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  • September 27, 2021, 10:50:58 PM

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Author Topic: Propellers  (Read 915 times)

Offline Al Williams

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Propellers
« on: March 02, 2021, 07:50:49 AM »
I have been reading old threads in the 1/2A section and there are many threads of plane and engine performance with propeller size changes.  For example,  plane  flies well level but the vertical part of a loop the engine looses power, change the propeller, how?  what happens when a  5.5x4 is changed to a 6x2?  What is the affect of propeller diameter in relation to pitch and engine speed?  Some threads talk about cutting propellers down to get a specific improvement.

Can someone please explain this and/or point me to an article that can shed some light on this subject.

Thanks

Online Dan McEntee

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Re: Propellers
« Reply #1 on: March 02, 2021, 08:57:11 AM »
   The propeller is the engine's primary load. There is a small range of sizes that will work best for each engine size and the airplane's size and weight along with other physical features also add to the load. The diameter affects the run by the weight and drag caused by the blades as they get longer, more diameter equals more weight and drag. The pitch or angle of the blade adds the other part, more pitch (steeper angle) more load. The pitch number you see is the theoretical distance that the prop will travel forward in one revolution. The more pitch, the farther forward but also more load.
   The easiest way for you to see this is to set up an engine on a test stand that you are used to starting and handling. Pick a prop that you know works good, install it and start the engine. Adjust the needle to nice setting and measure the RPMs with a tachometer. Shut the engine down, and install a prop with the same diameter but different pitch (higher or lower) and restart the engine, don't touch the needle and measure the RPM after the engine warms up. Make note f the difference.
   Now do this test again, and this time, change the prop for ne with the same pitch but different diameters. This will illustrate the difference in prop loads. To use your example of a 5.5X4 changed over to a 6X2, you will probably see a higher RPM at peak setting due to less pitch.  You do this to help the engine get to it's optimum horsepower or as is sometimes called peak power setting. All engines have a power curve and different props affect this curve in different ways.
   Different brands of different props run differently also. Repeat the test above with a prop that you like. Then try different props of different manufactures but stay with the same diameter and pitch. You will see a difference in RPMs. You will sometimes find that a prop of one brand and sizes works well on your model but then another doesn't pull as well. Part of this is in the blade design and airfoil shape. Another part is in manufacturing tolerances, where a prop may say 10-6 on it's markings, but if measured on a pitch gauge will only actually be a 5 inch pitch more or less.
    To find out which prop works best, the best way to find out is to just test it on a bench or to install on a model and fly it and see how it works. The proof is in the pudding so to speak. Through experience and testing you will develop a like and dislike for what brands work and which ones don't meet your needs.
    Type at you later,
  Dan McEntee
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Online Dan McEntee

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Re: Propellers
« Reply #2 on: March 02, 2021, 10:07:40 AM »
   I forgot to add that the material they are made from makes a difference. There are wood, plastic, (and MANY different types of plastic) fiber glass, carbon fiber, and many variation of each. They weigh considerably different and prices range from 3 dollars to 30 dollars and more! All can be modified by clipping or reshaping the blades. Trimming the tips of a prop can give you a few more RPM if you need it, but it needs to be done evenly and precisely, then rebalanced. Balancing a prop is pretty important and that takes some practice. I mentioned a pitch gauge before and while not absolutely necessary, is a very handy tool especially if you want to get into re-pitching props. The science of a propeller is very interesting and experimenting with the different kinds is very educational and no matter what discipline of the hobby you are participating in, learning something about them helps you get the best performance out of your engine and the performance of your airplane.
  Type at you later,
   Dan McEntee
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Offline Al Williams

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Re: Propellers
« Reply #3 on: March 02, 2021, 12:11:31 PM »
Thanks Dan

Offline john e. holliday

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Re: Propellers
« Reply #4 on: March 02, 2021, 12:53:43 PM »
Also start a log book and try to keep it current. D>K
John E. "DOC" Holliday
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Offline Air Ministry .

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Re: Propellers
« Reply #5 on: March 02, 2021, 09:34:43 PM »
The prop is a bit like a gear in a vehical .

Motors gunna be happy at a certain range , so needs to be ' geared ' to match that , model, and airspeed = gearing.

The pitch is the ( theoretical ) inches forward per turn . so a two would pull you two inches, a ten would ten inches. If it didnt stall, overload and so on . No pitch itd go nowhere .

Offline GregArdill

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Re: Propellers
« Reply #6 on: March 03, 2021, 02:13:14 AM »
Anyone with interest in props, (and aren't we all at some level?) read "Propellor Dynamics" by Joe SuperCool (AKA Stuart Sherlock).

This covers every aspect of propellors for model planes, the maths is a bit taxing but worth the effort.

Online Brett Buck

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Re: Propellers
« Reply #7 on: March 03, 2021, 10:14:40 AM »
I have been reading old threads in the 1/2A section and there are many threads of plane and engine performance with propeller size changes.  For example,  plane  flies well level but the vertical part of a loop the engine looses power, change the propeller, how?  what happens when a  5.5x4 is changed to a 6x2?  What is the affect of propeller diameter in relation to pitch and engine speed?  Some threads talk about cutting propellers down to get a specific improvement.

Can someone please explain this and/or point me to an article that can shed some light on this subject.

     This is a huge topic, and (for IC engines used in stunt, at least) has many poorly-understood "rules of thumb" beyond the basics.

     The real simple basics - pitch controls the speed and the diameter controls the thrust, and both affect the load on the engine, and how the load on the engine changes with speed.

     If you ran a *fixed* RPM (and using 1/2A sort of numbers), say, 18000 RPM - more pitch would be faster, and more diameter would be faster. That is, at a fixed RPM of 18,000, a 5-3 might pull the airplane at 45 MPH, and 5-4 might go 55 mph. It does that because more of the engine power is being transferred to the airplane, that is, the *efficiency* is better. It is more efficient because the prop (which is just a spinning wing) flies at a higher angle of attack. Same with the diameter - a 6-3 at 18,000 will go faster than a 5-3 at 18,000. Same reason - it's more efficient, in this case, because it is moving more air (the disc area of 6" prop instead of a 5" prop).

     However, you already know, it *will not* be the same RPM, because the *drag* from your prop goes up with diameter as well. This is where it gets complicated, because how the engine reacts to the prop varies and can be adjusted to vary how you might want it.

    Full-scale airplanes are generally interested in the most speed for a given power, so they choose props that load the engine give the peak horsepower in flight at the highest possible efficiency. For full-scale airplanes that maneuver and change speed, this means that the best prop for takeoff is not the same as the best prop for maximum speed - so they have *variable pitch* propellors, low pitch for takeoff and high pitch for cruise at high speed.

    We don't have variable-pitch props (er, at least not commonly, not yet...) so we pick a prop that gives us the desired maneuvering characteristics that has to compromise various factors like load, RPM, power curve, etc. In stunt, the drag of maneuvering slows the airplane down, and you want to either not lose as much, or recover that speed quickly, so depending on how the engine reacts to load, we adjust various prop parameters and engine parameters to make it work in an ideal manner.

    A big problem with explaining it for stunt  is that *no one agrees what "ideal" means* so we all argue endlessly over how to adjust it. How the engine/prop combination works is *absolutely critical* to how the airplane flies, by far the most critical factor in the airplane performance, far more important than the design of the airplane, as long as there is nothing to crazy about it. Just getting something that will work is pretty easy, getting something that will work better than your competitors is a lifetime of work that we never completely master.

     Brett


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