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Old Feb 10, 2013, 09:05 PM
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Help trimming elevons down

Hello i was wondering if i should trim down these elevons to make the plane handle more agile. Its a stryker C fuselage with depron fins and elevons and made real light with a small motor.
I built a lightweight stryker before following this thead. This one is a little different with added rudders and much over sized elevons. As well as a front mounted motor instead of the standard pusher setup.

Im trying to have fun and learn 3D style and float around really slow flipping around every which way. Im wondering if there are some fundamental aerodynamics i could follow. That would give a ballpark idea what shape elevon would work best for this setup. What angles to cut and what % of the total size maybe. The elevons are roughly 3x14". The leading edge of the flying wing is about a 55 degree angle swept back.

I need to trim the inside of the elevons to make room for the rudders. Hoping for advice on if i should trim the outsides as well to make for a more enjoyable flying experience. Added agility and stability would be ideal. Will lean towards more stable though. Thank you!!
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Old Feb 10, 2013, 09:08 PM
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The elevon pivot is around 15 degrees swept back.
Thanks again!
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Old Feb 10, 2013, 10:17 PM
B for Bruce
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Proper 3D style flying requires the prop blast to pass over the ailerons and tail surfaces to replace the lack of normal airspeed found at near hovering like speeds. The Striker design fails this requirement as the elevons are well outboard of any prop blast. So to have control authourity the model has to be flying at some minimal amount of airspeed.

Now if you want something simple then a delta wing or even a simple D shaped "pizza box flyer" would actually be a better option. There's also an all Depron Ryan Vertiplane out there somewhere that does 3D style stuff quite nicely.
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Old Feb 12, 2013, 12:37 AM
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Thank you!!!!

The elevon inside edges are about 9" apart. I can run a 9 inch prop with this setup.
~Does the prop blast go back in the form of a cone?

Still sitting here wondering if i should trim down the shape. Will be crashing a lot so i want to get rid of the sharp pointy angle at the outside tip of the elevon. Was hoping you smart guys could break down some basic fundamental building techniques.

~Does changing the tail edge angle of the elevon, relative to the pivot angle, effect the way a flying wing will roll?
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Old Feb 12, 2013, 01:18 AM
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Originally Posted by wopachop View Post
Thank you!!!!

The elevon inside edges are about 9" apart. I can run a 9 inch prop with this setup.
~Does the prop blast go back in the form of a cone?
The prop blast actually contracts a bit so that a few inches downstream it is a smaller diameter than the propeller. More funnel shaped than cone shaped.

Quote:
Will be crashing a lot so i want to get rid of the sharp pointy angle at the outside tip of the elevon.
Go ahead, round those corners off. It probably won't hurt much


Quote:
~Does changing the tail edge angle of the elevon, relative to the pivot angle, effect the way a flying wing will roll?
Yes, tapered elevons with the wide end at the wing tip are a little more efficient in cruise but that won't help any in 3D since it is opposite of what you are contemplating


--Norm
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Old Feb 12, 2013, 04:38 PM
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Interesting stuff. So the prop blast is more like a tornado. Does a slow fly prop blast stay wider for longer?

Really glad i didnt cut the elevons shorter at the wing tip. The light strykers are super floaty and fun and easy for beginners to fly. Im not really trying to make it a 3D plane. But it might fly a lot different than the regular setup because of the front mounted motor and rudders. I was able to get about 50 degree deflection. It might be hilarious to haul butt and then slam those suckers to the side. Or see if i could fly really slow and low to the ground and turn it with the rudders while keeping the wings level. I could make a tire screech sound to make it even funnier.

Max speed is around 35-40 with an 8/4 prop.

Is there a ratio for trimming down the elevons wider at the wing tip? These are 3x14. From the thread you linked it seems like 1/2 inch on a 2 inch wide elevon seemed groovy.
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Old Feb 12, 2013, 06:02 PM
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Your wide elevons are beneficial during slower flight. SOME response, it may or may not be a lot, will be lost by trimming them If you're worried about catching the ailerons and stripping the servo gears what about fitting the wings with "tip mounted AIM-9 missles on launch rails" or something funky like that which simply act as guards to keep things from striking the tips of the elevons.
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Old Feb 12, 2013, 06:02 PM
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The only way I can see to get those ailerons in the prop blast is to convert it to a twin engined plane. Or putting canards right behind the prop in place or in addition to the elevons.
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Old Feb 12, 2013, 07:05 PM
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What about using those little fins on the leading edge of the wing to direct air flow? Saw them on a different plane. I guess next time around it would be better to have a single rudder in the center so the elevons can be closer to center.
Not even sure how the front motor will fly. Just trying to have fun and mess around building. My original plan was to make it a front motor glider of sorts with tiny motor and small 2 cell battery. Then decided to make big rudders. With the added weight i opted for the larger motor and battery. Now its sort of a mixture of planes. Super floaty lightweight stryker. But big control surfaces with lots of deflection and a nice power setup. Should be about 10-11 ounce plane and 16-20oz thrust depending which prop it can safely swing.

Still curious if i was to trim the elevons. Are there any guidelines to size and angle? Or just take a 1/2 inch off the inside edge and call it good?
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Old Feb 13, 2013, 06:27 PM
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There's simply no guidelines to size or angle. We see everything from narrow strip ailerons with only a little throw for slower handling models or small ailerons with very little throw on racing models up to 30 to 50% ailerons with massive 40 or more degree throw on slow flying 3D models that are looking to darn near turn themselves inside out. So you can leave them as they are or cut them down. The key is that the slower you want to fly the model the more benefit you'll get from wider surfaces. ESPECIALLY since you won't have any sort of prop blast over the control surfaces.
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Old Feb 13, 2013, 11:41 PM
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Thank you B for Bruce!!!
I get so worked up on trying to build the plane the best i can. Always have a million questions that develop and slow me down. This forum is so cool really glad it exists.
Thanks to everyone who helped me.
At the moment im thinking of taking off 1/4 to 3/8 of an inch from the inside. The elevons are so huge compared to what we normally run that trimming them should not hinder slow flight performance.
My current plane has 1.75" elevons. This plane has 3".

Is there a point that the elevons become too wide and screw up flight performance?
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Old Feb 14, 2013, 02:13 AM
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Is there a point that the elevons become too wide and screw up flight performance?
Yes. On a swept 'wing the elevons can have a very long chord, say 30 or 40%, but the span should be limited to outboard of MAC because any elevon surface inboard of MAC will just reduce lift and pitch authority. On a plank that rule of thumb is pretty much reversed. A full span elevon of 15 to 20% chord will produce the greatest change in pitching moment per change in lift ie delta Cm per delta CL. Your stryker type could go with either system because of the low aspect ratio and low trailing edge sweep. If I were starting from scratch with that planform I'd go with the short span elevons as per Edward Udens' research.

--Norm
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Old Feb 14, 2013, 07:21 AM
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If I recall correctly, sometimes on flying planks or forward swept trailing edge wings it is also advantageous to have separate ailerons and elevators, mainly at slow speed and high angle of attack. With separate inboard elevators deflected upward the wing will automatically acquire some aerodynamic washout. and at the same time the moment arm for the elevator and the ailerons is increased.
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Old Feb 14, 2013, 06:25 PM
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Thanks Norm and Brandano!! I read up what MAC means and checked out the pictures of Edward Udens elevon positions.

Is the MAC of a Stryker C where the fins attach?

Think im gonna keep the long span elevon. Take about 1/4-1/2" off the inside to create a taper. They will be 3" long and i think the MAC is around 12". So i guess im around 25%.

Thanks so much for all the help!!!


One other thing i noticed on Wiki page about Chords. So the leading edge of the wing is not the furthest point forward. But where the curve of the front of the airfoil stops? Its not huge but it would change my measurements on other planes a bit.
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