Robinson R-22 Manifold Pressure Online Ground School 

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Robinson R-22 Training Manifold Pressure Online Ground School. Hey, this is David Redmond, at Odyssey Aeronautics in Auburn, California. We had a member on Helicopter Online Ground School ask about Robinson R-22 manifold pressure and what that means. I'm going to make a little video today on manifold pressure performance, and we're going to take that right into why we have the engine we have in the Robinson R-22 Helicopter. When you're dealing with manifold pressure the first thing you really need to understand is what the heck is it measuring. It's measured in inches of mercury. Most people don't understand what that actually means. Back in the day when they had to measure what is the pressure of the atmosphere ... You know they had to measure this thing. They built a device that looks just like this and they filled it full of mercury. When you see something that says, inches of mercury, Hg is just the 2 letter identifier for mercury on the periodic table. What they did was they pulled a vacuum in this tube and this mercury would get pulled up right here. What's actually going on is it's the outside pressure pushing down that's pushing the mercury up. Well if you pull out all the air so it's a complete vacuum up here this mercury will only rise so far, and then you literally just measure it. If you go outside and it says, oh today is 30 inches of mercury, like your altimeter setting, that literally means that the mercury is getting pulled up 30 inches. If you have a lower pressure system coming in and you don't have nearly as much pressure coming in this level will go down to let's say like 28 inches, or a higher pressure system comes in and you have the mercury go up to let's say 31 inches. It's just literally how high up mercury goes in a tube with a vacuum and an open pool. When they say high, low pressure, inches of mercury, this is what they're talking about. Now on to our manifold pressure gauge. This is just measuring pressure. Now here we have kind of a cartoonish drawing of the engine and we have ... Air would come through here. This is our throttle, so this lets more or less air into the engine. Then it goes from there into the engine. Obviously we have venturi and fuel and all that kind of stuff going on, but for this we're not worried about that. Do you ever notice when you get into the Robinson R-22 helicopter and you haven't started the engine yet, your manifold pressure is up to almost the end of the scale. The manifold pressure will be reading something over here like 30 inches. Well with the engine off, and I just ... This doesn't actually exist, but I just made up my own manifold pressure gauge for on the other side of this butterfly valve. Right now this is all equal. No air is flowing anywhere, so your pressure inside going to the engine is actually the same as your outside atmospheric pressure. You can see these 2 are the same. The engine starts turning over and it starts pulling air in, well this throttle is mostly closed. You're actually going to pull a vacuum in here and this manifold pressure, you'll notice when the engine starts it's going to drop way down. This will drop down to somewhere around 10 to 12 inches at idle. Which means you still have 30 inches of pressure out here in the outside atmosphere. This butterfly valve is mostly, so it's not allowing much air to get by, so you have far less air pressure down here. That's why your manifold pressure is down low when the engine is at idle. Once you start actually flying this valve will start to open up depending on your power needs and let more air through, so your needle will begin to rise reflecting that up to, let's just say like 22 inches or more. This Robinson R-22 manifold pressure is just simply measuring how much air pressure is going into the engine. That's a direct proportional to how much power the engine is producing. As a note, before you start the engine. You're sitting there the engine is dead, wherever this manifold pressure gauge is reading is actually where the engine would be at if you fired it up and ran it wide open throttle. When you do your limit Robinson R-22 manifold pressure checks and you look at your chart and say, okay given this altitude and this temperature I can pull, let's just say 23 inches of power. That 23 inches of power will be back here, but your engine could actually pull this much power. We're just not letting it go wide open throttle.