IO-390 Manifold Pressure Knob?

[bkervaski]

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.

[C-FAH Q]

There is, it's black and usually called the throttle.

[BillL]

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.

[bkervaski]

Got it, I misunderstood how adjusting manifold pressure worked. Thanks.

[salto]

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.

[frghtdg]

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.

[mturnerb]

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)

[BobTurner]

Originally Posted by mturnerb

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.

[BillL]

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.

[mturnerb]

Originally Posted by BobTurner

Actually, with fixed throttle setting higher RPM= lower Manifold pressure. Higher air volume demands result in a bigger loss thru the induction system.

I stand corrected