Hi Forum, I am a fairly new user to APM2 (made about 4 successful flights on a skywalker.) And I am looking at building a twin boom aircraft for taking Aerial Pics to be used for GIS, land surveying etc.. I initially built an off the shelf twin boom called the URSUS, but due to its wooden spa construction, it broke in half in mid air. I have added glue, carbon and ply reinforcements, but now its too heavy to fly. Therefore I thought i'd try and build something myself using its power system
Here are my objectives
- Must be able to accommodate a roll gimbal (I made this for the URSUS)
- Must fly for 45 + mins
- Must be shower/rain proof
- Must Fit in the back of a Volvo V50!
- Must be able to fly slow or fairly fast into wind
- Airframe without power system should weight less than 1.5kg (based on weights taken from other airframes in this size range)
Based on some research, I have discovered that aircraft in the 2.2 - 2.6m range can achieve this. I have a moderate amount of building experience, using polystyrene, depron. But have never tried to build anything that performs well.
I am going to make the Fuse out of fiberglass, and the wings out of EPP.
Some technical details...
Wings
- Wingspan - 2200mm
- Chord - 250mm (giving an aspect ratio of 1:8.8 )
- Profile - Clark Y (noticed a lot of these aircraft use a clark-y profile)
Fuselage
- box section - 120mm x 120mm x 600mm (excluding nose cone and tail cone)
- Booms - 51cm, From trailing edge of wing to start of vertical fin
Power System
- Esc - Castle Creations Phoenix 45
- Motor - Ultrafly F/18/10 1000KV Brushless motor (model number uf-mtr-f1810)
- Prop - 10 x 7
- Lipo - 2 x 5000mah 3s 11.1v
Some pics... there will be a nice cowl on front and back of fuselage, but i cant do that with my limited CAD skills!
This thread and idea is a product of realizing that the lower end off-the-shelf airframes are not capable of what I want to achieve. I have reached that conclusion in my Arial Photography thread here.. The Skywalker is great, but can't fit a roll gimbal in it (I keep getting pictures at different angles). The Ursus is nice, but the construction is too weak for my needs. Whilst working on a new airframe, I will be continuing to use the overweight ursus to perfect my roll gimbal. However a new airframe is needed to get longer flight times. I quite like the medium priced hugin, but thats a little out of my price range right now.
I can get EPP wing cores cut for £40, and a fiberglass fuse made for next to nothing.
I have a few questions -
- What distance should there be between the booms?
- What is the best aspect ratio?
- What thickness should the carbon booms be?
- What height should the rudders be?
- Is it best to go with an inverted V or H tail?
If anyone with aerodynamic knowledge has any fairly simple calculations to see if this will work, please could you let me know! or any other thoughts for that matter, positive or negative!
Cheers
Replies
good idea
A few tweeks to airframe v1...
Rear firewall removed and rear of fuse tapered in the help aerodynamic-ness! this has reduced the weight of the plane bya few hundred grams. I have worked out, If i could build a tail to weigh 215g the plane will balance and fly....current tail is 357g.
I am going to try to use blue dow foam instead of eps for another tail to see if i can make it lighter. less resin as the surface will be smoother. fingers crossed that'll be good enough to get it off the ground for a bit of fun... no serious photography will be taking place though!
Aircraft weights broken down into sections...
Quite a large post...
But the below is in a working draft format.. trying to plan Version 2...
Lessons Learnt from twin boom Aircraft V1 - also plans and improvements for V2.
Here are my objectives - They have largely remained the same from my first attempt.
Fuselage
My Comments
- Remove wooden switch panel on side of plane, also remove APM USB connector lead & light switches.
Fuselage construction & Materials
I may look at making a fuselage mold, and then making the fuselage out of a Fibreglass layup. this will probably be a bit lighter than the existing fuselage, but will be a lot stronger.
Increase Length of fuselage by 70mm to aid COG. Unsure of thickness of Fibreglass cloth for layup, possibly 135 g/sq meter.
The wing will not go all the way through the fuselage, but instead push flat onto the side of the fuselage. connecting the wing half to the fuselage will need to be designed.
Wings
Comments from Blog
My original wing design was 2500mm long and 300mm wide (and was going to be a Clark Y but the foam cutter changed it to a different shape with a curved bottom) It flies well and just mushes out when trying to stall and doesn't drop a wing which is great. I will ask the cutter what the shape is.
The wings only use a internal dual tube and spar set up that only runs about 1/3 of each wings length,
The rest of the wing is just reinforced with the upper and lower flat carbon piece, which when vacuum bagged the whole wing in a light weight fibreglass really stiffened up the whole wing. I also used the slots cut into the wings for the top and bottom flat carbon to run wires out to the ends of the wings for lights and servos, so give some thought to that now as its a real bitch when you forget a particular wire run,(dont ask me how I know).
Think how you are going to transport and assemble this bird at the field during the design phase. Having put up with our arrangement, I long for the alternative where the tail, booms, and mid wing all stay in one piece for transport and assembly.
The Clark-Y is easy to build and has good stall performance, but it's not the most efficient airfoil. There's been a lot of work done in the last decade on low reynolds number airfoils, which is what RC stuff is mostly operating in.
My Own comments on the current build...
- Too much filler - make smoother wings in future out of blue foam
- adding plastic channels in the wing for cabling, next time I will use simple slices in the foam
- large full depth spruce spa in wing can be removed in favour of a full width single cf tube and glass wing surfaces
- lighter wing servos, currently 39gm Futaba s3003, use hitec hs81 19g in future.
- remove wing section from inside fuselage.
Other airfoils to consider
Wing Construction & materials
XPS/Blue foam cut clark-y cores Clark-y, 295mm chord, as opposed to 310mm.
Two CF tubes to keep wings rigid, 600mm long, going 225mm into each wing. Wings will be fixed onto the fuselage with nylon screws from inside of fuse into wings.
Wings will have 2 CF strips (10mm x 0.5mm) top and bottom at thickest point, and will also be covered in 30g sq/mt Glass, and vacuum bagged. possibly painted in resin mixed with micro balloons and sanded for smoothness. only if I can afford the extra weight though
Wingspan will be increased to 2400mm, currently 2200mm - for a lighter wing loading. wings wil be made in two halves.
Tail
Comments from Blog
I used a general rule of thumb of %15 of main wing area for Elevator surface and %7 for the rudders,
The further back it goes the more stable the plane will be and the less pitch sensitive the elevator control will be, give a nice stable flight platform.
For V tail we used a V-tail mixer we use the normal rudder / elevator APM arrangements. The rudder output goes to to the nose wheel, then the rudder and elevator go to a V-tail mixer. Neither the pilot or the APM notices the difference. - Don't forget to test it on fresh batteries as well as low batteries.
Future construction & materials
Possibly using the existing tail and 19g Servos. I will keep the distance between the booms 600mm. The Tail with Servos and wires and fixing weighs 370g. Which I am told by others on here is pretty good, it is also super strong.
Booms & joiners
Comments from Blog
I used 12mm Carbon rods with a 1.2mm wall section and mating 14mm carbon rods that allowed me to make sockets to plug the tail section on and off.
For booms look at ex carbon fibre golf sticks.
My comments
- boom joiners in wing could be formed from laying up fibreglass, instead of using lots of ply, gorilla glue and heavy plastic tube
- use cf booms instead of alloy
Landing Gear
Comments from Blog
Plan your nosewheel undercarriage carefully. It is going to be long, and with the need to steer it, becomes a bit difficult and vulnerable to a bad landing. You need either crash resistance or easy in field replacement.
My Comments
Construction for V2 will use the same Inverted V Tail
Reference books and links
General Aircraft design notes from the MIT website
Model Glider Design by Frank Zaic
Glider wing design
Wing Profiles, flow analysis
Alternative off the shelf airframes
Bormatec MAJA
X8 Wing - available on Hobbyking.
Quick update...
I thought seeing as the airframe is not going to be a usable platform, I decided to hack away at the front and add a box section fuse of 70mm. this has enabled me to balance the plane at the thickest point in the wing. Gave up on using calculators, and will simply find CG using trial and error glides down a slope....
I do need both flight batteries and the RX Batt in order to balance it, but i'm going to give it a try this weekend. I am in the process of compiling all the advice I have received during this thread, and am going to organize and collate the info, and post here as a basis for my next build....
It's going to fly like a brick...but am getting tired of building, so will spend the day flying instead! - Prior to commencing Airframe number 2.
In my design the tail had wooden booms, very heavy, but that just allowed me to load it up with a huge battery up front. It flew great!
Rich, you may have seen this link in my blog but if not, here it is again. Foam wing cutting using gravity pull.
https://www.youtube.com/watch?v=FiHA6K82tFQ
As mine has no taper, both wires attach to the same place on the bar. In the past, I have just used the hot wire by hand but this gravity system worked great.
The airfoil templates are a bit of a pain to make. I used Formica type stuff. It may be worth investing in one of the foil-plotting programs. I use profilli, it does loads more than just plot, including full flow analysis using Xfoil.
http://web.mit.edu/drela/Public/web/xfoil/
Must get this. Excellent, complicated and FREE!
http://www.xflr5.com/xflr5.htm
Thinking ahead to my next project.
Joins add problems and weight. Twin booms are good but as we have both discovered its not simple to make.
Most of us do these odd designs because we dont want a propeller on the nose to get in the way. I dont like pushers with a low tailboom that limits prop size.
So.... am currently thinking of a twin motor design. High wing, motors on the leading edge. Carbon tube tail about 20mm dia.
Should be less likely to end up with an aft C of G.
What do you think?
Wing Loading of 24.4 oz/sq ft - Is that too much?
Unfortunately the plane is still tail heavy...I am considering my options for sorting the problems out...
1. Replace the 19g servos for 9g servos
2. Cut off the Fiberglass resin boom surrounds that allow the tail to fit onto the booms, and simply use a single screw to keep the tail on each side.
3. Cut the booms 100mm shorter each side
4. Change the 96g each alloy booms for CF booms
I am leaning towards point 3, how much lighter does the tail get the closer it is to the fuselage?
In other news the final weights are below....
AUW with a single 5000mah 4s lipo is 4567g - Wing Loading of 21.9 oz/sq ft (7.34 Reynolds)
AUW with 2 5000mah 4s lipos - 5121g - Wing Loading of 24.4 oz/sq ft.
very cool project, subbed
Almost ready for Maiden flight!
There are a few problems though, which I think may be a bit difficult to sort out. Mainly around COG. I was going to do some test glides throwing it into wind downhill to establish the correct COG. On the 10 or so rc planes I have owned, the COG is normally around 1/3rd chord of the wing.
However with all the batteries up front, it seems the COG on this plane is too far back - about 50% chord. During the build i tested the balance point a handful of times, and all seemed well, however after doing the final touches to the tail, the COG is too far back.
Does anyone have any thoughts on COG, or how to estimate it? I'm sure there are plenty of complex equations, but i'm just looking for some guestimates...
It is going to be quite a lot of work to move the COG further forward, and would like to avoid it if i could.
The tail turned out quite well and is super strong, Building the tail took longer than the rest of the plane put together, I did have to learn how to fibreglass, and build a hot wire cutter though which took some time.. some pics of tail...
And a couple of the plane...(I am aware the prop is on backwards!)
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