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Do I really need to see MAP INHG?

rileyspoon

Active Member
I have the dual Dynon HDX configuration and I have the engine instrumentation on the bottom band of my PFD. The system is configured based on the Van's documentation but one of the items shown to the right of the fuel pressure is for what I assume is manifold pressure. The scale is all green. No yellow or red areas and unless I missed it somewhere, I can't find any reference to it in the Van's documentation or the HDX manual.

Why is it even shown? Nothing in the POH to tell me to do something if it has a specific reading. What parameters represent good or bad? Is there an upper and lower limit? And what actions am I supposed to take for either one? The Dynon pilot user's guide doesn't even show it on the engine band.

If I remove it from the display will Dynon still give me a warning if it is out of bounds? Not that I'll know what to do.
 
Manifold pressure is handy for setting power. When changing power settings, RPM takes a while to settle down. Once you are used to your airplane, you can adjust power by using MAP much faster.
 
Manifold pressure is handy for setting power. When changing power settings, RPM takes a while to settle down. Once you are used to your airplane, you can adjust power by using MAP much faster.

This is Rotax 912 fixed pitch prop. MAP is virtually useless for RV-12 airplane...
 
This is Rotax 912 fixed pitch prop. MAP is virtually useless for RV-12 airplane...

You can still set power faster with MAP than with RPM. MAP is definitely useful with F/P prop. I flew my RV-6 with F/P for over 25 years making power adjustments with MAP.
 
Manifold pressure is handy for setting power. When changing power settings, RPM takes a while to settle down. Once you are used to your airplane, you can adjust power by using MAP much faster.

This is Rotax 912 fixed pitch prop. MAP is virtually useless for RV-12 airplane...

I disagree for the reason Mel mentioned.
Once you know your airplane and know what MP gives you a typical RPM in cruise you can just set it to that value and not have to move it again.
If you only use RPM to set power, you will be adjusting the throttle multiple times as the airplane accelerates and eventually stabilizes in cruise.

I have an RV-6A with a fixed pitch prop and I almost never look at the tach. reading.


To the OP's original question... do you need a MP indication? No, but it is very useful to have. Even on a fixed pitch airplane.
 
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If you are descending, it's a useful guide to the increased power due to the thicker air as you descend. It lets you know how far to continue to reduce throttle to keep power constant.

Dave
 
Wow, that’s an eye-opener... Maybe I’m the only one not using MAP for power settings while flying RV-12? I've flown my 12 for 800 hours without looking at MAP. I don’t know anyone else that is actively using MAP in a RV-12. It would be interesting to start a poll and ask the fraternity this question... I don’t see where VAF has ability to create a thread with polling option.

It would be nice to have EMS display % Engine Power based on MAP and RPM. Dynon doesn’t have option for Rotax 912 engine….
-
 

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Wow, that’s an eye-opener... Maybe I’m the only one not using MAP for power settings while flying RV-12? I've flown my 12 for 800 hours without looking at MAP. I don’t know anyone else that is actively using MAP in a RV-12. It would be interesting to start a poll and ask the fraternity this question...

Put me in the "always reference manifold pressure" column. The only time I reference the tachometer is when I notice the engine sound change due to density, temperature or throttle creep.
 
I always use manifold pressure to set the throttle, usually at 25" for local flying.
I only look at RPM to make sure that upper and lower limits are not exceeded.
 
I'm going to fly again on Weds. Maybe I'll cover the tach with a sticky note and give it go. Could be I'm letting the best in life past me by....
 
I'm going to fly again on Weds. Maybe I'll cover the tach with a sticky note and give it go. Could be I'm letting the best in life past me by....

I wouldn't cover the tach. It's still a good reference.

It's just that the MP reacts more quickly.
 
Manifold pressure is handy for setting power. When changing power settings, RPM takes a while to settle down. Once you are used to your airplane, you can adjust power by using MAP much faster.

Mel
Is there a chart for the 0-360? I have never really looked at MP as I thought it was not relevant with a FP prop. Learn something new every day.
Figs
 
There is the old rule of thumb:

24 square = 75% power

The application is any combination of rpm and map the adds up to 48 is 75% power.

No doubt an engine-appropriate chart will refine that rule but it might be a starting point in absence of a chart.
 
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MAP pressures for 912 ULS

You can still set power faster with MAP than with RPM. MAP is definitely useful with F/P prop. I flew my RV-6 with F/P for over 25 years making power adjustments with MAP.

On my RV-12 a previous owner installed a 3-bladed Whirlwind prop. I understand the 3-bladed prop is supposed to be smoother and quieter. I don't have any experience with the 2-bladed standard version so I am not sure of the pros and cons.

One of my issues with this prop is that since it was installed it was setup such that on the ground WOT results in 4900 RPM. I think that is lower than the usual 5200-ish that you get with a standard prop. Takeoff performance is great and it doesn't have any problem reaching higher RPMs in flight so I am disinclined to try to adjust it. So my quandary has been how to set power. With a standard prop high cruise is 5500 RPM and low cruise is 5000 RPM. But 5500 RPM "seems" too high for me. Fuel flow, MAP, and air speed all seem higher than expected.

My approach then has been to look at MAP pressure rather than RPM for setting power. Like Mel, my experience has also been that it is easier and quicker to set power using MAP rather than RPM.

Which raises another question I've been meaning to post. What MAP do you get with a standard 2-bladed prop setup by the book? What pressures do you see at 5000 and 5500 RPM? The Mooney I used to fly was very happy at 23" but the Rotax seems to want to run higher. If I don't exceed 5500 RPM any idea what max continuous MAP is for the 912 ULS?
 
MAP with a fixed pitch sort of like RPM in a car with a automatic transmission but it still does tell you something. Mel is correct, MAP is quicker and isn't flopping around like RPM can do.
 
There is the old rule of thumb:

24 square = 75% power

The application is any combination of rpm and map the adds up to 48 is 75% power.

No doubt an engine-appropriate chart will refine that rule but it might be a starting point in absence of a chart.

Does this rule apply with a geared engine? The 912ULS is 2.43 to 1.

I use RPM in the -12, I find the resolution to be more precise changing only 10 - 20 (engine) RPM than trying to set .1 of manifold pressure. Actually it is 2.43 times more precise than using propeller rpm on non-geared engines.

In the 172 I use manifold pressure because it is more precise than propeller (engine) RPM.
 
well.....lets get to the elephant in the room since it brought up the original question. what map numbers for the 12?
 
well.....lets get to the elephant in the room since it brought up the original question. what map numbers for the 12?

It depends on what prop pitch setting you are using and what power setting you like to cruise at.
Just note the map value at your typical cruise rpm, and then set that value when you level off (it is influenced by DA and altitude you are choosing on a particular day but with just a little experience it shouldn’t take long before you are able to do a single setting of the throttle and hit very close to the rpm you usually use.
 
I am going to show my ignorance here.
Can someone please explain why should I use map instead of rpm when the goal is to set rpm? It seems map is the preferred here. Is the reason that it is old school from direct drive props? I read some say it is quicker to set the rpm with map. I went up and tried it yesterday and I was able to hold a more consistent rpm by adjusting for rpm during climb and descents. In cruise the using rpm it adjusts instantly and I can set it to hold within 10. Maybe it is personal preference, I’m good with that. If the reasoning is that one or the other is better for the engine please explain.
 
So, I flew my 12 today with the thought that I would be enlightened by using the MAP. I have to confess… I don’t get it. I see zero benefit in referencing MAP to set engine speed. I did a round-robin to three outlaying airports. I used WOT for climb, pushed-over at cruise altitude, pulled throttle back about 1” and was very near 5500 RPM as the airplane accelerated to 115 knots at 2500MSL. No fuss and no dick’n. Happy engine / happy airplane / happy pilot....
 
Then maybe use it like this...... With a fixed pitch adjustable prop, set your prop so you will have the highest MP at the max allowed RPM at the same DA. Then you know you've got all you can get out of that motor.

With a non-geared motor (Lycoming) you can now set power with 48 being 75%.... MP+RPM=48=75%. is important for leaning without damaging the engine.

The Rotax may not be affected by MP/RPM abuse. I am sure someone that understands the operation of the Rotax motor can provide direction in the use of MP and the proper operation.
 
I usually just position the throttle to the point that past experience gives my desired cruise. With a fixed pitch prop the tach is my primary power indication. Since MAP is dependent on altitude using the tach seems easier. I do look at MAP to ensure it doesn’t vary from what I expect.
 
Yes, many ways to skin the cat and they vary a bit based upon how you set up for any given flight regime. Personally I do most power setting via FF. I have a target cruise / FF that I want and set the throttle only to get the EGTs into a range that works well with the FF I have set. That is for the 10 with the CS and often get better flows below 10K by using less than WOT. On the 6 with the FP, I am always at WOT at cruise and then it is just mixture to get desired fuel flow.

I rarely look at MAP, however do look at the % power display with is a value derived from RPM and MAP.
 
Setting the throttle by reference to either the MAP or the tachometer, you will still experience some "creep" as the engine speed and aircraft speed increase or decrease.

I'm not trying to persuade anyone here, but in my very few hours with my -12 I have found fuel flow to work as well as RPM (not showing MAP at all on Dynon). My target for fun cruising is 4.0 gph. Yes, it will also creep as speeds change. There isn't a single right answer.

(this part is for carbs only?) Since the mixture is automatic, fuel flow is closely correlated to power. I don't recall who pointed it out (Ross?) but the mixture (A/F ratio) is quite flat in the cruising range. For a given A/F ratio, power is the result of fuel flow. Fuel flow is the result of RPM, air density, throttle, temperature, etc. MAP is mostly the result of pretty much the same list. For non-precision flying, fuel flow, power, MAP are equally informative but RPM is perhaps a bit less so.

Actual power (which, divided my maximum power is %) is the result of RPM and MAP. For a given MAP, more RPM is more power. Various combinations of RPM and MAP will give same power. But fuel flow is power in this airplane and it is also, for me at least, a target setting. The Rotax documentation tells fuel flow and percent-power. My wallet is a function of fuel flow.

When, in my 7A, I could lean out the mixture, then MAP was more interesting and more important - even with my FP prop. But this is a different case.
 
Some “talk” in this thread about chasing engine speed with throttle… The Rotax flat-four engines have a significant gear reduction – for RV-12 it is 2.43:1. It is a “hunting” ratio so gear wear is spread evenly… an excellent design consideration. The 912ULS makes 100 HP from 1352 cm³ by running fast where it develops maximum torque. Some of us are old enough to have experienced (owned) an air-cooled Volkswagen Beetle. Same logic for making HP. Those were the days….

Because the 912ULS base engine runs roughly 2.5 times faster than the propellor, minor changes in prop speed (slight climb or decent) will be magnified by the reduction ratio. This is why the tachometer shows changing speed during flight. Most Continental and Lycoming engines are direct drive and behave very differently. A slight change in prop speed is directly proportional to the same slight change in engine speed... barely noticeable. Direct drive aircraft engines make gobs of torque at slow propellor speeds because they have large displacement (read large diameter pistons) with relatively large stroke. For comparison… 912ULS (100HP) is 1.4 liter displacement and Lyc 0320 (150HP) is 5.2 liters.
 
Yes MAP is handy even for fixed pitch and no reason to not connect and display MAP. It adds little weight and cost. You have glass right. It is useful in determining power. Altitude affects MAP. Also many install ground adjustable props.
 
(this part is for carbs only?) Since the mixture is automatic, fuel flow is closely correlated to power. I don't recall who pointed it out (Ross?) but the mixture (A/F ratio) is quite flat in the cruising range. For a given A/F ratio, power is the result of fuel flow. Fuel flow is the result of RPM, air density, throttle, temperature, etc. MAP is mostly the result of pretty much the same list. For non-precision flying, fuel flow, power, MAP are equally informative but RPM is perhaps a bit less so.

Went flying again today and utilized your "fuel flow" method. This is a truly remarkable method! Climb at 6.5 gph and cruise at 4.0 gph. Descent is just pull the throttle back but I did notice there was a range of fuel flows that was between 2.2 and 2.9 gph for the various descents.

This is on the fuel injected Rotax.
 
Fuel Flow and Altitude

This may be obvious to most, but wouldn't fuel flow rates vary with altitude?

If so, would that be a downside (or additional challenge) to using fuel flow values for throttle settings?

Are there different fuel flow vs. altitude implications for the ULS vs. IS?
 
This may be obvious to most, but wouldn't fuel flow rates vary with altitude?

If so, would that be a downside (or additional challenge) to using fuel flow values for throttle settings?

Are there different fuel flow vs. altitude implications for the ULS vs. IS?

I am not an expert but I've spent a lot of time studying this issue, mostly for Lyco engines.

It's pretty well known that the IS version of the engine has better BSFC but that's really all that I know about it. When you introduce full FADEC including spark timing and mixture then it's not simple anymore.

The fuel flow relationship to power should be insensitive to air density (altitude, etc.) BUT it should be sensitive to the torque curve. In addition, prop efficiency varies with RPM and TAS but not as much as you'd think. That will be more with CS than FP, oddly.

The efficiency of the prop and of the engine (with or without boost) will vary with Density Altitude. But how much it varies depends on many other things.

If the air is thinner then, at a given air-fuel mixture, there cannot be as much fuel used and, of course, the maximum available power is thus less. But the fuel flow at a constant mixture will still be the big determinant of power. We make power by burning fuel.

The air-fuel mixture primarily, and other factors less so, will have a close relationship with engine power. The power as it affects performance is engine power modified (reduced) for prop efficiency. Some engines have quite flat torque curves in the regions at which we cruise. Some less so.

To the best of my limited knowledge, the 912 ULS has a reasonably flat torque curve and a reasonably flat mixture as controlled by the carbs. That is why fuel flow reflects actual power with reasonable accuracy. I define reasonable as useful for normal flying such as XC.

All gasoline engines have BSFC (Brake Specific Fuel Consumption) which we usually see expressed in pounds per HP per hour. My 7A with manual leaning, etc. could run at about .40-.42. Lower is better. The 912 ULS runs at around .44-.45. You can compute this from the data in the Rotax specs. I'm writing this from memory. BSFC is a measurement; you can't directly control it. It's the result of many things, even, especially, compression ratio. Not that you care, but my 1975 C-150 had a BSFC of about .44-.45 over a very large range of RPM's and Density Altitudes. I calculated that from the POH.

If you think briefly about the above paragraph you will see that a given BSFC means that fuel consumption "equals" power. I was able to see computed BSFC on my GRT in my 7A but it would be useless without the ability to control mixture. My Lyco IO360 had a BSFC curve in the "book" and it showed me what I was able to observe experimentally - that my best engine efficiency was at 8GPH (lowest BSFC). Unsurprisingly, the best cruise performance was at 8GPH and 8000'.

I'm sorry if this is TMI, but I tried to take your question seriously. It's fun to study this stuff, but simple read-outs like fuel flow are very good for low performance, fun airplanes.
 
So, H.... Do you fly your 12 with Map or Tach?

Or maybe just wet our finger and stick it out the cabin air vent....
 
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So, H.... Do you fly your 12 with Map or Tach?

Or maybe just wet our finger and stick it out the cabin air vent....
I just listen to it and keep my eyes 👀 outside, of course. Wet fingers readout varies too much with ambient humidity.

Fwiw, I used % power on the 7A.
 

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This point about MAP may have been missed. For a given MAP, power is a function of rpm. MAP by itself is not telling you much about power and also, it is not linear with power. You can demonstrate this by observing MAP at full throttle while static.
 
I "did the math" for clarity.

RPM is not needed. MAP is not needed. Mixture is held constant and is not used.

HP x BSFC / pounds-per-gallon = Gallons per hour.


I am very skeptical of these claims but Rotax says that the maximum continuous power is 97.3 HP and that 75% uses 4.9 gph. That requires a BSFC of 0.40 for the 912 ULS.

(97.3*.75=73) x .40 / 6 = 4.9 gph

The good folks at GAMI would be amazed. Even the best big Continentals with balanced injectors, leaned past peak would be doing well to get that.

https://www.aviationconsumer.com/uncategorized/rotax-912-is-efficiency-better-than-claimed/

Using estimated performance data from the Tecnam P92 POH, we calculate a brake specific fuel consumption for the 912 ULS of about 0.44 pounds per horsepower/hour. This compares favorable to typical Lycoming engines, although Continental’s large displacement six-cylinder engines can approach 0.39 BSFC when running lean of peak.

I like .44 as a realistic number for BSFC. if I use 4 gallons per hour I'm at around 55%.

The Dynon doesn't compute % power for the 912 ULS.

(97.3*.55=53.5) x .44 / 6 = 3.9 gph

If you read the article it will explain why the 912 iS is much more efficient.
 

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Hi all, I will chime in on the original question....I don't use the MAP data. It's there on the panel but not a part of my 'scan'. I hope nobody thinks less of me (if that's possible, haha). I am not saying it's not relevant, I just don't use it. Other than full power for takeoff and initial climb, I am normally operating in the 4800 - 5300 range on the tach (2000 - 2200ish prop speeds). My scan is oil and fuel pressures, engine temps. This Rotax 912 continues to amaze me. I just did 10 hours on Thursday with a passenger and some gear, burning about 4GPH and TAS 110-115Kts (and that's with the ULS). In my 6 years of flying my -12, I am still amazed. Cudos to Van's for designing this incredible airplane. By the way, if you ever get a chance to fly the Hudson River Corridor (NYC), do it!! What an unbelievable experience for this ex-Californian. I will attach a couple pics.

forum2.jpg

forum1.jpg
 
Thanks for the trip report and don’t worry about my feelings regarding your use of MAP. GREAT to see you out flying your plane to some really cool places.
Safe flying to you my friend.
 
Correction to my earlier post

It's hard to find authentic torque curves for these engines but this would strongly suggest that the curve is NOT FLAT. I goofed on that. The screenshot is listed as applying to the 912.

But please note in the attached power chart that the chart is for CS props. With that caveat, if you want to see how MP relates to HP this offers some data points.

The 3-frame picture shows the relationship between fuel consumption and other parameters including torque.

I edited it with red and blue lines to show the HP vs GPH. If I understand the graph correctly, HP rises a bit faster(with RPM) than GPH but the relationship is close. I would have to check how the numbers are used on the vertical axis to be sure; it could be a matter of how the axis is scaled. Both are pretty flat in the range of speeds we use for cruise.

Since torque increases with RPM I would expect power to rise faster than RPM, proportionally. T x R / 5252 = HP. If T were flat then HP would rise in proportion to RPM alone. If T rises at the same time as R then HP rises as the product.

No one gauge on my Dynon is perfect for this but I still like FF for simplicity, appropriateness and being easy to understand. If I had % Power I'd use it.
 

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Went flying again today and utilized your "fuel flow" method. This is a truly remarkable method! Climb at 6.5 gph and cruise at 4.0 gph. Descent is just pull the throttle back but I did notice there was a range of fuel flows that was between 2.2 and 2.9 gph for the various descents.

This is on the fuel injected Rotax.
When you talk to FF set this is it.... amazing, I will add.... RPM & MAP 1st and FF 2nd. FF is definitely an indirect indication of power. WE are talking Rotax I assume which automatically adjust mixture. However Lyc with RED knob FF ends up being more important for setting mixture. There is no way around it RPM + MAP + FF (+EGT for mixture) gives compete picture. Fixed pitch plane like all basic trainer's RPM was it

On fixed pitch MAP could be foregone but as I said you have glass? The MAP transducer/ sensor and wiring is nominal cost, space; weight. Put it in and let the Glass give approx % Power. Is FF needed? That cost more to buy, install more complex, but gives you Approx range / MPG as well as fills in the variable that validates power.

On a Rotax RV12 you don't NEED MAP or FF, but with glass and it being 2023 why not?


PS For air cooled tight low drag cowls CHT for every cylinder is a must IMHO. I remember early on renting and if rental plane (C172, Cherokee) had one CHT it was rare. Leaning was to misfire/rough and push it back in a "bit". No MAP, no FF. We can do better now.
 
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Comments embedded red..GMC Pilot's remarks are always worth careful consideration.🙇
When you talk to FF set this is it.... amazing, I will add.... RPM & MAP 1st and FF 2nd. FF is definitely an indirect indication of power.

WE are talking Rotax I assume which automatically adjust mixture. However Lyc with RED knob FF ends up being more important for setting mixture. There is no way around it RPM + MAP + FF (+EGT for mixture) gives compete picture. Fixed pitch plane like all basic trainer's RPM was it

Completely true but there is a complex formula using those parameters.

On fixed pitch MAP could be foregone but as I said you have glass? The MAP transducer/ sensor and wiring is nominal cost, space; weight. Put it in and let the Glass give approx % Power. Is FF needed? That cost more to buy, install more complex, but gives you Approx range / MPG as well as fills in the variable that validates power.

For FP Dynon Classic, at least, % Power is not available. Mine came with FF; isn't it standard for the 12? On mine, I included M/g which is an indirect measure of efficiency, too.

On a Rotax RV12 you don't NEED MAP or FF, but with glass and it being 2023 why not?

Why not is personal preference for not cluttering up a small display with low value data. FF is much more useful and closer to accurate than MAP when either is taken alone.For those with two screens, the clutter is much less important, I would think.

PS For air cooled tight low drag cowls CHT for every cylinder is a must IMHO. I remember early on renting and if rental plane (C172, Cherokee) had one CHT it was rare. Leaning was to misfire/rough and push it back in a "bit". No MAP, no FF. We can do better now.

I would prefer to have both EGT and CHT for each cylinder but for the Rotax, CHT is water cooled so that's lower value than for Lyco-Con style engine. On the Rotax, pilot can't control either while in flight so useful only for noticing non-nominal data. Is individual cylinder data even offered without some additional mods>
 
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