Mixture/weather rpm question
07-05-2003, 12:56 PM
#1
Thread Starter
My Feedback: (2)
Join Date: Jul 2002
Location: Hawthorne, CA
Posts: 5,839
Likes: 0
Received 0 Likes
on
0 Posts
Mixture/weather rpm question
I should probably know this but I'm still a little confused and I think I'll get a more knowledgeable answer here than in the engine forum.
OK, cold air denser and more oxygen engine should run faster yet it seems during the course of a day the rpm's go up as it gets hotter.
Gas engine was running a little rich yet when it got hotter out the engine seemed to lean out with apparently less oxygen from the thinner hot air. (I know this is the pylon forum but it pertains to glow engines also, I think.)
Does the heat of the ambient air have as much effect on engine cooling as the mixture or is it negligible and mostly effected the density as opposed to just the temperature?
I did a search since I'm sure this has been explained before so if someone can point me to a thread or explain it simply or recommend a P.H.D. program in physics I'd appreciate it.
OK, cold air denser and more oxygen engine should run faster yet it seems during the course of a day the rpm's go up as it gets hotter.
Gas engine was running a little rich yet when it got hotter out the engine seemed to lean out with apparently less oxygen from the thinner hot air. (I know this is the pylon forum but it pertains to glow engines also, I think.)
Does the heat of the ambient air have as much effect on engine cooling as the mixture or is it negligible and mostly effected the density as opposed to just the temperature?
I did a search since I'm sure this has been explained before so if someone can point me to a thread or explain it simply or recommend a P.H.D. program in physics I'd appreciate it.
07-05-2003, 02:19 PM
#2
Banned
Join Date: Dec 2001
Location: Monroe,
NC
Posts: 1,332
Likes: 0
Received 0 Likes
on
0 Posts
Mixture/weather rpm question
I think I can partially answer your question. As density altitude goes up, the prop meets less resistance. This means that the prop is more free to turn and the engine needs more fuel/air for the higher rpm. Even though the rpm's are up, depending on the prop, you may or may not get more speed. This is where prop selection becomes important.
Density altitude can be checked via METAR's for full scale aircraft. Basically a hot, dry day will have a significantly higher density altitude than a cool day.
For more information, find a site or book for General Aviation prop planes (Cessna's, Piper's, etc) and read up on density altitude.
If I'm flying a full scale plane at 5500 MSL (Mean Sea Level) early in the day, and lean the engine using the EGT (Exhaust Gas Temperature), I have to watch the EGT in the instrument scan, because as the density altitude goes up, the EGT will also, and the mixture will need to be richened to keep the EGT in an acceptable range. In a plane with a constant speed prop, I'll actually get more airspeed. With a fixed pitch prop, true airspeed will decrease.
Hope that gets you started on a new quest for info.
Density altitude can be checked via METAR's for full scale aircraft. Basically a hot, dry day will have a significantly higher density altitude than a cool day.
For more information, find a site or book for General Aviation prop planes (Cessna's, Piper's, etc) and read up on density altitude.
If I'm flying a full scale plane at 5500 MSL (Mean Sea Level) early in the day, and lean the engine using the EGT (Exhaust Gas Temperature), I have to watch the EGT in the instrument scan, because as the density altitude goes up, the EGT will also, and the mixture will need to be richened to keep the EGT in an acceptable range. In a plane with a constant speed prop, I'll actually get more airspeed. With a fixed pitch prop, true airspeed will decrease.
Hope that gets you started on a new quest for info.
