So, I haven't posted anything here in several days but that should not mean that I have been goofing off, though maybe some of the time I have been doing just that.

In between the goofing off however i have gotten some work done on the POP wing. Got the other side done, the CF tubes cut and glued in. Sense it has worked so well in the past, I dissered the fin with some CF tow. If anyone was wondering what the foam inserts were for in the tail, this is it. Needed something there that would provide backing for the tow and light enough to not have an impact on weight. We'll see how it turns out tomorrow when I pull it from the "wax paper bag".

Mark

Starting to look like something that might actually fly. The Tail came out shiny, weighing 6.4g going in and 7g coming out. Stiff and flat, no warp. A little more work on the fuse then it's wing mounting time. i really don't want to use the motor I have now as it is more than it really needs, It's an emax 2822 and with the airframe only weighing 3.2oz so far it has enough power to prophang the thing to the stratosphere. I have a "blue wonder" type, but it's about used up, and it's a coin toss as to which moments during the flight it will just stop spinning, not good. Then I have a little 10g job but I don't think it will be enough, so getting this one in the air might be a few more weeks waiting on a new motor.

Mark

As light as this thing is, you could put a bungee hook on it, launch into a thermal and fly for a 1/2 hour. It might be easier to test fly without power just to see what it will do without worrying about thrust lines and such. No surprise getting such advice from me.

Have you decided on a CG for the test flying yet?

Kent

Originally Posted by Knoll53

As light as this thing is, you could put a bungee hook on it, launch into a thermal and fly for a 1/2 hour. It might be easier to test fly without power just to see what it will do without worrying about thrust lines and such. No surprise getting such advice from me.

Have you decided on a CG for the test flying yet?

Kent

Actually, no CG yet. Haven't crunched any numbers (you have a good starting point in mind?) and you know, I just might carve out a nose and glide it around at first. What I am aiming for though is a 2-3m E-glider (power assist) at the end of it all if this works out. The fuse needs much work as it is now just an ugly box, but not knowing IF it will even be flyable, I can live with an ugly box for now.

Thrust lines, we don't need no stinken thrust lines. Just slap a prop on and give her he!!, right? Really though, I thought I would start out straight and shim up from there if needed. Haven't done anything with motor / prop sense my foamie days, so any advice in the motor alignment department would be shiny.

Mark

I could take a guess at the CG, but I'd like to see your method.

I just use a graphic method based on a straight taper to create a Mean Aerodynamic Chord, then a percentage of that, so I'd probably break this one up into 2 tapered panels. Have you posted a CAD file of the top view yet?

If you build a 3m version of this, with a cool fuselage, I'll build the Samurdactyl.

Kent

Originally Posted by Knoll53

I could take a guess at the CG, but I'd like to see your method.

I just use a graphic method based on a straight taper to create a Mean Aerodynamic Chord, then a percentage of that, so I'd probably break this one up into 2 tapered panels. Have you posted a CAD file of the top view yet?

If you build a 3m version of this, with a cool fuselage, I'll build the Samurdactyl.

Kent

This is what I came up with using a straight taper - http://fwcg.3dzone.dk/?wing_span=35....ow_mac_lines=0. That's how I was figuring on doing it my own self. It's worked OK so far and there is always the toss to the bed and then the tall grass before I try to send them skyward.

The post HERE has a pdf with a few measurements and the tip chord is centered on the root.

I got this by "averaging" a line through the l/e.Interesting to see how these compare to Kents

Not sure what your measuring point is, but it looks like we have different estimates.

I was able to convert the PDF to a DWG. I came up with a CG, measured at the root rib face at the fuse, from the LE back 2 1/8" for the test flight and 2 3/8" for Expert setting.

We'll know soon enough.

I'm voting for a responsive elevator.

Kent

Aerodynamically the moments operate about the CG.

So, the more forward the CG is, the more likely it is that you will get some pitch effect from the pitcherons. I used a three panel wing to rationalize your design. This analysis indicates that the Neutral Point is 2.14 inches back from the centerline leading edge. If it was mine, I'd probably start trying to fly this with a static margin of 10%. That puts the CG at 1.63 inches. The smallest static margin I'd attempt to begin with is 5%. That would put the CG at 1.89 inches.

A little flat foam test bed might be a good idea - or as Kent suggested, skip the motor and get some glides in over a safe terrain. I would not trust a hand launch. A good flying plank will dive right into the ground from a bad hand launch. A light weight bungee launch would be best. For a model this size I just use 1/8th inch wide rubber powered model motor rubber. If you need some, PM me and I'll send you 20 feet or so. I have a lot on hand.

OK , I got 1.92, Kent has 2.125 and Stuart has 1.89.... So that's somewhere around ~2.0 to start then????

Mark

OK Herk, you and Kent have talked me into it, glider version first. Still at the point that adding a hook will be easy so there's that. now the question is, should I place the hook at the "standard" place, just forward of the CG or should I move it more forward?

I'll see if I can't get the nose built so it can be changed out easy if I want to add a motor once I get it flying. maybe make it hollow and fill it with epoxy and shot to get a run on the balance.

Mark

For the towhook location, in a side view, my vote is to put it at a 45 degree angle forward of the CG. This should give a launch between a flat bungee launch and a steep high start. For the test flight, it's nice to have a little more ground clearance than a true bungee launch provides, yet not such a steep launch that control becomes a problem. Just a short and tiny bungee force. Similar to a hand toss. This plane would probably launch with a 2 lb. pull....which is nothing (I bungee launch my 50" ships starting at 20 lbs).

If you survive that, you can jack things up from there.

Kent

OK, so if I'm getting you correctly, you would place it a bit more forward than a standard sailplane but not out on the nose. Got ya. maybe 3/4" - 1" or so froward of the CG instead of ~3/8" or so if it were being set up to launch up high.

Mark

Originally Posted by Quick61

OK , I got 1.92, Kent has 2.125 and Stuart has 1.89.... So that's somewhere around ~2.0 to start then????

Mark

It is still not clear what the forward measuring point is.
I used the intersection of the LE and the side of the fuse.
I'll bet others used forward point of the wing at the centerline of the fuse.
Due to the sweep of the wing, it makes a difference.

My CG estimate is much further back than others. Keep in mind that a very forward CG will be flyable and safe as long as you have lots of elevator throw. My favorite example of this is my Komet's maiden flight. It flew well with a CG that was eventually move back 1 3/4" ( and I'm still moving it back).

Herk says that the more forward the CG is, the more effective the pitcherons will be as an elevator. I don't understand this, but it does matter. It counts as another vote for a forward CG. Now that you have done a survey, my vote is the most forward CG estimate, which is 1.63". Just be ready to pull full up elevator upon launch....as I did in this video.

Kent (who is endlessly interested in such things)

ME 263 Komet - maiden voyage (1 min 39 sec)

BTW, have you seen this???
It's a CG calculator that allows you to input your gear too to check a design prior to building. Very nice.

Click the example
Kent