Restrictor Plate racing

[JfromJAGs]

Mic, don't get me wrong. My suggestion is not to make the boats slow. But I suggest to use the equipment within its design limits and this obviously means for the given motors that the limited boats will need to run a bit slower than now. If you want to keep the speed then use appropriate motors - which need to be bigger than those selected now.

Lohring, as written in the last paragraphs: the coefficients are correct, I don't question them. But as shown above, they only apply under certain constraints: they assume that all props you compare got the same pitch ratio. If thats the case, yes, power increases to the fifth with size. But again, this increase is split: to the second based on diameter and to the third because of the increased absolute pitch. So pitch behaves similar to RPM - also to the third - which makes sense to me and which I had guessed in an earlier post. The above formula #6 describes pretty much my practical experience.

Joerg

[lohring]

Joerg, I'm a slow learner and my math skills are a little rusty. I'm slowly working my way through your arguments. I think I'll calculate some examples from the Rolla data for our prop sizes. I also need to get more data from some real boats. Currently I have three sizes of basically the same hull: an 1/8 and 1/10 scale Executone as well as an electric sport 40 with the 8255 design. All were designed to be limited power plant prototypes, not open class boats. The 1/8 scale boat holds the NAMBA two lap record, and I have the data from that run. Brian is running the sport 40 this weekend and I'll get data from that boat. The 1/0 scale boat is ready to test. They should make great prop test hulls.

I'll start reporting on their performance data.

Lohring Miller

[lohring]

In my area the scale racers are the most active in investigating spec rules. The latest from RCU where they are trying to get electrics to race equally with 11 cc nitro boats are:

Motor- NEU 1527 1.5Y. or equivalent. (825 Kv)
Batteries - 14.8V 4S lipo cells (2 per set for a total of 8s)
ESC – Open. (Minimum of 150 amp continuous)
Prop - up to 55MM diameter

The first race of the year results were:
R/C U's 1st race is in the books. 1st place Dave Brandt driving His E-lam, 2nd- myself (John Olson) with my Bud Light, 3rd went to David Newton driving an electric Formula boat 2.

Those specs are very close to my NAMBA two lap record holding 1/8 scale boat with a Scorpion 800 Kv motor. It looks like they are accomplishing their goal of equal racing, but the season is young. RCU has some of the best racing drivers in the country with years of experience building and driving scale boats. It should be an excellent test of propeller restrictions added to spec Kv motors.

Lohring Miller

[JfromJAGs]

What does "or equivalent" mean? At how many amps are these motors usually run in these setup?

Joerg

[lohring]

I thing "equivalent" means the Kv needs to be similar. These are trial rules, so they will be tweeked.

"In the spirit of good sportsmanship, any FE boats having a noticeable speed advantage over the nitro boats will be asked to detune their boats to ensure competitive racing. If they do not comply, they shall be disqualified from the class for the race weekend."

Some data from my 1/8 scale boat during its record run as well as a heat from Brian Buaas driving my sport 40. The Sport 40 runs:
Scorpion HK 4025 1100 KV motor
Turnigy 180 amp speed control
6S 5000 40 or 45C batteries

Below is a graph of the Mutt's data for an entire heat from the launch at 350 seconds through the mill to the start at 400 seconds to the finish around 490 seconds. Note that the rpm at around 20,000 (that's about 80% of the nominal 1100 Kv x 22.2 volts no load rpm) is a little low compared to nitro engines, the average current is around 130 to 140 amps, and current spikes go to over 190 amps, especially when you hit the throttle at the start. Not as visible on the graph is the voltage. Freshly charged it was 25.18 volts. At the start it fell to 20 volts and then held to between 20 and 23 volts for the rest of the race depending on load. The final voltage after the heat was 21.16.

The 1/8 scale boat is 20+ years old equipped with a Scorpion HK 4035 800, a Castle ICE 200 lite ESC with added water cooling, and two Gens Ace 5300 4S packs in series. Both boats were designed as "spec" class prototypes using low cost components.

Lohring Miller

[JfromJAGs]

1. You are running rather low RPMs. They are not the preferred playing field for 2 or 4 pole motors. Because of IR (12mOhm) and idle current (1.5A) the efficiency of a 1527/1.5Y at 8s maxes out at about 60A. So running this motor at 150A or more is not the preferred solution.

Higher pole motors are preferred for such low RPM applications. For example a 1912/1Y got the same RPM, only 11mOhm and the same 1.5A idle current, but weights only half. So if available, I would try to get a 1924/0.5Y. Double length and half the winds will ensure same RPM, but only half of the IR. A 1924/0.5Y weights pretty much the same as a 1527/1.5Y, but will have its maximum efficiency around 100-120A - which is more appropriate for what you are doing.

2. Are you running the same 55mm props on the nitro boats? If thats the case, then the electric motors should have an advantage concerning acceleration over the nitro boats, because of higher torque under load. If you find this to be the case and if you want to make the electric boats more equal, then I would suggest to use smaller props and higher rpm motors for the electric boats.

Besides the formulas we had above, which indicate that at speed there are many solutions to use props for a given power level, acceleration is more a question of static thrust. And static thrust is higher on bigger props - actually the efficiency of bigger props is higher at such static conditions. So going to smaller props and higher RPMs for electric boats will make them "softer" out of the corners.

Joerg

[Randy Naylor]

After coming back from the Nat's P LTD classes are doing great and was a lot of fun. I think if I get a hearing aid I could enjoy the race better. Brian and I were talking about limiting 1/8 scale and did speak about the 1527 1.5Y motor. I think I really like the 8S part that would slow the speeds down and help stop damage to hulls and for the racer that wants to go balls out leave it open in T Sport and you could run the same boat with a battery change if so inclined. I like ware you guys are going with this.

[lohring]

Brian ran the same boat in 1/8 scale and T hydro. It fits the RCU electric 1/8 scale rules including prop specs. He won both classes. The boat is a 20+ year old Dave Frank Executone originally raced by Jerry and Paul Dunlap in the late 1980s. I rebuilt it first for nitro, then for electric. Brian refined the details, drives really well, and the boat just keeps getting faster. It and the advances with the P limited boats are a lesson in how hard it is to successfully restrict a class. The best racers can take any set of rules and still go faster.

Lohring Miller

[JfromJAGs]

Originally Posted by lohring

It and the advances with the P limited boats are a lesson in how hard it is to successfully restrict a class.

As pointed out in the above discussion, what you got now is only a small restriction. A similar feature to "restrictor plates" would need further limitations - if one wants it.

Originally Posted by lohring

The best racers can take any set of rules and still go faster.

I doubt this has ever been a question and I doubt anyone ever wanted to change this. For restricted classes it is accepted and actually wanted that the best driver wins - not the one with the deepest pockets. So nothing wrong with this. A side effect of restrictions is usually a more leveled field - which is not bad for the moral of the drivers who do not win.

Joerg

[lohring]

how about the power sensing circuit at the bottom of this page?

Lohring Miller

[JfromJAGs]

1. The question is - what do you want to achieve? If you want to simulate a fuse, then this circuit together with a low IR shunt (0.1-0.2mOhm) is for sure better - kind of an electronic reusable fuse.

But would I want this behaviour of a fuse, that is, shutting everything down once I'm over a certain power limit? Me, no. First, too dangerous for the ESC. Second, too dangerous for the following boats. If one would want to have a power limited system, then I would suggest a overpower counter - a system that counts for how long you are over the limit. After 2s a first red LED comes on, and after 5s a second LED - then you are out. Change the times to whatever you like.

2. I have thought about such analog multipliers for an energy limiter. This would solve the problem of the error that happens at partial throttle when you first averaging voltage and current and then multiply them in the micro controller. But so far I have not found any of such analog multipliers with reasonable linearity and accuracy - and a reasonable price.

If one would use a 0.2mOhm shunt then 250A would equal 50mV shunt voltage. There you already have 0.8% typical inaccuracy. Going down to half throttle currents (50-100A) the inaccuracy is 2% and becomes much higher below this. The total accuracy of an energy limiter should be more in the 1% range - including all error sources.

Joerg

[lohring]

Still, the MAX4211 type circuits are close to what could work. The cost for 1 is around $4, and 1.5% accuracy is also pretty good. At the least, it could be connected to an alarm type circuit running a flasher, as an example, that would announce an over power condition. Contest supplied units could be calibrated for accuracy near the power limits. That could result in a suitable penalty during a race without affecting anything else.

Lohring Miller

[JfromJAGs]

If you want some logic behind this circuit then you need a micro controller. Then you will end up very close to an energy counter or limiter concerning hardware - the rest is just software.

Joerg

[bbr]

It,s great to see Franks boat back action.Raced with Paul & Jerry back in the 90s on the RCU circut. Hats off to Buass & Lohring.
Wild Bill U-62 RACING