled using Bernoulli's equation for dynamic pressure.

Conditions are: sea level, zero humidity, moderate (~55 F) temperature.

Control linkages have zero offset at hinge line and are perpendicular to horns at neutral.

Control mechanisms are frictionless and surfaces are mass-balanced.

The wing, stab, fuse, and control surfaces are basic scale shape.

No aerodynamic counterbalances are used. (Account for these manually, if desired.)

The pushrods are longer than the servo and control horns.

Please note:

The calculations are completely theoretical. No empirical "tweaking" has been done.

The assumptions (except #6) should generally yield conservative (high) predicted torques.

Extreme control throws are probably not practical at high speeds.

This model is best used for comparisons. No guarantees are made of its validity.

Maximum required servo torque may occur at LESS than maximum throw.

...and now off to the shed to do some measuring.

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oipigface

Then I researched the specs of the MKS, and they claim torque of up to 8.2 Kg.

It's actually worse than you think, Chris. The MKS6125e has claimed stall torque of 3.25Kg/cm, which, according to the formula, is marginal when everything about the elevator and the linkages is perfect. Since everything about the elevator and the linkages is not perfect, it looks like I am going to have to think again. I could replace it with the MKS 6125, which DOES have claimed stall torque of 8.2Kg/cm, or I could install another servo symmetrically. Whether this is best done using two Rx slots or just one and a y-lead is the subject of many discussions among the large model boys.

Has anyone any experience of either, or both?

By the way, Chris: Thanks for pointing this problem out. It's not the sort of thing that one looks forward to finding out about after the model has been launched.

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Woodstock

It's actually worse than you think, Chris. The MKS6125e has claimed stall torque of 3.25Kg/cm, which, according to the formula, is marginal when everything about the elevator and the linkages is perfect. Since everything about the elevator and the linkages is not perfect, it looks like I am going to have to think again. I could replace it with the MKS 6125, which DOES have claimed stall torque of 8.2Kg/cm, or I could install another servo symmetrically. Whether this is best done using two Rx slots or just one and a y-lead is the subject of many discussions among the large model boys.

Has anyone any experience of either, or both?

By the way, Chris: Thanks for pointing this problem out. It's not the sort of thing that one looks forward to finding out about after the model has been launched.

Sorry, I didn't realise it was the "e" version.  

 

For two elevator servos there might be a problem with a Y-lead in that one servo may need to be reversed in direction.  So, it's either two channels, or one channel and use a JR Matchbox or similar (I use the EMCOTEC DPSI V-Match @ EUR 29.90) electronic "splitter":  these give you adjustable direction, centre and throw on one servo of a pair on the same channel.

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oipigface

Another issue is that the Powerbox switch I have got regulates to a fixed 5.9v, while MKS recommend not exceeding 5.5v. Looks like I'm going to have to get another switch (suggestions?) or ditch the whole idea of an elevator mounted servo, and go for a pushrod with a big servo in the fus.

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oipigface

How fast does a Graecalis go? I have figured out that an MKS6125 will be OK with the current setup, with 10 degrees movement (probably more than I will actually need), as long as I'm not exceeding 130mph. Any danger of that?

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