Large rigger

[pda75]

Hi

I am thinking about a large rigger. The hull will be home made, with the following set up

10S 2P 5000 mA cells

ESC : YGE 200

weight of the hull : 6 kg

Length : 1,20 m (eg 45")

The objective is to run 5 minutes at 130 km/h (eg 80 mph). My assumption is that I need 4000 W for this weight and this speed.

Given the price, I will not buy a motor for "a test" and I need to choose the right KV with limited risk of mistake. So I would like to get your advice on the best KV .

kv = 750 (lehner 3040/14). Speed would be 80 mph with an 1667 octura prop assuming 15% slipery and 3,4 V per cells

Kv = 808 (Lehner 3040/13). Speed would be 80 mph with an 1467 octura prop assuming 15% slipery and 3,4 V per cells

Kv = 875 (Lehner 3040/12). Speed would be 80 mph with an 1462 octura prop assuming 15% slipery and 3,4 V per cells

The only true data point I have comes from the nitro guys. They get 80 mph with 25 000 rpm and a 1667 octura with an equivalent hull.

off course, I can target more rpm and a smaller prop but I am not sure that it will lead to sufficient propeller traction and that slipery will not dramatically increase.

Thanks for your help.

cheers

Pierre

PS: a lehner 3060/9 has a 778 kv. It could be an option but it adds some more weight.

PS : I have already post this topic inthe rigger section but I got the advice to post it there.

 

[jayt]

I don't know anyone else who is trying to meet your goals of 80 mph AND 5 minutes of run time. If your estimate of an average of 4KW of power output is correct, then you will have difficulty in meeting the 5 minute goal. With an average of 37 volts available and a motor efficiency of 90%, you will exhaust a 10,000 mAh battery in 5 minutes causing permanent damage.

I am not certain if the 4KW power estimate is correct either. I run a 41" cat with a 3060, and in ~2 minutes I use about 4000 mAh of battery power for an average of 120 amps and 60-65 mph. Watercooled motor temperature is usually ca. 115F. A rigger would certainly be more efficient than a cat, but at 80 mph it might draw even more amps.

IMO the 3040 will be too small for this task - it has insufficient mass to absorb the heat for 5 minutes of running. I believe that a 3060 would be a better choice, and a 3080 is not unrealistic.

I know an FE racer who has an 80 mph 10S rigger, but it is for sprint racing with heats of well under 2 minutes duration. He runs a 3040/10 spinning a 60x76 modified prop powered by 4500mAh 30C LiPos. This is reliable for him, but he runs for much less than half the time you desire.

Using nitro props as a guide is wrong for a boat this size. BIG Nitro motors are rpm limited so they need larger props to get their speed. FE motors have relatively unlimited rpm potential, and they usually work best at higher speeds by spinning a higher rpm. My cat usually heat races with an x460/3, but my motor is quite conservative....yet plenty fast enough to pass most all the gas cats I race against. I have no need to run faster or for a longer time. You have your challenge set up for you, and I wish I could have been more help. Good luck!

,

 

[pda75]

Thanks a lot Jayt for your very interesting point of view

I agree 130 km/h may lead to less than 5 minutes. If I have to choose, let's reduce the running time and keep the speed.

The question is now how many rpm ? 35 000, 30 000 or 25 000 ?

First of all, I have ploted some of the existing data point between hull weight and prop diameter :

Though, I do have more data point on mono hull that on rigger (H in french), it seems that the hull weight is link to the square ot the prop diameter. the issue on this chart is that I missing rigger point in the 200 (6 kg) once range.

One proxy would be to start from what I know :

I do have a 10 S rigger which weights 3,7 kg and run 118 km/h with a 50 mm propeller.

For 6 kg, it would leed to 63 mm : 63 = 50 x (6/3,8)^0,5

This rigger has a Kv of 960 (lehner 2250/12). getting 130 km/h with a bigger prop would lead to 825 Kv with 63 propeller

I know that this is a very approximative approach because I look at the prop diameter and not to the pitch. My assumption is that increasing the Pitch/ diameter ratio leads to increased speed up to a certain limits. For most hydro, I have seen, you're comfortable with a 1,4 - 1,6 ratio and going higher works .. sometimes and creates instabillity of the hull.

This approach would lead to a lehner 3060/9 with 778 Kv

feel free to comment.

cheers

Pierre

 

[jayt]

Actually Pierre, for many years I have tried various mathamatical models for predicting props and speeds, and I have come to the conclusion that when comparing the math model to reality - reality wins. Since we have a good case of reality in the large 10S 80 mph rigger using a motor with a Kv around 1000 rpm and a prop with a diameter of 60mm (in my previous post), we have found a solution. I believe there are several paths to any solution, this is just one proven path. Under load with the 1000 Kv motor we would not see 37,000 rpm on the water - probably closer to 34,000.

Based upon my experience with the 3060/10, I would not recommend a 3060/9 for 80 mph speeds. I believe that a Kv between 900 and 1050 rpm would get you where you need to be speed-wise. The motor size would depend on the heat sink requirement - longer run times mean more motor mass.

Assuming the increasing the diameter/pitch ratio is seamless can lead to error. Prop diameter is more directly related to motor load and thrust - pitch determines the speed. But increasing pitch alone also increases the load, and this may require a reduction in diameter to maintain rpm and speed. To find out the theoretical speed, use the pitch and motor rpm under load, then increase diameter until it is large enough to move the model but not so much as to slow the motor. By concentrating on prop diameter alone you may miss an opportunity for more speed. Good luck!

.

 

[SReesor]

Pierre,

I agree completely with Jay.

My 10S rigger was designed around the 30 series lehner motors.

At 45" long and weighing 16lbs it is not a light weight boat. It does however handle heat race water extremely well. It does 80+ with the right prop on it. For heat racing high 70's is usually enough for what I do with it. 1-2 mill laps, 6 heat race laps, one post race lap. I use 10S2P 30C 4500's from FlightPower.

I typically take out ~2300mAh in the above scenario. Cells come out at 125F. Motor and Esc are usually at 110-115F.

I have tried many different props on my prototype hull and it likes the ABC 60x76, best is a 60X80 cut down in diameter a bit and detounged.

My first runs were with a 3040/10T and it was easily in the low to mid eighties. Right now I am working on a set-up for a customer with a 3060/8T with excellent results and lower heats but just a bit slower.

Running for 5 minutes at those speeds is IMO going to be a real challenge. I know I couldn't do it with the current set-up I have, but then again, I don't need to run that long for the type of racing we do here.

The prototype was built heavy by design (for rough water). Next generation will be down around the 13-14 lb range all up weight.

As Jay has said, I have also tried models of pitch to speed to select propellers for FE boats. I have found that different types of BL motors "like" different styles of props. For Lehner motors I like the ABC props (old style) with a lower pitch ratio between 1.2-1.3. The blade area to diameter at that pitch ratio just seems to unload better and carry speed through the corners well.

My findings with the Neu motors is that they like less blade area, especially at the tips and more pitch.

I am always trying to find the best compromise between speed and heat. As Jay pointed out. Going these speeds with such a big hull requires a motor of a given mass to handle the heat generated within the motor. It also requires a balance with respect to cell heat. I set my rigger up to be able to run at that speed consistantly and just push the equipment hard enough to compete, while not hurting anything. These are expensive rigs, and should be set-up to "live" a long time.

One last thing. After my experience with the 10S rigger and some testing this fall, I know I can build a 6S2P rigger to run just as fast and reliably for a LOT less $$$$. This is one of my winter projects.

Good luck with your build.

 

[pda75]

Thanks guys. there is converging point toward a 3060/8 (or 3060/9 ?). that's will be my choice.

Here is a drawing of the set up.

If 5 minutes is too much, never mind. When I was saying 5 minutes; I was thinking that 5 ' means pulling amps at 12 C of the Lipo cells, which is a way to stay in the safe zone of the pack.

I agree. Such a set up is expensive and I want to stay in a safe area of the components.

cheers

Pierre