RCU Forums - Engine Sagging

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RE: Engine Sagging

Quote:

ORIGINAL: av8tor1977

You can "take this to the bank." Hot and humid weather makes an engine richen. The air is thinner and you have to lean the engine to compensate if you want optimum mixture setting.

Cool, dry air is more dense, and will lean an engine out. With large temperature changes to cooler air you need to richen your engine to regain optimum mixture.

Barometric pressure and field altitude also make differences. Especially field altitude. An engine loses approximately 3% power for each 1000' increase in density altitude. Density altitude is a function of true altitude, barometric pressure, and temperature. An engine at a density altitude of 7000' will have about 21% less power than the same engine at sea level, 59 degrees, and 29.92 barometric pressure. (ISA) This sounds like a high altitude, but it is commonly encountered in the western United States due to high field elevations and high temperatures. I have seen many 7000' foot days in Tucson, Arizona.

At high density altitudes; over 5000' due to high temperatures and true altitudes, Continental and Lycoming recommend leaning their engines for take off power! In these conditions, you can add full power, and start leaning the mixture on a full size airplane, and just watch the tach start climbing; gaining power because the mixture was too rich. Baseline full rich mixture is set for standard conditions, (ISA) of sea level, 29.92 barometric pressure, and 59 degrees. Any thing above that in temperature, altitude, or lower in barometric pressure, or any combination thereof, causes a richer than optimum setting. Humidity has the effect of lowering air density as well, but is less of an effect than temperature or altitude.

AV8TOR

<hr />AV8TOR....you got to be related to TOM or Zeeb....or Senator Blow jaws!http://www.rcuniverse.com/forum/js/f...sn/biggrin.gif

RE: Engine Sagging

Quote:

ORIGINAL: vanguard
. Enough said, I have contacted the moderator and asked him to end the thread.

I would like to step in and comment on this briefly. RCU's policy is to allow threads to remain open whenever possible. We do not close them simply on request from the OP or anybody else participating in the discussion. We prefer to allow threads to simply "fade away" over time as interest in the thread diminishes.

Ken
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RE: Engine Sagging

Quote:

ORIGINAL: vanguard

guys read my original post. I didnt ask how to tune an engine. I normally tune them when new and forget them. Hate to mess with needles unless really necessary. Our weather has been very very hot and humid the last two weeks. I noticded that my DA85 started to sag a little after about 10 minutes in the are. Normally sagging would indicate a lean condition..

guys

Sagging is different than loss of performance. If you notice this after about 10 minutes you are probably getting it a little hot. Reasons have already been mentioned for this but I would bet it has more to do with air flow and cooling than the needles. You should not crutch or compensate the needles for poor cooling.
I have posted this many many times, it is useful for every modeler to look at.

http://www.prme.nl/forum/viewtopic.php?p=506#506

RE: Engine Sagging

Lets make this real simple. On a cold morning do you need a choke to start your car? Do you need a choke to start you care on a hot day?

Case closed.

RE: Engine Sagging

DA 85 sagging could be due to other reasons, such as air leaking in one of the gaskets. This happened to my copy multiple times, when the ambient temperature is around 96 degree. It worked well for a while and suddenly the high rpms dropped.

RE: Engine Sagging

Quote:

ORIGINAL: RTK

Quote:

I am building a round cowl setup right now. When done, I will expand the cooling post with pictures. I still must solve the carb/air filterintallation. Plane is for towing duty, so the engine runs prolonged WOT at lower speeds.

RE: Engine Sagging

Vanguard, I would look at your cooling. I had the same problem, on a Yak, and solved it with baffling. Pe is correct. On a hot day, you have less air, and therefore need to lean your mixture, if anything, but not normally that critical, from day to day.

Check your baffling in your engine Bay. ;)

RE: Engine Sagging

Pe, I found that if you split the cowl horizontally from front to back mid cylinder blocking only the bottom half of the cowl, you will have great success. If I can find the pics I will post them.

RE: Engine Sagging

Here is what I did. It would work much easier if you have a split cowl. With this setup I was always below 300*F no matter how long I ran at WOT even on the ground. All bottom louvers were opened up (not the top) and on the other stock opening I put a low pressure lip.
Oh, all other open areas were sealed off on the motor box. The air came in the top and only had one way to go (by the cylinders) to exit out the bottom
Don't laugh at my looongg stand-offs, they did work well but allowed a fair amount of vibration so I had to tie them together

RE: Engine Sagging

Thanks for the pics. This setup always works good.
In a round cowl with a single cylinder engine inverted it is easier to split vertically in a fore/aft fashion, or even put the baffle forward of the engine. This provides a lot of surface and options for air extraction. Only attention point leftis the rear mounted carb, that needs a cool air space of it's own, like maybe a box around the standoffs, open to the fuselage.

OTOH, that could be done along the lines of the lengthwise split cowl method as well.

RE: Engine Sagging

Excellent job Pe!!! With that air extraction it's gonna run like a refrigerator

RE: Engine Sagging

wait till you see tha carb fridge. (pic to be here soon.)

RE: Engine Sagging

Quote:

ORIGINAL: pe reivers

wait till you see tha carb fridge. (pic to be here soon.)

The carb fridge is meant to isolate the rear carb from the engine hot air. Purpose is to prevent vapour lock, and keep the carb functioning well. In the last two pictures I show how to block off the last tiny bit of hot air. MVVS does not use a carb isolator block. It is not needed as long as the carb resides in cool air. However, the combination of warm air and engine heat may cause problems, so I rather be safe than sorry.
PS
Material used for the fridge is 5-ply balsa, which I stumbled across in a hardware store. It holds screws very well, and is stable in all directions.

Every part can be disassembled in order to access engine and throttle setup. The last air blocking parts are best glued to the engine rear mount. First tests will be without blocking the last bit of hot air to the carb. If all is well, no further actions will be needed.

RE: Engine Sagging

It's more complex than your system, but with one evening extra work I have more options later on.