Do I really need to see MAP INHG?

[rvbuilder2002]

Originally Posted by bobnoffs

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.

[seagull]

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.

[Piper J3]

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....

[Mich48041]

I think it is a matter of personal preference. If the plane is happy and you are happy, we are all happy. :-)

[gasman]

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.

[RFSchaller]

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.

[lr172]

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.

[hevansrv7a]

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.

[Piper J3]

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.

[gmcjetpilot]

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.