1/8 Scale rules question

[Doug Smock]

I'm all about those watts, bout those watts, no dimensions.

Yes some real testing needs to be done with motors with VERY similar output.

My scale weighs nearly19lbs and the Neu1527 1.5Y and Etti 200 pro don't even know they are in the boat. IMO if a guy can't get it done with (Like Don said) 21-2500 watts he should consider building a T Hydro.

Just another .02 FWIW

 

[Hydro Junkie]

Terry, why does a scale need to be at 10S? We race at 8S up to 6000 mAH @ up to 4.2 V/C(LiPo) or 4.35V/C (LiHV) and are consistently running at speeds comparable to nitro scales using 57mm props. Are you considering using larger props or just want longer run times or is there some other reason for the larger packs? As Doug said, his boat is heavy and it's apparently getting it done with out going to the larger packs.

 

[Doug Smock]

Sorry I should have stated I am running 10s (less load over 8s for the same 2100 watts) and a 450/3. When I set this boat up I was and still am running with Rick Bellinger's and other nitro scales in D13. I haven't changed a thing even to run with FEs.The boat is competitive, but not over the top.

 

[raptor347]

Mark,

These guys are running IMPBA rules.

Not NAMBA or RCU club rules. They have a different rule set with a longer history.

 

[Hydro Junkie]

I understand that, Brian. I'm just trying to understand why Terry says they HAVE TO RUN 10S. I know what our, as some have called them, "spec" boats can do, just curious as to why those in IMPBA see the need to run bigger packs? It might be something that NAMBA or R/C U might want to look in to if the reasoning is sound

 

[raptor347]

The current IMPBA rules allow S and T power, so 7-10 cells. 10S is likely an artifact of the conversion from round cells to LiPo. It's the power level boats were historically built to.

 

[Don Ferrette]

I understand that, Brian. I'm just trying to understand why Terry says they HAVE TO RUN 10S. I know what our, as some have called them, "spec" boats can do, just curious as to why those in IMPBA see the need to run bigger packs? It might be something that NAMBA or R/C U might want to look in to if the reasoning is sound

Mark go back and re-read Doug's last post. Do we HAVE to run 10S? No. Why do we do it? Simple and the point I made twice earlier that keeps getting "ignored" - with FE a given application takes a certain amount of wattage to get the job done. With the number of FE scales I have built or helped build I have found for them perform reliably and with decent speeds we need to be in the range of 2100- 2500 watts. You have 2 choices how to get there volts or amps as the formula of volts x amps = watts DOES NOT CHANGE. Higher volts (like 10S) will mean a lower overall amp draw. Higher amp draw means more heat, heat is the number one killer of electronics. A properly set up 10S scale will run in the 60s, not break a sweat temps wise and can get it done on a 1P battery set up at 5000-6000mah with a 65-70C rated pair of 5S packs.

 

[RaceMechaniX]

Don did you finish that carbon Velasco scale as an FE?

-Tyler

 

[LohringMiller]

It's true that current is the limiting factor. A higher voltage requires lower current for the same power. However, how are you going to regulate the maximum current? KV, voltage and prop size pretty much set the current in an indirect way. Also anything over 8S requires an HV speed control.

These days either the motor or the speed control is the fuse. At the point of maximum efficiency, motor torque is pretty much proportional to motor weight. Motor power is torque times rpm. Motor can size could be a somewhat inaccurate stand in for motor weight. The list of available motors is going to be very long. The only way I can think of to make the process of power train design simple is to test several suitable motors and make their use required. The list will need to be updated frequently.

Lohring Miller

 

[Doug Smock]

The only way I can think of to make the process of power train design simple is to test several suitable motors and make their use required. The list will need to be updated frequently.

Lohring Miller

I agree FWIW.

 

[LohringMiller]

My apologies if my comments offended anyone.

Lohring Miller

 

[K.R.Joye]

Don your right on point with all your comments, been running FE scales since the beginning. 10S is the way to go these guys that want to run the same boats on less voltage will be going through motors, controllers and possibly lipos.

 

[LohringMiller]

Once again, what do you propose to limit current? Motor burn out? ESC failure? Common sense? I promise a 10S, 10,000 mah lipo battery pack can deliver 600 amps. The simplest method is a propeller diameter limitation.'

'Lohring Miller

 

[Don Ferrette]

Once again, what do you propose to limit current? Motor burn out? ESC failure? Common sense? I promise a 10S, 10,000 mah lipo battery pack can deliver 600 amps. The simplest method is a propeller diameter limitation.'

'Lohring Miller

LMAO! You are so obsessed with your limitations that your "examples" are bordering on ridiculousness. 10S 10,000mah @ 600amps??? That's over 25 thousand watts, about TEN times what a 10S 1/8 scale will draw at full song. Not to mention who the hell wants to haul or even try to fit 6 pounds of batteries in a scale boat?? ( a typical 5S 5000mah pack weighs about 24oz) Time to get real and stop with this nonsense. As I've said numerous times a 60+mph FE scale will pull between 2100-2500 watts on 10S with a 40-45mm can motor, I have PROVEN this repeatedly with boats I've either run myself or helped with. Solid reliable heat race set ups that run cool and get it done all day long on 5-6000mah no problem. You guys want to place a bunch of limits in NAMBA knock yourselves out but let's keep that out of this IMPBA FE scale discussion. Can size limits in my opinion are the best route as it also limits the armature size which limits the wattage potential. Simple, effective and EASY to tech in the field and opens up a ton of motor options (also as I've said before a 56mm can has no business in an 1/8 scale). And lastly, as for your prop diameter limit I destroyed the IMPBA FE scale SAW record at just over 100mph with a 50mm prop on a pair of 5000mah packs that went back into my heat race T rigger afterwards. Ok so it wasn't your average 50mm prop but the point stands, it was 50mm in diameter.

 

[LohringMiller]

My understanding of what's possible comes from experience and data from various electric power plants, not just my opinion. I helped design some of them. From the 1/8 scale and T hydro two lap record holder that also won the 2012 NAMBA Electric Nationals:



From the current full size electric boat world record holder:



The voltage and current is from one 2008 6S battery. Today's batteries are much more capable. More modern data from current record holders is below.

Data from the current world record holding model boat designed and built by [SIZE=11pt]Jörg Mrkwitschka:[/SIZE]
[SIZE=11pt]According to the data logger, the acceleration performance was at 298.6 km/h Pass at about 17kW (590A at 29.0V), startup load after about 4 sec at about 9.5kW (320A at 29.5V). The speed increased up to 63.900 RPM to - jumping over their own wake of the previous pass briefly at 70.000 RPM. [/SIZE]

[SIZE=11pt]Even a 4S power plant has run 140 mph in a rigger. Data from that boat designed by Brian Buaas:[/SIZE]

Hull: Raptor tub, SAW front sponsons and no rears
Motor: TP3650 2Y 3080kV
ESC: Castle Creations Edge Lite 200 with added cooling tubes and conformal coating
Batteries: Dinogy Graphene (4S2P 3300's)
Prop: ABC 1823-19-38 (2) cll S/B (snowman prep)
Driveline: .150 double square cable with teflon liner into Hyperformance outboard stub shaft

13.4 volts, 422 amps peak

[SIZE=11pt]40.500rpm, 9% slip[/SIZE]

Lohring Miller

 

[raptor347]

The 1/8 scale rule proposal passed for NAMBA. So it's moot point for us.

 

[Buddy Benedict]

There again, I am talking about the IMPBA 1/8 Scale FE Rules.

Also Brian, you have a PM.

 

[T.S.Davis]

You need 10s for the reasons Don explained. Approximate watts out needed for a scale. Watts = volts X amps. Less volts will generate more amps at the same watts. The knee jerk is prop size limit. Really? With a prop anvil we can modify a prop such that the diameter becomes irrelevant. Don's record is but one example.

I disagree with Don on the 56mm motors. If you're running a fat pig boat (like my own) it would be nice to have the option of putting a torque beast in there to compensate for the weight. 56 mm has a big rotor and more importantly more poles. Torque is our friend on a heavy boat. Use them to overcome the weight barrier and get up to the aerodynamic limitations of the hull.

Ken was involved in scale right from the beginning. The first FE scale heats that I was ever aware of were run right here in MI at the Cup race. The guys ran and baked a myriad of setups before they settled on 10s and the 1527/1.5y as a staple. In fact, if you look at the motor list in the IMPBA rule book it include the motors that the guys running in the great lakes region found worked. It was 2005 though. They were running 32 sub C's at first. Then 12c LiPo. Then 20c. Woohoo. Very little guesswork went into that motor list. These guys ran it, broke em, ran em some more. They were racing them where ever they could. Randle simply proposed the settled on list.

The only real wild card on the list was the Plett. Nobody really pushed past that low to mid 60 range with it because the boats can't handle it anyway. They're scale. You can't delete the aerodynamic characteristics of the boats no matter how many watts you throw at them. Push an 8255 to 75mph and you might want to put a tail on it because it's actually a kite at that point. We ran a fresh Plett down in Atlanta a few weeks ago. Ran cool as a cucumber. Has potential. Can't keep the boat in the water. Aero may not allow more speed. We'll keep fiddling with it. Makes me want to build a light modern turbine now. Ugh. Just say no Terry. Just say no.

Not everyone agrees with me obviously but in my gigantic head the limitation is/should be ........reality. Reality is the equalizer. We call them "Unlimiteds" but they have a built in limitation. Much like our full scale counterparts. What happens to a modern full scale unlimited when they push faster than the hull can handle....................?

 

[LohringMiller]

You need 10s for the reasons Don explained. Approximate watts out needed for a scale. Watts = volts X amps. Less volts will generate more amps at the same watts. The knee jerk is prop size limit. Really? With a prop anvil we can modify a prop such that the diameter becomes irrelevant. Don's record is but one example.

It's always better to run higher voltage with lower current for the same power. Just tell me where in the rule this lower current is required. It will be determined by whether the motor or the ESC fails first. The batteries have a lot higher capacity.

The power absorbed by propellers has been understood by engineers for over a century. The power absorbed varies approximately as the first power of the pitch and blade area; the cube of the rpm; and the fifth power of the diameter. That means that doubling the blade area (think 4 blade vs 2 blade) or the pitch doubles the power absorbed. However doubling the rpm will increase the power absorbed 8 times and doubling the diameter increases the power absorbed 32 times. I can't imagine bending (increasing the pitch) more than doubling the power absorbed. This same effect can be gained by increasing the diameter 1.15 times or from say 50 to 58 mm. At the same voltage this diameter change would double the current. That's why a diameter limitation is one simple way to limit current. Dedicated current limiting devices (standard or electronic fuses) would be better.

Fortunately, electric motors aren't particularly rpm limited. I have a lot of experience making IC engined boats go faster with rpm limits. That means more pitch at the cost of efficiency. Electrics can vary rpm, pitch, and prop diameter at a particular power level for the most efficient solution to convert that power into speed.

Lohring Miller

 

[LohringMiller]

That's over 25 thousand watts, about TEN times what a 10S 1/8 scale will draw at full song. Not to mention who the hell wants to haul or even try to fit 6 pounds of batteries in a scale boat?? ( a typical 5S 5000mah pack weighs about 24oz)

You finally are beginning to understand the problem with the present electric rules that only limit voltage. By the way, my 8S boat carried 4-5000 mah packs in heat race form. We only needed 2 in series for two lap record trials. As a nitro boat it weighed 14.9 pounds. As an electric it weighed 18.3 pounds with 4.9 pounds of batteries. The electric version was faster. See the picture below.

If you look at the graph I posted, you can see the motor draws around 150 amps in the corners. Something like that is reasonable and is set by the combination of motor KV, voltage, and prop diameter. We could easily run a bigger prop, go faster, and draw more current. What do you propose to set a reasonable current limit?

Lohring Miller