I would like to dispel a myth here.
 

I don't know how many times I've heard this, and it makes absolutely no sense from a physics standpoint.
People will say- "Make sure you run your engine a few clicks to the rich side, because it will lean out at altitude".

Completely false. In full-scale flight, we lean our engines as we gain altitude. In fact, when taking off from high altitude fields, especially on hot days (density altitude), we will lean the engine to peak RPM before takeoff. It works like this: The more altitude, the less dense the air, therefore the higher relative volume of fuel in the mix.

I will say that it is good to run a little rich for other reasons; you will get leaner as the tank empties, and with a nose-high attitude, and a little extra oil never hurts. I just get tired of hearing that altitude makes you leaner, when it actually does the opposite!

Theoretically, one could tune their fully warmed engine to peak with an almost-dry tank in a nose-up position, and it would always be safely on the rich side without being overly so. Probably see a reduction in fuel consumption as well.

RE: I would like to dispel a myth here.
 

OK Evil Homer!! You Did IT !! Run your engine wherever you want, rich, lean or whatever you need to do in order to not be [" completely false" ] They are after all, your engines. Please don't try to tell me how to set up MY engines. Thank you.
Bob Bauer

RE: I would like to dispel a myth here.
 

never heard the part about altitude myself, just that the engine will lean out. unloading and getting up "on the pipe" are the two main reasons.



dave

RE: I would like to dispel a myth here.
 

Homer,

Have you run many engines?

It is fairly common knowledge that competition schneurle engines will pick up a couple of thousand RPM when "unloaded" in the air and you need a little more fuel and oil to provide power and keep the engine lubricated.

Back in the '50's I used to avoid lean engine runs by opening the needle a little and that was on the older loop-scavenged engines.

Everyone has their own reasons for running engines their own way but the guys who richen the needle a bit are the guys who still have running engines.

That's my story and I'm stickin' to it.

RE: I would like to dispel a myth here.
 

Flycfii,

You are correct in saying that the altitude thing is a myth, a model engine will NOT lean out with alitude. But MOST importantly, models are NOT flown high enough to ever start worrying about mixture changes. Say we fly to 500 feet which is pretty high for a model, 500 feet just is NOT enough to ever make any difference in mixture one way or another. Even in a full scale aircraft, 500 feet does not require any mixture adjustment. So altitude is just NOT an issue with models period.

What are big issues in flight is, G-loading, vibration, changes in aircraft position. All these things can change the mixture in a BIG way. So its best to be on the rich side, because in flight lots of things can make your engine lean out. But the altitude is NOT one of them.

JettPilot

RE: I would like to dispel a myth here.
 

As indicated below, a bad read on my part.

RE: I would like to dispel a myth here.
 

Wow, what a childish reply.[sm=confused.gif]


Zagnut & Dan- you're right. I failed to mention unloading at speed, and there may be more reasons. I definitely still run my motors rich of peak, because I want them to last. I just want people to know that they do not lean out because of "altitude", that is a false statement.

RE: I would like to dispel a myth here.
 

In most peoples minds "at altitude" means when you level off, the plane accelerates and unloads the prop increasing RPM and the mixture goes lean.

RE: I would like to dispel a myth here.
 

Oh, goody- another attacker.

Read my post, and you will see that I still advocate rich running. This thread was started with the specific intent of debunking a myth that exists in the r/c community, namely that altitude has a leaning effect on engines. Whether they be glow, gas, diesel, whatever- more altitude = richer mixture, pure and simple. You can attack me and pull hair all you like, but physics will be the last one standing! Lighten up, dude.

Jettpilot- of course 500ft makes no difference. And there are quite a few factors in why we all should still run rich of peak- like the $1K that I have tied up in 4-strokes:D. I just keep hearing people say that the plane will lean out with altitude..... maybe its a midwest phenomenon...:)

RE: I would like to dispel a myth here.
 

All right, more constructive feedback! (sarcasm, for those of us that are not smart enough). Did we get up on the wrong side of the bed this morning?

Why not say "at speed" when that is what you really mean? Hmmm? Either way, misleading comments like "at altitude" -altitude being a variable just like speed is- do nothing but add to the confusion. Lets clear it up. Without the catfight, please.

RE: I would like to dispel a myth here.
 

I have heard the myth you are refering, the trouble is the blind acceptance of some people with regards to what they hear or read. I get what you are saying and I agree. If people read properly what you wrote in the first post there shouldn't be a problem(with starting a myth I mean).

RE: I would like to dispel a myth here.
 

Clearer, I set my engines 200 rpm rich of peak with the tank half full to insure protection against any variables whether they be altitude, changes in speed or other, I've never found it necessary to run them richer than that. I've seen where others run them 1,000 rpm rich but that seems to me to be overkill and wasteful. I personally never heard that altitude was responsible for the leaning only the increased engine rpm from unloading.

RE: I would like to dispel a myth here.
 

Fact: An engine WILL run leaner moving through the air than stationary on the ground. I'm not buying the altitude thingy, a few 100 feet doesn't make that much difference in pressure, unless you're beneath a school of sharks in a scuba suit, with your air running out. <grin> What DOES make a difference is the slipstream from the prop+the airspeed moving by the carb throat. Also, it's easier for the prop to pull through moving air than stationary air. That's what "unloading" is. Check your physics textbook more closely. Who says this? I say this. A guy who has over 25 years of glow experience in all types of planes.

Dr.1

RE: I would like to dispel a myth here.
 

I suppose that when you pull the throttle back on your model to land, you re-trim the elevator for a neutral feel at the proper approach speed? Then control approach speed with the elevator and altitude with the throttle?

I suppose you also keep the nose aligned with the runway and control crosswind drift by cross controlling your model with the ailerons?

The old continental powered Cessna 150's and 172' were initially unresponsive to requiring the mixture be leaned until you had gained 6,000 ft. The later model Lycoming would show an increase in speed by leaning at around a 3,000 ft gain.

Someone here that started this tale, has not flown models or real airplanes enough to talk proficient about either?

The reason for making the model engine rich is not altitude gain but the unloading of the engine in flight.

Alcohol type fuels have a lessor air fuel ratio than a gaoline powered engine. Therefore, they are very sensitive to mixture adjustment. That's whay they have needle valves instead of fixed jets. You make changes in your engines mixture on the ground relative to the current density altitude. The difference in height that the model flies is not significant.

Enjoy,

Jim

Jim

RE: I would like to dispel a myth here.
 

Homer,

I think we all mis-read your original posting...or at least we though you meant something else.

Don't think badly of us, we've all inhaled a lot of fumes over the years and I was sniffing fuel fumes before they took that dangerous stuff out that can affect your memory..can't remember what it was called. It eliminated that crackling noise at high RPM.

RE: I would like to dispel a myth here.
 

Thats NOT true. That line is a just simplistic way to get students to understand that a correct power setting is important in a correct approach, but its not true. Elevator is the most effective, responsive, and most immidiate way to control altitutde period, even on approach. To get a correct glidepath to landing, you must fly the airplane at the correct pitch, and you correct pitch with elevator. If you adjust power, it will have some effect on your sink rate and EVENTUALLY affect your pitch, but pitch is by far the most effective way to keep the descent rate you want to landing. If you pitch the airplane 10 degrees nose down you are going to go down no matter how much power you add [X(]. If you are doing a deadstick landing, again elevator controls altitude even though you cannot contorl power. If you want to climb, all the power in the world will not make the airplane climb if you do not pitch it up with the ELEVATOR ! Now power and elevator and speed are all realated, but that gets complicated for some people to understand , so they try to over simplify it for students and keep them from overcontrolling the airplane by telling them the what W8YE said... So even though I understand why they say that, the statement above is just not correct.

RE: I would like to dispel a myth here.
 

More clearly, I establish a glideslope and control it with both pitch and power.

Yes, I do. Every time

Personal attack, made through a desire to discredit- very unbecoming of a moderator. Don't assume that you know me, or that you know everything.

My point, exactly. Thank you!

RE: I would like to dispel a myth here.
 

Thanks, Dan! It was lead..... or was it LED ??? "I never inhaled, I swear!"


Jett- the relationship between pitch and power is definitely a synergy, and balancing the two is important for any precision manuever. Ever fly any chandelles? That is a great way to learn pitch/power management outside of the pattern.

RE: I would like to dispel a myth here.
 

Elevator does NOT control altitude! Try giving "up" elevator at idle and see how long your plane climbs.

Dr.1

RE: I would like to dispel a myth here.
 

Dr.1 -- You are correct. Elevator only controls angle of attack, or pitch.

RE: I would like to dispel a myth here.
 

Yeah but he said a combination of controls does.

"Now power and elevator and speed are all realated, but that gets complicated for some people to understand , so they try to over simplify it for students and keep them from overcontrolling the airplane by telling them the what W8YE said"

RE: I would like to dispel a myth here.
 

Here is a way to visualise the pitch/power relationship: Say you are flying straight and level at 100knots, and you reduce power for a 500fpm descent. If your aircraft is trimmed for 100 knots, and you do nothing but reduce power, it will relatively quickly establish a 500fpm descent while maintaining the same airspeed.

Now- if you re-trim your aircraft for a 60 knot airspeed, and keep you power the same, you will briefly slow/stop your descent while decelerating. But this is the point- once established at 60 knots, the plane will resume its initial descent rate of 500fpm (approximately, with corrections needed for drag, etc. involved in the configuration change).

So what you are really doing on final is a subtle juggling act between descent rate and ground speed, controlled with pitch and power.

RE: I would like to dispel a myth here.
 

Example #2:

You are straight-and-level at 100 knots, and you add full power for a climb. You will briefly see an increase in speed, but the aircraft will quickly settle down to 100 knots again, and you will have established a climb rate of "X"fpm. Hey, better lean that motor.....[sm=eek.gif]

RE: I would like to dispel a myth here.
 

"Say you are flying straigh and level " I think you mean "at altitude";)

RE: I would like to dispel a myth here.
 

Better lean that motor.....:D