[Turbofan]

I have rv4 w airflow fuel injection I'm cleaning all up, why did builder install MAP gauge w a fixed pitch prop? Any purpose? Considering removing as I install new panel

 

[RVDan]

I had a fixed pitch RV and you can get away without MAP without any problem. However if you start playing with things that impact power it is a handy reference.

If you never expect to muck with the engine, just fly, I see no reason you really need it. Perhaps for troubleshooting. It will be interesting to see what others say.

 

[fixnflyguy]

I did it too

I put a MAP in my F/P -4 also, for no reason other than keeping same gauges as a C/S ship. For my type flying, I don't use it religiously, and would likely sacrifice it if I needed the space.

 

[Mel]

I fly mostly by the MAP. It's much quicker to make power settings than waiting for the rpms to catch up.

 

[gasman]

Understanding the MAP will show you the capability of your motor AND your prop.

Without one you can't troubleshoot performance.

Based on fixed pitch prop.

 

[curtis]

I need some more info / education on how to fly a fixed pitch carbureted engine by the MP gauge. Doesn't MP change with RPM so if you would set the MP waiting for the RPM to catch up wouldn't the MP setting change? I always thought that in a FP engine / prop setup the RPM and MP were directly related that and that any RPM setting would always yield the exact same MP reading and vise versa.

I have have a GRT EIS 4000 with MP and I have just never really used the function. If I wanted to fly at 2,450 RPM I would just move the throttle to make it happen.

I'm always up to learning new things especially if it makes life easier.

 

[curtis]

The thing is that if you leave the climb for cruise at lets say 110mph and you set 2450rpm... a bit later you will go 140mph and your rpm will be way over 2450 if you have not monitored it.

Agree 100%, but will not MP move / change in the exactly the same way? This is my main question. Doesn't MP change as RPM changes so regardless of which one you elect to set it will change with the transition from climb to cruise and any other changes within the flight.

 

[gasman]

Your motor can only produce a given amount of RPM. But that RPM does not show you how much power you are getting from the motor.

You need a manifold pressure gauge to see how the motor is working. Let's not fly by RPM setting, let's fly by % of power. On takeoff with a FP prop it's simple.... full throttle check static rpm and go. The CS guys set their power to 75% by going to what they call 24 square. 2400 rpm and 24" mp. 24+24=48.

So now let's set our fp prop to 75% power..... Any combination of rpm (first 2 numbers) and mp that adds up to 48 will give you 75%.
And 65% is 46. 55% would be 44. As you climb, your mp will decrease. So to keep your power setting, you will need to increase your rpm.

 

[Raymo]

What gasman said plus knowing that you are at 75% power or less allows for safe LOP operation.

 

[curtis]

To answer the question: the only significant change of MAP is outside air pressure (Altitude). With the same throttle position you will see the same MAP at 110kt as at 160kt..

This is my thoughts exactly and that they are directly related.

 

[gasman]

To answer the question: the only significant change of MAP is outside air pressure (Altitude). With the same throttle position you will see the same MAP at 110kt as at 160kt..

How do you go from 110kt to 160kt without changing the position of the throttle in level flight?

 

[rvbuilder2002]

How do you go from 110kt to 160kt without changing the position of the throttle in level flight?

Easy

Start in slow flight..... 100 MPH, 14 " MP

Push throttle open to 23 " MP

Keep adjusting pitch trim nose down as you excellerate to 170 MPH

The rpm will be increasing the whole time you are excellerating.
The MP will change very little.

I rarely look at the tach on my fixed pitch RV.

 

[gasman]

Easy

Start in slow flight..... 100 MPH, 14 " MP

Push throttle open to 23 " MP

Keep adjusting pitch trim nose down as you excellerate to 170 MPH

The rpm will be increasing the whole time you are excellerating.
The MP will change very little.

I rarely look at the tach on my fixed pitch RV.

AH! the key...... "same throttle position"..........