Thanks chaingang man.

@ Jeffie,
your post was on the verge of a troll, but you knew that didn't you
I am glad you posted the question like you did. Many go astray on the prop efficiency road, while all the time it is just a matter of matching real prop pitch to plane speed requirements. Again, real prop pitch is not always what it says on the propeller you buy.

Actually Pe I really am not a very experienced modeler and I want to know everything I can to make the best decisions I can. I have had bad luck in the past with an R/C Trainer than flew itself into the ground. It was a multitude of factors , mostly all my fault.
I do have a lot of experience with engines in general since I have an Applied Science Degree in Diesel Mechanics. Glow engines are really just a very simple form of Diesel. (oh I can hear it now, I shouldn't have said that)

Quote:

ORIGINAL: jeffie8696

...Glow engines are really just a very simple form of Diesel. (oh I can hear it now, I shouldn't have said that)

No, you shouldn't have, Jeffie...

While there are several attributes that define a Diesel engine; I believe the most prominent one is its reliance solely on compression to ignite its fuel/air charge, by the heat that results from compression.

Glow engines have been termed 'semi-Diesel' in the past, because the heat of compression intensifies the catalytic reaction of the platinum in the glow-element, on the methanol in the fuel, which in turn causes the glow-element to glow hot enough to initiate the combustion. ...And because some Diesel engines (mostly older ones) employ glow-plugs for cold starting, which are not maintained lit afterward, like those of glow engines...


But in reality, this discussion belongs elsewhere in this forum.

...And surly not in a thread dedicated to propellers...

Dar. I couldn't have said it better myself. [8D]

Dar, I am With Jeff?
I am a ship's engineer, and like IC engines as the spring of my life. A glow engine IS like a diesel, but also much like a Spark ignition engine.
With the model diesel it has in common, that the moment of ignition start is not all that much defined. Indeed this is not the thread to rave about it. The tongue should not always be where the heart goes.

I am surprised no one bit on Dar's comment that when a prop is turning on a plane in the air at a speed that does not generate thrust, the drag is higher than when the prop is stationary. He said the result is that a dead stick landing has a shallower glide path than one at idle. I'm not sure that's true.

This is also really counter-intuitive, so if true, what is the explanation?

It also contradicts the long-standing practice of scale modelers who use rubber bands for power. These guys are constantly trying every trick to squeeze out a little more endurance. And they use free-wheeling props, even when using balsa props which do not have enough momentum to cause much reverse rotation. All they all wrong?

Jim

Quote:

ORIGINAL: buzzard bait

I am surprised no one bit on Dar's comment that when a prop is turning on a plane in the air at a speed that does not generate thrust, the drag is higher than when the prop is stationary. He said the result is that a dead stick landing has a shallower glide path than one at idle. I'm not sure that's true.

This is also really counter-intuitive, so if true, what is the explanation?

It also contradicts the long-standing practice of scale modelers who use rubber bands for power. These guys are constantly trying every trick to squeeze out a little more endurance. And they use free-wheeling props, even when using balsa props which do not have enough momentum to cause much reverse rotation. All they all wrong?

Jim

Jim,


That's because it is true.

The longitudinal drag produced by stopped, aerodynamically stalled props blades, is smaller in magnitude than the longitudinal, 'negative lift' produces by this prop, as it spins significantly more slowly than needed, for forward flight at the same speed.


If you want the longitudinal parallel; a plane descending in a harrier maneuver, will lose altitude much faster than the same plane gliding...

This is because the stalled wings (in a harrier) make less lift than the lifting wings (in a glide).


Windmilling props make even less drag than a stalled prop; certainly less than a prop forced to spin significantly more slowly than necessary...
This is why the rubber guys let the prop windmill, rather than holding it stopped.


An idling prop will not be spun much faster by the ambient air it is flowing through - it does not windmill!

A windmilling prop, on the other hand, will approach pitch RPM speed.

Of course an idling prop doesn't windmill. But at some rpm an engine on idle will be equivalent to a windmilling prop. And of course there is always some friction involved, so obviously the rubber fliers prefer a prop turning at some speed that creates drag, but less than what is created by a prop in a stalled condition. I didn't really expect to have to explain that.

So picture an engine running at low rpm, not producing thrust, equivalent to what it would turn if it were windmilling in a low friction condition. Now slow it down just a little. Obviously it will produce a little more drag than at windmill speed. Is that amount necessarily more than the drag of a stationary prop in a stalled condition? Of course not. One can slow it down slightly and there will be more drag than when it windmills, but less than when it is fully stopped.

Now slow it down some more. More drag? Well, if it is still not in a stalled condition, there will be more drag. To use your words, more "negative lift". Now, it may be that a stopped prop in a stalled condition produces less drag than a moving prop in an unstalled condition if the rpm is low enough. But not at ANY rpm below what is required to produce thrust.

Also, at some rpm, a prop turned by an idling engine will nevertheless be in a stalled condition. Is a prop that is turning in a stalled condition draggier than a prop that is stationary in a stalled condition? I don't know, but I can't see why it would be.

I am willing to believe that under the right conditions an prop turned by an idling engine could be draggier than a stopped prop, but if so, it will be true only under particular conditions that depend on the prop characteristics, including its pitch, and the speed of the plane, and will only be true through a particular rpm range that may be much narrow than the range we consider to be at idle.

Jim

Boy, Dar....I really wanted to stay out of this one and you make it too hard to do that.

Okay, I'm sort of nickpicking here, but it probably needs to be said...your statement is untrue for a rotor blade in autorotation. In fact, doing an autorotation with 180 degree turn will normally accelerate a properly tracked rotor blade past redline. The forces buildup is dangerous, and you never forget the whining/screaming sound as blades and drive train approach limits. It is a sequence of events beginning with overspeed, increased load on the blades in the turn, and coning of the blades (coreolis effect).

So, can this happen in some miniscule fashion with an airplane propeller?

Barry,


I have edited my previous post...

A prop on an idling engine will obviously speed up somewhat; increasing the RPM of the engine with it.

But by no means will it spin as fast as it would when windmilling (when the plane is flying significantly faster than in 'idle flight'), or like a helicopter rotor during auto-rotation.


In fact, I have emphasized this in some past thread... If you use a prop with too low a pitch, your plane could (aerodynamically) stall on the final leg of the landing approach, as you chop the throttle; if 'idle flight' speed is lower than the model's stalling speed.