is this too much for a st 90? its for a h9 show time? i dont wanna find out the hard way

The ST90 will spin a 15x4 but don't expect stunning performance, especially if you plan to 3D that bird.

The ST90 is *not* a good 3D engine (lousy midrange/transition).

I'd also give the 14x4W a try (you'll probably get the same static thrust but more speed because of the higher RPMs) although my choice would be for a decent 14x6 wood prop.

Why wood?

Well, once you start getting up to 14" diameter and above, plastic props (like the APCs) get really heavy and that can affect the spool-up time of the engine (an important issue for a 2-stroke in a 3D plane). I've also found that some of the newer wood props (like Xoar) are actually quite a bit more efficient than the APCs.

If I were you I'd just go buy a bunch of props from 14x4 to 15x6 and try them out to see which is not only best for the engine but which is also best for your flying style.

I know that if I plan to do a lot of hovering, harriers etc a bigger low-pitched prop is good. However, once you get into maneuvers where you want to have a *lot* of very rapid tail authority (waterfalls, yak-offs, flips, etc) then a little less diameter and a little more pitch makes the rudder/elevator far more effective when necessary.

i tached my st with a 15x4 apc funfly prop and got 9800 rpm safely and around 10000 peak on 10% mach7 with stock muffler.... ill take you advice an buy some wood 14x4 props and i have some 14x6's that i had for my 1.08 that ill try on the showtime

i have a feeling that ill like the 14x6 best.. im not a great 3der yet, and i have a couple of flying buddies that like to race around

The only way to be sure is to fly the props.

Terry in LP

I would second the 14x6. I have tried a few on a few different ST 90's. The 14x4W is not enough and the 15x4W is a little much.

David

I forgot to mention. A MA 13x6 gives great spool up on a ST 90. I have one on a 90 powered Pizzaz. The verticle is stunning to say the least.

Hi!
I use a 15x4 APC in my Great Planes GEE BEE R-2 powered by a MVVS 15cc engine (.91).
It works a little better than the standard 14x6.
14x4 is too small for models weighting around 5kg.
I even use a 15x6 APC in my U-Can-Do .60 with same engine.

Yep, it's been my experience that the ST90 likes the MA Scimitar 13x6 above all others. People say they're a torquey engine that likes to be loaded down but if you throw a 13x6 on them you'll see that they really do like to sit at around 12K-13K on the ground. It also seems to help with the miserable mid-range too.

Several years ago I ran a Super Tigre S91K (cowled/inverted) on a 8.2 lbs (dry) tail dragger MT Joss Stik (mid wing mod) with an older Zinger 14~8 and a Magic Muffler that flew about 75 mph straight and level, vertical out of sight and straight down. Plan on building another like it.

Terry in LP

XJet, if you ever get over to the ST support forum. The mod over there,BAX, likes his Tigres to turn a bunch of RPM. If you look at what he uses I have been over propping my ST's since day one.

David