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Author Topic: Flaps design  (Read 4393 times)

Offline Matt Piatkowski

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Flaps design
« on: February 18, 2018, 04:12:36 PM »
Hello,
Please see the attached.
The full balsa flap is made for 60" wing span plane. Material: 1/4" medium-hard balsa with local reinforcements around the flap horn mount.
Weight as photographed: 32 grams. The cross-section of this flap is rectangular and will stay this way (I will not taper it).

The other flap (1/4" thickness too) is lighter: 27 grams as photographed, with the MonoCote covering on one side and six broken plastic hinges but:
1. It is too wide for the modern stunt planes
2. The torsional stiffness of it (before the covering was removed on one side) was about 50% smaller than the full balsa flap without covering.

I can remove about 15% of balsa from the full balsa flap by making the line of the 1/2" holes in it. Then, I can dope it with medium Silkspan and this will add some torsional stiffness.

I can also remove 35% of balsa from the full balsa flap converting it into the "modified other flap" (truss like design) and dope it with medium Silkspan.

Which solution is better from the flaps effectiveness in corners?

We all know that stiffer is better but unfortunately it is also heavier. I smell another compromise between weight, stiffness and the corners quality.

Greetings from MP Stunt Academy in Toronto,
M

Offline Tim Wescott

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Re: Flaps design
« Reply #1 on: February 18, 2018, 04:47:31 PM »
How does this light, wide-chord, narrow-span, wet noodle of a flap compare to this long skinny rigid flap.

Well, gee, Matt, when you're comparing apples to oranges in the supermarket, do you ever wonder if you should be comparing McIntosh to Valencias, or Gravensteins to Mandarins?

Yes, rigid flaps matter more than weight.  Probably not so much that you should machine them out of tungsten, but way more than the extra weight of using solid balsa vs. using something off of a 3D RC plane.  If you want rigid and light, start including carbon fiber in your thoughts.
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Offline Howard Rush

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Re: Flaps design
« Reply #2 on: February 18, 2018, 05:44:14 PM »
I’d do whatever Igor does.
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Offline Randy Cuberly

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Re: Flaps design
« Reply #3 on: February 18, 2018, 06:49:04 PM »
Stunt Flaps are definitely not the place to save weight and sacrifice any stiffness!!!

Silkspan and dope is not the best way to gain torsional stiffness.  Think Fiberglass and epoxy or Carbon veil and epoxy and several layers!

I typically use two layers of 1/2 oz glass and epoxy over solid hard balsa flaps.  I also use at least 5/16 thick balsa not 1/4.  3/8 is even better if the wing thickness will tolerate it!

Surprisingly not that much heavier!

I broke this rule on a recent I beamer I built and will never get talked into it again!  "Wimpy" flaps will completely make an airplane into a piece of pretty "Wall Hanging Junk"!

Randy Cuberly 
Randy Cuberly
Tucson, AZ

Offline Dave_Trible

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Re: Flaps design
« Reply #4 on: February 18, 2018, 07:01:23 PM »
All good suggestions so far.  I would think 1/4" is pretty light for a 60" wing unless you can make them pretty stiff.  another problem is keeping them from warping while finishing or later with a few flying sessions out in the sun.  My flaps look quite a bit like your built up one but with a few more ribs but then 1/16" sheeting on both sides for a total thickness of 3/8".  Even these can warp if not careful.  I've replaced several recently and so now install them with lucky boxes to make changing a slightly easier job next time.  I'm now using harder wood and not being too concerned about weight here.  Carbon is also good but found carbon over too soft wood won't prevent them from warping over time. 

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Offline Tim Wescott

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Re: Flaps design
« Reply #5 on: February 18, 2018, 07:27:19 PM »
Carbon is also good but found carbon over too soft wood won't prevent them from warping over time.

What are you using for the resin?  Epoxy?  Polystyrene?  Veil + dope?
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Offline Paul Walker

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Re: Flaps design
« Reply #6 on: February 18, 2018, 09:16:59 PM »
Wood selection is the key here. They need to be C grain, and contest balsa. I buy 1/4 " in bulk and select the primo pieces for flaps. The flaps on my new P-47 weigh 15 grams each, and are VERY stiff in torsion.

So, stiff does NOT have to be heavy. The correct grain and quality balsa is the key.

Throw away those built up flaps covered in $coat.

Offline Brett Buck

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Re: Flaps design
« Reply #7 on: February 18, 2018, 09:51:20 PM »
We all know that stiffer is better but unfortunately it is also heavier. I smell another compromise between weight, stiffness and the corners quality.

  For a given type of construction, that is kind of true, however, if your stiff flaps are 3x as heavy as your flexible flaps, you add maybe an ounce- ounce and a half to a 64-ounce airplane. The difference between 64 and 65 on full-size airplanes powered by 2018 powerplants is negligible - and if you keep your flaps stiff, they might deflect more at the tips, meaning you might gain more lift than you lose weight.

    Brett

Offline Dave_Trible

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Re: Flaps design
« Reply #8 on: February 19, 2018, 04:22:07 AM »
What are you using for the resin?  Epoxy?  Polystyrene?  Veil + dope?
Tim I use non-tautening dope over .02 veil for the flaps.  They will sure be pretty stiff but once in a while still warp over time but not as bad as silkspan.  I'm sure Paul is right-it's mostly about wood selection.

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Offline Scott Richlen

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Re: Flaps design
« Reply #9 on: February 19, 2018, 06:58:24 AM »
in general, should built-up flaps be avoided and solid wood always used?  Or is that too broad a statement?

If you were to build up your flaps. would changing the grain direction of the 1/16th sheeting improve stiffness (for instance, instead of the grain running lengthwise, have it run diagonally, or chordwise)?

Quote
I’d do whatever Igor does.
  Specifically, what does he do?

Thanks,
Scott

Offline TDM

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Re: Flaps design
« Reply #10 on: February 19, 2018, 07:12:24 AM »
You can make something both light and stiff using alternate construction methods.
Soler composites sels what is called carbon fibers biaxial sleeves.
The build construction is done like this
This is a built up balsa construction method with a special Flaps LE spar
Take the balsa LE and shape the 30-45 degrees bevels
Next is to remove absolutely all the dust from the balsa
Take the biaxial sleve and wet it with epoxy thoroughly then blot out the excess epoxi with paper towels
Slide the balsa LE inside the sleve then pull on the ends os the sleve to have it colapse on the balsa stick
Tie up with string at the ends to hep keep it in place
Insert the balsa carbon inside shrink tubing (tubing will become a press) heat the tubing till it colapses on the balsa nnd squeses the carbon over the bals. Don’t worry it will not get glued.
Put on a flat table maybe some straight edges to keep this straight

Next day cut out the shrink tubing you are set the rest is business as usual normal bild
Because the biaxial sleve has the fibers orientation +_45 it ads a tremendous torsional rigidly which is what you need.
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Offline TDM

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Re: Flaps design
« Reply #11 on: February 19, 2018, 07:23:14 AM »
Another way to get a light part with high stiffness is to use a carbon tube. Aloft Hobbies has a nice selection of carbon tubes that are light Use the .5mm wall thickness like 3ID 4OD). You can make a few triangular ribs glued to the tube (NO TE) then you put the sheeting on top and a LE made of light balsa which you shape to make the bevels for the hinge.
https://alofthobbies.com/carbon-fiber-tubes.html

In both these examples the idea is to make or buy a torsional stiff LE for the flaps.
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Offline Matt Piatkowski

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Re: Flaps design
« Reply #12 on: February 19, 2018, 07:54:52 AM »
Hello and thank you for your comments.
Please see the attachments.

Paul,
I have many C grain planks with "contest" stamped on them. Most of them were initially reasonably straight but, after half a year in my basement, they are all bent and some are also twisted to different degrees. I gather they were sold not properly dried and only now they dried, bending in the process. It is very hard to find a good quality balsa in Toronto and the problem is growing. I may order the presumably high quality balsa from one of the warehouses in the US but only the time will show if this wood is more stable.

Everybody,
My flaps are dead straight at this moment and reasonably stiff in torsion before covering.
Unfortunately, there is no numerical criteria for this stiffness so, like in many other cases, we are left with opinions based on experience.

I have a limited experience in building the 60" wings span planes. Specifically, MP Max Bee is only my second plane of this size.
The first one (Intrepid XL called Great White) was smashed to splinters before I could fly corners and the flaps in it were probably too wide anyway for the modern stunt.

The main question again: if I remove about 15% of the flaps weight by drilling the holes (see the proposed holes pattern on the photo, please) and cover the flaps with Silkspan with Doculam on top.....Will they be stiff enough for the good quality corners? The torsional stiffness of the rectangular cross section (J) can be calculated but this number is very conservative. When the flaps are covered with Silkspan and Doculam, the diagonal tension developed in these coverings will stiffen the flaps in torsion but how much?

Are we talking 10%, 50% or more?

Again...it would be good to know the test results of similar flaps from the well flying plane to establish the numerical criteria.

Regards,
MP Stunt Academy

Offline Dane Martin

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Re: Flaps design
« Reply #13 on: February 19, 2018, 08:21:17 AM »
I'm not a great stunt flyer (yet) but my builds come out very light. In my experience, it's not because I cut away or skimp on materials. It's the methods I've learned from the generation before.
You say you want to cut a bunch of holes in the flaps and reduce the overall weight, but then silkspan AND doculam to make up for the torsional strength? Why not leave the flap alone and cover it only in doculam? I would wager the result would be stiffer and the weight would end up the same. While I was learning to build, I thought cutting out holes or sanding a lot would save weight. It's very very minimal. The real weight savings is in the building and finishing.
I think it's cool you're looking into stuff like this. Innovative and forward thinking always helps. Build a few examples, make your flaps removable and test. I'm thinking the generation before us did that already though.

Offline Matt Piatkowski

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Re: Flaps design
« Reply #14 on: February 19, 2018, 11:10:08 AM »
Hi Dane,
I may do what you are suggesting, namely no holes and Doculam only. I know that Doculam adds a lot of torsional stiffness for very little weight.
I will also measure the torsional stiffness before and after covering with Doculam.

By the way: the generation before us used wider flaps and the pattern flying was as competitive as it is now.
In other words: there is, in my opinion, no direct comparison. It would be nice, though, to know the ball park numbers measured before,
Thanks,
M

Offline Dane Martin

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Re: Flaps design
« Reply #15 on: February 19, 2018, 11:23:33 AM »
Well, I think then, the generation before me is the guys figuring out giant changes in the 90's and 2000's. I could write names for days of who specifically has written articles on things like this. But to point out some like Paul and Brett who have replied on this thread. The giant wide cord flaps were a few generations back I would say.
My whole point is, that you have a very long piece of whatever. It's being controlled by a horn at one end of it. If not built stiff, it'll twist. RC planes get rid of twist by adding more servos per wing half. We don't have that option.

Offline Howard Rush

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Re: Flaps design
« Reply #16 on: February 19, 2018, 12:08:46 PM »
If you are trying to duplicate a successful design, you probably want flaps with the same aeroelastic properties as the original.  Igor probably has quantitative data, but you can certainly reproduce his materials and dimensions.

That said, I have a history making Impact flaps with essentially infinite torsional stiffness.  They are reproducible,  but may not work as well as the design flaps. I'll do something different next time.

The JCT is coming to town.
We're plenty stiff from the flaps on down.
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Offline Scott Richlen

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Re: Flaps design
« Reply #17 on: February 19, 2018, 12:38:23 PM »
Dorin:  Would it work to make the whole flap built-up of 1/16th and then put the whole sleeve over it?  Maybe use the lightweight 2" dia. sleeve?

Howard: Yes, but how did you construct the flaps?

Offline Howard Rush

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Offline TDM

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Re: Flaps design
« Reply #19 on: February 19, 2018, 02:00:38 PM »
Dorin:  Would it work to make the whole flap built-up of 1/16th and then put the whole sleeve over it?  Maybe use the lightweight 2" dia. sleeve?

Howard: Yes, but how did you construct the flaps?

That would be too heavy as the sleeve in question is 3K tow probably over 200g/M2.

I mentioned that just for the LE spar part of the flaps. You can cut the LE out of foam (foam stick pentagons shape) and dress it with the bi-axial. An easier way is to use the tube. With either construction full sheeting of the part is unnecessary.
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Offline Scott Richlen

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Re: Flaps design
« Reply #20 on: February 19, 2018, 06:28:27 PM »
Thanks Dorin!

Thanks Howard!

So, I'm  thinking that the only way I'm going to get really stiff flaps is to hang out around the L-Pad during the Nats waiting for a possible "accident" at which point I yell "Salvage rights" and run out and pull the flaps off the the partially dismantled/destroyed airframe and then hope that I can outrun the JCT Jivettes as I run down the taxi-way....

Offline Guy Markham

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Re: Flaps design
« Reply #21 on: February 20, 2018, 07:05:59 PM »
Hi Dane,
I may do what you are suggesting, namely no holes and Doculam only. I know that Doculam adds a lot of torsional stiffness for very little weight.
I will also measure the torsional stiffness before and after covering with Doculam.

By the way: the generation before us used wider flaps and the pattern flying was as competitive as it is now.
In other words: there is, in my opinion, no direct comparison. It would be nice, though, to know the ball park numbers measured before,
Thanks,
M
Are you using the doculam (  DI Laminating Film - 3 Mil .26 oz per square foot or 5 Mil .46 oz per square foot   after the final finish is done  to the flap ?  ???
You only have to floss the teeth you want to keep!

Offline Igor Burger

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Re: Flaps design
« Reply #22 on: February 21, 2018, 10:11:01 AM »
MY feeling is that it is much easier to find one plate of good wood than 2 sheets of identically good wood. That is reason why I did those flaps from one 6mm plate and not glued from thin plates. It could be lighter, but not worth of troubles with stability etc. Plus LE of flap is shaped, so it was much easier to sand it from one plate.

Flaps which have some flexing asymetry in positive / negative or twisting with humidity etc. make lot of problems while trimming, so I think plus few grams does cannot make result much heavier, but imprecise flaps can kill model potential.

Offline Tim Wescott

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Re: Flaps design
« Reply #23 on: February 21, 2018, 11:36:26 AM »
So, I'm  thinking that the only way I'm going to get really stiff flaps is to hang out around the L-Pad during the Nats waiting for a possible "accident" at which point I yell "Salvage rights" and run out and pull the flaps off the the partially dismantled/destroyed airframe and then hope that I can outrun the JCT Jivettes as I run down the taxi-way....

There are suitable CF tubes out there if you want to use Howard's method without having a complete CF laminating shop.  You may need to settle for flaps a bit bigger than 1/4" think, though.  I found some 5/16" OD fabric tubes at Rock West Composites that looked like the CF was on a diagonal bias of some sort.  Everything except laminating the tubes should be an ordinary model shop thing.

I think, were I going to laminate my own LE tubes instead of buying them, I'd be tempted to make a pair of balsa dowels and put a sleeve on them.  It'd depend on whether I could figure out how to hold them straight while they cured, and if I really felt I needed the exact right solution when there's a good-enough one out there for purchase.
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Offline Matt Piatkowski

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Re: Flaps design
« Reply #24 on: February 22, 2018, 05:35:28 AM »
Hi Guy,
It is 3 Mil. Doculam.

Hi Igor,
The flaps indeed have presently some torsional asymmetry, with the inside flap being a bit softer. This one is 1.0" longer as per your design and I took it into consideration measuring the stiffness. Please see the attached. The bending and twisting length of both flaps was 24.25" and the load was 0.776 oz.
Load offset for twisting tests was 4.375" from the flaps LE.

Bending stiffness is the same for both flaps: 1.55 - 2.07 oz./inch.
Torsional stiffness: 0.50- 0.55 oz./degree of twist for the outside flap and 0.60 - 0.65 oz./degree of twist for the inside flap.

At this moment, I am considering two solutions:
1. Add 3 mm x 0.8 mm. carbon composite beam on epoxy to the TE of the flaps. Weight penalty: about 6-8 grams per flap.
2. Cover the flaps completely with carbon veil on dope. Weight penalty unknown.

In both cases I am planning to top cover everything with Doculam. Final weight penalty unknown.

I am also considering re-making the flaps using 1/4" C grain contest balsa (if I find true C grain contest balsa anywhere) and cover them completely with carbon veil attached by dope. This method was used by Paul Walker in his P-47. His flaps have smaller area, though: about 80% of the Max Bee II flaps and I do not know the P-47 flaps slenderness ratio and the way their LE is supported in the model.

Hi Tim,
I am also considering to use the slightly tapered carbon composite tubes ( we can call it modified Howard's method) as the flaps LE and glue the 1/4" C grain contest balsa to it. The flaps cross-section will remain rectangular like in Igor's Max Bee.

To sum it up: there is more interesting work ahead and I truly enjoy the challenge.

Everybody,
I believe it would be great for our sport to establish the required flaps stiffness range for the 60" wings span plane.

Your comments and friendly critique are always welcome,
MP Stunt Academy Toronto.

Offline TDM

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Re: Flaps design
« Reply #25 on: February 22, 2018, 07:28:00 AM »
Your best bet is to buy the tubes from aloft make some ribs and built up the flaps. Dress the LE in balsa so youcan shape the beveled LE and you are golden.

3mil Doculam is stiff but twice the weight of any .......Kote might be of value on the built up construction but with the tube you can go back to the thin Dcoulam.
Carbon veil does Nothing for Torsion strength it just adds ding protection that is it. Carboweave is a much better solution that can be applied over closed cell foam for the lightest construction method. CW20 or CW30.
But for that you need laminating resin etc.
Your best bet is to go with the tubes built up the flaps and call it a day. That is light very stiff torsionally and easy to build. A 2mm tube for trailing edge is also a easy way of doing this.

Mold a piece of soft a grain balsa over a rod with ace bandaids. Windy make a mold with a steel rod and he taped a piece of balsa with masking tape.
Look at the picture and you can tell where I am going with it.
The idea is to dress the LE carbon tube in balsa to create the bevel LE. All the ribs are a flat rectangular balsa sheet and are sanded to finish size at the end. To make ribs basically you drill two holes and rough cut oversize top and bottom. TE has to be at proper height to ensure symmetry and that is what the support is for. After all is glued block sand like in the picture to finish size. Actually sand till you touch both tube the LE and the TE tube. Obviously sand both top and bottom. Should you rough cut before sanding? Of course you can.
The tube will provide all stiffness you need and you can cover with anything you want too. Because the tube is super stiff you don't need the stiffness from the covering anymore.
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Offline Matt Piatkowski

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Re: Flaps design
« Reply #26 on: February 22, 2018, 07:44:00 AM »
Hi TDM,
Thanks for the sketches and additional comments.
I am still collecting the opinions and will decide what to do soon.
Regards,
M

Offline Matt Piatkowski

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Re: Flaps design
« Reply #27 on: February 22, 2018, 08:40:25 AM »
Correction: my Doculam is 1.5 mil.
M

Offline Howard Rush

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Re: Flaps design
« Reply #28 on: February 22, 2018, 03:27:33 PM »
TDM,

You can use the carbon tube itself as the LE.  It's easier, but a smaller tube would have the benefit of less bending stiffness, while still being torsionally stiff.  You can get those properties by rolling your own, in which case the larger, the easier to make.

A little carbon tube at the back would help prevent dings, but the tube at the LE provides all the torsional stiffness you need, a roundy TE is possibly a bad idea aerodynamically, and you sure don't want weight aft of the hinge line on a control surface.

If you poke the holes in the balsa sheet first, you can cut triangular ribs by cutting tangent to the holes. Then you don't need to sand them down.
The Jive Combat Team
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Offline Igor Burger

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Re: Flaps design
« Reply #29 on: February 22, 2018, 03:52:25 PM »
Mat,

1/ If you look to flap and elevator airfoils, I do not think you will find usefull any of those tricks to save weight. I have also no idea how you want combine built up  or drilled flaps (and keep them strong enough) or tubes and keep the airfoil properly shaped.

2/ I found old photos from first Max Bee which has balsa flaps.

First picture shows sanded flaps <40g
Second picture shows all movable parts (flaps, elevator, rudder) covered by paper, 6 x doped and sanded and ready to paint <80g

Offline Matt Piatkowski

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Re: Flaps design
« Reply #30 on: February 22, 2018, 04:48:18 PM »
Hi Igor,
If I had my flaps bare below 40 grams there would be no problem.
If I decide to re-do the flaps in order to save ~20 grams on the wood only, I would have to find the 1/4" thick true C grade contest balsa that is dry and straight.

Like I wrote before: in my home town there is no balsa of this kind at all at this moment and the only choice is to buy it in the US, hoping that I would be getting true C grade.

Regards,
M

Online Lauri Malila

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Re: Flaps design
« Reply #31 on: February 23, 2018, 09:58:25 AM »
Hi.

I would really really really like to know if we really need these "Aeroelastic properties", as Howard put it, in our flaps.
So far I have seen it about a million times that when structures are made stiffer, models perform better and are more easy and logical to trim and fly.
I mainly see it with free flight but I've seen a similar trend in my cl airplanes. Why would CL be an exeption?
Could it rather be so that, like in Howards case, the Impact is a result of long evolution but with traditional construction techniques? If you improve the torsional stiffness of parts that like to twist during flight, maybe you must re-optimize their layout (flap width, lenght, travel..).
Or do we really need this uncertain Wash-out in wingtips during maneuvring?

Just pondering,

Lauri

Offline Vitalis Pilkionis

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Re: Flaps design
« Reply #32 on: February 23, 2018, 03:34:45 PM »
This is how I'v made elevators for my new model. Balsa and carbon fiber construction. Light and stiff enough even without covering. The only issue - a process is very time consuming, that's why for flaps I'm going to use full sheeting method with glass fiber at 45 degrees.

Offline Brett Buck

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Re: Flaps design
« Reply #33 on: February 23, 2018, 04:35:29 PM »
Could it rather be so that, like in Howards case, the Impact is a result of long evolution but with traditional construction techniques? If you improve the torsional stiffness of parts that like to twist during flight, maybe you must re-optimize their layout (flap width, lenght, travel..).

  That's certainly the premise I would start with. This came up a long time ago in Howard's case, it's not at all surprising that if you radically change a parameter in one area, you might alter the balance between a lot of other things.

   A more egregious and common case happens with weight.  Taking a design from the past, that weighed, say, 45 ounces, and get "really good wood" and very careful construction to get it to weigh 36 ounces radically alters the balance between a lot of other parameters. Yes, you might be able to retrim or partially redesign it to perform better at 36 ounces than at 45, but now without changing something, maybe a lot of things.

  Same thing with power - the tradeoff between weight, wing loading, flap travel/size, etc, changes radically when you change engine types.

     Brett

Offline Matt Piatkowski

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Re: Flaps design
« Reply #34 on: March 02, 2018, 10:46:51 AM »
Hello,
I have built another (and hopefully final) set of flaps for my Max Bee. 

Because I could not find the 1/4" thick true C grade contest balsa that was dry and straight, I sliced one of my curved contest balsa pieces longitudinally and flipped a half of it 180 deg. about the slice line. Then, I weighted both halves (one curving up, the other down) flat and glued along the slice line. The weight penalty: 1.5 grams. The pieces stayed weighted on the flat surface for two days and the glued plank was dead straight after the weights were removed.

The weight before painting and covering (yes...in this order) is 36 grams (1.3 oz.).

Nice surprise: the torsional stiffness of the new flaps is about 15% higher than the old, heavier (56 grams) set (!).

This stiffness will further increase when 1.5 mil. Doculam is used for covering. I will measure it and publish these results.

Thank you,
M

 

 

Offline Matt Piatkowski

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Re: Flaps design
« Reply #35 on: March 02, 2018, 12:18:50 PM »


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