3D Design Question

[Gaz]

I am in the process of trying to design a plane for electric 3D flight.
What would be the best option for the wing placement, directly inline with datum line or just below ?
I have not built this section of the fuse yet so I can go either way,
below may be a little more practical for mounting electronics.

Gary.

[Gaz]

Progress so far

[BMatthews]

Those parts look like they want to float away! ! ! Lovely work that pulls at my free flight background.

On the datum or slightly low. Not a big deal one way or the other.

[Gaz]

Thanks for the reply, appropriate you should mention free flight.
I'm currently working from How to Design & Build Flying Models by Keith Laumer c1960, I have the revised 1975 edition, full of rubber endurance models, pendulum-controlled F/F and lots of other interesting topics.

[vintage1]

If you want the plane to be as neutral as possible...

(i) thrustline, wings, tailplane all in line

(ii) as much fin and rudder vbelow datum as above, or nearly.

(iii) plenty of fuselage side area, with about twice as much total area behind CG as in front.

That will give you a plane that is pretty stable at any bank angle through knife edge to inverted, and will tend to fly where you point it.

Lovely piece of structure by the way. I hope its got LIPOS in its future...

[shaneyee]

Vintage1,
Why "twice as much area behind CG as in front"? I understand that in a knife edge, the fuselage is acting like the wing but that rule of thumb puzzles me....

Shane

[BMatthews]

The 1/3 2/3 split (as in twice as much to the rear) reflects the area distribution of the typical wing with the balance point being at the 30% mark. Otherwise it may be unstable when in knife edge... or in yaw while level for that matter.

[shaneyee]

ok. that makes sense....

[DICKEYBIRD]

Originally posted by Gaz
Thanks for the reply, appropriate you should mention free flight.
I'm currently working from How to Design & Build Flying Models by Keith Laumer c1960, I have the revised 1975 edition, full of rubber endurance models, pendulum-controlled F/F and lots of other interesting topics. Hey Gaz, I'm green with envy! I grew up lusting after Keith Laumer's designs in the model magazines. Matter of fact, I scaled up and modified his "Twin Lizzie" from the 1959 M.A.N. magazine plans and converted it to 2 engines.

I've always wanted a copy of that book! Where did you find it?

ps: Luv that lightweight construction you're doing!

[Gaz]

Dickybird, It is one of my father's books so I'm not sure where it came from originally, it could be worth a search on amozon.com if you are looking for one. The twin sound like a good conversion.
In my book there are plans for the Push-up F/F and also Flea-Wizz control line intended for cox .020 power that could be scaled up - may do that sometime

Gary.

[Gaz]

I have another question, regarding prop size, it seems that most 3d designs opt for a large prop diameter; my understanding is that this is to create a wide air flow over the control surfaces as possible that is important at low air speeds (hover). Is this correct and are there any other reasons for it ?

Working on this principle I have selected a MPJET 25/25-26 MK II motor in a GWS EPS 300 gearbox with a 12x6 APC Slow fly prop , Motocalc shows 751gm of thrust at 10.5 amps with 3s Etec’s. I am hoping for AUW of around 400grams so the Etec’s shouldn’t be working too hard.
Thanks for the replies so far, very helpful.

[opualuan]

what do you guess the empty structural weight would be?

[Gaz]

Around 150gm or less uncovered, at the moment as a guide the wing has a span of 100cm and cord of 22.5cm and is 30gm.
It also has 10 metres of braided fishing line to add strength and retain the airfoil shape.