Big prop-low rpm or small prop-high rpm
I just finished reading Jim Bourke's "Understanding Electric Power Systems" articles and feel like an enlightened individual with many many questions that I don't have the answers to.
One that has been bugging me is as follows:
I now understand how you might go about achieveing a particular output power with motor/cell/prop combinations. However, given that you need an output of 500 W, is it better to swing a large prop at a slow rpm or a small prop at a fast rpm? The output power of the system is theoretically the same, so what it the benefit of one over the other?
Thank in advance. I'm finally getting a grasp (maybe) on my power systems.
IMO the overall efficiency of a larger prop is likely to be somewhat higher. Apart from anything else less of it is hidden by the spinner or masked by the fuselage.
But "better" always depends on what you are looking for. E.g. you may also need to take things like pitch speed (basically pitch x revs) into consideration. If you are looking to fly fast then you will need a high pitch speed and that's often easier to acheive with a smaller high revving prop. There is also the question of whether a larger prop will physically fit, especially on multis. Just as on full size aircraft it's all compromise.
HTH - Steve
So for higher top speed, a smaller, more "pitchy" propeller should be used? I think I once read that a higher pitched prop makes it more difficult take off as it doesn't become as effective until higher speeds are reached. Is this correct?
From playing with motocalc, it seems like the large props spinning slowly give great static and low speed thrust, but low pitch speed. The small props spinning fast have a low static thrust, but a higher pitch speed meaning a higher flying thrust and higher top speed for the model. For a slowflier, the large props work great, but I couldn't imagine having a P-51 with a 7mph top speed!
The really awkward combination is a high stall speed and a prop that is ineffective at low speeds. It's either very hard to build up speed (ROG) or you need a very good throwing arm.
You have to look at the entire package, and often past glow experience won't help all that much - they are usually so vastly overpowered, it doesn't matter as much what prop is used.
Basically - fast models will use a direct driven smaller prop, a slower model will prefer a larger, slower revving prop. My two 300W models are opposite ends of that spectrum - one burns the sky up behind a 8.5 x 7 on direct, the other acts like a slow Sumo-parkflier on a 12 x 10 geared 3.7:1. Both use the same battery packs, BTW. The fast one is an Aveox Embat - sleek, thin winged and hand toss launches real easy for a 54oz model. The other is my design, with a fat, stubby wing, UC, open cockpit fuselage and much bulkier all round.
Low thrust at low speed is embarrassing if the model only flies at a very high speed. The FAI class pylon racers are a fine example - they take the first lap of a race to get going enough to unstall their little, high pitch props and start really hauling. Not a fun model to fly for fun, I'd reckon, and an Olympic level javelin thrower may be essential for launching to stand a chance.
If you have wheels on a model, you can stand slower initial acceleration - the ground doesn't mind holding the model up a little longer (unless the runway is short :eek: ). If you can't hand toss a model to a speed it can reasonably fly at, misery and rebuilds await you.
Props are interesting devices. My beloved CAP 232 is a converted glow model. As intended, it would normally fly with around a 10 or 11" prop on a two cycle, maybe a 12" on a fourbanger. My usual 14 x 10" (geared MaxCim brushless) barely rates a second glance in an e-power setting, but wet power fliers are amazed at that much prop on the model. I once flew it - very well, too - on a 12 x 11 at an electric site and was ribbed for flying with a tiny little prop! With a 14 x 10, the initial take off roll is slow, but things soon get fun and the wings take over real fat. Good job - hand launching 6.5lb would be overly taxing.
Best tip - rely more on examples seen in reality than in *calc programs!
Regarding the small fast prop vs. the large slow:
There are two fundamental laws regarding impulse and energy that -when understood- clear up things a lot.
The thrust from the propeller is achieved by taking an amount of air and thrusting it backwards. This propels you forward.
The thrust is determined by the mass flow multiplied by its increase in velocity:
F = m * V.
The power required to attain the propulsion is given by the formula:
P = ˝ * m * V^2
F = Newton
m = mass flow (kg per second)
V = increase in velocity (meters per second)
P = power (Watts = volts * ampere)
Now comes the golde clue:
To achieve a certain thrust you can either choose a little fast prop or large slow prop.
You get the same thrust as long as the product of m and V is the same.
But the power needed to drive the system is not independent of the two measures:
If you choose a little propeller the mass flow reduces and the velocity must increase likewise.
This means that if the mass flow is reduced by a factor of two - the V^2 terms is quadrupled.
This means that the power needed from the motor and battery is doubled!!!!!!
The power is inversely proporional to the size of the propeller.
I recently noticed this when a friend of mine was boosting his little EDF. He kept increasing the number of cells onboard but instead of getting vertical performance his plane got heavier and showed less and less performance. The way to go had in hs place been to choose a larger fan as this would have decreased the V and therefore power consumption.
A prop with a pitch to diameter ratio of greater than 2 to 3 will be stalled until it is moving at some forward velocity. The higher the pitch to diameter ratio the higher the speed where the prop unstalls. You can see this in the "inflight" analysis of one of the calc programs.
One interesting side effect of the small versus larger prop tradeoff is that since power equals RPM times torque, and the larger prop spins slower for a given power, you have more torque to counteract with a large prop.
If you take the small high pitch prop to extremes you have something very close to a ducted fan. The average EDF model has the characteristics we've been discussing, it won't ROG and it's almost impossible to hand launch. Bungees are the order of the day to get the beast up to flying speed and even then it's quite normal to have to be careful with up elevator until it's built up more speed. After a while they do get moving and then you're away :)
I guess a pylon model must be fairly similar but I've never flown one.
F = m * a
where 'a' is the increase in velocity (meters per second per second).
E = ˝ * m * V^2
where E is the energy (in Joules or kg m^2/s^2), and V is the velocity (in meters per second), <u>not</u> the increase in velocity, and m is the mass (in kg).
The formula for power is:
P = F * V
Where P is the power in Watts, F is the force (thrust) in Newtons, and V is the velocity in meters per second.
Actually Max had this right – Newton stated that a Force is equal the “time rate of change (d/dt)” of Mass times Velocity!
In Kinematics we think of F = m * a, where acceleration is d/dt velocity.
In fluid dynamics its d/dt (m), mass flow; so its mass flow * velocity – this is known as impulse, just like he stated.
Neither thrust nor velocity (pro wash) alone will tell you how a plane will fly. In a hover thrust is important; in level flight speed might be.
One rule of thumb has you start with prop pitch, determined by your desired level flight speed and then get to diameter based on power where P = k * p * d^4 * rpm^3.
If we hold prop power absorbed constant then we can trade thrust for velocity by changing diameter for thrust and pitch for velocity
Sometimes its useful to think of this old RCM rule of thumb – prop power is proportional to 2*d + p. This shoe that an 8x6 loads the same as a 9x4 – 16 + 6 = 22 = 18 + 4.
In the end its what happens in flight that counts and by using a variable ratio GB (MEC or GP) and a selection of props you are far more likely to match a motor/battery to your plane’s performance.
Direct drive only allows you to play within a narrow selection of props.
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