Lockheed 1049H Constellation (Beta Release)

[simbabeat]

Hey can someone explain the whole mixture thing to me? I've never really gotten it.

[Yakko]

Its easy - an engine needs a pretty precise ratio of air to to fuel in order to run properly - this is the air-fuel mix or simply the mixture. Now, as you ascend in the atmosphere the air gets thinner and oxygen content decreases, so you end up with a ratio higher in fuel content than in oxygen (the mixture is getting richer in fuel) so you have to adjust it - this process is called leaning out. If you don't eventually you reach a point where the fuel does not have enough oxygen to combine with and stops burning (the engine chokes out).

Conversely, as you descend the air gets thicker and more oxygen, so you have to increase the fuel aspect of the ratio (richen the mix) to maintain the proper ratio. Slight misadjustments cause the fuel to not burn at optimum rate which is why you see the power and EGT values drop off when you are too rich or too lean.

[Tuxklok]

Also, during takeoff and landing, except at higher altitude airports, you usually want full rich. The idea is that it's better to waste a little gas and know your ok, rather than to get it wrong, or forget to re-adjust at such a critical time.

cheers!

[ethan3391]

Why does everybody forget to mention the manifold pressure? It is an indicator, more or less, of the power output. Keep an eye on that, and that is how you will know when to change the mixture.

[Yakko]

I normally recommend monitoring the EGT and BMEP instruments - the BMEP is a direct measure of engine rotational power. The EGT is basically the fuel burn temperature - when the mixture is ideal and the fuel is burning most completely the EGT instruments will read a peak value. However, this does not always relate directly to peak power - normally between 50-70 degrees centigrade rich of peak EGT is where this particular model shows max power.

I never noticed a direct relation between the manifold pressure and the mixture - but I will certainly play with it and see.....

[simbabeat]

Ahhh well thank you guys!

[Buckaroo]

A footnote: In the real Constellations, I understand that flight engineers would often run the engines on the lean side of rich depending on the policy of the operating airline, the condition of the engines, and the skill of the engineer. The advantage was better fuel economy. As all cylinders typically can't be uniformly monitored, it's possible for one or more cylinders to run hotter than the sampled temperature and risk damage or failure, so it's safer to run rich of peak. But fuel distribution to cylinders of a well-maintained radial engine is fairly uniform, so it was possible to trust cylinder head readings as representative of all cylinders and so run lean of peak for the better economy, which over many long flights could add up to a lot of savings. I'll wager that Pratt & Whitney and Lockheed did not recommend this.

To add to what Tuxklok said, running full-rich during takeoff and landing and ground operations is an additional means to cool the engine. Piston engines are generally designed to maintain temperature at a certain airspeed for the optimal flow of air over the cooling fins at maximum continuous power. At lower airspeeds this cooling factor is greatly reduced and the engine will run hotter. Dumping additional fuel through the cylinders can take away a considerable amount of heat and help keep the engine running within design parameters.

[Farmboy]

Buck's right on the money with his explanation. One thing many low hour pilots don't understand is how the temps are related to mixture. If you're engine's running hot feed it more fuel, it'll cool it down. On many radial engines they used a water injection system to allow a leaner fuel mixture without overtemps.

In regards to mixture settings, if you go back to the seat-of-your-pants flying, the general rule of thumb in general aviation was establish your level flight (which was always below 10,000 feet anyhow), and cruise speed, then pull the mixture control back until rpm's start to drop. Push it back in/forward until rpm's stop climbing, then add a touch more for safety. A touch can be a 1/2" of the lever, or whatever you were comfortable with. This rule of thumb was generally used on A/C without EGT gauges, or other tell-tale instruments.

Peter

[Andreas]

A footnote: In the real Constellations, I understand that flight engineers would often run the engines on the lean side of rich

Right! One simple explanation would be:
Running an engine at the stoichiometric "correct" balance would produce the best power output (=peak), because all the fuel chemically reacts with all the air and leaves nothing but hot exhaust gas. But this would also damage the engine (the valves for example) by overheating.
So you have to cool down the process by providing some extra fluid which will remain after the combustion. This can be extra fuel (running the engine rich of peak), or extra air (lean of peak).
Running lean of peak safes fuel, which is very important for long range operation - but running rich of peak keeps the engine in a more flexible state: a sudden increase of throttle would be responded by a quicker increase of torque. That is why in low level flight a rich mixture is necessary.

I'll wager that Pratt & Whitney and Lockheed did not recommend this.

Yes, they did!
http://www.enginehistory.org/Wright/TC%20Facts.pdf
At page 31 ff. Curtiss-Wright themselves give instructions how to manually lean the mixture in cruise flight.

BTW, being an injection engine, the real world R-3350 had an "Auto-Rich" and "Auto-Lean" setting. I think that for takeoff and landing, the injection governor took care about the right (rich) mixture.
The booklet notes that for cruising the mixture should be adjusted manually, because the Auto-Lean mechanism is too coarse.

As Yakko and Buckaroo already said in the README-files, it's hard to realistically simulate all the fuel and temperature- related stuff with Flightgear 1.9.1, simply because the program is not capable of doing that. I know, I've tried really hard and it really drove me mad

There is a simple way to demonstrate the bug in FGPiston1.9.1:
When you press N to turn the propeller blade to a coarser angle, the engine has to overcome a higher torque and slows down. That means there is less air pumped through the engine and, as we didn't change the mixture, the fuel flow should decrease.
The opposite happens! On the Fuel Flow gauge you can observe that it increases! There is a logical mistake in the whole thing.

But here's the good news !
Last november, the CVS-version of FGPiston changed! And as far as I can tell by now, at least this bug has been cured. So there is hope that one fine day we may simulate a realistic trans-atlantic flight with Buckaroo's and Yakko's gorgeous propliner model!

[Yakko]

I played with this a little bit last night - I also recall reading about running engines lean of peak EGT for better fuel savings, but in Flightgear this doesn't happen accurately (I set up the autopilot for a 11500 cruise altitude and RPM/manifold pressure to maintain 250 knots, then noted the BMEP reading. I tried adjusting the mixture lean of peak EGT and increasing manifold pressure to maintain the same BMEP, which did in fact result in maintaining the same cruise speed, but the fuel flow was actually HIGHER than it was with the normal rich of EGT setting. This is in FG1.9.1 so it may show up more realistic in the next release - we will see.

I also did not see any real direct coorelation between power and the manifold pressure - even with the engines leaned to cut-out or riched to choke-out the manifold pressure cheerfully reacted to throttle movement as though it were developing power. One other thing I find as an interesting in Flightgear is that, if en engine runs out of fuel from the tank going empty it will cut out and spin down to 0 RPM - if you pull the mixture back to cut out the engine will cheerfully windmill forever......in real life running out of fuel or leaning to shut-off should have the same effect.

If I remember correctly, running lean of EGT also had a risk of pre-detonation in the cylinders if not done very carefully, and the EPA is not particularly happy with the method because it generates some interesting combustion byproducts. Of course in the era of the Constellation the EPA wasn't a major factor, but predetonation could turn a million dollars worth of engine into junk weight hanging off the wing pretty quickly. If memory serves. one of the purposes of water injection was to reduce the risk of that.

[Tuxklok]

I hadn't really thought about the cooling aspects of mixture before, though it make sense, learn something new every day.

[Buckaroo]

Great PDF Andreas! That's a resource I didn't have.

Did I really say Pratt & Whitney? Ack, 3am in the morning and too much time working with the Goose's P&W R-985 engines these days. Somebody whack me with a giant spoon.

[Yakko]

Someone whack me with a giant spoon??

You REALLY should know better than to say such things around cartoon characters...........now where's the Giant Spoon section in the ACME catalog? Oohhh they come in large, extra large, freight truck and cruise ship sizes......

[simbabeat]

Someone whack me with a giant spoon??

You REALLY should know better than to say such things around cartoon characters...........now where's the Giant Spoon section in the ACME catalog? Oohhh they come in large, extra large, freight truck and cruise ship sizes......

That added the perfect amount of comedy into my day.

[ethan3391]

@Yakko

I was clued in on the manifold pressure because of a magazine article which mentioned the importance of that over throttle position when compared to power. Then while flying the Piper Cherokee it seemed to be confirmed. I later tried it on the PA 22-160, in a KLAX-KSFO attempted flight, and as far as the manifold was concerned there wasn't much difference between 0 and 6000 feet, only losing power when I accidentally ran one of the tanks dry. On the other hand the difference between the Cherokee and the Constellation, I'm assuming that's what you used to test it, may be the variable pitch propellers. Maybe I'm wrong, but that's the only way I can account for the difference, unless there was a problem with the model prior to a resent update.

I hope this was clear enough. Whenever I get relatively long winded in a post it seems to turn out confusing when I proof read it.