Three bladed r/c prop?

I don't have a bass boat. Don't have the patience to fish. :) Depending on design, a prop will cavitate more easily in air than a prop will in water. Air is less dense than water, as I'm sure you know. Dr.1 Driver "There's a Hun in the sun!"

SNIP

SNIP

Ah, I beg to differ with you fidelity comment. . significantly.

There is absolutly no way any of the model aircraft you fly are any where near 'scale' in the power to weight ratios and THAT is what you are implying with your "fidelity" remark. Here is some fast and easy proof you can look up on the web.

The Cessna 152 has a gross weight around 1600 or 1800 pounds (been a few years) and a 110 hp engine on the front of it. That means (being generous now) it has a 1.6 to 1 weight to power ratio, or as is more commonly spoken about a 1/14 to 1 thrust to weight ratio, where I actually suspect that the p/w ratio is actually more like a 1/16 to 1 value. Most early fighters had large engines, but the p/w values could quadruple and STILL not match those of the average trainer (1 hp and 5 pounds or 1/5 to 1 p/w ratio). Let's see, 4 times 1/14 to 1 is about 1/3:1. Bet your DR1 p/w ratio is much higher than that.

That is the important issue, rather than all the interesting but boring discussion about the (politically?) "correct" prop. To truely get 'scale fidelity' your take off run would have to be something like 40 times the length of your fuselage. Measure and calculate that number for a real eye opener. The first time I watched one of these conversations, I did and got a laugh. Many folks don't even fly from fields that would support SCALE take offs and landings. Something about obstacle clearance. . .

Now as for some model possibly being unmanageable with a multi blade prop on it, that actually speaks to piloting skills rather than power or prop issues. If you are smooth enough and observant enough, you CAN fly a model nearly like a full scale bird. The first problem is that doing so is NOT FUN. The second problem is that you need more on board information than you currently have, in general. The list of impediments goes on, but I think you get the gist. That makes your final remark in the quoted segment on target if you add "for you" at the end.

SNIP

There has been much reference to "thrust" in this thread, plus swept area etc. in the argument between propeller efficiency.

I concur that fine pitch props give greater acceleration and high pitch more speed for the same diameter with the same engine.

However, there is an opinion (not mine) that a propeller is a rotating wing, and, like a wing, develops lift which creates the forward motion.

This might be born out by one of my models - a Sopwith Pup of around five foot wing span. The power plant is an ancient Merco 61 swinging a 12 x 6 prop. The cowl diameter is at least 8 inches so that leaves two inches of each blade clear of the cowl - not much to generate thrust.

However, this very draggy biplane flies very well and, IMO, in a scale like manner, and full throttle is only used for take off.

Just my two pennorth FWIW

Malcolm.

Cavitation, n. 1. the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure. 2. such a pocket formed in a flowing liquid.

FWIW, air in liquid phase is not generally of much interest in a discussion of the dynamics of airplanes and their propellors.

Anyhow, if we can go back to the query that started this thread, the topic was availability of 3-bladed propellers in model scale, posed by a fellow who clearly stated his interest was in scale fidelity of his P-40 model. The simple, factual answer is YES, and I unfortunately embellished my reply with words to the effect that it will work okay. Having ventured beyond that simple reply to the question asked, perhaps I should have qualified it further, thusly: " A 3-bladed propeller sized as I recommended to produce equivalent power loading to the engine mfgr's recommended 2-blade prop may require a slightly higher power setting to produce equivalent thrust. I have made the assumption that the engine you have chosen is capable of producing power somewhat in excess of the bare minimum required to keep the model airborne."

Does that work for you?

Abel

Ah...air is considered a compressible fluid for most purposes. It DOES cavitate. That came from a degreed chemical engineer with many years in the design field.

Actually, it does. By using a larger-than-recommended engine, a 3 blade prop may be properly sized and probably used successfully. Dr.1 Driver "There's a Hun in the sun!"

Too bad YOU didn't catch that the first time, wizard. Dr.1 Driver "There's a Hun in the sun!"

I'm not "implying" anything, I spoke clearly and distinctly, and said what I meant. I referenced scale "fidelity" to what the original question was about; the appearance of the plane on the ground and in the air. I referenced the word "fidelity" to nothing else than appearance. Dr.1 Driver "There's a Hun in the sun!"

It does generate lift which combines with with the AOA (read pitch). These two work together to generate forward motion.

Actually, that sounds like a good setup. Dr.1 Driver "There's a Hun in the sun!"

Well then, maybe your degreed CE can tell you what difference it makes when air transitions from vapor phase to vapor phase, or alternatively whatever else it changes into??? Perhaps he can also explain that fluids by definition include both liquid and gaseous substances.

I glad of that.

Abel

CG,

It seems to me as YOU are the one with the arrogant attitude. If all who disagree with you must leave before the door hits their backside, who is being arrogant? I for one, learned a lot from the preceding discussion and although it went far beyond the original posters intentions, the points brought out were pertinent and, at least to me, interesting enough to keep reading this thread. If you don't like what is being said, please find a more tactful way of expressing your displeasure as I'm sure others may have contributed more had you not castigated others in the scathing manner you chose. There are enough brusque people in world without adding to the pool.

Just my thoughts

Jim W

I didn't catch it because it didn't matter. I have no interest in analyzing your irrelevant numbers, they don't matter. If you'd bother to read, rather than pontificate, you would have figured this out already.

The first time I said that it was in response to DR1's arrogant game of lecturing and then saying "end of discussion". Polite people do not state their opinion and then tell everyone else they are not going to listen to their responses. The second time I used that line was in response to rudeness from flyrcalot. If someone is rude to me, I will respond in kind.

You can learn just as much, in fact more, by searching archives. It's all been discussed many times before.

I am quite tactful when people are polite. I am less than tactful when people are rude.

Understand.

SNIP..... you, with your

Plonk

Whatever

snipped-for-privacy@aol.com (Dr1Driver) wrote in message news:...

Ah, but pitch and AOA are two different things. Pitch is the theoretical distance a prop would move through the air in one revolution if there was no "slip." Angle of attack is the angle formed by the blade chordline (at any given point along the blade) and the direction of the incoming air. When a prop is turning but not moving forward, the AOA is max, but not quite as high as the blade's pitch angle, since the air is still moving into the blade and reducing AOA somewhat. The inboard area of the prop is stalled in most cases, since the pitch near the hub is so high and the AOA is past the stall angle. This hurts takeoff performance. In cruise the AOA drops to between 2 and 4 degrees, at which the prop is most efficient, and any further speed must come by increasing RPM which increases AOA and therefore thrust. There's a point at which the airplane will go no faster simply because the AOA is too close to zero. And that's the reason for variable-pitch or constant speed props. Their blades rotate in the hub to give a low pitch for takeoff and allowing the engine to reach redline and therefore max horsepower. As forward speed increases, the blades increase in pitch to maintain a higher, more useable AOA and a resultant higher cruise speed. A fixed-pitch prop is like having only one gear in your car's transmission. Has anyone built an RC constant-speed or variable-pitch prop yet? The results would be outstanding performance. (Not that models don't already have spectacular performance!) The CS prop would make scale three- and four-blade props much more worthwhile and sensible on scale models. And expensive, too.

Dan

Take the rotation and straighten it out and damned if that pitch ain't AOA then. It generates "lift" in a direction perpendicular to the axis of rotation. Dr.1 Driver "There's a Hun in the sun!"

Dan- Don't you go creating confusion here by presenting facts (oops, looks like you already did). Re variable pitch props, yes somebody has built them. They have even turned up in the marketplace a couple of times, but soon disappeared. My old Futaba FP-8SGAP, which was popular among F3A fliers late 80's to early 90's, has a mixing mode specifically for controlling a variable pitch prop. The manual shows a set-up example with a VP prop made by MK.

Abel

AOA changes with three factors: prop pitch (which is fixed in most cases), propeller RPM and forward speed of the airplane. AOA, for instance, goes negative when the engine is idling and the airplane is gliding. It gets really negative when the engine quits. It is at highest positive values at full RPM and the airplane isn't moving. Or maybe even higher at a full-throttle tailslide. Changing pitch allows us to change AOA somewhat, but pitch isn't the same as AOA. You are right that it generates lift. The net lift is actually perpendicular to the chordline of the blade, not the axis of rotation, and with the drag vector added into it, the lift angle tips back even farther.

Dan

Thanks Dan for the clear explanation.

To confuse matters even more, AOA changes from the root of the prot to the tip, except for a single condition, where the speed of the airplane matches the pitch for a constant pitched propeller. Note that at rest, the angle of attack at any point on the propeller is equal to the arctan(Pitch/2*pi*r). This means that the angle of attack changes all along the blade. I don't think Gerald really meant angle of attack.

-Fritz