I know you are telling the truth, XJet, but the newbies (those that started flying after 1985) won't believe you. They have been brainwashed into thinking that the more efficient larger props are actually superior to running a toothpick prop, regardless of the airspeed/rpm requirements.


Ed Cregger

It all depends on the engine, the prop, and what you want to get out of it. Xjet hit it right about propping for the power curve.

Select the engine that best fits the model you are flying and how you want to fly it.
If you take two APC type propellers lets say an 11/7 and a 12/6
If you spin a 12/6 at 12,000 RPM it will require about 1.3 horsepower, it will produce around 8.5 pounds of static thrust with an estimated top speed of around 69 MPH
If you spin the 11/7 at 12,800 RPM it will also require about 1.3 horsepower, it's static thrust drops to around 7.9 pounds but it's estimated top speed will jump to 85 MPH.
This spread will become greater at the size difference in the prop gets greater for any given horsepower.
So the bottom line is if you like to go fast pick an engine that turns a smaller diameter at a higher rpm, if you like to hover and 3-D pick one that spins a larger diameter at a lower RPM.
The problem is most companies don't publish RPM's with different props, so places like this are the only real source of information and it tough to figure out.
You would not want to prop your ROSSI to turn a maximum of 12,000, and I don't care what you do with an LA your not going to get it to work at 17.500.
Read through the posts on the different engines and pay special attention the what prop and RPM numbers are being reported, do the math then pick the one that fits you the best.

Seems we are all mostly saying the same thing, but in differant languages.

Dar's explinations like above are a pleasure for me to read.

Where I get lost is when someone says something that doesn't make sence to me, and then doesn't elaborate. The other thing that really gets me is when someone tells ya something, expects you to believe it, but then can't tell ya why? This is not dirrected at anyone other than to thank guys like DarZeelon, RaceCity, Hobbsy and some others for sharing their knowlege.

Xjet, I did think about it Sir...I wrote it!

OnPipe- The ringed engines I've had were all Foxes and all baffel/bbng. engines. (Eagle Is) They required little "break-in". I would expect any ringed engine to seat pretty quick. The non-ringed engines I've had have all run better the longer they were run. (OS,Fox,T/Tiger) One good thing about a ringed engine is that rings can be replaced once worn for little cash. Once a non-ringed engine is worn out it's liner/piston time. Good luch whichever you choose. Just follow the mfg's. break-in procedure and you will have a good running engine for a long time.
BTW, if yer considering an LA-.65, ya better getcha one now before they are gone.[]

In the air, the 14x6 will make the plane go faster than the 12x6 prop in most cases. This is if you are measuring RPM on the ground and find them the same, as was the original question. That is because the 14x6 will usually become more efficient at flying speeds due to the larger diameter. There is a simple formula that shows this without any engine horsepower considerations. When you measure RPM on the ground, you are only seeing one part of the formula. The airspeed of the model is the other part, and you only get that by flying. This explains why things seem so much better or worse once you fly it.

Barry,


The AOA of the blades is one factor in efficiency.

The CL does become greater, so the smaller blades of the 12x6 prop do produce more thrust (compared to a flatter AOA). But their CD, or coefficient of drag increases to a greater extent, causing their L/D (lift/drag) ratio to decline, as AOA is increased.

This is a major factor of prop efficiency and one reason a larger diameter, identical pitch prop, will produce a higher speed at identical RPM.

...But this is really not an issue. The same engine will not be spinning a 12x6 and a 14x6 at the same RPM...

If the model you are flying isn't the sleekest around, using a relatively low pitch prop will provide your model with better performance.


Using a 10x6 prop on your .40-.50 trainer is nearly the worst choice you can make (better only than using a 9x8, or an 8x10 on the same trainer...).
An 11x5, an 11.5x5, or a 12x4 even, if ground clearance permits, will load the engine down a bit, but will provide better take-off performance (shorter run, much better climb-out, never a ground-loop) and better off-throttle breaking, which makes landing easier and shorter.

The cost; a lower top speed, which you have no need for anyway.


So, if the LHS guy tries to sell you 10x6 props with a new trainer, you can tell him that he's a complete idiot...

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And I am not saying otherwise.

Most folks acknowledge that if you want all-out speed from a model airplane, you will end up using a two-stroke engine. How many of us can explain in engineering parlance WHY this is true, without referring to the years of experience they have under their belt? Even many of the acknowledged experts in our forums would be at a loss to explain this commonly known truth in verifiable, scientific, easily repeatable and verifiable terms. Does this mean that their opinions have no merit? Of course not. Experience coupled with proper observation and analyzing habits can yield a terrific amount of information. Yet, we have some folks, especially since the advent of the internet, that always use the "show me in demonstrable terms or reliable information sources" argument in an attempt to put down other folks' opinions with which they do not agree.

Let's put an end to this type of argument. The goal is transparent and has nothing to do with furthering our knowledge on any topic.

From the second paragraph on, this was not referring to you, Barry. And I do not mean it as a put down to anyone in particular. As far as I'm concerned, we are all friends here.

Ed- I was not referring to any specific type of engine.

Dar- yes, this would be best to compare similar props.

Dar, thats what I was trying to say in my post.
Thats why I spun one prop at 12,000 and the other at 12,800, so they would both consume the same horsepower.
I thought that would approximate the effect the prop change would make on the same engine.
Any yes I know there would be a variable because of porting, that was the comparison between the Rossi and the LA.
Now if we could only get the manufactures to publish the torque curve so we know where to start.

Of course...


It would be easier re-inventing the wheel than to ask engine manufacturers to advertize an actual torque curve, which they will need to guarantee...

Claiming huge HP numbers at RPM levels 99.9% of their users will never approach and of the remaining 0.1% 0ver 99% will never bother to calculate the actual HP output.... Yeah, that's much easier.

The HP (and thus the torque) curve can be plotted from testing an engine with various props. And then using the Reivers PropPower calculator you can put actual number on a graph.

Go to [link=http://mvvs.nl/prop-power-calculator.xls]this web site[/link].

You can either use it on-line, or download it and use MS Excel, or the equivalent 'Open Office' application to use it on your PC.

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I know you weren't, Barry. Read the disclaimer at the end of my post.


Ed Cregger