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Author Topic: Cross-section for P-30 motor  (Read 934 times)
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calgoddard
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« on: April 22, 2016, 04:38:07 PM »

Most P-30 flyers (including me) use a rubber motor made up of six strands of 1/8 inch rubber.  So that is a 6 x 1/8 inch rubber motor.

I have read that Don DeLoach flew his Polecat P-30 with six strands of 3/32 rubber.  He supposedly gets 2,000 winds and a 2 minute motor run. He says this is a good set up for light winds.  Of course Don's building and flying success are legendary.

My P-30's never come out near the 40 gram minimum.  They are more like 45 grams.  Two of my P-30 are set up to carry an RF tracker and I accept the weight penalty for the comfort of having a high chance of retrieving these models. I am building a P-30 with a pop-off wing DT and hope that I can leave out the RF tracker with a more certain, and rapid, descent when the DT is triggered.

Don's Polecat was probably right at the minimum weight. It has a pop-off wing DT.

I am thinking of trying out a 6 x 3/32 rubber motor.  The GG prop assembly that I use in a couple of my P-30s will hopefully prevent this longer motor from bunching. So there is no need for me to build a P-30 with a longer hook-to-peg length or to braid the 6 x 3/32 rubber motor.   However, I am concerned that my P-30s won't climb very much, if at all, with a 6 x 3/32 motor.

I am wondering if anyone has any experience with a 6 x 3/32 P-30 motor.

I can try it myself, but I am just curious what other P-30 fliers think.

It would be a hassle to strip rubber to, say, 0.110 inches to make up a P-30 motor that has a cross-section intermediate a 6 x 3/32 inch rubber motor and a 6 x 1/8 inch rubber motor.

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Rewinged
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« Reply #1 on: April 22, 2016, 05:16:47 PM »

I have been flying 6 x 3/32 for a few years.  My current model is 41.5 grams.  From memory, I recall Don writing (P-30 FFQ issue) that he doesn't think the long motor run approach is good if a model is much above 41-42 grams.

Flying with the GG front end, I usually get about 1800 turns, and I wind pretty hard.  My motor runs are 1:35 to 1:40.  You need to really wind hard with the smaller cross-section, because much of the climb is in the (weak) burst.  The big issue with the smaller cross-section and a heavier model is that you can lose the climb during cruise.  Then you may need lift to make the max.

Weight of the model is not the only consideration, but also the area, which of course weight correlates to.  There are tradeoffs between how much the prop is loaded, how long the motor runs, and the efficiency of the prop and motor while it is running.  I don't know all of that, just that it exists.  All sorts of relatively extreme models have been successful, but it is the total airframe, weight, prop, and motor together that determines success in neutral air.

I think I can do about 3 minutes, or close to it, in dead air, but of course that is really hard to check.  A big negative of the smaller cross-section is if there is down air near the ground, you may never get through it (I know that from experience in a situation where a bigger cross section motor would have been much better.)

The biggest benefit of the smaller motor, IMO, is the potential--with the right airframe and weight-- of putting up 4 or 5 maxes (3 at 2 minutes, 1 at 2:30, and maybe 1 at 3:00 minutes) early in a contest before there is up or down air.

I am using the full length of the fuselage for the motor.  It is good because there is less bunching, but bad because if there is bunching it can have a worse CG impact than with a shorter hook-to-peg length.  John Kamla's Marie, which was designed for 6 strands of 3/32, has the peg just in front of the fin or stab if I recall, and I think the Polecat is similar.

--Bill
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DerekMc
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« Reply #2 on: April 22, 2016, 07:29:40 PM »

Last contest I used 10 strands of 1/16".  I got about 1600 winds into them and they worked well. I use the GG front end.  Just another option to consider Smiley  Decent burst and pretty long motor run.  Could be a nice fit with 43+ gram P30's.
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« Reply #3 on: April 22, 2016, 07:37:04 PM »

I investigate this in
http://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=142
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calgoddard
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« Reply #4 on: April 22, 2016, 08:31:07 PM »

Thanks everyone.  These are all helpful comments.

Ricardo - thanks for the reference to the analysis.

I hadn't even thought of the simple 10 x 1/16 inch option.

These calculations might be helpful.

6 x 3/32 = 0.5625

10 x 1/16 =0.625

6 x 1/8 = 0.75

4 x 3/16 = 0.75

I am really wary of under-powering my P-30 and removing its ability to climb through down air. With a 45 gram P-30 I will probably not go below the 10 x 1/16 option.

You can try to pick good air. Some are really good at this, even without instruments. I am not into free flight enough to have a real time electronic monitoring system for detecting thermals.

You can also get lucky.

The very first P-30 contest in which I competed I flew a Square Eagle P-30 that I had built. I was just a novice at outdoor flying at the time but I had about five years of indoor flying experience.  My Square Eagle caught good air and I placed first in the contest.  Many veterans were competing in that contest.  I have also lost contests when downers seemed to suck my airplane down Sad



 


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