1995 200hp Variable oil ratios.

[TownsendsFJR1300]

Why does it have to be less?

Ever played with a pinwheel as a kid? The propeller is not connected to or turning anything. When held out of a car window at 30 mph the spinning blades will create muccho drag. Moreso when spinning than if not spinning.

A propeller in a free turbine gas turbine engine is not directly connected to the main engine. Walk up to one on a ramp and you can turn the propeller with a single finger. If the engine fails the propeller will wind mill just like the pin wheel. In an air plane if the propeller can not be feathered to stop it from turning the outcome will not be good.

I was referring and posted a direct drive prop to engine(crankshaft) set up, like in most boats.

Are you saying that a spinning, free wheeling prop (or spin-wheel) causes more drag than a static one (while underway) ?

And its even worse under water as the density of the water is MUCH greater than the air..

[boscoe99]

[QUOTE=TownsendsFJR1300;116016

Aviation with a DIRECT DRIVE PROP TO ENGINE, it won't free wheel with a dead engine obviously and cause major drag. [/QUOTE]

Oh but it will. In a single engine direct drive piston or gas turbine engine the forces on the propeller are more than enough to rotate the engine. This is exactly what happens. In a single no problem. Just more drag and the plane won't glide as far due to the wind milling drag. In a multi-engine air plane the propeller has to be feathered to stop the rotation of either the propeller/engine combination or just the propeller alone.

[boscoe99]

Yes of course (smack to side of face), a gyrocopter has a windmilling propeller that is not driven by an engine. It produces lift when rotating; one cannot have lift without drag on an aerofoil. Does that also mean drag produces lift?

You see I have difficulty associating a water screw (propeller) as a having blades with aerofoil sections. If blade strength was minimal a water propeller would only consist of thin blades proportionally curved to push the water (solid medium) the same distance from the centre out to its fullest diameter.

A water propeller is designed to screw through a solid medium. It's push or thrust is very much purely derived by pushing a certain amount of water mass out at a certain velocity. An aircraft propeller I would think derives a significant pull (vacuum created on front of aerofoil section) as well as the push of air (significantly lower mass than water and compressible that water isn't).

Thus on the point of drag, I would think where a water screw is milling the water speed over the blades would be far less than when it is not turning - almost zero if there is no slip. this very small amount of water flow over the blade therefore produces less drag. Drag being friction between two surfaces exponentially increasing with speed.

Conversely, a stationary prop has a large water flow over the blade surfaces and therefore must create a lot of drag. There must also be a lot of turbulence behind these blades as well adding to the drag!

However, do I hear someone say, turbulence on an aerofoil reduces drag (vortex darts, rough paint etc on leading edges. are commonly engineered into wings).

I say water does not act like air because it is incompressible, so no comparison can be made.

Things to consider:

Both water and air are fluids. They are not solids. Above the freezing point of water that is. Propellers work well in fluids. Not so much so in solids. Although a wood screw will move forward in a solid, a water or air propeller achieves its thrust mostly by grabbing the water or the air and forcing it violently rearward. High pressure on the face of the blade and low pressure on the back side of the blade helps.

Starting at the inner most point of a boat propeller blade slice out a very narrow section. Maybe a mm thick. It will be air foil shaped. Repeat the cuts until all the way to the tip of the blade. The sections will all be air foil shapes. Different shapes of course, because the tip of the blade moves faster than does the root of the blade for a given RPM.

[99yam40]

dang this oil ratio stuff is confusing, aint it

[boscoe99]

dang this oil ratio stuff is confusing, aint it

I am working a thread on a different forum wherein fuel flow rates and spray patterns as a function of different fuel pressures are being discussed. Wish I had studied physics as a career.

Now in the case of the oil ratio we got engine rpm changing, the intake manifold pressure changing, the oil pump volume changing, and goodness only knows what else variables there may be. Someone wants a simple answer such as "what is the oil to gasoline ratio at various rpms?" Hell, this stuff is not easy to answer as one might expect.

[TownsendsFJR1300]

Bottom line,

its a 2 stroke, it smokes more with pre-mix, very simple...

Either hook up the oil injection or get a 4 stroke, simple!!!

[zenoahphobic]

I am working a thread on a different forum wherein fuel flow rates and spray patterns as a function of different fuel pressures are being discussed. Wish I had studied physics as a career.

Now in the case of the oil ratio we got engine rpm changing, the intake manifold pressure changing, the oil pump volume changing, and goodness only knows what else variables there may be. Someone wants a simple answer such as "what is the oil to gasoline ratio at various rpms?" Hell, this stuff is not easy to answer as one might expect.

If I remember correctly, the OP engine oiling system (atleast prior to when he purchased it) was a crankshaft driven (RPM dependent) oil pump, carburettor linked to vary the volume output, oil tubes from the pump to the crank bearings, and oil tubes to the throats after the carburettors. How on earth does this system know what fuel to oil ratio is? It doesn't mix all the oil with fuel anyway! So Boscoe is absolutely right, it is not easy.

[zenoahphobic]

Things to consider:

Both water and air are fluids. They are not solids. Above the freezing point of water that is. Propellers work well in fluids. Not so much so in solids. Although a wood screw will move forward in a solid, a water or air propeller achieves its thrust mostly by grabbing the water or the air and forcing it violently rearward. High pressure on the face of the blade and low pressure on the back side of the blade helps.

Starting at the inner most point of a boat propeller blade slice out a very narrow section. Maybe a mm thick. It will be air foil shaped. Repeat the cuts until all the way to the tip of the blade. The sections will all be air foil shapes. Different shapes of course, because the tip of the blade moves faster than does the root of the blade for a given RPM.

Boscoe you got me thinken a while back when you said a spinning prop appears like a disc (presumably perpendicular to air/water flow and therefore a direct face on resistance). Looking down a boat prop from behind it does appear to be so, but absolutely not so with a aeroplane prop.
My initial thinking was a prop blade width is design to allow the air/water to leave on its way including turbulence before the next blade comes along to do the same.
I look at windturbines, three very thin blades (yes I know you can climb inside them) with a lot of space between them (80%?). It is the turbulence that affects efficiency and stability of speed.

One way I understand an aerofoil is that the distance on one side is different to the other and both ends are somewhat pointy. Boat props not! O/B props have a flat perpendicular trailing edge! Can't do that with an aeroplane prop.

I am with Townsend here, purely based on two observations I had made many years ago.
1. anchored on the edge of a sandbank with a tide flowing I was better abled to steer away from the bank with the engine in gear (prop locked).
2. my father when approaching shore (WOT) would have me poised on the kill switch awaiting his order to cut the engine. His motor (evinrude) had separate leavers for gear and throttle. When he yelled cut he would immediately throttle back and the boat slowed very quickly. He did this to be more precise; whereas previously he would not only throttle back but put it in neutral, the extra drift was taxing on his judgement (he landed on a road once the boat kept going).

I am prepared to be proven wrong and will setup a test with my model props (2inch dia) and see if the rubber band that I will attach the shaft stretches differently with the small prop turning c/f locked.

Nothing like a con*****ed experiment. Oil to fuel ratio you will need flow meters for both oil and fuel as well as a load measurer (dynometer) on the prop shaft, and graph with RPM and throttle position.