How much does altitude affects engine power? At 5k I'm told I need to take the highest recommended engine and then add 25% so if its rated to a 60 mad i need an 85 or so what do you guys think

Here in Colorado we use a Saito 91 (100 larger mount) for 40 size planes. if we want unlimited vertical. I'm putting a YS 110 for a 90 size plane (excite 90) and I hope it's enough.

Im at 7k in Wyoming we just use a engine at the high end of what the plane calls for and it works good . I like to put a 60 2 stroke on a 40 size plane . I have herd that at around 5k-6k you lose about 35%. No air !

rik1024,
That's why we're all 1/2 crazy... no oxygen to the brain!

A carbureted, non supercharged engine will produce about 75% of its sea level rated power with wide open throttle at 7500 feet on a standard day (59Deg F/25 deg C at sea level), so for 7000 feet elevation, using the largest recommended engine size +25% sounds fine.

Cheers!

Jim

An accepted rule of thumb is that power decreased 3% for every 1000 foot gain in density altitude.

The thing that looks strange to some people is the fact, that when the air is less dense, the engine uses less fuel and makes less power, but the prop is spun at about the same RPM, as it does at sea level.

The thinner air also affects the load on the prop, allowing it to spin faster with the same power and at the same speed with the lower available power. Aircraft performance, however, is directly affected by the power, not the RPM.

Real planes with S/L turbocharged engine(s), make the same power, to 15, or 20 thousand feet, but still take a longer runway to take off and land, because lift is also a function of air density and more speed is needed to take-off.
The IAS will be the same at take-off speed, but the TAS will be higher.

I agree with the last post. I use an ASP 40 on my very heavy telemaster 70 and it flies just fine at 5280 ft. I own 3 "fun fly" 40 size planes with 40s in them and have not had a problem. I also own a cap 232 40 size with an os 65LA and it has too much power so going too far is easily possible even at higher altitudes. A rule of thumb at the club is to go one step up from the engine you plan on using. i.e. 40 to 46, 46 to 52, etc. I kinda wish I would have put in the 46 size engines in my fun fly planes for a little more performance, but they still fly fine with 40s. Also, I use a 15-18 blend on fuel mixed by guys at my club.

Dave

You lose about 3% for every 1000ft. I bbacked this up once with a specific plane i run which I get about 120mph. There were a few at sea level with identical plane engine and setup getting around 145 mph which is a 17% increase for them .
My rule of thumb on the glow engines are to install the larger of the recommended range. Got a 60-90 plane, put a 90 in it. Some of my stock have 90s in .40s. 1.08 in 60s and the whole thing.