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Old Dec 16, 2007, 04:41 PM
trying to kick the habit
United States, CA
Joined Mar 2006
1,396 Posts
Build Log
Drive65 PW51 plank (Was: Anhedral/Dihedral and washout in planks) +video!

Hey all, I am am going to be scratch building my first plank. Yay for me. I am going with something similar to the Bluto planform with the PW51 throughout (ya, pretty much a Drongo). I have been reading (for days actually) the PW articles in RCSD as well as many threads on RCG about plank design and while I think I know just about enough to start cutting foam, I have a few questions. For me, this is a project to learn from and I anticiapte more than one revision. Hope I dont offend anyone by scratch building this for my personal use.

So far, I am planning to keep the wing straight, no washout like PW suggests. Does this sound right or should I add a few degrees to compensate for the taper?

I am sure this depends on the last question, but what about anhedral/dihedral. I was planning to keep the chord at a constant height above the bottom surface of my foam (keep "h" in Profili constant) which I guess would be considered flat. Because of the multiple tapers in the planform, I dont think this will work out though when it comes to a straight TE. So I guess I have to play around with the dimension. Anyway, should I try to keep it as flat as possible or give it some an/dihedral.
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Last edited by Crashaholic; Mar 17, 2008 at 10:32 PM. Reason: changed thread title to reflect the build
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Old Dec 16, 2007, 04:44 PM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
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I would build it so the "peak" of the max thickness would be on a flat plane on the top of the wing.

Adam
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Old Dec 16, 2007, 04:53 PM
trying to kick the habit
United States, CA
Joined Mar 2006
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So this would give it some dihedral, the same way a Weasel has it kind of "built in". Right?

I guess I should mention that this will be for front side flying. I am not is to DS'ing... at least not yet.
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Old Dec 16, 2007, 04:56 PM
Just fly it!
wyowindworks's Avatar
Cody, WY
Joined Nov 2007
6,915 Posts
Quote:
Originally Posted by Crashaholic
So this would give it some dihedral, the same way a Weasel has it kind of "built in". Right?

I guess I should mention that this will be for front side flying. I am not is to DS'ing... at least not yet.
Yes, that would give it a little bit of dihedral, but still have very good inverted flight.
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Old Dec 18, 2007, 09:35 AM
trying to kick the habit
United States, CA
Joined Mar 2006
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So the attached pic shows what happens when I make the top "flat". This is the problem that I was hinting at in my first post referring to the multiple tapers. The red line is drawn from the point where the bottom TE meets the root to the point where the bottom TE meets the tip. You can see that it does not follow the TE. Since this airfoil is to be bottom hinged, this wont work. If I leave out the dihedral, this problem goes away (not completely, but the line is within 0.2mm of the TE the whole way). I can also adjust the height of the center section to make the red line conform to the bottom edge.

So what does everyone think?
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Old Dec 18, 2007, 05:20 PM
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Kopenhagen
Joined Apr 2005
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Dear Crashaholic

You need to chose the hinging line as a reference point, if you want to make a somewhat flat upper side, due to the too trapezes. If you dont, you will get some problems to move the flaps.


Yours Peter
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Old Dec 18, 2007, 06:38 PM
trying to kick the habit
United States, CA
Joined Mar 2006
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Quote:
Originally Posted by Peter Wick
if you want to make a somewhat flat upper side
And that is the input I am really asking for here. Do I really want any dihedral with this kind of planform? I am not really a plank guy and am just starting to learn about aero-design.

and thanks Peter
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Old Dec 19, 2007, 02:32 AM
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the flight performance won't change much, whatever you do - I'd build it flat upside down on the board to give just a sniff of dihedral - looks better!
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Old Dec 19, 2007, 03:13 AM
Registered User
Heathcote, Australia
Joined Jan 2007
579 Posts
Some more reading

http://www.aerodesign.de/english/pro...le_s.htm#hs520
http://www.mh-aerotools.de/airfoils/flywing1.htm
http://pagesperso-orange.fr/scherrer...h/fwairfe.html - this one has the zaqgi foil too.
Try some of the more modern foils, they may have a reduced bobbing effect when flying. The bobbing is produced by pitch instability and short control coupling.

Good luck.
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Old Dec 19, 2007, 04:56 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,457 Posts
Quote:
Originally Posted by ENGINETORQUE
the flight performance won't change much, whatever you do - I'd build it flat upside down on the board to give just a sniff of dihedral - looks better!
That's the way the Drongo was built!!,it has 15mm in total dihedral.




Rick
off the edge
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Old Dec 19, 2007, 05:00 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,457 Posts
Check out the build thread (am I doing my self out of a sale )




Rick

Drongo
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Old Dec 19, 2007, 12:16 PM
trying to kick the habit
United States, CA
Joined Mar 2006
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Thanks guys. I think I have a pretty good handle on it now. Playing around with the CAD model to get the trailing edge straight and the location of the spar fairly flat put me right around the amount of dihedral that BMtech recommended.

Rick, I recently built a hotwire rig and bought a bunch of EPP and have been looking for something to build. Some of the local flyers in Ventura have Blutos and Bluto like planes. I always liked the planform due to the lack of a fuselage. Just seemed to make sense. I wanted to go with something like that but with an updated airfoil (not that there is anything wrong with the Bluto foil, I just wanted to do something different). The PW51 is the hot setup right now so I figured I would go with that. A quick search for the terms "Bluto" and "PW51" led me to your Drongo build thread. I am always a few years late with my great ideas So anyway, there was unfortunately never a sale to be had. If any of the local flyers like the plane, I will be sure to point them in your direction though. Hope that is enough consolation.

I guess I could always build the public domain AZ2003 with a different foil...
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Old Dec 19, 2007, 01:30 PM
SlingWinger
San Bernardino, California, United States
Joined Oct 2004
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When considering dihedral, it is good to think about the relationship of dihedral and directional stability.
As an example, consider the rudder/elevator gliders. They have no ailerons, but when a side slip/skid is introduced the portion(s) of the wing that has dihedral actually becomes, in effect, an aileron.

Now consider the free-flight model. In this case, dihedral isn't being used to provide roll control, but is being used to provide roll stability. If the free-flight enters a turn, we know that the high,or outside, wing will have a higher airspeed than the lower, or inside wing, and will have more drag as the result of the higher airspeed. It will also have more lift, which will tend to cause the bank angle to increase and possibly result in a spiral dive into the ground.

However, the increased drag on the outside wing will cause what is referred to as "adverse yaw", the inside wing will be flying forward, or in advance of the outside wing. We would say that the aircraft is "slipping"
towards the inside wing, and, the dihedral will cause this wing to have a higher angle of attack (AOA) than the outside wing, this in turn will cause the aircraft to want to roll out of the turn.

Now the amount of adverse yaw that we get is limited, or controlled by, the amount of directional stability we have. So let's look at our free-flight model, which we agree is roll stable. Now suppose we increase the directional stability by making the vertical tail twice as large as it was.

This will increase the directional stability, we won't get as much adverse yaw, the dihedral won't produce that differential in AOA that we needed to roll the aircraft back to level (or at least limit the bank angle to one we can live with), and we now have a free-flight model that is spirally un-stable and probably busted up pretty bad.

So from the above we can recognize that if our rudder/elevator glider with poly-hedral doesn't respond as quickly as we like to rudder input, we can increase pro-verse yaw by making the rudder bigger, or we can increase the di- or poly-hedral, or do both.

So what does all this have to do with your plank-style flying wing?
When we make an aileron input to the plank we increase lift, and drag, on one wing, and we get the opposite on the other one. The aircraft will try to yaw the wrong way, although we can reduce or eliminate this tendency by using differential on the ailerons, where the upward moving aileron is configured to have more movement than the downward moving one. This will tend to reduce the adverse yaw, and if you have a computer radio will probably make you happy.
I mention the radio because if you're a Po'boy like me all you got is a cheap Hitec Focus 3, in which case if your aircraft is a flying wing with elevons having differential movement will likely cause the glider to want to pitch up when all you wanted was to roll.

So without the differential, we're going to see some adverse yaw, and if our glider has dihedral, we'll see a reduced roll rate because the dihedral, coupled with the adverse yaw, will produce a rolling moment opposite to the moment produced by our aileron deflection.

To sum up, if you want as much roll rate as you can get, strive to keep either dihedral to a minimum, or adverse yaw to a minimum, or directional stability to a maximum (to limit adverse yaw) by using (a) a large fin or (b) move the fin aft, farther from the CG, with a tailboom.
Or both a and b.
I've found that I can get a noticeable improvement of roll rate on my chevron style combat wings by bending in some anhedral and securing it with a strip of strapping tape under high tension. It should be kept in mind that a swept-back wing has an aerodynamic dihedral inherent within it, it runs about 1 degree of dihedral for every 5 degrees of sweep.
So you have to have a noticeable amount of anhedral bent into it, just to reach neutral.
If you want the aircraft to handle the same inverted as upright, then just build it flat.
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Old Dec 20, 2007, 11:50 PM
trying to kick the habit
United States, CA
Joined Mar 2006
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Thanks for the excellent info everyone. Here is what I have come up with so far. It has most of the Bluto/Drongo dimensions. It really looks like the outer panels want to be longer doesnt it? Think I am going to extend them a bit. Plus, my foam is in 48" sections so I may as well use it up (the outer panels are currently at around 21"). The tail on there is a placeholder although I like the shape. Sorta "eta"ish like PW recommends. I have to use the formula he references in his series in RCSD to figure out the exact volume.
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Old Dec 21, 2007, 03:04 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,457 Posts
The original two Drongo's were 56'' & I now make them 60'' to use the foam.

I think the fin needs to have some more volume on the underside, the Drongo is 50/50 but overall it looks great.

Rick
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