Basic launcher

[Jean Claude]

Jazzenjohn, Here's the current I'm calculating for the basic launcher I'm now considering.

Do you think this battery would be suitable for this job (4 launches expected before recharging)?

 

[Aerofoam]

How many rrpms are you looking for?
I really like the small form factor of that starter, but it seems a bit small for the task if it is a conventional brushed motor.
The brushless 3 phase hobby motors are a lot more efficient and lighter. I would be looking at about 4hp. out of the same size motor, but
would be running it off of 2 of those 4 to 5 AH batteries in series. I would expect 2 to 4 spin ups from that arrangement..
Also, the enclosed nature of that starter is not going to promote cooling and running a starter more than 10 seconds it outside of it's design parameter. I think you may burn up the motor after a few launches. Maybe the case could be ventilated with a hole saw if it is not integral to the magnets, or windings?
There are some very small LiPo charge controllers, so you could run a wire for in flight charging and probably drop down to a 4AH batt.
Not sure about doing it on 12v though, I really don't think the starter will be enough, or last very long if it is a conventional starter.

 

[Jean Claude]

How many rrpms are you looking for?

With this very low power, I can only expect 130 rpm on my 21' rotor. But the main thing keep that I can settle in calmly before pressing the button.

In add the low torque applied on the rotor allows keep the launcher engaged during the run without noticeable effect on yaw.
So, the run would be 600' with no wind, but 370' with only 6 mph of headwind.

Of course, the throttles will have to be managed

I really like the small form factor of that starter, but it seems a bit small for the task if it is a conventional brushed motor.

Yes, but no mecanical to add other than the 9” diameter crown gear

 

[Jean Claude]

Also, the enclosed nature of that starter is not going to promote cooling and running a starter more than 10 seconds it outside of it's design parameter. I think you may burn up the motor after a few launches. Maybe the case could be ventilated with a hole saw if it is not integral to the magnets, or windings?

You're right,
While the low efficiency (0.6) of this brush motor is not too detrimental to the rotor, it could be unacceptable due to motor overheating.

 

[Aerofoam]

You're right,
If the poor efficiency (0.6) of this brush motor wouldn't hurt too much, it could be eliminatory because of overheating due to losses.

Is the motor a separate unit inside the case, or is that case the actual structure holding the magnets, or windings?
If the motor is a separate unit that slides into the case, you should be able to cut some large ventilation holes, or slots.
I think this is the "Achilles heal" because starter motors are designed for very short duration operation with no aspect of cooling.

 

[Jean Claude]

I'm going to drill 3 holes 22 mm in diameter on the brush side and 3 holes 15 mm in diameter at the bottom of the bell as shown in the photo.
This will allow centrifugal ventilation. Not sure if this will be enough, but will much better.

 

[Aerofoam]

I'm going to drill 3 holes 22 mm in diameter on the brush side and 3 holes 15 mm in diameter at the bottom of the bell as shown in the photo.
This will allow centrifugal ventilation. Not sure if this will be enough, but will much better.

View attachment 1162674

That should help a lot. Is that starter from a motorcycle?

 

[Jean Claude]

Yes, Starter Mitsuba SM-5 of motorcycle
Now that the holes are drilled, I'm going to look for a mini-fan to force internal cooling.

 

[Aerofoam]

Yes, Starter Mitsuba SM-5 of motorcycle
Now that the holes are drilled, I'm going to look for a mini-fan to force internal cooling.

How about a radio control EDF unit?

 

[Jean Claude]

Unfortunately, the signal required for rpm control is not
DC

 

[Aerofoam]

Unfortunately, the signal required for rpm control is not
DC

You can use a servo tester and the proper ESC to control it.

This one is about $14.00

 

[Jazzenjohn]

Sorry i missed this thread JC. That battery is a very good choice, and you can expect more prerotations than 2-4 with it. If you are sure about using that motor, I'd consider a 4 cell. After voltage drop, and for the brief time of prerotation, it would be ok I think. I wouldnt worry too much about motor overheat. It has plenty of mass for the brief intermittent load if you are modest in the ramp up speed. I agree with Aerofoam that brushless is better and that is what I use. The advantage of the starter motor is easy and cheap exchange, also the bendix/ring gear system makes it easy to put a rotor brake on. I use a one way bearing or sprag clutch to get rid of the heavy bendix, but it makes adding a rotor brake more difficult. The servo tester Aerofoam suggested will work but I might look for a more robust one than that. Before you decide on that, you need to make a decision on the speed control, because some accept a simple 5 or 10k potentiometer.

 

[Aerofoam]

you need to make a decision on the speed control, because some accept a simple 5 or 10k potentiometer.

Are you saying there are ESC's that don't need a PWM signal?

 

[Jazzenjohn]

If you venture outside of the brushless RC stuff, yes. If he goes with the starter motor, as he said, he likely will be able to since it is probably a DC brushed motor. something like a Kelley controller or like this one from Amazon often come with a variable pot https://www.amazon.com/10-50V-Contr...5a-be5d-96ee094b4892&pd_rd_i=B072LXX7G2&psc=1

 

[Aerofoam]

If you venture outside of the brushless RC stuff, yes. If he goes with the starter motor, as he said, he likely will be able to since it is probably a DC brushed motor. something like a Kelley controller or like this one from Amazon often come with a variable pot https://www.amazon.com/10-50V-Contr...5a-be5d-96ee094b4892&pd_rd_i=B072LXX7G2&psc=1

Ok, didn't realize you were back on the brushed motor. I think the old brushed type ESCs would still be a good choice, smaller and lighter, but maybe hard to find.
The rheostat for low V high A is usually quite large and heavy......the one you listed is not small.....

I wonder if he could pull the brushed motor out and find a brushless that would fit. Worse case scenario would be having to use a collar to adapt to the shaft size, but the case mods. and mounting shouldn't be too difficult....

 

[Jazzenjohn]

Aerofoam, I'm with you on brushless motors. I've been using them my prerotators for the last 12 or 14 years. I'm sold on them, but many others are reluctant, and there is room for other opinions on the subject. I wasn't recommending the controller from Amazon, i was using it as an example. You are correct in that the low voltage high amperage controllers are going to be heavy, but that is a result of the choice of motor. You might have better luck finding an older brushed motor controller like a Jedi or something like that but they are hard to find.

 

[Aerofoam]

Aerofoam, I'm with you on brushless motors. I've been using them my prerotators for the last 12 or 14 years. I'm sold on them, but many others are reluctant, and there is room for other opinions on the subject. I wasn't recommending the controller from Amazon, i was using it as an example. You are correct in that the low voltage high amperage controllers are going to be heavy, but that is a result of the choice of motor. You might have better luck finding an older brushed motor controller like a Jedi or something like that but they are hard to find.

Yeah, I still have a pile of brushed speed controllers, but they are all small. I be there are a number of high output ones for the off road RC cars from about 24 years ago, most are probably in a landfill.......

 

[Jazzenjohn]

I've looked over the motor you've chosen and it appears to be about 300 watts (.4x750). If I put it into the graph from Prerotator power vs RRPM i get about 80- 90 RRPM. You say the motor is 1300 rpm, and virtually all the ringgears I've seen use 108 teeth and have pinions of 9 teeth for a final ratio of 12-1. That gives a theoretical RRPM of 108 at the motor max of 1300 RPM. This might inhibit the max RRPM because it is a bit overgeared. Which speed control are you looking at using?
Is that enough RRPM? it is in the ballpark of what you might expect from simply hand starting, Can you help myself and Aerofoam understand why there is so much resistance to using brushless motors?

 

[Jean Claude]

Jazzenjohn,
Brushless motors are very interesting, but mechanical adaptation to a Bendix is out of my reach

With a diameter of 6.45 m, a chord of 0.18 m and a standard pitch, my calculation indicates that this 52 kgm2 rotor driven by this starter can reach 130 rpm in less 35 s (unless I'm mistaken)
I doubt that hand starting exceeds 60 rpm without wind, so 130 rpm would be already much better. But what I'm mainly looking for is to launch after having buckled up and calmly carried out the usual checks, without losing any extra rrpm.

I'd like to remind you that the starter will be connected directly to the battery by a simple traditional contactor. Hence the required current in my post #1 due to the inertia of 52 kgm2 and aerodynamical torque (green parts)

Now, before I cut the 10' ring gear (by high-pressure water jet), I need to assess the overheating of the starter windings.
As the Joule effect due to the current is known (0.07 Ohms), it's already possible to estimate the temperature rise, assuming no ventilation at all, just from the thermical capacity of rotary parts (0.15 kg heated)

It shows the winding could be destroyed after only 20 sec. if the insulating enamel cannot withstand temperature above 150°C (usually)

Then I have to add forced-ventilation cooling, which removes heat in proportion to the temperature difference between the ambient air and the winding.

I've ordered a small Pc fan to measure the cooling produced at a known temperature and take its effect into account.
That way I'll know what the temperature will be through the winding at the end of the launch.

Concerning the low ratio of the crown gear. The reason is to provide an extra effective energy boost during the run. The Bendix self releasing at around 220 Rrpm. See my post #3

 

[Jazzenjohn]

How certain are you of the output of that motor in watts and rpm? Are these test numbers or manufacturer?