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Engine output variation with ambient temperature Empty Engine output variation with ambient temperature

Post  balogh on Sun Jan 18, 2015 8:25 am

I was trying to replicate the 27+k rpm of my TeeDee 010 which I sound-tached on the ground on a hot summer day of 35C or so temperature.

I understandably could not replicate that today when the ambient temperature in my garage is around 8C i.e. some 27C colder than it was in the summer field. This also translates into some 10% more air density.

One would think air density has a two-fold effect on engine output:

1. Thicker air will burn more fuel in a given air/fuel volume, so one would expect the engine output to rise with the air density.
2. In airplane engines the load from the prop varies linear with the air density and on a cube-base with the rpm.

So one could easily calculate - without the engine output effect under point 1 - that the cold-to-hot weather prop speed ratio is the cubic root of (1/1,1) i.e. 0,968, meaning the prop speed will drop at 8 C ambient to about 96,8% of what it was at 35 C ambient temperature. This would mean some 26,1k rpm at 8C instead of the 27k rpm at 35C.

In fact the prop speed I measured was around 25,5k only, meaning that the effect in point 1 above was not at all in place.

Any idea of someone on the above?
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Post  KariFS on Sun Jan 18, 2015 9:18 am

It could have something to do with air humidity and/or engine temperature. Humidity could make air more dense and cause extra load for the prop. Your engine also most likely runs cooler in a cooler ambient temp and that will reduce the output as a greater part of the combustion energy (=heat) is absorbed by the cylinder and head.

On the other hand, you only got 600rpm less (right?), that could be caused by many things, slight variation of fuel quality, minute difference in the needle setting, castor oil viscosity at current temp, position of the Moon related to Jupiter at the moment, that sort of thing Smile
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Post  crankbndr on Sun Jan 18, 2015 9:23 am

Humidity make air less dense, all that arithmetic goes right over my head, you and Roddie should work on it. lol! Popcorn Beer Cheers

http://www.pilotfriend.com/calcs/calculators/density.htm
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Post  TDbandit on Sun Jan 18, 2015 10:22 am

Yup With cooler air, density goes up therefore requires a richer mixture to run properly prop load goes up as well because there is more air present for it to bite into. Warm air, density goes down which requires a leaner mixture and prop load goes down due to less air to bite into which means more rpm. Now insert humidity into the mix and it screws it all up since it reduces density and also hampers engine performance. There is another factor however and that is Engine Cooling. The Cox 1/2A's have more cooling area per ci than any other air cooled engine which is one of the reasons why they like nitro. On cold days they tend to over cool unless somewhat shielded and their performance suffers due to it. Huh... (Bandit)


Last edited by TDbandit on Sun Jan 18, 2015 10:52 am; edited 1 time in total (Reason for editing : Wrong smiley)
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Post  RknRusty on Sun Jan 18, 2015 10:35 am

Remember cold fuel has higher viscosity oil, especially castor, which inhibits flow. Your 8 degree garage is pretty chilly, so that might be a factor too.
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Post  balogh on Sun Jan 18, 2015 11:29 am

Thanks to all,

I also have thought about it and the explanation lies in where the prop characteristic curve intersects with the engine break horse power curve .
At higher air density = in winter the prop load curve, and the engine break-horse power curve shift higher, and their intersection i.e. the equilibrium engine/prop speed is lower in winter than in summer, see the two red spots in the chart below..(sorry I do not have a scanner at home so I scribbled the additionalk curves on the chart by MS windows, and I can only host it here as an image...)

Engine output variation with ambient temperature Td01_010
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Post  balogh on Sun Jan 18, 2015 12:00 pm

RknRusty wrote:Remember cold fuel has higher viscosity oil, especially castor, which inhibits flow. Your 8 degree garage is pretty chilly, so that might be a factor too.
Rusty

I guess the higher viscosity of fuel that blocks flow in the fuel line is compensated by a wider open NV so it may not have a major effect?
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Post  TDbandit on Sun Jan 18, 2015 1:14 pm

balogh wrote:
RknRusty wrote:Remember cold fuel has higher viscosity oil, especially castor, which inhibits flow. Your 8 degree garage is pretty chilly, so that might be a factor too.
Rusty

I guess the higher viscosity of fuel that blocks flow in the fuel line is compensated by a wider open NV so it may not have a major effect?
Cold fuel effects the whole engine really due to the castor as Rusty stated. in extreme cold conditions, castor can thicken up to the point where it can come out of solution especially in high nitro fuel. I keep my fuel in a warm spot just before using. It also helps to shake it up good before use to make sure everything is mixed good. (Bandit)
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Post  Paulgibeault on Sun Jan 18, 2015 5:58 pm

Hello Balogh,

I think you have to take in to account the ENTIRE weather picture, as opposed to just ambient air temperature. But in order to do that it is most useful to have a portable weather station which gives you this information. You will see these devices all over the pit area at a world C/L speed championship. Some of these sophisticated devices combine many variables including, temperature, humidity, air density and oxygen content together. Sometimes these combined variables are read out on a gauge in per cent of 100. So anything above 100% will give you a faster performance & anything below 100 will give you a slower performance. The truly talented fliers will adjust their engines (usually the mixture needle, glow plug heat range, or even tank position (which adjusts mixture without having to move the needle), based on the reading of their weather station. On occasion they will refuse to fly and pass on the round knowing full well that the weather conditions will not allow them to go as fast as in a previous round. OR conversely, they will pass on their normal round just to re-fly later on in the day when they predict the weather will be (hopefully) better and therefore faster.
At one of my very better championships I was lent a very expensive electronic weather station to use. It takes practice and understanding to use such a device, but it is THRILLING to make an engine adjustment in the pit solely based on your weather station readings & then going faster than ever. It almost feels like cheating that test flying is no longer necessary when using scientific equipment to help out. Of course weather sensing devices are standard at drag racing events, but I don't know if you have drag racing in Hungary. In any event the Hungarian C/L speed fliers all know this & are masters and world champion speed fliers. Good luck with your research.

Cheers, Paul
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Post  RknRusty on Sun Jan 18, 2015 6:31 pm

Thanks for that peek into a speed event, Paul. It's amazing how different the challenges are for executing the various disciplines of control line.
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Post  Cribbs74 on Sun Jan 18, 2015 10:18 pm

Rusty,

I feel the weather has a huge effect on stunt as well. Actually it will play a role in any IC engine run in any discipline.
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Post  RknRusty on Sun Jan 18, 2015 11:00 pm

Cribbs74 wrote:Rusty,

I feel the weather has a huge effect on stunt as well. Actually it will play a role in any IC engine run in any discipline.
Absolutely, but just comparing weather analysis, I have a spinning garden daisy on a cold roll pole and a cellphone. I set the needle till it sounds right and hopefully get a test flight to gauge fuel consumption. The carrier guys have some really strange trim settings and mechanical doodads that I'd hate to have to depend on. Combat. I just meant there's so much you can learn about each type of flying.
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Post  Cribbs74 on Mon Jan 19, 2015 12:14 am

Sorry, I misunderstood.

I see now that you were stating a point not asking for info.


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Post  balogh on Mon Jan 19, 2015 8:38 am

Paulgibeault wrote:Hello Balogh,

I think you have to take in to account the ENTIRE weather picture, ...................I don't know if you have drag racing in Hungary. In any event the Hungarian C/L speed fliers all know this & are masters and world champion speed fliers. Good luck with your research.

Cheers, Paul

Thanks Paul, quite interesting...one would think that adverse wheather conditions i.e. deviations downwards from the 100% you mentioned would equally hit all competitors in a race thus there is no use in picking on the weather parameters to justify why one refuses to fly....

I know quite a number of Hungarian C/L competitors are world-renowned experts, but unfortunately I am a lonesome R/C freak (do not attend modeling events here) and even more lonely as far as my COX obsession is concerned (all nitrofreaks in the field here run much larger engines) so I do not discuss these topics elsewhere than the CEF, and try to invent theories every now and then.
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