Hi guys,
I'm a pretty accomplished F3C and 3D RC heli pilot who has decided to give 3D planes a go for times when I feel like need something different.
I had my first flights last weekend on a Sig Fazer and can takeoff, land, circuits, inverted, basic aerobatics etc.

I've brought my first plane, a 70inch Edge 540 (don't want to muck around with smaller ones) and I have a spare OS 91 lying around that I was thinking of putting in it. I also have a spare Hatori 700 tuned pipe lying next to it.

A mate and I found a calculator for calculating props on the net and we found that a 15x5 prop spinning at about 12,500rpm would give 20lb's of thrust.

As I am new to planks, I'd appreciate any forsight you can give. While helis will remain my priority, I'd really like to get into 3D fixed wing.

Thanks very much!
Si

OS 91FX(?) w/ 15x5 will give you about 15lbs static thrust(Usable / Realistic). So your 540 needs to weigh in at less then 13lbs (Complete).

OS91FX
Displacement: 0.912 cu in (15.0cc)
Bore: 1.091 in (27.7mm)
Stroke: 0.976 in (24.8mm)
Practical RPM: 2,000-16,000
Output: 2.8 bhp (ps) @ 15,000 rpm
Weight: 19.3 oz (550g)
Includes: Muffler, Safety Propeller Locknut Assembly, #8 glow plug
Requires: fuel, mount & prop
Recommended Propellers: 13x8-9

Personaly id go with a 15x4. The 15x5 seems to overprop it a bit.

It's the OS 91 SX-H which has the same specs pretty much.

What kind of thrust to weight ratio is required for hovering/3D aeros?

Si

For good 3d performance figure 1.5hp for every 5lbs or a 1.5/1 ratio. Much lower and you cant reliably pull out of a hover.

So if I could get 15lbs thrust out of a 10lb model, I'd be in the ball park?

Yup you'd be in 3d land all right.