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Post  SuperDave Wed Sep 14, 2011 10:09 am

Seems a topic little comprehended.

Props are, in effect "gears", which must be carefully selected to suit the application. Some would say that propeller application is all about the achiement of the highest RPM. I would hardilly disagree but I'd like to initiate discussion rather than state my own opinions.

The considerations are many; what are they?

SuperDave


Last edited by SuperDave on Wed Sep 14, 2011 10:52 am; edited 1 time in total
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Post  GermanBeez Wed Sep 14, 2011 10:19 am

so what would you recommend for slower flying?
larger diameter, more angle? or the other way around?
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Post  nitroairplane Wed Sep 14, 2011 10:29 am

large diameter lower angle maybe a 6 by 3
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Post  nitroairplane Wed Sep 14, 2011 10:30 am

3d planes use large props with low pich because pitch slows down the climb efficiency.
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Post  GermanBeez Wed Sep 14, 2011 10:32 am

ok
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Post  microflitedude Wed Sep 14, 2011 10:37 am

It's simple, right? Smaller propeller = more RPMS, more speed, less torque, on same power source and vice versa.
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Post  nitroairplane Wed Sep 14, 2011 10:40 am

microflitedude wrote:It's simple, right? Smaller propeller = more RPMS, more speed, less torque, on same power source and vice versa.

sort of but pitch increases air resistance so the prop turns lower rpm.
eg a 5 by 2 turns faster that a 5 by3 of the same basic construction.
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Post  SuperDave Wed Sep 14, 2011 10:49 am

You "students" are missing the several elements; what about the weight load to be carried?

Socrates (aka: SD)
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Post  nitroairplane Wed Sep 14, 2011 10:58 am

SuperDave wrote:You "students" are missing the several elements; what about the weight load to be carried?

Socrates (aka: SD)

students are we students?
from what i have sadi here it would appear i am the teacher(a bad one) and y'all are i studenti.
and MF dude is the other teacher.
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Post  SuperDave Wed Sep 14, 2011 11:26 am

nitro:

As I clearly stated it was my intention to initiate a discussion (rather than arguement). (Some have trouble unstanding difference) I happen to have a reasonably solid knowledge of the subject under discussion.

I know that asking questions rather than speculating answers antagonizes some yet that was the matter in which I was taught in my schooling days and I am the better for it. In the end Socrates was sentenced to death but took his own life by ingesting poison hemlock.

Not choosing that fate, I chose to drop the subject of propeller dynamics.

SuperDave


Last edited by SuperDave on Wed Sep 14, 2011 11:32 am; edited 1 time in total
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Post  nitroairplane Wed Sep 14, 2011 11:28 am

its ok Dave i was kidding.
i like this conversation it is sure to throw up some interesting info.
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Post  RknRusty Wed Sep 14, 2011 11:48 am

I'm always experimenting with props, and these are only my observations, not science. If you need hauling power for a heavy plane, a longer prop like a 6x3 on an .049 will get it moving. When I want to do stunts at a controllable speed I also use a 6x3 on my lighter plane. A 5x3 on a light plane is a good prop for most any type of flying. If you want to scream around like flying a rocket, a 5x4 will do the job. I learned some things I didn't know before I read an article on Stuka Stuntworks last night.
I used to use the Cox 5x3 tri-blades because they look cool. But I've come to learn they under-perform on all of my planes.

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Post  nitroairplane Wed Sep 14, 2011 11:53 am

RknRusty wrote:I'm always experimenting with props, and these are only my observations, not science. If you need hauling power for a heavy plane, a longer prop like a 6x3 on an .049 will get it moving. When I want to do stunts at a controllable speed I also use a 6x3 on my lighter plane. A 5x3 on a light plane is a good prop for most any type of flying. If you want to scream around like flying a rocket, a 5x4 will do the job. I learned some things I didn't know before I read an article on Stuka Stuntworks last night.
I used to use the Cox 5x3 tri-blades because they look cool. But I've come to learn they under-perform on all of my planes.

yes a 5 by 3 triple blade prop gives way less power because it is heavier it may be good on a 0.074.
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Post  hlsat Wed Sep 14, 2011 12:23 pm

The propeller is very important place for the proper functioning and engine performance.
I never operate the machine to the maximum to do more damage.
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Post  Kim Wed Sep 14, 2011 12:25 pm

6x3's and 5x3's do it for me in the .049 department...although I DO have a ridiculous 4.5x4 that I got from Bernie and am saving for a 2 channel, near-suicidal-short-winged thing that keeps peeking at me from the dark recesses of yonder airplane cabinet.

Another consideration is the brand/airfoil section of the prop, and it's material and weight. Two props of the same diameter and pitch, but from different makers, may give you some radical differences in performance. Blade planform and undercamber at certain stations along a prop's diameter play into how it drags an airplane around.

The precision stunt control line guys in my club are ALWAYS experimenting with combinations to give them just the right balance between outright thrust and engine speed in their stunt sequences. It's also not unusual at all for two "identical" props from the same manufacturer to produce measurably different results.


Last edited by Kim on Wed Sep 14, 2011 12:29 pm; edited 1 time in total
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Post  microflitedude Wed Sep 14, 2011 12:29 pm

When I first started out, I knew nothing about airscrews other than they pulled or pushed the airplane. Just like I knew nothing about balancing, airfoils, etc. But I scratchbuilt my first two planes in that stance, and they flew great.

Here's my understanding ~

The "Pitch" is the distance the propeller will travel in one revolution. The higher the pitch, more speed - the lower it is, more power. (torque)

So if you have a low diameter/high pitch, it will be very fast with not much torque behind it. Then you can flip that around.

So max RPM is not always desirable - it's just your preference. But if you can get your engine to put more out, that's always a good thing. lol!
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Post  nitroairplane Wed Sep 14, 2011 12:33 pm

yes i even got a pee wee .020 to turn a 7" by 5"
model diesel engine.
kind of like a little texaco
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Post  Cz10 Wed Sep 14, 2011 12:40 pm

Interesting testing here
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Post  hlsat Wed Sep 14, 2011 12:45 pm

I think the Top Flite Nylon propeller are the best for Cox engines.
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Post  GermanBeez Wed Sep 14, 2011 1:02 pm

yeah, some people actually prefer them to the original cox propellers.
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Post  andrew Wed Sep 14, 2011 1:09 pm

My observations:

Larger diameter, lower pitch props tend to be more desirable when we need constant speed and the ability to move a heavier aircraft (or draggy aircraft) thru most flight attitudes. The lower pitch limits the top end speed, but the greater volume of air being moved by the larger diameter improves "hauling" ability. The disk area of a 6" prop is around 44% greater than the disk area of a 5" prop.

Small diameter, high pitch props will give a higher top speed for a given RPM, but do poorly with draggy airframes.

All props exhibit some inefficiency in usage, i.e. they slip, so even if we are able to turn a 6" pitch prop at 18000 rpm, you will not reach theoretical pitch speed ~ 102 mph. Another problem that rears its head with high pitch high rpm props is blade stall. Cavitation usually is defined as a conversion for a liquid to a gas in areas of extremely low pressure, but tip cavitation has been used to explain some phenomenon with supersonic blade tips. However, at the speeds we run our engines, tip speeds seldom reach supersonic. When stalled, rather than move the expected volume of air, high pitch propellors turning high rpm will "break away" from the airflow and become very inefficient at low IAS. As the airspeed increases, this high speed stall loss diminishes and the prop enters an efficient airspeed envelop. Parasitic drag also becomes a factor as airspeed goes up, since parasitic drag increases exponentially with speed. Consequently, while we can improve speed characteristics by using a higher pitch prop, we also need a larger diameter prop to overcome increasingly greater drag forces imposed as we fly faster. To drive a larger diameter prop in the upper pitch ranges, we need more horsepower -- and that becomes one of the limiting factors. There are many others, but speed requires a slick airframe, high pitch and enough power to drive the prop at a high rpm.



That's ma' story and ah'm stickin to it.

andrew


Last edited by andrew on Thu Sep 15, 2011 4:09 pm; edited 1 time in total (Reason for editing : Correction of some wording)
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Post  nitroairplane Wed Sep 14, 2011 1:10 pm

GermanBeez wrote:yeah, some people actually prefer them to the original cox propellers.

i like apc props but have also carved my own props in the past but i found it very labour intensive.
they were not as good as cox props but good enough for the flying that i did.
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Post  SuperDave Wed Sep 14, 2011 1:17 pm

Thank you, Andrew! I've given you a + because you have captured much of what I hoped would be drawn out from this discussion.

May everyone here read Andy's observations and learn from them.

SD
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Post  Cox International Wed Sep 14, 2011 2:22 pm

Ran our yellow 4.5x4 racing prop on a 049 Killer Bee II a few days ago for testing purposes. Holy cow - 23,800 RPM!

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Post  GermanBeez Wed Sep 14, 2011 2:25 pm

oh wow, a prop that size probably has a hard time even turning the engine over!
but i bet it can't run at low speeds with that prop, right?
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