Author Topic: Is Thrust King? (Read 1719 times)

Ken Culbertson · « **on:** January 13, 2019, 10:07:56 AM »

I am perplexed by all of the variables that go into electric. I am running a constant RPM timer (Huben). It is my understanding that RPM that I tell it is what the ESC will deliver up to the limits of the battery. If a battery will deliver sufficient amps from a 3s battery to drive a motor at "X" RPM, how would having a 4S battery make any difference other than the motor and battery running a bit cooler and the battery lasting longer. For example, why would a 3S 3000mha battery not be as effective as a 4S 2200 if both can deliver a steady 9000 RPM on the same prop for 5.5 min and still have about 20% left?

The way I see it a prop turning at 9000 RPM will produce the same thrust if it is being driven by a battery, a piston or a kid in a bicycle.

No argument, the 4S is the better choice, BUT the 3S battery is much lighter, cheaper and smaller and it will FIT in an already cramped conversion where I simply cannot move the battery back any more than it already is. I hate adding tail weight. I already have to add a full oz with the 4S just to get the plane to be slightly nose heavy. Every OZ I save on battery is 2 OZ on the weight and 2 OZ less thrust the motor has to give me.

Help me out here - Ken

Igor Burger · « **Reply #1 on:** January 13, 2019, 10:59:16 AM »

If you use constant RPM setup and your motor can deliver needed RPM on ground (loaded prop) on end of its capacity (almost discharged), Then you have enough voltage and you not need more cells.

If you use active timer which has to do some RPM boost, then you need some voltage on top of that, that is why active timers need usually +1 cell more. The rule is that it has to be able deliver maximum RPM on ground (loaded prop) on end of its capacity (almost discharged).

The size of the battery is defined in Watthours. Means voltage * capacity in Ah. Means if you have 3 cell battery at some size and you will go to battery of the same capacity in Wh at the same efficiency (what is not necessary to be true) then you will get battery of the same weight, no matter how many cells it will have. By other words, battery with higher number of cells will need smaller capacity in Ah for the same flight time on the same model if properly combined with motor and prop. So you are right, if you feed motor which needs some power, does not matter what voltage you use, you will need to deliver the same amount of Wh for given time. Therefore the battery will be of the same weight does not matter what the voltage is.

And regarding the thrust - ground (static) thrust does not mean too much. Prop with stronger static trust can easily loose it in flight at cruise speed. And also very good prop which will make perfect flight can loose its thrust in static regime. Do not read R/C tests with static tests, they are may be valid vor 3D airplanes, but not for us.

Tim Wescott · « **Reply #2 on:** January 13, 2019, 11:56:26 AM »

Quote from: Ken Culbertson on January 13, 2019, 10:07:56 AM

If a battery will deliver sufficient amps from a 3s battery to drive a motor at "X" RPM, ...

Not to detract from Igor's response, which is entirely correct in a nicely practical way and should be heeded in every regard, I have a physics/electrical nit I must pick:

Batteries are, roughly, constant voltage sources. The voltage they can supply drops with increasing current, but a LiPo cell's voltage stays within +/-20% of it's rated voltage throughout a flight. Motors, to a similar degree of roughness, put out a given RPM for an applied voltage (or if they aren't, they're so overloaded they quickly burn up). So you really want to think about whether the battery can deliver sufficient voltage to drive a motor at "X" RPM.

But you need to think about whether the battery can do this when it is at the end of a flight, and at those portions of the flight envelope where the most current is being demanded (this happens even with a constant-RPM setup, when the plane gets significantly slower than cruise speed). Battery voltage sags with current, it sags more with current when it's close to discharge, and it has less voltage to start with when it's close to discharged. This is why Igor suggests his ground test with a battery close to the state of charge it'll be at the end of a flight -- running the plane on the ground asks for a bit (but not a bunch) more current than the worst-case load in the air, so if the thing will hold RPM with a battery at end-of-flight charge when it's on the ground, you're fine.

I'm not sure how this works with the Igor timer. I think it has the RPM command at maximum on the ground, because it knows the plane isn't moving, but I wouldn't guarantee it -- particularly because Igor is right here to clarify things.

Ken Culbertson · « **Reply #3 on:** January 13, 2019, 12:54:46 PM »

Am I correct then that the Huben is not an active timer therefore, if the 3s battery will deliver the required rpm at 80% life I should be able to use it? The motor in question is a Cobra 2826/12. I have a 2820/12 with a higher rpm @ volt rating. Maybe a swap is in order?

Thanks for the input. I stand to gain 4-6 ounces if I can pull it off. (FYI when I control the design it is for a 5S!).

Ken

Igor Burger · « **Reply #4 on:** January 13, 2019, 01:35:49 PM »

Quote from: Tim Wescott on January 13, 2019, 11:56:26 AM

This is why Igor suggests his ground test with a battery close to the state of charge it'll be at the end of a flight -- running the plane on the ground asks for a bit (but not a bunch) more current than the worst-case load in the air, so if the thing will hold RPM with a battery at end-of-flight charge when it's on the ground, you're fine.

I'm not sure how this works with the Igor timer. I think it has the RPM command at maximum on the ground, because it knows the plane isn't moving, but I wouldn't guarantee it -- particularly because Igor is right here to clarify things.

It is more question of ESC, not the timer. Speaking about Spin, which has setting for max and min RPM, all is clear, simply timer must be set to maximal throttle. Other ESCs without such settings, for example Schulze which has one fixed regulation slope from extremaly small to extremally large RPM, it is necessary to set timer so that it asks for aproximately 10% (usually 1000rpm) more than necessary for level lap time. That looks like good power boost for usual models.

How to do it on particular timer depends on that timer. It is necessary to learn how it works. My timer with Spin ESC will need simply target throttle dialed to 200. That will ask for max RPM in Jeti ESC. Another way is to use Jetibox as a servo tester and set pulse width 2ms. But way how to do it with particular timer does not change result if it does what I described in first paragraph.

Tim Wescott · « **Reply #5 on:** January 13, 2019, 02:16:24 PM »

Quote from: Ken Culbertson on January 13, 2019, 12:54:46 PM

Am I correct then that the Huben is not an active timer therefore, if the 3s battery will deliver the required rpm at 80% life I should be able to use it?

Unless Will Hubin has been busy, his timers are all non-active (passive?). So, yes.

Quote from: Ken Culbertson on January 13, 2019, 12:54:46 PM

The motor in question is a Cobra 2826/12. I have a 2820/12 with a higher rpm @ volt rating. Maybe a swap is in order?

If I remember correctly, even the 2820/12 only has a Kv rating of 960 RPM/volt. Unless you're spinning the prop at 8000RPM, three cells is going to be marginal.

Quote from: Ken Culbertson on January 13, 2019, 12:54:46 PM

Thanks for the input. I stand to gain 4-6 ounces if I can pull it off. (FYI when I control the design it is for a 5S!).

Those numbers sound correct if you're comparing (for example) a 3S 2800mAh pack against a 5S 2800mAh pack. But if you're going to a 3S pack from a 2800mAh 5S pack, you need a 4000mAh pack.

I just checked the Thunderpower site. A 5S, 2800mAh pack in their "lite" series is 290 grams. A 3S, 4000mAh pack, same series, is 269 grams. That's only a difference of 21 grams (2/3 ounce), not 120 to 180 (your 4-6 ounces).

Ken Culbertson · « **Reply #6 on:** January 13, 2019, 03:10:50 PM »

You guys are getting over my pay grade. I may need to look into Thunderpower Lite.

Weight and size are the issues and it is just on conversions. Maybe the answer is there. Anybody know of anybody that has made up a dimensions chart for batteries?

Thanks Guys - Ken

Tim Wescott · « **Reply #7 on:** January 13, 2019, 03:50:13 PM »

Ken, this is not a Thunderpower thing. It's a Lipo cell in general thing.

Could you please list the 5S pack and the 3S pack that you're comparing? It will help us to help you.

Ken Culbertson · « **Reply #8 on:** January 13, 2019, 05:22:42 PM »

Quote from: Tim Wescott on January 13, 2019, 03:50:13 PM

Ken, this is not a Thunderpower thing. It's a Lipo cell in general thing.

Could you please list the 5S pack and the 3S pack that you're comparing? It will help us to help you.

It is a 4s not a 5. The lightest and smallest 4s I could find was a HRB 2200 @ 220 grams. Still what I am finding is that weight within a MFG appears to be the same per MPH so dropping cells and upping MPH doesn't produce a lighter battery. RATS

Ken

Tim Wescott · « **Reply #9 on:** January 13, 2019, 06:13:44 PM »

Quote from: Ken Culbertson on January 13, 2019, 05:22:42 PM

It is a 4s not a 5. The lightest and smallest 4s I could find was a HRB 2200 @ 220 grams. Still what I am finding is that weight within a MFG appears to be the same per MPH so dropping cells and upping MPH doesn't produce a lighter battery. RATS

Ken

What do you mean by "MPH"? mAh?

The reason that dropping cells and upping mAh (if that's what you mean) doesn't produce a lighter battery is because the amount of energy that a battery can hold is a function of how much energy the stuff inside the cells can hold, and that's going to be constant for a given battery technology. The term is called "energy density", for rechargeable batteries, LiPo is about as good as you can get on the open market right now. If it weren't, someone would be shouting it from the rooftops.

Quality costs money, and a cell's capacity is a soft number (because you can always suck it a bit drier, at the cost of fewer charge/recharge cycles). So the variation from manufacturer to manufacturer in weight vs. rated capacity is going to be a combination of actual quality variation, and spin doctoring (or outright lies, if you buy eBay specials). Folks like Thunderpower will apply your battery dollar to buying cells with slightly better innards, or cells that have more active material vs. the amount of cruft, or the pack assembly being made lighter weight (package, wires, pretty labels, etc.). Nameless strangers on eBay will just lie, or believe the lies that they're told by no-name battery manufacturers and pass those lies on to you. Other people will be somewhere in the middle.

Brent Williams · « **Reply #10 on:** January 13, 2019, 06:18:42 PM »

Quote from: Ken Culbertson on January 13, 2019, 05:22:42 PM

It is a 4s not a 5. The lightest and smallest 4s I could find was a HRB 2200 @ 220 grams. Still what I am finding is that weight within a MFG appears to be the same per MPH so dropping cells and upping MPH doesn't produce a lighter battery. RATS

Ken

ThunderPower 4S 2200. 197g (approx)

http://www.thunderpowerrc.com/AIR_BATTERIES/2200-mAh/TP2200-4SPX25

Tim Wescott · « **Reply #11 on:** January 13, 2019, 06:21:25 PM »

Quote from: Brent Williams on January 13, 2019, 06:18:42 PM

ThunderPower 4S 2200. 197g (approx)

http://www.thunderpowerrc.com/AIR_BATTERIES/2200-mAh/TP2200-4SPX25

Good for a 35 ounce plane, if I'm doing my math right. Not much more (with an eye to my comment about the numbers being soft).

Ken Culbertson · « **Reply #12 on:** January 13, 2019, 06:25:31 PM »

Quote from: Brent Williams on January 13, 2019, 06:18:42 PM

ThunderPower 4S 2200. 197g (approx)

http://www.thunderpowerrc.com/AIR_BATTERIES/2200-mAh/TP2200-4SPX25

On top of that it is 2cm shorter! I will have to give one a try next time they go on sale.

Ken

Ken Culbertson · « **Reply #13 on:** January 13, 2019, 06:42:11 PM »

Quote from: Tim Wescott on January 13, 2019, 06:21:25 PM

Good for a 35 ounce plane, if I'm doing my math right. Not much more (with an eye to my comment about the numbers being soft).

Running a 2200 Turnigy on a 42oz profile (w/battery) with excellent results using a 2820/12 12 x 6 Prop. I had to shorten the flight time to 5:20 to stay around 20% but there is no power loss anywhere in the pattern. The TP appears to be nearly 2 oz lighter.

Ken

Brent Williams · « **Reply #14 on:** January 13, 2019, 07:54:10 PM »

I run the Cobra 2820 970kv, APC 11x5.5e (standard rotation) and a TP 4s 2800 on my 45 oz ('ish) Gieseke Nobler. It has 30+% left in reserve after a 6 minute flight. 66ft (total) .015"lines. It is very strong in the wingover, vertical 8/hourglass and overhead 8. Not lacking for thrust, at all.

My budget doesn't allow for it, but it would be very nice to go to the next size smaller everything, motor, battery, ESC, prop, ect, to drop several ounces directly off of the nose. It is pretty nose heavy currently. I will have to extensively cut into the nose/bottom block and move this battery aft to properly balance the plane without significant tail weight.

Ken Culbertson · « **Reply #15 on:** January 13, 2019, 09:19:07 PM »

Quote from: Brent Williams on January 13, 2019, 07:54:10 PM

I run the Cobra 2820 970kv, APC 11x5.5e (standard rotation) and a TP 4s 2800 on my 45 oz ('ish) Gieseke Nobler. It has 30+% left in reserve after a 6 minute flight. 66ft (total) .015"lines. It is very strong in the wingover, vertical 8/hourglass and overhead 8. Not lacking for thrust, at all.

My budget doesn't allow for it, but it would be very nice to go to the next size smaller everything, motor, battery, ESC, prop, ect, to drop several ounces directly off of the nose. It is pretty nose heavy currently. I will have to extensively cut into the nose/bottom block and move this battery aft to properly balance the plane without significant tail weight.

My Nobler has the 2826/12 which I think is 720kv. I plan to run around 9000 RPM so the 2200 is going to be a stretch. Tried an 11 x 5.5 on my Twister running a 2820/12. I had to up the rpm's by 500 and it put me over the 20% limit so I went back to the 12 x 6. It is almost as hard to get the right power combination as IC. But, once you have it, it's "Punch the little red button" and have a perfect run every time.

Ken

john e. holliday · « **Reply #16 on:** January 14, 2019, 11:42:12 AM »

But, does this dependable power setting work from day to day like windy or calm days, also hot to cold days??

Ken Culbertson · « **Reply #17 on:** January 14, 2019, 12:59:40 PM »

Quote from: john e. holliday on January 14, 2019, 11:42:12 AM

But, does this dependable power setting work from day to day like windy or calm days, also hot to cold days??

So far it has. IMHO electrics don't seem to be anyway near as sensitive to the weather as IC.

I have flown mine in all of the above and my only change was to shorten the run time by 10 seconds. Air density matters and I am prepared to change my RPM's but so far I have not needed to do it.

Ken

Tim Wescott · « **Reply #18 on:** January 14, 2019, 01:50:19 PM »

Quote from: john e. holliday on January 14, 2019, 11:42:12 AM

But, does this dependable power setting work from day to day like windy or calm days, also hot to cold days??

Hey John:

I'm a circuit designer who sometimes builds motor drives, and a stunt pilot flying slime. I've been sitting on the fence for about five years no about whether to change, so I've been reading everything that goes by.

According to what I see, and to my understanding of the underlying process, the electric systems should be less susceptible to changes in weather, but not entirely insensitive. This is because the power system itself isn't as affected by wind, heat, humidity, etc., but the airplane still is.

Most notably, if you size your batteries in "stunt heaven" conditions you may find that you don't have enough oomph at the extremes. I'm not sure about flying in really windy weather, but I've seen reports about folks flying in totally still air and using considerably more charge than in a light breeze.

Igor Burger · « **Reply #19 on:** January 14, 2019, 02:45:38 PM »

Hot or thin air needs more RPM, that has clear relation. I must add little bit as temperature rises or if I travel to another altitude. I have Arduino program for that (throttle as function of conditions).
http://maxbee.net/download/AirDensity.pdf
http://maxbee.net/download/AirDensity.apk

The same is with wind, windy conditions need less RPM.

Temperature also makes higher battery resistance and smaller capacity.

So calm cold air in low altitude will discharge battery much more.

Ken Culbertson · « **Reply #20 on:** January 14, 2019, 03:15:27 PM »

Quote from: Igor Burger on January 14, 2019, 02:45:38 PM

Hot or thin air needs more RPM, that has clear relation. I must add little bit as temperature rises or if I travel to another altitude. I have Arduino program for that (throttle as function of conditions).
http://maxbee.net/download/AirDensity.pdf
http://maxbee.net/download/AirDensity.apk

The same is with wind, windy conditions need less RPM.

Temperature also makes higher battery resistance and smaller capacity.

So calm cold air in low altitude will discharge battery much more.

All of this is quite true but, they are fairly easy to adjust to. With IC the odds of getting it right on the 1st try are much less and a crappy electric run is nowhere as dangerous. I don't get to travel much so altitude never enters in and we normally fly about the same time of day so I am lucky, flight prep for me consists of strapping in the battery and hitting the little red button!

Thanks for all you are contributing, I will put those .pdf's in my flight box.

Ken