How does a cox texaco spin a bigger propeller

Thanks i was just curious. But I thought it was something to do with the cylinder head.

mad monky wrote:Hi

Basically, i just want to know how a texaco can turn a bigger prop and that's it.

Thanks

Slowly  

mad monky wrote:Thanks i was just curious. But I thought it was something to do with the cylinder head.

That is just for increased cooling due to more load.

mad monky wrote:Hi

Basically, i just want to know how a texaco can turn a bigger prop and that's it.

Thanks

From what I know.. "Texaco" is a free-flight endurance event. Any of your Cox .049 engines are capable of turning a larger prop. The "purpose-built" Cox Texaco engine's venturi is sized/designed to "sip" fuel from a regulation size integral tank, and I believe has a special glowhead with extra fin(s) for additional cooling because of the load of the larger diameter prop. I also believe that the engine's drive-plate has a "boss" to fit the larger prop's hub.

Check out this link; http://cmac.synthasite.com/cox-babe-bee.php

I have bushed the hubs of wooden 8" x 4" props and run them on Cox .049 reed-valve engines flying small control line biplanes for exhibition purposes only. The airplane won't have much power to "stunt"... but it flies with a more realistic scale speed and sound. The downside to running a prop this big, is that it will make the engine run hotter and also stresses the ball/socket joint of the piston/connecting rod. I would usually run the engine on the "rich" side when doing this. The ball/socket joint can be reset with a special tool that Cox used to sell. Resetting the joint is something that should be done if excessive "play" is noticed when you have the engine taken apart on the bench.

With 5%-15% nitro fuel, Texaco can run nicely with longer duration. I also run my Black Widow with 8X4 prop, and add 3 extra shims under the glow head. Almost any reed valve engines can do this.

Extra gaskets gas???

i know they will reduce de compression, but how does it affect the engine?? i been using some cox with 3 gaskets, as i really love slow planes, so i fly them with 3 gaskets on plug, 10-15 % nitro and 7x4 props. but... again, how affect to reduce the compression?

noticed that sub induction Cylinders are harder to start.... any idea??

Best regards from mexico!

I used to run 8" diameter (8"d x 3p) on my Babe Bee engines. I used only "one" head-gasket. The props were wood; "J-Zinger" 8 x 3's.. with the prop-hub bushed to fit the Cox 049's 5-40 prop-screw. Fuel was 25% nitro.. and "needling" with the stock needle was "touchy". I had no problems starting the engine though. It will "4-cycle" with enough fuel. You need to close the needle-valve very slowly to achieve a 2-stroke cycle "on the ground". The adjustment between 4-stroking.. and 2-stroking is VERY small. You may need to "richen" the mixture slightly.. to allow for when the engine "unloads" in the air.

IMHO overpropping any engine will cause it to run in off-optimum condition i.e losing power....the Texaco duration contest - as far as I understand -  may have been about the length of the flight on a metered amount of fuel, and the reduced performance of the engine will cause it to run for a longer time on the same amount of fuel.




If you mount a prop on an engine that will allow it to run anywhere below (see my theoretical, overpropping  red prop curve intersecting the TD049 engine curve at 70 Watts output  at 17,5k rpm) where the peak of its performance curve is shown, you are losing power...this is why e.g. in the TD manual sheets it is mentioned that your TD is the happiest when it runs at high rpm so do not overprop it..(the optimim prop fit to the TD049 engine yielding maximum output of of 80 Watts at about 22,5k rpm would be the dark blue one..with smaller dia and/or pitch than the overprop represented by the red curve)

The extra fins on the Texaco are needed to boost cooling, IMHO not because of the increased load (the engine will in fact generate less output because overpropped, see the curves above), but because of the longer cycle  time (due to lower rpm)the hot gas remains inside the cylinder before the exhaust begins, and the longer time allows more heat to be dissipated by the cylinder wall and glow head, turning these components hotter than at higher rpm-s...another factor reducing the thermal efficiency of the Carnot cycle that best works in an adiabatic (i.e. zero heat communicated between the expanding gas and the cylinder it is captured in) expansion..

Sorry for the theoretical explanation

No need to apologize, balogh, I see where you are coming from. Being the nerd that I am, probably explains why I like the technical. Also might explain why I like playing the sax.

I think a misnomer in efficiency is considering engine only and not the entire system to include the aircraft, flight envelope, engine and atmospheric conditions (wind, elevation, temperature, humidity, etc.).

A larger, draggy aircraft flying at slower speeds might perform better with a larger propeller at slower rotational speeds even though the engine is not operating at its optimal performance HP. This of course is without a suitable reduction gear box. Thus, I think is the impetus for the birth of the Cox .049 Texaco.

Back in the 1970's, I observed my rudder only 27" (685mm) span Top Flite Schoolboy flew at the same speed without trim changes but with about a minute more flight time by fitting the Cox .020 Pee Wee with a Top Flite white nylon 5.25x3 prop instead of the more "optimal" Cox 4.5x2 prop.

Thanks but do not say you are a nerd because you definitely are not  ... but if you say so it makes me think my explanation was an offense though I simply wanted to interpret the effect of overpropping based on what I learnt in school...

I will not involve myself in technical stuff here anymore because I may be seen as too bookish. One may want to blame it on written communication that is far more prone to misunderstanding than oral, especially because I am a non native speaker, being at a disadvantage in properly explaining myself on a predominantly US forum.

(As I said this I expect a truckload of red marks on my post as a sign of new misunderstanding   )

roddie wrote:I used to run 8" diameter (8"d x 3p) on my Babe Bee engines. I used only "one" head-gasket. The props were wood; "J-Zinger" 8 x 3's.. with the prop-hub bushed to fit the Cox 049's 5-40 prop-screw. Fuel was 25% nitro.. and "needling" with the stock needle was "touchy". I had no problems starting the engine though. It will "4-cycle" with enough fuel. You need to close the needle-valve very slowly to achieve a 2-stroke cycle "on the ground". The adjustment between 4-stroking.. and 2-stroking is VERY small. You may need to "richen" the mixture slightly.. to allow for when the engine "unloads" in the air.

That's why you need head shims. The optimum compression setting is highly dependent on the prop load. The larger prop needs a much lower compression setting. This is very obvious to anyone that is used to model diesel engines.

Ideally even glow engines should have an on-the-fly adjustable head.

balogh wrote:Thanks but do not say you are a nerd because you definitely are not  ... but if you say so it makes me think my explanation was an offense though I simply wanted to interpret the effect of overpropping based on what I learnt in school...

I will not involve myself in technical stuff here anymore because I may be seen as too bookish.

(As I said this I expect a truckload of red marks on my post as a sign of new misunderstanding   )

Hey Balogh,

I'd highly encourage and request that you continue your posts as before, as your thoughts and opinions have always been a plus here.

GallopingGhostler's reference (I'm sure) to being a nerd is just humorous way to for us to point out that we DO like technical posts, and wasn't intended as a complaint.

This really is an international group of enthusiasts, and I'm confident that over the years, we've all learned to interpret our members' intentions, regardless of the sentence structure of their posts, and at most just asking for a clarification for the purpose of better understanding. I'm sure (and appreciate) that you have the same challenges with the way we form our posts.

This has turned into a particularly interesting thread (speaking for myself), and I DO hope you choose to participate just as before.

Kim

Kim,

thank you for the encouragement, in fact I am a bit embarrassed as I was really not fishing for compliments. I will stay as a reader of the CEF posts and will focus my occassional contributions to where I believe it fits a wider circle of CEF member's interests.

When posting on any technical issue I always hesitate how to phrase that - and my Hunglish needs to be factored in as well for the best clarity - not to seem arrogant or too bookish.

This is a great site and I learnt and learn a lot - on COX and other issues too.

András

balogh wrote:I
The extra fins on the Texaco are needed to boost cooling, IMHO not because of the increased load (the engine will in fact generate less output because overpropped, see the curves above), but because of the longer cycle  time (due to lower rpm)the hot gas remains inside the cylinder before the exhaust begins, and the longer time allows more heat to be dissipated by the cylinder wall and glow head, turning these components hotter than at higher rpm-s...another factor reducing the thermal efficiency of the Carnot cycle that best works in an adiabatic (i.e. zero heat communicated between the expanding gas and the cylinder it is captured in) expansion..

Sorry for the theoretical explanation

That doesn't sound right though. The lower rpms means that you have fewer events per unit time, hence less power and heat is produced. The real reason the engine gets too hot is that the ignition comes too early when the prop is too big (and the compression ratio is not readjusted for that prop).

These thing are pretty obvious when one runs a diesel engine. A larger prop is a lower load and cooler engine if the compression setting (a lower one) is optimised for that prop. If one were to simply keep the compression setting of a small prop when hanging to a larger one, the engine will overheat simply because the timing is wrong.

This has very little to do with the carnot cycle...

OK, you guys....its alright if you explain things that go right over my head! I will learn something just by reading the posts, so don't fret about getting to technical or nerdie about something. Just remember....I like pictures and videos too! Like I just learned how to start a 1/2A engine from that post about SMALL. Keep it up!

Thanks Kris.

IMHO yes and no. The lower speed with larger prop increases cylinder and head temperature ( imho as I tried to describe above) and this will cause ignition to occur sooner..not the other way around ....I agree the cure is to reduce compression by additional shims.

The temperature rise of the cylinder wall is proportional with the heat flux and the time the wall is exposed to heat flux from hot gases in an expansion cycle which is longer if the rpm is lower...thus the heat loss during expansion is more if the rpm is lower and the ideal adiabatic expansion is thus compromised, reducing the thermal efficiency of Cannot cycle ....at least the way I deduce it for myself.

But let's stick to our own explanations as we like it.

Hi Guys! this had clear my mind a lot! and make a lot of sense to put a diesel engine in comparation....

i really love FF and RR go slow....

somebody wanted videos?!?!?!











ahhhh!!!! by the way!!! why engines with no subinduction work better with this kind of Texaco mods???

BUT

THIS IS S L O W !

a nice FF rubber model with HUGE propeller!
love my korda!



enjoy!

This discussion with larger props and reduced compression is interesting.

My Fuji .099S-II ABC RC engine (cross scavenge), essentially an Enya clone internally (same style cylinder liner) has only a 5.5:1 compression ratio, whereas my similar Enya .09-III TV has a higher 7.5:1 ratio. Bore x Stroke: The Fuji has a 0.500x0.500 inches (12.7x12.7 mm), Enya 0.512x0.480 inches (13.0x12.2 mm).

Fuji with lower compression has a slightly longer stroke than the Enya. It is interesting that although props for the Fuji are 7x4 to 7x5 for application, for break in an 8x3 is recommended. I'm thinking an 8x4 might be a good ticket considering the lower compression, even though manufacturer does not show it.

Or, run it on racing fuel.

balogh wrote:
The temperature rise of the cylinder wall is proportional with the heat flux and the time the wall is exposed to heat flux from hot gases in an expansion cycle which is longer if the rpm is lower...thus the heat loss during expansion is more if the rpm is lower and the ideal adiabatic expansion is thus compromised, reducing the thermal efficiency of Cannot cycle ....at least the way I deduce it for myself.

I guess your are thinking of the Otto cycle, and not the carnot?
(carnot is the heat cycle that one has in sterling engines)

The heat loss to the walls in the adiabatic exansion is often neglected as the gas is cooling during the expansion anyway. The most important effect in terms of the heat cycle in our engines is the maximum temperature reached in the cycle, the combustion temperature. This is something that we affect when we turn the needle valve setting. A rich setting will not give give good burn, thus lower temperature at the high point in the cycle and a low power from the engine. I.e. lower rpm and lower engine temperature etc

Thanks Kris,

I did not mean the Otto cycle but the Carnot cycle (of internal combustion engines, thermal power plants, open cycle gas turbines etc....where thermal energy converts into mechanical energy as per the 1st  law of thermodynamics) whose thermodynamic efficiency rises if the average combustion i.e heat input temperature increases (and/or the heat dissipation temperature sinks.). Efficiency = 1 - (Tout/Tin)... I think we are talking about the same thing.

If the expansion is not adiabatic the heat loss from the gas will cause its pressure to drop (more than in an adiabatic expansion)  and thus the mechanical work it exerts will also drop...you convert the internal energy of the gas partly not by drawing mechanical work from it but by drawing heat through convective dissipation.

I am afraid these theoretical debates are getting too boring to the readers on non-technical baclkground here...

Where to start?   In order to not get too technical, a practical application of the Carnot Cycle is with refrigeration equipment. Refrigeration is basically the Carnot Cycle in reverse.

thermodynamics-engineer.com Reversed Carnot Cycle

Otto Cycle best describes our gasoline internal combustion engines, but I believe it also describes our glow engines, because combustion occurs with sufficient methanol comes into contact with the glow element made of, alloyed with, or plated with an element such as platinum (Please note, I will not get off on a tangent about other similar catalytic elements, rhodium, etc.) resulting in ignition, versus a pressure process such as the Diesel Cycle.

Basic explanation of the Otto and Diesel are in (but also has a brief on Carnot):

quora.com What are Carnot Otto and Diesel Cycle

Otto better describes the internal combustion engine process because the combustible fluid media enters (intake) and leaves (exhaust) the cycle.

Regarding heat gain through transfer by the fluid media, the expended glow fuel / air charge:

For example, take the Cox .049 Black Widow. Following link has Peter Chinn's August 1974 Aeromodeller engine test:

sceptreflight.com Model Engine Tests/Cox Black Widow

The BW exhaust is open 132 degrees of the crank rotation (66 deg. before bottom dead center (BBDC) to 66 deg. after bottom dead center (ABDC). Transfer port is open 108 degrees (54 deg. BBDC to 54 deg. ABDC). That means the exhaust port is closed 228 degrees (114 deg. before top dead center (BTDC) to 114 deg. after top dead center (ATDC).

We'll make a simple assumption that ignition occurs at top dead center (TDC, because we don't have that value and for the purposes of academic discussion). The 100% ignited fuel charge has 114 degrees of crankshaft travel prior to the start of being exhausted. then the fuel charge remaining it tapers off until the crankshaft travels another 132 degrees. For academic discussion, we'll approximate the linear relationship by assuming all the exhaust remains until half that travel or 66 degrees, for a total of 114+66 = 180 degrees.

At 15,000 RPM say with a 6x3 prop, ignited fuel charge gases would have a contact time of 0.002 seconds. At say 7,500 RPM say with a 8x4 prop, time would be 0.004 seconds.

OK, we have half twice the time exposure for heat to be transferred from the exhaust gases, but then we have half the "ideal" fuel consumed, too. Logically, one would think that because the stored heat was reduced in the 0.002 seconds left with the larger prop, there is less heat being transferred to the engine prior to exhaustion.

Of course this explanation ignores the adiabatic cooling of the rapidly expanding gases when allowed to exhaust quicker with faster RPM, and no doubt contributes to heat gain by lengthening the exhaust process with the slower RPM of a larger prop.

One thing not discussed is the closed system as a whole, which includes the propeller and cooling fluid, the air. An aeroplane flying at the same velocity with two different propellers will experience different velocities of air across the engine cooling fins.

An 8x4 prop has an area of 3.1416/4x8^2 or 50.2 sq. in. 6x3 has 3.1416/4x6^2 = 28.3 sq. in. To move the same mass of air, less velocity is required of the 8x4 versus the 6x3. It is not hard to imagine that the amount of air molecules travelling over the engine cooling fines is almost halved by the larger propeller.

The accelerated flow of air caused by the smaller prop does help to contribute to cooling.

balogh wrote:
I am afraid these theoretical debates are getting too boring to the readers on non-technical baclkground here...

What, me??  Confused??