small engines in hot weather

Ted shuffled out of his cave and grunted these great (and sometimes not so great) words of knowledge:

If you want the same performance in July that you had in March go to a higher nitro fuel.

The air is denser when it is cooler (colder ), therefore the engine is getting more oxygen for combustion. By increasing the nitro content of your fuel (usually ) by an additional 10% (going from 10% nitro to 20% nitro ) will frequently "put back" the lost power. As with anything, don't go overboard on this as it can lead to other problems.

I suggest increasing the nitro content INITIALLY by 10%. Then, based on the performance received AND HOW THE ENGINE IS RUNNING, adjust the nitro content from that point in 5% increments. You will most likely have to adjust both the idle and the high end in order for the engine to perform properly.

This also is very effective if you go from flying at sea level to flying at a substantially higher altitude, since the air is less dense at the higher altitude.

Try adding 3 -- 5% more castor oil, and dropping the prop pitch 1".

Small engines need additional cooling in hot and humid wx.

(top posting "fixed" -- what's the group standard?)

In addition the lower air density caused by both the heat and the humidity (water vapor is less dense than air) your aircraft will have to travel faster to get the same lift at the same angles of attack. The lower air density will increase your induced drag, and possibly make you run out of power. Hot, humid weather is notorious among full-scale pilots for causing problems. It's why they have this thing called "density altitude".

I've never experienced the problem (at least not to the extent that an airplane wouldn't fly for me), but the suggestion of playing with the propeller choice as well as the nitro seems to be a good one.

It's funny, I wasn't looking for suggestions. I was just complaining about the weather a little bit, but I ended up getting some good ideas anyway. I never thought about nitro content, because I'm one of those guys who buys fuel at swap meets and whenever the hobby store runs a special. I got a bunch of fuel three years ago when a local guy sold his RC gear off. I have about ten gallons in my basement now, ranging from 5 to 15%, and what I usually do is pour them a little bit at a time into my field jug, observing the strange ugly colors that form when they are mixed. Yeah, I know what you're going to say, but the engines always run beautifully regardless of what fuel I use. Anyway, maybe if I use some of the 15% exclusively I would see a boost in performance. I have an RCM Sporty Forty biplane/OS 52 Surpass that gets rather dull in warm weather. Maybe that would liven it up.

I was thinking about fiddling with the propellers a bit, although I never got around to it. Mostly it's just better to wait for cooler weather because around here you don't even get a breeze when it's like this, and you end up sweating off half of your body water content in ten minutes. Cooler weather usually comes back in a few weeks anyway. But who knows, I may just go out to fly some next week. If I can get my house sold I won't have so much real work to do for a while.

On Sat, 23 Jul 2005 19:58:02 -0700, Tim Wescott wrote in :

Random. ;o)

I prefer heavy snipping and bottom posting myself, but I'm not going to try to inflict my tastes on others.

YMMV.

Marty

Many people forget that models operate on the same principles as full-scale aircraft. In hot weather, you need to concern yourself with density altitude. You can do a look up on Google for a full explanation.

Essentially the heat moves the molecules of air further apart resulting in less lift for the wings. The engine is also affected to some degree, but the major problem is loss of lift. Also on very hot days you'll often find that the winds are very mild or calm further compounding the problem of the aircraft getting off the ground.

The moral - keep it in the hangar on extremely hot days!

Mr Akimoto

Yeah, I think we all know this stuff about air density. It's just that it sometimes amazes me when I see the principles in action anyway. You never think of how delicate of a balance you are working with until a 20 degree change in temperature keeps the plane from taking off.

I tend to do the same thing as you. The planes stay in the basement and collect dust and cobwebs, and I go to the flying field again when it gets cooler. I've never been one to try to squeeze the last bit of power out of an engine anyway.

Maybe I should build that 10 foot Paragon sailplane I've been meaning to put together. It would be perfect for a day like today.

Speaking of hot weather, when I was working as an A&P mechanic in Dallas we had a guy at our company who put a Cessna together in his spare time. He worked long and hard on it, and when it was finished he took it out for a hop with three of his friends. Unfortunately, he had failed to check the charts for takeoff weight and ambient temperature, so the plane ended up in the fence at the end of the runway. This fellow was really smart. When he got the plane fixed after his mishap, he got the same three friends in the airplane and did it again.

Mr Akimoto wrote: