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Old May 26, 2011, 05:11 PM
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FPV twin boom, help needed...

I would like to make my FPV plane from scratch. This plane will be two planes in one (see pictures) the short wing span version to have fun and the longer wing span version in order to have more weight and more flight time. The short version dimensions are very similar to the easystar are the long version is inspired from the easyglider. I have never built a plane from scratch and I hope I did not make and error in the design. If one of you could review my design, it would be very much appreciated.

Thank you in advance for your help,

Francois

PS: if you do not know what you are talking about, please read (do nor comment)
PS2: "build it from a plan" or "buy a kit" is no fun at all... thanks
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Old May 26, 2011, 05:22 PM
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so here it goes:

length.................100 cm
nose lenght .........25 cm
wing span...........140 cm (short) 210 cm (long)
wing area............30dm2 short 41.55 dm2 long
dihedral...............0 midle secction, 2d first sections, 5degrees last sections
wing profile..........MH 32 with 1d washout
tail area..............6.6 dm2 (horizontal projection) 2.7 dm2 (vertical projection)
twin boom...........44 cm apart
weight................about 2lb for the short version, maybe 3lb for the long version

do you have any comments? is this going to work?
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Old May 26, 2011, 05:35 PM
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Dallas Tx USA
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Get the CG correct and it will fly.
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Old May 26, 2011, 05:36 PM
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Ohio
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Looks like it will fly to me. Have never used a V tail, lateral stability may be a problem

if not big enough. Might be dangerous to hand launch, that PROP is near.

Also the CG must be optimum.
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Old May 26, 2011, 08:21 PM
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what about the position of the tail? Do I need to move the tail back in the longer wing span version?
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Old May 27, 2011, 07:59 AM
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Quote:
Originally Posted by francois 3.14 View Post
what about the position of the tail? Do I need to move the tail back in the longer wing span version?
I would do so. You have to calculate the length of tail booms according to main wing area.
1. MH32 profile is for the glider so you can expect additional unwanted effects like unstability when pushing throttle or other side effects.

2. Positioning ailerons there will do the job while using short wings but with big wings they will be ineffective. I would use them as ailerons with short wings and as flaps with additional ailerons in blue pieces wen using larger wings.

3. It would be better to extend fuselage (to the front) because with such short nose you will have problems balancing CG

4. If you will need autopilot it will be easy to put it into a plane with classic Hstab + 2x Vstab. Some AP dont have an option to use V tail.

5. Think about fuselage near the engine. Looking from the front it is square shaped with big surface covering most of the space of propeller. I would make the fuse as tiny as it is possible to avoid turbulent airflow near prop.

Reasumming it will fly but you need to pay attention to the "details".

Good luck
Michal
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Old May 27, 2011, 10:33 AM
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thank you so much Michal, this is exactly the type of advice I was looking for.

could you suggest another wing profile? (this is really hard to choose)

I was worried about the ailerons, that's why I made them so large and I am going to try to slim down the fuselage.

Thank you again!

PS: do you think the CG will have to be adjusted between the long and short wing span version?
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Old May 27, 2011, 05:02 PM
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Francois,
As far as CG placement as a function of main wing area - yes you should correct it by a small amount. There might be another solution - you can make adjustible length of the tail booms.
Here is a good CG calculator that could help.

Here is a very good website to make a scientific guess about some aircraft dimensions. I would pay special attention to aileron dimensions and constrain between main wing chord and distance between horizontal surfaces.

Here you have great place to choose proper airfoil. I am sorry but I am a purist here and I will not point you to particular one. I believe the airfoil is one of the most important part of the design and it depends on what you want. It is a good opportunity to begin reading airfoil graphs. Believe me it will pay back in better behaviour in air.

Michal
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Old May 30, 2011, 02:13 AM
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You will also need to consider your launch options.

Pusher props can be interesting to launch if you want to protect your fingers. As the weight/size of model increases this becomes more of a problem.

This type of model flies very well:

http://www.rcgroups.com/forums/showthread.php?t=844614



As mentioned the CG can be hard to achieve with a lot of weight in the rear of the model - so you need to pay attention to the length of the nose and how you are going to obtain a suitable balance.

Tim
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Old May 31, 2011, 04:54 AM
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I've just finished building a twin boom design, and the nose looked pretty long to me, I hought 'yeh CG will be no issue there!' , anyway LUCKILY it balenced with the battery literally against the nose.... it surprised me! So just check that!

How I test my 'rough' CG is to build the wing first, then i attach a garden cane under the wing, attach some kind of horizontal stabaliser to the back, then i move a weight around on the front of the cane until it 'glides' , you just get an idea, a little hit and miss. Once i'm happy ive found the 'sweet spot' I then see where the pivot is under the wings, and i try and get my maiden to balence roughly in that same area.

Just my method!!
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Old May 31, 2011, 08:15 PM
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Thank you guys for your help. Right now I am still working on the airfoil (thanks Michal of the links). It's easy to find airfoils for gliders, harder for or FPV. I would like this plane to be able to glide in the long version but I do not want the plane to be unstable when I push the throttle.

I am also working on an excel file: plane's dimension, CG calculation... but it's going to take me a few days to finish it. I hope that you guys can review it when it will be done.


In any case, I am planing to build a simple foam version of this plane and see how it fly, then I'll build a nicer version. I am planning to build it out of kevlar and this by itself will be a whole different story, I have never vacuum bagged a wing. Will see how it goes!

Thank you again,

Francois

PS: leonr, do you have the dimensions of your plane, maybe pictures (even videos) you want to share?
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Old Jun 01, 2011, 03:05 AM
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It may be a week or so before I can get any proper test flight/pictures im afraid... I will try and get a picture up at the weekend, not sure if I will be testing it though!

I can't remember the dimensions now, the wing span was 110 or 120cm, 24cm chord (i think) , trailing edge to end of tail was about 50cm I think?? (sounds wrong, ill have to check!)
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Old Jun 03, 2011, 09:20 PM
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no problem, i am overloaded with work anyway... I am sure it's going to take ma a few weeks before I am done with the final design.

thank you
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