Manifold pressure problem - single engine

[Great Ozzie 101]

As CoolP mentions, this can be counter-intuitive

 

yeah it sure is... and counter RL ops btw.

 

afa FSX is concerned... I use automixture as I really see no point in doing so otherwise (exceptions are for aircraft like the A2A Cub & B377 etc.).

 

Brett Henderson addressed this "FSX anomaly" in a very interesting post here: http://forum.avsim.n...-orion-m7-260c/

 

Scott my man... I simply do not know what to do with the rest of your statement. Suffice it to say, if Tom had a "Buckwheat" emoticon I would have used that.

 

but may for example lead to false 'high' FF readings in the case of an clogged injector, actually meaning that less fuel enters the engine.

 

RW... you are correct here... one reason one might observe a high fuel flow indication is due to a restricted nozzle... (not sure what specifically that has to do with leaning...)

 

This will decrease the actual fuel flow, adding to the power loss of the sheer altitude gain.

 

fuel flow won't decrease (unless there is something like some sort of automatic mixture control unit built into the injector)...

 

If you gain altitude... obviously DA decreases increases (thanks coolip) -> and this is what causes decreased power (besides the fact of a relative increase in the F/A ratio) so a decreasing FF would be the "proper thing" i.e. this would actually be leaning... to return closer to best power... or even decreasing more... to a best economy FF.

 

So FSX may give you the max FF at the max HP developed, which happens at the fuel/air ratio for best power.

 

Maybe a great way to determine for FSX, not RW.

 

Guys... for RW engine leaning... I would suggest a couple articles:

 

"The Fuel Air Mixture" by Flight Magazine: http://flighttrainin...al/mixture.html

 

"Mixture Control" by the Light Plane Maintenance Staff: http://www.aerotrain...gif/mixture.htm

 

"Engine Leaning" http://www.principal...gineleaning.htm

 

"Ask a Flight Instructor" http://www.askacfi.c...g-technique.htm

[CoolP 283]

Rob, you did get that we were, still are, on the FSX case?

If you gain altitude... obviously DA decreases -> and this is what causes decreased power

If your 'DA' means density altitude, it increases in that case. So the sentence makes sense.

 

You tend to partially quote folks and, therefore, spoil the context at times. I doubt that it happens intentionally, but since we were on the FSX fuel flow stuff, well, we still are. The OP was wondering about a low FF rate, among other things.

 

As you may have noticed, the FSX engine doesn't even offer a distinction between carbs, some mixture automation being present or some fuel injection system. The turbine guys have different problems, even more so on the geared turbines. Some devs work around the issue with running engine parameters outside of the sim.

 

So, yes, that game has it's quirks, as some FF reading in the rw may have, when not knowing about the actual methods of measuring. That was the intention of my example.

 

I've also tried to explain why the FSX fuel flow meter behaves as it does. It most likely relates to power output and I'm a aware that the rw units don't do this. Hence my sentence before your quoted one 'I may add that the sim's rendition of the fuel flow may be power based.'

 

So the wise sim pilot will try to avoid forming up bad habits, while every sim pilot has to handle the realism downsides at some time. Thanks for the rw links. I'm sure they help some folks understanding the differences some more. A good read for sure.

[tttocs 301]

Scott my man... I simply do not know what to do with the rest of your statement. Suffice it to say, if Tom had a "Buckwheat" emoticon I would have used that.

 

Well, it was a nice explanation and all - it was just wrong. Too many hours flying FSX and too many hours since I flew the real thing I'm afraid.

 

Anyway, mea culpa.

 

A Buckwheat emoticon? A dunce cap wasn't good enough, eh?

 

Scott

[Great Ozzie 101]

Rob, you did get that we were, still are, on the FSX case?

 

Real world and sim.

 

The rw stuff has it's quirks too.

 

Dunno... you tell me... you intermix RW with sim and I am left wondering which is which... why I asked for clarification

 

If your 'DA' means density altitude, it increases in that case. So the sentence makes sense.

 

yes thanks... you guys with your "it does this in the sim... and rw" had me doing a number of scooby doo "huhhh????" I'd imagine I was initially thinking of pressure (decreasing) then realized it was going to be based on temp too and brain farted my wording.

 

You tend to partially quote folks and, therefore, spoil the context at times.

 

Yes intentionally... just to capture what was said and not the whole post... as I find that very lazy. If one is concerned about context the original post is always there to be read.

 

A Buckwheat emoticon? A dunce cap wasn't good enough, eh?

 

No as a Buckwheat thingy I find humorous... and no need for the dunce cap as I know you aren't a knucklehead and (as can be seen) I brain fart stuff too.

[mryan75 281]

Still on the throttle, eh? :P Perhaps your instructor should show you what that throttle actually does, next time being on the plane. I really mean the mechanical ways of that lever and where it ends. Differs a bit on the fuel injected engines, but perhaps it's an eye opener.

 

I always read some 'I'm worried about over-stressing the engine' into your statements, mryan and, while I like the cautious attitude, seeing that you may relate engine stress levels to the position of the throttle lever alone.

 

Well, that MP gauge is your power instrument on that kind of plane. Together with the rpm, those are your factors of defining how much power you are actually getting out of that engine. See both red lines? Match both and you will receive the rated engine power, your 100% defaulting to some HP value. You may not always be able to achieve it or, at colder days, you may be able to overshoot more or less.

 

So while you are ascending, the achievable MP will reduce, as described in the previous posts. With that, you are getting away from the max MP while the rpm may stay at max for way longer as the prop arrangement adjusts itself for the lower engine output, keeping the rpm steady.

This means that you yield lower than 100% either way. With the throttle lever fully opened, you allow for 100% of 'what's there' or 'what's left' which is perfectly save at altitude or even very shortly after climbing away from your starting point as the pressure drop within e.g the first 5000ft is very noticeable.

 

So there is no need to worry as long all your gauges show nice temps, nice oil pressure levels and a MP and rpm combination being within the limits the manufactures has defined. For example, the stuff does not like high MP settings with low rpm running, due to the torque levels developing in that situation. So there's a certain envelope (plane specific) to keep in mind. To find out about that one, the POH is the very fist source of data.

 

On the fuel burn. I don't know how accurate the Carenado plane is, but your table shows that you will achieve those 16 gal/hr up to 7000ft on a standard ISA day. Anything higher or warmer will reduce it as your achievable MP will drop, while lower and colder may be able to increase it because you can run a higher MP there.

 

Also mind the mixture setting of yours. You have to lean to get more power out and also see a rise in fuel flow. This may sound counter-intuitive at first. If you stick to the mixture levels from the takeoff and climb to 7000 feet, you will lose some more power just by running to much fuel in comparison to the volume of air.

 

Some 'smart' engines lean by themselves, but I guess yours has an extra lever for that one. And FSX needs a huge amount of leaning, perhaps too large in comparison to the rw engines.

 

Ok, that clarifies a lot for me. Yeah, I was wondering about the wisdom of having the throttle firewalled the entire time. Also, I know all about leaning, watching EGT go up, lean a little more, wait for it to stop going up, then enrichen the mixture until the EGT drops 50 degrees. No confusion there, I had the engine leaned properly.

 

Thanks for your answer!

 

An excellent point here, along with a lot of other good advice. FSX exaggerates how much leaning needs to be done, so you're going to find you'll have to pull the mixture back more than you would in reality. But even in the real world, you're going to be running fairly lean at these kinds of altitudes and the available power settings.

 

Power and heat are what your engine cares about in terms of stress and should be what you care about - you really, really (really, really!) need to stop worrying about the position of the throttle lever and pay attention to MP/RPM, and setting them according to the POH for your desired power. 75% power is 75% power (65 is 65 and so on) in terms of engine stress. Watch your leaning technique and cowl flap settings to help manage temps and you're good to go. The gauges are what you pay attention to - the physical throttle lever ends up where it ends up - and that's usually firewalled at altitude in a normally aspirated plane). "75% throttle" is meaningless.

 

Cool, as I said above, this clarifies the question for me. I appreciate your input.

 

Oh, and on the gear-up landing - sorry if this sounds preachy, but as you're in training, yes the sim is great but it can also teach you bad habits. A quick GUMPS check would've saved you here. If you're going to use the sim as a trainer don't allow yourself to develop bad habits and forget standard procedures. :-)

 

Scott

 

I understand what you're saying, but you have to bear in mind, this was my first flight in this airplane; all of my real-world and sim time has been in fixed pitch aircraft with the gear down and welded. I posted that story more because I thought it was funny. My instructor is a huge, huge proponent of checklists, and I have mine for the Cherokee ("fuel pump on, mixture rich, seatbelts are secure"). I'm not so much worried about what happened in terms of forming bad habits, but you make a good point in that now I can start incorporating the GUMPS checklist into my sim flying of this airplane. This is exactly what I'm shooting for: using FSX to learn.

 

Thanks again for your input.

[CoolP 283]

My standard excuse for gear up landings always is that I'm used to sea planes.

[Great Ozzie 101]

My instructor is a huge, huge proponent of checklists, and I have mine for the Cherokee

 

Yeah but like Scott said, once you fly retract planes you will add that GUMP check (besides using the checklist)... at least twice... once on downwind and on final. I also ALWAYS do a "cleared to land" on short final.... at which point I will probably flash check for 3 green.

[tttocs 301]

and no need for the dunce cap as I know you aren't a knucklehead and (as can be seen) I brain fart stuff too.

 

Well, maybe not a complete knucklehead, anyway.

 

Scott

[Ken_Stallings 2]

Pardon me again, Ken, if this is a stupid question, but as was stated above, full throttle at altitude doesn't necessarily mean full power. So is it ok to keep the throttle firewalled if I'm getting 21.7 inHg as I was and 2,400 RPM, or should you back the throttle down to 75% (or some lower setting)? Is it bad for the engine to have the throttle at 100% if you're not getting 100% engine power? On this flight I just left the throttle in full, backed the prop to 2,400, and putted along.

 

It is somewhat complicated, but once understood becomes quite simple actually. Power is power. Manifold pressure and RPM both have an input into power since power is simply thrust.

 

Manifold pressure is simply a measure of how much air pressure is being sent to the cylinders to support the combustion cycle of the four-stroke engine (intake, compression, combustion, exhaust).

 

RPM is simply the measure of the revolutions of the propellers.

 

Now, thrust (power) requires one to consider both manifold pressure and RPM. But, the RPM is a bit complicated. The faster you go the flatter the prop blades have to be to achieve the same thrust. Meaning, at standstill on the runway for takeoff, the prop produces optimal thrust when the blades are relatively fine in angle (meaning the prop control levers are full forward).

 

However, once you get airborne and accelerate to your cruise airspeeds, due to something called "relative angle of attack" on the prop blades to the airflow, you have to flatten out the blade angles some to achieve the same thrust that the blades were able to achieve at the same manifold pressure setting when you went from standstill to takeoff speed.

 

In fact, this is why prior to the development of what were called variable pitch propellers that the fastest aircraft on earth tended to be floatplanes. This was because props back then were fixed pitch and you could carve (or mold) them to produce best thrust at low speed or high speed. If you made them at a fixed angle for best thrust at high speeds, then the drawback was they did not produce best thrust at low speeds and this meant you needed a much longer runway to get airborne from. Since very long runways were not prevalent back then, the best alternative was large bodies of water such as long rivers, lakes and estuaries, were the best option -- ergo, floats put on the planes!

 

Now, let's return to manifold pressure.

 

All the throttles on your airplane do is open up a butterfly valve installed inside the air intake manifold. The valve goes to its max closed setting (which is still not fully closed) when the throttles are pulled full aft, and open up full when the throttles are moved full forward. With the butterfly valves closed, the engines can only suck a small amount of outside air -- same as if you try to suck a drink through a soda straw and pinched the straw.

 

With the butterfly valve fully open, the engine can suck all the air pressure available to it. This means with a non-turbocharged engine, that max air pressure is no more than outside air pressure. With a turbocharged airplane, the turbocharger can use vanes to compress (or pressurize) the outside air. Either way, with the butterfly valve full open, the engines get all that's available to it.

 

With more air pressure in the cylinders, more fuel can be sprayed into the cyliders. Regardless, optimal combustion is only achieved with the ideal ratio of fuel to air mixture. If the fuel is too rich, combustion is reduced. With the full too lean, combustion is reduced. There are other impacts, but beyond the scope of this post to talk about here.

 

So, as I wrote earlier, you need to understand that once the outside air pressure reduces when you climb, even with the butterfly valve full open, the manifold pressure reduces. If you recommend cruise power setting is 24 inches of mercury (manifold pressure value), then at about 6,000 feet, you can expect that with the throttle full open, you won't be able to achieve anything higher than 24 inches of manifold pressure. So, above that altitude, it won't make a bit of difference in power (thrust) whether you have the throttles wide open or not. You can certainly pull back the throttles until such a point where the manifold pressure reduces below outside air pressure. This is because at some point, you finally close the intake manifold butterfly valve to the point where it constricts (reduces) the manifold pressure below even the reduced air pressure present at the higher altitude.

 

Hope this helps!

 

Ken

[mryan75 281]

Yeah but like Scott said, once you fly retract planes you will add that GUMP check (besides using the checklist)... at least twice... once on downwind and on final. I also ALWAYS do a "cleared to land" on short final.... at which point I will probably flash check for 3 green.

 

By the way, went out for touch and goes this morning, guess what my checklist says under "Before landing"?

 

G.U.M.P.S.

 

 

It is somewhat complicated, but once understood becomes quite simple actually. Power is power. Manifold pressure and RPM both have an input into power since power is simply thrust.

 

Hope this helps!

 

Ken

 

You've more than helped, Ken. Thank you so much for your time and knowledge.