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Old Oct 04, 2012, 04:48 AM
WestTexasKing is offline
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Twisted, several guys run 4S on the JPower, and I believe some of them use the original prop.
Here's what you need to know about pitch and going up a cell...and if you don't care for technical stuff just read the last paragraph LOL

Pitch is measured in inches, so a 12" pitch means it will travel one foot forward in one rotation, but there's a catch to that.
It's geometric pitch that's measured, which means that it will only go 12" forward theoretically, as if you drew out the actual flight path of the blade.
In reality, the blade won't go that far due to the fact that it slips through the air (just the same as a flat wing won't maintain altitude without some sort of deflection and positive angle of attack).
The difference between geometric pitch and what you actually get is called blade slip.
The effective pitch of a blade depends on several factors, most notably what the blade is attached to and how fast it's going.
If it's bolted to a very large, draggy airframe, it'll never gain enough speed to hit geometric pitch.
Bolt an EDF to a parachute that hits terminal velocity at 20mph and it'll never reach the geometric pitch of the EDF's blades.
The other factor is the RPM of the blade...transonic and supersonic flight is inefficient, so when a blade tip goes supersonic it more or less rips through the air projecting a shockwave rather than propelling the air rearward.

Ideally, if you encounter excessive slip, you would increase the diameter of the blade while maintaining pitch (and if there's limitations on diameter, you increase blade count...ever wonder why warbirds have 3, 4, and 5 blade props?).
The reason for blade slip is because there is insufficient thrust to match the drag of the airframe at those speeds.
Increase the thrust (blade diameter or count) and you generally get closer to the geometric pitch of the blade.
For example, I had an EDF that would hang around stalling speed throughout the flight because it lacked sufficient thrust...yet the EDF/motor combo should have netted a flight speed of 100+ (airspeed from EDF was well over 100mph).
It had the pitch to theoretically fly around like a missile, but not enough thrust to counteract the airframe drag.
Likewise, I also had a pitts with a 4.7" pitch prop that would hang around stalling speed...but that was because the prop/motor combo was reaching its max geometric speed, not because of a lack of thrust.
It would hit max speed pretty much instantly on full throttle, much like leaving a car in 1st gear and flooring it.
Think of thrust like tire traction and pitch like drivetrain gearing.
Think of larger diameter props as wider tires and multiple blades as additional driving axles (or dual wheels, whatever) to envision the similarities...both net you more traction in the air.
You can have the power and gearing to hit 200mph in a car, but if you have bicycle tires you'll just spin them like crazy once aerodynamic drag takes effect.
Likewise, you can have the power and traction for 200mph, but if your car is stuck in 1st gear you'll never hit that speed.
Suppose you have a car with more power than necessary, the gearing to get up to 200mph, but just enough traction so that at 200mph you're riding on the verge of spinning the tires.
Throw out a drag chute and you now begin to spin your tires, so regardless of how much power you add, you'll never get back to 200mph without cutting the drag chute off.
That's how blade slip can be defined in simple terms, and the only way to combat it is to add traction.
One drawback...bigger diameter and multiple blades increase drag and reduce efficiency.
You don't see Bonneville racers with giant semi-truck tires or 6 driving axles because they create a lot of drag.
They're as narrow as they can possibly be without losing traction.
Large diameter blades are also more susceptible to going supersonic because they travel faster at the tips.
So, the fewer blades you have and the smaller the diameter they are the more efficient it'll be, so long as it provides enough thrust to propel the aircraft along without excessive slip.
You can see the dilemma the engineers were faced with in WWII when aircraft were fast approaching the speed of sound, all the while trying to maintain prop clearance AND finding a way to efficiently sink thousands of HP into a prop to turn it into speed.

Just as a side note, when you double the speed of an aircraft, you quadruple the drag.
So, if your model flies at 60mph on 300W, it would take something like 1200W to hit 120mph, everything else being the same.
It's a neat little tidbit I learned from an unlimited air racer at Reno.

Back to the subject...
Increasing voltage:
1. You will gain speed
2. Your amp draw will increase at full throttle
3. Your flight times will increase as long as you maintain the original wattage you flew at

-1. Added voltage increases RPM, increased RPM effectively increases pitch.
6" pitch will net you 5,000ft/m at 10,000RPM, or 56mph
Double the voltage with same prop and you get:
6" pitch will net you 10,000ft/m at 20,000RPM or 113mph
Double the pitch with same voltage and you get:
12" pitch will net you 10,000ft/m at 10,000RPM or 113mph
Double the pitch and double the voltage and you'll get:
12" pitch will net you 20,000ft/m at 20,000RPM or 227mph

So, doubling the RPM (double voltage) does the same as doubling the pitch, in a theoretical sense.
What does an extra cell equate to in pitch then?
In a very crude example...if a 1,000kv motor is used on 3S with an 8" prop:
8" pitch, 12,800RPM(3S)=8,533ft/m
8" pitch, 16,800RPM(4S)=11,200ft/m
Running 4S should give you roughly 1.3x the speed of 3S, or the equivalent of going from an 8" pitch to a 10.5" pitch (8x1.3125=10.5):
10.5" pitch, 12,800RPM(3S)=11,200ft/m
My experience is that adding a cell is much like adding 2" of prop pitch, and the above calculations mirror those findings.

-2 For the reason listed above, increasing the voltage is similar to increasing pitch.
Added pitch makes the motor work harder because the blade is moving the air faster (or moving more air, depending on perspective).
As a blade's RPM is increased, so is the drag and force required to maintain that RPM (remember how drag increases 4x for double the speed?).
Also good to note that it's not increasing thrust, but rather the effective expect to see more amp draw and be sure your ESC is more than capable of handling the additional load.

-3 If your plane flew with 300W on 3S, reduce power until you're in the 300W range on 4S and you'll theoretically increase flight times by 33%, here's why:
A single cell 1,000mah battery contains 4.2 watt/hours of energy (1A/h x 4.2V=4.2W/h), meaning you can draw 4.2 watts per hour, 8.4 watts for 30min, 16.8 watts for 15min, 33.6 watts for 7.5min, etc. until you reach max discharge rate.
A 2S 1,000mah battery contains 8.4W/h
3S ~~~~ 12.6W/h
4S ~~~~ 16.8W/h

So if you're drawing out 300W on 3S, your theoretical flight time would be 2.52 minutes.
On 4S, maintaining 300W, your theoretical flight time would be 3.36 minutes.
Of course, that's assuming that the battery will put out a minimum of 300W all the way until it's spent, there's precisely 1,000mah available in the battery, and the plane pulls precisely 300W at all times.
Or, imagine it in another're running a 3S battery and decide to go jump straight to a 6S battery maintaining the same mah capacity.
Well, what's a 6S battery compared to 2 separate 3S batteries?
Just the wiring and a bigger wrap around the battery.
You're basically flying with two 3S batteries stuffed in the plane, so if you reduce the extra power you have to original 3S levels, it's effectively like having twice the battery capacity.

The problem lies in the fact that it's difficult to fly at that much of a reduced power setting when you know you have so much extra power available (especially when the throttle stick is at the halfway point for original power settings).
You might only use it in spurts and go back to sane flight, but that short amount of added power draw from full throttle on 4S adds up quick.
It seems that the balls to the wall tendency in most of us balances out the added power being carried by an extra cell.
Not only that, but there is a weight penalty for carrying the extra cell and additional power is used to counteract all pretty much comes out in the wash I suppose.

To sum everything up...yeah, you're going to get marked improvements in the JPower by going to 4S.
The blades might flex a bit to a lower pitch setting, but all that's going to do is lower the full throttle potential at slower speeds (the blades unload somewhat when approaching geometric pitch speeds and deforming is reduced or eliminated)
Think of it as a variable pitch prop
A stiffer 2 blade will net more speed and quicker acceleration provided thrust isn't reduced.
It's been noted in the thread that significant improvements in power and speed have been realized just by adding a cell and the added amp draw hasn't blown any ESC's to date
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Last edited by WestTexasKing; Oct 04, 2012 at 04:54 AM.
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