I have run a Saito 91 at 10,00 feet + density altitude. I normally fly at 4200 feet but a couple times a year at a 7200 foot altitude field, usually during the heat of summer. I started with a 14 x 6 and the plane did not fly fast enough. The air is pretty thin and engine power is down. The plane had unlimited vertical with the 14 x 6 prop. I tried a 13 x 10 and the forward flight was better, but the prop loaded down the engine and the vertical suffered. I stepped down to a 13 x 8 prop and the plane maintained it's unlimited vertical (not as well as the 14 x 6) but it had good forward speed. I have found that to be the best overall prop for the plane-motor combination.

I have run the small engines at higher altitudes for some time. I have the best success cutting an inch off of the pitch or diameter. I would recommend against putting a larger (one that would load the motor more) prop on the engine. From experimentation, the loss in power has a much greater affect than the decreased load on the prop due to the lower air density. Infact, I have seen engines overheat because they are running on the upper end of the manufacturer's recomendations. The ones I have seen is the OS 25 FX with a 10 x 6 prop and a 46 FX with a 12 x 7 at altitude in the heat. The prop was replaced with a more reasonable one and the problems went away.

As an example. I have a modified Quickee Jet 50. It is available on special order. The motor needs to run at 19,000+ static RPM to be in the tuned range and unload in the air. I flew at a field in the spring time down around 3000 feet when the temp was in the 60's. I flew with an 8.79 x 9 prop and got 20,500 RPM static. The plane was incredibly fast. I would say flat and level around 160-170 MPH. I then took the plane home and flew it in the hot summers. I had to step down to an 8 x 9 prop to run at the same RPM. The plane is probably around 25 MPH slower.

I think you would do better with a 120 on that plane at that altitude if you need any nose weight. There is a big weight jump between the 100 and the 120. I'm a speed freek and sometimes go 1.5 to 2 times the recommended displacement (2 times is on the high side. I wouldn't recommend it to everyone, sometimes the planes need to be reinforced and throttle management needs to be used). I have had fun landing the models in the thin air and I realize you will want to keep the weight down. I would also recommend a .90 2 stroke engine, but it sounds like you will be doing the 4 stroker. The 91 FX would give you more perfomance than you will get with either the 91 or 100 Saito.

Props are cheap.......I love to experiment. I have a prop collection of around 50. Every time I go the the field with a new plane I take a couple of props and see which one suits the engine-plane combo the best.

OK, OK I'll stop typing.......I'm sure I'vesaid something that will be disagrred upon. Good night.