Rubber powered propeller RPM?

[NcGunny]

Yes Psy..but now its retired Gunny in the Suck. 23 1/2 years and they said.."Sorry Gunny, but we are having drawdowns. We have to many Gunnies. Your out" Lol..I wonder how many people even know what the "suck" means? Anywho...I am wondering if a cheapo laser couldnt be utilized to count rpm? Blade breaks the beam and counts..a simple program could count breaks and record then transfer that to a graph to get high/lo?

[PsyberPhlier]

Sorry to hear that Gunny. At least you got the 20+ retirement out of them.

An opto sensor is like a laser but instead uses infrared light waves. That part is easy. The difficult part is designing the software without a known baseline range of rpms.

I made a command decision last night and said 1000 rpm is the max rpm in my software side... That gives me a range of 0 to 1000 over .0001 second increments. Now I can plug actual test data in as soon as I acquire some props and rubber.

Ted

[tom arnold]

You mentioned that you wanted to have a starting point rather than re-invent the wheel, well, what you are attempting to do has been done by Stew Meyers of the Maxecuters model club of Washington D.C. He rigged up a very clever testing device that was monitored by a computer and came out with some very interesting curves. He ran an extensive article in their newsletter, Max-Fax, along with photos and diagrams of his set up for others to build. I would go to their web site, contact him and buy that past issue of the newsletter. Stew is a wonderful source of engineering data and techniques and was the electric columnist for the old Flying Models magazine. He would be delighted to help, I am sure.

There is one more variable: Blade width.

[PsyberPhlier]

You mentioned that you wanted to have a starting point rather than re-invent the wheel, well, what you are attempting to do has been done...

Thanks for the info, Tom. I had seen part of Stews work but not the details. The has been done part, I don't agree with. I certainly agree that it is a good start in the right direction but it is missing what I think is a critical variable.

There is one more variable: Blade width.

oooh now thats a good tidbit I completely overlooked. The chord of the blade would have a significant effect on every aspect of any given parameter test. Thank you.

Ted

[Chris A.]

Another variable is propeller condition, is it smooth, no burrs and is it balanced. Also the shape and type of the shaft hook may make a difference on how fast rubber unwinds unwinds. Is the rubber to be run dry or lubricated?

[Mitch]

I was just re reading the chapter on Rubber Motors in Don Ross' Book. When I do my flight tests on Sunday I will be counting turns, but also be watching my torque meter. As the motor gets near the breaking point the torque goes way up. Then, that is used for the initial burst of power to get the model up in the air, after that, the motor will run for a while at a slower rpm keeping the plane flying. Of course when that is over the model glides. I do not know the rpm's but if I can be of help. I plan to take notes on my flight results on Sunday.

I think it will be interesting to see the difference between my FW190 and the Me109, as the Me109 is quite a bit lighter. Both planes will be using a 9 inch Peck prop and the same size motor from the same batch of rubber.

Mitch

[David Lewis]

You want each blade segment to see the angle of attack that maximizes L/D. That's easy to find. It's just the pitch that yields maximum thrust.

PsyberPhlier wrote: "This is a solid part of the test and the rest is to determine the exact right number of turns, given the variables, to establish thrust periods."

Isn't the right number of turns as many as possible?

[Mitch]

A quote I will always remember when I blew my first rubber motor in my beautiful Messerschmidt, at my first contest, WESTFAC IV...

from Phil... he said...

"Wind it up until it breaks, then back off one turn!"

[David Lewis]

You could measure thrust versus prop diameter, holding all other things* except prop speed equal.

As far as determing ideal pitch, we can either 1. assume a pitch-to-diameter ratio between 1.0 and 1.5 is close enough for rubber props, or 2. optimize P/D experimentally through a second iteration after we know what the prop diameter needs to be.

* The constants would be torque (i.e. rubber motor length, thickness, number of strands, and number of turns), air density, blade airfoil, planform, aspect ratio, and angle of attack. (Aspect ratio and pitch will probably decrease more slowly than diameter due to Reynolds number and cube loading effects.)

[David Lewis]

I'm looking for a way to match the prop diameter to the rubber motor. Designing the rubber motor itself is fairly simple. You just make the rubber weigh a certain percentage of the gross weight. The actual percentage varies depending on whether you'd prefer more propulsive energy, or a lower wing loading, but in any case the range is pretty narrow and well established through experience.

Similarly, we've found pitch-to-diameters ratios between 1.0 to 1.5 work well. You will adjust it a bit depending on advance ratio, but it's likely going to be within those limits. So far so good. However, a small change in diameter can have a big effect on performance. And diameter doesn't seem to be determined scientifically.

So I plotted a graph of prop speed vs. prop diameter (the black curve). The horizontal axis in this graph is prop diameter, and the vertical axis is prop rotational speed. Then applying the thrust formula T = n^2*D^4, I derived the thrust curve (the red curve). The vertical blue line is the optimum prop diameter.