Props what do i need to know?
 

Ok so how do you determine witch prop to use? A 10x6 will have higher RPMs the a 10x7. But i was told that becouse of this a less pitch prop will not always mean slower flight. I dont understand props at all. lol.

RE: Props what do i need to know?
 

The props work in a similar way a transmission in a car works. Props have two numbers: The diameter and the pitch. 10-7 = 10" diameter 7"pitch. The diameter is pretty much explanatory. The pitch works this way: This is the amount of air that would move through the prop arc in one revolution. One inch moves 7 inches of air, or the prop would, in theory, move 7 inches through the air in one revolution.

So, take the revolution of the engine, say for easy definition, rotates at 10,000 RPM at full throttle (maybe, maybe not, but for explanation purposes, say 10,000RPM).

So, that means that in one minute, the prop will move 70,000 inches or 4200000 inches per hour or 350000 feet per hour orjust over 66 miles per hour. That is in theory.
Also, think of the prop acting as a transmission on a car. If you want to climb a steep hill, you downshift to a lower gear and the engine goes higher in RPM at the same speed, but you have more pulling power. Back off to 10,000 RPM and you have a slower traveling plane but with more pulling power and faster acceleration. Increase the pitch, you have a faster plane at the same RPMbut with less power to get you there so it tkes longer to do the same work.

Hope that helps a bit.

RE: Props what do i need to know?
 

i think i understandhttp://www.rcuniverse.com/forum/js/f...y/msn/idea.gif lol, a 10x7=faster speeds, less power? so going with a higher pitch prop will make it harder for the plane to hold a climb. whereas a 10x5=slower speed, but the plane will be able to hold a climb better.... im such a newb http://www.rcuniverse.com/forum/js/f...sn/biggrin.gif

RE: Props what do i need to know?
 

Actually, you would go up to a larger diameter prop when going down in pitch. There is a certain amount of prop that your engine can turn. You can use that resistance in either pitch or diameter. A bigger prop grabs more air, therefore it pulls harder. But to keep it from overloading your engine, you have to go down to a lower pitch. A higher pitch prop moves more air, so it can get the plane going faster at the expense of all out pulling power. Going with a low pitch and small diameter (underpropping) lets the engine get its RPM up quickly and operate at a faster RPM. That comes in handy when you're doing 3D.

For a typical .40 size engine, common props would be 10x7, 10x6, 11x6, 11x5,  and 12x4. From left to right, you're going from top end speed props to acceleration and vertical performance props.

RE: Props what do i need to know?
 

it all about matching the power to the load just like gears on a car.
the simplest calc I've seen is

Diamerter sqarded times pitch. So a 10x6 prop wouls have a load factor of 600 a 10x7 would have aload factor of 700

the reason a 10x6 would be faster is the load factor is less. So it accalerates faster and run a higher rpm than a 10x7 prop. Kinda like when you down shift in a car and punch it you have more power in the lower gear, the rpm gain can overcome the speed differance.

basically you need to take a couple things into consideration when selecting a prop

what sized motor - manufaturer gives you recommended props and recommended rpm ranges. follow themhttp://www.rcuniverse.com/forum/js/f...bout_smile.gif

next decide what you plan on doing wwith the plane. Is it scale? what kind a lumpering cub. ya want a big low pitch prop to just truck around the sky with
for a 46 12 4 or 5. On smaller pattern ships a prop like this is ideal also. It won't go fast but It'll go straight up.

you doing quicky stuff, speed is all your worried about. you want a small high pitched prop. 9x8 or 8.75x10. you'll run like a bat outta hell but your acceleration will suck, may even have problems cavitating the prop at slow speeds. notice the calc on the 9x8 = 649. Split between the 10-6 and 10-7. This is backed up with the RPM with either prop being within a couple hundred rpm, get out a tach and try it.

or is it a sport model you want decent climb decent speed. a 10x6. A 10x7 probaly won't increase the speed if it's a weaker 46. It usually recommended on break in to limit the RPM. You don't wanna exceed red line when breaking in a motor or ever.

RE: Props what do i need to know?
 

besically your props for a 47 sized engines are
9x8, 10x7, 11x6 (or 11x5 depending on engine) and 12x4 (or 12.25x3.75 apc prop)
in general for every inch in diameter you add to the prop, you take two away from the pitch

the 9x8 is the fastest prop. the 12.25x3.75 prop will have the most torque.
fastest prop= fastest model. however, to make this simple, if you put a 7x12 (following the above rule) you may have to much speed and not enough torque to fly the model (like starting your car in 5th gear)
but with speed comesless vertical.
the more torque you have the more vertical you will have. but the lower your model will fly

basically for a trainer you will have two props, 10x7 or 11x5 (or 11x6 if your engine can handle it)


as for that load factor where 10x6 = a 60 load factor. that doesnt really work

example. 12.25x3.75 = 46
and 12x4 = 48
if your theory was correct my engine could swing a 15x4 prop at the same rpm as a 10x6 because they both have a 60 load factor.
a 47 engine has no hope of swinging a 15x4</p>

RE: Props what do i need to know?
 

I would just like to add one more variable to consider. And that is the airframe the prop is being used on. If the airframe is very sleek and slippery, you can use more pitch than if the airframe is blunt and draggy. For example, my Kougar , which was fairly slippery used a 10X8 prop on a OS 50. It might have been able to handle a 10X9. I would never try to use that much pitch on a dirtier airplane. Again, this was for top end speed and meant giving up some climb performance. But with enough speed, who cares or notices.

RE: Props what do i need to know?
 

The other day a pilot at the local club tried a Top Flite 10X8 on his Enya 40 powered trainer. It would not even get out of it's own way!
He changed it to a 10X6 and it found it's power.
In addition , just to confuse the issue different brand or styles of prop will load the engine differently.
I think just loading up a lot of different props and trying them all will learn you a lot.
Just remember to adjust the carb for any prop changes.

RE: Props what do i need to know?
 

Jimmyj
I don't know if the load factor that Red was talking about works or not but it was dia. squared X pitch so a 15x4 would have a load of 900 and a 10x6 would have aload of 600. 15x15=225x4=900 10x10=100x6=600 So they are not the same at all the 15x4 would place1/3 more load on the engine.

RE: Props what do i need to know?
 

Prop load factor is length to the fourth power times pitch:

Load Factor = Length^4 x Pitch

15x4 load factor = 202,500
10x6 load factor = 60,000

So a 15x4 prop places 3.375 times the load on an engine as a 10x6.

RE: Props what do i need to know?
 

I'd like to toss out one more consideration not discussed above. I think it absolutely falls in the category of "need to know" when it comes to props.

All of the information above is good stuff...and no doubt, it's all important to consider when selecting a prop.

However...one issue remains unaddressed.

The best prop for your airplane is the one that makes it fly the way you want it to fly.

Props can and do make some surprising, unexpected, often dramatic, and always HIGHLY subjective differences to any given airplane. Even 2 of the "exact same" props from the same manufacturer can make subtle differences to how an airplane performs.

Its all well and good to talk about one prop having 'more power" or 'less speed"...but what, exactly, do those mean? And how do they express themselves? By "less speed", do we mean the airplane's top speed is lower? Or do we mean that it accelerates slower? Or perhaps we mean it slows down more quickly on downlines or landings? Could we mean that it takes longer to reach takeoff speed?

And what about all those things...is one of them 'better" or 'worse" than another?

Without question...learn all you can about the technical aspects of prop performance and design...they are, without a doubt, a fine starting point for prop selection. And, certainly, understand at least the basics of how a prop loads an engine, and how to recognize when an engine is too heavily or lightly loaded, so as to avoid damage.

But once you're within that range...EXPERIMENT. Grab a handful of suitable props, and GO FLY. Out of any given 5 props, they might all suck canal water, or they might be varrying degrees of 'ok, but not perfect"...or maybe you'll stumble on one that's JUST RIGHT.

Didn't find one that just made the airplane shine for you? Fine...grab 5 more...try again.

The one "rule" to all of it is this : Once you find THE prop for you, your flying style, and your airframe...don't let ANYONE tell you yu have the 'wrong prop" just because they don't like the size, or material, or manufacturer.

Once again...

If it makes the airplane fly the way YOU want it to....IT'S THE RIGHT PROP.

RE: Props what do i need to know?
 

I was thinking the same thing. My mach2 has a 11x7 on a TT 46 and it has huge speed and will go vertical till I can't see it.

RE: Props what do i need to know?
 

Hello guys, I kind of know the basics of the props, my situation is the following,

I have an extra 330L from nitroplanes with a 46AX and pitts muffler, my plane is heavier than average, not that much, but still, anyway.. I am currently using a 11x7 prop and it took me a lot to take off, when I started to climb, I had to pull elevator very gently while the plane gained speed, once it was on the air, the plane was a rocket, flying very fast even at 60% throttle, however, when decreasing below 50% throttle, the plane does not seem to have enough power and it looses altitude during turns.......

I also noticed that when landing, if coming too slow the plane just seems unpowered and one of the wings stalls......

I was thinking that if I changed to 11x5 (or 11x4), this would give less top speed to my plane but maybe would make nicer the low speed landing approaches??? However, I am not sure and that is why I would like to ask to the experts of this hobby, given my case what prop is recommended???? My plane (extra300!) can not float, reducing speed makes it stall and then due to the prop (I guess), my plane does not speed up fast enough to recover, I have been lucky to land it without problems but I do not like that feeling of the plane tending to stall while approaching to land if I reduce speed too much......

Opinions please....

RE: Props what do i need to know?
 

http://www.rcuniverse.com/forum/js/f...n/rolleyes.gif
thats what i get for simming over posts. sorry

RE: Props what do i need to know?
 

Love the Kougar, it will be on my shopping list. And I have so many suitable engines. http://www.rcuniverse.com/forum/js/f...ngue_smile.gif

RE: Props what do i need to know?
 

Hello guys, I kind of know the basics of the props, my situation is the following,

I have an extra 330L from nitroplanes with a 46AX and pitts muffler, my plane is heavier than average, not that much, but still, anyway.. I am currently using a 11x7 prop and it took me a lot to take off, when I started to climb, I had to pull elevator very gently while the plane gained speed, once it was on the air, the plane was a rocket, flying very fast even at 60% throttle, however, when decreasing below 50% throttle, the plane does not seem to have enough power and it looses altitude during turns.......

I also noticed that when landing, if coming too slow the plane just seems unpowered and one of the wings stalls......

I was thinking that if I changed to 11x5 (or 11x4), this would give less top speed to my plane but maybe would make nicer the low speed landing approaches?? or would it be too slow and my plane would easily stall at low speeds(consider my plane is slightly heavier that it should be)? However, I am not sure and that is why I would like to ask to the experts of this hobby, given my case what prop is recommended???? My plane (extra300!) can not float, reducing speed makes it stall and then due to the prop (I guess), my plane does not speed up fast enough to recover, I have been lucky to land it without problems but I do not like that feeling of the plane tending to stall while approaching to land if I reduce speed too much......

Opinions please....

RE: Props what do i need to know?
 

To reiterate what has been said above,  fly and try.  This is the only way you will know what prop flies your particular airplane the best.   When you get a similar airplane and think the same prop will be the best, maybe so.  But it is still worthwhile to do a little fly and try.  I've been surprised several times. 

RE: Props what do i need to know?
 

For a long time, the 11x7 was considered a good starting point for .61 two strokes. Over the last several years, more and more people say they are running them on .46 engines. My brother had a TT46Pro on a Big Stik 40 and the prop that he felt gave the best performance was an APC 11X5.

Like gboulton wrote, you have to experiment to find what YOU like because your opinion is the onlyone that counts when it is your plane and your money involved.

RE: Props what do i need to know?
 

Matador

It sounds like you have an airplane with a high wing loading and possibly under powered. If the numbers I got off their web site are correct, your airplane has a wing loading of around 31 oz/sq ft. For a 40 sized model that is extremely high. So expect a model that has to be flown to landing at a rather high speed.

RE: Props what do i need to know?
 

thanks for the inputs, the manual recommends 11x6-11x8, that is why I have always used 11x7 for this engine, I had before a mustang pts and an edge, and they flew great with it, but as you said, this plane has lots of weight for the engine size......... do you think that the 11x5 would be too low? why does not the manual recommend it??? I would like that this plane would fly more solid at low speed and that is why I was wondering if a prop with lower pitch would help or if that would not improve the plane stalling at low speed.... would not a lower pitch prop help it to float a bit better?????

For take off, the plane really needs lots of speed to take off..... would a lower pitch prop would make it take off in a shorter distance at lower speed???

RE: Props what do i need to know?
 

I'd try the 11-6 first and see if the engine increases it's power output.  However, changing the prop is not going to change the stall speed of the airplane.  Only reducing weight will do that.

As to take-off, a lower pitch prop will pull harder and accellerate the airplane faster, since more of the blade is working.

RE: Props what do i need to know?
 

From what I read on RCU (so you know it must be true), OS prop recommendations are for noise reduction due to regulations in Japan. Props that really load the engines keep rpm down which reduces engine and prop noise.

The recommendations are a starting point for your experimentation. You may try 11x5 and hate it.

RE: Props what do i need to know?
 

Hello All -

I'm not adding fuel to a fire as I think everyone is right about the idea of experimenting w/props to find the right one for you. that's just all around a good idea and a great way to learn. From my experience with props and motors I've found this:

More often then not we rc hobbyists overpower our aircraft to perform unrealistic as compared to real aircraft (i.e. brett65's comment on unlimited vertical). this goes for both engines and props. I've overpowered some of my birds; guilty as charged! Yet, often times I've realized the manufacturers specs for both prop and engine/motor are set to obtain 'equal' performance of what the real world aircraft would do. I guess what I'm saying is that if your classic yellow Piper Cub can't maintain inverted flight - there's probably a reason behind it. :)

I've found flying more realistic is way more complex and hair raising for me vs. flying completely over-powered. And then there are times when going completely over-powered is just plain fun....

RE: Props what do i need to know?
 

Yes, there is a reason....no inverted oil and no inverted fuel system.

RE: Props what do i need to know?
 

HighPlains -

LOL! Sweeeet....... :D