Robinson R-22 Manifold Pressure Online Ground School

Hey, this is David Redmond at Odyssey Aeronautics in Auburn, California, and I'm going to make a video. Today we had a member on helicopter online ground school ask about manifold pressure and what that means. So I'm going to make a little video today on manifold pressure performance and we're going to take that right into, you know, why we have the engine, we have and derating and all that kind of stuff. When you're dealing with manifold pressure, the first thing you really need to understand is what the heck is it measuring. And it's measured in inches of mercury. And most people don't understand what that actually means. Back in the day when they had to measure, you know, what is the pressure of the atmosphere. You know, they had to measure this thing. So they built a device that looks just like this and they filled it full of mercury. So, so when you see something that says inches of mercury, HG is just the two 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. And 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 of the air, so it's a complete vacuum up here, this mercury will only rise so far. And then you literally just measure it. And so, like, 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 going to be pulled up 30 inches. So 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. So when they say, you know, high, low pressure, inches of mercury, this is what they're talking about. So 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 and 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. Now, you ever notice when you get in the helicopter and you haven't started the engine yet, your manifold pressure is up to almost the end of the scale. So 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. So you can see these two are the thing are the same. Now once we start to start the engine, the engine starts turning over and starts pulling air in. Well, this throttle is mostly closed, so you're actually going to pull a vacuum in here. And this manifold pressure you'll notice when the engine starts is going to drop way down. So 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 closed, so it's not allowing much air to get by. So you have far less air pressure down here. And that's why your manifold pressure is down low when the engines at idle. Now once you start actually flying, this valve will start to open up depending on your power needs and let more air through. And so your needle will begin to rise, reflecting that up to let's just say like 22 inches more. So this manifold pressure is just simply measuring how much air pressure is going into the engine. And that's a direct proportional to how much power the engine is producing. So as a note, before you start the engine, so you're sitting there, the engine's 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. So when you do your limit 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. Alright. So now that you have a basic understanding of the manifold pressure and what it's actually measuring, we're going to talk about performance and the horsepower and the D rating of the engine. So in the R22 we use a maximum continuous horsepower of 124. Now I know that we have a 5 minute 131, but for the sake of what I'm going to be teaching, we're just going to stick with the max continuous power124. So we run an 0360 reals engine in the R22 beta 2. Now, this engine is capable of producing a lot more than 124 horsepower. So why do we do this? Well, the first step is to understand that when you derate the engine, you know, when you're sitting in the cockpit and you look at your chart that says, okay, at this altitude and this temperature, I can pull, let's say, 23 inches of power, you're actually finding where 124 horsepower is so that you don't put too much horsepower through the system. Part of this whole thing is that the system of the helicopter is designed for 124 horsepower. Now, if you're at sea level, you could actually put in a much smaller engine. So let's just say Robinson picked an engine that only produced 124 horsepower. And you put that side by side with a beta 2 with its O360, which were looking at the manifold pressure and getting it to 124 horsepower. If you were to fly those two helicopters, the one with the smaller engine would actually outperform the beta 2 because the engine would be lighter. However, the engine would be at wide open throttle. So if I drew a helicopter right here, and this had a 124 horsepower engine, its manifold pressure would be completely wide open versus our beta 2, which we are pulling 124. So let's say the beta 2 will actually make 180 horsepower, but we're only using 124, so we're not using the engine at full throttle. So our manifold pressure would be somewhere like this. At sea level, the two, it wouldn't matter. They were both getting 124 horsepower. But what happens is now we're going to start going up in altitude. So in this scenario, let's just say that right now at sea level. So we're down here at sea level, there is 30 inches of pressure. So our outside ambient pressure is 30 inches. This engine is a wide open throttle. So it's at 30 inches of manifold pressure on your manifold pressure gauge. This one isn't using wide open throttle. The engine has extra power. So it's down to. I'm just going to make up a number. Let's just say it's at 24 inches. Now, the thing to understand is that there's a pressure lapse rate, which means as you go up in altitude, this 30 inches of ambient pressure drops obviously top of Mount Everest. The air is really thin. That's why, you know, hikers struggle to get up there because there's low pressure. So let's say we go up 3,000ft. This drops one inch per thousand feet of altitude. So if we go up 3,000ft, there's only going to be 27 inches of pressure. And again, I'm just coming up with easy numbers. It could be a little different, but we're just going to use this for this example. Well, this helicopter with this engine, it was running at 30 inches. Now up here at 3,000ft, it's only going to be able to use 27 inches, which, guess what, it's not going to make 124 horsepower. I mean, just a guess, maybe it's only going to give you 100 horsepower. That's not enough to fly the O360 because it has reserve. You know, that's what it could pull. That's what it's using. It goes up to 3,000ft, it can pull 24 inches. So at 24 inches, it's still giving you 124 horsepower. Now your reserve is less because there's only 27 total available. But you can see this engine will still give you 124 horsepower. That's why they put a bigger engine in the helicopter. And then you derate it so you have consistent performance going up in altitude. Obviously eventually it will start to drop off, but that's why they put in a bigger engine. So that all the way up to about 5,6000ft, you will be getting 124 horsepower all the way up. So just to take our manifold pressure a little bit further, we're going to look at our limit manifold pressure chart. Again. When you look at this chart, so you find your altitude and your temperature and sometimes you have to interpolate in between. You're finding where 124 horsepower is. Obviously you would add 0.9 for the 131. Now, something to note on this chart, if you're getting into down here, see how it says full throttle. If you're getting into these numbers right here, you need to watch out because you're getting to the point where you're going to start losing performance with altitude. So for example, if we are at 6,000ft and it's 30 degrees Celsius, you can see it's saying we can pull 21.6. So this is our performance chart for out of ground effect hover takeoff power, full throttle, 104% RPM. Now, you can see I gave you the example of 6,000ft and 30 degrees Celsius along this line you see how these lines come up, and then right along here, they stop and they shift their angle like this. This is where you're getting to your wide open throttle. Up until here, the engine's giving you 124 horsepower. 124 horsepower all the way up to this point. Now, this is where your engines at wide open throttle. So any more altitude is going to result in lower than 124 horsepower. So it's going to be giving you less and less power, which means your performance is going to dramatically start dropping off. So if you go up 1,000ft down here, you only drop like 20 pounds. If you go up 1,000ft up here, you're going to have to drop a lot more weight because the engine is not capable of giving you that 124 horsepower anymore.