Propeller dynamics

large diameter lower angle maybe a 6 by 3

3d planes use large props with low pich because pitch slows down the climb efficiency.

ok

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

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.

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

Socrates (aka: SD)

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.

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

its ok Dave i was kidding.
i like this conversation it is sure to throw up some interesting info.

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.

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.

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.

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.

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.

yes i even got a pee wee .020 to turn a 7" by 5"
model diesel engine.
kind of like a little texaco

Interesting testing here

I think the Top Flite Nylon propeller are the best for Cox engines.

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

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

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.

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

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!

Bernie
www.coxinternational.ca

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?