Wood Frame V Tail Quad Build. Maiden flight video 18.08.13

[JT]

 Having been really impressed with the v tail quad videos on youtube from flitetest and simplecopter I figured I would have a go at making my own. I have no idea really about designing such things so with a tiny bit of knowledge gleaned from internet land (angles of rear motors and thats about it) and whatever common sense still lingers in my brain I have started my project.....

The first step for me was to design the top and bottom plywood plates, I';m not the best at technical drawing so I first had to download and figure out how to use google Sketchup. This was pretty easy after discovering how to add exact sizes to lines and the protractor tool  made laying out the angles simplicity itself.

Here is the design I made for the top and bottom plates.


When I came to print the templates I discovered that my printer had decided to die..... Luckily I have a vinyl cutter so I managed to get my design into .svg format and cut the templates from self adhesive vinyl...




More to follow........ ::) ::)

[JT]

Arms and balance...... Here is where I try to use a bit of that common sense  

Multirotors in general are balanced by way of having the motors equally spaced (90 degrees in a quad, 120 for a tri etc.) and the motors laying an equal distance from the desired COG, that is, around a circle that could be drawn from the desired COG and this works well for a normal multi.
  However this is not the case with a v tail quad as the motors are not equally spaced at 90 degrees.  Now most of the designs I have seen ignore this and have all four motors an equal distance from the desired COG (around that circle), this results in the rear motors, as they are separated by a much smaller angle than the fronts, overhanging the (desired)COG by a much greater distance than the front ones. This results in a tail heavy quad and can';t be good for efficiency or stability.
In my design I have allowed for this by having shorter tail booms to keep the front to back balanced on the point I';m putting the FC board. Hope that makes sense...???

Anyway here are the cut plates and arms ready to be assembled...

[guest325]

If you take a look at my plan I';ve put the FC at the intersection of the perpendiculars off the centre of the lines between the motors - this gives you the centre of thrust which seems to me to be the best thing to do; the cog will be determined by the rest of the parts.

[JT]

 Hi Darrell, I';m no authority on any of this stuff but I think it best to have the FC on the centre of gravity as this is the the natural pivot point of the craft in flight and where the sensors can do the best job, as they are in effect only tilted and not lifted or dropped by the movement of the craft. I know you can move the COG to be under the FC by placing the weight of the battery etc forward or back a bit but for me it just makes sense to keep it all simple by having the centre of the battery under the natural COG directly under the FC. 

[guest325]

You could well be right thinking that way, I';m not 100% sure though, it';s a difficult one really; have to try it and find out the hard way  ::)
If it flies like a dog then I was wrong 

[JT]

If it flies like a dog then I was wrong 

I';m sure it won';t, like I said most designs I';ve seen seem to be. ';tail heavy'; with many based on a tri design with a v added at the end of the full length tail boom!! In any case if it was troublesome it would only require the FC to be moved to the midpoint between front and rear pairs of motors.

[guest325]

I thought that, so I have not cut the top plate out for the FC and rx, just hedging my bets!

[JT]

 All put together, I couldn';t find anything good laying around to make brackets from so I measured out what to remove from the tail booms to give 40 degrees and took it off with me black and decker power file hopefully accurately enough....

[JT]

 The battery tray is made along the lines of the one on my tricopter but with big wire loops to act as both vibration dampening for the gopro and part of the undercarriage to keep the rear props off the ground.



Electrics time now.....

First job is to flash the ESCs with simonk. Some really fiddly soldering especially with my crap iron, this was the last one though, so those poor little wires had been soldered desoldered 4 times...

[guest325]

Soon have it up in the air then, can';t wait to see it fly!

[Biffa]

Looking good JT

I have to say when I made mine I just threw it all together didn';t pay any heed to CoG or board placement

[JT]

 Thanks chaps    Its got its wiring on now so just the KK2 to suss out, the receiver to stick on, and a little  something to keep the rear props from touching the ground and it should be ready for a try......

I need to go to bed

[JT]

 The wife and kids went to see the MIL just before lunch so for me it was on with the build... ::)


I had to bodge up a cable to flash 1.6 to the Kk2 from some female header pins, not too pretty but done the job



I stuck the kk2 on with some pieces of draught excluder and decided to use the sides of the spongy box thing that the kk2 comes in as a bit of crash protection by removing the bottom of it with a scalpel and super gluing it over the board. On with the receiver, plugged everything in and thats about it done!








I';ve left all the cables long so that any changes can be made to the frame without the hassle of needing to extend/replace wires...

[JT]

 I';d been waiting since yesterday afternoon for the wind to drop, it finally did around 8 o';clock tonight. So with the light fading fast I got my wife to film (hence why its not that good ) and took it for its maiden flight.
This is all done with the KK2';s standard settings for v tail and with 9" props on the rear and 10" on the front. It flew pretty OK with good maneuverability but needs tuning as it seems to ';bob about'; a bit and drifts too much. Its certainly different to quad and tri but some of this is most likely due to using a kk2 for the first time instead of multiwii which for me flies like a dream. Time to learn to tune a kk2 ....

I was pretty chuffed to see it fly as although it is the second multi I';ve built from wood it';s the first that is completely designed by myself ::)

V tail KK2 Quadcopter Maiden Test Flight

[guest325]

Looks pretty good to me - hope mine flies anything like that! Mine';s still in progress - decorating the lounge has got in the way  but hopefully by next weekend I should have something.

[callaghan48]

Well done, Very impressed 

[JT]

- decorating the lounge has got in the way 

I';m having to get all my fun in quickly before my time is booked up..... We have got a new house that needs a lot of work doing, key day is overdue(good old solicitors) so any day I';m going to have a lot less time for multirotoring (I';ll make some though lol), not to mention I daren';t buy any fatsharks (that I';m dead keen to do) for fear of bringing the roth of SWMBO, you know the "YOU COULD HAVE SPENT THAT ON CURTAINS!!!!!" and angry looks over the breakfast table 

[heli]

Hi JT,

Great build and the vid looks good. Great choice of music.

I was wondering what wood you used for your arms?

Nick

[Biffa]

Well done JT

[JT]

I was wondering what wood you used for your arms?

Its just 12x12mm pse pine from the local diy place, less than £3 for 8 foot! It just took a bit of rummaging through the racks to find some straight lengths.

[guest325]

I daren';t buy any fatsharks (that I';m dead keen to do) for fear of bringing the roth of SWMBO, you know the "YOU COULD HAVE SPENT THAT ON CURTAINS!!!!!" and angry looks over the breakfast table 

Hmmm kindred spirits - know that look very well, if she had any idea how much I';d spent since Christmas I would be excommunicated  ::)  ::)  ::)

[Rob2160]

If you take a look at my plan I';ve put the FC at the intersection of the perpendiculars off the centre of the lines between the motors - this gives you the centre of thrust which seems to me to be the best thing to do; the cog will be determined by the rest of the parts.

Hi all,

I realise this is an old thread.

Rob here, and 36 years in RC Planes, 4 with Helis and recently got the Quad bug.

I am also looking at building a self designed V-Tail. 

The biggest question I have is location of CG / FC board.   I read your comment about putting the CG at the perpendiculars. 

This makes perfect sense to me.... except....

The tilted rotors provide less overall thrust in the vertical plane of the quad, by a factor of the cosine of the tilt.

IE, a V tail angle of 30 degrees, means the rear motors only provide 0.866 the thrust of the front motors. (with side forces cancelling each other out in a hover)

To find the actual point where front and rear motors provide similar rotating moments I did a little math.

Assume distance between front and rear motor is 100cm.

The perpendicular point is at 50cm if both engines are level.

Rear engine is tilted at 30 degrees.  Cosine 30 = 0.866

Force from rear engine = R
Force from front engine = F

IE R=0.866F

To find the balance point (where equal moments apply on the line between front and rear motor)

X = distance from rear motor to this point.
Y - distance from front motor to this point.

IE X x R should equal Y x F

or X/Y = F/R

F/R = 1.155

Therefore X/Y=1.115

or X=1.115Y

Since X + Y = 100cm

Also 1.115Y + Y = 100

Therefore 2.115Y = 100

So Y = 46 cm

X would equal 54 cm

This means you would measure along the straight line from front motor 46 cm, then run perpendicular.

This would place the CG slightly further forward and would be the ideal place to mount the FC board as this is the pivot point of equal moment from front and rear motors.

So for a 30 degree V - Place FC at intersection of a perpendicular between front and rear motor, (46% from front motor)

For 40 degree V - 43% (from front motor)

45 degree V - 41% (from front motor)

Happy to stand corrected if I missed something. 

[guest325]

Hi all,

I realise this is an old thread.

Rob here, and 36 years in RC Planes, 4 with Helis and recently got the Quad bug.

I am also looking at building a self designed V-Tail. 

The biggest question I have is location of CG / FC board.   I read your comment about putting the CG at the perpendiculars. 

This makes perfect sense to me.... except....

The tilted rotors provide less overall thrust in the vertical plane of the quad, by a factor of the cosine of the tilt.

IE, a V tail angle of 30 degrees, means the rear motors only provide 0.866 the thrust of the front motors. (with side forces cancelling each other out in a hover)

To find the actual point where front and rear motors provide similar rotating moments I did a little math.

Assume distance between front and rear motor is 100cm.

The perpendicular point is at 50cm if both engines are level.

Rear engine is tilted at 30 degrees.  Cosine 30 = 0.866

Force from rear engine = R
Force from front engine = F

IE R=0.866F

To find the balance point (where equal moments apply on the line between front and rear motor)

X = distance from rear motor to this point.
Y - distance from front motor to this point.

IE X x R should equal Y x F

or X/Y = F/R

F/R = 1.155

Therefore X/Y=1.115

or X=1.115Y

Since X + Y = 100cm

Also 1.115Y + Y = 100

Therefore 2.115Y = 100

So Y = 46 cm

X would equal 54 cm

This means you would measure along the straight line from front motor 46 cm, then run perpendicular.

This would place the CG slightly further forward and would be the ideal place to mount the FC board as this is the pivot point of equal moment from front and rear motors.

So for a 30 degree V - Place FC at intersection of a perpendicular between front and rear motor, (46% from front motor)

For 40 degree V - 43% (from front motor)

45 degree V - 41% (from front motor)

Happy to stand corrected if I missed something.

You are right and the situation is worse if you use smaller props at the rear like some do BUT there is a setup on the KK2 board specially for V Tails in the 1.6 & 1.6++ firmware which I think probably compensates for the difference; I could be wrong but I think it';s that way. I do know that several people have not been too fussy about where the FC is and have still have models which fly well.
To be pedantic the Sensors should be at the centre of thrust rather than the FC to be completely correct.
HTH

[barneyg]

Multiwii and baseflight have this option too ... They even have the ability to put on custom mixes so if you want to do the math you can work out exactly what the ratios are for your quad and then just place the FC in the middle

[JT]

You are right....

Not really.......

I made this for the centre of gravity to be the point that balances with the v tail unpowered. Now if you want to factor in the thrust etc that would be vastly more complicated. Firstly the angles of a vtails arms are not set out equally at 90 degrees, they are based on 120 degrees with the rear motors being like a single rear boom on a tricopter rather than a quad.
So to work this all out properly(perfectly) you would need a configuration that both balances unpowered and has prop/motor sizes worked out to give exactly the same thrust at the 3 points( the combined tail arrangement being just one). This would be a nightmare to work out correctly.
If you look at most vtails they tilt when in a fast vertical climb due to these imbalances, the smaller props on the rear are there to try and address this.
I came to the conclusion with mine that they are just an inefficient design and although cool looking, a tri is better at being a tri and a quad is better at being a quad

[Rob2160]

Not really.......

I made this for the centre of gravity to be the point that balances with the v tail unpowered. Now if you want to factor in the thrust etc that would be vastly more complicated. Firstly the angles of a vtails arms are not set out equally at 90 degrees, they are based on 120 degrees with the rear motors being like a single rear boom on a tricopter rather than a quad.
So to work this all out properly(perfectly) you would need a configuration that both balances unpowered and has prop/motor sizes worked out to give exactly the same thrust at the 3 points( the combined tail arrangement being just one). This would be a nightmare to work out correctly.
If you look at most vtails they tilt when in a fast vertical climb due to these imbalances, the smaller props on the rear are there to try and address this.
I came to the conclusion with mine that they are just an inefficient design and although cool looking, a tri is better at being a tri and a quad is better at being a quad

Thanks for the replies JT, barneyg and DarrellW.  I have spent the afternoon researching this and I think there were some flaws in my last post.

The perpendiculars work for an essentially square quad, but the theory falls apart for a very wide short quad with rear motors close together (visualise the perpendiculars and they intersect forward of the front motors).

I am thinking the CG should actually be closer to the equidistant point between the front and rear motor pairs   as per this diagram of the TBS discovery though there is argument in this thread that it shows the Centre of Thrust incorrectly.

http://fpvlab.com/forums/showthread.php?12404-TBS-Discovery-CoT-and-CoG-diagram

I have also attached a CG diagram of how they balance the Hunter V-Tails, I can see the logic for a tricopter but this position would make the front motors work much harder than the rear in a V-Tail.  (With 2 motors angled at 30 degrees the rear arm is generating 1.73 times the thrust of the front motors if operating at the same RPM so the Tricopter CG position is too far forward.

Looking at it from a different perspective (thanks to your post) I can also see that using smaller props / motors on the rear V, so that the combined thrust of the two motors equals one of the front motors.   In this case the Tricopter CG would work, I had not previously considered that.

If using 4 equal props and motors I believe the CG should be as per the TBS diagram but modified forwards as per my calculations above for the reduced thrust of the angled rear motors.   

I look forward to experimenting when I build mine.   What started me thinking about all this was the Simplecopter V-Tail here.

http://www.simplecopter.com/simplecopter-v-tail-the-tilt/

I';ll keep researching...  I guess the FC is very tolerant of incorrect CG and will just compensate as needed.

[Hands0n]

... I guess the FC is very tolerant of incorrect CG and will just compensate as needed.

Yup. Thinking about the likes of MultiWii, Naza, APM 2.5, Naze32 etc ... They will all compensate for relatively inaccurate CG although the optimum is to be at it.  The cost may be to a few milliamps of charge in having to drive one pair of motors harder than the other (in a four-motor config).  The appliance of stringent analysis will result in optimal placement and efficiencies of course. I';d only go as far as to suggest that it is not absolutely critical to get it spot on.

Interesting reference to TBS - I';ve just built a Discovery and the COG and the designed/suggested placement of the FC are different by quite a bit - the FC is typically about an inch or so in front of the indicated COG. I';m using APM 2.5 instead of Naza and have placed in the same location - the quad seems to fly very evenly and accurately without any apparent stresses on the motors (even temperature) even after flights using maximum power excessively.  Which is a good thing.

[JT]

.......I am thinking the CG should actually be closer to the equidistant point between the front and rear motor pairs.......

Yes, I like to think of a circle that the motors sit on with the centre of that being the centre of gravity( or if it makes more sense, each motor being the same distance from the FC/COG) this will give you a good front to rear balance. The angle of the arms/shape of the body doesn';t really matter. With this configuration and actually balancing it on that cog point with your gear(I do this by suspending it from a string attached to what will be the centre of the FC) you will get a nice balanced machine. The difficulty with a vtail is always going to be getting the thrust equal if you want it to fly perfectly. Flight controllers are pretty good at balancing out the power to keep it all level though, especially in level modes.