|Oct 03, 2012, 10:10 PM|
Joined May 2012
Ya, I think adhering to the prop chart by matching an electric motor size to a gas motor should get us a ball bark figure for prop sizes and pitch. The next discussion is regarding battery power....so I can say moving up from 3S to 4S really makes a difference on my other planes performance without changing props.....however I have NOT seen my flight times change that much.
Having noted this....it sounds like the JP P-38 flies just fine on 3S....4S may not provide any noticable difference if the props are not changed in some manner. So I am really curious to get the plane in the air and try to note the difference in flight times and performance between 3S and 4S.
|Oct 04, 2012, 05:48 AM|
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...
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 pitch...so 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 way...you're running a 3S battery and decide to go nuts...you 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 this...it 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
|Oct 04, 2012, 09:45 AM|
Joined May 2012
Outstanding write up thanks for posting. I am printing this off and placing it in my manuals.....Now I am setting up for arrival of the P-38 and definately will operate with counter rotating props on 4S....really appreciate your data regarding motor power, props, battery..
Next step is to continue practicing for the Monahans Manouver...I am still a long way off from getting close to pulling it off...the sttiP is the only plane I have to dare this...LOL
Thanks again WTK - great post. The other threads should see this data. I will be suprised if we don't see some additional remarks from the electrical engineering cloud....although I can't see any space to add more data!!!!
|Oct 06, 2012, 10:03 AM|
On stock prop and motor on 3S, I see small speed increase between 2/3 and WOT. I've got an APC 6x5.5. The speed increase compared to stock is amazing on 3S. Haven't tried 4S on it yet...
I might be wrong but that's how I understand it.
|Oct 06, 2012, 04:32 PM|
Joined May 2012
Well, I am into the build of this model....got more parts for the prop/spinner than I know what to do with. Thought I had the wiring matched for rudder and elevator....wrong. Just flipped them on the transmitter input plugs all good....landing gear are not working, all three will not deploy......the four forward screws for the cockpit pod will NOT seat into their respective receiver holes of the center wing....one of my fuselage sides is pretty sloppy on the wing....one of the landing gear doors is not glued on....
I figure two or three more pots o coffee and I'll have all this sorted out.....the dog has completly lost interest in this one.....this build is indeed a little bit different alright.
|Oct 07, 2012, 02:22 AM|
Joined May 2012
Completed the build, found glue on the front wheel had it stuck to the interior wheel well!!! This was preventing it from deploying, had to counter sink the forward cockpit pod plastic brace, the screws could not get enough bite, glued on the landing gear doors that had come loose from hinges, had to fix both engine covers as they would not find thier respective screw receivers on each sides, had to install two additional magnets to the rear cockpit cover, easily made switches to the elevator rudder swap, I have a slight warp on one of the rudder-stabilizers. Hope to prepare this model for 4S batteries. My aircraft is pretty loose...I thought it was going to be very rigid....but it flexes all over... nothing exceptionally bad.
Also I make note, the nose gear will be actuating any time we input rudder movement since it is tied to the rudder for steering. so even though the nose may be retracted...it is not moving side to side or turning....but forward and aft. I notice the steerable servo is binding each time rudder movement is made while it is retracted.....I can not think of any way to disarm this while it is retracted....
May try to maiden tomorrow....well later today after I nap awhile....LOL
|Oct 07, 2012, 06:23 AM|
What radio do you have? If its a Computer type (JR, Spektrum, Turnigy 9x, etc), you can put a Mix in to disable and centre the rudder servo when Gear is up. You would have to put the Nose Gear Steering Servo on its own channel of course.
|Oct 07, 2012, 10:44 AM|
Joined May 2011
Twistedgrin, you'll need at least 6ch Rx, but it's worth it. I've done the mix to shut off the steering and I'm sure it has made the steering servo and the undercarriage last a lot longer.
|Oct 07, 2012, 12:02 PM|
Joined May 2012
I am operating with a Spektrum DX6i and always use the Spektrum SPMAR7010 DSM 7ch receiver (I like the dual axis reciever radio feature). So I have everything but the brains to do this.
I'll have to study up and learn all about mixing and get this done.
Thanks for posting the idea and capability regarding putting the nose gear turning servo on its own channel and mixing it out when gear is up!!!!!
One more thing on this build....did you guys glue your main wing together or glue any of your wings to fuselage? It still seems "flexy" overall.
|Oct 07, 2012, 05:24 PM|
I only got mine on the 25th sept for my B'day. I went to put it together an found the glue joints on the centre Pod and the Booms to be crap. So I decided to cut/pull them apart and reglue with uhu por. I was a bit shocked how easy they pulled apart! I am going to carbon rod the lot now and reassemble.
|Oct 08, 2012, 01:54 AM|
The faster the relative airspeed, the less resistance on the prop (reduced AOA).
The less resistance on the prop, the faster it'll spin.
The faster it spins, the more AOA the blade experiences.
It all balances out though, and what you see is a steady rise in RPM until you get close to geometric pitch, and at that point the determining factor is airframe drag.
The more aerodynamic the airframe is, the closer you'll get to geometric pitch speeds.
Top speed is a balance between geometric pitch and airframe drag, provided the powerplant is up to the task.
However, pitch does not square the relative airspeed of the airfoil, therefore, it does not square the thrust.
In other words, pitch doesn't increase thrust in the same ratio as lift and drag.
So, simply changing the pitch won't net you the same returns in thrust like increasing RPM would (ie higher voltages).
For some reason I'm confused now...I think I had a point to this post...what were we talking about again
|Oct 08, 2012, 02:41 AM|
Joined May 2011
here's the instructions for shutting off the nose wheel steering in flight. I'm not sure if it all works the same on the DX6i - I'm using a DX7. Your Rx is fine.
RX set up
Disconnect nose gear steering servo from the Y cable combining it into the rudder.
Nose wheel steering servo connect to Aux1/FLAP on Rx. Now the servo which steers the nose gear is controlled by it's own channel.
Steering with Gear down:
MIX1 (this mix connects your rudder channel to your nose steering channel.)
Set RUDD as Master and FLAP as Slave
Set RATE to +100%, +100% (full steering together with rudder stick input. % can be less if one wishes less steering as compared to rudder)
Set SW to FLAP2 (Function is on when Flap 3pt switch is on FLAP2 position. If you flip the switch to FLAP0 or N, then your rudder input does not affect the steering servo anymore)
OFF SET: 0
getting the Gear up:
MIX2 (basically you want to assign the Gear up function to the Flap switch on your radio, since when the switch is on FLAP2 the gear is down and the steering coupled to the rudder. Otherwise, you would probably have to flip 2 switches to get the steering shut off - move from FLAP2 to N or 0 according to MIX1, and flipping the Gear switch up to get the undercarriage retracted. This way I assigned both functions to the Flap switch together.)
Set GEAR as Master and GEAR as Slave
Set RATE to -125%, +125%
Set SW to FLAP0 (by flipping the FLAP switch up to FLAP0 on the TX after take off, it pulls up the gear)
OFF SET: -70 (value to be checked, but purpose is to get retracts up tight w/out straining)
FLAP Sys: 0% for Norm, Mid and Land
Gear: +150%, -100% (also to be checked. Purpose is to make sure undercarriage is fully extended or fully pulled up)
FLAP: D 50 <---- Use this to adjust the nose-wheel center track
Note: all the rate and % values might be reversed on your model depending if one or more channels are reversed. What I mean is if I have a +% Value on my radio, it might be a -% value on yours and vice versa. So please always check everything to make sure it all works as it should.
With this setup the GEAR Switch on your radio still pulls the undercarriage up or down when flipped. However, you should leave it on the down position and not use it any more. Instead you now use the FLAP switch on the Radio to raise or lower the undercarriage.
Mixes are a bit complicated, and I have to always check them a few times to see if all is working as it should, but in the end they are a very usefull tool to get aroung certain things. So well worth going through them and try to understand the logic. Sooner or later you will need them. I also use them for differential ailerons or differential thrust on other models. Never really mixed ailerons and rudder, since I prefer a manual input to an automatic mix. Same with flaps and elevator. Seems to keep my flying/stick skills up, but many good flyers swear to it and use them often.
|Oct 08, 2012, 02:56 AM|
Joined May 2011
Otherwise, yes, the wings are very flexible. Do a loop or a tighter turn and you'll see the wing tips flexing quite easily. The first time I saw it I thought they'd snap off, but they didn't and haven't had any damages until now.
|Oct 08, 2012, 06:19 AM|
Mix1 RUDD > FLAP
Rate 100% 100%
This is so the nose gear servo moves with the rudder
Mix2 RUDD > FLAP
Rate -100% -100%
This is when the gear switch is toggle up and the gear retracts the rudder nose gear servo from mix 1 is now disable and centered.
My servo monitor show the AUX1/FLAP channel moving with the rudder (steerable nose gear) and when the gear is retracted AUX1/Flap channel is no longer moving.. and you can use the subtrim on the flaps channel to center the nose wheel.
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