Hi there,

I know its not exactly a speed plane, but i won a 40-size EXTRA 300s at my local club. I have an OS .75 lying around that i want to over-power it with (i know it may be a bit heavy, but i want to experiment).

What is the best, highest pitch, prop i can buy for it? Can you all offer me advice and links to where i can purchase these props? Local dealer only has 12-pitch props...

How fast can i get this thing moving?<br type="_moz" />

I'd get to know the plane first with a 6 pitch prop. Learn to land it in the shortest distance. Get the CG dialed in that allows good stability, but also allows you to flare the plane right before touchdown. Flying an over weight plane with a 8 pitch prop really makes controlling the landing speed more of a job. If you fly in a small field, small runway your prop choices have to be based on your abilitys with that plane. If you fly at some huge paved area where you can afford to make 30 MPH touchdowns anywhere in any direction, then obviously your prop choices are just about how fast you want the engine to turn and what works best for the plane's aerodynamics.
I would guess that a 10x8 prop for a start for speed.

I have plenty of space for hot landings so that shouldnt be a problem...

10x8 for an OS .75AX ?? Thats it ??

I would think something over a 12 pitch would be good, no?<br type="_moz" />

the ax 75 isnt that powerful of a motor a 10x8 isnt that much of a investment to tryout.....CP has been doing stuff like this for years, if you are asking for advice from those who know.......and by the way if you are trying to go fast with this model, be very careful about linkage slop, as you could easily flutter it and lose it all...

Just adding pitch does not necessarily add top speed. There is a limit. The goal is to run pitch just high enough to achieve the top speed that is limited by drag and horsepower, with the diameter that allows the engine to turn that pitch at rpm where it develops good horsepower.

- Decide what rpm you want to run the engine (likely 14-15k on the ground for most hp)
- Estimate how fast you think that aircraft can truly fly in practice
- Get an idea how much pitch you need via this: http://www.rcpro.org/rccalc/PitchSpeed.aspx calculator using your target rpm
- Figure out what diameter it will spin that much pitch at that rpm, then use that as a basis for experimenting.

My best guess is that you will gain nothing with pitches as high as you suggest. As you mention, it is not a speed airframe. Same deal as a car, just because a car could go 150mph in 5th grear at redline does not mean it will, that depends entirely on horsepower and drag. I would be surprised if you can push it too far north of 100, maybe 110 or a hair better. Maybe a 10-8 as CP suggests, maybe 10-9, that's where I would start.

Watch yer flutter.

To do this project the right way......wouldn't it require moving the firewall back?

At least as much as the spinner fit allows?

IIRC, I've seen the stock firewall on ARFs hacked out of the way, but not until the new firewall was installed behind it. The idea is to lay a spacer block behind the original firewall to act as an automatic allignment jig for gluing in the new firewall.

Hi Combatpigg,

Just a quick dumb question, why would i have to move it back? CG adjustment that would require less tail weight?<br type="_moz" />

because the ax 75 is a boat anchor compared to a 45 sized os engine.......so if you move the big ax75 back you need less tail weight to make it balance.....