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        Discussion Plank 101

#1 Knoll53 Sep 28, 2012 03:32 PM

Plank 101
 
http://i567.photobucket.com/albums/s...01-9-26-12.jpg


Any questions? :)

Kent

#2 Quick61 Sep 28, 2012 04:23 PM

First thing that comes to mind when I look at that is the Revert. However with the swept leading edge, maybe it will be less susceptible to proposing / hyperstall than the one I have.

So, the only question I would have at this moment is........... have you started on the build yet? :D

Mark

#3 Knoll53 Sep 28, 2012 04:34 PM

The Revert looks like a fun ship...but this one is sporting a PW51 airfoil modified to 12.46% thick.

The build has started...inasmuch as I have ordered materials and sent the files to the laser cutter.

With the half span elevons, it may be hard pressed to actually do a hyperstall. I'm shooting for only modest elevator control, as in fine control. More of a thermal ship than sloper, but it will do both.

Kent

#4 HerkS Sep 28, 2012 05:06 PM

1 Attachment(s)
If you are not going to use active rudder control, I'd recommend two smaller fins near the tips.

This layout worked really well in terms of stability, control and general handling qualities.

#5 Quick61 Sep 28, 2012 05:42 PM

Quote:

Originally Posted by Knoll53 (Post 22860958)
The Revert looks like a fun ship...but this one is sporting a PW51 airfoil modified to 12.46% thick.

The build has started...inasmuch as I have ordered materials and sent the files to the laser cutter.

With the half span elevons, it may be hard pressed to actually do a hyperstall. I'm shooting for only modest elevator control, as in fine control. More of a thermal ship than sloper, but it will do both.

Kent

Yea, I was referring to the over all looks, the way the tail is arranged, more than anything else. i figured you were going with something more than a KF airfoil. :)

Mark :popcorn:

#6 Knoll53 Sep 28, 2012 06:12 PM

1 Attachment(s)
Quote:

Originally Posted by HerkS (Post 22861210)
If you are not going to use active rudder control, I'd recommend two smaller fins near the tips.

Thanks Herk. So you're thinking that due to the sweep, that the winglets would be more effective than the central fin??

The Fin is fixed, no rudder. I'll start with the central fin and see if I have any complaints. I'm planning on a wing loading from 9 to 15 oz/s.f. so it should really move out...at least compared to the Manatee. I'm more concerned about pitch control than yaw.

Attached is my guess at the CG range.

Kent

#7 HerkS Sep 28, 2012 06:39 PM

Quote:

Originally Posted by Knoll53 (Post 22861721)
Thanks Herk. So you're thinking that due to the sweep, that the winglets would be more effective than the central fin??

The Fin is fixed, no rudder. I'll start with the central fin and see if I have any complaints. I'm planning on a wing loading from 9 to 15 oz/s.f. so it should really move out...at least compared to the Manatee. I'm more concerned about pitch control than yaw.

Attached is my guess at the CG range.

Kent

Hi Kent - no the fin you have drawn will be effective - though such a tall fin does introduce a rather undesirable rolling moment when it encounters yaw - particularly on a fairly low AR wing.

I like the fins at the tips because they increase the effectiveness of the elevons, have a favorable effect on induced drag, and by splitting the fin area into two surfaces reduce the roll effect mentioned above. All quite subjective on my part.

With that CG range the SM would be ~3% aft and 8 % forward. Looks right to me. I'd probably start a model like that at about 5%.

The model shown was an experiment to see how a flying wing could handle DLG launching. It did quite well - responded very well in that high yaw environment. I included it in the discussion because the layout was similar to your plan.

#8 Jon Snow Sep 29, 2012 01:03 AM

Hand up at the back of the class-can you you give a rundown on your reasons for modifying the the airfoil.From what I've read it's a well established plank section,I presume you are looking for something to suit your own preferences?
Regards Stuart.
:popcorn::popcorn::popcorn:

#9 Knoll53 Sep 29, 2012 09:52 AM

10 Attachment(s)
Thanks Herk. I've always considered the rolling moment from the fin above the CG to be minor, but as you point out, it has more of an influence due to the small wing span. The attached sketch shows the first fin with a low portion that is to be used at a bungee launch handle and skid. Although it adds a little weight, I'll go with it.

Those are some good reasons for winglets. Any thoughts on why most planks sport a central fin? Forward sweep would be one reason.

I'm sure, at some point, I will try to Discuss Launch this glider. Thanks for the reminder...this plane needs a throwing peg.

@ Stuart, I thickened the airfoil solely for the reason of fitting the big carbon tube spar into it. It is rare that a wing has no dihedral and so little sweep that a straight stick can serve as the spar, but with a plank it's quite do-able. Barely in this case. The spar is 1/2" diameter and beside providing bending strength it also provides all of the torsional strength (no D tube), not to mention serving as a handy jig for aligning the ribs. See sketch.

Kent

#10 Knoll53 Sep 29, 2012 10:08 AM

1 Attachment(s)
New fin and throwing peg

Kent

#11 HerkS Sep 29, 2012 10:28 AM

1 Attachment(s)
Quote:

Originally Posted by Knoll53 (Post 22866382)
Thanks Herk. I've always considered the rolling moment from the fin above the CG to be minor, but as you point out, it has more of an influence due to the small wing span. The attached sketch shows the first fin with a low portion that is to be used at a bungee launch handle and skid. Although it adds a little weight, I'll go with it.

Those are some good reasons for winglets. Any thoughts on why most planks sport a central fin? Forward sweep would be one reason.

I'm sure, at some point, I will try to Discuss Launch this glider. Thanks for the reminder...this plane needs a throwing peg. Kent

Hi Kent, I've always used a center fin when I wanted an active rudder control. I never wanted to deal with the complexity of running rudder controls to the wing tips. Since I'm mainly a thermal flyer, I've found that these planes need aileron-rudder coordination to fly at slow thermal-circling speeds.

Most of the fixed fin planes seem to be primarily slopers where most of the flying is at lower AOA - don't feel the need for an active rudder control. So why do they put the fixed fins in the center - convenience? imitation?, - added tail moment????

Regarding discus launch. I don't know if you've done much of that, but if it's structurally feasible, you probably want to move the peg aft a bit. At the start of the spin the plane lags pretty far behind your arm and the tip structure will dig into your hand if the peg is too far forward.

Here is where I put the peg on that DLG experiment -- worked well because the trailing edge of the tip slipped between my fingers instead of digging into them.

#12 Knoll53 Sep 29, 2012 10:47 AM

1 Attachment(s)
I see what you mean about the rudder. For slow thermal turns it may be very useful. Easy to cut in a rudder if needed later.

Here's the new throwing peg location. Should be do-able with the help of a little carbon tow. This will be my first discuss launch glider.

Kent

#13 HerkS Sep 29, 2012 11:06 AM

Looks good -

The sub fin will help with DLG - especially if you have no active rudder.

You don't want a super sharp trailing edge out there at the tip. It can cut into the web of your fingers if the plane lags too much during the DLG rotation.

#14 Knoll53 Sep 29, 2012 05:46 PM

I think I'll bring a sand block with me the first time I try the DLG thing. I suffer enough for this hobby....I don't need to bleed too. Thanks

Any other comments from the glider guiders ??? Looks like I might be able to get started on the build next week.

Kent

#15 Quick61 Sep 29, 2012 10:04 PM

While were talking DLG wings, I was looking for something else and came across this cute little guy.

And what i was looking for was this thread here - http://www.rcgroups.com/forums/showthread.php?t=1412503 Don't know if you have seen it Kent, but it might be worth a look. Of note is the way the fin is done.

Mark


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