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Twin boom builder after advice

#1
A big thank you too FT 1st and a big thank you to all you kitchen table/workshop aviation engineers.
Well I'm a newb to fixed wing and a complete novice at scratch building.
My FT Goblin was kida successful .A little fast for my armature fixed wing sausage thumbs , but with the help of hot glue and gaffa tape gave me a few hours of fun.
However the old Cobra quad motor finally proved why I ditched it. A stall and a snapped fuse.
A hand full of electronics(minus motor) one sheet of FT foam board , a new 3D printer,1mtr of 8mm carbon and a child like grasp of Fushion360 CAD.
I thought I would go for the FT Mighty Mini Trainer but front prop and one sheet of foam and being an FPV kinda guy this would be a problem.
So a pusher was decided on. Oh thrust angles and all that associated with a Bixlar type design eek I'm a neb remember.
A twin boom job it was was then. The mighty mini wing with the MM rudder/elevator with rudder surfaces halved for each surface and 8mm carbon booms.
So far so good a long night with the good old pencil and paper and a CAD package.
IMG_1906.JPG
Now I realise the fuselage and C.O.G will need to be well foreword of the wing .I am assuming probably wrongly that the distance between the trailing edge of the wing and the elevator should be kept at similar a ratio to the original mighty mini specifications ( the FT M.M.T paper print fuse is on the pic).My changes being a longer fuse compensating for the heavier tail end.
Will the C.O.G be a constant and will the boom length ( I imagine it will) need adjusting ?
IMG_1905.JPG
Thoughts , advice and ridicule all welcome .
Thanks in advance M.I.A.S.
 
#3
The fuse is on ,Used the mini trainer glider as a base and added an inch or two.
Lots of roome for a F/C but I'm going to see if the thing actually fly's first before that stage.
Seeing as my only spare RX is Sbus and my PPM converter is only 4 channel the rudder is going to be fixed for the test flights.
I'm still unsure as too the basic geometry for this type of plane .C.O.G and boom length .
Not being sure the extra length of carbon and servo connectors will be kept for adjustments . IMG_1912.JPG
IMG_1913.JPG
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
The fuse is on ,Used the mini trainer glider as a base and added an inch or two.
Lots of roome for a F/C but I'm going to see if the thing actually fly's first before that stage.
Seeing as my only spare RX is Sbus and my PPM converter is only 4 channel the rudder is going to be fixed for the test flights.
I'm still unsure as too the basic geometry for this type of plane .C.O.G and boom length .
Not being sure the extra length of carbon and servo connectors will be kept for adjustments . View attachment 131796
View attachment 131795
The CG point is a function of wing design in the majority and as you borrowed the wing from another design you can also borrow the CG.Balance point as well.

Boom length is widely variable! A long boom is powerful in operation but slow and smooth whereas a short boom can be a little weak especially at slow speed but can be so rapid in maneuver that it can seem "Twitchy".

Initially use the boom length to assist in achieving balance and adjust for desired flight performance from there!

Have fun!
 
#5
The CG point is a function of wing design in the majority and as you borrowed the wing from another design you can also borrow the CG.Balance point as well.

Boom length is widely variable! A long boom is powerful in operation but slow and smooth whereas a short boom can be a little weak especially at slow speed but can be so rapid in maneuver that it can seem "Twitchy".

Initially use the boom length to assist in achieving balance and adjust for desired flight performance from there!

Have fun!
Thanks for the help Hai-Lee.
I'm just adding the ESC and RX .So hopefully will get the maiden in a day or two.Just hope it isn't as disastrous as my previous maidens from tri-copters too the FT Edge.
I will get it right one of these days .
 

Merv

Well-known member
#6
Now I realise the fuselage and C.O.G will need to be well foreword of the wing.
You do not want your CG forward of the wing. Your CG should be at about 25-30% of the wing area. That is you want 25% of the wing area forward of the CG & 75% aft. @Hai-Lee makes a good point, if you if you take a wing from another plane, the CG for that wing will stay the same, regardless of the plane you put it on. The exception would be if you put it on a tandem wing plane such as the Ashley Peace Drone.
 
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