Help with speed setup: high kv, 4s, or fast prop

[Amiromaan]

Hello all!
I'm planning to build the nnP-39 and I'm trying to find out some stuff about the setup. I would like speed, but the 120mph is a bit overkill for me I think, as I have a lot of trees around the field and I'm not super skilled yet.:black_eyed:
I'm still a bit of a noob in this stuff, although I understand the basics. I would like to learn some more about it.

I'm seeing different setups to get speed:
- high kv motors
- 4s-6s batteries
- or fast props like a 10x10

I understand why these setups would provide more speed, but I don't understand the differences between them.

If you use 3s lipo on a 1600kv motor, would that actually be the same as putting a 4s on a 1200kv? They both result in 4800...something rotations (don't know the exact units or voltages). Or do I misunderstand this?

And using a 8x4 prop on a 2000kv or a 8x8 prop on a 1000kv? I understand that the first one rotates twice as fast and I can imagine that that does something with the available thrust, but also maybe with efficiency? But the progression through the air theoretically is the same right?

The RECOMMENDED ELECTRONICS I'm looking at are:
ADVANCED (60mph)
MOTOR - NTM 35-36 1400kv
PROP - 9x6 APC
ESC - 60a Dynam
BATTERY - 2200mah 3S

I'm wondering if this could be upgraded to a 4s lipo once I'm skilled enough for more speed. Is that possible? Or maybe a 8x8 prop instead? I really am confused!

 

[Foam Addict]

Yes that should work fine, the 7x10 prop that was just mentioned in the P-39 thread would be a good speed hop up, but test it thoroughly with the 9x6 first. The 7x10 won't have nearly the acceleration of the 9x6 prop, and if you have a bad launch you might not have enough thrust to fly out of it.

 

[Amiromaan]

Thanks for your reply. I understand that a 7 or 8x10 would be faster than the 9x6. But why choose a faster prop en not choose the 4S instead of the 3S?

 

[dayve]

Thanks for your reply. I understand that a 7 or 8x10 would be faster than the 9x6. But why choose a faster prop en not choose the 4S instead of the 3S?

I won't pretend to be an expert, so others can jump in to correct me if I'm wrong.

One reason for choosing a 3S would just be from a practical standpoint. For instance, if all your other planes use 3S, it would be much more convenient to use that so packs could be swapped. From a purely performance standpoint, you need to balance voltage vs. current. To get 100 watts from a 3S(11.1V) pack, you need to draw about 9 amps. To get that same 100 watts from a 6S(22.2V) pack, you only need to draw about 4.5 amps. Since every wire, connector, component, etc. in the plane has some amount of resistance, consider that the formula for power dissipation (power that is turned into heat) is P = I² + R. In English, that says that the power(AKA wasted heat in this case) is equal to the current squared plus the resistance. Since the resistance is some fixed value, the current is the only thing that's going to change in that equation. Since it's an exponential component, every time you double the current through the system, you quadruple the waste heat. At some point you start blowing up ESCs, melting connectors, etc. This is why(although some voltage range will be specified) ESCs, connectors, and wires are all rated in amps, not volts. It's that I² that gets you.

Choosing a prop is a balancing act between swept area, pitch, tip speed, etc. to get enough thrust at the desired speed. I know more about electronics than fluid dynamics, So I'd suggest checking out the WarpDrive prop calculator page. They actually go over some of the considerations there. Besides considering the most efficient tip speed, there are also RPM limits, above which props sort of come apart. I think those would all be well over the optimum pitch speed, but RC planes are known to take some abuse.

If your prop is spinning twice as fast, that means your motor is, too. Motors have bearings. Bearings have friction. Now there's more waste heat to consider.

 

[nerdnic]

Hi there,

I think the jump to 4s is actually more important than a faster prop on 3s. From my experience even though the math says a 3s setup with a high pitch prop will give you the same numbers as a 4s with lower pitch, in reality the motor won't actually spin at the theoretical speed. Your true, in air pitch speed is lower than a simple kv x pitch x voltage / 1056. A 4s will be closer to the math than a 3s because you're actually changing input power (voltage) and not just making the motor work harder (increased pitch). As the motor has to work harder, the kv goes down, so you end up with a 1200kv motor at full throttle instead of the rated 1800kv. That is just an example, but the reasoning is sufficient.

With that said I suggest a setup that will let you jump from 3 to 4s. This gives you the room to grow without too much hassle.

So on the note of props in general. It's true a smaller prop spinning faster is ideal for speed but not always. Some of the fastest plans spin big high pitch props. There is some fancy speed curve that I could never understand that factors weight, drag, voltage, pitch speed, rpm, etc etc. And it probably explains all the math behind this, but I'm not smart enough for all that. I like to keep it more simple.

On a 40" plane with the weight and make up like the nnP-39, top speed is only half the equation. I would not run smaller than a 8x prop on this plane. Why? You'll have no thrust. The immediate acceleration power that you have with a larger prop is key for recoveries, hovering, and general feel of POWER. Top speed is great, but having that thrust is what makes the plane fun to fly, in my opinion at least.

This is why I run the 10x10 on a 1100kv motor. All the thrust you want with all the top speed you need. Best of both worlds

The NTM you list is a great fit and can run a 8x prop on 4s when you're ready. However, I would recommend the NTM 35-48 1100kv with a 10x10. On 3s it will be just a tad faster (75 ish) than the 1400kv but have way more thrust. Then you can jump to 4s with the same prop when you're ready and get around 100.

Doing the same thing with the Tacon would give you 85 on 3s and 120 on 4s.

 

[Amiromaan]

Thanks guys! For this very broad explanation from multiple sides. It is all much clearer to me now. I understand the logic behind the low kv - high pitch combo. However, the high pitch props seem to be a bit harder to find for me, which also means they are quite expensive and I'm trying to keep a small budget.
Furthermore, maybe I'm a bit intimidated by the speed. I fly with a very simple 4-channel transmitter that came with the Hype Funtastic. So no expo and no possibility for flaps. Is this a problem?

I also found the 1300kv EF-1 combined with a 60a ESC (and 2200 3s which I already have). But I cannot find to much about this motor and I don't know if a 60a ESC is large enough, otherwise I have to step to the 80a which is again more expensive.

I made the speed calculation using Warp Drive (using 12 and 15v for rpms). All from HK europe:

NTM Prop Drive Series 35-36A 1400Kv / 550W EU20.73
Hobby King 60A ESC 4A UBEC EU18.74
Prop: 2*10x5 EU 2.11
9x6 - 11.1v - 432W - 39A 115km/h
10x5 - 11.1v -510W - 46A 96km/h
11x5.5 - 11.1v - 543w - 49A 106km/h
8x4 - 14.8v - 621W - 42A 96km/h
Total: EU41.58

NTM Prop Drive Series 35-48A 1100kv / 640w EU22.83
Hobby King 80A ESC 4A SBEC EU33.33
Prop: 2*10x10 EU 16
10x10 12v 151km/h
10x10 15v 189km/h
Total: EU72.16

NTM Prop Drive Series EF-1 Pylon Racing Motor 1300KV / 930W EU27.63
Hobby King 60A ESC 4A UBEC EU18.74
Prop:2*8x8 EU 2.32
8x8 - 11.1v - 260W - 24A - 0.9kg thrust 143km/h
8x8 - 14.8v - 560W - 38.8A - 1.45kg thrust 178km/h
Total: EU48.69

All advice welcome!

 

[Craftydan]

in the plane has some amount of resistance, consider that the formula for power dissipation (power that is turned into heat) is P = I² + R. In English, that says that the power(AKA wasted heat in this case) is equal to the current squared plus the resistance.

Niggling point . . . Sorry Dayve, my inner nerd is screaming and won't let it go . . .

P = I²xR not I²+R . . .

P = IxV (formal definition)
V = IxR (Ohm's Law)

P = Ix(IxR) = I²xR

Which further emphasizes your point, so otherwise, your analysis is spot-on.

 

[dayve]

Niggling point . . . Sorry Dayve, my inner nerd is screaming and won't let it go . . .

P = I2xR not I2+R . . .

Which further emphasizes your point, so otherwise, your analysis is spot-on.

:black_eyed: Well, that's embarrassing. Thanks! I honestly don't even know how I messed that up. R is a constant, so it doesn't matter as I goes to infinity, right?

I enjoyed seeing the dichotomy between my reply and nerdnic's. It's the academic answer vs. real world answer. I should have started with "Assume your plane is a perfect sphere..."

 

[nerdnic]

Thanks guys! For this very broad explanation from multiple sides. It is all much clearer to me now. I understand the logic behind the low kv - high pitch combo. However, the high pitch props seem to be a bit harder to find for me, which also means they are quite expensive and I'm trying to keep a small budget.
Furthermore, maybe I'm a bit intimidated by the speed. I fly with a very simple 4-channel transmitter that came with the Hype Funtastic. So no expo and no possibility for flaps. Is this a problem?

I also found the 1300kv EF-1 combined with a 60a ESC (and 2200 3s which I already have). But I cannot find to much about this motor and I don't know if a 60a ESC is large enough, otherwise I have to step to the 80a which is again more expensive.

I made the speed calculation using Warp Drive (using 12 and 15v for rpms). All from HK europe:

NTM Prop Drive Series 35-36A 1400Kv / 550W EU20.73
Hobby King 60A ESC 4A UBEC EU18.74
Prop: 2*10x5 EU 2.11
9x6 - 11.1v - 432W - 39A 115km/h
10x5 - 11.1v -510W - 46A 96km/h
11x5.5 - 11.1v - 543w - 49A 106km/h
8x4 - 14.8v - 621W - 42A 96km/h
Total: EU41.58

NTM Prop Drive Series 35-48A 1100kv / 640w EU22.83
Hobby King 80A ESC 4A SBEC EU33.33
Prop: 2*10x10 EU 16
10x10 12v 151km/h
10x10 15v 189km/h
Total: EU72.16

NTM Prop Drive Series EF-1 Pylon Racing Motor 1300KV / 930W EU27.63
Hobby King 60A ESC 4A UBEC EU18.74
Prop:2*8x8 EU 2.32
8x8 - 11.1v - 260W - 24A - 0.9kg thrust 143km/h
8x8 - 14.8v - 560W - 38.8A - 1.45kg thrust 178km/h
Total: EU48.69

All advice welcome!

The calcs usually give you an idealistic picture. Looking at the numbers they are definitely on the high side and don't seem to account for the fact that you're lugging a plane behind the prop

Having personally run all those setups the 1100kv is still my suggestion. Any of them will work well though so I say just go for whatever you feel will be best.

 

[Amiromaan]

Thanks Nerdnic!
I found the 10x10 in a LHS, and after some consideration: I only live once! So I'll go for the 35-48. Speed! :applause:
I'll start with 3s so I can have an upgrade when I feel the need for more speed.

 

[nerdnic]

Awesome! What size and C rating is your 3s?

 

[Amiromaan]

I have two times these: ZIPPY Compact 2200mAh 3S 25C Lipo Pack
and two 1300mah lipos: 30c and 35c

I really hope these will do....

 

[finnen]

Since we have some smart people here, how about this:

As long as the esc is rated for it, none of the components should really care about what battery voltage is used (in a sense), what they don't want is too high current (within reason of course, really high voltages will lead to all kinds of problems). I have kind of assumed this, but is it correct?

If that is correct, lets assume that we have an esc, motor and cables rated safely for 30 amps. Now we put on a prop that produces 30 amps at 3S, 11.1 volts. That will give us 30*11.1 = 333 watts.

Now, if we step up to 4s, and use a prop that gives us 30 amps (not necessarily the same prop), we will get 30*14.8 = 444 watts

So by stepping up to 4s, we actually get more power (33% more), without giving the esc or motor too much current. I would think that is the benefit of using 4S, or is there a fallacy in my reasoning?

 

[nerdnic]

I have two times these: ZIPPY Compact 2200mAh 3S 25C Lipo Pack
and two 1300mah lipos: 30c and 35c

I really hope these will do....

Your 25c batteries will not hold up. Do you have a watt meter or voltage telemetry for in flight data? I expect your battery to dip below 3.4v per cell under load. This will cause damage to the battery after repeated use.

This battery holds up great:
http://www.hobbyking.com/mobile/viewproduct.asp?idproduct=9942

 

[nerdnic]

Since we have some smart people here, how about this:

As long as the esc is rated for it, none of the components should really care about what battery voltage is used (in a sense), what they don't want is too high current (within reason of course, really high voltages will lead to all kinds of problems). I have kind of assumed this, but is it correct?

If that is correct, lets assume that we have an esc, motor and cables rated safely for 30 amps. Now we put on a prop that produces 30 amps at 3S, 11.1 volts. That will give us 30*11.1 = 333 watts.

Now, if we step up to 4s, and use a prop that gives us 30 amps (not necessarily the same prop), we will get 30*14.8 = 444 watts

So by stepping up to 4s, we actually get more power (33% more), without giving the esc or motor too much current. I would think that is the benefit of using 4S, or is there a fallacy in my reasoning?

This is correct. Just make sure your esc is rated for the desired voltage. Most are rated up to 6s. After this you need high voltage compatible esc.

From my understanding the frequency at which the esc runs at above 6s requires higher quality material and different spacing between the fets.

Here is a good video that talks about escs:
https://www.youtube.com/watch?v=22CpRl2_TRI

 

[Amiromaan]

Your 25c batteries will not hold up. Do you have a watt meter or voltage telemetry for in flight data? I expect your battery to dip below 3.4v per cell under load. This will cause damage to the battery after repeated use.

This battery holds up great:
http://www.hobbyking.com/mobile/viewproduct.asp?idproduct=9942

Oh that's terrible. I already placed the order. Maybe I can arrange fast that they put it extra in my order. But before I do that, I have a question:
What if I put my 2 2200 25c batteries in parallel? Would that be an option? Off course the weight is not very optimal, but would it work? Preventing the voltage drop.

I have a voltage alarm, no watt meter.

 

[finnen]

Putting batteries in parallel will double the amp draw capability. The battery is still 25C, but you are multiplying with 4.4A instead of 2.2A to get the acceptable current. I have no idea if that weight is ok for that airframe though.

 

[nerdnic]

Oh that's terrible. I already placed the order. Maybe I can arrange fast that they put it extra in my order. But before I do that, I have a question:
What if I put my 2 2200 25c batteries in parallel? Would that be an option? Off course the weight is not very optimal, but would it work? Preventing the voltage drop.

I have a voltage alarm, no watt meter.

Parallel will work and be a great way to tackle this issue. Don't worry about weight, the nnP-39 can handle more 2200 3s batteries than you could even fit in the fuse. I fly with a 500g 4000 40c 4s and it doesn't care one bit.

One suggestion though is to make a modified powerpod. You might not have enough room with the long, shallow FT pod. I use a stubby, deeper pod, and run two skewers through the fuse like on my nnChipmunk 800mm and Sonic Series nnP-39.

The flaps and a little bit of right thirst angle is all you'll need to launch and land at pretty much any weight or power setup.

 

[Craftydan]

. . . To be more precise . . .

The only things that care about what the voltage is are the things that directly draw on it for power:

- Your motor -- it will spin faster with higher the voltage. You'll run into two issues with this: over-prop, and over-speed. Keep the prop the same and boost the voltage and the motor will demand more current. Whenever you increase your voltage, generally, you reduce the prop to accommodate this, but if you're careful about it, the smaller prop will generate more thrust. You can also overspeed the bearings, which will shorten their life . . . hard to say from one motor to the next how big of an issue it is.

- Your ESC's UBEC (regulator) -- even opto ESCs have an onboard regulator. Their microcontroller brain doesn't run from 7-24v . . . but their regulator that drops it down to a smooth, constant 3 or 5v does. If it's a switching regulator, typically they're good to some very high voltages. If it's linear, they're typically good to modestly high voltage, but they're also dumping the excess voltage to heat -- too much of that or too warm on the FETs next door and the regulator stops working . . . and your ESC's brain goes into a drunken stupor which might mean shutting down . . .might mean open the FETs up wide

. . . there is one more place it can go badly . . . the LVC sense circuitry. Most ESCs have a voltage-dividing circuit attached to an ADC line (Analog-to-digital converter) to read the battery voltage. The idea is if it's too low, the ESC can gracefully shut down and allow the pilot to land. This voltage divider is sized to work over a particular input voltage range . . . if your input voltage is too much higher than this, it can burn out the A/D. I'd have to research the ESCs to see how they've sized the divider and the chips to know when and how gracefully they fail in this case, but typically it's not pretty . . . one more reason an LVC free ESC isn't such a bad thing . . . but that may just be the multirotor builder in me speaking

 

[Amiromaan]

Parallel will work and be a great way to tackle this issue. Don't worry about weight, the nnP-39 can handle more 2200 3s batteries than you could even fit in the fuse. I fly with a 500g 4000 40c 4s and it doesn't care one bit.

One suggestion though is to make a modified powerpod. You might not have enough room with the long, shallow FT pod. I use a stubby, deeper pod, and run two skewers through the fuse like on my nnChipmunk 800mm and Sonic Series nnP-39.

The flaps and a little bit of right thirst angle is all you'll need to launch and land at pretty much any weight or power setup.

Ok great! I'm proud to start understanding this stuff and having thought of this solution myself! :applause:

Bigger powerpod is a good idea, I will look into that!

The flaps.... a no no for me, as I only have a simple four channel transmitter. I was thinking of a landing gear with wheels, or maybe some sort of static landing gear using 'glide bars' that can take the beating, being just steel wire running along the fuselage and sticking out a bit. But that's just an idea for now...

"a little bit of right thirst angle" -> Do you mean thrust angle?

Would it be possible to put the 1300mah 35c and 2200mah 25c in parallel? Or even a 500mah 25c with 2200mah 25c? Or the last option: a 1300mah 35c with a 500mah 25c in parallel?