Manifold pressure problem - single engine

[mryan75 281]

Hey everyone,

 

I've encountered a problem relating to engine performance in a controllable pitch propeller airplane that I just can't figure out. When I get to cruising altitude, I can get the prop set to where I want it and the mixture correct, but even with the throttle maxed, the manifold pressure won't go above 22 inHg.

 

I did research online and found that for 75% power you should go for 24 inHg and 2,500 RPM (another suggestion I found was 25 inHg and 2,400 RPM). But I can't get anywhere near those inHg settings. Another thing to note is that the fuel flow gauge underneath the manifold pressure gauge has two lines, one for 65% power and one for 75%. I don't even get near the 65% line, so I know I'm not generating even 65% power.

 

Yesterday I was flying at 8,000 ft msl and had the mixture set at 35%, the prop at 2,400 RPM and the throttle firewalled (at 100%). For this I got 21.7 inHg of manifold pressure, which got me about 120 mph. I found that if I pulled the prop way back to like 1,300 RPM, the manifold pressure went up to like 22.3 inHg, and this was still with the throttle at 100%. Something has to be wrong.

 

So I put along at 120 mph in an aircraft (Saratoga) that should be able to do a good bit more than that.

 

Any ideas or suggestions?

[Jetstream96 20]

Which aircraft you are flying? Those data you found should be specific for an altitude. 75% of power will give you 24 inHg at certain altitude. If you fly higher, the number could be less than 24inHg. This is also the same for the engine RPM. At sea level you may get 2700rpm fro full throttle, but at 5000ft you may only have 2500rpm. It's quite normal.

 

Another possible reason is that the aircraft's performance data are not accurate in FSX. Sometimes this could happen.

[ryanbatc 12,803]

Which aircraft you are flying? Those data you found should be specific for an altitude. 75% of power will give you 24 inHg at certain altitude. If you fly higher, the number could be less than 24inHg. This is also the same for the engine RPM. At sea level you may get 2700rpm fro full throttle, but at 5000ft you may only have 2500rpm. It's quite normal.

 

Another possible reason is that the aircraft's performance data are not accurate in FSX. Sometimes this could happen.

 

Temperature would also play a role in the published chart performance.

[CoolP 283]

Yesterday I was flying at 8,000 ft msl and had the mixture set at 35%, the prop at 2,400 RPM and the throttle firewalled (at 100%). For this I got 21.7 inHg of manifold pressure, which got me about 120 mph.

Sounds ok to me. You are starting at sea level pressure (assuming a standard day), which will get you those 29.92 on the manifold pressure. With every 1000ft altitude gain, you will lose aprox one inch, so those 8000ft MSL will eventually end up being your 21.x value.

 

This is for a naturally aspirated engine. The turbo or super charged thingies of course yield way higher MP values, rendering them an option for flying higher and faster.

 

Keep in mind that, the higher you climb, the lower your indicated airspeed will be while your true airspeed goes up. Well, that's as long as the engine produces enough power of course, but those low IAS values you are seeing do not represent the speed of the air actually flowing around your aircraft. The deviation increases with altitude.

 

I did research online and found that for 75% power you should go for 24 inHg and 2,500 RPM (another suggestion I found was 25 inHg and 2,400 RPM). But I can't get anywhere near those inHg settings.

I can only guess in the direction David has given. Your charts may not only show some cruise values, but will also have an altitude level attached to those. The higher you go, the lower they will get because of the lack of a turbo or supercharger helping.

 

If you would constantly lose power over time (at the same altitude), you may experience that carb icing bug (or real carb icing) in FSX. That one will reduce the MP too since the engine develops less power due to the icing. If that problem has occurred, press 'H'.

 

Is that the Carenado Saratoga you are speaking of by the way?

[mryan75 281]

Sounds ok to me. You are starting at sea level pressure (assuming a standard day), which will get you those 29.92 on the manifold pressure. With every 1000ft altitude gain, you will lose aprox one inch, so those 8000ft MSL will eventually end up being your 21.x value.

 

This is for a naturally aspirated engine. The turbo or super charged thingies of course yield way higher MP values, rendering them an option for flying higher and faster.

 

Keep in mind that, the higher you climb, the lower your indicated airspeed will be while your true airspeed goes up. Well, that's as long as the engine produces enough power of course, but those low IAS values you are seeing do not represent the speed of the air actually flowing around your aircraft. The deviation increases with altitude.

 

Ah, ok, that makes some sense. (Can you tell I'm a fixed pitch pilot?). The part I still don't get, though, is that it can't be right that I have to fly along with the throttle firewalled the entire time. Like I said, I can't even get to the 65% indication line on the fuel flow gauge. Is it right that I wouldn't be able to get to 65 or 75% power at 8,000 feet msl?

 

I can only guess in the direction David has given. Your charts may not only show some cruise values, but will also have an altitude level attached to those. The higher you go, the lower they will get because of the lack of a turbo or supercharger helping.

 

If you would constantly lose power over time (at the same altitude), you may experience that carb icing bug (or real carb icing) in FSX. That one will reduce the MP too since the engine develops less power due to the icing. If that problem has occurred, press 'H'.

 

Is that the Carenado Saratoga you are speaking of by the way?

 

Yeah, it's the Carenado Saratoga. Also, it's not carb icing, I checked carb heat to see if it did anything. But what you said above makes sense, I'm still just wondering about the power issue. For example, I'm burning about 12 gph at 8,000 with the prop at 2,400 and the throttle firewalled. You would think I would be burning more fuel than that, so something still isn't adding up.

 

In the meantime I figured out that the power chart is actually under the sun visor in the airplane. See below.

 

http://farm9.staticflickr.com/8016/7598984084_989cb115ee_b.jpg

 

If you could help me make sense of that I'd really appreciate it.

[tttocs 301]

The part I still don't get, though, is that it can't be right that I have to fly along with the throttle firewalled the entire time. Like I said, I can't even get to the 65% indication line on the fuel flow gauge. Is it right that I wouldn't be able to get to 65 or 75% power at 8,000 feet msl?

 

Yes, it's quite right especially on a warmer day. Believe me, if you fly at altitude (my home airport is at 4650') in normally aspirated airplanes, you get used to this. Your plane's engine simply can't make full power as it climbs. Eventually, you hit a point where your chosen power settings require full throttle - but note that at altitude, full throttle is NOT full power and is perfectly normal and OK.

 

This occurs with fixed pitch props as well, but as you don't have a MP gauge on typical fixed pitch prop airplanes, it isn't so immediately obvious.

 

Scott

[Ken_Stallings 2]

This has nothing to do with whether the propeller is fixed pitch or constant speed. It has to do with the fact that as you climb, air pressure lowers. The rate at which it lowers is about one inch of mercury per one thousand feet climbed. On the ground at a sea level airport, the manifold pressure gauge is actually another barometer. In fact, a pilot flying an actual aircraft should, as a matter of habit, establish the pressure correction for the airport he flies out of (subtract one inch per thousand feet above sea level) and compare the manifold gauge or gauges to the reported airport altimeter value (which does not take into account altitude above sea level).

 

Say you fly out of an airport at 4,000 feet field elevation. This means you subtract 4 inches from the reported altimeter setting on the airport's AWOS report. If it said the altimeter setting is 30.00, then your pressure at the airport is 26 inches of mercury. Note: your altimeter on the panel reports 4,000 feet when you set 30.00 and this is done by the altimeter making the exact same "correction" based on measured pressure.

 

In this case, your manifold gauges prior to engine start should be reading right at 26 inches. Once you start engines, the manifold pressure gauge is changed by the suction of the engine and now reads the pressure of air feeding into the engine's intake manifold aft of the intake butterfly valve (which is opened and closed by adjustment of the throttles.).

 

So, as you climb, with the throttle full open (and the butterfly valve in the intake full open) unless you have a turbocharger that artificially increases outside air pressure, your maximum manifold pressure cannot exceed outside air pressure -- which again reduces by one inch per thousand feet in climb.

 

Ken

[CoolP 283]

This occurs with fixed pitch props as well, but as you don't have a MP gauge on typical fixed pitch prop airplanes, it isn't so immediately obvious.

I guess the last sentence from Scott is vital and perhaps explains the surprise the phenomenon established when not being used to the MP display. As he said, warmer temps add to it, means you even lose some more power. Density altitude is your buzz word on that one.

 

I may stick to your example from above, with you being at 8000ft and getting some 21.7 inHg of manifold pressure. Now the older Carenado planes may not be the benchmark when it comes to FDE details, but lets look at the table you've found.

 

On a standard day, those 8000ft MSL would equal to some -1 degrees Celsius (15°C from SL, approx 2°C loss per 1000ft altitude gain, so you'll lose 16°C) and if you look into the line where it states that value, you will find your 'low' MP being very much spot on. With temp variation at the same altitude, the achievable MP goes slightly up when it is colder than ISA and down when the surrounding air is warmer.

 

Either way, as Scott said, that full throttle does not equate to full power, more to full available power, which isn't that much due to the lower pressure at altitude. But, lucky you, that low density air also establishes some relief on the overall airplane drag the engine power has to overcome. So, up to a certain limit, you will achieve the best cruise speeds when flying higher.

 

Mind the note on the economic cruise mixture setting, which is a thing to e.g. achieve with EGT gauges in place. Running too rich, you may lose power, same goes for too lean.

 

I guess the POH will offer more detail on that aspect, but heading for peak EGT and then going back into rich a bit (mind the 'how much' on that expression) aims for the economic mixture, while adding some more may yield the best power ratio. As said, those are POH details and I don't know if Carenado and FSX allow for the full detail on the older planes.

[tttocs 301]

Also don't forget the huge role that temperature plays in all of this. That 4650' airport I mentioned in my previous post, has, on this blazing hot (36C) day a current density altitude of 7,800'! On an above standard temperature day, that 8'000' cruise altitude is effectively higher, and on a hot summer day, a LOT higher.

 

Scott

[Ken_Stallings 2]

Also don't forget the huge role that temperature plays in all of this. That 4650' airport I mentioned in my previous post, has, on this blazing hot (36C) day a current density altitude of 7,800'! On an above standard temperature day, that 8'000' cruise altitude is effectively higher, and on a hot summer day, a LOT higher.

 

Scott

 

True, but I'm trying to "keep it simple" for him! LOL!!

 

Ken

[tttocs 301]

Two excellent (and far more complete) responses sandwiched in while I made my second one. Good stuff, guys!

 

Hope all of this helps.

 

Scott

[mryan75 281]

I guess the last sentence from Scott is vital and perhaps explains the surprise the phenomenon established when not being used to the MP display. As he said, warmer temps add to it, means you even lose some more power.

 

Yeah, that is really the issue now that I'm reading these responses. I'm a soloed student pilot in real life, and my trainer does not have a MP gauge. There's a lot of great information on here for both real and virtual flying, and I can't thank each of you enough. I'll need to take some time to read through these posts, but this is fantastic information. Thanks guys.

 

* Edit: As I'm reading through the next few answers, again, I can't thank you guys enough, but I wanted to add one additional piece of information that may explain a few things: the airport I fly out of (KRME) is at about 506 feet msl and I basically have no experience flying at anything over 3,500 feet msl. I obviously know about density altitude in theory, but it doesn't play the role in my life that it does for you guys taking off at 4,500 msl. On hot, humid days here, the AWOS will often say things like, "Remarks: density altitude 1,800." So obviously a bit different than what you guys deal with on a regular basis.

[PIC007 82]

the airport I fly out of (KRME) is at about 506 feet msl and I basically have no experience flying at anything over 3,500 feet msl

 

Take your trainer up to 8K and you will see about the same results as you are seeing in FS :-)

[mryan75 281]

Take your trainer up to 8K and you will see about the same results as you are seeing in FS :-)

 

Exactly! And this is exactly why I wanted to find a complex aircraft to fly in FSX, so I could start getting a handle on new concepts before ever actually setting foot in one. And boy has it worked out better than I had hoped, thanks to you guys. I've learned a lot from doing this. I think FSX can be a great learning tool in this regard. For example, I learned VOR navigation in FSX before I ever flew under the hood in the airplane. Guys in my flying club kept saying, "It's not going to be as easy as you think," but my first time under the hood, my instructor told me if he hadn't known any better he would've thought I was an instrument rated pilot. I was able to fly VORs, straight and level, turns, constant speed climbs and descents, constant speed climbing and descending turns, all no problem, all well within PTS standards, all without ever having a minute's experience under the hood in an actual airplane. We even shot the ILS approach to KRME.

 

Anyway, this is what I was hoping for, to use the Saratoga and FSX to learn. And boy have I ever. Thanks again to everyone!

[CoolP 283]

Good report, mryan. I guess you've hit the nail on the head on what that simulator game can do, supporting some rw training stuff or even offering new impressions to then learn about.