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IO-390 Manifold Pressure Knob?

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bkervaski

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IO-390 Manifold Pressure?

Hey guys ...

Working with SteinAir on a panel design, RV14A IO-390 (thunderbolt). They didn't put a manifold pressure control on the drawing.

Being a fuel injected engine and no fadec that I'm aware of is there not a manifold pressure control on these engines?

I've never had a fuel injected engine or a constant speed prop, sorry if this is a dumb question.
 
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The intake manifold pressure is measured downstream of the throttle valve. The throttle control can lower the MP below atmospheric due to restriction i.e. partial throttle. At wide open throttle, WOT, the pressure is the result of a small pressure drop from ambient pressure. Ambient is the result of altitude.

You might say the throttle control is the MAP knob for a normally aspirated engine, non -turbocharged.
 
Prop and power are joined at the hip.. Go for an hour in a constant speed aircraft with an instructor and you will see its not hard to get your head around the operation, enough for you to build on with confidence.
 
With a fix pitch prop, you set power by RPM.
The engine efficiency is pre-determined, normally with a cruise pitch since most of the flying is during cruise. A towplane (gliders) however spends its time climbing, so we install a prop with slightly less pitch (climb prop).

With a CS Prop , you will set the RPM normally already established for the phase of flight ( TO, Climb, Cruise) . 23mp, 2300rpm for cruise as an example ("23 square").

BUT!!! the prop will always seek the pitch (Constant Speed) selected with the prop control, the throttle contols gas, in turns power. Power then thrust with the prop increasing pitch to maintain .....RPM.

So, rule never to break......always keep the prop ahead of the throttle (knobs).
We go 'full' prop on final in case of a go around. Poor on the gas with a high pitch cruise setting forgotten during a distracted descent, could overstress the engine. Checklist.

Manifold Pressure is the gauge for the throttle, for the most part. You will learn more and all the nuances between MP and RPM and operating them later.
 
As everyone says, throttle is the primary control for manifold pressure (which is an indirect measure of power). The prop speed control can also control manifold pressure to an extent - at any given throttle setting, higher RPM = higher manifold pressure, and vice versa.

One interesting thing about prop speed (RPM): below a certain power, throttle controls prop speed: in descent/approach if you pull the power back (with throttle), at some point the prop speed will fall below the "set" speed. During run-up on the ground, prop is full forward, but speed will be low (say 700-800 RPM) at idle power. To do the run-up, throttle is advanced to a desired prop speed (RPM) just as is done with a fixed-pitch prop.

Another way of thinking about it: Manifold pressure is an indication of power, which is a combination of throttle and rpm settings. To reduce power, you can reduce throttle or prop speed. As power is reduced, at some point, prop speed will fall "on its own". It makes more sense when doing it than it does on paper. During a climb, manifold pressure will fall gradually if all other settings remain the same.

(As a side note, another "power" control is mixture, especially lean of peak. But that's another topic for another day)
 
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As everyone says, throttle is the primary control for manifold pressure (which is an indirect measure of power). The prop speed control can also control manifold pressure to an extent - at any given throttle setting, higher RPM = higher manifold pressure, and vice versa.

One interesting thing about prop speed (RPM): below a certain power, throttle controls prop speed: in descent/approach if you pull the power back (with throttle), at some point the prop speed will fall below the "set" speed. During run-up on the ground, prop is full forward, but speed will be low (say 700-800 RPM) at idle power. To do the run-up, throttle is advanced to a desired prop speed (RPM) just as is done with a fixed-pitch prop.

Another way of thinking about it: Manifold pressure is an indication of power, which is a combination of throttle and rpm settings. To reduce power, you can reduce throttle or prop speed. As power is reduced, at some point, prop speed will fall "on its own". It makes more sense when doing it than it does on paper. During a climb, manifold pressure will fall gradually if all other settings remain the same.

(As a side note, another "power" control is mixture, especially lean of peak. But that's another topic for another day)

Actually, with fixed throttle setting higher RPM= lower Manifold pressure. Higher air volume demands result in a bigger loss thru the induction system.
 
Just to clarify - - Let's think of power differently.

Some good comments.

It should be noted that engine power is RPM x Torque, and since the RPM and MAP can be controlled separately with the CS prop, each has the proportional effect on power.

Manifold pressure is, more precisely, an indicator of engine torque, and the CS prop controls RPM. At least in flight range of engine speeds. So, under the right conditions, either prop speed or MAP can control power. Up to the engines ability to deliver of course.

With the FP prop, the RPM and MAP are linked and only controlled by throttle.

Secondary effects: If the engine at WOT and RPM is reduced via prop control, the MAP will increase slightly, but the primary effects remain dominant.
 
Poor on the gas with a high pitch cruise setting forgotten during a distracted descent, could overstress the engine. Checklist.

Not really relevant to this discussion, but this is an old wives tale that is for the most part (in regards to how people commonly operate their engine) false.

Anyone with a fixed pitch prop is in this condition every time they take off.

BTW I set my fixed pitch airplane power with the MP also. That way I don't have to keep readjusting as the aircraft speed stabilizes. It is easy to do with any airplane (as long as it is equiped with a MP gauge) once you get to know the specific numbers of that airplane.
 
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Not really relevant to this discussion, but this is an old wives tale that is false.

Anyone with a fixed pitch prop is in this condition every time they take off.

Not so false, and not quite.

Consider the "Limiting Manifold Pressure For Continuous Operation" found on any Lycoming power chart. It clearly prohibits, for example, 2200 RPM at full throttle.

As for fixed pitch, note that the line allows full throttle at 2400.

 
Not so false, and not quite.

Consider the "Limiting Manifold Pressure For Continuous Operation" found on any Lycoming power chart. It clearly prohibits, for example, 2200 RPM at full throttle.

As for fixed pitch, note that the line allows full throttle at 2400.


Figures....;)
As I wrote my post it initially said .... for the most part false, but I removed it (I am at home sick and didn't feel like expounding on the specifics).
So I will edit my post to say "for the most part in regards to how people commonly operate their engine".
 
I vote with Scott! Note the table above says 'continuous' operation. Lots of certified 172s turn less than 2400 rpm at the start of a full throttle takeoff. The OP seemed to imply oversquare was always a bad idea. As is often the case, the truth is somewhere in between-some oversquare is allowed, but there are limits, both in time and settings.
 
Don't want to try it

If you leave the CS control where it was during cruise, and go to full throttle, what does happen?

I am not suggesting anyone do this, but I am curious what happens when it is done.

You are giving the engine full gas, but not asking for much RPM in return. Does that mean you are running way too rich, and risk fouling your plugs or is it worse than that?

And does the answer change in any way if you are running with EFII?
 
If you leave the CS control where it was during cruise, and go to full throttle, what does happen?

I am not suggesting anyone do this, but I am curious what happens when it is done.

You are giving the engine full gas, but not asking for much RPM in return. Does that mean you are running way too rich, and risk fouling your plugs or is it worse than that?

And does the answer change in any way if you are running with EFII?

The issue is manifold pressure, not throttle position. Altitude dictates how much manifold pressure at full throttle. Down low you get a lot of MP. Up high you don't.

Lots of manifold pressure at some low RPM is one contributing factor in detonation. Others are F/A ratio, CHT, fuel octane, and inlet temperature. Give it a bad combination (say, 28.5" MP, 2400 RPM, leaned to just on the rich side of peak EGT using fuel barely meeting the 100LL standard, CHT at max on a hot day) and it will detonate.

Full throttle does not mean "full gas". In simplified terms, fuel metering is designed to match airflow in a proportional manner, by weight. For example, if the proportion is 12.5 lbs of air to 1 lb of fuel at some throttle position, it should be the same at any other throttle position. More air, more fuel, less air, less fuel, but always in proportion.

The mixture control varies the proportion.

The EFII-brand system is a fuel metering device. It delivers whatever it is set to deliver, as do carbs or constant flow injection. The metering components and method may be different, and accuracy is arguably better under many conditions, but a knothead can still put the knobs in the wrong places, just like the rest.
 
Not really relevant to this discussion, but this is an old wives tale that is for the most part (in regards to how people commonly operate their engine) false.

Anyone with a fixed pitch prop is in this condition every time they take off.

BTW I set my fixed pitch airplane power with the MP also. That way I don't have to keep readjusting as the aircraft speed stabilizes. It is easy to do with any airplane (as long as it is equiped with a MP gauge) once you get to know the specific numbers of that airplane.

Scott....I believe you to be up there with GOD in terms of knowledge, smarts, and wisdom. If you say I can have my RPM back at 2200 from a high altitude cruise, and shove the throttle forward to avoid hitting Sweet Pee crawling on the runway and not have a concern in the world, that it's an old myth....I believe you.
I was only passing on how I was trained to operate high powered turbocharged Cessna twins 400 series years ago.
"Keep the prop ahead of the throttle you knucklehead " the old geezer would yell.
Another thread, plus we're putting Bob to sleep.
 
Scott....I believe you to be up there with GOD in terms of knowledge, smarts, and wisdom.
Far from it.
The God of this world has never made a mistake..... I make multiple ones per minute.

If you say I can have my RPM back at 2200 from a high altitude cruise, and shove the throttle forward to avoid hitting Sweet Pee crawling on the runway and not have a concern in the world, that it's an old myth....I believe you.

I didn't say that, and if it is my edited post that you read (it likely was because I did that only minutes after writing it), you already know that.

You said
rule never to break......always keep the prop ahead of the throttle (knobs)

I and many others break it every time we fly. The very first control I move after takeoff is the prop RPM control to reduce RPM to 2500 to be a bit more noise friendly (RV's are noisy).

This practice is fully within Lycomings recommendations in two ways (it is within the RPM limitation and is not a continuous operation condition).

If it is not yet obvious, my comment was in regards to your use of the word never.
 
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The very first control I move after takeoff is the prop RPM control to reduce RPM to 2500 to be a bit more noise friendly (RV's are noisy).


That's my M.O. too. As I'm climbing over the runway I start twisting the prop control back to 2500 RPM. At the end of the runway I'll bring the flaps up into reflex and then start pulling the throttle back to 25 inches for the climb. I also lower the nose to get some more speed (read cooling). I'll then bring the mixture back to somewhere around 16 GPH. This is at my home airport which is basically sea level.

-Marc
 
Can anyone point to an official Lycoming definition of "continuous operation"?

As I'm climbing over the runway I start twisting the prop control back to 2500 RPM. At the end of the runway I'll bring the flaps up into reflex and then start pulling the throttle back to 25 inches for the climb. I also lower the nose to get some more speed (read cooling). I'll then bring the mixture back to somewhere around 16 GPH. This is at my home airport which is basically sea level.

Interesting...straight to LOP at 25/2500 to keep CHT down?
 
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