Locating CG placement on a Triplane

[Steve-S63]

I'm trying to find the calcs for determining where the CG should be on a triplane. Should I use the LE of the top wing & the TE of the bottom wing (they are different spans) and then calculate as if it's a monoplane? The plane is a Ziroli DR1.

Thanks for any help

[MarkusN]

Calculate ACs of each individual wing, then give each wing a weight corresponding to its area and from that calculate the common AC.

This is not absolutely precise, as the wings are influenced by the circulation of the other wings, but it should give you a good start.

[vintage1]

If using a calculation program, take the centre wing (if of equal size to the rest) - and multiply its span by three.

That nets you the right point in relation to the center wing ..

[MarkusN]

Since you say thet the span of the wings is not equal:

Starting from Vintage's approach of using a calcualtion program: Enter the plan form of the wings as if they were not above each other, but aligned spanwise:

i.e. start with defining the lower wing, then add wing panels for the middle one at the tips of that, then for the upper one outside of that composite wing. Gives you a weird planform with a staggered appearance, but the CG calculated from that is about right.

There will be errors in terms of induced angle of attack (because aspect ratio is too high), but adjusting AOA on a multiplane is a handful anyway.

[hul]

the Gordon Whitehead article in Flying Scale Models a couple of years ago says: for a biplane, the MAC is taken from the l.e. of the top wing to the t.e. of the lower wing, and vice-versa for back stagger.... For the calculation, wing area is taken as total area of both wings, and for a triplane, the total area of all three wings; not simply the area as projected in plan view.

Hans

[Julez]

Check winlaengs 4:
http://home.arcor.de/d_meissner/schwerp.htm

[yoyoML]

Some caution here. You'll want a more forward CG than an equivalent monoplane. This is because the main wings deflect & slow down the air quite a bit before it reaches the horizontal stab, then the stab's effective airspeed and effective AoA are both lower -> less stable. In a monoplane this is not as pronouced, but in bipes and tripes it is easily noticed.

[Steve-S63]

Thanks for all the replies. I've taken the LE of the top wing & the TE of the bottom wing as the chord and balanced for that. Made the range from 20% to 30% MAC from LE and currently have it balanced at about 23-24%. Hope to maiden it tomorrow. Knock on wood.

Steve-S