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Old Feb 11, 2014, 09:37 AM
less is more
Knoll53's Avatar
United States, CA, Marina
Joined Sep 2006
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I"m hangin tough for the drawing, but until then......
Quote:
Originally Posted by guitarcher View Post
Should the CF spar caps be thicker on the top than the bottom?
From my humble testing, I'd say yes. At nearly a 2 to 1 ratio. Most any symmetrical wing design will fail in compression.Here's a little testing from the shop.

Kent
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Old Feb 11, 2014, 10:17 AM
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iron eagle's Avatar
United States, MA, Middleboro
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If you look at the link Kent provided he pretty much has it locked down as how to do it.
The only thing I would add in regard to the CF caps is this:
In most cases 2 to 1 is great with the only exception to that being if your particular airframe is going to be pulling strong negative G's on a regular basis, a very aerobatic airframe for example. But keep in mind that adds weight and the easiest thing to do is overbuild, heck I do it all the time.

Our club is pretty much like any other model aircraft club anywhere on the planet. You have guys that know what they are talking about, some that think they know, and others without a clue. We have a great bunch of Toy Airplane Enthusiasts that have the main goal of having fun playing with their toys, and we can usually come up with a way to fix almost any airplane.
There is a wild mix of aircraft at our club from CL to real drones, a few sailplanes and yes both supras and allegros have been spotted in the air. lol But most of the sailplane guys like to go out by the water or up in the mountains where the air is better for them. I have plans for both the supra and allegros (they are available on the Charles River web site Mark Drela is a member of their club) that I plan to build sometime in the future.
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Old Feb 13, 2014, 04:40 PM
Whiskey Tango Foxtrot
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Australia, QLD, Maryborough
Joined Jan 2013
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Thanks again for your input Kent and Paul. You've given me plenty to think about.

Its been a big week at work and I've just started 5 weeks leave so its full steam ahead on the wing revamp.

I've just been rereading Some of Drela's stuff as well as Phil Barnes' and will try to come up with a spar plan today and maybe make a test sample. I now see after looking for about the tenth time that the supra spar plan has the CF attached to the spar core and then wrapped with glass.
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Old Feb 13, 2014, 05:50 PM
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You sound like a man with a plan.
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Old Feb 18, 2014, 03:14 PM
Whiskey Tango Foxtrot
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Australia, QLD, Maryborough
Joined Jan 2013
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Biting the bullet.

On the weekend I loaded the wing onto the back of my ute and took it the local club site. Ha ha, how different it is to fly where there's no trees and a flat open landing zone. Had some really good flights, hooked a few thermals. The club members were all impresses as there is nothing else like this wing at the club.

Also got to see a Brolga flying overhead for the first time. These things look just like a pterodactyl. Very impressive.

Then it was time to get on with making some spars. For ease of construction I made a hollow 1/8" balsa box then joined on the joiner box. Capped with 2 layers of 200gsm CF mat ( don't have any unicarbon yet ) on the bottom and 3 layers on the top. Then wrapped with 2 layers of 45 bias 200gsm fiberglass with an extra layer at the top. All wetted out the clamped in a jig overnight.

After much deliberation I went ahead and took to the wing joins with an angle grinder:. The separation was pretty painless. I laid the new spars in position on the wings then marked them. Then onto the bandsaw and cut out the old spar.

As a test first thing this morning I cut an old wing section and glued in a spar piece with PU to see how much it expands as I'm a little concerned that the wing halves may be forced apart as the glue foams.

I'm also in the process of making a new fin with CF sandwiched in the middle of 2 bits of foam. I may cover with 100gsm glass or 3 mil laminating film.

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Old Feb 18, 2014, 05:53 PM
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It must of hurt to cut up your plane like that.
Awesome!
Sounds like you need wing molds to install the new spars.
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Old Feb 24, 2014, 12:22 AM
Whiskey Tango Foxtrot
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Australia, QLD, Maryborough
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CG optimization.

Quote:
Originally Posted by iron eagle View Post
It must of hurt to cut up your plane like that.
Awesome!
Sounds like you need wing molds to install the new spars.
Not really Paul, once I'd made my mind up I just went for it, the whole thing is a learning tool for me and very enjoyable.The wing cores were put back in the original foam beds to glue the spars in place.

Its been fairly windy since I finished the new spar installation so no thermal flying but today there was a fair bit of east in the wind so it was off to the beach slope to relaunch.

I finally got to experience what my mate Tim has been telling me in regards to CG optimization.

The launch was smooth but it soon became apparent the the CG was too far aft. One click of elevator trim made a huge difference and when the tiniest bit of down elevator was applied the wing went into a nasty snap dive. Easy enough to recover but unnerving all the same. It was however super responsive to lift.

After adding 2 washers (35 grams) to the nose the tucking had disappeared but handling was noticeably more sluggish. It was easy to fly like this but the cartwheeling from the previous landing had loosened the new fin mount assembly so it was time to call it quits for today.

When I got home I set the wing up on the cg stands and was not able to accurately differentiate between the 2 cg positions that I flew with today. Less than 1mm I reckon. The battery is as far forward as I can get it so I will extend the firewall 12mm so the motor is further forward and hopefully I can then move the battery around to get the best cg position now that I have seen how much more responsive to lift the wing can be.

The new spars have added some weight to the wing and it is now 2600 grams which equates to just under 12 oz/ft wing loading. It has been well worthwhile hacking into the wing to make it transportable and has been a terrific learning experience.
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Old Feb 24, 2014, 03:02 AM
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gold coast australia
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Bingo
From what you have said , sounds like removing one of the washers and thats probably where the CG will be happy.
On that high AR you probably only have 1mm or less elevator at the perfect CG.

A lot of planks fly nose heavy only because people don't have the set up perfect ! Servo slop etc.
I have my gizmo's so finely tuned not I had to change my trim adjustment setting to the minimum . Which is now about 20+ clicks I think.
Other wise one click down it would tuck and one click up it would balloon. Now with many many more adjustments I can keep taking out lead and flying it with the smallest amounts of trim. The difference is amazing .
Over the years it has become more and more apparent to me that you can only tune a plank as good as the set up.

Glad you getting it flying better and better. I'm looking forward to having a fly of it soon

Tim
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Old Feb 24, 2014, 09:04 AM
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Greg sounds like you were having some fun with it.
CG range for a plane like that is pretty narrow but how they fly makes it worth it.
As far as the learning experience, you get a lot of that when you build or design.
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Old Feb 24, 2014, 01:40 PM
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Australia, NSW, Bellingen
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Plank CG trim.

That's a good report Greg! Well done.
Like Tim has indicated above, you may have made the CG adjustment to much in one step. I'm in favour of a small weight taped to the fuse (10 - 20 grams) which is moved forward of the Cg (or aft) in small increments, like 30 mm or so. Test fly each time, move more if necessary. Keep doing so until you observe improvements stop and the plane starts to become sluggish again (in the case of too far forward CG).
If you get it right to the nose and its still improving, leave it there, place a second one @ 30 mm in front of the CG and repeat the process with that.
Of course, in most cases, all this will be done in reverse; the plane will start with a forward CG and the trim adjustment will be moved back.

But I'm only repeating what Tim is indicating in another way..............I hope I am not laboring the point too much here!

And, as he says, elevator deflection rate will need adjustment, usually reduced as the sweet spot is approached. This is where the payoff is really obvious.

Don't break it before we see it fly at Manilla!
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Old Feb 24, 2014, 02:44 PM
Whiskey Tango Foxtrot
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Thanks for the positive feedback guys, I really appreciate the support you've all given throughout this project. Its been great to have such a successful outcome and I look forward to ongoing tweaking to maximize performance now I know what can be achieved.

I've still got 3 1/2 weeks leave ..... so what's next????

Greg.
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Old Feb 24, 2014, 03:41 PM
ETS....Energy retention system
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5m for manilla
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Old Feb 24, 2014, 04:14 PM
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Three piece wing 6 meter span.
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Old Feb 24, 2014, 05:00 PM
Whiskey Tango Foxtrot
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You guys crack me up. Pissed myself laughing.
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Old Feb 24, 2014, 07:51 PM
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Wouldn't want you to get bored with nothing to do...
Expand upon your BWB center section, just set it up to plug the wings you have onto it., something with a motor for those days you don't have any thermals to chase.
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