Hello All:

     Today, my friend and I had a terrible time to get good engine runs.  The big change was the change from in a high pressure environment to a low pressure one.  Does barometric pressure have a large impact on our engines?

      Stay well,

      Frank

   

     It may affect your needle setting a bit, but not affect your engine runs. Look for issues in the typical places like fuel, tank, fuel filter, etc..

     Type at you later,
    Dan McEntee

     The answer really depends on several factors; The important number is really "Density Altitude " which is effected by temperature, altitude relative to sea level, barometric pressure, and to a small extent by humidity. A small change won't have much effect but a big swing will definitely change the needle setting required to achieve the same type of run.
     For example: our local field at Whittier Narrows in So Cal is at about 210 ft ASL. the average DA is probably between 600ft and 1200ft. On a cold crisp winter morning it can be -400ft and on a hot humid summer day it can sometimes get above 3500ft. A big change like that will affect needle setting (mixture), lift/drag of the airframe, and the efficiency of the propeller. I have seen it change +2000 ft from early AM to afternoon requiring the needle to be turned in to achieve the correct mixture.
     The mixture will vary (higher density altitude will require leaner setting) and even after you correct the mixture, the power will be less at the higher DA. You can retrieve some or all of the power by raising the Nitro content, compression, or venturi size, but each of those can have side effects, and the nitro adjustment is the easiest and most practical. Even with those adjustments, the propeller will still be less efficient and the aircraft will produce slightly less drag and less lift (turn authority in our application)
     Highly tuned smaller engines (F2D .15's and/or 1/2A speed/combat engines) will be affected more than larger mildly tuned broad range stunt engines like .40 thru .77 and engines running very low or no nitro (0-5%) will be effected more than those running 10 to 20% due to the broader mixture range of higher nitro fuels.
   All that said; day to day changes and even week to week in most locations will only require a slight needle adjustment one way or the other, or maybe a slight bump in nitro content for midsummer higher DA's.

Temperature can really effect castor based fuels . Up here in the Great White North  - when I sometimes have a flying withdrawal moment and we get a calm day ,digging out a beater for a couple of flights sometimes seems like a good idea …for a while .

Castor based fuels will not flow unless warmed up prior to flight . Klotz and Ucon are your friends in sub 0 conditions . -10 c and lower means that it’s officially a building day .

Brad

Weather will affect engine performance. But to what degree might be a topic of discussion.

I usually start my day flying pretty early in the morning. It can be 50F at 8am, then increase to 80F by noon, and 95 by 3pm. Not unusual where I live. If I set my needle in the early morning, I do notice the engine becoming 'slightly' richer as the day heats up if I don't readjust the needle. But I do mostly sport flying and the change is really not enough to affect my flying. I do sometimes reset the needle a bit. For sport, I normally don't tach my engine on every flight. Some flyers tack and adjust to the prop speed they want on every flight. 

To me (maybe I am wrong) it is all about fuel-air ratio (or oxygen-fuel ratio). As the outdoor air heats up, the air is less dense and there is less oxygen available to fully burn the set amount of fuel. Therefore, the richened effect. Probably the same thing happens w/ changes in atmos pressure.

For sport flying, not a big deal. But for competition, probably a big deal.

[quote author=Colin McRae
To me (maybe I am wrong) it is all about fuel-air ratio (or oxygen-fuel ratio). As the outdoor air heats up, the air is less dense and there is less oxygen available to fully burn the set amount of fuel. Therefore, the richened effect. Probably the same thing happens w/ changes in atmos pressure.


Correct, but perhaps the richening comes more from the fuel viscosity change as it warms up during the day. Difficult to say which has a stronger effect. L

Agree!
Thx

When it is really cold you have to wear gloves to flip the prop and that makes it harder to start.

Ken