Airframe measurements for a semi unibody H frame

The new control algorithm for AutoQuad flight controllers.

Airframe measurements for a semi unibody H frame

Postby Mullet » Wed Aug 19, 2015 6:15 am

I have a few questions as I embark on my first Quatos based build.

Here is a pic of the frame to be used...
BlackBird180_1.jpg


The main part of the frame is unibody. The arms aren't separate. There is a small protrusion on the front where a pico camera will sit. On top there is a protective piece of carbon fiber held up by 4 standoffs. Then there is a lower level cage, which consists of a battery holder in back and camera holder up front. The arms are on standoffs which lifts the craft up.

Excuse me for my ignorance. Math and geometry isn't my strong suit.

Being that the arms are attached how do I determine the weight (mass) of each arm in the XML? Should I just measure the weight of the unibody and divide it by 4?

What would the body center be? My guess is if I put all 4 motors on and tied string from corner to corner that would give me a center. In terms of looking at the bodies dimensions, I'm going to assume that I'd measure the center portions length, width, and height -- this includes the unibody sans arms and the bottom cage. Is my thinking correct? Or is the unibody (main carbon layer) considered on entity and the cage another?

What if the ESCs are not in the same place on the back as on the front. How would this be represented in the XML in terms of distance from center?

Do I need a caliper to do measurements or is a simple ruler sufficient?

Thanks for helping out!
Mullet
 
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Re: Airframe measurements for a semi unibody H frame

Postby aBUGSworstnightmare » Wed Aug 19, 2015 11:32 am

Hi Mullet,

well, you Need to make some assumptions with this frame!
Here is a complex model of a RCTimer Spider frame that I've created as an example: viewtopic.php?f=48&t=4456&start=40#p33002

For your frame I would slice the unibody to see if it gives some weight for the arm. If not, just use 0g for the arm or add a little weight to the motors. The rest is pretty much up to you! You can make it complex, weighting/modelling all Major parts or just treat it as a box.

Where will you Mount the battery?
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Re: Airframe measurements for a semi unibody H frame

Postby Mullet » Thu Aug 20, 2015 5:11 am

Joerg as always thanks for your guidance. Your spider frame looks quite complex! I'm going to have to spend some time to analyze this.

The battery fits into the bottom level battery / 808 #16 v3 camera holder shown below. It's actually 3 carbon fiber pieces held together with the blue dampers.

BlackBird180_2.jpg


I plan on using the holder as one cube and then the battery and camera as separate cubes within that cube if that's possible.

I'm not sure what you mean by "slice" the frame. My assumption is to weigh the piece and divide by 4 or 8. Otherwise, I could try 0 for <arm>.

Looking at a few XMLs I did see a <geometry> option that I didn't see in the wiki. It looks like it's based on the custom config. What is the syntax for this - <motor port="1" rotation="+1">+0.175, -0.180</motor>

I understand the first two variables but not the last two.
Mullet
 
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Location: Fairfield County Connecticut

Re: Airframe measurements for a semi unibody H frame

Postby aBUGSworstnightmare » Thu Aug 20, 2015 6:57 am

Mullet wrote:Looking at a few XMLs I did see a <geometry> option that I didn't see in the wiki. It looks like it's based on the custom config. What is the syntax for this - <motor port="1" rotation="+1">+0.175, -0.180</motor>

I understand the first two variables but not the last two.


This is the distance from COG. The reference is this: http://autoquad.org/wiki/wiki/quatos-ad ... tribution/

For the motor 1, which is rotating clockwise it's the same as with the standard settings.
Next two values define the distance of the motor from COG, means arm length in case of standard mixers.
In this example the motor's offset is
175mm = 17.5cm in +X direction (to the front of the craft)
185mm = 18.5cm in -Y direction (to the left side)
--> this is the front left motor = 'A' when you look at the mixer table http://autoquad.org/wiki/wiki/quatos-ad ... tribution/

Mullet wrote:I'm not sure what you mean by "slice" the frame. My assumption is to weigh the piece and divide by 4 or 8. Otherwise, I could try 0 for <arm>.

Added the explanation to the .xml file of the new version with ALU-arm (also checking the weight and calculations again)
Code: Select all
            <!-- RCtimer SPIDER FPV frame body components are 2 GFK sheets with 1.6mm thickness.
             Because the frame is totaly assymetrical it is split into several sub-parts to get a better assumption of the total
             weight distribution.
             The weight for each sub-part is then calculated based on its volume.
             Total weight upper sheet: 50g
             Thickness is always the same -> calculating the grammage is sufficient to get the weight of each
             sub-part
             Total area = 2316mm^2 ->
                    grammage = weight of 1mm^2 = total plate weight / total area
                    grammage = 50g / 2316mm^2 = 0.21589g
             -> multiplay the area (dimx * dimy) of each sub-part with this value to get it's weight
             
             Total weight lower sheet: 61g
                   grammage = 61g / 2560mm^2 = 0.23829g
             
             Frame height needs to be considered for z-axis offeset calculations:
             Internal: 30mm
             External: 30mm + 2* 1.6mm = 33.2mm
            -->
            <cube name="topframe_front" dimx="0.070" dimy="0.084" dimz="0.0016" offsetx="+0.075" offsety="0" offsetz="-0.0158">12.694</cube>
            <cube name="topframe_center" dimx="0.080" dimy="0.080" dimz="0.0016" offsetx="0" offsety="0" offsetz="-0.0158">13.817</cube>
            <cube name="topframe_intermidate_r" dimx="0.034" dimy="0.140" dimz="0.0016" offsetx="-0.0570" offsety="0" offsetz="-0.0158">10.276</cube>
            <cube name="topframe_aft" dimx="0.102" dimy="0.060" dimz="0.0016" offsetx="-0.125" offsety="0" offsetz="-0.0158">13.212</cube>
           
            <cube name="botframe_front" dimx="0.040" dimy="0.060" dimz="0.0016" offsetx="+0.130" offsety="0" offsetz="+0.0158">5.719</cube>
            <cube name="botframe_intermidate_f" dimx="0.070" dimy="0.084" dimz="0.0016" offsetx="+0.075" offsety="0" offsetz="+0.0158">14.011</cube>
            <cube name="botframe_center" dimx="0.080" dimy="0.080" dimz="0.0016" offsetx="0" offsety="0" offsetz="+0.0158">15.251</cube>
            <cube name="botframe_intermidate_r" dimx="0.034" dimy="0.140" dimz="0.0016" offsetx="-0.0570" offsety="0" offsetz="+0.0158">11.342</cube>
            <cube name="botframe_aft" dimx="0.102" dimy="0.060" dimz="0.0016" offsetx="-0.125" offsety="0" offsetz="+0.0158">14.7679</cube>


I hope this helps.
aBUGSworstnightmare
 
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