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#1
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MAP behavior question
Here's a question ...
When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Does it indicate barometric pressure? Does it pin? (Go to max)? Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? ....and why? |
#2
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MAP behavior question
"Bob F." wrote in message . .. Here's a question ... When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Does it indicate barometric pressure? Does it pin? (Go to max)? Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? ...and why? Reading my own question, I guess I need to qualify it. 1. Prop is still rotating. 2. There is a MAP gauge in the aircraft (172rg, Arrow, etc.) -- Regards, Bob F. |
#3
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MAP behavior question
"Bob F." wrote Here's a question ... When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Does it indicate barometric pressure? Does it pin? (Go to max)? Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? From a theoretical point of view, I vote for the last choice. Why? I would think because, if the prop is still turning, the engine is still an air pump, just as much so as if it were still running. The "suck" part of the suck, squeeze, bang and blow is still there. If the throttle is closed, the throttle butterfly is closed so the suck is trying to pull a partial vacuum. I would think that because the engine is turning more slowly, the ability to pull as low of a MAP will be lessened, (higher MAP-just to clarify) than the pressure you would normally see at the same throttle setting with the engine running. -- Jim in NC |
#4
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MAP behavior question
"Morgans" wrote in message ... "Bob F." wrote Here's a question ... When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Does it indicate barometric pressure? Does it pin? (Go to max)? Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? From a theoretical point of view, I vote for the last choice. Why? I would think because, if the prop is still turning, the engine is still an air pump, just as much so as if it were still running. The "suck" part of the suck, squeeze, bang and blow is still there. If the throttle is closed, the throttle butterfly is closed so the suck is trying to pull a partial vacuum. I would think that because the engine is turning more slowly, the ability to pull as low of a MAP will be lessened, (higher MAP-just to clarify) than the pressure you would normally see at the same throttle setting with the engine running. -- Jim in NC Correct, at least your first paragraph. With respect, I don't understand your second paragraph without reading into it. I ask this Q. because I often ask a student to tell me how to recognize and engine fuel starvation. I often get the incorrect answer that the MAP goes to zero. Years ago, before best practices frowned upon this, you could set up a glide at about 400 fpm down, 80 kts, and 2200 RPM, pull the mixture and you would not hear or feel any engine loss. And if you pushed the throttle in, MAP would increase, just like when it's running ok. So this is not a good indication of engine fuel starvation. You could set up the same scenario on a ME and find the right speeds and settings to demo this. -- Regards, Bob F. |
#5
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MAP behavior question
"Bob F." wrote Correct, at least your first paragraph. With respect, I don't understand your second paragraph without reading into it. All I was getting at (perhaps incorrectly) that the engine would be running slightly faster if it were producing power compared to the speed of the engine at the same throttle setting if the engine was dead. That lower windmilling RPM should therefore produce a slightly higher MAP. I would expect that the difference would be nearly unobservable, but theoretically should be present. Certainly not enough difference for the student to be able to troubleshoot whether the engine is still running or not, now that I know what situation you were getting at. -- Jim in NC |
#6
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MAP behavior question
"Bob F." wrote in message
... "Morgans" wrote in message ... "Bob F." wrote Here's a question ... When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Does it indicate barometric pressure? Does it pin? (Go to max)? Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? From a theoretical point of view, I vote for the last choice. Why? I would think because, if the prop is still turning, the engine is still an air pump, just as much so as if it were still running. The "suck" part of the suck, squeeze, bang and blow is still there. If the throttle is closed, the throttle butterfly is closed so the suck is trying to pull a partial vacuum. I would think that because the engine is turning more slowly, the ability to pull as low of a MAP will be lessened, (higher MAP-just to clarify) than the pressure you would normally see at the same throttle setting with the engine running. -- Jim in NC Correct, at least your first paragraph. With respect, I don't understand your second paragraph without reading into it. The windmilling prop will turn the engine slower than it was turning under power. As a result the rate at which air is pumped out of the intake manifold is reduced, which allows the air flow through the throttle body to fill the manifold up to a somewhat higher pressure. -- Geoff The Sea Hawk at Wow Way d0t Com remove spaces and make the obvious substitutions to reply by mail When immigration is outlawed, only outlaws will immigrate. |
#7
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MAP behavior question
Bob F. wrote:
Here's a question ... When a normally aspirated engine quits while at cruise, how does the MAP react with respect to the throttle? Does it indicate zero? Only in outer space. Or if the gauge is broken. Does it indicate barometric pressure? With an open throttle and stopped prop. Does it pin? (Go to max)? Why would it do that? If you are 'cruising' on the taxiway, you might get the 29.9-30 inches reading, if it dies there. Does it follow the Throttle (high MAP with a high throttle setting , low when set to low throttle setting)? Yes. except as noted above. ...and why? Because an engine is an air pump. If the pistons are moving air, then the air pressure can be measured by the MP gauge. The throttle will manipulate the air pressure as the engine pumps air past it. Closed throttle, low pressure. Open throttle, high pressure. |
#8
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MAP behavior question
Bob F. wrote:
Reading my own question, I guess I need to qualify it. 1. Prop is still rotating. 2. There is a MAP gauge in the aircraft (172rg, Arrow, etc.) Item two is purely for fluff. Manifold pressure has no way of knowing its being measured. You can discuss manifold pressure, and its effect, without a gauge installed. But I'm guessing you knew that. |
#9
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MAP behavior question
Capt. Geoffrey Thorpe wrote:
As a result the rate at which air is pumped out of the intake manifold is reduced, which allows the air flow through the throttle body to fill the manifold up to a somewhat higher pressure. Careful.. there is very little "filling" going on. Unless you are in a Mooney, at wide open throttle, with the ram air door open. Manifold pressure is almost solely the effect of the engine sucking air in by piston action past the air filter, and ram air difference is negligible in the typical trainer. Respectfully, you guys are trying to measure something with a micrometer (ram air contribution), that was cut with an axe (throttle) (to quote Jim Weir).. I think some of the best reading on MP was in John Deakin's series of articles on Fire Breathing Turbos. The first two I think were dedicated to fundamentals...with and without turbo charging. Anyone interested, go to www.avweb.com and search for his material. Dave |
#10
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MAP behavior question
Bob,
Same indication as with engine running. The important thing to always remember is that "Manifold pressure sucks", which is also the title of John Deakin's brilliant Avweb column on the topic - absolutely required reading. -- Thomas Borchert (EDDH) |
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