The Components of Thrust; How to choose a propeller (Tutorial)

Discussion in 'Scratchbuilding!' started by LukeWarm, May 17, 2013.

  1. LukeWarm

    LukeWarm Top Gun

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    The Components of Thrust; How to choose a propeller.

    What is thrust?
    Thrust is a mechanical force, so the propulsion system must be in physical contact with a fluid medium to produce thrust. A good example of a fluid medium is the gasses that make up the air in our atmosphere. So basically, thrust is air pushing against air. There are two Components of Thrust; The "Size" (power) and "Velocity" (speed) of that push. A 8.5 inch has twice the area as a 6 inch pipe. A 8.5v inch pipe with air coming out of it at 40mph will produce the same thrust as a 6 inch pipe with a air coming out of it at 80mph. A 7.25 inch pipe with a air coming out of it at 60mph will be a balance between the two extremes. All three pipes are moving exactly the same amount of air and have the same amount of thrust. If like motors (with the same amp draw) produce the thrust in each pipe; the 8.5 inch pipe’s thrust will be powerful and slow, the 6 inch pipe’s thrust will be weak and fast, and the 7.25 inch pipe will have a balance of power and speed. Your decision is to choose how you wish to use the power that the motor has to offer. But when you throw a motor into this mix, there is a couple of other things that you must also look at:
    • The motor that would do well with a 8.5 inch prop will have a lower KV than a motor that would do well with a 7.25 or 6 inch prop. The propeller pitch would control the "Velocity" (speed) of that push, and the propeller length would control the "Size"(power) of the push.
    • For same types of motors; the more the motor weighs, the more powerful it is. A heavier motor will have more sustainable power or drive a more aggressive prop (a longer or higher pitched prop).
    Static thrust and Dynamic thrust:
    In some discussions you might hear the terms “Static thrust” or “Dynamic thrust”. Very few people understand these words. To understand why these terms are so important, let’s look first look at the definitions:

    Static thrust: The thrust developed by an airplane engine that is at rest with respect to the earth and the surrounding air. There is no air moving towards the propeller due to the lack of the aircraft's forward velocity. Because of this, the propeller must create its own inflow velocity instead. Static thrust is the measured thrust produced when movement is prevented.

    McGraw-Hill Dictionary of Aviation's definition of Static thrust:
    The thrust produced by an engine when the aircraft is stationary on the ground.

    Dynamic thrust: The thrust developed by an airplane engine that is moving with respect to the earth and force fed by the incoming air. There is air moving towards the propeller due to the aircraft's forward velocity, so as the propeller has to create from part to none of its own inflow velocity. Dynamic thrust is the difference between speed of the air the prop moves (slipstream velocity) and the aircraft's forward velocity with respect to the air's movement around the plane.

    McGraw-Hill Dictionary of Aviation's definition of dynamic thrust:
    The work done by a propeller or a fan in imparting forward motion. It is equal to the product of mass flow/second and the difference between slipstream velocity and aircraft speed.

    Now let’s take a look at how this applies to a foam plane that will go 60 mph in a dive, with zero wind, where the motor’s max output is a pound of thrust: (this is a simplification)
    • At zero mph, the motor-prop combo produces a pound of static and dynamic thrust,
    • At 30 mph, you’re down to half a pound of dynamic thrust,
    • In a dive, at 60 mph, you have no dynamic thrust at all; input equals output.
    • The faster the air comes at the prop, the smaller the pressure difference between "in front of" and "behind" the propulsion system, and the less motor amp draw (load) and dynamic thrust is produced.
    If you want to read more: http://www.answers.com/topic/dynamic-thrust#ixzz2TYXOh85Z

    This world we live in is all about compromises:
    You have to choose a motor and propeller combination that fits your needs. A bulky heavy cargo plane would not work well with a speed combo. The losses would be too high because the smaller propellers would not have enough bite. Now let’s look at a sleek light aerodynamic airframe; a speed combo will work great here. Weak and fast is OK because a sleek plane slips through the air much more easily. Powerful and slow would be a waste for this plane.

    You do not have to pick an extreme combo:
    There is a whole world in-between. “Powerful and slow” and “Weak and fast” are a byproduct or thrust Size and thrust Velocity (speed). To make this next part easier to see, look at the Prop Test Data Below. Let’s look again at that sleek light aerodynamic airframe again. A 6x4 prop is great for a speed run, but maybe that is not what you want the plane for, maybe acrobatics is more you cup of tea. A 6x4 prop will make the motor run a little hot, may quickly lose momentum during a vertical climb and stall, and it may recover speed slowly after each stunt. With a 6x3 prop, this motor will never overheat, will easily sustain vertical climb and will recover speed quickly after each stunt. It will still give good speed but will have a slower maximum speed. With a 5x5, or 4.75x4.75 prop, and a 3 cell battery, this motor will never overheat. These both would be fast and weak.

    The Prop Test Data you see after the motor specs is very useful, if you know how to read it:
    Motor Specs:
    Model: NTM Prop Drive Series 2836 2700kv
    Kv: 2700rpm/v
    Poles: 3
    Motor Wind: 6T
    Max current: 50A
    Max Power: 520W @ 11.1/V (3S) / 595W @ 15v (4S)
    Weight: 87g
    ESC: 40~60A
    Cell count: 3s~4s Lipoly

    Prop Test Data:
    4.75x4.75 - 11.1V / 311W / 28A / 0.7kg (24.7oz) thrust
    4.75x4.75 - 14.8V / 594W /40A / 1.12kg (39.5oz) thrust
    5x5 - 11.1V / 400W / 36A / 0.85kg (30oz) thrust
    6x4 - 11.1V / 520W / 46.8A / 1.2kg (42.3oz) thrust

    Why this prop or that prop:
    The 6x4 generally seems to be the best prop for this motor because it has great thrust. The only problem is the 46.8amps it draws is very close to the 50amp maximum current rating. To keep the motor from overheating, it will need some heat management. It would be easier on the motor if it is installed in a sleek light aerodynamic airframe. The 6x4 is not tested with a 14.8 4cell battery because the amp draw would be way too high and with it, the motor would have a very short life.

    The 4.75x4.75 uses a 14.8volt 4cell battery, this is the fastest prop in the test. The 40amps it draws will let it run very fast and cool. It will be fast and weak, and will fill more-so that way because of the weight of an additional battery cell. This 4.75x4.75 is faster than the 6x4 prop but will not have the punch the 6x4 has because it is a shorter prop.

    The 5x5 using a 14.8volt is not shown in the chart. A 4cell battery is useable, but not without a pretty good amount of heat management. In the right plane, this is the fastest prop.

    Prop Types:
    All prop manufacturers engineer the design of there prop uniquely. They have different strengths and weaknesses. Some are designed to where they do a certain job better than any other prop. In designing a prop, when you take from one area, you give to another.

    Lets look at an APC prop; It has consistent power through out its entire RPM range. This makes it a great all around sport prop. It is one of the noisiest and most highly engineered props you can buy. Its weakness; A jack of all trades is a master of none. For a camera platform plane, you want a quiet power prop that is very efficient and powerful at its target RPM range; A GWS prop would work well here. The problem with the GWS prop is, if its not in that target RPM range, it is weak and inefficient. To get the most out of a prop, you need to buy the right one, then take advantage of its strengths by using it the way it was designed to be used.

    The Motor:
    It’s the motor–prop combo that supplies the thrust. To maximize their effectiveness, you need to make sure that the aircraft is set up so that there is good airflow through the power system, and there is a sufficient amount of current (amperage) available from the ESC and battery. The Motor turns electrical energy into a mechanical force. The rate this motor consumes energy at is measured in amps or watts. Amps is current, watts is amps times the volts. The weight of the motor, the allowable current, and the wattage rating are each good indicators of the available power the motor can use to spin the prop. Use the motors Max current rating to pick a compatible the ESC and battery; it is safer to choose ones that are rated at least 5 to 10 amps over the motors Max current rating. The KV rating is RPMs per volt; this will tell you the RPM range that the motor is designed to operate at, and will help you to choose the correct prop. "Motor Wind: 6T" this tells us there are 6 turns of wire around each poll. A low number of turns (like 6T) is typical of a high KV motor (this one is 2700 KV). A 14T (turn) motor, I saw on the web, had a 1350 KV rating. The number of turns and poles, and the size of the wire, are what determines all of the other numbers. As the KV gets lower, the windings have less turns, and the useable current and wattage goes down. When going to a lower KV, you trade torque for speed. On a light plane, there is a point at which you are wasting power. This >> EMAX GT2815/05 1500KV is a good motor, but it is a waste on a light plane. But the more torque and power the system has, the more sustainable the thrust numbers are.

    After you’ve shopped around and seen prop data on a few motors, and you've been through a few motors of your own; From experience, you can look at the weight and the KV of a motor, and tell about what prop to use. Heavier motors are more powerful, and can drive more prop. Now look at the load the plane will impose upon the motor and make an adjustment on your first choice. From here, the heat test is the only way. I am usually very close. Read here about the heat test. >> Some ways to Managing the Heat. If the heat is good no matter what you do, that is a safe prop; but if it's too safe, you are not getting the most out of the motor. If you go up one in pitch or prop length, do the checks, and the motor is a little too hot; you have found the edge. Another inch of length uses more power than another inch of pitch. With a little heat management, this is the prop that will give you the most thrust. When choosing whither to increase prop pitch or prop length, this is a good rule of thumb; Prop pitch gives you speed, prop length gives you power. If you are using a prop that pushes the motor to the edge, give it the power it needs to do its job. If you have a good quality ESC and battery at a sufficient amperage rating to where the motor will never starve, the motor will run cooler faster and more powerfully. After every flight, check each component of your power system with your finger or a laser thermometer. If you cant hold your finger on it for at least 2 seconds, that component is to hot. If the battery or ESC is too hot, upgrade it. If the motor is too hot, use a lesser prop.

    A watt meter and real world testing of your power system will let you safely get the most from your motor. Manufacturer Specifications will put you in the ball park, but it's no substitute for checking to see what that motor will do on a particular plane, power system and prop, on that day, in that weather. A watt meter is a nice tool to have, but frequently checking the heat is the number one tool. It will give you a good sense of what you need to do to take care of your motor. The best case scenario is to use a prop that is aggressive enough to where you have to use a little heat management to maintain it at an exceptable temperature. The motor’s self-cooling can only dissipate so much heat. Anything beyond that point, the motor will overheat. The more load, the more amp draw (current), and the more heat. Heat management is flying the plane in such a way as to control the frequency and severity of the heavier loads that are placed on the motor, therefore control the heat. The max amp draw and max heat occurs only when you hear the motor struggling the most. The weather matters, flying on a hot day will require more Heat management than a cool day. High alphas are hard on a motor, use a conservative power prop for this type of flying.

    The Voltage:
    Because of their power to weight ratio, we use Lipo batteries. We primarily use these Lipo batteries in three voltages; 7.4 volts for a 2 cell, 11.1 volts for a 3 cell, and 14.8 volts for a 4 cell. The airplane and its electronics need to be designed to work well with your battery's weight and voltage. The higher the voltage, the more weight, power, and heat you must deal with. A higher voltage battery will also spin the propeller faster, this will give your plane more speed.

    The losses:
    The fluid dynamics of the air is both your best friend and your worst enemy at the same time. Without it, your plane would not move, yet it interferes with the planes movement by causing friction. The faster the plane goes, the denser the air feels to it. Example: At fast speeds, it takes 50% more horsepower to go 15% faster. What I'm saying is, the loss numbers are not so cut and dried, they are velocity and aerodynamically sensitive. The shorter and faster a prop is, the more losses you will see. With the faster props, you compromise power for speed. And poor aircraft and power system aerodynamics will also increase the losses.
     
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  2. Grey

    Grey Ace Pilot

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    Very high quality as per usual!

    Thanks Mr Luke Warm Sir!
     
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  3. squishy

    squishy Top Gun

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    It's nice to see you take on this subject luke, I bet you are doing some good research. In order to explain this well, and in a real world application, we truly need to give real world examples and show the difference between ratings and real world. Can you run a few motors with a series of props on a watt meter and write down all the results to analyze? I find that people really see it come to life when this is done. I think some more emphasis on efficiency is needed, it's the goal, the end result we are trying to achieve. Reading the motor rating and theoretical numbers is one thing, but putting that prop in the air and measuring it's efficiency is really the only way to know. Everything else is a guess, especially when trusting Chinese motor ratings. I have seen many instances of motors not living up to their ratings and not even being the correct weight that was listed when I purchased them.
     
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  4. squishy

    squishy Top Gun

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    Oh and different prop brands and designs make a huge difference as well. Because a motor is rated for a 9x5 might mean several things depending on the prop used. In multi rotors we need to know this because such variation throws off the tuning and gain adjustments of the flight controller so much. As an example I will tell you that a 9x5 Grupner prop has the equivalent thrust of a 8x5 APC at the same RPM. This is kind of an important fact to know, and emphasises the fact that a watt meter and real world testing of your power system is important.
     
  5. LukeWarm

    LukeWarm Top Gun

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    With this post, you can learn a lot about motors. This is the same motor in 4 different configurations. All they changed is the size of the wire, and the number of turns around each poll. When they decrease the number of turns, they can increase the size of the wire; thicker wire can hold more current.
    http://www.hobbyking.com/hobbyking/store/uh_power_Search.asp?strSearch=28-36
    All 4 of the NTM Prop Drive Series 2836 have these stats in common:
    Poles: 3
    Shaft: 4mm
    Cell count: 3s~4s Lipoly
    Bolt holes: 16mm & 19mm
    Bolt thread: M3
    Connection: 3.5mm Bullet-connector
    ===================
    http://www.hobbyking.com/hobbyking/...p_Drive_28_36_3000KV_755W_USA_Warehouse_.html
    Model: NTM Prop Drive Series 2836 3000kv
    Kv: 3000rpm/v
    Motor Wind: 5T (That's 5 Turns of wire around each pole)
    Max current: 55A
    Max Power: 396W @ 11.1V (3S) / 755W @ 15V (4S)
    Weight: 87gu
    ESC: 35~45A

    Prop Test Data:
    4.5x4.5 - 11.1V / 334W / 30.1A / 0.82kg thrust
    4.5x4.5 - 14.8V / 755W / 51A / 1.31kg thrust
    4.75x4.75 - 11.1V / 396W / 35.7A / 0.836kg thrust
    ===================
    http://www.hobbyking.com/hobbyking/...p_Drive_28_36_2700KV_595W_USA_Warehouse_.html
    Model: NTM Prop Drive Series 2836 2700kv
    Kv: 2700rpm/v
    Motor Wind: 6T
    Max current: 50A
    Max Power: 520W @ 11.1/V (3S) / 595W @ 15v (4S)
    Weight: 87g
    ESC: 40~60A

    Prop Test Data:
    4.75x4.75 - 11.1V / 311W / 28A / 0.7kg thrust
    4.75x4.75 - 14.8V / 594W /40A / 1.12kg thrust
    5x5 - 11.1V / 400W / 36A / 0.85kg thrust
    6x4 - 11.1V / 520W / 46.8A / 1.2kg thrust
    ===================
    http://www.hobbyking.com/hobbyking/...p_Drive_28_36_2200KV_696W_USA_Warehouse_.html
    Model: NTM Prop Drive Series 2836 2200kv
    Kv: 2200rpm/v
    Motor Wind: 7T
    Max current: 50A
    Max Power: 310W @ 11.1V (3S) / 696W @ 15V (4S)
    Weight: 87g
    ESC: 40~60A

    Prop Test Data:
    5x5 - 11.1V / 232W / 20.9A / 0.64kg thrust
    5x5 - 14.8V / 516W / 34.9A / 1.021kg thrust
    6x4 - 11.1V / 310W / 28.0A / 0.945kg thrust
    6x4 - 14.8V / 696W / 47.0A / 1.17kg thrust
    ===================
    http://www.hobbyking.com/hobbyking/...p_Drive_28_36_1800KV_438W_USA_Warehouse_.html
    Model: NTM Prop Drive Series 2836 1800kv
    Kv: 1800rpm/v
    Motor Wind: 9T
    Max current: 30A
    Max Power: 438W @ 15v (4S)
    Weight: 84g
    ESC: 30~40A

    Prop Tests:
    5x5 - 11.1v - 123W - 11.1A
    5x5 - 14.8v - 303W - 20.5A
    6x4 - 11.1v - 182W - 16.4A
    6x4 - 14.8v - 438W - 29.6A
     
  6. LukeWarm

    LukeWarm Top Gun

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    All True.
    Because of your post, I added this to post one.
    Thanks for the help Squishy.

    A watt meter and real world testing of your power system will let you safely get the most from your motor. Manufacturer Specifications will put you in the ball park, but it's no substitute for checking to see what that motor will do on a particular plane, power system and prop, on that day, in that weather. A watt meter is a nice tool to have, but frequently checking the heat is the number one tool. It will give you a good sense of what you need to do to take care of your motor. The best case scenario is to use a prop that is aggressive enough to where you have to use a little heat management to maintain it at an exceptable temperature. The motor’s self-cooling can only dissipate so much heat. Anything beyond that point, the motor will overheat. The more load, the more amp draw (current), and the more heat. Heat management is flying the plane in such a way as to control the frequency and severity of the loads that are placed on the motor, therefore control the heat. The max amp draw and max heat occurs only when you hear the motor struggling the most. High alphas are hard on a motor, use a conservative prop for this type of flying.
     
  7. LukeWarm

    LukeWarm Top Gun

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    That is a real complement. Someone that writes as much and as well as you, knows best of all, what it takes to write something like post one.

    When talking to someone, you can see on there facial expressions and body language, so you know whither you've helped them or not. When writing, you don't have that, that's why wording is so important. There are 1000 ways to get your point across, 995 of them do a poor job.

    A lot of what I write takes some deep thinking. Most of it, I get off the top of my head, one plus one equals two. But I also do research and draw conclusions from it. I often refer back to my own tutorials to get and give advice. Revisiting the tutorials like that gives me the opportunity to update the article as my knowledge increases and writing skills improve. Its nice to have friends that challenge my intellect, it's how we improve each other.
     
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  8. squishy

    squishy Top Gun

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    EXACTLY!!!
     
  9. Airforce101

    Airforce101 Top Gun

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    Well, if you put it that way...... ;)

    ...might I suggest adding the word "square" to the pipe dimensions in the first post? The way I read it (maybe it's just me, if so I apologize), you're comparing pipe diameters, in which case simply reducing the pipe diameter by 50% doesn't give you the twice amount of thrust velocity. As you may very well know, the formula for calculating the area is [​IMG]r2 , rather than 2[​IMG]r which calculates the circumference.
     
  10. LukeWarm

    LukeWarm Top Gun

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    I thought it might be close, but I just did the math, and its not. I'll find a way to fix it.

    Pi = 3.14
    Pi x R squared = the area or the circle
    .25 x 3.14 x 6 inch pipe squared = 28.26
    .25 x 3.14 x 7.25 inch pipe squared = 41.26
    .25 x 3.14 x 8.5 inch pipe squared = 56.72
    This works, I'll use it.
     
  11. LukeWarm

    LukeWarm Top Gun

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    I've have never seen a chart like this. Assuming no losses,
    this chart shows the volume of air that goes through each prop during one revolution.
    This number is important because it is directly proportional to the motors amp draw on like props.
    I made this chart to answer some prop relationship questions.

    This paragraph below is edited into this post due to Grey and others. Thank you all for your comments and help.
    The chart's intent is to give each prop a number that corresponds with the general load that prop will inflict upon the motor. If your motor is running too hot, the volume calculations chart can tell you what props are a step down, and how much of a step down the lesser prop is. The opposite is true for a cold running motor. That is the chart's only purpose. Some plane's designs favor power or speed, so some listings are inappropriate for some planes. Prop length is power, prop pitch is speed. This chart works best when comparing props of the same design and manufacturer. The Load is determined by the energy that is used to overcome the resistance of moving a volume of air. Load, energy usage, work done (air volume moved), all 3 are related. But the resistance is a variable that is determined by many things, and that is the problem with this list . If you are familiar with the variables and there affects, like props of different designs and manufacturers, Prop pitch and length, power system strength, and aircraft aerodynamics and weight; you can make the appropriate adjustments to the placement in the chart for your particular application.

    In order by prop size:
    Prop Size ---- cubic inches per turn
    4.5x4.5 ------------ 71.53
    4.75x4.75 --------- 84.13
    5x5 ----------------- 98.125
    5.25x4.75 --------- 102.77
    5.5x4.5 ------------ 106 86
    6x4 ---------------- 113.04
    6x5 ---------------- 141.30
    6x5.5 -------------- 155.43
    7x3.5 -------------- 134.63
    7x4 ---------------- 153.86
    7x5 ---------------- 192.33
    7x6 ---------------- 230.79

    In order by volume or amp draw
    Prop Size ---- cubic inches per turn
    4.5x4.5 ------------ 71.53
    4.75x4.75-----------84.13
    5x5 ----------------- 98.125
    5.25x4.75 --------- 102.77
    5.5x4.5 ------------ 106 86
    6x4 ---------------- 113.04
    7x3.5 -------------- 134.63
    6x5 ---------------- 141.30
    7x4 ---------------- 153.86
    6x5.5 -------------- 155.43
    7x5 ---------------- 192.33
    7x6 ---------------- 230.79
     
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  12. squishy

    squishy Top Gun

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    But what brand of props was used to create those numbers?
     
  13. LukeWarm

    LukeWarm Top Gun

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    For all brands. Those numbers come from the prop size.
    The area of the prop times the pitch gives you the cubic inches per turn.
    For a 6x4 prop *.25 x 3.14 x 6 squared x 4 pitch = 113.04 cubic inches per turn

    If your point is, you can't compare a GWS to a APC prop on that chart; I agree with you.
    The number is directly proportional to the motors amp draw on like props.
    Compare apples to apples, and you will do fine.
    For a prop that is shorter or has more pitch, the losses will be higher.
     
  14. squishy

    squishy Top Gun

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    I know for a fact that different brands of props with the exact same numbers provide different amounts of thrust. I have to battle this fact on my quad and went through many brands till I found the one I wanted which worked best with my motors and flight controller settings.

    A wood 10x5 nitro prop will not produce the same thrust at the exact same RPM as a APC 10x5, and if you used the Grupner props, you would need to go to a 11x5 to get the same thrust. This is well known tribal knowledge in the multirotor world. We seek balanced props not just thrust and sometimes thrust suffers based on the brand choosen.

    I know this may be hard to believe, but it's true. All props are not created equal. This is another reason for real world testing, I no longer rely on the numbers and stats given to me by manufacturers, I only trust my own data with a wattmeter...
     
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  15. Sharpy

    Sharpy Cadet

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    Does this work similar to a camera lens? Mutliply by 1.4 to give as near as twice the area? eg f4 x 1.4 = f5.6 5.6 X 1.4 = f8 (actual 7.84) x 1.4 = f11 . Simplifies it to make an assessment?
     
  16. Airforce101

    Airforce101 Top Gun

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    Ehm, I think you have mistaken the diameter for the radius. To use the diameter, use the formula 1/4[​IMG]d2. A 6 inch pipe would have an area of roughly 1/4 * 6 * 6 * 3.14 = 28.26 square inch

    Just my 2ct's...
     
  17. LukeWarm

    LukeWarm Top Gun

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    I've made the correction.
    Thanks for the help!!
    It didn't change the order,
    it just made the numbers a little smaller.
     
  18. LukeWarm

    LukeWarm Top Gun

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    6x4 ---------------- 113.04
    6x5 ---------------- 141.30
    The difference ---- 28.26

    6x4 ---------------- 113.04
    7x4 ---------------- 153.86
    The difference ---- 40.82

    The main reason I started this chart in post 11 is to see the relationship between "going up one inch in length", verses "going up one inch in pitch". In this case, going up an inch on the length moves 30% more air (thrust) then going up one inch on the pitch.
    With respect to motor load; 1 inch on the length = 1.5 inches on the pitch.
     
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  19. LukeWarm

    LukeWarm Top Gun

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    I was hoping that you would post and do a rant about the different types of prop, the weaknesses and strengths of each one. You know more than most people about this (including me).You got off to a good start; If you would do a detailed post on the different prop types, you would help a lot of people. All prop manufacturers engineer the design of there prop uniquely. They have different strengths and weaknesses. Some are designed to where they do a certain job better than any other prop. In designing a prop, when you take from one area, you give to another.

    Like an APC prop; It is a great all around sport prop. It has consistent power through out its entire RPM range. This is one of the noisiest and most highly engineered props you can buy. For a camera platform plane, you want a quiet power prop that is very efficient at its target RPM range; A GWS prop would work well here. The problem with the GWS prop is, if its not in that target RPM range, it is weak and inefficient. To get the most out of a prop, you need to buy the right one, then use it the way it was designed to be used.
     
  20. squishy

    squishy Top Gun

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    I really wish I could give specific details but my knowledge stops where my needs are met.

    The APC prop is designed and manufactured right down the road from me and they really hook me up with great bulk deals. I have a box of about 100 APC props sitting here, and can you belive I need to run to the hobby shop today to pick up some more 6x4's for combat tonight? dang it. I have settled on APC slow fly props for my multirotors but they are still not the best for that application, but they are balanced and provide lots of thrust at speed.

    Like luke just stated, props work best at certain RPM's or ranges but there's many other factors such as static thrust performance vs dynamic thrust performance. In multirotor we have a prop spinning and not moving, it's just hovering so the current design of props, all of them, suck for us quad pilots, we are simply trying to find the best thing that works. Companies, including APC are in the process of redesigning props for our uses but this fact alone is very telling. This means props are designed for a certain power output, rpm and also airspeed. Like a EDF that requires speed to reach max efficiency and "unwind". There's simply many many factors that go into prop design, and to be frank, much of it is way above my head. There are few engineers who can explain it in laymen's terms but I try to catch what I can in the fpv forums, there's some smart cats in there.

    For our uses here, it's pretty simple. We shouldn't be recommending anything other than APC 6x4's, they are by far the most efficient props with the highest thrust for their size, that's a well known fact. They achieve this by computer aided design and testing, each part of the prop, every curve and bump and even the shape of the tips is highly engineered. I actually don't think you can improve much on the APC speed design, at least not for parkjet use. They suck at static thrust, meaning from a full stop, but then again they are not designed for that, they are designed to fly through the air at speeds.

    There's factors like airfoil, angle of attack and even flex that go into those goals. APC is currently working on designing a prop for quads and it will be quite different than anything they currently produce, l I am curious what the computer will make it look like.

    Nitro props, which is something I know even less about, are made for a higher RPM, less torque, they have a straight blade and thicker airfoil. The design is very very different than an APC prop or any electric scimitar style prop.

    Like I said before, for our purposes here we should stick to one prop, depending upon availability, and my recommendation for all brushless motors is APC props. WHY? because I have actually tested. If you put you airplane's nose on a bathroom scale laying sideways and go full throttle you get a measure of the thrust, it may not be accurate but it's accurate enough to compare propls side by side. All the props I have tried, mostly from china, suck compared to APC, not to mention you have to balance them all by hand. I have seen the difference in thrust from one brand to the other, and APC always edges out the competition. If you are able to standardize the props you speak about then the numbers presented in the forums and info threads would be consistent and accurate, with a disclaimer about brands of course.

    The people over at APC are very nice, I don't know if you guy realize this but you can buy directly from the factory and they ship it quick. Here's a link: http://www.apcprop.com/default.asp

    You might want to email them and get more technical data that can be published here, they would be able to explain themselves better than I could.

    And here's some information about their RPM limits luke, this might be helpful. They are not going to publish info about static thrust or the max efficient speed because it's simply not important to RC plane pilots...YET...

    APC Suggested RPM Limits:

    Glow Engine and Speed 400 Electric Props
    Maximum RPM=190,000/prop diameter (inches)
    (For example, a 10x6 glow engine prop should be limited to 19,000 RPM)

    Thin Electrics and Folding Electric Props
    Maximum RPM=145,000/prop diameter (inches)

    Slow Flyer props
    Maximum RPM=65,000/prop diameter (inches)

    Racing Props
    8.75 N,W and 8.8 series 40 Pylon props
    Maximum RPM=225,000/Prop diameter (inches)
     
    trying2fly and LukeWarm like this.
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