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Author Topic: New Limited Pennyplane (mostly)  (Read 21227 times)
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Olbill
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« Reply #100 on: December 07, 2016, 11:03:40 AM »

The solid spars referenced above are stiffer than .025 tubes but I'm not sure how they would compare to .028 tubes. My first line of investigation would be the attachment from wing posts to spars. If there's any flexibility there the whole wing can twist regardless of how strong the spars are.
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cglynn
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« Reply #101 on: December 07, 2016, 03:00:33 PM »

Thanks Bill.  I thought that too after the first flight, so checked the post to tube fit.  No slop in the joint or the fit so that was not the culprit. 

I am still thinking it is the spars.  When I lay the model upside down resting on its tip plates (not going to lie, it is a close version of your design) the wing deflects about .75" without a rubber motor.  Add ~2g of rubber to the overall weight and I just don't think they are sturdy enough.

I had thought about adding struts or more ribs, or both to help with wing twist.  Adding ribs wouldn't be an issue as the model ended up 400mg underweight, so could stand to gain a few mg's.  Struts would most likely stiffen the wing more, but in my mind would defeat the purpose of using the .028" carbon, and that is to keep the drag to a minimum. 

Also, in an attempt to reduce drag I tried a thinner foil section, which didn't fly for anything.  Using the same motor and prop I was flying on last season's model, same torque and turns, I was only doing half the time.  So need to rebuild the wing with the usual 4-5% camber, instead of 2.5%.  Bad choice, and I payed for it with time and OS film.

I still have the .028" tubes, and am going to try them in a 4.5% cambered wing.  I think the 2.5% wing just needs to fly too fast to make lift, which requires more torque, which is putting undue stress on the wing, and the whole thing ends up in a death spiral, literally and figuratively.

I am not keen to give up on the .028" tubes yet.  I do want to see if they actually are strong enough as they are very easy to build with and could possibly offer a slight advantage in drag reduction.  Even if they don't, I can definitely see why you like carbon for wing spars.

Chris
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Olbill
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« Reply #102 on: December 07, 2016, 03:43:21 PM »

It's the tube attachment to the spar that is critical. You don't want any bending in that joint.

I know I harp on this a lot these days. I just think it's a source for a lot of problems in a lot of different kinds of models. I think it may have been the culprit in a lot of flyability problems that I've had in the past. F1L in particular comes to mind.
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Tapio Linkosalo
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« Reply #103 on: December 08, 2016, 01:33:21 AM »


I suppose when Chris mentions putting the model upside down resting on the tip plates, the deflection he observes is indeed bending the spar, as in that position there should not be any load on the spar-to-post joint.

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?

 
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Olbill
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« Reply #104 on: December 08, 2016, 09:26:29 AM »


I suppose when Chris mentions putting the model upside down resting on the tip plates, the deflection he observes is indeed bending the spar, as in that position there should not be any load on the spar-to-post joint.

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?

 

Yes - I was referring to the flyability problem. As far as the stiffness of the spars goes, I use an .026" tube for the LE spar on my F1M's and have no problem with it.

Sandwiched spars could be stiffer but for LPP I like the solid carbon rectangles. For LPP the weight is not much of a problem and they are very convenient to use.
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mkirda
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« Reply #105 on: December 08, 2016, 09:35:51 AM »

I wonder if making carbon-balsa-carbon sandwich spars would make a lighter and stiffer wing than solid carbon tubes or rods?


I think the answer to this is "Of course". With the Prepreg carbon coming out of Europe, I think an ideal carbon/balsa/carbon LPP spar might be something like 0.065" thick center tapered to 0.045" or so, with one layer over the entire length and possibly one more on the top for the center 5-6". The layup under vacuum and heat would be simple. The hard part is cutting to the right width. Tapered width might also be attractive.
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Olbill
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« Reply #106 on: December 08, 2016, 10:41:51 AM »

But for LPP would this have any advantage over solid carbon spars? Other than learning some new skills I can't see any.
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Tapio Linkosalo
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« Reply #107 on: December 08, 2016, 12:23:42 PM »

I think the answer to this is "Of course". With the Prepreg carbon coming out of Europe, I think an ideal carbon/balsa/carbon LPP spar might be something like 0.065" thick center tapered to 0.045" or so, with one layer over the entire length and possibly one more on the top for the center 5-6". The layup under vacuum and heat would be simple. The hard part is cutting to the right width. Tapered width might also be attractive.

The CCT pre-preg - at 19g/m2 - would IMO be an overkill for PennyPlane. I molded some F1D spars from it, and the weight is only slightly higher than balsa-boron. And that was pre-cured carbon, which I glued on with some extra epoxy. Once I master using the pre-preg, I think the weight would be the same as for boron reinforced wings.

From some sources (at least R&G in Germany) you can also get 31g/m2 spread-tow tape (same material as used for the 60 gram cloth commonly used in light, hi-tech constructions), which to me seems ideal material for carbon-balsa-carbon spars for PennyPlane or F1M. So far I have only used it to mold wing pylons for F1D, and honestly feel that carbon is not needed for wings spars in F1M, but if I'd choose to build a carbon wing for one, that is the material I'd use.
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cglynn
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« Reply #108 on: December 08, 2016, 12:29:43 PM »

I would think that while super cool, the sandwiched balsa/carbon spar would add an unnecessary level of complexity to LPP.  

Bill, I will have to continue working with the .028" tubes.  It was suggested that I add a few inches worth of boron to the top and bottom of the tube to stiffen it.  I am thinking of going that route, or leaving the spars as they are, and trying a few other things.  One, I am going to make my tip plates taller.  When designing the model, I was trying to reduce drag to an absolute minimum.  I may have went too short on the tip plates, so no enough effective dihedral.  Also, I am wondering if the stability issues weren't because of CoG placement.  The model was balancing around 75% of the leading edge without rubber.  

Next one will get taller tip plates, more camber, and a slightly more forward CoG.  I will also build another wing with the .028" tubes and top and bottom borons to compare.

Thanks
Chris
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leop
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« Reply #109 on: December 08, 2016, 07:55:21 PM »

Bill is correct in suggesting using the 1mm (0.040") tall carbon rectangle shapes.  The 0.7mm OD carbon tubing has but 34% of the bending stiffness of the 1mm (x 0.4mm) carbon rectangle shapes.  Adding 0.004" boron on the top and bottom of the tube will not get one to the stiffness of the 1mm shape.  I use 0.004" boron on the 0.063" (1.5mm) thick balsa leading edge spar on many of my LPP's.  This makes for a spar that is about the same stiffness (or a bit less) as Bill's LPP carbon wing spars.  My latest design LPP's had a terrible dive problem when upset or steered (leading to much laughing by the spectating crowds).  Adding boron to only the leading edge wing spar solved the problem.

LeoP
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cglynn
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« Reply #110 on: December 12, 2016, 12:31:57 PM »

Time will tell if this works or not, but I put 6" lengths of boron on the upper and lower tangent of the .028 tube, LE and TE spar.  Weight gain was negligent.  Using the .028 tubes with the boron, covered with OS film, the finished model needs about 300mg of clay to make the minimum weight.  I would rather have the weight in the structure, but now I guess I will be able to really play around with CoG by moving the ballast.  I am flying again this weekend and will report back with my findings, then most likely hopping on the 'puter to order a bunch of .040" rectangles from CST.  Even if the .028 tube with the boron works, the .040 tubes while possibly heavier, have got to be easier to build with than mucking around with boron.

CG
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cglynn
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« Reply #111 on: February 09, 2017, 12:46:33 PM »

So having flown this model a few nights now, and putting it through its paces, here is what I have found.  The wing most likely does not need boron, and most likely was not causing the flight failures of the previous model.  In my testing, I began to experiment with CoG a lot, and found that the CoG of the previous model was too far aft.  This was causing the climb issues, the stability issues, and pretty much everything else.  Once I moved the CoG forward by moving the clay ballast, the model became a whole new beast.  It started to climb like a proper plane should, recovers from ceiling hits like they didn't happen, and the forward CoG lowered the average prop. RPM to around 150. 

The model also handles torque very well.  I have been launching at .55inoz (quarter motors in the gym) and it shows no bad behaviors at this launch torque. 

When I get around to building another of these penny planes, I will most likely use the .040" rectangle, as I can afford some extra weight and will take all the stiffness I can get.

Have a good one all.  Looking forward to seeing everyone at West Baden.

CG
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Olbill
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« Reply #112 on: February 09, 2017, 02:24:52 PM »

As I've said before my LPP's have launched at around 1 in-oz without bad stuff happening - most of the time. Occasionally there's a few laps of racing at the beginning which is death in a high site like Lakehurst. I can usually tune this out.

I  had an interesting experience at St. Lukes last month. I broke the front tube off of the wing and then had to do a field repair to get it reattached. I did a low power test flight after the fix to check the decalage and then went back to launching at .4 in-oz. The model looked seriously underelevated and was racing around the room at a much higher speed than normal. But it turned 7:47 which was the best time of the day at that point. I decided another flight was in order and did 8:48 on the next flight. Both flights were bumping the wall of the basketball goal channel about a foot above the low ceiling for several minutes.

I'm planning to experiment a little with this trim at West Baden. It goes against what I thought was correct but......
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cglynn
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« Reply #113 on: February 10, 2017, 10:28:01 AM »

...you can't argue with the stop watch.  Perhaps the new decalage is allowing the prop to work at a better angle of attack throughout the flight? 

Did you happen to record your avg RPM for that flight?  This may be old news to those with more experience, but I have recently noticed that when a model is in proper trim with optimum CoG, the prop RPM's are lower than when the model is in poor trim, and the flight time increases. 

I can wrap my mind around the model flying longer when in proper trim, but moving the CoG forward and having the RPM's go down is counter intuitive to me.  Though I guess if the model isn't constantly stalling and dropping the nose, the RPM's would remain constant. 
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Olbill
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« Reply #114 on: February 10, 2017, 12:19:00 PM »

The actual average was 161 for my best flight. My first flight with more normal trim averaged 150 rpm which is about what I expect for my models. I was flying my small prop.
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cglynn
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« Reply #115 on: February 10, 2017, 01:17:18 PM »

That is more along the lines of what I would expect.  Drop the nose a bit and increase RPM's.  In this case do you think that the flight was longer because even though the prop was spinning faster, it provided usable thrust through a greater portion of the torque curve, or was more lift generated by the flying surfaces due to increased speed?  Or perhaps some other indoor/ultra low Re phenomenon?

CG
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Olbill
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« Reply #116 on: February 10, 2017, 03:05:18 PM »

I'll have to plead ignorance.Would the flight have been longer with a more normal trim? I don't know.This is a pretty low drag model and maybe it's just happier flying faster.

One of my cardinal no-nos about indoor flying is to not ever use a decalage change to decrease rate of climb. But with .4 in-oz of launch torque and my usual flight trim the model would be high up on the side walls of the channel just about every circle. On these flights the model never got up more than a foot or so on the side walls.

But maybe the most promising thing about that day of flying is my use of 1" long sleeves at the rear hook of my LPP and A6. I wasn't going to divulge this but it's so weird that no one else will try it anyway!
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frash
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« Reply #117 on: February 10, 2017, 04:47:49 PM »

Yes, Bill, it is weird (BROAD GRIN), but you have brought a lot of innovation and record times to indoor models by trying ideas that everyone knew would not work. Congratulations!

Fred Rash
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Hepcat
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« Reply #118 on: February 10, 2017, 09:12:45 PM »

response to #112 and #114

Bill,
It is probably obvious to you already but if the trim was such that the flying speed increased then the propeller angle of attack would decrease.  If, prior to the trim change that increased the flying speed, the propeller had been anywhere near to stall then the lower propeller angle of attack would increase the rpm and thrust and give a slightly higher climb.  I think this also agreeswith the figures you mentioned: Normal trim, 150 rpm, faster trim, 161 rpm, higher climb and duration.
John
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Olbill
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« Reply #119 on: April 05, 2017, 02:20:02 PM »

One of my props was destroyed at West Baden. It would have been repairable except that a large piece of one blade was missing. A wing bit the dust as well but losing a prop had to be attended to before I fly again. Here are a few pics of the process.

The lost prop was the smaller of my two LPP props. The old prop blades were 1.75" wide. I decided to make the new blades 1/4" wider than the old ones. I started with two sheets of indoor wood in the 4psf range and about .030" thick. These were glued together lengthwise with thinned Duco, then sanded to about .025" thick. I formed the blades on my F1M prop block from PropBlocks.com.

I didn't have a camber form for this blade shape so made one from 3/16" balsa. The camber form is about 1/8" wider than the blades all around. I sanded an arc into the camber form with the long edges of the form going to almost zero thickness.

The hub from the broken prop looked in good shape and had been working well so I reclaimed it by cleaning the balsa and CA from the carbon rods.

I glued it all together in my pitch gauge with the pitch set to 23. Finished weight of the new prop came out at 702mg which is about 40mg lighter than the old one.
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ceandra
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« Reply #120 on: June 04, 2017, 05:00:39 PM »

Bill:

As a newbie, I have to ask. Way back on page 1 you show making props from 0.030 balsa, with two sheets glued side-by-side. Why not use one wider sheet? Is it because the C grain does not carry through on the wider sheets? Specialized Balsa shows wide sheets of 1/32, but I am not sure what widths are available in C-grain, contest weight.

Also, for my kids, I have some Ultrafilm from Ray's shop. For these lighter planes, I assume I should go to the lighter OS film?

Thanks
Chuck
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Olbill
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« Reply #121 on: June 04, 2017, 11:20:48 PM »

I used two sheets b/c I used indoor balsa which is normally about 1 1/8" wide.

My model is tight on the 3.1g desirable weight. OS film will give you a few more milligrams to play with. For a first model it really doesn't make much difference.
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ceandra
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« Reply #122 on: June 19, 2017, 11:37:47 AM »

Bill:

What is a good source for the prop wood? Having trouble finding 1/32 in anything much lower than 6#, maybe 5.5#. I am not yet well plugged in to indoor supply sources.

Thanks
Chuck
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Olbill
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« Reply #123 on: June 19, 2017, 04:26:04 PM »

There's a supplier in Colorado who may be able to supply something usable. I'm on the road for a couple of weeks and don't remember the name. Maybe someone from that area can help

Yoy can use wood of that weight and just sand it thinner but it would be better to hold out for something a little lighter.
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ceandra
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« Reply #124 on: June 19, 2017, 04:33:29 PM »

OK, thanks. That is probably Specialized Balsa, in touch with them now...

Chuck
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