Jesse

Jesse

Well Known Member
I know traditionally fixed pitch prop planes don't have a manifold pressure gauge, but in the day of digital engine monitors, I think it is getting much more common. I have MAP in my -6A, and when I fly a FP plane without it, it drives me crazy, feeling like I really have no idea of my true power setting. Am I the only one? I think this might make a nice Poll.
 
I am very glad I have a MP gauge on my FP. My GRT Sport calculates % power which was helpful for break-in and is also helpful when leaning, and for getting a baseline of performance numbers at different power settings.

On another note, was that you in N399DC that gave me a shout-out on Friday with Jax center on my way to Charleston?

Chris
 
Jesse said:
I know traditionally fixed pitch prop planes don't have a manifold pressure gauge, but in the day of digital engine monitors, I think it is getting much more common. I have MAP in my -6A, and when I fly a FP plane without it, it drives me crazy, feeling like I really have no idea of my true power setting. Am I the only one? I think this might make a nice Poll.
Click to expand...

Same here. I really like having both pieces of information necessary (RPM & MP) to gauge the power setting.
 
YellowJacket RV9 said:
I am very glad I have a MP gauge on my FP. My GRT Sport calculates % power which was helpful for break-in and is also helpful when leaning, and for getting a baseline of performance numbers at different power settings.

On another note, was that you in N399DC that gave me a shout-out on Friday with Jax center on my way to Charleston?

Chris
Click to expand...

That was me. I didn't push it with Jax as they were a little slow on the uptake and were a little busy. I love meeting other RV'ers in the air. Next time please stop by X35.
 
Jesse said:
I know traditionally fixed pitch prop planes don't have a manifold pressure gauge, but in the day of digital engine monitors, I think it is getting much more common. I have MAP in my -6A, and when I fly a FP plane without it, it drives me crazy, feeling like I really have no idea of my true power setting. Am I the only one? I think this might make a nice Poll.
Click to expand...

I never fitted MAP to my -9A with an FP prop, and it doesn't bother me because I've never flown a plane with MAP....except for a -7A with a CS prop I flew in transition training.

If I'm going somewhere, say, more than 50 miles away, I just make sure I'm above 8000' DA and run between 2500 and 2550 rpm at 40-50 degrees F LOP. If I can hit 145-150 KTAS I'm happy; exact power doesn't concern me as long as the engine is not in danger of detonation and everything's running in the right temp range.
 
Last edited:
I prefer a manifold pressure gage when running a FP prop. My first experience with one was when I bought my T-18. When you are cruising along and begin your decent to your destination, the RPM hardly reduces at all, so the manifold pressure gage is the 'go-to' gage.
In the pattern, the Tachometer is the 'go-to' gage, but once I'm out of the pattern, the faster we go, the more I use the MP gage. It's also helpful when flying cross country in formation with friends.
So I installed one in my RV-8 when I built it. When I install an Engine Monitoring System, I'll get one with an MP gage to figure HP like Chris mentioned.
 
rvbuilder2002 said:
I rarely look at the tach indication on my fixed pitch RV-6A. Power setting is primarily done based on MP.
Click to expand...

That's exactly what was coming to my mind. RPM means very little, just to avoid an over speed in descent. MAP is the more useful gauge.
 
I've had an MP gauge with my fixed pitch from the get-go and cannot imagine flying without it. Most important for me is it's use in flying LOP. The % power tables in the GRT system are based on altitude, MAP, and RPM; there is no reflection of lean vs rich operation. Don Rivera indicated at FI 101 that Lycomings "do not like to be run lean above 24" MAP." Sure enough, if you look at at the related Lycoming graph of "Fuel flo vs % Rated Power", it shows that Lyco's are thirstier above 75% power. Not coincidentally, at lower altitudes 24" MAP gets you very close to 75% power.

http://aeroclub-lehavre.e-monsite.com/medias/files/pa28-lycoming-o-320-operator-s-manual.pdf (see page 60)

Accordingly, I run LOP at any altitude so long as the MAP is 24" or less. I don't use the % Power indication in my GRT system. At lower altitudes, that means throttling back quite a bit; at higher altitudes, not at all.
 
I agree with the above - I find the MP more useful than RPM for setting power, particularly in descents where I want to keep the engine burning fuel to prevent shock cooling but where the tach is all but useless due to the negative loading on the prop.
 
Yes, after you get to know your airplane, you'll get to know to pull X inches of MP to get Y fpm descent.

Also, Lycomings don't seem to be as prone to it as Contintentals, but a MP gauge is also handy for carb ice detection. Put the MP gauge in your scan when the temp/dewpoint spread is small, and just a couple of tenths of an inch drop without altitude or throttle change will indicate ice formation before you'll see it on the tach, and you'll still have sufficient heat to clear it.
 
I have to admit, first , that the charts in the engine manual are entirely greek to me. I have no idea how to read the darn things to get any useful information.

So when I fly, I primarily like to run about 2450 rpm and the MP is whatever the MP is. I use the GRT EIS to get to about 50 degrees LOP give or take .

So, that leaves me with the obvious question: What is it your MP gauge is telling you? Please describe more about how you use it.
 
LettersFromFlyoverCountry said:
I have to admit, first , that the charts in the engine manual are entirely greek to me. I have no idea how to read the darn things to get any useful information.

So when I fly, I primarily like to run about 2450 rpm and the MP is whatever the MP is. I use the GRT EIS to get to about 50 degrees LOP give or take .

So, that leaves me with the obvious question: What is it your MP gauge is telling you? Please describe more about how you use it.
Click to expand...

I am definitely no expert, but I focus mostly on the % power reading. The GRT EFIS can give this to you if you input some data points from the Lycoming charts. I also have checked it against the various rules of thumb for determining % power. There are several reasons it has been helpful to know. During break-in, I wanted to be in the 75% power range most of the time. Lycoming also gives guidance on when and how much to lean based on % power, and we know that below 60-65% power we really can't hurt it by leaning. So knowing my % power reassures me about when I can experiment with the red knob safely, especially as I play with LOP. Also, I add carb heat below 55% power.

Lastly, I can record performance data at various power settings to get accurate data and compare to others at the same % power.

Chris
 
rvbuilder2002 said:
I rarely look at the tach indication on my fixed pitch RV-6A. Power setting is primarily done based on MP.
Click to expand...

Same here. I typically cruise at 22" MAP and the RPM's are whatever they are. When changing altitude I set the 22" MAP about 200ft before I get to my new altitude and trimming becomes a lot easier. YMMV :cool:
 
Last edited:
The disadvantage of setting power level with a fixed pitch prop using RPM or % power is that it is not a one step process.

RPM does not stabilize on a fixed pitch prop until the airplane accelerates to its full level flight speed for a given power setting. That means the entire time that it is accelerating, the RPM and the indicated % power will be changing. Getting the RPM or % power you want will probably require a number of power changes until everything is settled.

If you get to know your airplane well enough, you can simply set the desired power setting based on indicated MP as you level off and then forget it.
The airplane will accelerate to its typical balanced speed/RPM and you wont have had to do another thing.
 
rvbuilder2002 said:
The disadvantage of setting power level with a fixed pitch prop using RPM or % power is that it is not a one step process.

RPM does not stabilize on a fixed pitch prop until the airplane accelerates to its full level flight speed for a given power setting. That means the entire time that it is accelerating, the RPM and the indicated % power will be changing. Getting the RPM or % power you want will probably require a number of power changes until everything is settled.

If you get to know your airplane well enough, you can simply set the desired power setting based on indicated MP as you level off and then forget it.
The airplane will accelerate to its typical balanced speed/RPM and you wont have had to do another thing.
Click to expand...

If you cruise lean of peak, fuel flow is also a good indicator of power setting - I know the fuel flow for a given power setting at normal cruise altitudes. (This is in a constant speed setup but should apply just as well to FP)
 
You must log in or register to reply here.