Manifold Pressure

[Moneybox]

Hi Guys, help me out here. I've never had a manifold pressure gauge. I understand how it operates and I understand what it's reading but I don't know my best settings.

 

 

When I'm cruising I have to adjust my propeller to a high pitch to prevent the engine running over maximum revs. 

 

What I don't understand is where my best/least fuel consumption comes from when matching RPM to Manifold Pressure while maintaining cruising speed.

 

Lets say I want to keep the Rotax 912ULS spinning at 5000 revs while getting my best economical cruise speed. Do I control this with manifold pressure?

[Thruster88]

To make use of your manifold pressure gauge you would need a power chart. Such charts for say a Lycoming O-360A1A will include altitude, rpm, manifold pressure and show fuel flow for each setting. 23-23 @3500 feet is not the same power as 23-23 @5500 feet.

 

Rotax do not seem to publish such charts, good luck finding one.

 

 

[skippydiesel]

Rotax do publish the manifold pressure/expected fuel flow/ altitude/ engine rpm charts;

 

Join Rotax Owners Forum for all the Manuals/Service letters/etc https://www.rotax-owner.com/en/rotax-forum

 

I strongly advise you read the Rotax 912 ULS Operators Manual file:///C:/Users/User/Downloads/OM_912%20Series_Ed4_Rev2%20(3).pdf

 

Some downloads that may interest you;

 

https://legacy.rotaxowner.com/si_tb_info/serviceletter/sl-912-016-r1.pdf

https://aquila-aviation.de/wp-content/uploads/2021/06/SL_912_016R2_914_014R2_Rotax_Notes_engine_handling.pdf

 

A couple of excerpts;

 

 

 

 

 

Note: The figures given are nominal ie will differ due to many factors eg airframe😈

[skippydiesel]

"When I'm cruising I have to adjust my propeller to a high pitch to prevent the engine running over maximum revs. "

 

Small but important point to avoid any confusions - Propeller pitch is expressed as Coarse to Fine. Terms relative to the angle of attack (like a wing) to the airflow.

What you seem to have described is Corsening the pitch to slow the engine rpm at a given power setting.

 

"What I don't understand is where my best/least fuel consumption comes from when matching RPM to Manifold Pressure while maintaining cruising speed."

 

In my aircraft (912ULS/Constant Speed prop) best speed/economy is in the region of 5000 - 5200 rpm. I usually Cruise at 5000 rpm,  25" Hg , 2500 ft ASL for a an average (whole of flight)fuel flow of about 14.25L/hr @ 130 knots true.

 

Your aircraft will deliver a diffrent True airspeed at similar settings.

 

"Lets say I want to keep the Rotax 912ULS spinning at 5000 revs while getting my best economical cruise speed. Do I control this with manifold pressure?"

 

Manifold pressure will change with density altitude. With an acurate graph/power chart for your aircraft/engine/prop combination, you can adjust power accordingly. I find the following easier;

On my last aircraft I had a reasonably accurate fuel flow meter, so was able to adjust power at any altitude to fuel flow.

My current aircrafts fuel flow meter is nowhere near as precise, so I fly to True airspeed - probably offends the real pilots but works for me.😈

 

 

[onetrack]

Here's a good instructive article -

 

What is Manifold Pressure? | Angle of Attack

WWW.ANGLEOFATTACK.COM

---

The manifold pressure gauge is staring you in the face every time you reach for the power and you have no clue what it means.

 

[skippydiesel]

Moneybox - check this out

SB-912-079.pdf 

 

😈

PS Your aircrafts POH should have the prop/engine inflight adjustment operating instructions.

Edited 19 hours ago by skippydiesel

[facthunter]

Don't think I've ever used one on a normally aspirated Engine. They do give an indication of engine condition but so does revs on take off for any fixed pitch prop., USE Coarse and Fine with Pitch to avoid confusion.  I got little if anything from that presentation, in  stryle or substance. Altitude  and air temp are the biggest factors in engine and aircraft performance for unboosted engines. You should have a fair idea what the Power at various Levels is on full throttle.. Nev

[Thruster88]

Skippy, not seeing altitude on any of the rotax charts you have put up. As you can see from this Lycoming chart there are numerous combinations of RPM (prop control), manifold pressure (throttle) and altitude to achieve a known fuel flow/performance. 

 

 

[skippydiesel]

https://www.aero-hesbaye.be/ROTAX/pdf/operations 912.pdf

Page 5-4

[facthunter]

What use is HP required at 10,000 feet?  Nev

[Moneybox]

6 hours ago, skippydiesel said:

Moneybox - check this out

SB-912-079.pdf 1003.73 kB · 18 downloads  

 

😈

PS Your aircrafts POH should have the prop/engine inflight adjustment operating instructions.

Great document Skippy and I've worked out how reading it makes you engine last longer.

 

The hours spent on reading are taking you away from flying therefore your engine lasts longer. 😀

[Moneybox]

I've just been in the air for a little over 3hrs and to be honest I could have done without a Manifold Pressure gauge. I used the variable pitch prop to avoid excessively high engine revs and I backed the throttle off by about a third to unload the engine. I can here the revs getting up into the yellow and I can hear it working unnecessarily hard. Variations in manifold pressure were next to nothing. I can't see the value in it.  

[Moneybox]

Can somebody tell me how much charge I get from the 912 ULS alternator?

How does that compare to the current drain to charge an iPhone 12 and a iPad series 6?

 

I started out with a flat battery. My nice new Odyssey battery might be stuffed since I left the Master on a few weeks back. It's been charged and flown since but after standing for a couple of weeks it was too flat to crank. I took it out and put it on 6A charge for a few hours before putting it back in knowing it was not fully charged but I need to fly while the weather was in my favour and I expected the Rotax to charge it.

 

This morning it fired up ok but after landing over 2 hrs flying when I stopped to refuel it was too flat to crank again. I'm charging the iPad and iPhone while I fly. I managed to reach up through the bottom hole on the cowl and attach a jumpstarter and flew off for another hour. Mid way the tacho shut down so I pulled the cigarette lighter plug and the tacho came good until the iPad dropped dead and I still had a long way to go so I plugged it back in. Once again the tacho dropped out so I continued on without it.

 

I've only just arrived home so I haven't had a chance to put a meter on it but the charge light says the Rotax is charging.

[skippydiesel]

"Can somebody tell me how much charge I get from the 912 ULS alternator?"

 

https://www.rotax-owner.com/en/912-914-technical-questions/10084-what-is-the-net-output-of-rotax-s-internal-generator_1

 

There is something not quite right about your charging system😈

[facthunter]

Some batteries can't be left FULLY discharged for very long at all. There are "special" digital chargers that can revive them but are pretty easy to damage if you don't set them up correctly and don't let anything get Hot. Having a suss Battery in a Plane is Not advised. The setup in a Rotax is a permanent Magnet type and not as easy to regulate as the 'Excited" type which won't work if shut Off when the Battery is flat. Making the Generator charge while all those extra services are On May be beyond the system's capacity. I'd sort the Battery FIRST.  It's a "known" unknown.  Nev

[BurnieM]

That is pretty disappointing 18-20 amps for a 912 ULS.

But still should probably be 'enough'.

 

Do you have an ammeter ? What is it putting out in flight ?

Note max output is at 5,000 rpm. 

Normally we cruise cross country at 5,200.

You probably do not want to be puttering around in the circuit at 4,500 with a low battery.

 

The 912Is comes with 2 alternators; 16A for the engine and 30A for the avionics/battery charging.

 

Edited 9 hours ago by BurnieM

[skippydiesel]

Hi BurnieM,

 

"That is pretty disappointing 18-20 amps for a 912 ULS.

But still should probably be 'enough'."

 

Yes it is more than enough for a basic day VFR aircraft.

 

The two 912 ULS powered aircraft, I have owned, never had a shortage of charging amps (assuming VR working correctly).

 

I do make a point of having ancillary navigation device (iPad/GPS/SE2 & any other systems) fully charged before flight. My simple thinking - reduces demand from charge system and heating of individual devices, reducing chance of going out on over temp.

 

As a rule of thumb - its best to fly/drive for at least 30 + minutes, to give the charge system enough time to make good any losses due to starting.

 

If you start adding lights and extra electrically driven systems (eg autopilot/flaps/trim/seat/retractable undercarriage/constant speed prop) you may want more. This is when the option for a second alternator comes in.😈

Edited 8 hours ago by skippydiesel

[Moneybox]

3 hours ago, skippydiesel said:

"Can somebody tell me how much charge I get from the 912 ULS alternator?"

 

https://www.rotax-owner.com/en/912-914-technical-questions/10084-what-is-the-net-output-of-rotax-s-internal-generator_1

 

There is something not quite right about your charging system😈

 

Yes, I just came in from checking it out. 10v in the battery so I started it with the jumpstarter. The red generator light comes on with ignition as expected and goes out once started but there's absolutely no battery charge. 9.8v at 3000rpm.

[Blueadventures]

5 hours ago, Moneybox said:

Can somebody tell me how much charge I get from the 912 ULS alternator?

How does that compare to the current drain to charge an iPhone 12 and a iPad series 6?

 

I started out with a flat battery. My nice new Odyssey battery might be stuffed since I left the Master on a few weeks back. It's been charged and flown since but after standing for a couple of weeks it was too flat to crank. I took it out and put it on 6A charge for a few hours before putting it back in knowing it was not fully charged but I need to fly while the weather was in my favour and I expected the Rotax to charge it.

 

This morning it fired up ok but after landing over 2 hrs flying when I stopped to refuel it was too flat to crank again. I'm charging the iPad and iPhone while I fly. I managed to reach up through the bottom hole on the cowl and attach a jumpstarter and flew off for another hour. Mid way the tacho shut down so I pulled the cigarette lighter plug and the tacho came good until the iPad dropped dead and I still had a long way to go so I plugged it back in. Once again the tacho dropped out so I continued on without it.

 

I've only just arrived home so I haven't had a chance to put a meter on it but the charge light says the Rotax is charging.

First checks after start includes voltage increase to 13.6 - 13.8v; if that does not occur then you will be doing a set of checks (usually faulty rec/reg or if lucky just poor connections to the rec/reg.  Do your volts increase after start?

Edited 6 hours ago by Blueadventures

[Moneybox]

8 minutes ago, Blueadventures said:

First checks after start includes voltage increase to 13.6 - 13.8v; if that does not occur then you will be doing a set of checks (usually faulty rec/reg or if lucky just poor connections to the rec/reg.  Do your volts increase after start?

No, slight voltage decrease.

[Moneybox]

The plane has no volt meter or amp meter. I've checked it with the multimeter.

[onetrack]

You need to do a load test on the battery. A pulse charger can be used to recover a sulphated battery, but if the battery doesn't pass a load test after charging, it's stuffed. How long did you leave the Master on for? A slow steady drain to a couple of volts will generally seriously damage a lead acid battery, and even pulse charging will often not make it recover. How old is the battery?

[Blueadventures]

23 minutes ago, Moneybox said:

The plane has no volt meter or amp meter. I've checked it with the multimeter.

Hook up a multimeter to a male plug for the 12v cig lighter type outlet and see what volts your making, needs to be 13.6 +.

[Moneybox]

13 minutes ago, Blueadventures said:

Hook up a multimeter to a male plug for the 12v cig lighter type outlet and see what volts your making, needs to be 13.6 +.

Did that 9.8v same as battery voltage.

[Moneybox]

31 minutes ago, onetrack said:

You need to do a load test on the battery. A pulse charger can be used to recover a sulphated battery, but if the battery doesn't pass a load test after charging, it's stuffed. How long did you leave the Master on for? A slow steady drain to a couple of volts will generally seriously damage a lead acid battery, and even pulse charging will often not make it recover. How old is the battery?

Yes I understand the battery may now be damaged however this is not a battery fault. I have no generator charge. I bought the battery new so about a year old but until completely flattened over several days it remained fully charged. It recharged fully after that so I'm hopeful the battery is ok. It may even improve with constant use.

Edited 5 hours ago by Moneybox