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Old Oct 15, 2012, 10:20 AM
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Knoll53's Avatar
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Considering the ever increasing weight of this prototype, using the fuse is looking very attractive. Saving 6oz. from AUW will be noticeable. I was hoping for a 9oz/s.f. wing loading but that mark has already been passed even with the fuse.

Kent
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Old Oct 16, 2012, 03:51 PM
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Added some CF tow and Kevlar cloth at stress points and gouged out all foam. Rigid and strong. Rough sanded and ready for flight testing.

Kent
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Old Oct 21, 2012, 09:53 PM
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First Flight - Couldn't be happier

Wow....this plane floats nice and moves out too!

Many thanks to Peter Wick for creating the airfoil. I've never seen anything like it. I'm hooked on planks. Even with the thickened version, this plane really flies fast. I can see why the DS guys like these planks. Can't imagine what will happen with 16oz. of ballast and a fast trim setting.....

As soon as I got it tweaked to be some what flyable, I flew the rest of the day at minimum sink just to attempt to slow things down. Very maneuverable ! Of course I'm used to the Manatee which is slow and "stately". Landings will be a challenge because it moves so fast. Even with today's head wind in the LZ, I could not get it to hover, which is normally easy. This plane has no drag. Glad I built it strong. Clearly there will be some high speed landings in it's future.

Even with the CG push way back from the initial flight, the elevons are still kick up, so I guess the CG is going back some more on another day. No control problems so far. Indeed I could man-handle it way deep in the LZ, at minimum sink without a tip stall. Zero twist in the wing. Go figure.

The conditions were light today, then as the sun set, it got weaker. I kept flying until the birds started to flap, then continue to fly after the birds quit flying. This plane really floats! Not sure why, but the birds just scatter when they see this plane. Maybe when I learn how to fly smoooooth we'll share the air

Here is the FIRST flight with CG way way WAY too far forward and elevator rates way too high. Glad I fly from a cliff. Almost stuffed it.
Plank 101 - 1st Flight (3 min 9 sec)



Shot tons of video today. Later after some tweaking, focused on landing practice. Here is one batch of landing attempts in absolutely perfect conditions when the wind was very light. Almost calm in the LZ.
Plank 101 - Min. Sink Landings (0 min 0 sec)




Kent
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Old Oct 21, 2012, 11:29 PM
skumgummi dave
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Nicely done...
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Old Oct 22, 2012, 01:17 AM
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Nice one Kent.You should borrow Richards "Quick Win"title for the heading.
Regards Stuart
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Old Oct 22, 2012, 01:59 AM
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Very nice Kent! what % MAC has the CG ended up at ?
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Old Oct 22, 2012, 05:30 AM
the answer 42 is
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Nice Kent, did you try and DLG launches?? mmm if you share the plans it will go on my to do list for the next winter building season

Regards

EZ
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Old Oct 22, 2012, 09:12 AM
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@miniphase: After yesterday's test flying, the CG ended up at 3 11/16" from the nose, which is a actual measurement. We probably have different methods of determining the M.A.C., so I'll leave it at that.

@EZ: Have not tried the DLG launch yet. I'll try that in Big Sur on a calm day. All the design info is in this thread. Hopefully I've encouraged you to create a spin off.
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Old Oct 22, 2012, 10:25 AM
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Here's a video that shows the acceleration possible with this light ship and the automatic landing once the tail touches terra firma.

Plank 101 - dive / short landing (0 min 0 sec)
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Old Oct 22, 2012, 02:56 PM
Herk
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Great flights Kent - looks like you put together a really good one.

I guess a fin here or there is not necessarily a bad thing.

Based on the calculation system I've been using since we started this thing your SM is 2.3%

That's consistent with my plank (with fins) flying nicely at 2.4%.

What's the all up weight at this CG???
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Old Oct 22, 2012, 03:12 PM
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Hi Herk,

the AUW is right around 32 oz.

So I'm guessing that you would suggest that moving the CG rearward from here would be "problematic". I bring it up because this well tested airfoil is flying with significant up trim to keep it from heading into the ocean. The only change I made was to thicken it from 8.9% to 12.46%, then let the sag of the covering between the ribs thin it down again. I would not think that this change would cause it to need up elevator (more reflex).....but maybe so.

I'm interested in moving the CG back to see if I can get the elevon to line up with the wing.
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Old Oct 22, 2012, 03:51 PM
Herk
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Quote:
Originally Posted by Knoll53 View Post
Hi Herk,

the AUW is right around 32 oz.

So I'm guessing that you would suggest that moving the CG rearward from here would be "problematic". I bring it up because this well tested airfoil is flying with significant up trim to keep it from heading into the ocean. The only change I made was to thicken it from 8.9% to 12.46%, then let the sag of the covering between the ribs thin it down again. I would not think that this change would cause it to need up elevator (more reflex).....but maybe so.

I'm interested in moving the CG back to see if I can get the elevon to line up with the wing.
Hi Kent - I felt that I could move the CG back on my plank too. At 2.4 % calculated - who knows what it is in reality. For me calculated SM is a reference and a starting point.

The way my plank flew made me think it would fly better with the CG back further - especially when gliding. Power with a tractor prop reduces the effective SM a bit. So when you have a power model with that kind of setup you might find that you need a smidge more forward CG when it's under power than when it's gliding. The gliding dive test on that model felt very safe. I haven't got around to moving it back, but will be doing so one day.

As you know the only real test is to do it and fly it. I say do it.
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Old Oct 22, 2012, 04:08 PM
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Thanks Herk. I was unaware of the power issues with your 2.4% static margin. Very interesting.

Oh, there's no stopping me on this one. I'll move the CG back, little by little. I did perform a dive test when the wing was trimmed for very slow flight and it pulled out of the dive very quickly. Then I did the same test again with it trimmed for speed and did not pull out of the dive at all.

Still wondering why it needs up elevator to fly. One reason is how the sagging covering between the ribs varies depending upon the curvature of the rib. More sag at the curvy parts. One thing is clear. The actual airfoil, between the ribs. is not a thickened PW51. Also, Koverall on the bottom and Monokote on top. These two covering shrink differently. Koveral mostly with the grain and Monokote uniformly.

It appears for this construction method, I could use a more reflexed airfoil for the ribs.
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Old Oct 22, 2012, 06:38 PM
Herk
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Kent - about the up elevator needed to trim the model;

I was not familiar with the PW51 - however I found the coordinates and checked it in XFLR5. The program says that even though it has some reflex it still has a negative moment coefficient.

In practical terms that means the airfoil by itself wants to nose down. Add to that the unbalance due to the CG being ahead of the neutral point and --- Yes you have to have up elevon to compensate for the inadequate reflex. The up elevon trim setting compensates for the fact that the airfoil doesn't have a positive CM.

I know that you increased the thickness of the section, but that would not affect the theoretical CM.

I did not do an exhaustive analysis and I could not find any design info on this airfoil, except for a set of posted coordinates - so someone else may have more to say about this.
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Old Oct 22, 2012, 06:56 PM
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Quote:
Originally Posted by HerkS View Post
you have to have up elevon to compensate for the inadequate reflex.
Very interesting. Thanks again Herk.

This negative CM thing surprises me a little bit. I just assumed that it would be zero. I have noticed that most planks have full span elevons so if they were deflected up a bit, you'd never really know.

I was unaware of this when I designed the partial span elevons. I may rethink the Samurdactyl. Sure flys good for a bad design less than optimal design. I wish I could find another RC pilot around here so we could get another glider (any glider) in the air at the same time just to show how great this little plank performs. It does it all. Floats great and moves out too!

It's raining this week and windy this weekend, but when the weather calms down I hope to SAL it into a thermal in Big Sur. That might be a first for me.
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