Rubber powered propeller RPM?

[PsyberPhlier]

Hey guys, I am toying with an idea but I first need to figure out the RPM range of rubber powered props. Anyone know anything about this? I did the Google thing but while finding lots of math, I found nothing specific.

I do understand that rubber type, length and winds all play into this, but there must be some sort of baseline for calculations, right?

Ted

[Mitch]

Ted,

...I think that would make a good science experiment for a young person... but I have NO idea...

I do know that we measure for length, number of strands and there is a maximum breaking point for what the rubber should take... I do know that when properly wound, the prop will spin very fast for the first few seconds. (That should get your model up) Then continue to spin at a slower rate for many more seconds. (I am referring to Tan rubber). I still wind to turns, most FACers wind with a torque meter.

But to know the RPM of the prop I have NO idea...

I just thought... you could wind up you plane in the stooge... to say 1000 turns. Keep the model in the stooge, and let the prop spin for say 6 seconds... 1/10 of a minute. Stop the prop and then put your winder back on and remove the rest of the turns (the counter counts both ways). Then you could calculate the RPM, of that initial burst of power.

May I ask why you want to know the RPM?

Mitch

[PsyberPhlier]

Well Mitch, it's like this. First and foremost I am a robotics guy. I love to tinker with electronics bit and pieces and of course write code.

As I am trying to get my head around this whole rubber thing it dawned on me that better flying comes through better data, maybe. To achieve better data I would like to have an idea of starter data. The idea I am toying with is to set up an opto sensor tied to a pic chip and not only read the winds in but also read the turns out and the time line of rpms. This would give a degradation of motive force rate at a fixed length given a number of turns. Basically a way to tune a rubber motor/prop combo before it ever goes in a plane.

Keep in mind, I am a guy that does RC, but the rubber thing has always intrigued me so I am going to build one. Since I am also an electronics/programming geek, I want to know more.

Thanks for the response.

Ted

[NcGunny]

Would a old school timing gun that has rpm work? Just a idea that occured to me.

[PsyberPhlier]

Unfortunately it would not Gunny. I need to see the fall over time and it would be measuring in small parts of seconds to prove a theory.

The circuit to count RPM over MS is a rudimentary, easy circuit. What I need to know most is some starting data. LOL I guess I am going to have to build one of these things gummie power and start from scratch but I will hold out hope that someone out there has some baseline data.

Ted

[Mitch]

Thanks for that and Happy Cinco de Mayo to All,

What you said is what I thought, but above my know-how... a good science project for the fair at middle school.

Basically what I know about rubber motors is we want to use as much as possible and flyers will measure the torque. If my plane was lighter I would try for 30% of the AUW to be the rubber motor alone. With the Guillow models being heavy (for contest flying) My FW-190 will be about 20% if I can get that much rubber in. I just ordered a bunch of Tan rubber and hope to have it before Sunday.

I also plan to fly my new Me-109 which is almost 20 grams lighter than my FW. This model is going to WESTFAC V. I hope to be able to pack in twice the rubber in that model and have the same AUW as the FW!

I plan to start using my torque meter this week also. I also plan to test wind a motor until it breaks, then I will know to go to 90% of that for competition flights. Although I do not know the rpm, if you think I can help you with any data, just let me know and I will take notes.

Mitch...Always Building, Always Flying, Always Improving!

[NcGunny]

Kk..the opti sensor makes sense. I had to hit Cherry Point once to check on some guys that were loaned out,and they were shooting a laser thru some Blackhawk blades. I had ask what they were doing and the explanation was way over my paygrade.

[PsyberPhlier]

NcGunny as in North Carolina Gunny? As in a Gunny in the suck?

If so, Semper Fi brother.

Ted

[PsyberPhlier]

Mitch, thank you and any data I can get from actual gummie models is abundantly helpful. I can feed it all in to the little program I am writing and if what I have learned, what I can measure and what real gummie users know works, I should be able to predict the exact sweet spot of a given number of bands at a given length at a given number of turns. All speculation at this point... just a theory.

Ted

[PsyberPhlier]

Near as I can figure, this is the list of variables:

-Type of rubber
-Length of rubber
-Strands of rubber
-Prop diameter
-Prop Pitch

Am I missing anything?

Ted

[David Lewis]

Your intention is to measure propeller speed in flight? You can mount a tachometer on the airplane with a radio telemetry downlink, or use a wind tunnel. Another useful dependent variable is thrust. It would be helpful to be able to calculate optimum prop diameter for a given rubber motor, and prop pitch that maximizes blade L/D.

[Mitch]

Rubber comes in different thickness also... 3/32, 1/8, 3/16... I will be using Tan, 3/16 for the 400 series. I will be glad to help with you science project. You caught me at the precisely correct time, so I can help!

[PsyberPhlier]

Cool thanks Mitch. I am working out software details right now.

David, in flight tach won't work for what I am trying to accomplish. Waaay to slow and too hard to repeat.

"It would be helpful to be able to calculate optimum prop diameter for a given rubber motor, and prop pitch that maximizes blade L/D." 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. In part, I think it is possible to know the exact performance sweet spot of a given set of variables as well as being able to 'tune' that sweet spot for a specific parameter ie. longer high thrust period or longer float thrust period etc...

I still need some sort of baseline... if anyone finds out an approximate prop rpm, please forward it along.

Thank you.

Ted

[dirk gently]

You could use a cheap optical RPM meter.
Please note, however, that stationary prop experiences different loads than a prop on a moving airplane (you can easily see the change in current draw for electric motors between static thrust measuremet and a moving model once it built up speed). This might have an impact on RPM (although I'm not sure if it will be significant).

[PsyberPhlier]

Thanks for the tidbit. I would be interesting to see what the weight of the model contributed.

Can't use a cheap tach as is has no serial out and does not support the timing necessary.

Ted