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Author Topic: Another new Penny Plane  (Read 1064 times)
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cglynn
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« on: January 20, 2014, 07:47:29 PM »

After playing with my Bank's LPP for a few months, and learning a TON about indoor flying technique, I decided to build something a little more competition oritented.  I dug into my spare RC parts bin and found a couple of nice pieces of carbon fiber rod to make the wing and stab with.  I split both rods so they were .040 x .020 or so.  For the stab I split one of the .040 rods so it was .020 x .020.  For anyone else considering this method, the carbon does pretty much split evenly.  I used a razor and cut it down the middle.  I went nice and slow and got all pieces pretty much even in terms of thickness.  Once the spares were cut, some ribs were cut from .060 C grain.  I made them .060 x .060 which I think is overkill (the next version of this model will use .060x.030).  To make the tip plates, I used some really thin carbon (don't have a micrometer or I would post dimensions) and bent it into a rough semi circle.  Using thing CA, I glued each end to a tip rib and held till it dried (fun when you glue your fingers to the rib).  This was done for both the wing and the stab.  Covering is polymicro from A2Z.  This was my first time using mylar for a covering.  I rolled it onto some upholstery foam, put it onto a 12 x 18 covering fram, and used some spray adhesive on the wing, stab, tip plate.  I found the polymicro cuts easily with a 25W soldering iron with the tip ground to a blade.  I have heard that the plastic films can be tricky, but honestly, using the above method, I got a very nice covering job with no foul ups.  For the rest of the model, I followed Olbill's dimensions.  I did not use a dropped boom on this model, and opted instead to use really tall wing posts.  Mine are 4 inches in the front and 3.75 in the rear.  These are the final lengths as the rear post was cut after test flying.  Prop is a Banks design salvaged from my older model.  I will be building some other designs to play with in the next few weeks as I know there is room for improvment in both design and construction.  She is a bit overweight (heavy MS, boom, and prop) comming out at 3.3g, but its lighter than my last one, and built much better.

Now for the flight report.  I set up the model with 0 offset in the wing, .35 wash-in on the left wing, and .35 wash-out on the right wing.  Wound a 22" length of 3/32 Tan 2 to around 3 in-oz and let her go.  She flew right off the board.  The circle may be a bit tighter than I want, and I will play with it.  The torque was just right for the 17 foot ceiling and I turned out a 3.33 on my next flight.  I am going to make a few new props and begin the process of optimizing the motor for the model.  Considering that flight and time was the second flight on the model, and I had really only flown it before to confirm trim, I am pretty pleased with it.  Getting the design ready for Kent will give me something to do this winter.  

Sorry about the pic quality.  It was taken from my phone, and my floor happens to be about the same color as balsa.  
Also, I want to ackknowledge the building techniques of Bill Gowen, who's model this closely resembles and thank him for posting all of his building and flying data.  I think he is doing for LPP flyers what Coslick did for Easy B flyers when he wrote the Hobby Shopper article.

Chris
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Another new Penny Plane
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cglynn
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« Reply #1 on: February 05, 2014, 10:22:11 PM »

Got to fly the Penny Plane today.  The heat was on full blast in my school gym, so I maybe had 15 feet to play with before the air went pear shaped.  Even keeping the model low, it was still getting pushed around pretty good.  At any rate, I tried a longer motor, a bit larger in cross section, and wound to .4 in-oz.  Flew a 3:45, which at 15 feet, isn't too terrible, but that model should be able to do more.  So I took a look at the important part, my prop.  Turns out when I built the pitch block used to make my current prop, I somehow managed to build a 36" block instead of a 24" block.  So I have been flying a 36" pitch prop, set for 24".  I am sure the angle of attack on that setup is far from optimum, and probably explains why I am not using as much of my cruise torque as I thought I should.

So these blades are going to get reformed on a proper 24" block, and I am going to get back to it.

Measure twice, cut once
Chris

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Olbill
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« Reply #2 on: February 06, 2014, 10:40:59 AM »

Chris
You could dump a lot of unnecessary weight by just shortening the wing posts.
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cglynn
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« Reply #3 on: February 06, 2014, 12:22:39 PM »

Thanks Bill.  I had thought about that, but was wondering how doing so would effect the stability of the model.  I will try it and if the stability is adversely effected, I can always make some new posts and glue them on.

Chris
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Olbill
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« Reply #4 on: February 06, 2014, 06:04:23 PM »

My advanced aerodynamic theory:
1. Long posts are more flexible than short posts. Flexibility is bad.
2. Long posts are heavier than short posts. Weight is bad.

I've always used shorter posts than most people and haven't seen a problem from that approach.
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cglynn
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« Reply #5 on: February 07, 2014, 10:41:28 AM »

Can't argue with that.  I shortened the wing posts by half.  That shaved 85mg off the model.  I did some light sanding to the motorstick, and though I hated to do it, took some area off the TE of the prop.  I know that is less than ideal, but this prop needs to be rebuilt anyways.  All in all she lost 160mg, and is still heavy.  When I am done learning on this model, I am donating it to the aeronautics program at my school so my students can use it to learn winding, rubber selection, trim adjustments, etc.

When I build V2 of this plane, I am going to use the tip plates from Bill's LPP plan.  The curved carbon ones look cool, and seem to work well, but need a really stiff rib to hold their shape, due to the springy-ness of the CF.  The only rib stock I had that could handle the load was also heavy, so I will save some weight there.  Also, I am going to use 1/32 for the ribs instead of 1/16th, and a lighter motor stick.  I am at the point where I have reached the "Ahah" moment and have learned to use my deflection gauge and mg scale for everything in indoor building and flying.  I wasn't doing that 3 weeks ago for whatever mental reason (read lazy), and now I am pretty obsessive about weight and strength. 

Despite the fact that V1 isn't competition worthy (way too heavy at 3.56g) it was a good build experience, taught me what not to do in many cases, and is still enjoyable to fly. 

Chris
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Olbill
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« Reply #6 on: February 07, 2014, 01:46:06 PM »

Chris
My LPP doesn't have any curved carbon rods.
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cglynn
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« Reply #7 on: February 07, 2014, 02:53:41 PM »

Bill,  I was referring to the curved tip plates on my model.  The carbon is springy enough went bent that I had to use super stiff ribs from 1/16 stock to keep the rib shape from deforming.  Your trapezoidal tip plates don't put any stress on the ribs, and if I am going to continue using carbon tip plates, I will use your design.
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mkirda
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« Reply #8 on: February 08, 2014, 08:43:24 AM »

You could also try carbon tow wetted out with epoxy. It will form to whatever shape you can make it. 1K tow from CST is inexpensive. All you really need is some wax and a flat surface.

You will need to experiment with the layup a bit to get them stiff enough, tapering four to three to two tows.
After the first set or two, you'll have an idea where you'll need more (or less).

Regards.
Mike Kirda
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Olbill
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« Reply #9 on: February 08, 2014, 07:23:36 PM »

Or you could try .010" rods like I used on my new F1M tip plates. I bent one to the shape required and then glued another one on top of the first one. This gives the same area as half of an .020" rod. They are very flexible. The second one will help hold the curve of the first one when they're glued together.

I'm planning to build an F1M prop using an .010" rod for the blade outline. I'm not sure if this will work but I think it's worth a try.
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cglynn
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« Reply #10 on: February 08, 2014, 09:15:00 PM »

Laminating two .010 rods sounds like it would work.  I don't have any .010 rods on hand, so I will ask, how does the weight of those compare to say 1/32 sq. balsa?  The balsa would be easy enough to form, but I have found the carbon to take impact better than balsa, and really like using it for spars.  I am surprised that more builders aren't using the stacked .020 rods or split .040 rod for their LPP spars.  It really is a great way to build.

Chris
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Olbill
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« Reply #11 on: February 09, 2014, 01:40:39 PM »

.011" rods from CST weigh 2.4 mg/in. Two glued together weigh about 5 mg/in. 5 pcf balsa 1/32" sq weighs 1.3 mg/in.
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cglynn
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« Reply #12 on: February 09, 2014, 02:43:05 PM »

Thanks Bill.  When I get around to building my next one, I think I will go with the balsa.  Even though it is a small weight savings, every little bit counts right?

Chris
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