2.4m UAV build log for AP, GIS, Photogrammetry - RC Groups
Jul 11, 2012, 03:33 PM
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Build Log

# 2.4m UAV build log for AP, GIS, Photogrammetry

Hi forum, thought i would take the plunge and post my build on here, I am in the early stages of planning/building my twin boom electric uav aircraft. It will be controlled by the DIYDRONES Ardupilot Mega 2 board. I initially had my build log over on DIYD, but thought here would be a more appropriate place to get feedback on the build etc. So far I have sourced a number of materials, done some equations, drawn a lot of plans - to get to this point...

Here are the important bits of information...

Ideally i would like the plane to be able to fly the following speeds:-

12 meters per second (26.8 mph) minimum - for no wind days for good photo runs
27 meters per second (60.39mph) max for photo runs in the hills where the wind can be quite strong ~40 miles per hour sometimes. (giving around 20mph ground speed into strong wind)
Anything in this range would be good, so ideally 19.5 meters per second (43.6 mph).

Reynolds Number:
399934.08 = Chord 30cm (0.98ft) x 19.5 meters per second (43.6 mph) x 9360
RE: Chord:- The 30cm Chord & wingspan 2400mm gives an aspect ratio of 1:8. I did some basic research, and it seemed that any aircraft with an aspect ratio in the range of 1:6 - 1:10 was okay for this type of plane, so i went in the middle. Would you advise I increase it to 1:10, giving a chord of 24cm?

The wing area is 684000mm sq (7.36 sq ft)
Total Weight is going to be around 4.5kg (158.7 oz)
Wing Loading = 20.88oz per sq ft
Calculation - 158.7oz (total AUW) / 7.36sq ft (Total wing area)

From the above numbers, I guess i'd like to know how much speed i need to maintain level flight, and how much power i need to maintain level flight. The motor I have for this build is going to change. Initially i was going to use the Ultrafly F/18/10 1000KV, but i would like to calculate what i need from the motor, ie how much thrust is needed in order to buy a motor suitable for the plane.

I have the following information below..
• a 3 view showing the dimensions
• Photos of materials aquired so far
• A table of weights

I have the following questions:
1. What motor should i get to power the thing, I have noticed other aircraft in this size range use props in the 14 x 7 / 14 x 10 range. But not sure how to select it
2. Is the Carbon strip top and bottom of 1mm x 4mm going to be strong enough to keep the wings for snapping?

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Jul 16, 2012, 06:19 AM
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# Should i move this thread?

Is this the best place for airframe design?
 Jul 16, 2012, 02:23 PM B for Bruce One further point. I would suggest that you want to resize and reangle the tails so that you obtain a Vertical Tail Volume Coefficient that is on the high side of acceptable. A long nose pod and boom style construction of this sort has a long and large nose with virtually no compensating fuselage side area behind the CG. It thus becomes part of the vertical tail area's job to overcome the destabilizing forces created by the generous side area of the pod which is located in front of the CG. My TLAR senses tell me that you do not have enough effective fin area.
 Jul 16, 2012, 02:23 PM A man with too many toys I will be interested in how you mount everything. Be sure and post plenty of photos. Do you realize that the APM2 has to have exactly 5v regulates power so make sure that you have a 5v BEC to power the receiver and APM2. .
Jul 17, 2012, 09:19 AM
Registered User

# Cheers for the advice...

Quote:
Hi BMatthews - I have just read your response (didnt get an email notification for some reason). I understand most of what you are saying, and will need to have a bit of a rethink regarding the booms, wing reinforcement, and tail.

Wing
I am not sure exactly what you mean by 'webbing' between the top and bottom reinfocement balsa pieces. If I slice the wing down the fat part (CG line). And glue in a thick piece of spruce stripwood the entire length of the wing, that should provide enought strength. I have already done this on a similar sized wing that was a one piece 2400mm wing. I will post pictures later showing the wing, and maybe i can use that. The joins between the outer wing panels and center wing are the problem area for me - how far should the joining tube go into the outer wing panel?

Another question regarding the booms, I guess the tail is potentially going to be 400g (ish including booms, surfaces and servos). Therefore When the aircraft is standing on the floor, the tail will drop and twist the wing towards the rear. How do you suggest I reinforce the center wing section so this doesn't happen?

I bought 10mm x 10mm alloy square tubing yesterday to lay into the wing center section at chord and 1/3rd from trailing edge, I would also lay this tube 30cm into the outer panels - then put a carbon tube of 1m length through the center section at chord and 1/3rd from trailing edge. I will post a picture tonight as what i have described sounds a bit difficult to visualise!

Booms
I take on board about what you are saying regarding the strength of the booms - I may change them for these 16mm booms. I did try bending them in the shop and they seemed to be very strong. I thought the 12mm dia tubes were okay, but the last thing I want is poor elevator response!

Tail surface
If the tail surface is going to cause yaw problems in its current size, I may reduce the height of the fuselage, from 150mm, to 120mm. As i dont need all that height in the fuse. would that be okay? It would reduce the fuse side by 800 x 300 = 240 sq cm.
Jul 17, 2012, 09:22 AM
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Quote:
 Originally Posted by RC Man I will be interested in how you mount everything. Be sure and post plenty of photos. Do you realize that the APM2 has to have exactly 5v regulates power so make sure that you have a 5v BEC to power the receiver and APM2. .
I will certainly post photos as and when I have something interesting to show. I have already made 10 successful flights with APM2 in my wing. I don't think i gave it enough power before and was getting brown-outs. which led to the demise of my skywalker and ursus!! - This aircraft will be equipped with a parachute recovery system and 'o s**t' button !
Jul 17, 2012, 03:30 PM
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# Update - some pictures

Hi all - Some pics - comments on pics.. easier that way....

One thing i didnt mention is that I will be running cables to each wing tip, including
• GPS
• Video Transmitter
• Nav Lights
• Wing Tip Camera

I have got lightweight U shape plastic ducting to embed in the wing. I will leave the exposed side on the underside, and use duct tape to cover cabling once inserted.

Wing is in pretty beat up condition, it is from a previous project.

Next steps - get large diameter booms - source 2 x 8mm ext dia 1000mm Carbon tube for wings.

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 Jul 18, 2012, 08:16 AM UAV Operator Wow, it's beautiful. Following your project worklog. Good luck!
Jul 20, 2012, 09:08 AM
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# Now that's what i call feedback... thanks!

Hi BMatthews.. Cheers for the info, haven't quoted it as I think the post would be rather large!

Wing and COG
I have certainly learnt a lot from what you have said, and will do my very best to include it into my current build. Regarding COG and thickest point of wing. this wing came from a previous aircraft i built a few years ago, and i remember the COG being at the thickest part of the wing, and it flew quite well....Is the COG always in the same spot for the wing, or does it take into account the nose and tail to determine its location?

I now understand what webbing is and get the concept, I would probably do it as you have suggested with a large 'I beem' and thinner webbing in future to reduce weight. I also understand what yo are saying regarding diagonal shear load. Hopefully the spruce will be okay, just a bit heavy. and should hold up in high G manouvers! 27kg is a lot - I may fiber glass over the top of the alloy rods and certainly connect them to the spruce in the outer wing panels, otherwise I can see the wings snapping at the end of the Alloy square tubes.

Booms
To clarify, I am using the alloy square tubes to maintain a clean channel in the wings, so that I may easily remove the outer wing panels for transport. The Carbon spa will go through the length of the alloy tubes for flight. the 2 carbon spas inside the alloy tubes are 8mm dia tubes, 2mm wall thickness, these seem pretty strong. My only concern is transferring the load between the alloy tubes and the spruce in the alloy wing panels.

I will ensure I secure the tail to the two alloy booms to avoid center wing section twist - I have attached a visual of how i plan to build the boom joiner boxes, basically two wing profiles with two thick blocks of wood holdings them together, with a large dia tube glued in (not on visual) to slide the booms into.

I think I will have to invest in some decent woven carbon tubes - 16mm dia for the booms, as i don't want to risk the alloy bending -Found them for ~£15 each here. It says they are pullwound. Does that mean pulltruded or woven? Any ideas?

Tail surface
I have looked at the reference photos you have supplied, and they do indeed show a considerably larger amount of tail surface. I may extend the tail surfaces length ways, and keep the angle the same (110 degrees)

Is there a rule of thumb for v tail surface area? I have read that the elev surface should be no less than 15 percent of total wing area.

Other
I am certainly glad I am discovering this stuff out now, and would much rather change the direction of my design slightly in favour of having a mid air break up! I have actually thought of making a small flying rc scale model of this to test it. my main issue is the wing profiles, I need to get them sized and cut by a chap in Scotland - which is quite a timely and costly process.

i may not have commented on all your feedback, but i will certainly read it over and over until i understand everything!

I have also attached some refined measurements - excluding tail change,

The tail will need some careful consideration, as Yaw stability is certainly an advantage!

I will post an update after the weekends progress - I am taking delivery of a scroll saw today to help cut the wing boxes - can't use a conventional coping saw!

Rich

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Last edited by richie967; Jul 20, 2012 at 11:30 AM.
Jul 21, 2012, 04:39 PM
Registered User

# Update

Hi Forum,

first of all .. @BMatthews - thanks for the second round of feedback, I am taking it all in and factoring it into the design process

The Center of gravity calculator looks fairly complex - I understand most of the number inputs, and i believe they are as follows (attached is the drawing with measurements attached)

Wing Root Chord (A): 315mm
Wing Tip Chord (B): 315mm
Wing Sweep Distance (S): 0
Wing Half Span (Y): 1100mm
Stabiliser Root Chord (AA): 112mm
Stabiliser Tip Chord (BB): 112mm
Stabiliser Sweep Distance (SS): 0
Stabiliser Half Span (YY): 255mm
Distance between both LE's (D): 1048mm
Stabiliser Efficiency*: ??? unsure on this one

When cutting the grooves for the two alloy tubes (before starting this thread), I did wonder if i should have stuck an alloy tube right next to the spa. I may gouge out the polystyrene between and put some spruce in there to transfer the load.

I have already tried the local golf shops for carbon shafts but to no avail. Ordered the 16mm o/d tubes 1.5mm wall from woolmer composites.

I am going to take your advice and alter angle of V tail to 90 degrees - and maybe double the length. The maths is the part I struggle with, so will probably pick back up on it once i have completed the fuse and wings!

As I wont be able to get wings for a small quick and dirty glider, I may make one out of balsa and traditional construction. The scroll saw is amazing and reduces building cuttign time massively!

A few pictures...

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 Jul 23, 2012, 01:17 AM B for Bruce I didn't really notice from the drawings but your ailerons are MASSIVE for the sort of performance that such a model would expect. Go real easy on the amount of aileron deflection angle or you'll have something that handles more like a fun fly model... There's no need to go overboard and double anything. All you need is to shift to a 90 degree angle and MAYBE increase the chord by an additional 10 to 15 %. If you did that then I don't forsee any issues at all with either the horizontal or vertical tail area equivalency. For a quick and dirty test glider flat plate balsa or something equally bend resistant is fine. The surface of the wings is pretty rough looking. Also judging by the red stripe on the leading edge I gather you're considering not using any outer covering. If so I have a couple of suggestions. First off fill in the waves and grooves with some light weight wall filler compound. You'll know you have the right product when you lift up the plastic tub it comes in and the container feels empty. Trowel it on to fill the wire cut grooves and then sand down to smoothen out the airfoil shape. You can also use the same filler in the joint rib gaps seen in the second picture of your last post. The stuff I'm talking about isn't all that much heavier than the foam you're replacing. Then when you have smooth wings you will want to find some clean newsprint to varnish onto the foam to act as a stiffening skin. Movers use this sort of newsprint to wrap up dishes and glasses with when packing for a move. Check around for a moving supply outlet which has this same newsprint paper. To "dope" it down to the foam I suggest you use some of the now common water based emulsion varnish. To skin the wings cut the paper slightly oversize and then soak it in water. Varnish the foam with the Water Based Poly Urethane (WBPU) varnish and while it's still "milky" looking lay on the newsprint. Brush a little more over the paper and work it into the paper well. Do BOTH sides of each panel within a short time and stand up where air can freely reach both sides to dry. Otherwise it can shrink a warp into the panel. But if both sides dry evenly you'll be fine. Overlap at the leading and trailing edges by about an inch Even this single layer will add a huge amount of torsional stiffness to the foam. And a wing can never be TOO stiff at resisting twisting in flight. And given the weight expectation of your project you definetly want a torsionally stiff wing. Not to mention that it'll greatly improve the surface finish of your model as well as reducing the drag of the final airframe over what the surface of the raw foam provides. Last edited by BMatthews; Jul 23, 2012 at 01:29 AM.
Jul 23, 2012, 04:51 AM
Registered User
The Ailerons are pretty big, they seemed to work ok when this wing was used for a previous plane - Video of prev plane at the end of this post...

I may glue in the current ailerons and re-cut them to allow for a more efficient wing shape.

Understood about tail size and angle - I will re-draw on plans and order the tail section from my wing cutter - as well as a wing for the smaller test glider.

Great minds think alike - I have already been using Polyfilla Onefill ultra lightweight filler for the wings - I have managed to smooth it over a small portion of the inner wing to test - before using on the main wing surface. It seems to fill small gaps well - however i did try using it to fill in the large damaged area on the damaged root of the outer wing panel, and it seems to crumble - So i am going to cut out a foam block and stick in and cut to shape.

I am going to the store today to try and find some of that paper you speak of - One question though, I would like this airframe to be heavy rain proof. I have though about how to waterproof most of the components yet still allow airflow to the motor and esc and batteries. Is water based varnish water proof? If not i was thinking of using oracover or monokote.

I would like to add some torsional stiffness to the wing so option A would be best using the varnish and paper.

Currently focusing on the wing/boom joiner boxes - I replaced the 12mm dia booms with larger 16mm dia booms.I am finding it difficult to source 16mm internal dia tube for the 16mm booms to slot into, thinking of getting some slightly larger plastic pipe, cutting a section out, and using a jubilee clip on it to secure the booms in place. Once the wing/boom joiner boxes are complete, I will move onto finishing the wings. Will post pics as and when...

Cheers - Rich

Video of wing being used on a plane i built a few years ago...

 Space Glider Mid Air Test Launch plane - first flight (0 min 49 sec)

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