I was wondering about prop diameter vs pitch, specifically whar is the difference between a 10X7 and a 11X6 I know that the larger the pitch the more thrust and lowers RPM but what advantages does a larger diameter prop have??

To oversimplify, the larger diameter prop should be more efficient in converting engine HP into thrust, compared to the smaller diameter prop.

Large diameter/lower pitch equals more thrust which is better for vertical maneuvers and slowing for a landing

Small diameter/bigger pitch gives more speed takes longer to get to takeoff speed and is harder to slow down for landing

Jim & Bubba have it right.

In your examples, the 11-6 will have greater efficiency, simply through having greater diameter, but it also has a slightly greater engine load than the 10-7. The engine will rev somewhat higher with the 10-7 & the model's speed will increase -- from the effects of both the rev increase & the steeper pitch. However, assuming the engine's revs don't fall below its usefull power band, the 11-6 will quite probably produce greater thrust due to the larger disc area & better overall efficiency.

For 2-stroke engines in the 40-75 size range, and assuming 2-blade props from the same manufacturer, made of the same material, & of the same style -- the rule-of-thumb for power load matching is: increase diameter 1", reduce the pitch 2". Conversely, reduce diameter 1" & increase the pitch 2" (1" of diameter is more-or-less equivalent to 2" of pitch in terms of engine load). This relationship obviously results from the fact that power load increase more rapidly with diameter, than it does with pitch.

Using the rule-of-thumb and taking your 10-7 as the baseline prop, an 11-5 would be a closer power load equivalent than the 11-6. The 11-5 would definately produce more thrust than the 10-7 (greater diameter & efficiency, same power transmitted). The model would accelerate & climb faster, but it would also decelerate faster & be slower in level flight.

Prop matching is a black art that comprises both theory & experimentation. The final "best-fit" is highly dependent upon the engine characteristics, the airframe configuration, the prop manufacturer, material & style, plus the intended outcome in the particular application. The theory will get you close & cut & try experimentation will finish the job. It helps to have lots of props in lots of sizes.

Do you want to go fast ? Or do you want to climb ? Think of the prop as the transmission in your car. Do you want to be in 3rd gear or 4th ? Now, what type of car do you have ? Sleek and slippery, or blunt and fat ? The airframe generally determines the top speed of an airplane. An airframe will go just so fast. Above that speed, diminishing return gobbles up horsepower with little effect.
I have a Kyosho Cap 232. I tried a number of props. Same pitch, different brands and shapes. I tried different pitches and diameters. When I put the APC 11x5 on the plane (OS Max 46 ), it was an obvious leap in performance, climb performance. My girlfriend could see the difference. And you know what ? That plane didn't seem any slower in level flight either. I am a big fan of the APC 11x5 for 40 size aerobats.
Now, my Kougar ran a 10x8 (OS 50 ). Its a sleek airframe. It is capable of higher speeds. I wasn't interested in improvng climb performance. ( had plenty ) And there was a ground clearance issue with an 11 inch prop.
So there are generally other issues that come into play besides effeciency. An airplane is a group of compromises flying in tight formation.

The pitch number equals the number of inches the prop will move through a solid material in one revolution. This is theory, since the prop slips in the air. GIVEN THE SAME RPM, more pitch, less diameter equals more speed and less pull. Conversely, more diameter, less pitch equals more pull, less top speed.

A small diameter, high pitch prop is like high gear on a car. A large diameter, low pitch prop is like low gear.

Airplanes with a lot of drag seem to fly better with large diamater, low pitch props than with smaller, high pitch props.

Dr.1

keep in mind the prop also affects how your plane lands. a higher pitch prop on a trainer can turn it into a bear to land, it might idle fast enough the trainer just floats along (or might actually be able to climb a little!)

spadeng,

PilotFighter and Dr1Driver gave the best answers.

I'll just ad that there is no one right prop for a given plane, just like there is no one right gear to drive your car in. It all depends on what you want to do and how badly you need to get maximum propulsion efficiency. A racer for example would use a different prop on a larger course than a smaller one. The plane will spend more time flying straight on a larger course, and more of the time turning on a smaller course. One pilot may enjoy short takeoffs and steep climbs, while another likes to run flat out straight and level.

Smaller pitch props are "climb" props. They give better pull at slow speed and better braking during glide. Larger pitch props are"speed" or "cruise" props. They give better pull at top speed, but climb and braking suffer. An in flight variable pitch prop is the best solution. For RC flying multi-pitch props are available, where the blades are shaped with a combination of pitches in a range. For example, instead of a 10x6 or a 10x8, you might find a 10x6-8. It won't climb or speed as well, but does a little better over all.

Notice I didn't mention diameter. That is because different operators may prefer to run the same engine at different rpms. One operator may be using a tuned exhaust and looking for the absolute screaming max power possible. Another operator may want to burn cheaper low nitro fuel, have a noise restriction to meet, or simply want to run softer so it lasts longer.

So you have to find the airspeed you want to have best prop efficiency at. Then decide what rpm you want your engine to spin at that speed. Then select the best prop for your choices.

Choose the pitch to match desired rpm with the desired speed, then choose diameter for engine load. It takes lots of experimentation to find the best prop for a specific application. Many experienced modelers have accumulated a large collection of test props to experiment with for each new plane.

Multiflyer

With high pitch props it can be hard to slow the plane down, especially if your engine does not have a slow idle but, big low pitch props can also make a plane hard to land for a different reason. When the engine is idleing, these props are acting like windmills and brake the plane. This causes the prop to block the airflow over the plane's elevator and the plane does ugly bounces upon landing because the elevator does not have enough control authority to flare the plane.
I have a plane that was a breeze to land deadstick but a bear to land under power, until I learned to open up the throttle a little right before touchdown so that my elevator would have enough control to flare the plane. When I used smaller high pitch props on it, this was not necessary.

As a novice I have much to learn. My knowledge of props was to use what the instruction manual said to use and then pick the one that was in the middle of the offering. This post is great. Now I have something to think about that doesn"t use a lot of eight cylinder words. Just what I needed. My thanks to all of you with experience and knowledge that are willing to take your time and help those learning.


Poppy2

I must say this is good info for someone like myself who is new to RC planes and is just learning. Thanks for taking the time to post this valuable information.

Thanks for all of the valuable info, I was planning on experementing, and I am used to dealing with constant speed props on full size airplanes but had no idea what diameter had, anyway, looks like others had simular questions

Have you ever sat on a bar stool that spins around 360 degrees, and spun your self with your arms near your body and then away from your body ?

Well if you didn't , the speed of you spinning lessons the further out you stick your arms. The closer you bring them back to your body the faster you begin to spin.

Apply this example to a prop. the bigger the diameter = less rpms.

That's conservation of angular momentum and the only time that needs to be worried about is with a helicopter doing an auto-rotate. But I'll keep it in mind if I ever use a bar stool to spin a prop

Diameter of a prop has more to do with thrust than anything else, or more correctly the area the prop disc covers. This area is the column of air that gets accelerated by the prop. If you go from a 10" prop to an 11" then the diameter goes up 10% but the area the prop sweeps out goes up by the square (or 21%). This takes more HP so if the engine is nearing its limit you have to reduce the pitch to reduce the load.

Hey RC-FIEND...

Spend a lot of time spinning yourself on barstools, huh? Is there like, a contest or a prize for this?

Dr.1

While all the given info is good, based on our experience, if you mention the plane and the engine in question, someone can give you an idea where to start. That way you can save some cash.

It would seem to me that given all the information here the novice would be better off going with a larger diameter/lower pitch prop because the take off would be faster, the glide would be slower, and the over all flying speed would be slower, three things that would seem to me to be in the novices favor. I am flying a Superstar 40 with an OS 46AX engine and have an 11x7 prop and it seems to do well. People say that I might be better with an 11x6 prop, but I try to find those props at the hobby stores and they are always sold out. It must be a very popular prop.


Poppy2

11x7 might be too much prop for a .46, so i would try to find an 11x6 or maybe even 11x5 for the first couple hours. but you are certainly correct in that a larger diameter/lower pitch prop is best for a beginner with a typical trainer. Those props aren't neccessarily just 'beginner' props, you can adjust later for how you like to fly. Personally all of my planes except one (lanier shrike) are 'geared' on the low end with large props/low pitch. i am usually flying much lower then anyone else at the field and like being able to more or less move in any direction at any speed, so having lots of low speed power is important.

smokingcrater wrote: It's quite O.K. to run a modern .46 with an 11x7 propeller. This size is usually included in the "recommended prop list" for OS 46 engines and they have more or less become "standard" propellers for my .46 engines.

/Red B.

What I have been running is a 11X6 on a S.P.A.D. Debonair I had a Thunder Tiger Pro .46 and it pulled it pretty good, i think i need more air time before I experiment too much

Was a game I played when I was very young, but the ice skater spinning also applies to the same examble. arms out she spins slower, arms in well you know the rest

Here is the general prop sizes in common use:

Most models fly at 60 - 90 mph wide open. Most glow 2 cycle engines are happy turning around 12,000 rpm range give or take. And so the 6 pitch is the most common for this combination.

A glow 4 cycle revs up a few thousand less, so 7 and 8 pitch is common for those. Larger gas motors turn even slower, typically 6,000 to 7,000 rpm, so 8, 10, and 12 pitch are common.

Then up or down a pitch or 2 to optimize for pull verses speed.

This all is for 2 blade props. Up the pitch one or 2 for 3 and 4 blades.

For model airplanes the diameter (the loading) pretty much follows the displacement. Glow 2 cycle .40s about 10 or 11 inch diameter, .60s about 11 or 12 inch, and so on....

And remember the pitch number is more of a manufacturer’s reference than an exact performance number. Blade design has a huge effect on prop performance. There are wide blades and skinny blades and sharp blades and blunt blades..... Your favorite prop size will often vary from one brand of prop to the next.

Multiflyer

Most models fly at 60 - 90 mph wide open. Most glow 2 cycle engines are happy turning around 12,000 rpm range give or take. And so the 6 pitch is the most common for this combination.

A glow 4 cycle revs up a few thousand less, so 7 and 8 pitch is common for those. Larger gas motors turn even slower, typically 6,000 to 7,000 rpm, so 8, 10, and 12 pitch are common.
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Multiflyer, what you say is true in generalization only. "Most" is nowhere near "all", and I'm not sure "most" can even be used as a generalization. In fact, there are so many types of engines, props, planes, and flying styles, I'm not at all sure a generalization like this can be made. Each engine/prop/plane combo is different and has different requirements, depending on how the pilot wants the plane to fly. Have you ever used a radar gun on an R/C model? Most do not fly anywhere near 90 mph. All engines are different and each has a different optimum RPM. The RPM requirements will change from plane to plane. I have a Webra Speed .32 and an OS .32 that are both happy turning 14,000. These engines use 10 x 4 props. I have a Zenoah G-23 that is happy with an 18-6 prop. 4 stroke glow engines can turn larger diameter props with less pitch, and are often used for this purpose.

True, there is usually a good base for prop diameter and pitch to start from. After that, experimentation is the only way to achieve the best combo for a specific plane's use and the pilot's flying style. The pitch and diameter may go way outside your figures, and even the manufacturer's recommendations. Blanket assumptions just can't be made.

Dr.1

I fly a high wing, high drag 900 sq in trainer (Sig LT-40) powered with a Thunder Tiger .46 Pro. It seem totally happy turning an 11x5 APC although the 11x6 is generally the recommended prop for this motor (assuming a sleeker model). I like having the thrust to quickly get out of a poor situation. Top end speed is generally something you don't need with a trainer type model.

I generally turn nearly 13,000 RPMs on the ground with 15% fuel on the 11x5.

Pipe, you made the right choice. Many trainers are much too fast using the "standard" 6" pitch prop most often recommended. I've taught beginners on planes they couldn't fly at over half throttle. Going up an inch on diameter and down a couple of inches on pitch makes a much nicer, slower-flying trainer bird. You're also correct when you say it's that low pitch and more thrust that will pull the plane out of trouble, especially at low speed.

Dr.1